Intrusive Rocks of the Holden and Lucerne Quadrangles, Washington— The Relation of Depth Zones, Composition, Textures, and Emplacement of Plutons By FRED W. CATER GEOLOGICAL SURVEY PROFESSIONAL PAPER 1220 A study of intrusive rocks ranging in age from Triassic t0 Miocene and/in dept/z zones from subvolcanic t0 calazone UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Cater, Frederick William, 1912— lntrusive rocks of the Holden and Luceme Quadrangles, Washington. (Geological Survey Professional Paper [220) Bibliography: 108 p. Supt. of Docs. No.2 [19.16 1, Intrusions (Geology)—Washington (State)—Chelan Co. 2. Geology, Stratigraphic—Cenozoic. 3. Geology, Stratigraphic—Mesozoic. I. Title. ll. Series: United States Geological Survey Professional Paper 1220. 551 .8’8’0979759 80—607844 QE611.5.U6C37 For sale by the Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Page Abstract ------------------------------------------------- 1 Tertiary intrusive rocks—Continued Introduction --------------------------------------------- 3 Early Eocene intrusive rocks—Continued Acknowledgments ------------------------------------ 3 Clark Mountain stocks ............................ 53 Previous work ---------------------------------------- 3 Late Eocene intrusive rocks ........................... 54 Tectonic setting ------------------------------------------ 3 Mineralogy ...................................... 56 Preintrusive rocks ---------------------------------------- 4 Textures ......................................... 57 Swakane Biotite Gneiss ------ . ------------------------ 5 Analytical data .................................. 58 Late Paleozoic metamorphic rocks --------------------- 6 Description of rocks .............................. 58 Pre-Tertiary intrusive rocks ------------------------------- ’7 Dikes ------------------------------------------- 58 Ultramafic rocks ------------------------------------- 7 Pawdite ..................................... 53 Dumbell Mountain plutons --------------------------- 8 Kersantite ................................... 59 Hornblende-quartz diorite gneiss ------------------- 11 Augite minette ............................... 59 Hornblende-quartz diorite augen gneiss ------------ 17 Spessartite ................................... 59 Gneissic hornblende-quartz diorite ----------------- 17 Other fine-grained, dark dikes ................. 60 Bearcat Ridge plutons -------------------------------- 18 Hornblende diorite of upper Rock Creek ........ 60 Leroy Creek pluton ----------------------------------- 20 Granitoid dikes .............................. 60 Seven-fingered Jack and Entiat plutons ---------------- 23 Duncan Hill pluton ............................... 61 Biotite-hornblende-quartz diorite ------------------ 24 Larch Lakes pluton ............................... 68 Hornblende~biotite-quartz diorite ------------------ 25 Rampart Mountain pluton ........................ 70 Hornblende-quartz diorite gneiss ------------------- 26 Old Gib volcanic rocks and associated dacite porphyry 72 Hornblende diorite and gabbro -------------------- 26 Jointing in volcanic neck ..................... 74 Contact complexes ------------------------------- 27 Intrusive breccia ................................. 75 Leucocratic quartz diorite and granodiorite ------------- 28 Hornblende-biotite-quartz diorite .................. 75 Tenpeak and White Mountain plutons ----------------- 31 Railroad Creek pluton and associated rocks ......... 77 Hornblende-biotite-quartz diorite and quartz diorite Copper Peak and Holden Lake plutons ------------- 80 gneiss ----------------------------------------- 33 Biotite-quartz monzonite and granodiorite .......... 82 Flaser gneiss --------------------------------- 33 Post-late Eocene intrusive rocks ....................... 84 Interlayered zone unit ------------------------ 34 Dikes of biotite dacite and rhyodacite ............. 84 Contact complexes --------------------------- 34 Rhyodacite .................................. 34 Petrography --------------------------------- 34 Dacite and rhyodacite dikes ................... 85 Sulphur Mountain pluton ----------------------------- 35 Miocene intrusive rocks ............................... 86 High Pass and Buck Creek plutons -------------------- 37 Cloudy Pass batholith and related rocks ........... 86 Riddle Peaks pluton ---------------------------------- 40 Chilled border rocks -------------------------- 87 Cardinal Peak pluton --------------------------------- 46 Leucocratic biotite-quartz monzonite ............... 89 Hornblende-biotite-quartz diorite and biotite grano- Contact complexes ....................................... 90 diorite ------------------------------------ r 47 General geologic problems relating to the intrusive rocks - 91 Calcic hornblende diorite and quartz diorite -------- 48 Relation of intrusives to host rocks and the room problem 92 Contact complexes ------------------------------- 49 Ages of intrusive rocks and related problems ........... 93 Petrography ------------------------------------- 50 Composition and differentiation ....................... 96 Tertiary intrusive rocks ----------------------------------- 53 Textures and depth zones ............................. 101 Early Eocene intrusive rocks -------------------------- 53 References ............................................... 102 ILLUSTRATIONS Page FIGURE 1. Index map of Washington showing physiographic provinces and locations of Holden and Lucerne quadrangles ------ 3 2. Map showing locations of the plutons in the Holden and Lucerne quadrangles ................................... 4 3. Triangular diagram showing modal rock classification system used in this report ................................. 5 4. Tectonic sketch map of north-central Washington showing the relation of the Holden and Lucerne quadrangles to the tectonic framework .................................................................................... 5 5. Photograph of interlayered late Paleozoic quartzite, biotite schist, and hornblende gneiss, upper Klone Creek, Lucerne quadrangle ............................................................................................... 6 ID IV FIGURE 13. 14—18. 19. 20. 21. 22. 23. 24—27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37—40. 41. 42. 43. 44. 45. 46. 47—50. 51. 52. CONTENTS . Photograph of gabbroic orbicules in peridotite from Holden mine, Holden quadrangle ---------------------------- . Photograph of Dumbell Mountain hornblende-quartz diorite augen gneiss containing inclusions with mafic margins - . Photomicrograph of gneissic hornblende-quartz diorite from the Dumbell Mountain plutons ----------------------- . Photomicrograph of oscillatory- and patchy-zoned andesine in hornblende-quartz diorite gneiss from the Dumbell Moun- tain plutons .............................................................................................. . Map showing location of modally analyzed samples from the Dumbell Mountain plutons ------------------------- . Triangular diagram showing plot of modes of rock samples from Dumbell Mountain plutons ---------------------- . Triangular diagram showing plot of norms of rock samples from the Dumbell Mountain, Bearcat Ridge, and Leroy Creek plutons ............................................................................................ Variation diagrams of major oxides in analyzed rock samples from pre-Tertiary plutons .......................... Photographs of: 14. Hornblende-quartz diorite gneiss from the Dumbell Mountain plutons ..................................... 15. Gneissic hornblende-quartz diorite, a less gneissic facies, from the Dumbell Mountain plutons ............... 16. Protoclastic biotite-hornblende-quartz diorite gneiss from the Bearcat Ridge plutons ........................ 17. Hornblende-biotite granodiorite gneiss from the Bearcat Ridge plutons ..................................... 18. Biotite granodiorite from the Bearcat Ridge plutons ...................................................... Map showing location of modally analyzed samples from the Bearcat Ridge plutons ............................. Triangular diagram showing plot of modes of rock samples from Bearcat Ridge plutons .......................... Photograph of gneissic biotite-quartz diorite from the Leroy Creek pluton ....................................... Map showing location of modally analyzed samples of biotite-quartz diorite gneiss from the Leroy Creek pluton - ~- Triangular diagram showing plot of modes of biotite-quartz diorite gneiss samples from the Leroy Creek pluton - Photographs of: 24. Coarse- and medium-grained hornblende gabbro from contact complex of Entiat pluton .................. 25. Contact complex of Entiat pluton showing pegmatitic hornblende gabbro, schist, and hornblendite -------- 26. Biotite-hornblende-quanz diorite from the Seven-fingered Jack pluton .................................. 27. Biotite-hornblende-quartz diorite from the Seven-fingered Jack pluton, showing strong lineation defined by pencils of mafia-rich material ...................................................................... Photomicrograph of protoclastic hornblende-biotite-quartz diorite from Seven—fingered Jack pluton showing cracks in fractured crystal of andesine filled with biotite, orthoclase, and quartz ........................................ Photograph of hornblende-biotite-quartz diorite from the Entiat pluton ......................................... Photograph of hornblende-biotite quartz diorite gneiss from the Entiat pluton ................................... Photograph of rotated inclusions of late Paleozoic gneiss in hornblende-quartz diorite gneiss from the Entiat pluton Photomicrograph of hypidiomorphic and protoclastic hornblende»quartz diorite gneiss from the Entiat pluton —————— Photograph of contact complex of the Entiat pluton showing gabbro chilled against quartz diorite ................. Map showing location of modally analyzed samples frOm the Seven-fingered Jack and Entiat plutons ............. Triangular diagram showing plot of modes of rock samples from Seven-fingered Jack and Entiat plutons ---------- Triangular diagram showing plot of norms of leucocratic quartz diorite and granodiorite samples and rock samples from the Seven-fingered Jack and Entiat plutons ................................................................. Photographs of: 37. Migmatite of leucocratic quartz diorite in biotite-hornblende-quartz diorite of the Seven-fingered Jack pluton 38. Leucocratic quartz diorite ............................................................................ 39. Hornblende- biotite- quartz diorite from the Tenpeak pluton ............................................. 40.1nterlayered biotite- hornblende- quartz diorite gneiss and hornblende schist of the interlayered unit of the Ten- peak pluton ...................................................................................... Photomicrograph of intergrowth of clinozoisite-epidote and quartz resembling myremekite, quartz diorite from the Tenpeak pluton .......................................................................................... Map showing location of modally analyzed samples from the Tenpeak and White Mountain plutons ............... Triangular diagram showing plot of modes of quartz diorite and quartz diorite gneiss samples from the Tenpeak and White Mountain plutons .................................................................................. Triangular diagram showing plot of norms of rock samples from the Tenpeak, White Mountain, Cardinal Peak, and High Pass plutons ........................................................................................ Photograph of gneissic granodiorite from the Sulphur Mountain pluton ......................................... Triangular diagram showing plot of modes of homblende-biotite granodiorite samples from the Sulphur Mountain pluton ................................................................................................... Photographs of: 47. Granodiorite from the High Pass pluton .............................................................. 48. Foliated granodiorite from the High Pass pluton ....................................................... 49. Granodiorite of the High Pass pluton in the core of an antiform in upper Paleozoic metasedimentary rocks - 50. Contact of granodiorite of the High Pass pluton and hornblende schist .................................. Map showing locations of modally analyzed samples of biotite-qua1tz diorite and granodiorite from the High Pass and Buck Creek plutons ....................................................................................... Triangular diagram showing plot of modes of biotite-quartz diorite and granodiorite samples from the High Pass pluton Page 10 11 12 15 15 16 17 18 18 19 19 21 22 22 24 24 25 25 26 26 27 27 27 28 28 29 29 31 32 32 32 34 34 35 36 36 37 37 38 38 38 39 40 41 41 FIGURES 53—56. 57. 58. 59. 60—64. 65. 67. 68. 69. 70. 71. 72—75. 76. 77. 78. 79. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. CONTENTS Photographs of: 53. Layered hornblende grabbro of the Riddle Peaks pluton and septum 0f gneiss in Riddle Peaks ............ 54. Hornblende gabbro from the Riddle Peaks pluton ...................................................... 55. Contorted layers in gabbro of the Riddle Peaks pluton ................................................. 56. Layered gabbro from the Riddle Peaks pluton ......................................................... Photomicrograph of gabbro from the Riddle Peaks pluton showing poikilitic crystals of bytownite that have grown around hornblende ............................................................................................... Photomicrograph showing chlorite having helminth structure in gabbro 0f the Riddle Peaks pluton ................ Map showing location of modally analyzed samples of gabbro from the Riddle Peaks pluton ...................... Photographs of: 60. Foliated hornblende-biotite-quartz diorite from the Cardinal Peak pluton ................................ 61. Quartz diorite from the Cardinal Peak pluton, showing a pseudoporphyritic appearance resulting from extreme protoclasis ........................................................................................ 62. Biotite granodiorite from the Cardinal Peak pluton, showing only slight flaser texture .................... 63. Contact complex of Cardinal Peak pluton, consisting largely of gabbro .................................. 64. Contact complex of Cardinal Peak pluton showing inclusions of metamorphic rocks, gabbro, and hornblendite in a matrix of quartz diorite .......................................................................... Map showing location of modally analyzed samples of quartz diorite and granodiorite from the Cardinal Peak pluton --------------------------------------------------------------------------------------------------- . Triangular diagram showing plot of modes of diorite, quartz diorite, and granodiorite samples from the Cardinal Peak pluton ................................................................................................... Photograph of hornblende-biotite granodiorite from the Clark Mountain stocks .................................. Map showing location of modally analyzed samples of biotite-quartz diorite and granodiorite of the Clark Mountain stocks --------------------------------------------------------------------------------------------------- Triangular diagram showing plot of modes of biotite—quartz diorite and granodiorite samples from the Clark Mountain stocks --------------------------------------------------------------------------------------------------- Variation diagram of major oxides in rock samples from Eocene plutons ........................................ Triangular diagram showing plot of norms of samples from small Tertiary intrusives ............................. Photographs of: 72. Marble-cake mixture of biotite-quartz diorite and granodiorite in Duncan Hill pluton at Fern Lake -------- 73. Protoclastic hornblende-biotite-quartz diorite near north end of Duncan Hill pluton ...................... 74. Biotite granodiorite from the Duncan Hill pluton at the south edge of the Lucerne quadrangle ............ 75. Miarolytic biotite-quartz monzonite from the Duncan Hill pluton on Stormy Mountain ................... Map showing location of chemically and modally analyzed samples of Duncan Hill, Larch Lakes, and Rampart Mountain plutons -------------------------------------------------------------------------------------------------- Triangular diagram showing plot of modes of rock samples from the Duncan Hill pluton ......................... Variation diagram of major oxides in samples from the Duncan Hill pluton ..................................... Triangular diagram showing plot of norms of rock samples from the Duncan Hill pluton, and isobaric lines mark- ing the position of the quartz-feldspar boundary at various water pressures .................................... . Photograph of biotite granodiorite from the Larch Lakes pluton ................................................ 81. 82—86. Triangular diagram showing plot of modes of rock samples from the Larch Lakes and Rampart Mountain plutons —- Photographs of: 82. Biotite-qualtz monzonite from the Rampart Mountain pluton ........................................... 83. Columnar joints in Old Gib volcanic neck and crosscutting basaltic dike ................................ 84. Vertical sheeting in Peléan spine in Old Gib volcanic neck ............................................. 85. Intermixed hornblende-biotite-quartz diorite, biotite-quartz monzonite, and inclusions of biotite schist, show- ing various stages of alteration, at the Holden mine .................................................. 86. Hornblende-biotite-quartz diorite ..................................................................... Triangular diagram showing plot of modes of homblende-biotite-quartz diorite samples ........................... Map showing location of modally analyzed samples from Eocene plutons ........................................ Photograph of biotite granodiorite from the Railroad Creek pluton .............................................. Photograph of porphyritic biotite-quartz diorite dike rock from the Railroad Creek pluton ........................ Triangular diagram showing plot of modes of granodiorite and quartz diorite samples from the Railroad Creek pluton Photograph of hornblende-quartz gabbro cut by biotite-quartz monzonite from the Holden Lake pluton ............ Triangular diagram showing plot of modes of hornblende-quartz gabbro samples from the Copper Peak and Holden Lake plutons ............................................................................................. Photograph of biotite-quartz monzonite ....................................................................... Triangular diagram showing plot of modes of biotite-quartz monzonite and granodiorite samples .................. Map showing locations of modally analyzed samples of granodiorite from the Cloudy Pass batholith ............... Triangular diagram showing plot of modes of granodiorite samples from the Cloudy Pass batholith ................ Diagram showing variation in contents of SiOe, K20. Na20, and CaO along the length of the Duncan Hill pluton -— Page 41 42 42 42 44 44 46 47 48 48 49 49 52 52 53 55 55 55 56 61 62 64 64 68 69 69 70 70 71 71 73 73 75 76 77 78 78 79 80 81 82 83 85 88 88 99 VI TABLE 1. 2. \IQUIAOD 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. . Modal analyses of samples of late Eocene hornblende-biotite—quartz diorite ......................................... 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. CONTENTS TABLES Chemical and spectrographic analyses of a composite sample of Swakane Biotite Gneiss ............................. Modal analyses of samples of hornblende-quartz diorite and hornblende-quartz diorite gneiss from the Dumbell Mountain plutons .................................................................................. L .................. . Chemical and spectrographic analyses and norms of rock samples from the Dumbell Mountain plutons ............... . Modal analyses of samples of quartz diorite and granodiorite gneiss from the Bearcat Ridge plutons .................. . Chemical and spectrographic analyses and norms of rock samples from the Bearcat Ridge plutons ................... Modal analyses of samples of biotite-quartz diorite gneiss from the Leroy Creek pluton .............................. . Chemical and spectrographic analyses and norms of a composite sample of biotite—quartz diorite from the Leroy Creek pluton ...................................................................................................... . Modal analyses of samples of diorite, gabbro, quartz diorite, and quartz diorite gneiss from the Seven-fingered Jack and Entiat plutons ............................................................................................... . Chemical and spectrographic analyses and norms of rock samples from the Seven-fingered Jack and Entiat plutons - >- 10. 11. 12. 13. 14. 15. Chemical and spectrographic analyses and norms of samples of leucocratic quartz diorite and granodiorite ............ Modal analyses of samples of quartz diorite and quartz diorite gneiss from the Tenpeak and White Mountain plutons - Chemical and spectrographic analyses and norms of composite rock samples from the Tenpeak and White Mountain plutons Modal analyses of samples of hornblende—biotite granodiorite from the Sulphur Mountain pluton ..................... Modal analyses of samples of biotite—quartz diorite and granodiorite from the High Pass pluton ...................... Chemical and spectrographic analyses and norms of a composite sample of biotite-quartz diorite from the High Pass pluton Modal analyses of samples of hornblende gabbro from the Riddle Peaks pluton ..................................... Chemical and spectrographic analyses of a composite sample of hornblende gabbro from the Riddle Peaks pluton ----- Modal analyses'of samples of diorite, quartz diorite, and granodiorite from the Cardinal Peak pluton ................. Chemical and spectrographic analyses and norms of composite samples of rocks from the Cardinal Peak pluton ------- Modal analyses of samples of biotite—quartz diorite and granodiorite from the Clark Mountain stocks ................. Chemical and spectrographic analyses and norms of a composite sample of biotite granodiorite from the Clark Mountain stocks ------------------------------------------------------------------------------------------------------ Modal analyses of samples of biotite— and homblende-biotite-quartz diorite and granodiorite from the Duncan Hill pluton Chemical and spectrographic analyses and norms of rocks from the Duncan Hill pluton ............................. Modal analyses of samples of granodiorite from the Larch Lakes pluton ............................................ Chemical and spectrographic analyses and norms of a composite sample of biotite granodiorite from the Larch Lakes pluton Modal analyses of samples of quartz monzonite from the Rampart Mountain pluton ................................ Chemical and spectrographic analyses and norms of a composite sample of biotite—quartz monzonite from the Rampart Mountain pluton ............................................................................................ Chemical and spectrographic analyses and norms of a composite sample of dacite from the Old Gib volcanic neck - Chemical and spectrographic analyses and norms of a composite sample of late Eocene homblende-biotite-quartz diorite Modal analyses of samples of granodiorite and quartz monzonite from the Railroad Creek pluton .................... Chemical and spectrographic analyses and norms of a composite sample of biotite granodiorite from the Railroad Creek pluton ...................................................................................................... Modal analyses of samples of hornblende-quartz gabbro from the Copper Peak and Holden Lake plutons ............. Chemical and spectrographic analyses and norms of quartz gabbro from the Holden Lake pluton ..................... Modal analyses of samples of biotite-quartz monzonite and granodiorite of post—late Eocene age ..................... Chemical and spectrographic analyses and norms of a composite sample of biotite-quartz monzonite ................. Modal analyses of samples of granodiorite from the Cloudy Pass bath01ith ......................................... Initial strontium isotope ratios of gabbro and quartz diorite from intrusive complexes associated with the Seven-fingered Jack plutons ................................................................................................ Radiometric ages of intrusive rocks .............................................................................. Page 13 14 20 21 23 23 30 31 33 35 36 38 40 41 46 47 51 52 54 54 66 70 71 72 73 74 76 77 80 80 82 82 84 85 88 92 94 INTRUSIVE ROCKS OF THE HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON— THE RELATION OF DEPTH ZONES, COMPOSITION, TEXTURES, AND EMPLACEMENT OF PLUTONS By FRED W. CATER ABSTRACT The core of the northern Cascade Range in Washington consists of Precambrian and upper Paleozoic metamorphic rocks cut by numerous plutons, ranging in age from early Triassic to Miocene. The older plutons have been eroded to catazonal depths, whereas subvolcanic rocks are exposed in the youngest plutons. The Holden and Luceme quadrangles span a-sizeable and representative part of this core. The oldest of the formations mapped in these quadrangles is the Swakane Biotite Gneiss, which was shown on the quadrangle maps as Cretaceous and older in age. The Swakane has yielded a middle Paleozoic metamorphic age, and also contains evidence of zircon in- herited from some parent material more than 1,650 m.y. old. In this report, the Swakane is assigned an early Paleozoic or older age. It con- sists mostly of biotite gneiss, but interlayered with it are scattered layers and lenses of hornblende schist and gneiss, clinozoisite-epidote gneiss, and quartzite. Thickness of the Swakane is many thousands of meters, and the base is not exposed. The biotite gneiss is probably derived from a pile of siliceous volcanic rocks containing scattered sedimentary beds and basalt flows. Overlying the Swakane is a thick sequence of eugeosynclinal upper Paleozoic rocks metamorphosed to amphibolite grade. The sequence includes quartzite and thin layers of marble, hornblende schist and gneiss, graphitic schist, and smaller amounts of schist and gneiss of widely varying compositions. The layers have been tightly and complexly folded, and, in places, probably had been thrust over the overlying Swakane prior to metamorphism. Youngest of the supracrustal rocks in the area are shale, arkosic sand- stone, and conglomerate of the Paleocene Swauk Formation. These rocks are preserved in the Chiwaukum graben, a major structural ele- ment of the region. Of uncertain age, but possibly as old as any of the intrusive rocks in the area, are small masses of ultramafic rocks, now almost completely altered to serpentine. These occur either as included irregular masses in later intrusives or as tectonically emplaced lenses in metamorphic rocks. Also of uncertain age but probably much younger, perhaps as young as Eocene, are larger masses of hornblendite and hornblende periodotite that grade into hornblende gabbro. These are exposed on the surface and in the underground workings of the Holden mine. Oldest of the granitoid intrusives are the narrow, nearly concordant Dumbell Mountain plutons, having a radiometric age of about 220 m.y. They consist of gneissic hornblende-quartz diorite and quartz diorite gneiss. Most contacts consist of lit-par-lit zones, but some are gradational or more rarely sharp. The plutons are typically catazonal. Closely resembling the’Dumbell Mountain plutons in outcrop ap- pearance, but differing considerably in composition, are the Bearcat Ridge plutons. These consist of gneissic quartz diorite and granodiorite. The Bearcat Ridge plutons are not in contact with older dated plutons, but because their textural and structural characteristics so closely resemble those of the Dumbell Mountain plutons, they are considered to be the same age. Their composition, however, is sug— gestive of a much younger age. Cutting the Dumbell Mountain plutons is the Leroy Creek pluton, consisting of gneissic biotite-quartz diorite and trondjhemite. The gneissic foliation in the Leroy Creek is characterized by a strong and pervasive swirling. Cutting both the Dumbell Mountain and Leroy Creek plutons are the almost dikelike Seven-fingered Jack plutons. These range in composition from gabbro to quartz diorite; associated with them are contact complexes of highly varied rocks characterized by gabbro and coarse—grained hornblendite. Most of the rocks are gneissic, but some are massive and structureless. Radiometric ages by various methods range from 100 to 193 m.y. Dikes, sills, small stocks, and irregular clots of leucocratic quartz diorite and granodiorite are abundant in the Swakane Biotite Gneiss and are locally abundant in the Seven§ l / TS 1% {\ \ ,Stehekin Grand if: < :\\ \ w: Lucer _Cou/ee/l/;,! o OLYMPIC? fg’c'b' Pk . . ea” "" )1“ | \1 MTS HLi/ enatchee Chelar/ Spokangq l T) \ Met ttle , d K I 0 Wenatchee 0 @Ta oma \\ / - COLUMBIA 47 _ \lw // MtRainie\r\>té \ PLATEAU i— 3 . WASHINGTON j ——/ OREGON l 200 KILOMETEHS 0 50 100 150 FIGURE 1.—Index map of Washington showing physiographic provinces and locations of Holden and Lucerne quadrangles. 121°00’ INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON 120°30‘ 48° 00' 1'0 KILOMETERS FIGURE 2.——Map showing locations of the plutons in the Holden and Lucerne quadrangles. Patterned areas are metamorphic rocks. out against the Hope—Straight Creek fault system. The northernmost extent of Cascade rocks west of the Straight Creek fault is not known because of the obliterative effects of the Coast Range Batholithic com- plex.” They further postulated that the Ross Lake fault or a precursor of it was the most likely place where this shift occurred. The pull apart from the North American continent is assumed to lie beneath the Miocene basalts of the Columbia River plateau. In any event the minicontinent is separated from the pre-Carboniferous continental shelf in northeastern Washington by some hundreds of kilometers, a gap now 1. Dumbell Mountain plutons 10. Buck Creek pluton 19. Homblende-biotite-quartz diorite 2. Bearcat Ridge plutons 11. Riddle Peaks pluton 20. Railroad Creek pluton 3. Leroy Creek pluton 12. Cardinal Peak pluton 21. Copper Peak pluton 4. Seven-fingered Jack plutons 13. Clark Mountain stocks 22. Holden Lake pluton 5. Entiat pluton 14. Duncan Hill pluton 23. Biotite-quartz monzonite and granodiorite 6. Tenpeak pluton 15. Larch Lakes pluton 24. Cloudy Pass batholith 7. White Mountain pluton 16. Rampart Mountain pluton 25. Leucocratic quartz monzonite 8. Sulphur Mountain pluton 17. Old Gib volcanic neck 9. High Pass pluton 18. Intrusive breccia consisting of large masses of island-arc and oceanic as- semblages intruded by very large amounts of granitic material. PREINTRUSIVE ROCKS The oldest plutons in the area intruded a terrain of metamorphic rocks of late Precambrian and late Paleozoic age. Younger plutons, of course, intruded not only the metamorphic rocks but also the older plutons. The oldest of the formations exposed in the area, and probably in the northern Cascades, is the Swakane PREINTRUSIVE ROCKS 5 Biotite Gneiss. The Swakane is possibly equivalent to at least part of the Custer Gneiss of McTaggart and Thompson (1967), which was originally called the Custer Granite Gneiss by Daly (1912, p. 525-526). The forma- tion has also been called the Skagit Gneiss by Misch (1952, p. 12-14) and others, but the name “Skagit” was preempted by Daly in 1912 for a sequence of volcanic rocks along the Canadian border west of Ross Lake. Overlying the Swakane is a sequence of metamorphosed eugeosynclinal, rather thinly laminated, complexly folded rocks of late Paleozoic age that is widely exposed in both quadrangles and elsewhere in the region. The un- metamorphosed Paleocene Swauk Formation outcrops in a few locations in the Chiwaukum graben (Willis, 1953) within the Holden quadrangle. SWAKANE BIOTITE GNEISS Quartz 100 50 Quartz . monzonite Granite Diorite and 5 gabbro o 7 / \ 0 5 35 65 Plagioclase system used in this report. 121°00’ BRITISH COLUMBIA 100 , The Swakane Biotite Gneiss is predominantly a light— Potassmm feldspar FIGURE 3.—Triangular diagram showing the modal rock classification brown to brownish-gray, fine— to medium-grained, strongly foliated rock. Intercalated with the gneiss, locally, are rather thin layers of hornblende schist and gneiss, gneisses predominantly of clinozoisite-epidote, and more rarely, thin layers of quartzitic rock. Some of these layers can be traced for kilometers and represent WASHINGTON OKANOGAN Mesozoic ocean floor "5 oceanic o X3‘dlu03 uonanpq lHSIVHlS M TH‘OW GRABEN Jurassic- Cretaceous NORTHERN CASCADES Precambrian-Tertiary plutonic complex Cascadia pug aiozoaled J 13an )lEIEHO HOLDEN OUADHANGLE 48° 00’ 51001 oiueaoo oiozosew CHlWAUKUM GRABEN Columbia Plateau Miocene basalt an original compositional layering or bedding. Kyanite occurs sparsely on Phelps Ridge but is rare elsewhere, ARC COMPLEX Triassic and Jurassic and garnet 1s common in many places. Streaks of blotlte accretionary terrain on the plane of foliation commonly define a marked Paleozoic-Jurassic llneation. Lenses and pods of quartz and 1rregular island arc and masses of leucocratic quartz diorite are notably abun- dant in some places and common elsewhere. Other than assemblages compositional layering, no original textural or structural features remain. Probably many thousands of meters of the formation are exposed in the northeast part of the Lucerne quadrangle, and equally great thicknesses are exposed elsewhere in the region; the base of the Swakane is not known to be exposed. The Swakane was assigned a Cretaceous and older age on the Lucerne and Holden quadrangle maps (Cater and Crowder, 1967; Cater and Wright, 1967). Mattinson (1972) obtained ages of more than 1,650 my (age of parent material) from zircons in the rock along the Columbia River, but he also got Paleozoic ages of about 415 my (metamorphic age) from zircons in the biotite gneiss along the Skagit River about 65 km northwest of the Lucerne quadrangle. On the basis of a minimum age of 415 my, the Swakane is considered early Paleozoic or older in this report. Staatz 0 10 20 30 40 50 KILOMETEHS ;l_l__l__L_l and others (1972, p. 8774, table 2) mapped these rocks as Custer Gneissof McTaggart and Thompson (1967), and FIGURE 4.—Tectonic sketch map of north-central Washington showing MISCh (1952’ 1966) mapped them as Skaglt Gnelss' The the relation of the Holden and Lucerne quadrangles to the regional tectonic framework. Named faults separating major structural ele- ments restored. Heavy lines are faults; lighter lines are contacts. Swakane on the Columbia River is directly traceable to the Swakane in the Holden quadrangle, which in turn is identical to rock mapped as Swakane in the Lucerne 6 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON quadrangle. Misch (1966, p. 107, fig. 7-1) showed the Skagit Gneiss on the Skagit River as continuous with the Swakane of the Lucerne quadrangle. Possibly metamorphism of the rocks in the area between the two localities has obscured the relation of the Custer to the Swakane. The nature of the material from which the Swakane Biotite Gneiss is derived is not known with certainty. Hornblende-rich layers may have been basaltic flows, and quartzitic and epidote-rich layers probably were sedimentary beds. Waters (1932, p. 616) considered it likely that the biotite gneiss was derived from mostly arkosic material, a possibility that still remains; but more recently Hopson (in Mattinson, 1972, p. 3773) con- sidered a volcanic derivation more likely. The accumula- tion of a nearly uniform pile of arkose many thousands of meters thick of the wide areal extent of the Swakane and its equivalents seems unlikely. Arkosic piles such as the Permian Cutler Formation in southwest Colorado and Utah tend to be segregated by sedimentary differentia- tion into interlayered beds of sandstone, mudstone, and shale farther from source areas (Cater, 1970). The Swakane and its equivalents are remarkably uniform in composition throughout their outcrop area and show no tendency toward original segregation into contrasting beds. The chemical composition of the biotite gneiss ap- proximates that of dacite; table 1 shows an analysis of a composite sample of representative specimens of biotite gneiss. With the foregoing in mind, it seems more likely that the Swakane Biotite Gneiss was probably derived from a huge, fairly silicic volcanic pile. Scattered basaltic flows now form the hornblende gneiss and schist layers, and TABLE l.—Chemical and spectrographic analyses of Swakane Biotite Gneiss, central Washington [N320 and K0 determined by flame photometer by Violet Merritt. Fegogi, MgO, and C30 determined by atomic absorption by Violet Merritt. FeO determined volumetrically by H. H. Lipp. SiOg and A120; determined colorimetrically by G. D. Shipley and G. T. Burrow. Spectrographic analysis by Harriet Neiman.] Chemical analysis (percent) Spectrographic analysis (percent) SiOg .................... 68.2 Ti ..................... 03 A1203 -------------------- 150 Mn -------------------- 07 F620;; ------------------- 3.45 Ba --------------------- .05 FeO ..................... 2.8 Be ..................... .0001 MgO -------------------- 1.60 Co ————————————————————— .007 CaO -------------------- 2.04 Cu ..................... .0015 Na20 ———————————————————— 3.01 Ga ..................... .002 K20 ..................... 1.96 Ni ..................... .002 Total .................. 98.06 Pb --------------------- .0015 Sc ..................... .0007 Sr --------------------- .03 V ---------------------- .015 Y ...................... .0015 Yb --------------------- .0002 Zr --------------------- .02 the quartzite and mica schist layers were probably thin sedimentary beds deposited in small, ephemeral basins in the volcanic terrain. The Swakane Biotite Gneiss has been warped into steep-sided but generally open folds having steep to ver- tical axial planes. Waters (1932, p. 621) noted that the same type of folds characterize the Swakane farther to the south along the Columbia River. Generally, attitudes of the gneiss are uniform through considerable thicknesses, but locally attitudes of foliation are irregular, particularly where disturbed by various intrusives. LATE PALEOZOIC METAMORPHIC ROCKS Overlying the Swakane Biotite Gneiss is a highly diverse sequence of metamorphosed eugeosynclinal rocks consisting largely of rather thinly layered, highly and complexly folded hornblende and biotite-horn- blende schist and gneiss, biotite schist, clinopyroxene- biotite schist, graphitic schist, quartzite, and lenses of marble (fig. 5). These rocks crop out on opposite sides of the Chiwaukum graben, which is mostly underlain by the Swakane. The rocks to the east of the graben were called “younger gneissic rocks of the Holden area” on the Holden and Lucerne geologic quadrangle maps, and the rock to the west, the “rocks of the Napeequa River area.” The rocks in the two belts are almost surely parts of a single series or group, but attempts at stratigraphic correlations between the belts have been thwarted, in part, probably, by rapid facies changes combined with FIGURE 5.*Interlayered upper Paleozoic quartzite, biotite schist, and hornblende gneiss, upper Klone Creek, Lucerne quadrangle. PRE-TERTIARY INTRUSIVE ROCKS 7 depositional wedging, but more critically by metamorphic changes, extreme structural complexities, and mangling by the various intrusions. Sedimentary features are preserved well enough to determine tops of beds in a few places on the ridge north of Holden, although the rocks are metamorphosed to amphibolite grade. Search elsewhere, however, failed to find similar unequivocal evidence for tops, despite the little- modified aspect of the beds in some places. Isoclinal folding and other structural complexities render thickness determinations highly uncertain, but in the Holden quadrangle the exposed thickness cannot be less than about 1.5 km, and it may well be at least twice that much. If the “rocks of the central schist belt” of the Glacier Peak quadrangle (Crowder and others, 1966) are also part of the same sequence, as seems likely, then the thickness could be very much greater. Zircons from a layer of metamorphosed keratophyre tuff in the Holden quadrangle were dated by Mattinson (1972, p. 3773) as 265i15 m.y. old; hence, the rocks are of late Paleozoic age. The structure of the upper Paleozoic metamorphic rocks differs radically from that of the underlying Swakane Biotite Gneiss and is far more complex. The major folds in the Swakane are broad, open, and mostly fairly simple, whereas those in the upper Paleozoic rocks are tightly compressed and overturned or actually in- verted. For example, sedimentary structures indicate that the antiform in these rocks that trends across the Lucerne quadrangle is an inverted syncline. East of the Chiwaukum graben, these rocks seem to have been thrust westward over the probably already metamor- phosed Swakane before they became metamorphosed. West of the graben, great thicknesses of beds are missing from the northeast side of the synform of upper Paleozoic metamorphic rocks, where these upper Paleozoic rocks rest on the Swakane, or where the rocks are separated from the Swakane, by a sheet of intrusive rocks. As is detailed in the text that follows, the structure of both se- quences of metamorphic rocks has had a profound in- fluence on the form of the various plutons. The metamorphic rocks are overlain by shale, arkosic sandstone, and conglomerate that were mapped as the Paleocene Swauk Formation. Later work (Whetten, 1976) indicated that the rocks are of late Eocene age; Whetten obtained a fission-track age of 44.1 i4.3 m.y. on Zircons from a tuff near the base of the sequence. In the study area, the sedimentary rocks have been preserved only in the Chiwaukum graben in the Holden quad- rangle, but they underlie large areas elsewhere in the northern Cascades. Within the Holden quadrangle, only a few dikes cut the formation, although farther south are a few small plugs and extensive dike swarms. Of par- ticular interest are numerous cobbles in the con- glomerates consisting of volcanic rocks. These volcanic rocks may be the extrusive equivalents of some of the late Cretaceous intrusive rocks described here. PRE-TERTIARY INTRUSIVE ROCKS ULTRAMAFIC ROCKS A number of small masses of ultramafic rocks ranging from only a few meters to a few hundred meters across crop out in the area. The smaller ones consist of serpen— tine, and some of the serpentine has been extensively altered to talc or tremolite. The larger ones, on the other hand, are highly variable in composition and consist of gabbro, hornblendite, and peridotite. The serpentine masses mostly occur in a northwesterly trending zone ex- tending through the southwest part of the Holden quadrangle to the northwestern part of the Glacier Peak quadrangle. The ultramafic masses of variable composi- tion are confined to the Holden mine—Buckskin Moun- tain area and are probably considerably younger and un- related to the serpentine bodies. Serpentine occurs as scattered lenses apparently tec- tonically emplaced in gneiss and schist or in the in- terlayered zone in the Tenpeak pluton. Rare masses only a few meters across have been seen as inclusions in various quartz diorite plutons scattered about the study area. The largest masses are near the divide at the head of Boulder Creek in the Holden quadrangle. Contacts, where seen, are sharp and either sheared or unsheared. The serpentine is light green to black and either schistose and sheared or dense and structureless. Where altered to talc or tremolite, the rock is light green or nearly white; such masses are mostly confined to quartz diorite. Thin-section examination revealed that the serpentine contains rare relics of olivine in a felted mat of an- tigorite; abundant magnetite occurs mostly as sagenitic webs of irregular grains. Most thin sections also contain talc and tremolite, and some consist almost entirely of talc. Scattered picotite grains are accessory. Ultramafic rocks in the Holden mine and on Buckskin Mountain differ considerably from the serpentine bodies and from other ultramafic masses that have been described in the northern Cascades (for example, Ragan, 1963, and Southwick, 1974). Most ultramafic masses in the northern Cascades are of Alpine type and consist of little but saxonite or dunite or their serpentinized products, whereas the ultramafic rocks in the Holden area consist largely of homblendite and hornblende peridotite that grade into hornblende gabbro. Saxonite and dunite are absent. Some of the peridotite in the Holden mine contains numerous coarse-grained gabbroic orbicules as much as 7 cm across (fig. 6). The rocks are dark green to black, coarse to very coarse grained, and some crystals are more than 2 cm across. The largest of 8 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON FIGURE 6.—Gabbroic orbicules in peridotite from the Holden mine, Holden quadrangle. the masses cropping out on Buckskin Mountain is about 300 m across; the size of those in the Holden mine are not known, but some exceed several hundred meters. Con- tacts With older rocks are either sharp and intrusive or faulted. Contact effects on intruded rocks are minimal. Microscopic examination of the ultramafic rocks con- firms the heterogeneity evident in hand specimen. Hornblende is the only major constituent common to all the rocks, and it is optically similar in all facies of the rock regardless of other constituents. Hornblende crystals are commonly anhedral and pleochroic from nearly colorless to light brown. The intensity of pleochroism varies considerably within a single crystal, some parts of most crystals being nearly colorless in all orientations; in others, the brown shades into light green. Some grains are loaded with Schiller-like inclu- sions that may be spinel. In many of the rocks, hornblende has been either replaced or at least partly rimmed by tremolite containing magnetite dust. Pyrox- ene occurs only in peridotites, and in these it may be either clinopyroxene alone or both clino— and orthopyrox- ene. Most crystals are either euhedral or subhedral, but some rounded grains occur as poikilitic inclusions in hornblende. Small amounts of olivine are scattered through some of the hornblendite, and as much as 30 percent is in hornblende peridotite. Olivine is generally much altered to antigorite and magnetite. Light-colored biotite or phlogopite is common in most of the rocks and makes up 20 percent of some of them. It is pleochroic from colorless to light brown and is mostly shredlike. Plagioclase in amounts approaching 50 percent dis- tinguishes the gabbros from the other rock types, although very small amounts of interstitial bytownite are found in some peridotite. Most of the plagioclase is bytownite, having an average composition of about A1180, but in some of the more felsic gabbro the plagioclase is sodic labradorite. Plagioclase in orbicular gabbro is bytownite, however. Regardless of composition, plagio- clase is subhedral to euhedral and shows both oscilla- tory and patchy zoning. Magnetite, apatite, and il- menite, which is commonly rimmed by sphene, are fairly abundant accessories. Mafic constituents in most of the rocks are variously altered to antigorite (replacing olivine and pyroxene), tremolite (replacing hornblende), and chlorite (replacing hornblende and phlogopite). In the Holden mine, various sulfides, including pentlan- dite, exsolved from pyrrhotite, locally replace both gab- bro and ultramafic rocks. Ages of the ultramafic rocks are not known with preci- sion; the serpentine masses in the Holden and Glacier Peak quadrangles are, of course, older than the intrusive rocks in which they occur and may well be the oldest ig— neous rocks in the area. The homblendic rocks in the Holden area are probably much younger and may be as young as late Eocene, inasmuch as they may be a facies of the mafic contact complex that envelops the north end of the Duncan Hill pluton. DUMBELL MOUNTAIN PLUTONS Rocks of the Dumbell Mountain plutons were described in detail by Crowder (1959). Since publication of Crowder’s report, however, these rocks have been reex- amined both in the light of what was learned from the in- tensive study of other intrusions in the area and from radiometric dating not then available. As a result, some of Crowder’s earlier interpretations and conclusions seem untenable. He concluded that the Dumbell Moun- tain plutons, in addition to others now known to be much younger, were merely facies of a large granitized mass that has been locally mobilized. The present study indicates that these conclusions were probably invalid. As Crowder (1959) showed, the Dumbell Mountain rocks within the mapped area consist of three separate but closely related units that I consider to be three dis- tinct plutons. From the north border of the Holden quadrangle they extend south-southeastward into the Lucerne quadrangle where they either pinch out or are cut off by the late Eocene Duncan Hill pluton. They possibly correlate with Misch’s (1966, fig. 7—1) “Marblemount Meta Quartz Diorite” to the northwest, which Mattinson (1972) showed to be of the same age. Misch (1966) considered the Marblemount a part of a pre-Middle Devonian “basement” consisting of regionally metamorphosed orthogneisses which he showed, citing theses maps of Tabor (1958) and Grant (1966), in thrust relation with the “Cascade River Schist,” presumably equivalent to “younger gneissic rocks of the Holden area” of the Holden and Lucerne geologic quadrangle maps. Inasmuch as Misch’s map (1966, fig. 7—1) is of an area extending well into the Holden quadrangle, he shows thereby quartz diorite of PRE-TERTIARY INTRUSIVE ROCKS 9 Dumbell Mountain thrust over “younger gneisses of the Holden area.” The Dumbell Mountain plutons are in fact neither older than nor thrust over the “Cascade River Schist” in the Holden quadrangle; rather, they are unrecrystallized or only incipiently recrystallized ig- neous rocks about 220 my old (Mattinson, 1972) that are intrusive into, instead of thrust over, the metasedi- ments in the Holden area. The evidence supporting these conclusions is presented, where appropriate, in the pages that follow. In any event, despite the resemblance of the “Cascade River Schist” to the “younger gneissic rocks of the Holden area,” either the-correlation is wrong or the “Marblemount Meta Quartz Diorite” is older than the Dumbell Mountain plutons. Inasmuch as Crowder (1959) described the distribu- tion, petrography, and contact relations of the Dumbell Mountain plutons in considerable detail, they are only summarized here. The Dumbell Mountain plutons are narrow, nearly concordant masses of quartz diorite that trend north- northwest, more or less parallel to the attitudes of the metamorphic host rocks. Sheets or screens of schists and gneisses, some of them of mappable size, are common in the plutons. These are particularly abundant in the hornblende—quartz diorite augen gneiss between Railroad and Ice Creeks. In addition to these, a thin screen of hornblende gneiss and schist about 8 km long separates the quartz diorite augen gneiss from gneissic hornblende-quartz diorite between Big Creek and Ice Creek, and a similar screen of about the same size separates gneissic quartz diorite from the Leroy Creek pluton. Contacts between the plutons and the metamorphic rocks are varied: sharp in some places, gradational in others, but more commonly characterized by lit-par-lit interlayering of quartz diorite and metamorphic rocks. Alternating layers in the lit-par-lit zones are several cen- timeters to tens of meters thick, and, indeed, some are almost the size of the mapped screens. Some layers of schist and gneiss have been wedged from the walls by quartz diorite and are inclined to the foliation of the quartz diorite. In general, contacts between layers are sharp, but layers of metamorphic rocks tend to fray out along strike and grade into and contaminate the in- trusive material. Some hornblende schist and gneiss layers and inclusions have dark, more hornblende-rich margins from which felsic material has migrated (fig. 7); in fact, many thinner layers and inclusions consist almost entirely of hornblende. Quartz diorite near contacts characterized by lit-par- lit injections tend to be crowded with inclusions of host rocks. As would be expected where host rocks are strongly foliated, inclusions of host rocks in the intrusive rock near contacts are mostly platy or discoidal, but many are more or less equidimensional and angular. As a 1 . ‘1 KLLLLLJQJLLLLULEUJJ Until H i l l i z z 17ch FIGURE 7.—Dumbell Mountain hornblende-quartz diorite augen gneiss containing inclusions with mafic margins, Holden quadrangle. rule, longer dimensions of inclusions—dimensions that usually accord with the planes of foliation in the in- clusions—are parallel to the foliation of the quartz diorite, but some are inclined at high angles. Had folia- tion of the quartz diorite resulted from postintrusion recrystallization, as Crowder (1959) earlier thought and as Misch (1966) assumed by correlating these rocks with his ancient basement rocks, it seems unlikely that the angular relation between foliation of the quartz diorite and foliation of inclusions defined by elongate and platy minerals could have been preserved. Furthermore, the intricately swirled foliation, particularly characteristic of the homblende-quartz diorite augen gneiss but occurr- ing in the other Dumbell Mountain plutons, is not the type likely to result from regional metamorphism; the host rock schists and gneisses are devoid of this type of foliation. Swirled foliation, in fact, occurs locally in most of the plutons regardless of age and results from flowage during intrusion. Locally contacts of plutons with metamorphic rocks are gradational, in most places through zones only a few tens of meters or less thick. In some places, however, as on the ridge south of the glacier on the east side of Cop- per Peak, the gradational contact zone is a few hundred meters thick. As contacts of plutons are approached, the grain size and proportion of felsic material in the hornblende schist and gneiss, of which the host rocks largely consist, increases, and thin seams of igneous material have been insinuated along foliation planes. Closer to the plutons, the amount of igneous material in- creases, and the metamorphic material becomes more thoroughly incorporated in the rock mass. Dikes and sills 10 of quartz diorite cut these gradational contacts just as they do contacts marked by lit-par-lit zones where there is a marked difference between igneous and metamor- phic material. Gradations are equally evident in thin sections, as is discussed later. Although each of the Dumbell Mountain plutons is closely similar to the others in chemical and mineralogic composition, they differ significantly in outcrop ap- pearance. These differences are mostly a function of gneissic foliation; its nature, its degree of segregation into light and dark laminae, and its persistence of at- titude or prevalence of swirling. For each of the plutons there is a unity of gneissic characteristics that is more or less distinctive, although there is considerable overlap of these in the different plutons. Thus scattered masses of augen gneiss occur in both the gneissic hornblende- quartz diorite and the hornblende-quartz diorite gneiss plutons, but augen texture is the characteristic feature of the hornblende-quartz diorite augen gneiss pluton. Despite the similarities among the different plutons, contact relations point to a sequence of emplacement. Oldest is the hornblende-quartz diorite gneiss; this rock was intruded locally by gneissic hornblende-quartz diorite, but more commonly the contacts between the two appear to be completely gradational and nothing suggestive of an age difference is apparent. Hornblende- quartz diorite augen gneiss, on the other hand, clearly intrudes quartz diorite gneiss almost throughout their contact lengths, yet in the one area where the augen gneiss is in contact with gneissic quartz diorite, on the north side of Railroad Creek upstream from Holden Creek, the relations are equivocal. The rocks in the Dumbell Mountain plutons differ lit- tle mineralogically; the principal difference is in the composition of the plagioclase, which ranges from. about A1133 to about An5o in the hornblende-quartz diorite gneiss and augen gneiss and from about A1130 to An50 in the gneissic hornblende-quartz diorite. Most abundant in all the plutons are rocks containing plagioclase having a composition of about A1133; considerable but lesser amounts of rocks in all the plutons contain calcic andesine of about An47, and in the gneissic quartz diorite pluton, about one-fourth of the thin sections examined contained andesine of about An32. Rocks containing andesine of compositions intermediate between these peaks are rare. Megascopic appearance or texture give no clue to the composition of the plagioclase of any given rock, although biotite tends to occur more commonly in rocks containing less calcic andesine. Since the Dumbell Mountain, Leroy Creek, Entiat, and Seven-fingered Jack plutons were described by Crowder (1959), the Dumbell Mountain pluton and rock of the Chelan Complex Entiat and Seven-fingered Jack plutons have been radiometrically dated by Mattinson (1972), and these rocks have been more thoroughly ex- INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON amined petrographically. The textures seen in many of the dated rocks where there is no question of recrystal- lization, particularly some of the older and deeper seated rocks, has made apparent the need for a reappraisal of what Crowder called “crystalloblastic” textures in the Dumbell Mountain and, for that matter, of the texture in the younger Seven-fingered Jack, Entiat, and Leroy Creek plutons. In my opinion, none of the rocks in any of these plutons are crystalloblastic; many, however, did crystallize under stress and most of the gneissic rocks show varying degrees of protoclasis. Plagioclase is the mineral most notably affected by protoclasis because it was the first to crystallize and seems to have been the only crystalline component in the magma when protoclasis began~or at least these were the only grains of sufficient size to have effectively im- pinged on each other during the grinding process that ac- companied intrusion. Locally, however, larger crystals of hornblende have been broken and distorted proto- clastically and, in some places, protoclasis passed into cataclasis where stresses continued after the plutons had completely crystallized. Plagioclase grains in the protoclastic rock are rounded and abraded; many grains consist of a number of slightly rotated fragments (fig. 8). Fractures separating the fragments end at grain boun- daries, and later hornblende, quartz, or biotite grains commonly have formed or replaced plagioclase along these fractures (fig. 8), but the later grains themselves are undeformed. Nearly every thin section of Dumbell Mountain rocks contains a few plagioclase grains where a few faint oscillatory and patchy zonings may be seen (fig. 9). Oscillatory zones in many grains are truncated, which indicates that such grains are but fragments broken from larger crystals during protoclasis. In some FIGURE 8,—Gneissic hornblende-quartz diorite from the Dumbell Mountain plutons, Holden quadrangle. Note fine-grained aggregates of quartz and sodic oligoclase filling fractures in andesine. FEE-TERTIARY INTRUSIVE ROCKS 11 m . FIGURE 9.—Oscillatory- and patchy-zoned andesine in hornblende- quartz diorite gneiss from the Dumbell Mountain plutons, Holden quadrangle. See figure 14 for hand sample. crystals, oscillatory and patchy zoning are preserved only in untwinned or only weakly twinned parts of crystals, suggesting thereby that the general lack of both types of zoning merely reflects destruction of them by twinning. A great many individual plagioclase crystals are twinned according to multiple laws, a feature many authors have detailed as suggestive of igneous origin (Gorai, 1951; Smith, 1958; Vance, 1961, 1962; Turner, 1951). The occurrence of patchy zoning in most thin sec- tions of Dumbell Mountain rocks virtually precludes recrystallization following emplacement of the plutons and is strong presumptive evidence of magma transport from greater depths (Vance, 1965). Some sieve-textured hornblende crystals are clearly derived from partly as- similated metamorphic rocks, but some result from in- complete replacement of plagioclase by hornblende; such grains not uncommonly contain groups of simultaneously extinguishing inclusions apparently derived from a preexisting single crystal of plagioclase. Partly for the sake of convenience and partly because significant differences do exist between them, the plutons are described separately. It should be empha- sized, however, that although each pluton is a separate entity, they are closely related and are probably derived from a common magma in which differentiation had lit- tle effect—at least at the presently exposed level, which the evidence indicates is catazonal. The location of modally analyzed specimens of the Dumbell Mountain plutons are shown on figure 10, the modal analyses are presented in table 2. and the chemical and spectrographic analyses are presented in table 3. The modes are also plotted on a quartz- potassium feldspar-plagioclase triangular diagram (fig. 11) and the norms are shown on figure 12. Compositions are plotted on a variation diagram (fig. 13). HORNBLENDE-QUARTZ DIORITE GNEISS Hornblende-quartz diorite gneiss crops out from the north edge of the Holden quadrangle on Bonanza Peak to Mount Fernow where it pinches out, a distance in the mapped area of 11 km. It lies between the younger quartz augen gneiss of Dumbell Mountain on the east and probably still younger gneissic quartz diorite to the west. As already noted, its contacts with these younger rocks range from sharp and intrusive to gradational. The megascopic appearance of this rock is more uni- form than that of the other Dumbell Mountain rocks, although the amounts of schlieren and incorporated material vary considerably from place to place. Nearly all of it is strongly and rather uniformly gneissic (fig. 14). The hornblende forms flat clusters or long black strings of needles that give much of the rock a well-defined lineation that plunges southeastward at low to moderate angles. Unlike the other Dumbell Mountain rocks, this gneiss is virtually but not entirely unswirled, and the foliation maintains rather constant attitudes through considerable distances. Thin—section examination of the hornblende-quartz diorite gneiss showed it to consist largely of plagioclase, hornblende, and quartz, and in about half the sections a little biotite is present. Small quantities of magnetite, titanite, and apatite are accessory. In a few thin sections, very small amounts of potassium feldspar occur inter- stitially, or more commonly, as tiny veinlets. Chlorite, clinozoisite-epidote, and sericite, mostly in small quan- tities, are secondary. Plagioclase is mostly fresh and ranges in composition from andesine having an average composition of about Angg to sodic labradorite having an average composition of slightly more than Anso. In- dividual crystals range from subhedral to anhedral, and many are rounded and obviously abraded; a few grains in most sections show four or five oscillatory zones and equally faint patchy zoning. In a few sections showing hypidiomorphic textures, many crystals have well- defined oscillatory and patchy zoning, but the zones in any given crystal do not exceed four or five (fig. 9). Hornblende occurs as euhedral to anhedral grains; much of it forms aggregates resembling poikilitic hornblende, but such groups do not form single crystals, although this is commonly not apparent except under crossed nicols. The pleochroic formula for hornblende is X, straw yellow to light greenish yellow; Y, green; and Z, dark green or dark bluish green. The pleochroic formula for biotite is X, light yellow and Z, moderate to dark brown. 12 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON 48°15' 120°45’ EXPLANATION Gneissic hornblende-quartz diorite ‘ Hornblende—quartz diorite ‘ augen gneiss Hornblende-quartz diorite gneiss Contact 036 Sample locality HOLDEN QUADHANGLE LUCERNE QUADHANGLE 0 \\~. ‘ " 370 38 =/u\\“’/ .48 49. 0 5 KILOMETEHS 48° 00’ FIGURE 10,—Map showing location of modally analyzed samples from the Dumbell Mountain plutons, central Washington. Sample analyses are given in table 2. PRE-TERTIARY INTRUSIVE ROCKS 13 TABLE 2.—Modal analyses of samples of hornblende-quartz diorite and hornblende-quartz diorite gneiss from the Dumbell Mountain plutons [Values are in percent. Leaders (. - -), not present] Accessory Total Anonhite in Sample Potassium and second- mafic plagioclase crystals (percent) No. Plagioclase feldspar Quartz Biotite Homblende ary minerals minerals Average Core Rim Remarks Gneissic hornblende~quartz diorite 1 55.0 --— 13.2 13.0 16.9 1.9 31.8 44 --- --- Hypidiomorphic. 4 41.4 -- 4.5 --- 52.3 1.8 54.1 40 --- --- Hornfelsed. 5 49.9 --- 17.6 --- 24.3 8.2 32.5 40 --- --. Protoclastic. 9 43.7 - - 6.4 47.8 2.1 49.9 40 43 38 Xenomorphic. 13 55.6 5.4 38.5 .5 39.0 38 50 30 Hypidiomorphic. 14 61.7 --- 14.1 .6 18.6 5.0 19.7 40 45 37 Do. 15 52.8 --- 7.4 --- 31.1 8.7 39.8 38 49 32 Do. 17 60.2 -- 21.8 -»- 3.4 14.6 18.0 38 44 32 Do. 18 50.9 15.0 7.4 12.1 14.6 34.1 38 --- --- Do. 19 41.6 --- 24.8 8.9 18.6 6.1 33.6 47 --- --- Xenomorphic 21 59.0 --- 6.1 1.4 32.3 1.2 .9 42 48 36 Seriate; xenomorphic. 22 49.2 - 20.0 24.3 6.5 30.8 35 37 28 Xenomorphic; seriate. 23 62.5 4.1 32.6 .8 33.4 38 40 21 Hypidiomorphic. 24 43.5 --- 15.0 14.9 26.1 .5 41.5 29 --- --- Protoclastic. 29 57.0 - 14.7 5.3 11.4 11.6 28.3 32 --- --- Xenomorphic. 30 45.0 - - 12.3 - - 35.9 6.8 42.7 48 Do. 31 52.6 -- 8.4 —-— 32.1 6.9 39.0 32 --- --- Do. 36 47.8 11.6 --- 38.7 1.9 40.6 44 45 36 Hypidomorphic. 38 58.8 -- 7.3 --- 32.6 1.3 33.9 47 51 41 Xenomorphic. 41 56.9 --- 8.3 6.2 18.7 9.9 34.8 47 52 40 Do. 45 47.7 --« 12.6 --- 38.3 1.4 39.7 47 52 43 Protoclastic. 48 53.6 --- 19.2 11.2 15.3 .6 27.1 32 --- --- Hypidiomorphic. 49 40.0 --- 19.0 1.6 34.3 5.1 41.0 38 40 25 Hypidiomorphic- cataclastic. 50 48.5 --- 20.1 .7 30.1 .6 31.4 33 36 24 Protoclastic. 51 47.9 -- 4.1 2.5 35.6 9.9 39.0 38 41 24 Xenomorphic. 52 48.6 ~-- 9.4 4.1 31.7 6.2 42.0 55 61 44 Hypidiomorphic. 53 49.0 --- 8.2 2.1 38.4 2.3 44.8 31 32 29 Xenomorphic. 54 47.3 -- 13.3 8.5 27.6 3.3 39.4 35 39 26 55 47.4 ~-- 27.9 4.1 16.7 2.2 23.0 48 51 41 Protoclastic. 56 50.6 --- 30.7 10.1 7.5 1.1 18.7 32 --- --- Do. 57 46.4 -- 36.8 5.5 10.6 .7 16.8 38 39 33 Do. Quartz diorite augen gneiss 11 48.2 --- 35-8 7.1 "— 8-9 16.0 37 45 24 Protoclastic. 20 49.1 --- 26.5 3.1 18 3 3.0 24.4 38 53 35 Hypidiomorphic- cataclastic. 26 48.8 21.8 4.0 23.3 2.1 29.4 38 49 36 Hypidiomorphic. 27 44.2 36.1 4.2 14.6 1.9 20.7 45 Xenomorphic- protoclastic. 28 30.8 48.3 7.1 11.9 1.9 20.9 31 35 28 Protoclastic. 32 36.8 --- 36.7 --- 25.9 .6 26.5 43 28 40 Reversed zoning of plagioclase; hypidiomorphic. 33 39.7 41.1 18.1 1.1 19.2 45 35 50 Reversed zoning of plagioclase; protoclastic. 34 40.5 -—- 40.1 .9 17.0 1 5 19.4 36 33 38 Protoclastic. 35 42.1 --- 45.7 2.9 7.1 2.2 12.2 35 —-- --- D0. 39 48.2 1.0 30.4 2.2 16.8 1.4 20.4 33 32 39 Reversed zoning of plagioclase; protoclastic. 40 45.5 .4 40.6 10.9 2 6 13.5 33 35 23 Protoclastic. 42 41 6 9 34.5 4 2 16.1 2 7 23.0 32 29 49 Reversed zoning of plagioclase; seria’ce; xenomorphic. 43 34.6 .7 54.7 3.9 - - 6.1 10.0 33 ~-- --— Protoclastic. 44 43.7 .2 45.1 --- 7 9 3.1 11.0 36 38 34 D0. 46 52.4 .6 27.4 3.6 13 2 2.8 19.6 33 35 29 Porphyroclastic. 47 48.4 1.9 37.0 3.5 7 3 .2 11.0 42 44 38 Xenomorphic; protoclastic. Homblende quartz diorite gneiss 2 42.9 . 23.6 42.9 2.2 45.1 38 40 33 Xenomorphic. 3 44.3 . 27.1 21.8 6.8 28.6 42 46 29 Do. 6 33.5 - 19.5 6.9 26.9 13.2 47.0 55 Porphyroclastic. 7 47.5 --- 6.4 2.4 42.2 1.5 46.1 46 51 36 Hypidiomorphic. 8 41.6 . - 24.8 8.9 18.6 6.1 33.6 40 44 38 Protoclastic. 10 42.7 0.3 24.1 -—- 25.6 7.3 32.9 58 60 47 Xenomorphic. 12 37.4 1 24.2 .8 26.0 5.5 32.3 40 45 33 Hypidiomorphic. 16 46.7 14.7 4.9 21.0 12.7 38.6 38 40 33 Xenomorphic. 25 48.5 - 9.0 --- 36.8 5.7 42.5 49 65 35 Do. 14 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES. WASHINGTON TABLE 3.—Chemical and spectrographic analyses and norms of rock samples from the Dam bell Mountain plutons, central Washington [For samples 1—5, 7, and 8, SiOz, A120,, and TiOg determined colorimetrically; total Fe, MgO, C30, and MnO determined by atomic absorption; FeO determined titremetrically; Nago and K20 determined by flame photometer; analysts: G. T. Burrow, Wayne Mountjoy, H. H. Lipp, and Johnnie Gardner. For samples 6 and 9, rapid rock analyses by P. L. D. Elmore, K, E. White, and S. D. Butts. Spectrographic analyses by B. W. Lanthorn and K. E. Valentine. <, less than; N.d.. 'zot determined; leaders (- - -), below level of sensitivity] Sample -------------- 1 2 3 4 5 6 7 8 9 Chemical analyses (percent) SiO2 ................ 61.0 62.4 63.8 70.8 73.7 68.9 52.5 65.7 56.9 A1203 ................ 17.8 17.5 17.1 15.4 15.0 14.3 20.9 16.2 17.9 Fe203 ............... 1.6 1.8 1.6 1.4 .9 2.1 2.0 1.2 2.7 FeO ................. 4.46 4.20 3.14 2.46 2.79 2.6 6.45 3.87 4.2 MgO ................ 3.10 3.04 2.48 1.46 1.23 1.7 3.67 2.63 3.7 CaO ................ 7.2 6.5 5.7 5.2 4.1 4.7 11.1 7.1 7.3 Na20 ................ 3.43 3.35 4.06 3.88 3.52 3.5 2.74 3.59 3.4 K20 ................. .57 .79 1.03 .11 .68 .42 .23 .24 .66 TiOz ................ .67 .73 .56 .45 .41 .41 .93 .71 .59 P205 ---------------- N.d. N.d. N.d. N.d. N.d. .21 N.d. N.d. .24 MnO ................ .11 .10 .08 .09 .06 .11 .13 .10 .14 H20 ................. N.d. N.d. N.d. N.d. N.d. .76 N.d. N.d. 1.5 co2 ................. N.d. N.d. N.d. N.d. N.d. < .05 N.d. N.d. < .05 Total ------------ 100 101 100 101 102 100 101 101 99 Semiquantitative spectrographic analyses (parts per million) Ba .................. 300 700 700 150 300 30 200 150 100 Be ------------------ --- --- --— --- --- --- --- --- 30 co .................. 20 20 20 10 15 10 30 15 30 Cr .................. 15 30 30 5 10 3 30 5 10 Cu .................. 15 30 15 3 5 10 70 15 30 Ni .................. 20 20 15 5 5 15 15 Pb .................. 10 10 so .................. 30 30 30 30 20 10 70 30 10 sr .................. 700 1,000 1,000 200 200 300 300 500 1,000 V ................... 150 200 150 150 100 10 300 150 10 Y ................... 30 30 30 30 30 10 30 30 10 Zr .................. 50 100 70 150 200 10 30 30 10 Ga .................. 30 30 30 20 30 10 30 15 30 Yb .................. 3 3 3 7 5 1 3 5 1 Norms q ................... 17.05 16.91 19.40 33.16 37.40 34.25 4.75 23.83 12.98 or ................... 3.37 4.63 6.11 .64 3.92 2.51 1.35 1.40 3.99 ab .................. 29.02 32.26 34.49 32.40 29.07 29.89 23.02 29.96 29.40 an .................. 31.52 27.88 25.51 23.99 19.86 21.84 43.76 27.02 32.35 d1 ................... 3.46 3.32 2.35 1.22 6.72 6.87 9.03 6.34 2.23 by .................. 11.99 11.04 8.74 5.74 1.00 .17 13.45 8.40 13.22 mt .................. 2.32 2.59 2.33 2.01 1.27 3.08 2.88 1.72 4.00 11 ................... 1.27 1.37 1.07 .84 .76 .79 1.76 1.33 1.15 8p .................. .60 .68 Total ------------ 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Plagioclase composition an ------------------ 52.1 46.4 42.5 42.5 40.6 42.2 65.5 47.4 52.4 15 PRE-TERTIARY INTRUSIVE ROCKS TABLE 3.—-Chemical and spectrographic analyses and norms of rock samples from the Dumbell Mountain plutons, central Washington— Continued Sample descriptions 1 2. 3 4 5 6. 7 8 9 Quartz /' / r’ / ,, / . / / , l 213815 5113 ,9 23 ,I’ i i' Potassium 35 feldspar Plagioclase EXPLANATION '52 Gneissic hornblende-quartz diorite A12 Hornblende-quartz diorite augen gneiss D47 Hornblende—quartz diorite gneiss FIGURE 11.——Plot of modes of rock samples from the Dumbell Mountain plutons, central Washington, on a quartz-potassium feldspar-plagioclase diagram. Sample analyses are given in table 2. The amount of mafic minerals, mostly hornblende, averages about 35 percent but ranges from 10 to 45 per— cent (table 2). The microtexture ranges from hypidiomorphic to xenomorphic with varying degrees of protoclastic modifications. In general, however, the quartz diorite gneiss is the least protoclastic of the Dumbell Mountain rocks, and foliation seems to be largely a result of . Composite sample of gneissic hornblende-quartz diorite; composition of plagioclase about An45 to Ania. Composite sample of gneissic hornblende-quartz diorite; composition of plagioclase about Anag. . Composite sample of gneissic hornblende-quartz diorite; composition of plagioclase about Anaz. . Composite sample of hornblende-quartz diorite augen gneiss; composition of plagioclase about Ams. . Composite sample of homblende-quartz diorite augen gneiss; composition of plagioclase about Ams. Composite sample of hornblende-quartz diorite augen gneiss; composition of plagioclase Anag to Ann. . Hornblende-quartz diorite gneiss; composition of plagioclase about Anso. . Homblende-quartz diorite gneiss; composition of plagioclase about A1133. Composite sample of homblende-quartz diorite gneiss; plagioclase of various compositions. Potassium 5 35 feldspar EXPLANATION 05 Dumbell Mountain plutons A2 Bearcat Ridge plutons D1 Leroy Creek pluton Plagioclase FIGURE 12.——Plot of norms of rock samples from the Dumbell Mountain, Bearcat Ridge and Leroy Creek plutons, central Washington, on a quartz-potassium feldspar-plagioclase diagram. Sample analyses are given in table 3 (Dumbell Mountain), table 5 (Bearcat Ridge), and table 7 (Leroy Creek). flowage and of crystallization under stress, but of generally insufficient stress to produce the extensive protoclasis so evident in the other Dumbell Mountain rocks. The chemical composition of the hornblende-quartz diorite gneiss appears to be somewhat more variable than is true of the other Dumbell Mountain rocks, despite its generally more uniform appearance (table 3). m OXIDES, IN WEIGHT PERCENT A1203 FeO MgO CaO N320 K20 Ti02 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON l> EXPLANATION Bearcat Ridge plutons Leroy Creek pluton Dumbell Mountain plutons 25— — O 20— — (V) O O . . Q“ 15— 8 0'0 A0— 4: I 10 J 5’ _ <> :0 . ° . o O o o 0 . A at)“ 0 0 ° u. 10‘~ —‘ C 5— o . _ 90 ° u. 0. 0 . . . A <> 0 W’ 5— —\ O I . I . . g) o 0 .¢3 A. z 10* o w . ‘ Q . O 5— . o ‘— 0 ° . A 0° 3 0 | 5— <> 00 AV 0 o '. O o . u 0 0L % _ _ Z 0 0° ON 0 . . .. . 0 .|. A. x 2 (N J ' . |~ - . - .I-o .7 E 50 60 70 FIGURE13.—Variation diagrams of major oxides in analyzed rock samples from pre-Tertiary plutons, central Washington. EXPLANATION C Seven-fingered Jack and Entiat plutons A Tenpeak and White Mountain plutons <> Quartz diorite of High Pass 0 Quartz diorite of Cardinal Peak 20 F . o A .A 0 o 15* 10 10— 5.. . O A A O o 0 | 10 5— A I .A O O o o 1 I 10~ 5— o A 0A 0 o o o l l 10— O A A 5— ‘ O o o 0 l 5— O Q 0 o A 'A o l | 2 . Al 9A O O [0 0 2 O ' Al “3 lo 50 60 70 SiOZ, IN WEIGHT PERCENT FEE-TERTIARY INTRUSIVE ROCKS 17 FIGURE 14,—Hornblende-quartz diorite gneiss from the Dumbell Mountain plutons, central Washington. Thin section shown in figure 9. As is true of all the Dumbell Mountain rocks, the hornblende—quartz diorite gneiss is remarkably low in K20 for a rock of otherwise unremarkable composition. A possible reason for the paucity of K20 is discussed in the section on “Composition and differentiation.” HORNBLENDE-QUARTZ DIORITE AUGEN GNEISS The pluton of hornblende-quartz diorite augen gneiss extends south-southeastward from near the outlet of Holden Creek to about the junction of Ice Creek and the Entiat River, a distance of about 15 km. Unlike the other Dumbell Mountain plutons, this one contains some screens of hornblende schist and gneiss a kilometer or two long, much larger than the small and unmapped sheets that occur in the others. Much of the eastern con- tact of this pluton has been intricately intruded by dikes satellitic to the late Eocene Duncan Hill pluton and these dikes furnished part of the material that forms the contact complex surrounding the northern end of the Duncan Hill pluton. Most of the rock is characteristically an augen gneiss (fig. 7), the augen consisting of nearly pure aggregates of plagioclase and quartz or mixtures of both; more rarely an augen may consist of a single abraded grain of plagioclase. Unlike the hornblende-quartz diorite gneiss, the foliation of the augen gneiss is swirled in many places and attitudes vary greatly over short distances. Inclu- sions are numerous in the augen gneiss; many show little effect of immersion in the quartz diorite magma, but others have been almost completely assimilated and are barely distinguishable from the surrounding augen gneiss. Many are extremely flat and attenuated, par- ticularly on Spectacle Buttes where we found one only 1 to 3 cm thick and 13 m long. Most of these consist almost entirely of hornblende. Great numbers of inclusions have dark, hornblende-rich margins from which felsic minerals have been almost completely removed (fig. 7). Long dimensions of most inclusions parallel foliation of the augen gneiss, but some are discordant. Study of thin sections showed the augen gneiss to be mineralogically almost identical to the hornblende- quartz diorite gneiss, including the same range of com- positions of the plagioclase. The texture, however, differs considerably in that the augen gneiss is everywhere protoclastic and in most places decidedly so, although the range of protoclasis is from extreme to mild. In gradational contact zones and where screens and inclu- sions of metamorphic rock are numerous, textures range from metamorphic in the inclusions to protoclastic in the intrusive material; in fact, because some of the augen gneiss closely resembles some varieties of the host metamorphic rocks, only by thin-section examination can distinctions be made, or, for that matter, can the augen gneiss and metamorphic rocks be distinguished from much of the contaminated or granitized rock of the gradational contacts. Plagioclase is most profoundly af- fected by protoclasis, and in many thin sections is the only mineral to show the effects of grinding. Quartz characteristically forms lentils consisting of a mosaic of interlocking grains, but small quantities are interstitial or replace earlier minerals, mostly plagioclase. Where present, biotite most commonly occurs with the quartz lentils. The relationship of quartz lentils and biotite to the rest of the rock suggests that these late constituents were squeezed or filter pressed when still fluid from the mass of crystals being milled during intrusion. The hornblende-quartz diorite augen gneiss is the most siliceous of the Dumbell Mountain rocks (table 3) but shares with the other varieties an extraordinarily low content of K20. GNEISSIC HORNBLENDE-QUARTZ DIORITE Gneissic hornblende-quartz diorite forms the largest of the Dumbell Mountain plutons in the mapped area and has an outcrop area at least as large as the other Dumbell Mountain rocks combined. Younger Seven- fingered Jack intrusive rock nearly splits the pluton lengthwise; the western segment is cut off to the north by the Miocene Cloudy Pass pluton, but the eastern seg- ment extends an unknown distance north of the Holden quadrangle. To the southeast, the pluton narrows and either pinches out or is cut off by the Duncan Hill pluton beneath the glacial debris and alluvium in the Entiat River valley near the Cottonwood Guard Station. The quartz diorite is mostly medium grained and gray and ranges from massive and nearly nongneissic (fig. 15) to strongly gneissic. In many places, layers of highly foliated or gneissic rock a few centimeters to a few meters thick alternate with layers of nearly structureless rock. Swirled foliation is a common feature, probably even more so than in the hornblende-quartz diorite augen 18 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON FIGURE 15.—Gneissic hornblende-quartz diorite, a less gneissic facies, from the Dumbell Mountain plutons, central Washington. gneiss. Locally inclusions are common, but nowhere are they as attenuated as are some of those in the hornblende-quartz diorite augen gneiss. In thin section, gneissic hornblende-quartz diorite is seen to be mineralogically identical to other Dumbell Mountain rock, except that considerable quantities of it contain sodic andesine averaging about An32, but medium or calcic andesine still predominates. Textures range from hypidiomorphic through xenomorphic to protoclastic. Much is also cataclastic, especially near shear zones. The more gneissic varieties are more highly protoclastic, and in the interlayered gneissic and non- gneissic rocks, the gneissic layers are protoclastic. The chemical composition of the gneissic hornblende- quartz diorite is apparently more uniform than is the composition of the other Dumbell Mountain rocks, but the content of K20 is characteristically low (table 3). BEARCAT RIDGE PLUTONS The Bearcat Ridge plutons crop out in the Lucerne quadrangle between Lake Chelan and Railroad Creek. The larger of the two plutons has a length west of Lake Chelan of about 18.7 km and a maximum width of nearly 1.25 km. It is cut off to the north and northwest by the Railroad Creek and Cardinal Peak plutons. The smaller pluton has a length of about 4.8 km and a maximum width west of Lake Chelan of about 1.2 km. How far either pluton extends east of the lake, if at all, is unknown. Both plutons are of granodiorite and quartz diorite and are roughly conformable to enclosing metamorphic rocks but locally crosscut them, particularly where they are isoclinally folded. None of the observed contacts, however, were discordant within the limits of the out- crops examined. The smaller pluton and the prong of the larger one that cuts across the headwaters of Bear Creek have wedged the host rocks apart and, in fact, may have been responsible for the tight and complex folding of the host rocks in the area. All contacts are either sharp or gradational through a zone only a few centimeters thick. In some places, however, most noticeably on the spur ridge between Klone and Tumble Creeks, the contact was difficult to locate precisely because the highly protoclastic, fissile border rocks of the pluton so closely resemble the sheared, highly fissile enclosing gneisses of similar composition. Rocks of the Bearcat Ridge plutons are of variable ap- pearance and composition, all are gneissic or foliated to some degree, and all are nearly white to dark gray, depending on the content of mafic minerals and the grain size. Most abundant are medium-grained, gray rocks verging on augengneiss (fig. 16). The plagioclase forms porphyroclastic eyes or thin lenses, around which are wrapped thin folia consisting of aggregates of biotite and hornblende crystals. Intermingled with these gray rocks are rather large quantities of light-colored gneiss that form conformable lenses and anastomosing dikes rarely more than several centimeters thick; the dikes in- trude more or less aimlessly the darker rocks. Contacts between the normal and the light-colored facies are gradational, and attitudes of foliation cross the light facies without deviation. Foliation, however, is much less distinct in the lighter rocks. Dark facies also occur but are much rarer than the light facies and seem to be more common near contacts. Some of the darker rocks are actually more mafic, but most, particularly those near contacts, are merely finer grained and lack the 3». +7 r, ’ ,i M W WM . ~. 4‘» j . FIGURE 16. —Protoclastic biotite-hornblende-quartz diorite gneiss from the Bearcat Ridge plutons, Lucerne quadrangle. Dike is Tertiary granodiorite. PRE-TERTIARY INTRUSIVE ROCKS degree of segregation into felsic and mafic folia of rocks farther from contacts. The larger pluton consists mostly of hornblende- bearing rocks that range in composition from biotite- hornblende granodiorite to quartz diorite, but some hornblende—free rocks also occur. Granodiorite shown in figure 17 is typical of the less gneissic rocks. The smaller pluton is almost, but not entirely, devoid of hornblendic rocks and consists of biotite granodiorite and quartz diorite. Figure 18 illustrates a nearly nongneissic facies of granodiorite. As a general rule, hornblendic rocks seem to be slightly older and are intruded by rocks con- taining only biotite, but contacts between the two types of rock are mostly gradational in places through many meters. The field relations suggest that both types were mobile simultaneously. Because of the similarity of ap- pearance, the difficulty in recognizing rocks containing no hornblende (hornblende is commonly much subor- dinate to biotite in any case), and the erratic distribu- tion of the two types, no effort was made to separately delineate them in either of the two plutons. Thin-section examination of the rocks reveals many characteristics that are common to all rocks in the plutons, the most striking of which is the pervasive protoclasis that lends to the rocks their gneissic ap- pearance, but there are also some significant differences between rock types. Hornblende-bearing rocks contain considerable ilmenite and coarse titanite, and plagio- clase has an average composition of about Anm, whereas rocks lacking hornblende contain very little titanite, no ilmenite, and plagioclase has an average composition of about A1120. All the rocks contain varying quantities of quartz, biotite, and orthoclase, along with accessory magnetite and apatite. North of Graham Harbor Creek, near the southwest contact of the smaller pluton, the rock not only contains biotite but also considerable FIGURE 17.—Hornblende-biotite granodiorite gneiss from the Bearcat Ridge plutons, Lucerne quadrangle. 19 FIGURE 18.—Bi0tite granodiorite from the Bearcat Ridge plutons, Lucerne quadrangle. muscovite. A little chlorite was seen in all sections as an alteration product of the mafic minerals, especially biotite, and some epidote occurs in a few thin sections. Andesine in the hornblende-bearing rocks and oligoclase in the hornblende—free rocks occur as abraded, rounded, and commonly fractured porphyroclasts. These porphyroclasts may have tails of abraded material streaming away from them, but most commonly, the abraded material forms a mortar surrounding the porphyroclasts. A few porphyroclasts, in most sections, show faint oscillatory and patchy zoning, attesting thereby to their igneous origin. Myrmekite showing a variety of forms is rather common. Quartz occurs as in- terstitial anhedral grains in the least deformed rocks, but by far the greatest amount occurs as thin, lenslike folia consisting of mosaics 0f interlocking crystals; these folia commonly occur in the zones of granulated plagioclase. Orthoclase, much of it perthitic, is com- monly interstitial or is associated with the quartz folia, but some partly replaces both granulated and porphyroclastic plagioclase. Some of the larger grains of orthoclase are traversed by zones of perthite that follow lines of distortion in the grains. The amount of orthoclase ranges between rather wide limits; some of the biotite-hornblende quartz diorites contain less than 1 percent, whereas some of the leucocratic material may contain as much as 50 percent, but most rocks contain between 5 and 15 percent. Rocks containing sufficient orthoclase to be called granodiorite are at least twice as common as the quartz diorites low in orthoclase. The distribution of rocks containing more and less ortho- clase, however, is haphazard, and in outcrop the rocks are indistinguishable. Microcline is rare; accessory amounts were seen in only one thin section. The major mafic mineral is biotite, and it occurs as folia of anhedral shreds that follow planes of granulation. Biotite in all the rocks is optically identical, the pleochroic colors ranging 20 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON from straw yellow to dark olive brown. Hornblende, where present, usually occurs with biotite, and the longer dimensions of the grains are oriented in the plane of foliation; in this plane, however, the orientation is mostly random, but in a few places a vague lineation is apparent. Hornblende is anhedral and pleochroic from yellowish green to dark green. Biotite tends to replace hornblende, and hornblende adjacent to biotite is generally lighter colored. Titanite is a common accessory in the hornblendic rocks; much of it forms rhombs as long as 1 mm, but some forms irregular rims surrounding grains of ilmenite. The most characteristic feature of the Bearcat Ridge rocks is their pervasive protoclastic texture. As a rule, all the minerals show some slight distortion, i.e., un- dulatory extinction in quartz and warping of scattered biotite and hornblende grains, but only plagioclase, the earliest formed mineral, has everywhere undergone in- tensive and extensive grinding. No grains have escaped abrasion entirely, and many porphyroclasts consist of slightly rotated fragments of an original crystal. The un- distorted or only slightly distorted later minerals —hornblende, biotite, quartz, and potassium feldspar —that grew along planes of granulation, indicate that by the time they started crystallizing, the plutons had largely come to rest. The leucocratic lenses and dikes that are so common throughout the plutons and, indeed, cut the enclosing metamorphic host rocks are probably the results of this process of squeezing and grinding. The plagioclase in these lighter rocks is identical to the plagioclase of the surrounding granodiorites and quartz diorites and show the same abraded outlines, but they form a relatively small proportion of these lighter rocks, which consist largely of quartz and orthoclase. Thus, the still-fluid component that was to crystallize mostly as quartz and orthoclase seems to have been filter pressed from the crystal mush of the intruding plutons to form these leucocratic lenses and dikes. Directed stresses had not entirely ceased, however, by the time the leucocratic rocks crystallized. This is indicated by the rocks’ vague foliated texture defined by the parallel orientation of the biotite, in places parallel to the walls of the dike but elsewhere parallel to the regional attitudes of enclosing rock regardless of orientation of dikes. Modal analyses of the Bearcat Ridge plutons are given in table 4, and chemical analyses in table 5. Figure 19 shows the locations of modally analyzed specimens, and figure 20 is a plot on a triangular diagram of these modes. A plot of the norms is given in figure 12, and a variation diagram of major oxides in figure 13. LEROY CREEK PLUTON The Leroy Creek pluton, a mass of gneissic biotite- quartz diorite about 10 km long and 2 km wide, extends along the west side of the Entiat Mountains from upper Rock Creek to upper Phelps Creek. The pluton intrudes Dumbell Mountain gneissic hornblende-quartz diorite and is in turn intruded by quartz diorites of the Seven- fingered Jack plutons; hence, the ages of 45 and 54 my, determined on mica (Engels and others, 1976) are much too young and indicate heating by later intrusions. The Leroy Creek pluton consists of fine- to medium- grained gneissic biotite-quartz diorite (fig. 21). The rock TABLE 4.—M0dal analyses of samples of quartz diorite and granodiorite gneiss from the Bearcat Ridge plutons, Lucerne quadrangle [Leaders (- - --), not measured or not present] Accessory Total Anorthite in Sample Potassium and second- mafic plagioclase crystals (percent) No. Plagioclase feldspar Quartz Biotite Hornblende ary minerals minerals Average Core Rim Remarks 1 54.5 0.4 18.0 11.2 14.5 1.4 27-1 40 ProtoclastiC- 2 46.4 10.6 12.8 2.8 23.0 4.4 30-2 33 35 31 DO- . 3 53.9 9.1 14.2 9.7 10.8 2.3 13-1 33 40 26 XenomorphIC- 4 41.8 11.8 3.2 11.0 3.1 1.1 15.2 29 ProtoclastiC- 5 47.1 8.5 18.7 13.7 10.7 1.3 25~7 32 33 28 0- . 6 55.7 7.8 194 6.7 8.2 2.5 17.4 32 33 29 Xenomorph1c;sphene rimmed with ilmenite. 7 67.6 1.7 13.8 6.8 5.5 4.6 16.9 35 Xenomorphic. 8 54.5 3.4 23.4 16.6 1.3 .8 18.7 32 Do. 9 55.2 6.8 18.2 9.4 9.7 .7 19.8 28 32 25 Protoclastic. 10 56.8 4.1 19.2 6.2 12.6 1.1 19.9 27 29 25 Xenomorphic; slightly protoclastic. 11 55.0 6.2 30.4 7.9 .5 8.4 25 26 23 Protoclastic. 12 55.9 13.2 21.0 9.0 .9 9.9 23 27 15 Xenomorphic. 13 44.2 15.7 30.2 7.6 2.3 9.9 23 27 16 Some muscovite; hypidiomorphic- protoclastic. 14 36.4 33.3 25 1 4.5 7 5.2 22 23 18 Xenomorphic- protoclastic. 15 50.1 12.3 30.8 6.3 --- .5 6.8 24 Protoclastic. PRE-TERTIARY INTRUSIVE ROCKS 21 TABLE 5.—Chemical and spectrographic analyses and norms of rock samples from the Bearcat Ridge plutons, Lucerne quadrangles [For samples 1 and 3. standard rock analysis by E. E. Engleman. For sample 2, 8102 and A1203 determined colorimetrically; FeO determined volumetrically; F6203, MgO, CaO, and Mn determined by atomic absorption; Na20 and K20 determined by flame photometer; analysts: Violet Merritt, Wayne Mountjoy, J. D. Mensick, Claude Huffman. H. H. Lipp, G. T. Burrow, and G. D. Shipley. Spectrographic analysts: Barbara Tobin, H. G. Neiman, and B. W. Lanthorn. N.d., not determined; leaders (- - -), below sensitivity limit; N.p., not present] Sample ............... 1 2 3 Chemical analyses (percent) Si02 ................. 71.25 69.8 65.7 1x1203 ................. 15.99 16.2 17.1 Fe203 ................ .45 3.14 1.12 FeO .................. 1.06 1.00 2.68 MgO ................. .52 .48 1.3 CaO ................. 2.44 2.12 4.3 Na,o ................. 4.79 4.77 4.53 K20 .................. 2.55 1.83 1.96 H20+ ................ .29 N.d. N.d. H20- ................ .02 N.d. N.d. Ti02 ................. .23 N.d. N.d. 12,05 ................. .07 N.d. N.d. MnO ................. .04 .04 .076 co2 .................. .00 N.d. N.d. Cl ................... .00 N.d. N.d. F .................... .04 N .d. N .d. Subtotal ---------- 99.74 Less H20 --------- .02 Total ------------- 99.72 99 99 Semiquantitative spectrographic analyses (parts per million) B -------------------- 20 Ba ................... 1,000 500 1,000 Be ------------------- 1 --- --- Co ------------------- --- --- 15 Cr ................... 5 1 50 Cu ------------------- 3 5 5 Ga ------------------- 30 30 20 Nb ------------------- 10 - Ni ------------------- 20 Pb ................... 70 15 Sc ................... 5 15 15 Sr ................... 1,000 500 1.000 V .................... 30 20 100 Y -------------------- 10 10 Yb ................... 1 1 1 Zr ................... 100 70 50 Norms q .................... 27.51 30.00 19.24 or .................... 15.13 10.88 11.72 ab ................... 40.68 40.59 38.79 an ................... 11.40 10.58 20.80 hy ................... 2.57 1.20 .66 c ..................... .120 2.34 7.14 mt ................... .65 3.38 1.65 i1 .................... .44 .83 N.p. ap ------------------- 17 N.p. N.p. Total ............. 100.00 100.00 100.00 TABLE 5.—Chemical and spectrographic analyses and norms of rock samples from the Bearcat Ridge plutons, Luceme quadrangles— Continued Sample ............. 1 2 3 Plagioclase composition 21.9 20.7 34.9 Sample descriptions 1. Composite sample of biotite granodiorite gneiss. 2. Composite sample of hornblende-biotite-quaitz diorite gneiss. 3. Composite sample of biotite-hornblende-quartz diorite gneiss. 48° 120°30’ 12' EXPLANATION l:| Biotite-hornblende-and hornblende-biotite-quartz diorite gneiss and flaser gneiss Biotite-quartz diorite and grano- diorite gneiss Contact . Sample locality 0 5 KlLUMETERS l_l_l_l__;l FIGURE 19.—-Map showing location of modally analyzed samples from the Bearcat Ridge plutons, Lucerne quadrangle. Sample analyses are given in table 4. varies considerably in appearance from place to place; most of it is rather light colored, but where con- taminated with considerable mafic material derived from host rocks of hornblende and biotite gneiss and schist, it is dark. Some of it is nearly nonfoliated and medium grained, whereas at the other extreme rocks are so finely foliated as to appear almost schistose. Probably the most characteristic feature of the rock is the per- vasive swirled foliation. This foliation, defined mostly by planar parallelism of biotite flakes, trends generally northwest, but at the north end of the pluton, trends are northeast more or less parallel to the contact. Everywhere, however, are local changes in trends— swirls—on scales ranging from a few meters to many tens of meters. As with other gneissic intrusive rocks in the area, the foliation is the result of pervasive protoclasis, but here the foliation is generally far more highly swirled 22 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON Quartz / \ // r / l / / / ’ 50 / //// .11 .15 J3 .8 2 1/ ' .14 10 ' /° 09..6 .12 O .7 3 / 4 5 o I]! / Potassium Plagioclase 5 35 feldspar FIGURE 20.—Plot of modes of rock samples from the Bearcat Ridge plutons, Lucerne quadrangle, on a quartz-potassium feldspar- plagioclase diagram. Sample analyses are given in table 4. FIGURE 21.—Gneissic biotite-quartz diorite from the Leroy Creek pluton, Holden quadrangle. than in other plutons, indicative, perhaps, of more ir- regular movement of the crystal-packed magma during intrusion than seems to have been true in other plutons. Despite the variability of appearance, the composition of the Leroy Creek pluton is rather uniform except for the hornblende-bearing rocks at the north end. The hornblende-free rocks consist of 48 to 58 percent oligoclase, 33 to 37 percent quartz, and 5 to 11 percent biotite as essential minerals, and epidote, sericite, and chlorite in generally small but variable quantities as secondary minerals (table 6). Potassium feldspar, opaque minerals, titanite, muscovite, and apatite are ac- cessory. In the northern part of the pluton, garnet crystals as much as 1 cm across are fairly common. As with other quartz diorite plutons in the study area, the Leroy Creek pluton also contains many inclusions of hornblende gneiss and schist and Swakane Biotite Gneiss. Relicts of Swakane are less obvious than homblendic inclusions because of the similarity of the Leroy Creek gneissic quartz diorite to the Swakane. Inclusions of Swakane tend to form ghostlike wisps and streaks and larger irregular masses that grade into quartz diorite, whereas inclusions of hornblende gneiss and schist mostly form long, thin layers having sharp contacts with the enclosing quartz diorite. Scattered dark, angular, unon'ented inclusions do occur, however, and are largely restricted to the swirled facies of the rock. Contacts of the Leroy Creek pluton are mostly rather abrupt, the lateral contacts commonly consisting of thin zones of lit-par-lit injections. The northwest end is marked by a zone of migmatite a few meters thick, and here gneissic quartz diorite contains considerable hornblende in contrast to the rock elsewhere in the pluton. The pluton has had little megascopically dis- cernible effect on the enclosing wall rocks. Plagioclase is medium oligoclase in the hornblende- free rocks but ranges up to sodic andesine in the homblendic rocks at the north end of the pluton. Plagioclase is mostly anhedral and unzoned, but some grains are progressively zoned and a few show pronounced oscillatory and patchy zoning. Quartz forms anhedral interstitial grains, irregular lenses of interlock- ing grains, and matrices consisting of mosaics of ir- regular grains in which plagioclase and other minerals occur as partly resorbed islands. Nearly all the quartz is strained. Biotite occurs as shreds or aggregates of ir- regular grains; in some thin sections, biotite is mostly confined to zones of granulation. The biotite is characteristically olive brown and has been analyzed previously (Crowder, 1959, table 2). Hornblende forms anhedral blades or aggregates of small blades pleochroic in shades of light yellowish brown to green. Most clinozoisite-epidote is secondary, but in thin sections showing the best hypidiomorphic textures, large sub- hedral crystals, apparently of primary origin, occur separately or with biotite. Such clinozoisite-epidote is also characteristic of some other plutons in the study area. Textures of rocks in the Leroy Creek pluton range from typically hypidiomorhpic to xenomorphic and proto- clastic. Some of the rock is slightly recrystallized. Most characteristic are xenomorphic textures with subdued protoclastic modifications. Plagioclase grains show con— PRE-TERTIARY INTRUSIVE ROCKS 23 TABLE 6.—M0dal analyses of samples of biotite-quartz diorite gneiss from the Leroy Creek pluton, Holden quadrangle [Leadeis (- - -), not measured or not present] Accessory Total Anorthite in Sample Potassium and second— mafic plagioclase crystals (percent) No. Plagioclase feldspar Quartz Biotite Homblende ary minerals minerals Average Core Rim Remarks 1 40.3 0.2 48.3 2.9 5.4 3.9 12.2 28 38 16 Xenomorphic. 2 47.4 --- 33.0 2.2 13.2 4.2 19.6 30 31 26 D0. 3 48.7 --- 36.7 1.3 ~-- 13.3 14.6 20 27 16 Do. 4 50.8 --- 43.8 --- .7 4.7 5.4 20 21 17 D0. 5 48.5 --- 38.4 9.8 ~-- 3.3 13.1 23 26 21 Protoclastic. 6 57.2 .2 33.0 7.1 --- 2.5 9.6 22 27 19 Xenomorphic. 7 53.4 --- 28.4 3.8 10.9 3.5 18.2 23 30 18 DO. 8 54.3 ~-- 38.9 3.6 --- 3.2 6.8 20 23 12 Protoclastic. 9 49.7 --- 36.6 4.3 --- 9.4 13.7 22 23 15 D0. 10 41.2 --- 39.6 1.0 13.2 5.0 19.2 26 31 21 Xenomorphic- protoclastic. siderable rounding and some have been broken, and the individual fragments have been differentially rotated with respect to other fragments. Nevertheless, fine- grained comminuted material is mostly rather rare and seems to have been resorbed before complete crystalliza- tion. Some of the textures suggest mild recrystallization has occurred, but if so, it has been insufficient to destroy the swirled foliation indicative of irregular intrusive move- ments or even to obliterate oscillatory and patchy zon- ing. Furthermore, there seems little reason to believe that granitization was particularly effective in forming the pluton, as Crowder (1959) once thought. Repeated episodes of large-scale granitization or ultrameta- morphism, explaining each successive gneissic pluton that otherwise had little effect on the surrounding ter- rane, seem far less likely than magmas rising periodically from the lower crust. Chemical and spectrographic analyses and norms are shown in table 7; locations of modally analyzed specimens are shown in figure 22, and a plot of the modes in figure 23. A plot of the norms is in figure 12 and a variation diagram of major oxides in figure 13. SEVEN-FINGERED JACK AND ENTIAT PLUTONS The Seven-fingered Jack and Entiat plutons make up a group of intrusions cropping out from the Vicinity of Hart Lake south-southeastward across the Holden and Lucerne quadrangles. The Seven-fingered Jack pluton, consisting largely of biotite-hornblende-quartz diorite, is the northernmost of these plutons and extends into Ice Creek in the Holden quadrangle. The Entiat plutons, composing the rest of the plutons, are more variable in composition and are directly traceable into the Chelan Complex of Hopson (cited in Mattinson, 1972, p. 3772), and hence they are correlative with that complex. Within the two quadrangles, the plutons are closely as- sociated spacially with the Dumbell Mountain and Leroy Creek plutons and were considered by Crowder (1959) to be genetically related. Mattinson (1972, p. 3771—377 2) also correlated them with these older plutons and considered them part of Misch’s (1966) “Marblemount belt,” although he did not rule out emplacement during the Jurassic or Early Cretaceous (Mattinson, 1972, p. 3378). Although the Seven-fingered Jack and Entiat plutons do resemble the Dumbell Mountain plutons in many respects, they are com- positionally far more diverse, and they not only intrude the Dumbell Mountain rocks but contain considerably more K20 (tables 3, 8). Ages as determined by Mat- tinson (1972) are tens of millions of years younger for rocks collected from the Chelan Complex. These ages TABLE 7,—Chemical and spectrographic analyses and norms of a composite sample of biotite-quartz diorite from the Leroy Creek pluton, central Washington [8102 and A120: determined by X-ray fluorescence; total Fe, MgO, and CaO determined by atomic absorption; FeO determined volumetrically; N320 and K20 determined by flame photometer; T102 determined colorimetrically; analysts: P.L.D. Elmore, K. E. White, and S. D. Botts. Spectrographic analyses by H. G. Neiman] Semiquantitative Chemical analyses spectrographic (percent) analysis (parts per million) Norms Si02 ---------- 73.0 Ba ----------- 500 q ------------- 36.07 A1203 ---------- 14.0 Cr ——————————— 1.5 or ------------- 3.83 F8203 ————————— 1.24 Cu ........... 3 ab ------------ 39.74 FeO ——————————— 1.39 Mo ........... 3 an ------------ 14.82 MgO ---------- .72 Sc ----------- 10 hy ------------ 3.11 0210 .......... 2.95 Sr ----------- 500 c .............. .32 N820 —————————— 4.64 V ------------ 30 mt ———————————— 1.82 K20 ----------- .64 Y ............ 15 i1 ------------- .29 Ti02 ---------- .15 Zr ----------- 70 Total ------- 10000 Total -------- W Ga ----------- 15 Yb ----------- 1 5 Plagioclase composition an ----------- 27.2 24 121°52'30" 48° 10' 121°45’ EXPLANATION Contact Sample locality 48° 05’ 0 3 KILOMETERS l—l___.J—l FIGURE 22.—Map showing location of modally analyzed samples of biotite-quartz diorite gneiss from the Leroy Creek pluton, Holden quadrangle. Sample analyses are given in table 6. range from about 100 to 193 m.y. for zircons from sam- ples of these rocks to only 71 to 87 m.y. for sphene from the same samples (table 38). Although only one age, a PbW/Pb206 age of 183 m.y. for a medium-grained fraction of zircon, was clearly discordant, the Pb""/Pb206 ages ranging from 100 to 132 m.y. for other samples were either concordant or but moderately discordant “within the limits of the lead-lead ages” ( Mattinson, 1972, p. 3778). He considered it likely that these rocks were “remobilized” from rocks of the “Marblemount belt,” a possibility for which little evidence exists. Engels (Engels and others, 1976) obtained potassium/argon ages of about 60 and 64 m.y. from biotite and hornblende from the Entiat pluton, figures that are probably much too low because of argon loss. For these reasons, correla- tions of the Seven-fingered Jack and Entiat plutons with the Dumbell Mountain or Leroy Creek plutons are con- sidered erroneous. The plutons are elongate, narrow masses injected into fault zones or zones of structural weakness. None of the masses within the mapped area exceeds a thickness of about 1.5 km. The largest is the Entiat pluton, con- sisting of hornblende-biotite- and biotite-hornblende- quartz diorite; its length within the mapped area is about 16 km, but its total length is not known. The Seven-fingered Jack plutons are smaller but more diverse in composition, consisting of hornblende-quartz diorite and quartz diorite gneiss, hornblende-biotite- INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON Quartz / / / / / 5 7’ l / Potassium feldspar Plagioclase 5 35 FIGURE 23,—Plot of modes of biotite—quartz diorite gneiss samples from the Leroy Creek pluton, Holden quadrangle, on a quartz- potassium feldspar-plagioclase diagram. Sample analyses are given in table 6. and biotite-hornblende-quartz diorite; quartz gabbro, diorite, and gabbro. Some of the smaller masses, which are merely dikes as little as 30 m or so thick, consist of a single rock type. Others, particularly the larger plutons, consist of a variety of types, although a single type is commonly dominant. Unfaulted contacts of Seven-fingered Jack plutons are mostly sharp where not obscured by later dikes and ir- regular masses of leucocratic quartz diorite, but locally they are gradational through a zone a few meters thick. The Entiat pluton, on the other hand, is characterized by wild contact complexes, particularly where it cuts metamorphic rocks, but the contact is generally sharp where intrusive into other plutons. Similar complexes characterize the contacts of other quartz diorite plutons of different ages in the area; all consist of spectacular melanges of various intrusive rocks, inclusions, and coarse-grained hornblendite (figs. 24 and 25). In more gneissic parts of the complex, foliation laminae wrap around inclusions in a manner suggestive of rotation by the inclusion because of differential flowage. BIOTITE-HORNBLENDE-QUARTZ DIORITE Biotite-hornblende-quartz diorite of the Seven- fingered Jack pluton is mostly a gray, medium-grained rock ranging from massive to gneissic (figs. 26, 27). Gneissic varieties are erratic in distribution but are PRE-TERTIARY INTRUSIVE ROCKS 25 FIGURE 24.—Coarse- and medium-grained hornblende gabbro from contact complex of Entiat pluton, Lucerne quadrangle. a; ..:~ I FIGURE 25.—Contact complex of Entiat pluton showrng pegmatitic hornblende gabbro (Hgb), schist (sch), and hornblendite (hdld). generally more common near contacts; the westernmost dikelike mass paralleling the Entiat fault is gneissic nearly everywhere. Locally, the large mass between Hart Lake and Ice Lakes consists of layers of gneissic quartz diorite 0.3 m or more thick that alternate with nongneis- sic rock. Probably the most noteworthy feature of this mass, however, is a conspicuous lineation defined by closely spaced pencillike mafic streaks and schlieren having parallel orientation (fig. 27). In places, the rock has a gneissic appearance because of mafic xenoliths smeared out in the plane of foliation; angular inclusions are common locally. In thin section, the rock is seen to consist of andesine, quartz, and hornblende, and in most specimens, small amounts of biotite as essential minerals. Titanite, opaque minerals, apatite, and rare zircon are accessory; generally small amounts of clinozoisite-epidote, chlorite, and sericite occur as secondary products. 'Andesine ranges in average composition from about Ansa to nearly An5o, but most of it falls between Angg and An45. Crystals are subhedral to euhedral and, except where destroyed by twinning, show conspicuous oscillatory and patchy zoning. In contrast to plagioclase in the Dumbell Moun- tain plutons, no crystals of which show more than four or five oscillations, plagioclase in this quartz diorite com- monly shows a dozen or more oscillatory zones. This characteristic helps to distinguish rocks of these plutons from those of the very similar Dumbell Mountain plutons. Quartz is interstitial or replaces andesine and hornblende. Hornblende is anhedral to subhedral and much of it is zoned, the rims being deeper green than the cores. The pleochroic scheme of most of it is X, pale yel- low; Y, greenish brown; and Z, dark green or bluish green. Cores of some grains contain poikilitic inclusions of quartz and smaller amounts of plagioclase. Biotite oc- curs as irregular flakes or is secondary after hornblende. In a few thin sections, clinozoisite-epidote is coarse and euhedral and seems to have formed in equilibrium with hornblende. Textures, which in some of the gneissic varieties show slight protoclastic modifications, are characteristically hypidiomorphic; cracks in crystals of abraded andesine are filled with biotite (fig. 28), although most of the rock in the westernmost mass is decidedly cataclastic and sheared. This mass seems to have been intruded along the zone of dislocation marked by the Entiat fault. HORNBLENDE-BIOTITE-QUARTZ DIORITE Hornblende-biotite-quartz diorite of the Entiat pluton is also a gray rock but is rather more variable in grain size than biotite-hornblende-quartz diorite and ranges from medium to fairly coarse grained (fig. 29). Locally it is gneissic, in places because of protoclasis, elsewhere because of alined crystals resulting from flowage. Mafic streaks and lenses are common, particularly near con- tacts. Hornblende-biotite-quartz diorite is micro- scopically similar to biotite-hornblende-quartz diorite, the principal differences being the preponderance of biotite over hornblende, generally more sodic 26 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON FIGURE 26.—Bi0tite-hornblende-quartz diorite from Seven-fingered Jack pluton, Holden quadrangle. plagioclase, and the common occurrence of small amounts of interstitial potassium feldspar, and in a sec- tion or two, enough to classify the rock as granodiorite. Characteristic also is considerable titanite as rhombs as much as 4 mm long. The textures are hypidiomorphic granular with local protoclastic and cataclastic modifications. HORNBLENDE-QUARTZ DIORITE GNEISS Hornblende-quartz diorite gneiss of the Entiat pluton that cuts the southwestern part of the Lucerne quadrangle and adjacent parts of the Holden quadrangle is a light- to dark—greenish-gray, medium-grained rock having a pervasive and highly swirled but not strongly accentuated foliation (fig. 30). Light-colored segregation dikes and pods are common. Lenticular segregations consisting largely of biotite, and other mafic streaks and lenses are fairly common and, in places, particularly near contacts, angular blocks of amphibolite are numerous. Inclusions, except for the angular ones that are commonly rotated (fig. 31), are elongated along planes of foliation. The rock varies considerably in both composition and texture. It consists of calcic oligoclase or sodic andesine, quartz, hornblende, and biotite in various proportions. Biotite occurs in only accessory amounts in most of the rock, but in places it is more abundant than hornblende. In lighter colored rocks, muscovite is also common. Accessory minerals are magnetite, titanite, and apatite. Chlorite and epidote- clinozoisite are secondary. Textures range from hypidiomorphic to xenomorphic, variously modified by protoclasis and cataclasis (fig. 32). HORNBLENDE DIORITE AND GABBRO Hornblende diorite and gabbro of the Seven-fingered Jack pluton are confined to the Lucerne quadrangle, although small masses of these rocks do occur locally in plutons consisting dominantly of the other rock types. W “WWI”; T' l "‘ FIGURE 27.—Biotite-hornblende-quartz diorite from the Seven- fingered Jack pluton, showing strong lineation defined by pencils of mafic-rich material, Holden quadrangle. The contact complexes do, indeed, consist perhaps dominantly of hornblende diorite and gabbro, and along the western margin of this diorite-gabbro pluton, the contact between the pluton and the complex is gradational. The rocks are of varied appearance and range from gray to nearly black with a greenish cast and from medium to coarse grained, and some are slightly pegmatitic. The hornblende diorite is generally of more uniform appearance, is gray, medium grained, and mostly massive, but some has a slight foliation._Gabbro is darker and some is greenish black; grain size, arrange- ment, and shapes of crystals differ greatly from place to place. Some gabbro is rather coarse grained and has a panidiomorphic texture; much is medium grained and granitoid, but fair quantities are relatively fine grained and, in outcrop, are very similar to the pawdite of various dikes in the area. The various rock types, in- cluding the diorite, are intergradational and locally stirred together in mixtures that look like marble cake. PRE-TERTIARY INTRUSIVE ROCKS 27 “d“ x ' I .__ . I . v I. . M W J . l 1 I FIGURE 28.—Protoclastic hornblende~biotite-quartz diorite from Seven—fingered Jack pluton, showing cracks in fractured crystal of andesine (A) filled with biotite (B), orthoclase (O), and quartz (Q), Holden quadrangle. Inclusions of various kinds are abundant; among the more numerous are knots of hornblendite and clots of coarse-grained plagioclase-hornblende rock. Hornblende and plagioclase are the only major constituent minerals common to all the rocks, but some contain considerable biotite and even a few percent of quartz. Plagioclase in diorite is calcic andesine and in gabbro is sodic labradorite; both oscillating and patchy zoning are com- mon. Orthoclase occurs as a sparse interstitial mineral locally, and titanite, apatite, muscovite, and magnetite are accessory. Epidote, chlorite, and sericite are sec- ondary. Textures range from xenomorphic to pan- idiomorphic; both protoclasis and cataclasis are prevalent. 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I - - wAHNAw ................ cu. - . - SWM ....... 3:on .............. ea .8 ...................... ow- . . . 33$ .......... £86 £250 2.x ....... ow- . . . .......... ESE $35 an ...................... 2o- . . . $3.8 ................ 8- . . . “<3 -- . $5355: .............. on .oQ ...................... cu. - - - «.mnzdb ................ ow. . - - 2.x ....... 853m .............. on .oQ ...................... ow- . . . «.mfivdh ................ ow- . - . 2-x 33832 . ........ on .5 ...................... ow. . . . H.398 ................ ow. . . . 2-x mvnmfifiom .............. on an ...................... cu. . . . twig -- 85% SEE. 06320 E-M ....... Scam .............. on— an . . 3H2: ................ ow- . . . 2.x 33832 .............. on 96 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON data presented by Engels and others (1976). Not in- cluded are those determinations that they, for various reasons, considered suspect or for geologic reasons are untenable. All the data presented by Mattinson (1972) concerning the Dumbell Mountain, Seven-fingered Jack, and Entiat plutons are included, however, to show the spread that different methods give. The middle to Late Cretaceous period of metamor- phism proposed by Mattinson, during which he believed the upper Paleozoic rocks and the Dumbell Mountain plutons together were metamorphosed to amphibolite grade, was also a period when a number of larger plutons 0r batholiths in the region were intruded. These include the about-90-m.y.-old Mount Stuart and Black Peak batholiths and the Tenpeak- and White ' Mountain plutons (Engels and others, 1976). There can be no ques- tion that potassium/argon radiometric clocks were reset in many rocks 60 to 90 m.y. ago, but as is discussed later, there is little or nothing to suggest rather intense regional metamorphism at that time other than what has been interpreted as the metamorphic nature of the Dumbell Mountain and Seven-fingered Jack plutons and their correlatives (Mattinson, 1972, p. 3772; Misch, 1966; Crowder, Tabor, and Ford, 1966; Crowder, 1959). The upper Paleozoic rocks of the Holden area believed by Mattinson to have been metamorphosed to gneisses 60 to 90 m.y. ago were already gneisses when intruded by the Dumbell Mountain plutons 220 m.y. ago as proved by disoriented inclusions of gneiss and schist within the plutons. To regionally metamorphose the Dumbell Mountain (and Seven—fingered Jack) plutons to amphibolite grade and leave untouched the discordant metamorphic fabric of these inclusions seems unlikely. Rocks as susceptible to recrystallization as the contact complexes, characterized by abundant gabbro and very coarse grained hornblendite, that border the Entiat pluton (fig. 25) and other supposed premetamorphic plutons are equally unlikely to have completely escaped any metamorphic changes. All these complexes also con— tain abundant inclusions of disoriented fragments of metamorphic country rocks. More likely, the host rocks were metamorphosed before or near the time Dumbell Mountain quartz diorite was intruded. Potassium/ argon radiometric clocks were as widely reset in late Eocene time as during mid- and Late Cretaceous time, as is in- dicated by late Eocene potassium/argon ages obtained from rocks such as the Leroy Creek pluton and the Swakane Biotite Gneiss and its equivalents (Engels and others, 1976), but no one has credited this heating event with producing such dire effects on the terrane as had the proposed earlier event. Mattinson considered the Chelan Complex of Hopson (Mattinson, 1972, p. 3772) to be a metamorphosed, migmatized, and mobilized extension of Misch’s (1966) Marblemount belt, which includes the Dumbell Moun- tain plutons (Mattinson, 1972, fig. 1, p. 3770). Moreover, the Dumbell Mountain plutons, and hence the Marblemount belt, pinches out in the southern part of the Lucerne quadrangle, and the correlation of the Chelan Complex with the Marblemount belt is invalid. The Chelan Complex of Hopson probably correlates with the Seven—fingered Jack plutons, although a strip of Entiat pluton rocks, just southwest of the location of Mattinson’s sample 23 (1972, fig. 1, p. 3770), is designated as “Late Cretaceous and Tertiary.” This designation is based on potassium/argon ages of about 61 and 64 m.y. obtained from biotite and hornblende (Engels and others, 1976), roughly the same as the potas- sium/argon ages (60 to 82 m.y.) obtained from rocks nearby in the Chelan Complex from which Mattinson (1972) obtained zircon Pbm/U238 ages ranging from 100 to 183 m.y., and sphene ages of 71 to 87 m.y. No zircon Pbm/Um age determinations from the strip of Seven— fingered Jack rocks labeled as “Late Cretaceous and Tertiary” have been made, and until this is done, there is no more reason for calling these rocks Late Cretaceous and Tertiary than for calling the Chelan Complex Late Cretaceous and Tertiary on the basis of potassium/argon ages and ignoring the zircon lead/uranium ages. The Seven-fingered Jack rocks, including the Chelan Complex, are far more nearly akin, petrologically, to the older intrusives than to any of the Tertiary intrusive rocks, including the about—60-m.y.-old (Engels and others, 1976) Clark Mountain stocks. The lower beds in Paleocene Swauk Formation in the Chiwakum graben, in the Holden quadrangle, contain numerous pebbles and boulders identical to nearby out- crops of both the Dumbell Mountain and the Seven- fingered Jack intrusive rocks, indicating thereby that these rocks were exposed during that time. On the other hand, the formation contains pebbles and cobbles of ex- trusive rocks, but the affinities of these rocks are un- known, unless, perhaps, they are extrusive equivalents of the Clark Mountain stocks or somewhat older plutons. But it does seem unlikely that the Dumbell Mountain plutons, in particular, could have been eroded to catazonal depths during or so soon after the time they were supposed to have been metamorphosed about 90 m.y. ago. COMPOSITION AND DIFFERENTIATION Compositions of the various intrusive rocks in the study area were determined by 47 chemical analyses and more than 300 modal analyses. Of the chemical analyses, 7 were by standard methods and 40 by various rapid methods; all chemically analyzed samples were also analyzed by spectrographic means for semiquantitative determination of minor elements. Modes were deter- mined on thin sections of standard size with a point GENERAL GEOLOGICAL PROBLEMS RELATING TO INTRUSIVE ROCKS 97 counter of the type described by Chayes (1949). Analyses of rocks from the Cloudy Pass batholith have been reported earlier (Cater, 1969; Tabor and Crowder, 1969) and are not repeated here. Inasmuch as none of the rocks are particularly coarse grained, stained slabs were not used for modal determinations. For most of the smaller intrusions of relatively uniform compositions only single, representative, mostly com- posite samples were chemically analyzed, but for larger plutons that are zoned or are otherwise of variable com- position, a number of samples were commonly analyzed. The chemical analyses supplemented by numerous modal analyses are thought to give a fairly represen- tative view of the compositions of each intrusive. Even for the salic, quartz-bearing plutons, apart from the gab- broic contact complexes associated with some of them, the range is fairly wide; Sl02, for example, ranges from about 56 to about 76 percent. The range for some of the larger, compositionally variable plutons, moreover, is nearly as large as the range for the entire group of plutons. A number of plutons are closely similar in com- position, some because they are closely related in time and probably in source; but other compositionally similar plutons are probably unrelated, being separated in time by scores of millions of years. All, however, probably originated under somewhat similar conditions and from similar materials presumably from the upper mantle and lower crust. The older, narrow, quartz— bearing plutons mostly show much less diversity in com- position than do the latest Cretaceous and younger plutons. In fact, only the largest plutons, mostly Ter- tiary, are compositionally zoned or have considerable variability at exposed levels. However, as explained earlier, presumably only the roots or feeders of the older intrusions remain, and these narrow conduits would seem to be physically unfavorable locations for demonstrable zoning resulting from differentiation to oc- cur, particularly in more or less horizontal planes, although these narrow masses certainly seem to repre- sent levels in vertical zones. Probably the most characteristic feature distin- guishing the intrusive rocks of the area is their low con- tent of K20, relative to average intrusive rocks (Nockolds, 1954) of similar $102 content. This low K20 content is true regardless of age of an intrusive, although in general, the older the rock the lower the content of K20 even where SiOz is high. Indeed, the older pre- Tertiary plutons, even the most quartzose, are virtually devoid of potassium feldspar, and biotite is not common. Of the pre-Tertiary plutons, only the Bearcat Ridge, Sulphur Mountain, and High Pass plutons contain more than rare, interstitial amounts of potassium feldspar. The Sulphur Mountain pluton has a radiometric age of 70 my (Engels and others, 1976), indicating a Late Cretaceous age; the undated High Pass pluton cuts the Sulphur Mountain, and possibly the High Pass may be .as young as Tertiary. The undated Bearcat Ridge plutons both structurally and in megascopic appearance closely resemble the Dumbell Mountain plutons and were shown on the Luceme geologic quadrangle map as being contemporaneous. The Bearcat Ridge rocks, however, are not in contact with any of the older dated rocks, so their age relative to older intrusive rocks is not known. Norms and modal data indicate that, com- positionally, the Bearcat Ridge rocks are more nearly akin to far younger rocks, and it may be that these rocks are younger than was indicated on the map. Hietanen (1975) has proposed an explanation for the origin of the potassium-poor magmas in the northern Sierra Nevada that may explain the potassium-poor in- trusive rocks in the northern Cascades. Her explanation was based on the experimental work by Allen, Modreski, ' Haygood, and Boettcher (1972) and Modreski and Boett- cher (1972) on stabilities of amphiboles and biotite or phlogopite at high temperatures and pressures. Their ex- perimental work indicated that amphiboles become un- stable at depths of about 75 km, whereas phlogopite only begins to decompose at depths greater than 100 km and is unstable below 175 km. Hence, in a subducting lithospheric plate, amphibole would break down to release water and sodium to magmas forming at depths shallower than 100 km, whereas potassium would be retained in phlogopite or biotite and would only be released to magmas formed at depths greater than 100 km. Thus, possibly the magma for the potassium-poor intrusions in the study area were formed at relatively shallow depths. The Dumbell Mountain plutons, the oldest of the ma- jor intrusives in the area (there may possibly be some older metamorphosed dikes and sills in the Swakane Biotite Gneiss), have compositions that appear to be unique. In fact, a search of the literature, albeit not an exhaustive one, failed to disclose any analyses of igneous rocks having comparable contents of silica, iron, magnesium, and calcium where K20 was so astonish- ingly low; the rocks are extremely potassium-deficient quartz diorites (table 3). The perhaps not much younger but undated Leroy Creek pluton is about equally deficient in K20, but its trondhjemitic composition is not particularly unusual. Both younger pre-Tertiary and Tertiary plutons also are without exception poorer in K20 than is normal for rocks of otherwise comparable compositions. In fact, only in the porphyries and granophyres at the top of the Eocene Duncan Hill pluton does the content of K20 reach levels normal for rocks of similar silica contents. Rocks from the Dumbell Mountain plutons have a considerable compositional range, the analyzed samples containing from 52.5 to 73.7 percent SIO2. All other ma- jor constituents decrease as silica increases except Na20, 98 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON which remains fairly constant, and K20, which also re- mains constant but very low (table 3; figs. 12, 13). Hornblende—quartz diorite gneiss is compositionally the most variable of the Dumbell Mountain rocks, possibly because of considerable but locally variable quantities of partly assimilated hornblendic host rocks. The quartz diorite augen gneiss is considerably more siliceous than the slightly younger gneissic hornblende-quartz diorite and contains a little less K20. The variations of composi- tions within the plutons, as indicated by modal analyses of numerous samples (table 2; fig. 11), are erratic and in- dicate no systematic zoning. Systematic zoning, as in- dicated by modal analyses, also appears to be lacking in the rest of the pre-Tertiary intrusives within the area, although local variations of composition are appreciable and common. In the Glacier Peak quadrangle, however, Crowder, Tabor, and Ford (1966) mapped a dark, mafic facies in the northern part of the Tenpeak pluton; the other mapped units in the pluton differ from each other not so much in composition as in structure and texture. Tertiary intrusive rocks are compositionally more varied on the whole, and the distribution of com- positional variations is generally more systematic than is true of the older intrusive rocks. The smaller plutons are relatively homogeneous, and variations in content of K20 are rather small. The two largest plutons, the Cloudy Pass and Duncan Hill, on the other hand, con- tain rocks of widely different compositions. In the Duncan Hill pluton the compositional variations, in general, are systematic and show progressive changes along the length of the pluton, whereas in the Cloudy Pass variations are largely erratic and range between wide limits across short distances. The composition, the variations therein, and the reasons for these variations in the Cloudy Pass batholith were discussed at length by Cater (1969) and Tabor and Crowder (1969). The parts of the batholith described by each of these reports differ somewhat in the abundance of rock facies exposed and in the relative significance of differentiation processes operative in the batholith to produce these facies. The core of the batholith in the Holden quadrangle, described by Cater (1969), is characterized by rocks con- taining labradorite, the composition of which changes little from place to place. The abundance of orthoclase can range from none to abundant within distances measured only in a few meters. Rocks containing sodic plagioclase are rare and mostly confined to late dikes. To the west in the part of the batholith described by Tabor and Crowder (1969), much of the rock is a light-colored, oligoclase-bearing quartz monzonite, containing fairly constant amounts of potassium feldspar. Inasmuch as the batholith is only now in the process of being deroofed and is exposed only to depths not exceeding 1.6 km, it af- forded a particularly advantageous opportunity to study subvolcanic phenomena associated with the upper part of a batholith. The Duncan Hill pluton, on the other hand, affords an opportunity to View a large intrusion not only from a similarly shallow level but also to depths of a fair number of kilometers. Other plutons, because of their narrow vertical ranges of exposure or ranges, at any event, that show little or no consistent compositional variation, seem to correspond more or less to given narrow levels within the broad spectrum of depth zones observable along the length of the Duncan Hill pluton. Older intrusives, such as the Dumbell Mountains plutons, may indicate the kinds of rocks existing at deeper levels in the Duncan Hill. Not all plutons that are older than the Duncan Hill, however, seem to have been eroded as deeply as the north end of the Duncan Hill. The older Clark Mountain stocks to the west, on the opposite side of the Chiwawa graben, for ex- ample, have the general characteristics of rocks near the midpoint along the length of the Duncan Hill. In fact, in the Holden quadrangle, the terrane on the west side of the graben, in general, seems not to have been as deeply eroded in Tertiary times as that on the east side. The only lower Paleozoic or older Swakane Biotite Gneiss ex- posed west of the graben is in the roof of the Cloudy Pass batholith, and plutons older than or equal in age to those east of the graben have the characteristics of shallower intrusions. Because the compositional and textural difference present in plutons of progressively younger ages roughly parallel the changes that occur at increasingly shallow depths in the Duncan Hill pluton, probably the differen- tiation processes that were active in the Duncan Hill were equally active in the other granitoid intrusives of the area. In the Duncan Hill, however, the processes can be interpreted with reasonable assurance without recourse to a series of extrapolative leaps that would be necessary were the evidence only a series of fragments preserved in a series of separate intrusives. Hence, the Duncan Hill was investigated in more detail outside the mapped quadrangles because of information it probably gives on the history of differentiation of other plutons in the area. The plot of modes of rocks from the Duncan Hill pluton in figure 77 shows a wide scatter consistent with the wide compositional range of the rocks in the pluton; the plotted norms (fig. 79), on the other hand, although showing a considerable scatter, are confined to a narrow field extending from near the center of the triangle towards the albite corner. As shown by the variation diagram (fig. 78), in which major oxides are plotted against SiOz, only K20 increases as Si02 increases, although somewhat erratically, but as is well known, K20 is commonly the most variable oxide in granitic rocks. Na20 remains nearly constant; for that matter, it ranges between narrow limits in all the granitoid plutons of the area. Bateman and Dodge (1970, p. 414) also found GENERAL GEOLOGICAL PROBLEMS RELATING TO INTRUSIVE ROCKS that the Na20 content is about the same in the plutons in the central part of the Sierra Nevada batholith. As they pointed out, the content of Si02 in granitic rocks is a measure of differentiation, but it appears that the content of Na2O does not vary systematically with differentiation. Of particular interest is figure 98, a diagram showing the variations in SiOz, K20, Na20, and CaO along the length of the pluton. SiO2 and K20 are the only con- stituents that increase higher in the pluton, and Na20 remains virtually constant throughout. As indicated in figure 98, the only abrupt changes in composition of the Duncan Hill occur in the miarolitic and granophyric rocks at the south end of the pluton where the content of K20 about doubles and CaO decreases to a third of its value lower in the pluton. As shown in table 23, ferric ox- ide also increases slightly at the expense of ferrous oxide in the miarolitic rock, possibly the result of contamina- tion with atmospheric oxygen at these shallow levels. Although a gap of about 15 km separates analyzed- sample 7 from the analyzed samples at the south end of the pluton around Stormy Mountain, the reality of the abrupt change is confirmed by study of thin sections of samples in the intervening interval. The change in com- position is as abrupt and occurs in the same interval as 99 does the change in megascopic appearance in the rather narrow transition zone between biotite granodiorite and miarolitic quartz monzonite. Furthermore, unlike the underlying biotite granodiorite, the highly gas-charged miarolitic quartz monzonite appears to have been totally molten immediately prior to final crystallization. The oscillatory and patchy zoned plagioclase having labrodorite cores in the rock below the quartz monzonite is replaced by oligoclase showing progressive but little or no oscillatory zoning; cores of labradorite are either non- existent or where rarely present are highly saussuritized and nearly resorbed. That the Duncan Hill magma was moderately wet is suggested by the occurrence of hornblende and biotite to the total exclusion of pyrox- ene. Yoder (1969, p. 83—84) has pointed out that only at elevated water pressures can plagioclase of high anorthite content form at temperatures of less than 1250°C. The labradorite and a few bytownite cores in plagioclase having average compositions of generally sodic to medium andesine indicates high water pressures at the beginning of crystallization. Hence, a sufficient source of water at depth in the pluton for eventual transfer upward seems probable. Abundant evidence indicates the Cloudy Pass batholith also contained considerable water (Cater, NORTH END SOUTH END 76 — —16 O EXPLANATION 74 3 O Si02 — 14 0 NazO E A K20 Lu 72 — o CaO —12 g E E u.l f- 8 7o — _ _ , , —10 5 w Hornblende-blotvte-quartz lel’lte — n. Biotite granodiorite Miarolytic quartz monzonite g '— I _ to h .— I <2 68 2 2 023m _8 E Lu :3. a. 33.33. ‘ a s g EEEE °~ z w w own/1m U — 66 — a A '6 2 Oczl ‘ A m a7) 9; 64 — o Nazo . —~ 4 :‘zu X o T h A O CaO 3: 62 — —2 o l | | l l | 60 I I 1 I l l I II o 0 10 20 30 40 PLUTON LENGTH, IN KILOMETERS FIGURE 98.—Diag'ram showing variation in content of SiOz, K20, Na20, and CaO along the length of the Duncan Hill pluton, central Washington. Sample analyses are given in table 23; sample locations (fig. 76) are projected to the center line of the pluton. Hornblende- biotite-quartz diorite probably crystallized at least 8-10 km below the miarolytic quartz monzonite. Biotite granodiorite probably crystallized at least 3-6 km below the miarolitic quartz monzonite. 100 INTRUSIVE ROCKS, HOLDEN AND LUCERNE QUADRANGLES, WASHINGTON 1969), but the compositional changes in the rocks in the Cloudy Pass resulting from degassing of the cooling batholith are markedly different from those that occur- red in the Duncan Hill pluton. There are no indications, for example, that rocks in the upper part of the Cloudy Pass were remelted prior to final crystallization; only large-scale but erratic metasomatism occurred along with features indicative of violent expulsion of gas and extrusive activity. The remaining uppermost part of the Duncan Hill pluton shows no features suggestive either of explosive escape of gas or extrusion of lava. One seems forced to the conclusion that the roof of the Cloudy Pass was leaky, whereas that of the Duncan Hill was tight and permitted only slow escape of volatiles as the pluton crystallized. Had volatiles from the Duncan Hill been able to escape readily, there seems little likelihood that water-vapor pressures could have risen to the point where labradorite cores of the plagioclase crystals, with which the magma must have been charged, could have gone back into solution in a magma now almost com- pletely liquid. The miarolitic and granophyric quartz monzonite then finally crystallized from this almost completely liquid magma to a rock bearing little outcrop resemblance to the underlying granodiorite into which it grades. The depth at which the miarolitic quartz monzonite crystallized is not known, but if the water vapor was ap- proximately equal to the lithostatic pressure when the quartz monzonite crystallized, as indicated by the posi- tion of samples of quartz monzonite on the isobaric lines (Tuttle and Bowen, 1958, p. 74—75) in figure 79 (a pres- sure of about 150,000 kilopascals), then thequartz mon- zonite solidified at a depth of 8 or 9 km—a depth, however, that seems excessive for such volcanic- appearing rocks. Perhaps the water-vapor pressure con- siderably exceeded the lithostatic load. Certainly con- siderable water pressure was necessary to accomplish resolution of the plagioclase. The probable depths of samples of granodiorite and quartz diorite below the miarolitic quartz monzonite are interpreted from the dips of remnants of Miocene lava caps remaining on ridges south of the Lucerne quadrangle and are believed to be minimal. The progressive enrichment upward of Sl02 and K20 in the Duncan Hill pluton probably resulted from transfer of these components in supercritical aqueous solutions, rather than from differentiation by crystal fractionation. However, the decrease in CaO and mafic constituents higher in the pluton seems to be more than would be likely if the only changes were the addition of Sl02 and K20 strictly by metasomatism, so possible sinking of heavier minerals in the magma may have had a minor effect on composition. Furthermore, the protoclastic texture of the quartz diorite in the north- ern—and lower—part of the pluton suggests that filter pressing upward of the still fluid, more silicic fraction of the magma may also have been effective. In any event, the protoclastic textures do indicate crowding and mutual interference of crystals while the magma was still in motion. Metasomatic transfer of 8102 and K20 was equally ef- fective in causing compositional variation in the Cloudy Pass batholith (Cater, 1969), but the variations there were much more haphazard than in the Duncan Hill. The efficacy of potassium metasomatism in altering compositions of intrusions has been documented by many geologists as, for example, Boone (1962), Kennedy (1953), and Taubeneck (1967) and is supported by a large body of experimental work (Burnham, 1967; Jahns and Burnham, 1958; Kennedy, in citing Mosey, 1955, p. 498; Luth and others, 1964; Orville, 1963; Tuttle and Bowen, 1958, p. 90—91). Possibly this same process has operated effectively in other plutons in the area and may explain the extreme poverty of K20 in the Dumbell Mountain plutons. Potassium added to one rock or magma must have been taken from another somewhere else, and commonly only rocks enriched in potassium have elicited much attention because evidence of enrich- ment is far more obvious than evidence of depletion. The remaining parts of the Dumbell Mountain, Leroy Creek, and Seven-fingered Jack plutons may have been sites of potassium depletion. The numerous Tertiary dikes in the area of widely dif— fering compositions suggests that differentiation probably by crystal fractionation was also effective. Many of the dikes are aschistic and some are directly traceable into or are identical in composition and ap- pearance to some of the larger plutons, but others are diaschistic and unlike the rocks of any of the larger plutons as, for example, the swarm of mafic dikes and ir- regular masses associated with the south end of the Duncan Hill pluton. The localities or magma chambers where the differentiation occurred, however, either have not been recognized or are not exposed. The large number of late Eocene intrusives in and adjacent to the mapped area that differ but little in age certainly suggests a widespread and copious source for large quan- tities of magma. Inasmuch as available data from stron- tium isotopes (Hedge and others, 1970; Church and Tilton, 1973; Peterman and others, 1970; Zell, Peterman, written commun., 1975) indicates the source of most magmatic material is the lower crust and upper mantle, then plenty of room exists in the remainder of the crust below the presently exposed surface for magma chambers to have formed. Such chambers not only could have fed the exposed plutons but also were sites where differentiates could have accumulated that were later forced into the dikes and small, irregular masses of widely different compositions that cut the area. GENERAL GEOLOGICAL PROBLEMS RELATING TO INTRUSIVE ROCKS TEXTURES AND DEPTH ZONES The fact that textures of igneous rocks vary with rate of cooling and with depth of emplacement is well known, but much that has been written concerning textures ap- plied only to mesozonal or shallower intrusions—rocks with so-called typical igneous textures. Deep-seated ig- neous rocks, on the other hand, often have been in- terpreted as recrystallized or as having been formed by replacement, because the textures were considered metamorphic. Thus, Crowder (1959), from his study of thin sections of the Dumbell Mountain, Leroy Creek, and Seven-fingered Jack plutons, considered these rocks to be either largely recrystallized or to have formed by replacement or granitization. Van Diver (1967) con- sidered the rather deeply eroded 90-m.y.-old White Mountain pluton to be recrystallized; he interpreted the protoclastic texture to be a recrystallized cataclastic tex- ture, although the foliation of the protoclastic rock is conformable to the nearest contact of the pluton and over large areas transects the regional foliation of the metamorphic host rocks. Identical textures also exist in most of the Tertiary plutons, including the Miocene Cloudy Pass batholith, as previously discussed in the section on “Ages of intrusive rocks and related problems.” Mattinson (1972) also considered the Dumbell Mountain plutons and their correlatives and Hopson’s Chelan Complex—probably correlative of the Seven-fingered Jack plutons—to be metamorphosed, the Chelan Complex to the point of anatexis. Misch (1966, p. 106 and fig. 7—1) considered the northward correlative of the Dumbell Mountain plutons—his Marblemount Meta Quartz Diorite—a part of his “basement complex” that had undergone his “Skagit metamorphism,” an event, incidentally, by which he derived his “Skagit Gneiss”—our Swakane Biotite Gneiss—from the “Cascade River Schist” (p. 113). Mattinson (1972) has dated the Cascade River Schist as late Paleozoic and the Skagit Gneiss as Precambrian. Misch also showed the Dumbell Mountain plutons (his fig. 7-1) as overthrust by the upper Paleozoic gneisses, rather than intrusive into them but did concede (his p. 114), “the meta quartz diorite has relict gross igneous textures.” These examples illustrate some of the interpretations of deep-seated intrusive rocks in the area that have been published by various geologists, but there are, however, a number of reasons for believing these rocks have not been metamorphosed, or only weakly so, since they were intruded. These reasons are discussed below. As Crowder (1959, p. 843) noted, the hornblende- quartz diorite augen gneiss of the Dumbell Mountain plutons contains unoriented inclusions of schist and gneiss. He considered these inclusions to be mobilized breccias resulting from granitization. In addition to con- 101 taining these unoriented inclusions, much of the rock in both the Dumbell Mountain and Leroy Creek plutons has swirled foliation that is independent of regional foliation in the metamorphic host rocks; furthermore, foliation in the Leroy Creek pluton tends to parallel its contacts. Had regional metamorphism occurred after emplacement of the plutons, then the disoriented foliated fabric of the inclusions could not have survived regional recrystallization of supposed amphibolite grade. Were the inclusions confined to the central parts of plutons, it could be argued, perhaps, that the competent plutonic rocks shielded the inclusions from strong metamorphic effects; however, disoriented inclusions are most common near contacts, as other inclusions are. Also, it is difficult to account for the swirled foliation resulting from regional metamorphism where no swirled foliation exists in the enclosing gneiss and schist. Although the textures of the plutons resemble crystal— loblastic textures, most thin sections contain plagioclase that shows both vague oscillatory and patchy zoning. Furthermore, younger plutons such as the Seven- fingered Jack have textures that verge on crystalloblastic in appearance in most places but not to the extent that is characteristic of the older and deeper seated plutons; textures of some of the Seven-fingered Jack rocks are typically hypidiomorphic. It is equally difficult to ex- plain how the spectacular coarse-grained contact com- plexes associated with the Seven-fingered Jack and other presumed premetamorphic plutons and containing in- numerable foliated metamorphic rock in all orientations, completely escaped recrystallization while their con- sanguineous plutons were being reduced to amphibolite- grade gneisses. Rocks crystallized from magma intruded into material where conditions of amphibolite grades of metamorph- ism exist would probably differ texturally from rocks crystallized from magma injected into material where conditions of greenschist metamorphic facies or cooler conditions prevail. If, as Buddington (1959, p. 676) es- timated, rock temperatures at the top of the catazone are about 500°C, then plagioclase in a crystallizing magma, for example, would have ample opportunity to equilibrate, and oscillatory and patchy zoning, so characteristic of plagioclase in mesozonal and epizonal rocks, would be largely destroyed, as is true in rocks from the Dumbell Mountain and Leroy Creek plutons and in most of the rocks in the Seven—fingered Jack and Entiat plutons. Extremely slow rates of cooling of magma in- truded into regionally hot rocks should not be expected to yield rock having textures usually referred to as typically igneous. Most likely, textures more nearly resembling crystalloblastic textures should result. The generally pseudocrystalloblastic textures of the Dumbell Mountain and Leroy Creek plutons and much of the rock in other older pre-Tertiary plutons, and the gneissic 102 fabric of these rocks, can hardly have been formed by metamorphic recrystallization, leaving unaffected the attitudes of gneissic foliation in disoriented inclusions and slabs. The gneissic fabric of the quartz diorite is probably the result of overgrowths on crystals oriented by flowage at early stages of crystallization. It is true, nevertheless, that rocks in the area have been sufficiently reheated periodically to yield anomal- ously low potassium/argon ages for both metamorphic and pre-Tertiar'y intrusive rocks (Engels and others, 1976). 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S., Jr., 1969, Calcalkalic andesites—Experimental data bearing on the origin of their assumed characteristics, in Andesite Conference, Eugene and Bend, Ore., 1968, Proceedings, Oregon Department of Geology and Mineral Industries Bulletin 65, p. 77—89. ____1973, Contemporaneous basaltic and rhyolitic magmas: American Mineralogist, v. 58, p. 153—171. Yoder, H. 8., Jr., and Tilley, C. E., 1962, Origin of basaltic magmas—An experimental study of natural and synthetic rock systems: Journal of Petrology, v. 3, pt. 3, p. 342—532. Youngberg, E. A., and Wilson, T. H., 1952, The geology of the Holden mine: Economic Geology, v. 47, p. 1—12. A Page Actinolite ............................. 57, 59 Rock Creek ............................... 60 Duncan Hill Pluton ...................... 65 Age ........................... 3. 7, 23, 56. 58, 93 Sulphur Mountain pluton ................... 37 Swakane Biotite Gneiss ..................... 5 Agmatite. Duncan Hill pluton ................. 65 Alaskite ................................ 58, 60 Albite ............................ 56, 59, 84, 89 Buck Creek pluton ....................... 39 Clark Mountain stocks ..................... 53 Cloudy Pass batholith ...................... 87 Duncan Hill plu ton ........................ 64 High Pass pluton .......................... 39 Old Gib volcanic rocks ...................... 74 Allanite .............................. 57, 76, 84 Cardinal Peak plu ton ....................... 50 Duncan Hill pluton ......................... 61 Railroad Cred: pluton ....................... 79 Rampart Mountain pluton ................... 71 Alpine Creek ............................... 38. 92 Amphibole ................................. 81 See also Hornblende. Amphibolite ................................. 96 Andesine ........................ 56. 59, 60, 76. 84 Bearcat Ridge pluton .................... 18, 19 Cardinal Peak pluton .................... 48, 50 Clark Mountain stock ....................... 53 Cloudy Pass batholith ...................... 87 Dumbell Mountain pluton ................ 10, 11 Duncan Hill pluton ................... 62. 67, 99 Entiat pluton .............................. 26 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Old Gib volcanic rocks ..................... 74 Rock Creek ............................... 60 Seven-fingered Jack pluton .................. 25 Tenpeak pluton ......................... 33, 34 White Mountain pluton ..................... 34 Anorthite ................................. 34. 67 Anthem Creek ................................ 83 Antigorite .................................. 7, 8 Apatite ........................ 8, 57, 59, 60, 76. 84 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain ............................ 53 Dumbell Mountain pluton ............... . . i 11 Duncan Hill pluton ......................... 6 1 Entiat pluton ........................... 25, 26 High Pass pluton ......................... 39 Holden Lake pluton ........................ 81 Larch Lakes pluton ......................... 69 Leroy Creek plu ton ......................... 22 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks pluton ........................ 43 Rock Creek ............................... 60 Seven-fingered Jack pluton .................. 27 Swakane Biotite Gneiss ..................... 29 Tenpeak pluton ............................ 34 White Mountain pluton ,,,,,,,,,,,,,,,,,,,,, 34 Aplite, Duncan Hill pluton ...................... 64 INDEX [Italic page numbers indicate major references] Page Appinite ..................................... 90 Archean Stillwater Complex ..................... 44 Augite .................................... 59. 35 Cloudy Pass batholith ...................... 88 See also Pyroxene. B Basalt Peak .................................. 72 Bear Creek ................................ 18, 77 Bearcat Ridge pluton ............... 18, 77, 89, 92, 97 Big Creek ..................................... 9 Biotite ............ 5, 8. 56. 58. 59, 75, 76, 82, 84, 89, 97 Bearcat Ridge pluton .................... 19, 20 Buck Creek pluton ........................ 39 Cardinal Peak pluton ............ 46. 47, 48.50.51 Clark Mountain ........................... 53 Cloudy Pass batholith ...................... 87 Copper Peak pluton ........................ 81 Dumbell Mountain pluton ............. 10, 1 1, 17 Duncan Hill pluton ................ 61, 62, 64. 99 Entiat pluton .................. 24. 25, 26. 27, 29 High Pass pluton ......................... 39 Holden Lake pluton ........................ 81 Lamh Lakes pluton ......................... 69 Leroy Creek pluton ........................ 22 Old Gib volcanic rocks ..................... 74 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks pluton ........................ 43 Rock Creek ............................... 60 Seven-fingered Jack pluton ....... 24, 25, 26, 27, 29 Sulphur Mountain pluton .................... 37 Tenpeak pluton ...................... 32, 33, 34 White Mountain pluton ............... 32, 33, 34 Biotite dacite ................................. 84 iBiotite granodiorite ............................ 83 Cardinal Peak pluton ..................... 46. 47 Duncan Hill pluton ......................... 99 High Pass pluton .......................... 37 Larch Lakes pluton ......................... 68 Rampart Mountain pluton ................... 70 Black Peak batholith ........................... 96 Bonanza Peak ................................ 1 1 Boulder Creek ................................. 7 Buck Creek plutons ......................... 37, 92 Buck Mountain ............................... 37 Buckskin Mountain ....................... 7. 61, 83 Bytownite .................................. s. 59 Cloudy Pass batholith ...................... 87 Duncan Hill pluton ...................... 65, 99 Riddle Peaks pluton ........................ 43 C Calcite ................................. 35. 59. 60 Buck Creek pluton ......................... 39 High Pass pluton .......................... 39 Cardinal Peak ................................. 47 Cardinal Peak pluton ......... 18. 40, 46, 75. 82, 84, 92 Cascade River Schist ........................ 8, 101 Cascadia ................................... 3. 91 Chalcopyrite. Cloudy Pass batholith .............. 89 Page Chelan Complex ................... 3. 10, 23, 96, 101 Chelan Lake. See Lake Chelan. Chiwaukum graben ........... 5, 6, 7, 58, 72. 84, 92, 98 Chiwawa Ridge ............................ 29, 38 Chiwawa River ................................ 58 Chiwawa River valley .......................... 72 Chlorite ........................... 8, 57, 59. 60, 83 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain stocks ...................... 53 Cloudy Pass batholith ...................... 87 Dumbell Mountain pluton ................... 11 Duncan Hill pluton ......................... 62 Entiat pluton ..................... 25, 26, 27. 29 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 leroy Creek pluton ......................... 22 Old Gib volcanic rocks ...................... 74 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 7 1 Riddle Peaks pluton ........................ 43 Rock Creek ............................... 60 Seven-fingered Jack pluton .......... 25, 26, 27, 29 Sulphur Mountain pluton .................... 37 Tenpeak pluton ............................ 35 White Mountain pluton ..................... 35 Clark Mountain stocks ................. 5.2 92, 96. 98 Clinopyroxene ........................... 8, 60, 83 Cardinal Peak pluton ....................... 50 Cloudy Pass batholith ...................... 87 Sulphur Mountain pluton .................... 37 Clinozoisite ................................. 5. 57 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain stocks ...................... 53 Dumbell Mountain pluton ................... ll Entiat pluton ........................... 25, 26 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton .................... 78, 79 Seven-fingered Jack pluton .............. 25. 26 Sulphur Mountain pluton .................... 37 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 Cloudy Pass batholith rrrrrrrr 17, 53, 60, 67. 68. 72, 80, 85, 86, 91. 92, 98 Coast Range batholith complex ................... 4 Columbia River ............................. 5, 30 Columbia River plateau .......................... 4 Contact complexes .......................... 27, 90 Cardinal Peak pluton ....................... 49 Copper Peak pluton ........................ 90 Duncan Hill pluton ................... 61. 65, 90 Holden Lake pluton ........................ 90 Seven-fingered Jack pluton ............... 49. 90 Tenpeak pluton ............................ White Mountain pluton ................ 34, 49, 90 Copper Peak ............................... 9, 81 Copper Peak pluton ......................... 80, 90 Cordillera .................................... 91 Cottonwood Guard Station ................ 17, 61, 62 105 106 Page Crow Hill .................................. 85 Custer Gneiss ................................ 5 Custer Granite Gneiss .......................... 5 Cutler Formation .............................. 6 D Dacite, Old Gib volcanic neck .................. 72 Bikes ...................................... 58 Diorite ...................................... 90 Copper Peak pluton ....................... 81 Duncan Hill pluton ......................... 61 Dole Creek .................................. 47 Domke Lake .................................. 77 Donegal pluton ............................... 8 1 Dumbell Mountain plutons ........ 8, 23, 25, 30, 91, 92 a, 96, 98, 100, 101 Duncan Hill pluton ............ 8, 17, 54, 56, 57, 58. 61, 81, 82, 85, 89, 91, 92, 97 contact complexes .................... 61, 65, 90 pawdite .................................. 59 E Edi] mine .................................... 43 Ellicott City Granodiorite, Maryland .............. 35 Emerald Peak ................................. 50 Entiat fault ................................ 25, 72 Entiat Meadows .............................. 75 Entiat Mountains ............................. 20 Entiat pluton ........... 10, 23, 41, 58, 68, 70, 96, 101 Entiat River ............................ 17, 48, 61 Epidote ....................... 6, 57, 59, 60, 84, 89 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain stocks ...................... 53 Dumbell Mountain pluton ................... 1 1 Entiat pluton ..................... 25, 26, 27, 29 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks pluton ........................ 43 Seven-fingered Jack pluton .......... 25. 26. 27, 29 Sulphur Mountain pluton .................... 37 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 F Feldspar .................................. 59, 83 Buck Creek pluton ......................... 37 Duncan Hill pluton ......................... 64 Fern Lake ........................... 61, 62, 67, 82 Flaser gneiss .................................. 33 Fourth of July Basin ........................ 45, 46 G Gabbro ................................ 26, 32,90 Copper Peak pluton ........................ 8 1 Duncan Hill pluton ......................... 65 Garland Peak ........................... 58, 85, 86 Garnet ....................................... 5 Buck Creek pluton ......................... 39 High Pass pluton .......................... 39 Leroy Creek pluton ......................... 22 Tenpeakpluton ...................... 33, 34. 35 White Mountain pluton ............... 33, 34, 35 Glacier Creek ................................. 87 Glacier Peak quadrangle ........... 3, 7, 31, 60, 86, 98 Glacier Peak volcano ........................... 31 Gopher Mountain ............................. 83 Graham Harbor Creek .......................... 19 Granite. Duncan Hill pluton ..................... 64 I N D E X Page Granodiorite ............................ 59, 60, 82 Bearcat Ridge pluton ....................... 19 Cardinal Peak pluton .................... 47. 50 Clark Mountain stocks ................... 53, 54 Cloudy Pass batholith ...................... 86 Copper Peak pluton ........................ 81 Dumbell Mountain pluton ................... 18 Duncan Hill pluton ...................... 61, 62 Entiat pluton ........................... 28, 29 High Pass pluton ........................ 38. 40 Larch Lakes pluton ......................... 69 Railroad Creek pluton ....................... 77 Rampart Mountain pluton ................... 70 Seven-fingered Jack pluton ............... 28, 29 Sulphur Mountain pluton .................... 37 Tenpeak pluton ............................ 32 White Mountain pluton ..................... 32 Granophyre .................................. 84 Cloudy Pasa batholith ...................... 87 Duncan Hill pluton ......................... 64 H Hart Lake ........................ 23, 25, 86, 87, 93 Helmet Butte ................................. 38 High Pass pluton ........................ 37, 92, 97 Holden Lake .................................. 8 1 Holden Lake pluton ............................ 80 contact complexes .......................... 90 Holden mine ................... 3,7,8 56, 75, 81 83 Hope, British Columbia ...................... . .3 Hope fault .................................... 4 Homblende ......... 5.6.3.56, 58.59.60, 75, 76, 82. 98 Bearcat Ridge pluton .................... 19, 20 Buck Creek pluton ......................... 39 Cardinal Peak plumn ........... 46, 47, 48, 50, 51 Clark Mountain stocks ...................... 53 Cloudy Pass batholith ...................... 87 Copper Peak pluton ........................ 81 Dumbell Mountain pluton .............. 10. 11, 17 Duncan Hill pluton ................ 61,62, 64, 99 Entiat pluton ........................ 24, 25. 26 High Pass pluton .......................... 39 Holden Lake pluton ........................ 81 Leroy Creek pluton ......................... 22 Old Gib volcanic rocks ...................... 74 Railroad Creek pluton ....................... 7 7 Riddle Peaks pluton ..................... 41, 51 Rock Creek ............................... 60 Seven-fingered Jack pluton ............. 24, 25, 26 Sulphur Mountain pluton .................... 37 Tenpeak pluton ...................... 32, 33. 34 White Mountain pluton ............... 32. 33, 34 Homblende diorite ............................. 26 Cardinal Peak pluton .................. 46, 48, 50 Rock Creek ................................ 60 Hornblende gabbro. Duncan Hill pluton ........... 61 Homblendite ............................. 7, 8, 90 Cardinal Peak pluton ....................... 49 Copper Peak pluton ........................ 81 Duncan Hill pluton ...................... 61, 65 Entiat pluton ........................... 24, 27 Riddle Peaks pluton ........................ 50 Seven-fingered Jack pluton ............... 24, 27 Tenpeak pluton ......................... 32, 34 White Mountain pluton ..................... 32 Hypersthane, Cloudy Pass batholith .............. 87 I Ice Creek ................................ 9. 17, 23 Ice Lake ..................................... 25 Idaho batholith ............................... 91 Ilmenite ................................... 8, 57 Bearcat Ridge pluton ....................... 19 Duncan Hill pluton ......................... 61 Iron sulfide ................................... 43 K Page Kersantite ................................. 58, 59 King Lake .................................... 38 Klone Creek ............................ 18, 48. 75 Kyanite ....................................... 5 L Labrador-ito ........................ 8, 56, 60, 76, 90 Cardinal Peak pluton ....................... 51 Cloudy Pass batholith ................. 86, 87, 98 Dumbell Mountain pluton ................... 1 1 Duncan Hill pluton ...................... 65, 99 Holden Lake pluton ........................ 81 Old Gib volcanic rocks ...................... 73 Riddle Peaks pluton ..................... 43, 44 Lake Chelan ............................ 18, 77, 84 Lamprophyre, Duncan Hill pluton ................ 64 Larch Lakes ............................... 27, 70 Larch Lakes pluton ................. 56, 58. 62% 70, 85 Leroy Creek pluton ....... 9. 10, 20, 23, 96, 97, 100, 101 Limonite ..................................... 89 Little Creek . ................................ 77 Luceme Mountain ............................. 77 M Magnetite ....................... 7, 8, 57. 59, 60. 89 Bearcat Ridge pluton ....................... 19 Cardinal Peak pluton ....................... 51 Clark Mountain stocks ...................... 53 Dumbell Mountain pluton ................... 1 1 Duncan Hill pluton ......................... 6 1 Entiat pluton ........................... 26, 27 Holden Lake pluton ........................ 8 1 Old Gib volcanic rocks ...................... 74 Rampart Mountain pluton ................... 7 1 Riddle Peaks pluton ........................ 43 Seven-fingered Jack pluton ............... 26, 27 Marble .................................... 6, 43 Marble Meta Quartz Dior-its .................. 8, 1 01 Marblemount belt .......................... 23, 96 Methow trough ................................ 3 Mica ...................................... 6, 20 Microcline .................................... 59 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Clark Mountain ............................ 53 Entiat pluton .............................. 29 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 Railroad Creek pluton ....................... 79 Rampart Mountain pluton ................... 71 Seven-fingered Jack pluton .................. 29 Sulphur Mountain pluton ................... 37 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 Migmatite .............................. 22, 28, 58 Milham Pass ........................... 47, 84, 85 Miners Ridge ................................. 86 Minette, augite ............................. 58, 59 Mirror Lake ............................... 77, 89 Mordenite .................................... 65 Mount Femow ................................ 1 1 Mount Rainier National Park .................... 87 Mount Stuart batholith ......................... 96 Muscovite ................................... 84 Bearcat Ridge pluton ....................... '19 Duncan Hill pluton ......................... 62 Entiat pluton ........................ 26, 27, 29 Leroy Creek pluton ......................... 22 Railroad Creek pluton ....................... 79 Rampart Mountain pluton ................... 7 1 Seven-fingered Jack pluton ............ 26, 27, 29 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 Mylonite ..................................... 57 Page Myrmekite ............................... 76. 84 Bearcat Ridge pluton ..................... 19 Clark Mountain stocks ...................... 53 High Pass pluton ......................... 39 Railroad Creek pluton ....................... 79 Rampart Mountain pluton .................. 71 Tenpeak pluton ............................ 35 White Mountain pluton ..................... 35 N N apeequa River ............................. 6, 38 Ninemile Creek ....................... 41. 75, 77. 82 Nontronite .................. . . ............ 85 Cloudy Pass batholith ...................... 89 Old Gib volcanic rocks ...................... 74 North Fork. Entiat River ....................... 61 North Star Mountain ........................... 86 0 Okanogan terrain ............................... 3 Old Gib Mountain ............................. 72 Old Gib volcanic neck .................. 60. 72. 74, 85 Old Gib volcanic rocks ....................... 72 85 Oligoclase ........................... 56. 60. s4. s9 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak plu ton ....................... 48 Clark Mountain stocks ...................... 53 Cloudy Pass batholith ...................... 87 Duncan Hill pluton ......................... 62 Entiat pluton .......................... 26. 29 High Pass pluton ......................... 39 Leroy Creek pluton ......................... 22 Rampart Mountain pluton .................. 71 Seven-fingered Jack pluton ............... 26. 29 Sulphur Mountain pluton .................... 37 Olivine .................................. 7. 8. 45 Opaque minerals .............................. 57 Buck Creek pluton ........................ 39 Cardinal Peak pluton ....................... 50 Duncan Hill pluton ......................... 62 Entiat pluton .......................... 25. 29 High Pass pluton .......................... 39 Larch Lakes pluton ........................ 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton ...................... 78 Seven-fingered Jack pluton ............... 25. 29 Tenpeak pluton ........................... 34 White Mountain pluton ..................... 34 Orthoclase ............................ 59. 76, 83 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Cloudy Pass batholith .................... 87. 98 Duncan Hill pluton ......................... 62 Entiat pluton .............................. 27 High Pass pluton .......................... 39 Holden Lake pluton ........................ 81 Larch Lakes pluton ......................... 69 Riddle Peaks pluton ...................... 43 Rock Creek .............................. 60 Seven-fingered Jack pluton ................. 27 Orthopyroxene ................................. 8 P Pawdite ................................ 58. 60. 68 Pegmatite .................................... 38 Peléen spine. Old Gib volcanic neck ............... 73 Penninite ................................. 57. 72 Pentlandite .................................... 8 Pericline ..................................... 56 Peridotite ............................ 7. 8. 45. 90 Perthite ................................ 19. 83. 89 Duncan Hill pluton ......................... 62 INDEX Page Phelps Creek ................................. 20 Phelps Ridge ............................... 5 86 Phlogopite .............................. 8.56.97 Picotite ....................................... Plagioclaee ................. 8. 56. 57. 59. 76. 83, 90 Bearcat Ridge pluton ....................... 19 Clark Mountain stocks ...................... 53 Cloudy Pass batholith .................... 87. 98 Dumbell Mountain pluton ............. 10. 1 1. 17 Duncan Hill pluton ...................... 62. 67 Entiat pluton .......................... 25. 27 Holden Lake pluton ........................ 81 Old Gib volcanic rocks ...................... 74 Railroad Creek pluton ....................... 78 Riddle Peaks pluton ....................... 43 Seven-fingered Jack pluton ............... 25, 27 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 Pomas Creek ............................... 27. 29 Potassium feldspar .............. 54, 56, 60. 76. 84. 97 Bearcat Ridge pluton ....................... 20 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 47 Clark Mountain stocks ..................... 53 Cloudy Pass batholith ...................... 98 Dumbell Mountain pluton ................... 11 Duncan Hill pluton ......................... 62 Entiat pluton .............................. 29 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Seven-fingered Jack pluton .................. 29 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 Prehnite. Buck Creek pluton ..................... 39 High Pass pluton .......................... 39 Old Gib volcanic rocks ...................... 74 Riddle Peaks pluton ........................ 43 Pyramid Creek ................................ 61 Pyramid Mountain ............................ 47 Pyrite ................................. 59. 83. 89 Cloudy Pass batholith ...................... 87 Railroad Creek pluton ....................... 78 Riddle Peaks pluton ........................ 43 Pyroxene .................................. 8. 99 Old Gib volcanic rocks ...................... 74 Riddle Peaks pluton ........................ 45 Pyrrhotite .................................. 8. 89 Q Quartz ................ 5, 56, 58. 59, 60. 76. 83. 84, 89 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ....................... 37. 39 Cardinal Peak pluton ....................... 50 Clark Mountain stocks ...................... 53 Cloudy Pass batholith ...................... 87 Dumbell Mountain pluton ............. 10 11 17 Duncan Hill pluton ......................... 62 Entiat pluton ..................... 25. 26, 27. 29 High Pass pluton .......................... 39 Holden Lake pluton ........................ 81 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Old Gib volcanic rocks ...................... 74 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks pluton ........................ 43 Rock Creek ............................... 60 Seven-fingered Jack pluton .......... 25, 26. 27, 29 Sulphur Mountain pluton .................... 37 Tenpeak pluton ............................ 34 White Mountain pluton ..................... 34 107 Page Quartz diorite 111111111111 5. 7. 56. 58. 60. 7d 82. 90. 98 Bearcat Ridge pluton .................... 19. 20 Cardinal Peak pluton ............ 4s. 47, 48, so. 51 Clark Mountain stocks ................... 53. 54 Cloudy Pass batholith ...................... 87 Dumbell Mountain pluton ......... 9. 10,11, 17, 18 Duncan Hill pluton ...................... 61. 64 Entiat pluton ................... 24. 25.26, 28 High Pass pluton ........................ 37: 46 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton ....................... 78 Riddle Peaks pluton ........................ 43 Seven-fingered Jack pluton 1111111111 24, 25. 26 28 Tenpeak pluton ...................... 32. 33. 34 White Mountain pluton ................ 32. 33, 34 Quartz gabbro. Cloudy Pass batholith ............. 86 Copper Peak pluton ........................ 81 Holden Lake pluton ........................ 81 Quartz granodiorite ..................... . . 58. 75 Quartz latite, .‘ ............................... 84 Quartz mouzonite ......... 56. 58. 59. 60. 75. 82 83. 89 Cloudy Pass batholith ...................... 98 Copper Peak pluton ........................ 81 Duncan Hill pluton ...................... 62. 99 Larch Lakes pluton ......................... 69 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................ 70. 71 Riddle Peaks pluton ........................ 43 Quartzite ................................ 6. 34. 43 R Railroad Creek ..................... 9. 10. 18, 40. 47 Railroad Creek pluton ...... 46. 56. 57. 58. 76. 77, 84. 92 Rampart Mountain ......................... 58. 70 Rampart Mountain pluton ........... 56. 57. 58. 70. 85 Red Mountain ................................. 3 Rhyodacite ................................ 58. 84 Rhyolite ..................................... 61 Riddle Creek ............................... 7 9, 84 Riddle Peaks ......................... 77. 83. 91, 92 Riddle Peaks pluton ............. 40. 45. 50. 51. 65. 75 Ripidolite ................................. 43. 44 Rock Creek .......................... 20. 29. 60, 86 Ross Lake fault ................................ 4 S Saska Peak ................................... 47 Sausurite. Duncan Hill pluton ................... 62 Sericite ............................. 57. 59. 60. 89 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain ............................ 53 Dumbell Mountain pluton ................... 11 Entiat pluton ........................ 25. 27. 29 High Pass pluton .......................... 39 Larch Lakes pluton ......................... 69 Leroy Creek pluton ......................... 22 Railroad Creek pluton ....................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks plu ton ........................ 43 Rock Creek ............................... 60 Seven-fingered Jack pluton ............ 25. 27. 29 Sulphur Mountain pluton .................... 3 7 Tenpeak pluton ............................ 35 White Mountain pluton ..................... 35 Serpentine ................................. 7. 45 Seven-fingered Jack pluton ....... 10, 17. 20. 23, 26. 41. 49, 92. 96. 100 contact complexes ....................... 49. 90 Sevenmile Creek drainage ....................... 82 Shetipo Creek ................................. 27 Sierra Nevada hatholith .................. 91. 93. 99 Skagit Gneiss ........................... 5. 30. 101 See also Swakane Biotite Gneiss. 108 Page Skagit River ................................. 5 Snoqualmie batholith ...................... 91, 93 Snow Brushy Creek ................... 46, 48, 61, 67 South Pyramid Creek .......................... 61 Southern California batholith .................... 91 Spectacle Buttes ............................. 17 Spessartite ................................ 58, 59 Sphalerite .................................... 89 Sphene ............................ 8, 24, 33, 34, 37 Spinel ....................................... 8 Stilbite .................................. 39, 74 Stormy Mountain ............... 56. 61, 64, 67, 89, 99 Straight Creek fault ............................. 4 Sulfides .................................. 59, 83 Sulphur Mountain .......... ‘ ................... 37 Sulphur Mountain pluton ................. 35 38, 97 Swakane Biotite Gneiss ........ 5, 7, 22, 28, 30, 38, 92, 93, 97, 98 Swauk Formation ....................... 5. 7, 58. 96 T Talc .......................................... 7 l N DE X Page Tatoosh pluton ................................ 87 Tenmile Creek ........................ 42, 47, 75, 82 Tenpeak pluton ............... 7, 31, 37, 60, 92, 96, 98 Titanite .......................... 57, 59, so, 76, 34 Bearcat Ridge pluton ....................... 19 Buck Creek pluton ......................... 39 Cardinal Peak pluton ....................... 50 Clark Mountain stocks ...................... 53 Dumbell Mountain pluton ................... 1 1 Duncan Hill pluton ......................... 61 Entiat pluton ..................... 25, 26, 27, 29 High Pass pluton .......................... 39 Holden Lake pluton ........................ 81 Larch Lakes pluton ......................... 69 Leroy Creek pluton ........................ 22 Old Gib volcanic rocks ...................... 74 Railroad Creek pluton ...................... 78 Rampart Mountain pluton ................... 71 Riddle Peaks pluton ........................ 43 Seven-fingered Jack pluton .......... 25, 26, 27, 29 Tremolite ............................... 7, 8. 59 Triad Creek .................................. 38 Trondjemite .................................. 28 Tumble Creek .............................. 18, 89 Page W White Mountain pluton ............ 31, 49, 92, 96, 101 contact complexes .......................... 90 White River Orthogneiss ........................ 32_ Wilson Creek ................................. 82 X. Y, Z Xenoliths .................................... 33 Xenotime, Duncan Hill pluton ................... 62 Yellow Aster Complex .......................... 93 Zircon ................................. 57. 84, 89 Buck Creek pluton ......................... 39 Clark Mountain ............................ 5 3 Duncan Hill pluton ......................... 61 Entiat pluton ........................... 24, 25 High Pass pluton .......................... 39 Railroad Creek pluton ....................... 78 Seven-fingered Jack pluton ............... 24. 25 Tertiary Volcanic Rocks and Uranium in the Thomas Range and Northern Drum Mountains, Juab County, Utah By DAVID A. LINDSEY GEOLOGICAL SURVEY PROFESSIONAL PAPER 1221 The geologic setting and controls of uranium mineralization in a volcanic environment UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1982 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Catalog No. 82—60050] For Sale by the Branch of Distribution, US. Geological Survey 1200 South Eads Street, Arlington, VA 22202 CONTENTS Page Abstract 1 Description of rock units——Continued Introduction 1 Topaz Mountain Rhyolite—Continued Acknowledgments 3 Stratified tuff Stratigraphy 3 Structural geology Stratigraphic section 3 Thomas caldera Age and correlation 7 Dugway Valley cauldron Description of rock units 9 gasmtjangl-range structure Drum Mountains Rhyodacite 9 mm ‘" e"° “m“ . . Some unsolved problems Dlonte 10 Chemical com osition of volcanic rocks Mt. Laird 'I‘uff 11 P , Rock types Joy Tuff 15 . . Trace-element associations Crystal tuff member 15 . . . . . . Relationship of volcamsm to mmerallzation Black glass tuff member 17 . . . _ Origin of volcamc rocks Landshde breccias 17 Uranium occurrences Breccia at Spor Mountain and breccia at Wildhorse Uranium in fluorspar pipes Spring 17 Uranium in the beryllium tuff member of the Spor Megabreccia of the northern Drum Mountains 20 Mountain Formation Dell Tuff 20 Yellow Chief mine Needles Range(?) Formation 22 Uraniferous opal Spor Mountain Formation 23 Uranium in stratified tuff of the Topaz Mountain Beryllium tuff member 23 Rhyolite Porphyritic rhyolite member 24 A model for uranium deposits and some suggestions for Topaz Mountain Rhyolite 26 exploration Flows and domes of alkali rhyolite 27 References cited ILLUSTRATIONS FIGURE 3- 13 Photographs and photomicrographs showing: Drum Mountains Rhyodacite Mt. Laird Tuff 1. Map showing location of the Thomas Range and Drum Mountains. other geographic features, and mineral occurrences — 2. Geologic map of Tertiary rocks in the Thomas Range and northern Drum Mountains Crystal tuff member of Joy Tuff Black glass tuff member of Joy 'I‘uff Landslide breccia at Spor Mountain and Wildhorse Spring The Dell Tuff 599°99’92““ Needles Range(?) Formation 10. Beryllium tuff member of the Spor Mountain Formation 1 l. Porphyritic rhyolite member of the Spor Mountain Formation 12. Topaz Mountain Rhyolite l3. Stratified tuff' 1n the Topaz Mountain Rhyolite 14 Aerial photographs of the ring fracture zone of the Thomas caldera, showing the Joy graben and its northward exten- sion in The Dell 15. Geologic map of The Dell 16. Maps showing structural evolution of the Thomas Range and northern Drum Mountains Ill Page 29 29 30 33 33 34 35 38 38 44 45 46 46 47 50 51 52 53 55 10 15 16 18 19 21 22 25 26 27 28 30 32 37 IV FIGURE 17. 18. 19. 20. TABLE 21. 22. 23. . Analytical data for new fission track ages of igneous rocks in J uab and Millard Counties . Analytical data for uranium content of zircon in volcanic rocks of the Thomas Range . Chemical analyses of igneous rocks of the Thomas Range and Drum Mountains CON TE N TS Graphs showing chemical composition of igneous rocks compared to age Graph showing total alkalis (N a20 plus K20) versus SiO2 content of igneous rocks AFM diagram of igneous rocks Histograms showing the abundance of uranium and thorium in the beryllium tuff member of the Spor Mountain Formation Diagrams showing distribution of altered zones, beryllium, thorium. uranium. and thorium:uranium in the beryllium tuff member of the Spor Mountain Formation at the Roadside beryllium mine Histograms showing the abundance of BeO, uranium, and thorium, and a scatterng showing segregation of BeO and uranium in mineralized zones of the beryllium tuff member of the Spor Mountain Formation --—-—-————-———————— Diagram showing location of geochemical samples in two ore lenses in the Yellow Chief mine TABLE S Revised stratigraphy of the Tertiary rocks of the Thomas Range and northern Drum Mountains Summary of radiometric ages for volcanic formations of the Thomas Range and northern Drum Mountains Comparison of mineral composition of volcanic rocks of the Thomas Range and northern Drum Mountains Section of Mt. Laird 'I‘uff in drill hole east of Topaz Mountain Chemical analyses of samples from two ore lenses in the Yellow Chief mine Page 39 42 43 48 49 50 52 TERTIARY VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND NORTHERN DRUM MOUNTAINS, JUAB COUNTY, UTAH By DAVID A. LINDSEY ABSTRACT The Thomas Range and northern Drum Mountains have a history of volcanism, faulting, and mineralization that began about 42 my. (million years) ago. Volcanic activity and mineralization in the area can be divided into three stages according to the time-related occur- rence of rock types. traceelement associations. and chemical com- position of mineral deposits. Compositions of volcanic rocks changed abruptly from rhyodacite—quartz latite (42—39 m.y. ago) to rhyolite (38—32 m.y. ago) to alkali rhyolite (21 and 6—7 my. ago); these stages correspond to periods of chalcophile and siderophile metal mineralization, no mineralization(?), and lithophile metal mineralization. respectively. Angular unconformities record episodes of cauldron collapse and block faulting between the stages of volcanic activity and mineralization. The youngest angular un- conforrnity formed between 21 and 7 my. ago during basin-and- range faulting. Early rhyodacite-quartz latite volcanism from composite volcanoes and fissures produced flows. breccias, and ash-flow tuff of the Drum Mountains Rhyodacite and Mt. Laird 'I‘uff. Eruption of the Mt. Laird Tuff about 39 my. ago from an area north of Joy townsite was accompanied by collapse of the Thomas caldera. Part of the roof of the magma chamber did not collapse. or the magma was resurgent, as is indicated by porphyry dikes and plugs in the Drum Mountains. Chalcophile and siderophile metal mineralization. resulting in deposits of copper. gold, and manganese. accompanied early volcanism. The middle stage of volcanic activity was characterized by ex- plosive eruption of rhyolitic ash-flow tuffs and collapse of the Dugway Valley cauldron. Eruption of the Joy ’I‘uff 38 my. ago was accompanied by subsidence of this cauldron and was followed by collapse and sliding of Paleozoic rocks from the west wall of the cauldron. Landslides in The Dell were covered by the Dell Tuff, erupted 32 my. ago from an unknown source to the east. An ash flow of the Needles Range(?) Formation was erupted 30—31 m.y. ago from an unknown source. Mineralization probably did not occur dur- ing the rhyolitic stage of volcanism. The last stage of volcanism was contemporaneous with basin-and- range faulting and was characterized by explosive eruption of ash and pumice. forming stratified tuff, and by quiet eruption of alkali rhyolite as viscous flows and domes. The first episode of alkali rhyolite volcanism deposited the beryllium tuff and porphyritic rhyolite members of the Spor Mountain Formation 21 my. ago. After a period of block faulting. the stratified tuff and alkali rhyolite of the Topaz Mountain Rhyolite were erupted 6—7 m.y. ago along faults and fault intersections. Erosion of Spor Mountain, as well as explosive eruptions through dolomite. provided abundant dolomite detritus to the beryllium tuff member. The alkali rhyolite of both formations is fluorine rich, as is evident from abundant topaz, and contains anomalous amounts of lithophile metals. Alkali rhyolite volcanism was accompanied by lithophile metal mineraliza- tion which deposited fluorite, beryllium, and uranium. The structure of the area is dominated by the Thomas caldera and the younger Dugway Valley cauldron, which is nested within the Thomas caldera; the Thomas caldera is surrounded by a rim of Paleozoic rocks at Spor Mountain and Paleozoic to Precambrian rocks in the Drum Mountains. The Joy fault and Dell fault system mark the ring-fracture zone of the Thomas caldera. These structural features began to form about 39 my. ago during eruption of the Mt. Laird Tuff and caldera subsidence. The Dugway Valley cauldron sank along a series of steplike normal faults southeast of Topaz Mountain in response to collapse of the magma chamber of the Joy Tuff. Caldera structure was modified by block faulting between 21 and 7 my. ago, the time of widespread extensional faulting in the Basin and Range Province. Vents erupted alkali rhyolite 6—7 m.y. ago along basin-and-range faults. Uranium mineralization was associated with the stage of alkali rhyolite volcanism, extensional basin-and-range faulting. and lithophile metal mineralization; it occurred at least 11 my. after the end of the caldera cycle. Uranium, derived from alkali rhyolite magma. was concentrated in trace amounts by magmatic fluids and in potentially economic amounts by hydrothermal fluids and ground water. Hydrothermal fluids deposited uraniferous fluorite as pipes in carbonate rocks of Paleozoic age on Spor Mountain and uranium-bearing disseminated deposits of fluorite and beryllium in the beryllium tuff member of the Spor Mountain Formation. Uranium of hydrothermal origin is dispersed in fluorite and opal. Uranium in fluorite may be tetravalent(?) but that in opal is prob- ably hexavalent; no primary minerals of tetravalent uranium are known to occur. Ground water has concentrated significant ores of hexavalent uranium minerals in the beryllium tuff member of the Spor Mountain Formation at the Yellow Chief mine and is probably also responsible for widespread low concentrations (0.0X percent) of uranium that occur separately from beryllium ore in the beryllium tuff member. More deposits of the Yellow Chief type may occur in down-faulted sections of beryllium tuff beneath the Thomas Range. The ground-water ores show no evidence of a reducing environment; instead, precipitation of hexavalent uranium minerals occurred by evaporation, by decline in concentration of complexing ions such as carbonate, or by some other mechanism. Reducing environments for hydrothermal deposits must be sought around rhyolite vents and in a hypothesized pluton of alkali rhyolite composition beneath Spor Mountain; for ground-water deposits, reducing environments may occur in basin fill such as that of the Dugway Valley cauldron. INTRODUCTION The Thomas Range (fig. 1) contains important deposits of fluorspar, beryllium. and uranium. These mineral deposits are associated with a sequence of volcanic rocks that extends into the northern Drum 1 2 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH G REAT SALT LAKE DESERT A Du av Range Xqu drangle 9,, s 9 RAIDSLEIEIIAIN r Moudgn‘)?’ _ U 113° I EXPLANATION x89 Mineral occurrence SIMPSON MOUNTAINS DUGWAY VALLEY DUGWAY- NGE , p 44 Detroiteisrrimgffib-EEn, cu‘ Au 243(19040 _ _ _ __ T Eaz Moun‘t'ain ' 4/ “—— '——" -—- MILLARD COUNTY ouadranle ‘ DRUM €476 Lynndw I ‘ MOUNTAINS ’1’», 76‘ ' LITTLE DRUM 15° MOUNTAINS WHITE 9 I VALLEY RED Hinckley ' o 10 20 KILOMETERS D KNOLLS . Delta ' I. o I LONG @ I I RIDGE FIGURE 1.—Map showing location of the Thomas Range and Drum Mountains, other geographic features, and mineral occur- rences shown by a pick and hammer and the following symbols: U. uranium; Be, beryllium; F, fluorite; Mn, manganese; Cu, copper; Pb, lead; Zn, zinc; Au. gold; and Ag, silver. Mountains and contains intermediate-composition flow rocks and tuffs, rhyolitic ash-flow tuffs, and large volumes of alkali rhyolite. The area is part of an east- west belt of mineral deposits, volcanic and intrusive rocks, and aeromagnetic high anomalies called the Deep Creek—Tintic mineral belt (Shawe and Stewart, 1976; Stewart and others, 1977), which also contains the beryllium belt of western Utah (Cohenour, 1963). A similar sequence of volcanic rocks, but with no known mineral deposits, crops out in the Keg Mountains, east of the Thomas Range. The volcanic rocks of the Thomas Range were first mapped and divided into two groups by Staatz and Carr (1964). Shawe (1972) reclassified the volcanic rocks of the Thomas Range into three assemblages, (1) flows and agglomerates, (2) ash-flow tuffs, and (3) rhyolite flows and tuff, all of which he was able to map throughout the area of the Drum Mountains, Keg Mountain, and Desert Mountain. Geochronologic studies (Lindsey and others, 1975) confirmed much of Shawe's (1972) three-fold classification of the volcanic rocks of the region. Shawe also concluded that erup- tion of voluminous ash-flow tuffs of his middle assem- blage was followed by caldera collapse in the Thomas Range and at Keg Mountain and Desert Mountain. 'The Joy fault was interpreted as part of the ring frac- ture of the Thomas caldera, and such ring fractures were believed to have provided conduits that localized the deposits of ore-forming fluids. The northern Drum Mountains also were mapped by Newell (1971), who confirmed the general outline of the caldera model there. Recent mapping in southwestern Keg Mountain by Staub (1975) did not confirm the Thomas caldera ring fracture that was projected there by Shawe (197 2). The mineral deposits of the Thomas Range were studied by Staatz and Osterwald (1959) and Staatz and Carr (1964), who described the fluorspar pipes and uranium occurrences there. Beryllium deposits in tuff STRATIGRAPHY 3 were discovered at Spor Mountain in 1959, and studies of these deposits related them to fluorspar mineraliza- tion and rhyolite volcanism in the Thomas Range (Staatz and Griffitts, 1961; Shawe, 1968; Park, 1968; Lindsey, 1977). The manganese deposits of the Detroit district in the Drum Mountains have been studied by Crittenden and others (1961), and the area’s potential for gold, copper, and other mineral deposits has been examined by mapping and geochemical surveys (Newell, 1971) and geochemical studies of jasperoid found there (McCarthy and others, 1969). This report describes the geology of Tertiary rocks and uranium occurrences of the Thomas Range and northern Drum Mountains in detail, and proposes a model relating volcanism, tectonism, and mineraliza- tion for the area. The present study was conducted in response to the current (1978) high interest in uranium exploration in the Thomas Range and vicinity. Recon- naissance studies of the uranium potential of the area have been made (Leedom and Mitchell, 1978; Texas In- struments Incorporated, 1977), and the uranium potential of the area has been evaluated by drilling (Morrison, 1980), but recent geologic investigations (Lindsey, 1978a) indicated that the history of volcan- ism and tectonism in the Thomas Range and Drum Mountains was still not sufficiently understood to relate it to uranium mineralization and to provide reli- able guides for uranium exploration. A new strati- graphic framework, resembling that of Shawe (1972) in general outline but differing in many details, was developed from field mapping, geochronologic, petro- graphic, and trace-element studies, and a new geologic map of the area was prepared (Lindsey, 1979a). The results of the new mapping are summarized in figure 2 for reference here, but the reader should consult Lind- sey (1979a) for details. The present report supersedes an earlier open-file report (Lindsey, 1979b). ACKNOWLEDGMENTS Many employees of the US. Geological Survey pro- vided chemical analyses and assisted with other aspects of this study; they are acknowledged in the text and tables. D. R. Shawe, by many discussions of the area studied, J. C. Ratte' , by discussion of cauldron-subsidence concepts, and R. K. Glanzman, by obtaining samples for uranium and thorium analyses, have contributed importantly to this report. L. M. Osmonson and Ezekiel Rivera made mineral separa- tions, and Louise Hedricks prepared photomicro- graphs. I thank many industry exploration geologists, including M. J. Bright, W. A. Buckovic, R. H. Dorman, D. E. Gorski, S. M. Hansen, S. H. Leedom, P. L. Nieson, J. M. Pratt, and W. A. Spears, for information about the geology of the area; and I also thank W. L. Chenoweth and D. A. Sterling of the US. Department of Energy and Charles Beverly, M. C. Callihan, C. M. Freeman, and R. D. Cole of Bendix Field Engineering Corp. for geologic information and coordination with US. Department of Energy plans and programs in the area. I thank the employees of Brush Wellman for their cooperation and hospitality during fieldwork. STRATIGRAPHY STRATIGRAPHIC SECTION The Tertiary rocks of the Thomas Range and north- ern Drum Mountains are divided into nine formations (table 1). As revised here, the stratigraphy corresponds generally to the former subdivision of stratigraphic units into oldest (flows and agglomerates), middle (ash- flow tuffs), and youngest (rhyolite flows and tuffs) assemblages or groups (Shawe, 1972; Lindsey and others, 1975). Each of these groups is characterized by a particular style of volcanism and is separated by an angular unconformity. Minor unconformities occur between some formations, also. All of the volcanic rocks with the possible exception of the Needles Range(?) Formation have local sources. Flows of the Drum Mountains Rhyodacite were prob- ably extruded from local fissures and from central volcanoes in the Black Rock Hills and Little Drum Mountains (Leedom, 1974) about 42 my. ago. Quiet eruption of rhyodacite gave way to explosive eruption of ash-flow tuff from vents north of Joy townsite in the Drum Mountains and east of Topaz Mountain in'the Thomas Range. The first explosive eruptions, from the Drum Mountains vent, deposited tuff of intermediate composition (Mt. Laird Tuff), whereas all later erup- tions, from 38 my. to 32 my ago, deposited rhyolitic tuff. One or more of these eruptions deposited the crystal tuff member of the Joy Tuff over an area that extends from Fish Springs Flat to Desert Mountain, a distance of 60 km (kilometers). Collapse of cauldron walls 39—32 m.y. ago left landslide deposits of mega- breccia and breccia interbedded with lava flows and ash-flow tuffs. Near the end of explosive rhyolite vol- canism, about 30—31 m.y. ago, an ash-flow tuff of the Needles Range(?) Formation was deposited in part of the area. AH later volcanism consisted of explosive eruption of ash and quiet extrusion of alkali rhyolite lava as flows and domes 21 my. ago (Spor Mountain Formation) and 6—7 m.y. ago (Topaz Mountain Rhyolite). VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAIN S. UTAH 3 KILOMETERS m Alluvium and colluvium (Quater- nary) Topaz Mountain Rhyolite (Upper Miocene) Flows and domes of alkali rhyo- lite § § Stratified tuff Angular unconformity D Spor Mountain Formation (Lower Miocene) Porphyritic rhyolite member Beryllium tuff member Needles Rangel?) Formation (Oligocene) ’; Dell Tuff (Oligocene) Landslide breccia (Oligocene?)— Megabreccia in northern Drum Mountains, Age relations With other landslide breccias unknown Landslide breccia (Oligocene?)— Breccia at Spor Mountain Landslide breccia (Oligocene?)— Breccia at Wildhorse Spring Joy Tuff Black glass tuff member (lower Oligocene) Crystal tuff member (lower Oligo- cene and upper Eocene) Angular unconformity B ’ Mt. Laird Tuff (upper Eocene)— ' Ash-flow tuff and breccia Mt. Laird Tuff equivalent (upper Eocene)—lntrusive dike and plugs Diorite (upper Eocene?)—lntrusive plugs STRATIGRAPHY 5 ' Drum Mountains Rhyodacite (upper Eocene)—F|ows and breccias Angular unconformity A x I: Undifferentiated rocks (Devonian to " Precambrian)—Limestone, dolo- mite, quartzite, and shale Contact—Dashed where approxi- mately located —1'_ Fault, dotted where concealed, ball and bar on downthrown side; concealed faults located from aerial photographs (Lindsey, 1979a) >l< Probable vent area III—l Structural margin ofThomas caldera— Oueried where location uncertain — —I Structural margin of Dugway Valley cauldron—Queried where location uncertain *1 Mines and prospects in Tertiary rocks 1 North End (Be) 2 Taurus (Be) 3 Monitor(Be) 4 Roadside mine (Be) 5 Fluro mine (Be) 6 Rainbow (Be) 7 Blue Chalk mine (Be) 8 Oversight (Be, U) 9 Buena no. 1 (U) Claybank (Be, U) 11 Hogsback (Be) 12 Yellow Chief mine (U) 13 Autunite no. 8 (U) Sigma Emma (Be) FIGUiiE 2.——Geologic map of Tertiary rocks in the Thomas Range and northern Drum Mountains (modified from Lindsey. Four an lar unconformities record periods of cauldron co apse, faulting, and erosion in the Tertiary section; th e unconformities have regional extent throughout he volcanic field of the Thomas Range, Drum Mou tains, and Keg Mountain. Unconformity A, at the b e of the section, records some prevolcanic period or pe 'ods of uplift and erosion of uncertain age. Unconformi y B lies beneath the 38 m.y.-old crystal tuff membe of the Joy Tuff, and records subsidence of the Thomas caldera that accompanied eruption of the Mt. Laird ff about 39 my ago. Erosional detritus is lacking above the unconformity, indicating that it is 1979a). mainly constructional. Cauldron collapse resulting from eruption of the Joy Tuff 38 my. ago was followed by deposition of the Dell Tuff 32 my. ago and the Needles Range(?) Formation about 30—31 m.y. ago. Unconformity C represents a 9—m.y. period of quiescence after ash-flow eruption; it is partly con- structional and partly erosional, as indicated by ero- sional detritus in the Spor Mountain Formation of 21 my ago. Unconformity D, between the Spor Moun- tain Formation and the Topaz Mountain Rhyolite, formed during basin-and-range faulting between 21 and 6—7 m.y. ago. 6 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 1.—Revised stratigraphy of Tertiary rocks in the Thomas Range and northern Drum Mountains [leaders (—l. information not available or not relevant; <. less than; ~ . about] Description of rocks1 Age (m.y.)2 Map symbol (fig. 2) Alluvium and colluvium (Quaternary): Alluvial pediments and stream deposits of poorly sorted gravel, sand, and <1 Qal clay; colluvium covering slopes; playa sediments; beach sand and gravel deposits and lake-bottom clays deposited by Lake Bonneville at elevations below about 1,580 m elevation. Unconformity Topaz Mountain Rhyolite (Upper Miocene): Flows and extrusive domes of gray to red, topaz—bearing alkali rhyolite, 6.3-6.8 Ttm1 2 3 black vitrophyre, and interbedded units of tan stratified tuff. Tuff units (Ttmt) seldom exceed 30 m in ’ ’ thickness, are local in extent, and have unconformable bases. Rhyolite contains sparse crystals of quartz, sanidine, biotite, and plagioclase except locally at Antelope Ridge and lower part of Pismire Wash, where phenocrysts are abundant. Maximum thickness of rhyolite about 700 m. Angular unconformity D Spor Mountain Formation, porphyritic rhyolite member (Lower Miocene): Flows, domes, and plugs of gray to red 21.3 Tsp porphyritic alkali rhyolite; rhyolite contains abundant phenocrysts of dark quartz, sanidine, plagioclase, and biotite and abundant microscopic topaz in groundmass- Maximum thickness about 500 m. Spor Mountain Formation, beryllium tuff member (Lower Miocene): Stratified tan vitric tuff and tuffaceous breccia with abundant clasts of carbonate rocks. Tuff includes thin beds of ash—flow tuff, bentonite, and (21.3) st epiclastic tuffaceous sandstone and conglomerate in The Dell. Hydrothermal alteration of tuff to clay, fluorspar, and potassium feldspar widespread. Maximum thickness about 60 m. Angular unconformity C Needles Range(?) Formation (Oligocene): Simple cooling unit of pink to gray to red—brown ash-flow tuff with 31.43 an abundant small crystals of plagioclase, hornblende, and biotite. Partially welded. Tuff fills paleovalleys on northwest side of Drum Mountains. Maximum thickness about 30 m. Dell Tuff (Oligocene): Gray to pink rhyolitic ash-flow tuff that contains abundant crystals of euhedral quartz, 32.0 Td sanidine, biotite, and plagioclase in poorly welded to unwelded matrix of devitrified shards and pumice. Tuff resembles older Joy Tuff but may be distinguished from Joy by presence of abundant large quartzA bipyramids and loose, ashlike weathering aspect. Maximum thickness about 180 m at north end of The Dell. Landslide breccia, megabreccia of the northern Drum Mountains (Oligocene?): Megabreccia of Cambrian limestone (<37) Tld and dolomite overlying Joy Tuff in northern Drum Mountains. Megabreccia retains original stratigraphy of Cambrian strata but contains intensely brecciated and rotated clasts of Cambrian rocks locally. Maximum thickness about 60 m. Landslide breccia, breccia at Spor Mountain (Oligocene?): Breccia of Ordovician and Silurian dolomite, limestone, (32-42) T15 and quartzite. Breccia retains original stratigraphy of Paleozoic rocks near breakaway zone at crest of Spor Mountain but passes east into breccia with clasts of various strata mixed together and faintly stratified. Maximum thickness estimated very approximately at 80 m. Landslide breccia, breccia at Wildhorse Spring (Oligocene?): Ereccia of mixed angular to subround clasts of (32-42) le Paleozoic rocks and Drum Mountains Rhyodacite in a matrix of rhyodacite fragments. Breccia underlies and passes laterally into breccia at Spor Mountain and is stratified at top. Maximum thickness estimated very approximately at about 20 m. Unconformity Joy Tuff, black glass tuFF member (Lower Oligocene): fiimplo cooling unit of rhyollrlc ash-flow tuft with sparse ~37 ij crystals of sanldine, quartz, plaginrlnse, and biotite, and llthlc fragments of limestone and volcanic rock. Most of tufF intensely welded, with abundant anpaoted blnrk pumlro in lower part. Upper unwelded part is tan and contains abundant light-cnlnrod pumice. Maximum thickness about an m. Joy Tuff, crystal tuff member (Lower Oligocene and Upper Eocene): Cray-pink to red-brown rhynlitic ash-flow tuff 38.0 ch with abundant ouhedral and broken crystals Of quartz, sanidine, plagioclnse, and biotite in moderately welded matrix of devitrified shards. Lower 10 m of tuft contains abundant compacted black pumice; light-colored pumice present higher in section. Tuff contains abundant accessory sphene and rare cognate inclusions of lathlike plagioclase, biotite, and sphene that aid in distinguishing it from Dell Tuff. Welding strong near probable vents east of Topaz Mountain, where foliation near vertical and breccia occurs. Maximum thickness about 180 m at the type locality. Angular unconformity E Mt. Laird Tuff (Upper Eocene): Pink quartz latitic ash-flow tuff with abundant euhedral crystals of white .394 Tml plagioclase (10 mm long), bronze biotite (5 mm across), and hornblende. Quartz phenocrysts with resorbed outlines occur in tuff northeast of Thomas Range. Pumicenus breccia and hydrothermally altered tuff occur near probable vent north of Joy townsite. hikes and plugs of porphyry (Tmli) very similar to Mt. Laird Tuff crop out 3 km south of Joy townsite are included in unit. Maximum exposed thickness about 80 m, but 500 m of tuff interbedded with tuffaceous lacustrine sediments in subsurface of Dugway Valley. Diorite (Upper Eocene): Plugs of dark-gray, massive, fine—grained diorite intrude Paleozoic strata 3 km southeast (39-é2) Tdi of Joy townsite. Diorite contains abundant calcic plagioclase and hornblende. Drum Mountains Rhyodacite (Upper Eocene): Rusty weathering black rhyodacite flows and breccias with phenocrysts of -42 Tdr intermediate composition to calcic plagioclase and pyroxene in an aphanitic to glassy matrix. Modally, rock is bypersthene andesite, but chemical analyses show rock to be rhyodacite in classification of Rittmann (1952). Unit includes some interbedded tuffaceous sandstone and laharic debris flows in Black Rock Hills and some aphanitic flow or dike rocks near Joy townsite. Maximum thickness about 240 m in Black Rock Hills. STRATIGRAPHY 7 TABLE 1.—Continued I Description of rocks Age (m.y.)2 Map symbol (fig. 2) Angular unconformity A Undifferentiated rocks (Devonian to Precambrian): Limestone, dolomite, quartzite, and shale. Formations are -- Dp€ differentiated on maps of Staatz and Carr (1964) for Thomas Range and Newell (1971) and Crittenden and others (1961) for Drum Mountains. Maximum thickness exceeds 1,200 m. lflany units have unconformable tops, so that the original thickness has been reduced by erosion. Ages are averages of all valid potassium-argon and fission—track dates (table 3); ages in parentheses are inferred from stratigraphic relationships. Average of two ages of Needles Range(?) Formation of Little Drum Mountains is 31.4 m.y.; average age of the Needles Range Formation is 30.4 m.y. (Armstrong, 1970). 4A single age of 36.411.7 m.y. on the Mt. Laird Tuff was determined (table 3), but the true age of the Mt. Laird Tuff is estimated at about 39 m.y., because it underlies the 38.0-m.y.—old crystal tuff member of the Joy Tuff. AGE AND CORRELATION An attempt was made to date each rock unit in the Thomas Range and Drum Mountains, using both old and new data (table 2). The ages have been revised to reflect additional dating and recently adopted decay constants used in calculating ages by the potassium- argon and fission-track methods. Methods used for fission-track dates reported here are described by Naeser (1976) and Naeser and others (1978). The new fission-track ages extend the history of volcanism in western Utah to almost 42 my. ago (tables 2 and 3). A single age of 4181-23 my. on zircon from the Drum Mountains Rhyodacite is somewhat older than two whole-rock potassium-argon ages of 38.2i0.4 m.y. (revised from 37.3:0.4 m.y., reported by Leedom (1974), to account for change in constants) for flow rocks that overlie the rhyodacite in the Little Drum Mountains. The zircon age is considered reliable because it is not subject to the effects of weak altera- tion that pervades much of the rhyodacite; also, uran- ium and the resultant fission tracks are distributed uniformly in the zircon dated so that counting errors that may attend dating of zoned grains are not a problem. The age of the crystal tuff member of the Joy 'I‘uff is estimated at 38010.7 my. (table 2) by eight fission- track dates on sphene. zircon, and apatite. The date of the Joy Tuff marks the onset of extensive eruptions of rhyolitic ash-flow tuff. A single zircon age of 36.4:t1.6 my. on the Mt. Laird Tuff, which unconformably underlies the Joy Tuff, is probably not significantly different from the age of the Joy Tuff. Accordingly, the true age of the Mt. Laird Tuff is believed to be about 39 my. The age of the black glass tuff member of the Joy ’l‘uff, which overlies the crystal tuff member, was checked by a single determination of 37.01'41 my. on sphene. The age is supported by the close chemical and spatial association of the two members of the Joy Tuff and sets them apart from the younger Dell Tuff, which has an average age of 32.01: 0.6 m.y. as determined by 10 fission-track dates on zircon, sphene, and apatite. The age of the ash-flow tuff correlated with the Needles Range(?) Formation (Pierce, 1974) was deter- mined to test that correlation. Tuff of the Needles Range(?) Formation was described from outcrops south of the Little Drum Mountains (Leedom, 1974; Pierce, 1974), and mapped by me (Lindsey, 1979a) along the northwest side of the Drum Mountains. Fission-track ages of 30.6i1.2 my. on zircon and 32.2:35 my. on apatite are in accord with assign- ment of the tuff to the Needles Range Formation, which was estimated by Armstrong (1970) to be 30.4 my. old (age adjusted to account for different decay constants). The 21—m.y. age of the Spor Mountain Formation is confirmed by a new zircon date of 21.5111 my. on the dome of porphyritic rhyolite near Wildhorse Spring. H. H. Mehnert dated the porphyritic rhyolite flow at the Roadside mine at 21.2:09 m.y. by the potassium- argon method on sanidine (Lindsey, 1977). An age of 18.1 $4.6 m.y. was obtained for porphyritic rhyolite at the east side of The Dell but was not used in estimat- ing the average age (table 2) because only a single grain of zircon could be dated. An attempt to date zircon from bentonite in the Yellow Chief mine, a facies of the beryllium tuff member of the Spor Mountain Formation, was not suc- cessful. Detrital zircon from the bentonite makes up about 10 percent of the total zircon and is dated as 28.31- 1.8 and 40.0:70 my. old; it reflects the ages of the older volcanic rocks. The remaining 90 percent of the zircon has high track densities and could not be dated; it contains 3,700—7,500 ppm (parts per million) uranium, which is well above that of zircon from the older volcanic rocks and within the range of high uranium content typical of zircon in the overlying por- phyritic rhyolite member. Field relationships pre- 8 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 2.—Summary of radiometric ages for volcanic formations of the Thomas Range and northern Drum Mountains [Ages determined from some formations in nearby areas are included. All fission-track ages on zircon. sphene. and apatite have been recalculated using )‘D=L551X10"yr" and XF=7.03X10'”yr". All K-Ar ages on sanidine have been recalculated using decay constants for ‘°K of )\€=0.581><10"°yr" and XB=4.962X10“°yr“ and “K/K=1.167X10"] Rock unit Sample No. Mineral dated Age 120 Average age (millions of years) (i standard error of the mean) Topaz Mountain Rhyollte: Younger flow, Topaz Mountain—— 1771 Sanidine 6.1t0.4 Do -------------------------- TAO-TRAZ --do——- 6.3tO.A }- 6.310.1 n.y. Do —————————————————————————— TAO-TRA Zircon 6.310.} Older flow, Topaz Mountain-——- TSZ—TR—A --do-—- 6.2t0.3 } Do —————————————————————————— T52-TR-B3 ——do——- 6.4i0.3 6.3tn.1 m.y. Older flow, Pismire Wash ------ T50-TR-A3 ——do——— 6.810.} Spor Mountain Formation: Porphyritic rhyolite member—-— T53—TR-BA Sanidine 21.2t0~9 Do —————————————————————————— U26 Zircon 21.Sil.1 21.3tn.2 m.y. Needles Range(?) Formation, U229 -—do-—- 30.6i1.2 Little Drum Mountains. } 31.4in.8 m.y. Do -------------------------- U229 Apatite 32-2t3-6 Dell Tuff ————————————————————————— TA3-A3 Zircon 30.716.37 no _________________ T43—A Sphene 28-5t1-2 Do ----------------- TAB-A Apatite 32.8 Do ————————————————— TAZ—A Zircon 33.0tl.3 r 32.010.6 m.y. no _________________ T42-A3 Sphene 32.4;1.4 Do ————————————————— TAZ—A Apatite 33.3 Do —————————————————————————— TEA-A Zircon 29.4t1.3 Dell Tuff, Keg Mountain Pass ——————————————— x20-A3 Sphene 33-6t1-8 K4R—A3 --do—-— 32.511.6 RAB—A3 Zircon 33.811.34 Joy Tuff: Black glass tuff member ------- U141 Sphene 37.0t4.1 Crystal tuff member ----------- T51-A3 Apatite 40-0 \ Do ------------------- -- T51-A3 Sphene 38.512-0 Do ----------------- - U183 ——do——— 39.7:3.4 Do ————————————————— -- U32 ——do——— 39.412.8 38.010.7 m.y. Do ————————————————— - U34 -—do——— 38.414.” [ Do -------------------------- U56 ——do——— 36.412.8 Crystal tuff member, Desert Mountain ------------- U238 Zircon 34.Sil.3 Crystal tuff member, Picture Rock Hills —————————— U240 -—do-—— 36.9tl.7 Mt- Laird Tuff -------------------- U57 -—do—-- 36-Atl.6 Drum Mountains Rhyodacite --------- UlOA ——do——— 41.812.3 1Armstrong (1970, table 3). 2E. H. McKee, oral commun., 1975. Lindsey and others (1975). H. H. Mehnert, oral commun., 1976, 1978. sented in following sections of this report indicate that the age of the bentonite and the rest of the beryllium tuff member is close to that of the overlying por- phyritic rhyolite. The history of igneous activity is approximately the same in the Thomas Range, Drum Mountains, Keg Mountain, and Desert Mountain (Shawe, 1972; Lind- sey and others, 1975). Each range has local intrusive and volcanic rocks, however, and only the ash-flow tuffs provide stratigraphic markers that extend into all of the ranges. The “Keg Spring andesite and latite” of Erickson (1963) is 39.4:0.7 my old and is confined to the northwest part of Keg Mountain. The “Keg Spring” is overlain by the Mt. Laird Tuff north of Keg Pass; thus, it occupies the same stratigraphic position at Keg Mountain as the Drum Mountains Rhyodacite does in the Thomas Range. A newly recognized stock of granodiorite (Staub, 1975; H. T. Morris, oral com- mun., 1976), dated here at 36.6 11.6 m.y. by the fission-track method on zircon, intrudes the “Keg Spring andesite and latite” west of Keg Pass. Both of these rocks are unconformably overlain by the crystal tuff member of the Joy Tuff in the Picture Rock Hills, which suggests that the zircon age of the granodiorite stock may be about 1-2 m.y. too young. The Dell Tuff unconformably overlies the “Keg Spring andesite and latite” and the Mt. Laird Tuff north of Keg Pass, where three fission-track ages yield an average of 33.1:0.4 m.y. (Lindsey and others, 1975). The crystal tuff member of the Joy 'I‘uff is well exposed on the east side of Desert Mountain, where it has been dated at 34511.3 m.y. by the fission-track method. This age may be too young because the tuff has been intruded by the stock of Desert Mountain (Shawe, 1972); the DESCRIPTION OF ROCK UNITS TABLE 3.—Analytical data for new fission-track ages of igneous rocks in western Juab and Millard Counties [All ages determined by author by external detector method. Neutron dose determined by C. W. Naeser] Age tZa-(m.y.)1 Sample No. Rock unit Geographic Sample Mineral Number of Induced tracks area location grains Fossil tracks in detector Neutron dose Number Density Number 2X densit 2 counted (tracks/cmz) counted (tracks/cm ) (Neutrons/cm ) 026 ------- 5por Mountain Formation, North of Spor 1151/4 set. 9, Zirc0n--- 5 730 1.13x107 463 1.4111107 4.56x1n” 21.5:1.1 porphyritic rhyolite Mountain. T. 12 S., R. 12 H member. U7A ————————— do -------------------- The Dell ----- 5111/4 sec. 36, --do ----- 1 155 7.22.1106 119 1.10.1107 4.60.11015 18.114.6 '1. 12 s., R. 12 w. u12s-1--—— Spor Mountain Formation, --do ————————— 1011/4 sec. 36, --doz———— 1. 321 4.5711106 338 9.63x106 9.97x10” 28311.8 herymun tuff member '1. 12 5., 11. 12 w. (bentonite at Yellow Chief mine). --doZ---- 2 346 9.62x10" 257 1.43x107 9.97.110” 40.0:7.0 11229 ------ Needles Range(?) Formation South of ----- 1151/4 sec. 23, --do ----- 10 919 5.87x106 861 1.10x107 9.61x101" 30.6:1.Z Little Drum r. 16 5., R. 11 w. Mountains. 1122n . = A Apatite-- 15 234 2.6311105 1,002 2.25x106 21.62x10l5 32.213.6 r544 ----- 0e11 mff ----------------- The Dell ————— 51:1/4 sec. 26, Zircon--- 5 1,031 1.051(107 1,047 2.1311105 1.00x1015 29.4113 r. 12 5.,1z. 12 11. 11141 ------ Joy Tuff, black glass ————— Northeastern 1151/4 sec. 36, Sphene--- 12 262 1.0mm" 1,017 7.351110" 4.81x1n15 37 014.1 tuff member. Drum 1'. 13 5., 11. 11 w. Mountains. U188 ------ Joy Tuff, crystal tuff---- East of Topaz 511 1/4 set. 10, ——do ----- 12 458 1.56x10" 350 2.7011106 9.551110" 39.7:3.4 member. Mountain. 1'. 13 5., R. 11 U. u32 --------- do ---------------------- South of ----- 11w1/4 sec. 22, --do ----- 11 436 1.83x106 311 2.62x1n" 9.431.101" 39412.3 Tops: T. 13 5., R. 11 w. Mountain. 111 1 a 111:1/4 sec. 20, --do----— 10 366 1.70x106 1,3194 1.2811107 4.86x1015 38.414.0 r. 13 5., R. 11 w. 056 --------- do ---------------------- Northwest of— 1121/4 set. 21, --do ----- 10 369 1.72x106 1,466 1.36x107 4.84xlOl5 36,412.11 Joy townsite. '1'. 114 S., R. 11” uzas -------- do ---------------------- East of Desert 1110/4 sec. 29, Zircon——- 10 789 5.031(106 656 11.3:1x106 9.611(101" 371.511.; Mountain. '1‘. 12 5.. R. 6 H U240 -------- do ---------------------- Picture Rock~ 1011/4 sec. 23, --do ----- a 1,005 13.1mm" 782 1.37.1107 9.61.110” 36.9:1.7 111115. T. 13 5., R. 10 w 057 ------- m. Laird Tuff ------------ Northwest of— 111:1/4 sec. 21, —-do ————— a 966 6.63x106 886 6.08x106 1.123(1015 36.4114, Joy townsite. T. 14 8.. R. 11 W U293 ------ Granodiorite of Keg ------- Western Keg—— SEl/b sec. 26, --do ----- 6 727 6.A8x106 598 1.07x107 1.01111015 36.6tl.6 Mountain (Staub, 1975). Mountain. T. 12 5., R. 10 W. 01011 ------ Drum Mountains Rhyodacite The Dell ------ 5111/4 sec. 35, --do ————— s 513 3.00x10" 345 4.003(106 9.331110” 41.3123 *1. 12 5., R. 12 w. loonputed using 1.3-1.551x10‘w/yr and AF=7.03x10-l7/yr. 1. . total decay constant for 233v; A , decay constant for spontaneous fission of 238v. Zircon believed to be detritsl; abundant (90 percent) zircon with high U content could not be dated. granitic facies of the stock was emplaced about 28—31 m.y. ago (Lindsey and others, 1975; Armstrong, 1970), and that event may have reset slightly the zircon age of the tuff. Stratified tuff and rhyolite considered to be equivalent in part to the Topaz Mountain Rhyolite were erupted from Keg Mountain about 8-10 m.y. ago. Alkali rhyolite and basalt north of Fumarole Butte were erupted about 6—7 m.y. ago (Mehnert and others, 1978; Peterson and others, 1978). The last volcanic ac- tivity in the region was the eruption of basaltic lava at Fumarole Butte 0.88 m.y. ago (Peterson and others, 1978). DESCRIPTION OF ROCK UNITS DRUM MOUNTAINS RHYODACITE The oldest formation of volcanic rocks in the mapped area consists of dark, rusty-brown-weathering flows and flow breccias having the chemical composition of rhyodacite and, to a lesser extent, quartz latite‘. Nam- ed the Drum Mountains Rhyodacite for exposures in the Drum Mountains (Lindsey, 1979a), the rhyodacite crops out discontinuously around Spor Mountain and is well exposed in the Black Rock Hills, where it is believed to have been erupted from a small central volcano. It is about 240 m (meters) thick in the Black Rock Hills and about 150 m thick in the southern part of The Dell. Volcaniclastic sandstone and laharic brecJ cias are interbedded with the lower part of the rhyo- dacite in the Black Rock Hills. Some dark, aphanitic flow rocks were mapped with the more porphyritic rhyodacite near Joy townsite; these are seen under the microscope to have similar petrographic features. Much rhyodacite in the northern Drum Mountains and 1Nomenclature of Rittmann (1952) is used throughout this report. unless otherwise stated. 10 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH around Spor Mountain is broken into blocks about 100—300 m across of diverse orientation; these blocks may reflect shattering of the formation as it subsided into the Thomas caldera. The rhyodacite everywhere unconformably overlies rocks of Paleozoic age and is overlain by the Mt. Laird Tuff locally, by the Joy Tuff at many localities, and by landslide breccia, Dell Tuff, and the Spor Mountain Formation around Spor Mountain. The rhyodacite contains an average of 35 percent euhedral phenocrysts of plagioclase and pyroxene as large as 3—4 mm (millimeters) in a groundmass of plagioclase microlites and glass (table 4, fig. 3). Plagioclase is mainly andesine and labradorite, but sodic rims are common; pyroxene is mainly hypers- thene, but lesser amounts of augite are present, and pigeonite has been reported (Staatz and Can, 1964). Euhedral magnetite is abundant, traces of quartz oc- cur at a few localities, and very small amounts of ac- cessory apatite and zircon also occur locally. Along the east side of Spor Mountain, the mafic minerals in the rhyodacite are commonly altered to chlorite and brown hydromica(?). The texture of the rhyodacite indicates that the phenocrysts were not in equilibrium with the magma at the time of eruption and cooling of the groundmass. Intratelluric crystallization of the phenocrysts evidently proceeded in an andesitic magma; somehow, either by addition of another magma, contamination with wall rocks, or differentiation, the melt became more silicic and alkalic than the original magma, so that plagioclase phenocrysts were partially resorbed and overgrown with sodic rims. The interiors of some plagioclase have a sieve texture of holes filled with glass; this texture may indicate change in composition of the melt or, alternatively, of temperature and pressure. When erupted, the remaining lava cooled quickly to form microlites and glass. Chemical analyses confirm that the groundmass of the rhyo- dacite is much more silicic (60—67 percent Si02) than the phenocrysts would indicate. The modal phenocryst content suggests the rock is a hypersthene andesite, but most chemical analyses show it to be rhyodacite and quartz latite in the classification of Rittmann (1952). DIORITE Plugs of diorite crop out in the Drum Mountains 3 km southeast of Joy townsite; they were first mapped by Crittenden and others (1961) as quartz diorite and were examined only briefly by me. Fresh specimens of the rock are dark brown in color, like the Drum Moun- tains Rhyodacite, but the rock is phaneritic, unlike the aphanitic-porphyritic texture of the rhyodacite. In thin section, the rock is seen to contain about 56 percent elongate, subparallel grains of calcic plagioclase and about 22 percent hornblende that is highly altered. Other minerals include about 7 percent anhedral brown biotite, about 9 percent anhedral potassium feldspar, 5 percent magnetite, and accessory apatite. Although Crittenden and others (1961) called the rock quartz diorite, no quartz could be identified with certainty. The diorite is assigned a probable Eocene age on the basis of overall compositional similarity to Other rocks of that age and on the basis of crosscutting by dikes considered equivalent to the Mt. Laird Tuff. Critten- den and othrs (1961, pl. 20) mapped dikes of quartz monzonite porphyry cutting the diorite. Reexamina- tion of their quartz monzonite dikes 2—3 km south and southwest of Joy townsite showed them to be nearly identical to the Mt. Laird Tuff. Similar but hydrother- mally altered dikes cut one diorite plug, indicating that the diorite is older than the Mt. Laird Tuff. lfitL FIGURE 3.—Photomicrograph of Drum Mountains Rhyodacite showing partially resorbed plagioclase (P) and clusters of hypersthene (H) and plagioclase in a groundmass of microlites. Crossed polars. Sample from sec. 20, T. 13 S., R. 11 W. DESCRIPTION OF ROCK UNITS TABLE 4.—Comparison of the mean and range of mineral composition and the occurrence of accessory mine 11 rals in volcanic rocks in the Thomas Range and northern Drum Mountains [Mineral composition estimated from point counts on three to six thin sections and cobaltinitrite—stained slabs of each rock type by C. A. Brannon. Accessory minerals identified by binocular microscope and X—ray methods by the writer using mineral concentrates prepared by heavy-liquid and electromagnetic separation. Range of values are in parentheses. ’l‘r, trace; x. present; leaders (—). not present; < , less than] Rock unit —————————— 1 2 3 A 5 6 7 R 9 Mineral composition, in percent Quartz ------------- 0.1(0—0.6) 0.3(0-1) 20(22-34) 5(3—7) 20(14—25) 3(0-5) 19(13-27) 6(4-10) 16(12—19) Potassium—feldspar -- —— 24(15-35) 3(2—6) 21(16-26) -— 19(12—29) 7(3—10) 13(11—14) Plagioclase -------- 25(20—27) 17(9-29) 8(3—14) 5(1—7) 8(5—11) 31(26-37) 2(0—3) l< Probable vent prior to or during the Thomas caldera— includes Drum Mountains Rhyodacite —--- Structural margin of caldera or cauldron— and Mt. Laird Tuff Queried where location uncertain FIGURE 16.—Maps showing structural evolution of the Thomas Range and northern Drum Mountains. A, Surface of unconformity B. about 39—38 m.y. ago, after eruption of the Mt. Laird Tuff and subsidence of the Thomas caldera. B. Results of eruption of the Joy Tuff 38—37 m.y. ago, accompanying subsidence of the Dugway Valley cauldron, and col- lapse and sliding of cauldron walls. C, Surface of unconformity C, about 30-21 my. ago. The Thomas caldera has been filled with intracauldron Joy and Dell 'l‘uffs. The Needles Rangel?) Formation has been omitted. D, Volcanism and sedimentation at the time of eruption of the Spot Mountain Formation 21 my. ago. E, Surface of angular unconformity D, resulting from block-faulting and erosion 21—7 m.y. ago. F, Results of volcanism 7-6 m.y. ago after eruption of the Topaz Mountain Rhyolite. 38 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH cauldron. The horst did not exist 38 my. ago, when the crystal tuff member of the Joy Tuff flowed west to the wall of the Thomas caldera. Although the horst prob- ably owes much of its present relief to early basin- range faulting, its early existence seems likely. Two ex- planations for its origin are (1) uplift over resurgent magma, and (2) differential subsidence within the caldera complex. Uplift by resurgence is considered unlikely because no intrusive rocks or lava flows are associated with the horst. Consideration of differential subsidence leads to the interesting possibility that the entire caldera complex may reflect incomplete or vari- able degrees of subsidence. Drilling has revealed a high area of Paleozoic rocks beneath Topaz Mountain Rhyo- lite near Colored Pass, at the head of Pismire Wash in the Thomas Range. that may indicate another horst. If other positive areas are found within the caldera com- plex, and if these can be shown to be related to fault blocks that predate basin-range structure, then in- complete, differential subsidence will be verified. The problem of insufficient ash-flow volume for sub- sidence may reflect inadequate information about the true extent and thickness of ash-flow tuffs. The volume of the Mt. Laird Tuff, believed to be associated with subsidence of the Thomas caldera, is somewhat roughly estimated at 50—100 km“ (cubic kilometers). This volume seems small for the size of the Thomas caldera, which is at least 15—25 km across. The ac- curacy of the volume estimate is severely affected by lack of information about the extent of the 500-m-thick section east of Topaz Mountain. Addi- tional drilling or geophysical study is needed to deter- mine the volume of tuff beneath Dugway Valley. The combined volume of the Joy and Dell 'I‘uffs, which filled the Thomas caldera and overflowed it to the east, is estimated at approximately 150—200 km3 if one assumes a once-continuous distribution of 180 m of tuff from the Joy fault to Desert Mountain. Such a volume of tuff is compatible with the collapse of the Dugway Valley cauldron and perhaps additional un- discovered cauldrons. Individual formations of ash-flow tuff are absent from parts of the caldera; such distribution would not be expected if a single large area subsided at once. Although the distribution of individual tuffs is in- completely known because of cover and gaps in subsur- face inforrnation, enough is known to suspect that ad- ditional structures related to cauldron subsidence will be found. Such an example is the absence of the Mt. Laird and Joy Tuffs in the caldera throughout the west half of the Thomas Range, and of the Dell Tuff from the complementary part of the caldera. Such a distri- bution may be caused by a separate time of subsidence for the northwest part of the caldera, perhaps later than the 39—38 m.y. eruptions of the Mt. Laird and Joy Tuffs. Nevertheless, no structure delineating a separ- ate subsidence feature is evident. A second explana- tion is that of differential subsidence; the northwest part of the caldera may not have subsided as much as the south and east parts; only after these were filled with tuff could the Dell Tuff flow into the northwest part of the caldera. A third explanation is that long periods of erosion removed so much ash-flow tuff that the present distribution beneath rhyolite flows and alluvium is spotty. Long gaps in the record of volcan- ism, as shown by the ages of volcanic formations, pro- vided time for extensive erosion. If so, the detritus from such erosional epochs must have been carried be- yond the volcanic field, because no volcanic sediments in the area are of sufficient volume to account for the expected amount of detritus. Available evidence does not permit a definitive explanation for the absence of individual tuffs from parts of the Thomas caldera. CHEMICAL COMPOSITION OF VOLCANIC ROCKS ROCK TYPES Most of the volcanic rocks of the Thomas Range and northern Drum Mountains are believed to have been derived from local vents.'Only the ash-flow tuff of the Needles Range(?) Formation may be from a distant source. Thus, from geographic considerations, mag- mas beneath the Thomas Range and Drum Mountains could have produced the entire volcanic sequence and the small plutons associated with it, with the possible exception of the Needles Range(?) Formation. Accord- ingly, the composition of all volcanic units except the Needles Range(?) is compared (fig. 17). There are three time-dependent rock types among the indigenous rocks of the Thomas Range and north- ern Drum Mountains. In the classification of Rittmann (1952), the three types are (1) rhyodacite and quartz latite of 42 and 39 my. ago (Drum Mountains Rhyoda- cite and Mt. Laird Tuff), (2) rhyolite of 38 to 32 my. ago (Joy and Dell Tuffs), and (3) alkali rhyolite of 21 and 7-6 m.y. ago (Spor Mountain Formation and Topaz Mountain Rhyolite) (fig. 17). Rhyodacite and quartz latite, erupted as both flows and tuffs, are characterized by about 60—67 percent SiOz, 14-19 per- cent A1203, 4—8 percent total iron as Fe203, 2—4 percent MgO, 4—6 percent CaO, 2—4 percent each of Na20 and K20, and 0.6—1.1 percent TiO2 (major oxides have been recalculated to total 100 percent and to exclude vola- tile components). The rhyodacite and quartz latite are about the same age and close in overall composition to the intermediate-composition volcanic rocks in the Lit- CHEMICAL COMPOSITION OF VOLCANIC ROCKS 39 >Porpl1yritic rhyolite member, Spor Mountain Formation -Porphyritic rhyolite member, Spor Mountain Formation Crystal tuff member, Joy Tuff~ Crystal tuff member, Joy Tuff Mt. Laird hit I] Black glass tuft member, Joy Tuff Mt, Laird Tuff—l Black glass tuif member Joy Tuff ~In ruslve diorite (age estimated) . , | tru ive diorite (age estimatsd) . __ («E-{r ‘m fillountalns RhyodaCIte Tapaéhvafiflgmn [-IIIi-rurm IllISDuntains Rhyodacite Topaéhhcgliii‘ngaln RhyodgCI)-e;q{U lrtz ‘ rDell Tuff , _ thodaCIIe-qua z ' Dell Tuff . . Em‘ fihyolite Alkali rhyolite latitéI filhyolite Alkali rhyollte . ,___Afi i . 80 ‘ lI| I i i I l I I I 10 ' II I I I fl x X X 33$< xx >3“ XXX:&X&X E ‘(x M “ng Ax Lu A U E E Ax l m 70 — — fl- 5 —- X _ 0 Z 5 x —-” A x n. § C3 X z X U a A )(X X —. ‘K Q XX x X x ix 8’ 60 — i x — o l l ' "’ l— 5 5 Q | I T 35 flfi W l XXXX so I . ,1 I .. new x 9 l E A ”l X . XX 40 I l O 0 I I I l I 3' l— Z Z t”) 10 I I 0: Lu I- t 2,, _ — 5 E as * g * K a} - xx fig}? A 8 % 5W x xxxx‘pexxxx E 5 x >8;ng x x xx— .9.‘ z §x < —. Q If?“ ,I x I 0 I 3| X’SI ,0 I I 0 I I C53 . Li‘.’ )2 f X A 2 I I I ‘0 U-I 35X l— < 0 z m E 5 — “ LLI u. o. g _l “J A IS Z ‘ D. 1 _ X)‘ _ Z ,9 I $1M fix I IE??? Xxxx Xmmxxx :1 g M 0 l g I P, filly/l 4W XX XX: 10 I I I I l 0 l X I X ’2 3 A 0-2 I I I I o: a 5 _ - g g x X 0 ‘ X x (a; x E A x - _ X __ E "‘ i X X x z 01 A ”X A ll x x M“ M w («x X 7 g yx x fix Xx 4'0 X go 33“ some 9 ax A *3“ “W“ 2 X I 1 ”AGE IN MILLIONS ‘OF YEARS I XX i a I I k I I 9 0 22 1 1 . 1 7 050 40 30 20 0 Total publlshed analyses AGE IN MILLIONS OF YEARS I I II I I I , I I II I l l | 5 1 36 6 11 17 9 0 221 7 Total new analyses FIGURE 17.—Chemical compositions, including trace elements, com- pared to the ages of igneous rocks in the Thomas Range and northern Drum Mountains. Major oxides have been recalculated 1 1 Total published analyses I I II I I I 5 1 366 4 11 17 Total new analyses to total 100 percent and to exclude volatile components. Line pat— ides from the literature (shown by triangles) are from Staatz and tern along the base of some diagrams shows limit of detection Can- (1964, p. 110), Newell (1971, p. 38), and Hogg (1972, p. 180). when some samples were below the limit. Analyses of major ox- New chemical analyses (shown by x’s) are in table 8. 40 . VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS. UTAH —Porphyritic rhyolite member, Spor Mountain Formation Crystal 1qu member, Joy Tuff MI. Laird TufFI‘I Black glass 1qu member Joy Tull I—I [I u ive diorita (age estimated) IDI'UImI ountains Rhyodacite TopathIh/Aaoiligtain — RhyodlaciIte ua‘nz roan Tqu LIame'W RhyoliteI Alkali rhyolite r—‘IHI : A 2000 I II I I I I I E XXXX X n. O. E 1000 — X WW x _ cg X W XX XX x X§X X 0 J l x ( xxxlhmomm 40 l l X E n. E 20 - _ cu" XXX XXX m XXXX XX III M W 0 X288? I | I 40 l l E X X G. o. z 20 — X _ :1“ XX x O X X X X X 0 I Ex I 37A I 200 l l E X D. D. Z 100 — X _ “k x 0 XXX X ”)gé Xié M W 0 fl 3‘ | I I "t“ . 50 40 10 0 I I II I 5 1 366 4 11 17 Total new analyses FIGURE 17.-—Continued tle Drum Mountains described as shoshonitic by Lee- dom (1974) and Pierce (1974). Rhyolite, erupted as ash- flow tuffs, and alkali rhyolite, erupted as flows and tuffs, both contain 74—79 percent SiOz, 11-14 percent A120,, 1—2 percent total iron as Fe203, and less than 1 percent MgO; both are of calc-alkalic affinity. as is Porphvritic rhyolite member. Spor Mountain Formation Crystal luff member, Joy Tuff Mt, Laird Tuff—I I~Black glass tuff member, Jov Tuff I—Iln ruIswe diorite (age estimated] Dr m ountalns Rhyodacite TopanIhllI/llglutnetaln Alkali rhyolite thodaCIIel- u Irtz IDell Tuff I I I I I I I l I Lame Rhyolite [r—H zl—""—\ I | 151000 - 8 >0( E w 500 — — n. E XXXXX ; E 200 — X _ ":35: XX *4 :a 100 — Wm ~ U) 2 " 50 x l E 10 I 0. O- x Z , M O 2 0 xx I WW :vxIv 200 X I XXXX 2 CL 0. Z 100 _ W XX ‘“ .6 xx xxxxx 2 xxxxxxx xx . (ix x 0 , a I I 2 100 I I & x 2 :_ xx 2 x o x I I 2 100 I I I g x xxxx >o’ W X X X xx 6% 0— l I ”501521 I . I l I E 100 ' I I | & xxxxm xxx 2 X xx _ W W W * x >‘ 0 XI W XXX I | l _———I 50 40 30 20 10 0 AGE IN MILLIONS OF YEARS I I II I l I 5 1366 4 11 17 Total new analyses FIGURE 17.—Continued percent for the rhyolite). Alkali rhyolite also differs from rhyolite in that it contains as much as 0.77 per- cent fluorine (Staatz and Carr, 1964, p. 110; Shawe, 1966). The high fluorine content of alkali rhyolite is re- flected by the widespread occurrence of topaz in that rock. Rocks intermediate in composition between rhyoda- citequartz latite, rhyolite, and alkali rhyolite are not ~P0rphyritic rhyolite member, Spor Mountain Formation Mt Laird Tqu—I Black glass tuff member, Joy Tqu I—IIIIrusive diorite (age estimated) Crystal tufl member, Jov Tum-II Dlrum Mountains Rhyodacite TOP“ Mountain I Rhyolite Rhyodaqilq-quar 1 —Dell Tuff IratIte' tholitd Alkali rhyolite I 20 I I I I II I | I | E x>o< n. o. z 10— XXXXX _ ,0“ xx xx>oooo‘ WOW WX X X W * LX W 5?)“I I I xx M XXX x E k E 100 — x XWW xx Wxx _ I; XX XXXX XXW XXXXXX N x 0 I I E 100 I I I n. o. x 2 xx 3% tags I xx D W | l | w 20 I I {3; x xxx x XX 2 x o. §8§ x 0. xx E 10— x : — 5 xx X§§j X X xXX xxX X X X X 0 I l I E 100 I I I | e X %I E x W &xx 4:“ $60? I- 0 g"IX 93“ I m I 50 30 20 10 O AGE IN MILLIONS OF YEARS ' I l l l I I l I 5 1 366 4 11 17 Total new analyses FIGURE 17 .—Continued present (fig. 17). Shifts in composition from one rock type to the next are abrupt, not gradual. For example, the age of the Mt. Laird Tuff is intermediate between the Drum Mountains Rhyodacite and the Joy Tuff, but the composition of the Mt. Laird Tuff closely resem- 42 bles that of the Drum Mountains Rhyodacite and is distinctly different from the slightly younger rhyolite of the Joy Tuff. Comparison of total alkali versus silica contents of all three types indicates that they are within the field of subalkalic suites (fig. 18) (Irvine and Baragar, 1971). The rocks also fall within the calc-alkalic field of Irvine and Baragar on an AFM diagram (fig. 19). The three rock types tend to define separate fields in the alkali- silica and AFM diagrams, however, thus supporting the previous observation that volcanic products hav- ing compositions intermediate between these rock types are not present. The fields of rhyolite and alkali rhyolite occur close together and might be considered as one, but the rocks of each type plot as a coherent group having only partial overlap. Soda equals or ex- ceeds K20 in most of the rhyodacites and quartz la- tites, but K20 dominates NaZO in the rhyolites and alkali rhyolites. TRACE-ELEMENT ASSOCIATIONS The three rock types are sharply defined by their trace-element association; rhyodacite-quartz latite con- tains chalcophile and siderophile metals, rhyolite has a scarcity of most trace elements, and alkali rhyolite con- tains lithophile metals (fig. 17). Rhyodacite and quartz latite contain trace amounts of copper (5—50 ppm), VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH nickel (7 -100 ppm), and vanadium (100—200 ppm) that far exceed the amounts of these elements in rhyolite and alkali rhyolite. The most obvious characteristic of the rhyolitic tuffs is their relative scarcity of most trace elements. An exception is barium, which is more abundant (500—1,000 ppm) in rhyolite than in the alkali rhyolite (less than 200 ppm). Rhyodacite and quartz la- tite contain slightly more barium (LOGO—1,500 ppm) than the rhyolite. In contrast to the other rocks, the alkali rhyolites contain traces of lithophile elements such as beryllium (generally 3—15 ppm),.lithium (as much as 700 ppm), molybdenum (as much as 7 ppm), niobium (30—200 ppm), tin (as much as 50 ppm), yttri- um (20—100 ppm), ytterbium (3—10 ppm), uranium (generally 10—20 ppm), and thorium (50-80 ppm). The abundance of lithophile elements is slightly different in alkali rhyolites of two ages: the older alkali rhyolite of the Spor Mountain Formation contains more gallium, lithium, niobium, tin, yttrium, and ytterbium, and less molybdenum and uranium than the Topaz Mountain Rhyolite. The consistent associations of trace elements con- firm the classification of volcanic rocks in the area into three distinct types. In particular, trace-element abun- dances reveal significant differences between the char- acter of rhyolite and alkali rhyolite. Whether all three are cogenetic members of a series is not known. Sharp distinctions between the three rock types and a time- Na20+K20, IN PERCENT i l l \ . . xx :YAlkali rhyolite . I EXPLANATION Published New A analyses analyses + * X Topaz Mountain Rhyolite xx Spor Mountain Formation, por- phyritic rhyolite member Dell Tuff Joy Tuff, black glass tuff member +\ \ \ Joy Tuff, crystal tuff member Mt. Laird Tuff Intrusive porphyry, equivalent to Mt. Laird Tuff (somewhat altered) Intrusive diorite Drum Mountains Rhyodacite o D1x1000 10—25 25—50 50—100100—150150—200 BERYLLIUM,IN PARTS PER MILLION URANIUM, IN PARTS PER MILLION Hole Hole Hole 1 2 3 DEPTH, IN METERS 8 3 ,I:l <2 E ,; (>0\<>‘““e -- 3—4 >4 2-3 THORIUM: URANIUM FIGURE 21.—Distribution of (A) altered zones (Lindsey and others, 1973), (B) beryllium (Griffitts and Rader, 1963), (C) thorium, (D) uranium, and (E) thoriumzuranium in the beryllium tuff member of the Spor Mountain Formation at the Roadside mine. Uranium and thorium analyzed by delayed-neutron method by H. T. Millard, A. J. Bartel, R. J. Knight, J. P. Hemming, J. T. O’Kelly, and R. J. White. uranium had been concentrated by mineralizing fluids. Many samples selected as having been mineralized did not prove to be of beryllium ore grade, however, so that the samples cannot be regarded as representative of beryllium ore. Thorium is generally more abundant (about 100 ppm) in mineralized tuff than in the tuff overall (about 80 ppm), but the frequency distribution ,of thorium is not skewed positively (fig. 220). The most significant aspect of the data is the separate occur- rence of uranium from beryllium, as shown by a scat- tergram (fig. 220) of 48 samples having 0.1 percent or more BeO or 100 ppm or more uranium. The scat- tergram defines two distinct groups of samples, one having high beryllium (0.1—1.42 percent BeO) and one having high uranium (100—556 ppm). There is no cor- relation between beryllium and uranirim. This relation 50 3° a I I I I ,, 109 samples 20 5 (13 blanks) _ $ 0 L . ~ m [1331 In m n I In E 0 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 E BeO, IN PERCENT < 20 U) I I I LL 0 J, 122 samples 35 10 m 2 D Z 0 * m: m a: u s 0 00 200 300 400 500 600 URANIUM, IN PARTS PER“ MILLION 20 I I I 118 samples (4 blanks) 10 — ~ 0 , :1” _ 0 100 200 300 400 500 THORIUM,|N PARTS PER MILLION 500 I I I I T I I Z X \\\ EXPLANATION g 500 r \ . — j \ 48 samples havmg 2100 ppm U E x x\\ or 20.1 percent 820 n: 400 e Uranium X 1 sample _ E mineralized] * 2 samples 0) tuff I [— cc 300 — x / — < / CL X ~ x Z x / 2200 if / — 2 * / Z * X / NOI /— Beryllium mineralized $100 plotted /// X W\><\\ tuff — D —//X x x X \\\“ X * X X XXX x X \\ x x x X x x \x“ 0 | I I 1 I | I 0. 6 0.8 1.0 1.2 1.4 1.6 820, IN PERCENT 0 0.2 0.4 FIGURE 22.—Histograms showing the abundance of BeO, uranium, and thorium, and a scatterng showing segregation of BeO and uranium in drill cuttings of mineralized zones in the beryllium tuff member of the Spor Mountain Formation. Data from Glanzman and Meier (1979). corroborates that seen in the Roadside drill-hole cut- tings (fig. 21), where uranium tends to occur below beryllium ore. YELLOW CHIEF MINE The Yellow Chief mine is in a tilted fault block of volcanic rocks in The Dell (fig. 15). Tuffaceous sand- stone and conglomerate in the lower part of the beryllium tuff member of the Spor Mountain Forma- VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH tion is the host for uranium ore at the Yellow Chief mine (fig. 10D). Westward tilting of the fault block has brought the Spor Mountain Formation down against the Dell Tuff along the footwall of the fault that marks the west side of the block. The fault at the west side of the Yellow Chief has been active for a long time, both before and after 21 my. ago; at the pit it downdropped a small erosional remnant of the 21—m.y.-old porphy- ritic rhyolite and beryllium tuff members. Farther south in The Dell, the fault is exposed in the Dell Tuff but is covered by the Spor Mountain Formation, with no displacement of that unit. Small faults having less than l—m displacement cut the tuffaceous sandstone and conglomerate in the pit. The stratigraphic section at the Yellow Chief has been described in detail under the discussion of the beryllium tuff member of the Spor Mountain Formation. The ore at the Yellow Chief is beta-uranophane (Ca(U02)2(Si03)2(OH)2-5H20), a pale-orange-yellow mineral (Bowyer, 1963). It occurs in lenses as much as 6 m thick and 90 m long that are approximately con- cordant to the bedding in tuffaceous conglomerate and sandstone (fig. 10D). The yellow mineral weeksite (K(U02)2(Si205)9-4H20) occurs in lenticular zones less than 1 m thick and 10 m long in the limestone conglom- erate that overlies the tuffaceous sandstone (Staatz and Carr, 1964, p. 156). Both of these minerals occupy interstices and fractures and coat sand grains and clasts in the conglomerate. Schroekingerite (NaCa,(U02)(COa),(SO,)F'10H20) has been reported in veinlets in the tuffaceous sandstone (Staatz and Carr, 1964, p. 157) but has not been found in most of the ore. The host rock is partially altered to smectite, and some limestone clasts in the conglomerates have altered shells, but evidence for intense hydrothermal altera- tion is lacking. Small bits of earthy yellow jarosite are scattered in the tuffaceous sandstone and conglomer- ate, and coarsely crystalline calcite cement and vein- lets are widespread. ' Two lenses of ore exposed on the face of the Yellow Chief pit (fig. 10D) were sampled to check for geochemical haloes and possible associations of trace elements that might suggest clues to the origin of the Yellow Chief ores. One ore lens sampled contains week- site; the other, believed to be more typical of the Yellow Chief ore, contains beta-uranophane. The ore lenses were mapped in the field using a hand-held scintillator, and a series of samples was taken across the middle and ends of each ore lens (fig. 23, table 7). Equivalent uranium (eU), analyzed by a counting tech- nique, and uranium determined by delayed-neutron analysis, are about the same in the ore lenses, in- dicating that uranium there is approximately in equilibrium. Equivalent uranium exceeds the concen- URANIUM OCCURRENCES 51 TABLE 7.—Chemical analyses of samples from two are lenses in the Yellow Chief mine [Samples are located in figure 23. Values are in parts per million. eU by beta-gamma sealer by H. G. Neiman: U and Th by delayed-neutron method by H. '1‘. Millard, Jr.. A. J. Bartel, R. J. Knight. C. L. Shields, C. M. Ellis, R. L. Nelms. and C. A. Ramsey; F by specific-ion-electrode method by H. G. Neiman and Patricia Guest; Be, Li, Cu, V, Cr. and Pb by six-step semiquantitative spectrographic method by J. C. Hamilton. Ag and Mo not found at detection limits of 0.5 ppm and 3 ppm. respectively. by the spectrographic method. Leaders 1- - <), no data; < , less than] Sample No. Sample source eU U Th F Be Li Cu V Cr Pb Samples from vicinity of weeksite lens 1 ----- Bentonite ----------- 30 15 57 3,600 15 200 5 10 15 70 2 --do 90 100 68 6,100 30 1,000 7 30 30 70 3 ----- Ore in conglomerate. 910 1,002 —— 800 20 150 10 30 70 300 4 ----- Barren conglomerate. 50 34 14 1,400 15 100 7 50 100 300 5 --do 50 21 14 1,200 30 <50 7 70 100 100 6 ----- Sandstone ——————————— 30 10 14 600 10 <50 3 30 10 50 7 ----- Ore in conglomerate. 290 286 —- 800 15 <50 7 30 70 70 8 ----- Barren conglomerate. 70 49 21 1,300 15 <50 7 30 100 30 Samples of sandstone from vicinity of uranophane lens 1 ————— Outside ore lens. 40 21 28 1,200 50 <50 10 100 70 30 2 ————— Ore lens ------------ 120 76 35 1,400 20 100 10 70 50 30 3 -—-dc 80 39 26 1,500 20 <50 10 70 100 30 4 ----- Outside ore lens. 40 16 22 1,000 30 <50 7 7O 50 70 5 ----- Ore lens ------------ 3,130 3,343 —- 1,700 30 100 7 70 3O 15 6 --do 610 602 —- 1,300 30 100 10 70 30 15 7 —-dc 2,200 2,316 —- 1,400 30 <50 15 150 70 70 8 ----- Outside ore lens. 40 31 33 1,500 5 <50 15 100 70 150 tration of uranium outside the ore lenses because at low concentrations (100 ppm and less) the content of potassium and thorium accounts for a significant part of the eU. Fluorine, beryllium, and lithium, which would be expected to indicate intensity of hydrother- mal mineralization associated with beryllium deposits, show no systematic change in abundance across the middle or ends of the ore lenses. Fluorine and lithium are most abundant in the bentonite at the top of the pit wall, indicating that beryllium-related mineralization affected the clay-rich bentonite most. Uranium content of the bentonite is 100 ppm immediately above the weeksite ore, but only 15 ppm 1 m above the ore. Cop- per, vanadium, chromium, lead, silver, and molybde- num were analyzed by spectrographic methods to determine whether the ores might chemically resemble those of the Colorado Plateau; no enrichment of these elements, except perhaps lead, and no systematic changes were noted for either of the ore lenses. The uranium deposits at the Yellow Chief mine are of uncertain origin. The paucity of fluorite and beryllium in the uranium ore suggests that the Yellow Chief deposits were not formed by the hydrothermal miner- alization that produced the fluorspar and beryllium deposits. Overall, the ores do not resemble those of the Colorado Plateau, inasmuch as they do not contain uranous minerals, organic matter, or pyrite, although jarosite, noted in the host sandstone, might represent oxidized pyrite. Metals that are locally abundant in the beryllium ores (such as manganese, lithium, and zinc) and in the Colorado Plateau ores (such as vanadium, copper, and molybdenum) are not concentrated in the Yellow Chief ore. Uranium at the Yellow Chief may have been introduced by ground waters bearing silica and uranyl carbonate complex ions that dissociated when the calcite cement precipitated. URAN IF EROUS OPAL Uranium occurs in the structure of opaline silica in fracture fillings in tuff at many places in the Thomas Range. The best known and most uraniferous opal oc- curs in the crystal tuff member of the Joy Tuff at the Autunite No. 8 prospect on the east side of Topaz Mountain. Fracture fillings of uraniferous opal occur also in the Topaz Mountain Rhyolite above the Autu- nite No. 8 prospect, at the Buena No. 1 prospect (Staatz and Carr, 1964, p. 152—154), and west of Topaz Valley; in the Dell Tuff in The Dell; with massive urani- ferous opal in the beryllium tuff member of the Spor Mountain Formation at many locations; and in the 52 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS. UTAH Lens of weeksite ore X x8 2 ’ ongmme‘a 4 c \e>< and com)‘8 b 1 L\mest°“e p 5 “6 X6 o sands“) e -‘ fiac O Lens of beta-uranophane ore FIGURE 23.—Locations of geochemical samples (table 7) in two ore lenses in the Yellow Chief mine. Location of lenses is shown in figure 10D. A, Lens of weeksite ore at the top of limestone pebble and cobble conglomerate. B. Lens of beta-uranOphane ore in tuf- faoeous sandstone and conglomerate. dome of the porphyritic rhyolite member of the Spor Mountain Formation near Wildhorse Spring. Recon- naissance suggests that at least minute quantities of uraniferous opal can be found in any tuff in the Thomas Range. The fracture fillings are generally less than 1—2 cm wide and occur both singly and in zones less than 10 m wide and 30 m long. None has produced ore, and their small size and extent indicate that they are not of economic interest themselves. The opal in fracture fillings is generally zoned, the zoning being defined by varying sizes of fibrous crystallites oriented perpendicular to the walls of the vein. The opal fluoresces bright yellow green under ultraviolet light, so that it can be readily distinguished from ordinary opal in the Thomas Range, which does not fluoresce. Calcite, quartz, fluorite, weeksite, and perhaps other secondary uranium minerals are com- monly associated with the opal. Fission-track maps show that uranium is concentrated in the opal parallel to the zoning, so that there are large variations in uranium content between zones only a fraction of a millimeter thick (Zielinski and others, 1977). The uraniferous opal is probably of hydrothermal origin; more specifically, such opal :may have precipitated in hot springs. Both massive replacement opal and opal in fracture fillings occur in hydrothermal beryllium deposits in the beryllium tuff member of the Spor Mountain Formation. Uranium-lead apparent ages indicate that deposition of opal began 21 my. ago, during or soon after eruption of the host Spor Mountain Formation and beryllium-fluorite minerali- zation, and that deposition of opal continued episodic- ally until 3 my. ago (Ludwig and others, 1980). The close temporal relation between the onset of opal for- mation, beryllium-fluorite mineralization, and igneous activity indicates a genetic relation, also. Many of the fracture fillings show strong zoning of uranium con- centration, which suggests wide fluctuation in the sup- ply, rate, or conditions of precipitation of uranium. If silica, calcite, and fluorite were the major phases in ‘ equilibrium with the fluids, a likely mechanism for con- trolling the rate of precipitation was change in temper- ature or pressure. Fluctuating temperature or pressure and the presence of fluorite support a hydrothermal source. For opal at the Autunite No. 8 locality, temperatures of deposition in the range of 36°C or less were estimated from oxygen-isotope composition (Henry, 1979), but this opal may have been deposited far from its presumed hydrothermal source. URANIUM IN STRATIFIED TUFF OF THE TOPAZ MOUNTAIN RHYOLITE A survey of stratified tuff in the Topaz Mountain Rhyolite showed that its original chemical composition was nearly identical to that of the associated alkali rhyolite and that large areas of the once-vitric tuff had been altered to zeolite (Lindsey, 1975). Alteration was accomplished by open-system ground-water leaching of the major alkalis, Na20 and K20, and of minor elements including fluorine, rubidium, manganese, and lead. No analyses of uranium were available for the study reported in 1975, but the eU content, which is dependent on “’K, thorium, and uranium, was found to decline during zeolitization. The tuff was weakly af- fected by beryllium-fluorite mineralization, also. A random selection of 20 samples used in the 1975 survey was analyzed for uranium and thorium by the delayed-neutron method by H. T. Millard, J r., Cynthia McFee, and C. A. Bliss of the US. Geological Survey. These analyses show that the stratified tuff contains 7—23 ppm uranium (average of 16 ppm) and 44—71 ppm thorium (average of 55 ppm). Zeolitic tuff does not con- URANIUM OCCURRENCES 53 tain appreciably less uranium and thorium than vitric tuff, but the small number of samples may be insuffi- cient to detect differences. The analyses do show that the overall uranium and thorium content of the tuff is very close to that of alkali rhyolite. Seventeen samples of alkali rhyolite showed 6—20 ppm (average of 16 ppm) and 36—76 ppm thorium (average of 62 ppm). Although the wide range of uranium content in some of the tuff and rhyolite suggests that uranium has been mobile, study of similar tuffs at Keg Mountain (Zielinski and others, 1980) indicates that most of the uranium has not been flushed from the tuff to be concentrated in ore deposits. No concentrations in excess of trace amounts of uranium, except in uraniferous opal, are known in the Topaz Mountain Rhyolite. A MODEL FOR URANIUM DEPOSITS AND SOME SUGGESTIONS FOR EXPLORATION The uranium (and other lithophile metal) mineraliza- tion of the Thomas Range was associated with exten- sional block faulting (early basin-and-range faulting) and fluorine-rich alkali rhyolite volcanism beginning 21 my. ago. Uranium and other lithophile metals did not accompany the caldera cycle, which was complete by 32 my. ago. A hiatus of 11 my separates uranium mineralization at Spor Mountain from the caldera cycle; thus uranium clearly is not associated with the caldera cycle or with earlier magmas that contained abundant sulfur and chalcophile metals. Uranium in the Thomas Range has been concen- trated by (1) magmatic fluids, (2) hydrothermal fluids, and (3) ground water. These methods of concentration correspond respectively to the (1) initial magmatic, (2) pneumatogenic, and (3) hydroallogenic classes of vol- canogenic uranium deposits proposed by Pilcher (1978). All of these fluids circulated in an environment of extensional faulting and alkali rhyolite volcanism, however, and their only relation to the caldera cycle was their introduction through fractures that were formed during cauldron subsidence and later reacti- vated during basin-and-range faulting. Uranium was concentrated by magmatic fluids in the beryllium tuff member (about 20 ppm) and in alkali rhyolite (10-20 ppm) of both the Spor Mountain Formation and the Topaz Mountain Rhyolite. Particularly large amounts of uranium, beryllium, and fluorine were present in the magma of the Spor Mountain Formation, which under- lay the vicinity of Spor Mountain and which was erupted as the beryllium tuff and porphyritic rhyolite members. Uranium was further concentrated in hydro- thermal fluids rising through conduits that were opened by early basin-and-range faulting. Such faulting tapped fluorine and lithophile-metal-rich fluids in the top of alkali rhyolite magma that underlay the vicinity of Spor Mountain. The fluids deposited uraniferous fluorite in pipes along faults and fault in- tersections on Spor Mountain (Staatz and Carr, 1964, p. 130-148) and spread laterally into the beryllium tuff member, where they deposited disseminated fluorite, beryllium, lithium, and uranium (Lindsey, 1977). Uranium of hydrothermal origin was deposited in the structure of fluorite and opal; no tetravalent uranium minerals have been found. Uranium in the structure of fluorite at Spor Mountain may be tetravalent, but this possibility remains unproven. The oxidizing chemical environment of hydrother- mal uranium mineralization at Spor Mountain con- trasts with the reducing chemical environments at McDermitt, Nevada-Oregon (Rytuba and Glanzman, 1978), and Marysvale, Utah (Cunningham and Steven, 1978). Pitchblende, pyrite, and fluorite are common in uranium ores at both McDermitt and Marysvale, but pitchblende and pyrite have not been found at Spor Mountain. At McDermitt, hydrothermal fluids con- taining sulfur, fluorine, mercury and uranium mineral- ized volcanic domes and moat sediments of the McDer- mitt caldera complex. At Marysvale, uranium-bearing hydrothermal fluids contained abundant fluorine, which complexed with uranium, and minor concentra- tions of sulfur, which was in the reduced state; uranium probably traveled as a tetravalent fluoride complex and was precipitated as the fluids cooled and the pH rose by reaction with wall rocks (Cunningham and Steven, 1978). At Spor Mountain, uranium pro- bably traveled in hydrothermal fluids as hexavalent fluoride and SiO2 complexes; precipitation of uranium occurred in response to precipitation of fluorite and SiO2 and accompanying breakdown of complex ions. Fluorite and SiO2 were probably precipitated by cool- ing of fluids; reaction of the fluids with carbonate rock and porous tuff caused the pH to rise and resulted in widespread smectite and potassium-feldspar alteration (Lindsey and others, 1973). Ground water has redistributed hydrothermal uranium at Spor Mountain and concentrated it locally in deposits of secondary minerals. The potential for finding magmatic or hydrothermal deposits of pitchblende in the Thomas Range is uncer- tain. No pitchblende or other tetravalent uranium mineral has been found in the near-surface environ- ment exposed at Spor Mountain, indicating that no reducing agent or environment was present to precip- itate large amounts of uranium. No reducing agent, such as pyrite or carbonaceous matter, has been observed in the fluorspar pipes or in the beryllium tuff member by me, but such reductants might occur in deep environments. Some Paleozoic carbonate rocks on 54 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAIN S, UTAH Spor Mountain are fetid and might serve as a local reducing environment along the walls of fluorspar pipes and rhyolite vents. The discovery of carbonace- ous and pyritic lakebeds in the Mt. Laird Tuff of the subsurface northeast of Topaz Mountain indicates yet another reducing environment. The most favorable areas are near vents for alkali rhyolite lava, where the lava has passed through carbonate rocks, or Tertiary lakebeds that contain carbonaceous matter. A mineral- ized plug of porphyritic rhyolite, in the southern part of Spor Mountain, and vents of alkali rhyolite at Topaz Mountain, Antelope Ridge, and the northeastern Drum Mountains fit this criterion. A deep hydrothermal environment at Spor Mountain may be present near a hypothetical pluton of alkali rhyolite that was the source of the beryllium tuff and porphyritic rhyolite of the Spor Mountain Formation, and of the beryllium-fluorite-uranium mineralization that followed. The vicinity of intrusion of the hypothesized pluton can be predicted from the distri- bution of the Spor Mountain Formation around Spor Mountain and the intensity of mineralization associated with that area. Also, Spor Mountain may have been lifted trap-door fashion by the pluton after eruption of the Spor Mountain Formation 21 my ago. The hinge of the trap door would have been west of the mountain, and the region of greatest uplift would be along the Dell fault system where it displaces Spor Mountain Formation against Paleozoic rocks. Maxi- mum upward projection of the pluton might be ex- pected about 1 km west of Eagle Rock Ridge. The depth to the pluton is probably unpredictable without geophysical data, and the nature of the ores, if any, that might be associated with it seems equally difficult to predict. If sufficiently reducing environments are present at depth, there is a possibility of finding large concentrations of uranium and perhaps other metals. Anomalous traces of molybdenum, tin, and tungsten near the surface, concentrated in manganese oxide minerals near beryllium ore, may be generally indica- tive of other metals that would be expected below (Lindsey, 1977). The Spor Mountain Formation and the Topaz Moun- tain Rhyolite are the most favorable source rocks for uranium deposited by ground waters. In general, the alkali rhyolites contain 10—20 ppm uranium, which is approximately two to four times as much as other vol- canic rocks in the area. The beryllium tuff member probably contained 20 ppm uranium at the time of deposition and local concentrations of 2,000 ppm or more were added in hydrothermal fluorite and silica. The stratified tuffs of the Topaz Mountain Rhyolite also contained 10—20 ppm uranium. The tuffs of both formations are porous and were initially glassy, and they are between relatively impermeable (though not totally impermeable) rhyolite and other rocks, so that they provided a good conduit for fluids to leach and transport uranium. Thus, it is not surprising that many occurrences of secondary uranium minerals, in- cluding the Yellow Chief deposits, are in the beryllium tuff member of the Spor Mountain Formation. Much secondary uranium, probably deposited by ground water, occurs in the beryllium tuff member, as shown by the separate occurrence of uranium and beryllium in the tuff. The Yellow Chief deposit, which consists en- tirely of secondary uranium minerals, probably was formed by ground water also. The potential for finding oxidized uranium deposits formed by ground water in the beryllium tuff member of the Spor Mountain Formation is moderate. The scar- city of reductants to precipitate uranium in tuff indi- cates that deposits of reduced uranium minerals such as uraninite and coffinite, which usually occur in deposits of the Colorado Plateau and Wyoming types, will not be found unless a hydrologic basin favorable to entrapment and precipitation of uranium can be located; none is exposed. The beryllium tuff member southwest of Spor Mountain has been drilled exten- sively in the search for both beryllium and uranium, so it is not likely that large high-grade deposits could have been overlooked. There may be much potential, however, for uranium-bearing tuff of low grade (0.0X percent). The best possibility for discovery of addi- tional uranium deposits formed by ground water may be concentrations of secondary uranium minerals like those at the Yellow Chief. Favorable host rocks in The Dell may extend beneath the Thomas Range, where they may be expected in down-faulted blocks covered by Topaz Mountain Rhyolite. Finally, another area of possible but very uncertain uranium potential, as yet mostly unexplored, is in the subsurface of Dugway Valley north and east of Ante- lope Ridge. The potential hosts there are stratified tuff of the Topaz Mountain Rhyolite and lakebeds of the Mt. Laird ’I‘uff. Drilling and testing of Dugway Valley to 500—1,000 m will be necessary to evaluate its uranium potential. The cauldron environment there is a passive factor favoring occurrence of uranium; sub- sidence of both the Thomas caldera and the Dugway Valley cauldron created a depression filled with lake- beds, tuffaceous sediments, and ash-flow tuffs that could act as a reducing trap for uranium leached by ground water from adjacent highlands of alkali rhyolite and tuff in the Thomas Range and Keg Mountain. 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L., 1979, Variations in "Sr/“Sr ratios of igneous rocks along the Uinta trend. northwestern Utah [abs.]: Geological Society of America, Rocky Mountain Section. Abstracts with Programs, v. 11, no. 6, .297. Moore.p W. J .. and Sorensen, M. L., 1978, Metamorphic rocks of the Granite Peak area, Tooele County. Utah [abs.]: Geological Society of America. Rocky Mountain Section, Abstracts with Programs, v. 10, no. 5, p. 234. Morrison, B. C., 1980, A summary geologic report on the Spor Mountain drilling project in Juab County, Utah (preliminary): US. Department of Energy Open-File Report GJBX-l9t80), 211 p. Naeser, C. W., 1976, Fission track dating: US. Geological Survey Open-File Report 76—190, 68 p. Naeser, C. W.. Johnson, N. M., and McGee, V. E.. 1978. A practical method of estimating standard error of age in the fission-track dating method, in Zartman, R. E., ed., Short papers of the Fourth International Conference, Geochronology, Cosmo- chronology, and Isotope Geology: US. Geological Survey Open-File Report 78-701, p. 303-304. Newell, R. A., 1971, Geology and geochemistry of the northern Drum Mountains, J uab County, Utah: Golden, Colorado School of Mines M.S. thesis, 1 15 p. Noble, D. C., 1972, Some observations on the Cenozoic volcano- tectonic evolution of the Great Basin, western United States: Earth and Planetary Science Letters. v. 17, no. 1, p. 142-150. Park. G. M., 1968. Some geochemical and geochronologic studies of the beryllium deposits in western Utah: Salt Lake City, Univer- sity of Utah unpublished M.S. thesis, 105 p. Peterson, James, 'I‘urley, Charles, Nash, W. P.. and Brown, F. H.. 1978, Late Cenozoic basalt-rhyolite volcanism in west-central Utah [abs.]: Geological Society of America, Rocky Mountain Section. Abstracts with Programs, v. 10, no. 5, p. 236. Pierce, C. R., 1974, Geology of the southern part of the Little Drum Mountains, Utah: Brigham Young University Geology Studies, v. 21, pt. 1, p. 109—130. Pilcher. R. C., 1978, Classification of volcanogenic uranium deposits, in Mickle, D. G., ed.. A preliminary classification of uranium deposits: US. Department of Energy report GJBX—63(78), p. 41—51. Rittmann, Alfred, 1952, Nomenclature of volcanic rocks: Bulletin Volcanologique. ser. 2. v. 12, p. 75—102. Rytuba, J. J., and Glanzman, R. K., 1978. Relation of mercury. uranium, and lithium deposits to the McDermitt caldera com- plex, Nevada-Oregon: US. Geological Survey Open-File Report 78—926, 28 p. Shawe. D. R.. 1964, A late Tertiary low-angle fault in western Juab County, Utah, in Geological Survey research 1964: US. Geological Survey Professional Paper 501-13, p. B13-B15. ___1966, Arizona-New Mexico and Nevada-Utah beryllium belts, in Geological Survey research 1966: US. Geological Survey Professional Paper 550—C, p. C206—0213. __ 1968, Geology of the Spor Mountain beryllium dis- trict, Utah, in Ridge, J. D., ed., Ore deposits of the United States, 1933—1967 (Graton-Sales Volume): New York, American Institute of Mining, Metallurgical and Petroleum Engineers. v. 2, pt. 8. p. 1149—1161. 1972, Reconnaissance geology and mineral potential of the Thomas, Keg, and Desert calderas, central Juab County, Utah: US. Geological Survey Professional Paper 800-B. p. B67-B7 7. Shawe. D. R.. and Stewart. J. H., 1976, Ore deposits as related to tectonics and magmatism, Nevada and Utah: American Insti- tute of Mining Engineers, Annual Meeting, Las Vegas. Nev.. 1977, Transactions, v. 260, p. 225—232. Shreve, R. L., 1968, The Blackhawk landslide: Geological Society of America Special Paper 108, 47 p. Shuey, R. T.. Caskey, C. F., and Best, M. G., 1976, Distribution and paleomagnetism of the Needles Range Formation. Utah and Nevada: American Journal of Science, v. 276, p. 954—968. Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons, in Coats, R. R., Hay, R. L., and Anderson. C. A., eds., Studies in volcanology (Howell Williams volume): Geological Society of America Memoir 116. p. 613-662. Snyder. W. S., Dickinson, W. R., and Silberman, M. L., 1976. Tec- tonic implications of space-time patterns of Cenozoic magmatism in the western United States: Earth and Planetary Science Letters, v. 32. no. 1, p. 91—106. Sparks, R. S. J ., Self. S., and Walker. G. P. L., 1973. Products of ig- nimbrite eruptions: Geology, v. 1, no. 3, p. 115-118. Staatz, M. H., and Carr, W. J ., 1964, Geology and mineral deposits of the Thomas and Dugway Ranges, Juab and Tooele Counties, Utah: US. Geological Survey Professional Paper 415, 188 p. Staatz, M. H., and Griffitts, W. R., 1961, Beryllium-bearing tuff in ' the Thomas Range, Juab County, Utah: Economic Geology, v. 56, no. 5, p. 946—950. Staatz. M. H., and Osterwald, F. W., 1959. Geology of the Thomas Range fluorspar district, Juab County, Utah: US. Geological Survey Bulletin 1069, 97 p. Staub, A. 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Uranium in volcanic rocks from the central Andes: Journal of Volcanology and Geothermal Research, v.2, no. 3, p. 251-258. Zielinski, R. A., 1978, Uranium abundances and distribution in associated glassy and crystalline rhyolites of the western United States: Geological Society of America Bulletin, v. 89, no. 3, p. 409-414. Zielinski, R. A., Lindsey, D. A., and Rosholt, J. N., 1980, The distribution and mobility of uranium in glassy and zeolitized tuff, Keg Mountain area, Utah, USA: Chemical Geology, v. 29, no. 1, p. 139—162. Zielinski, R. A., Ludwig, K. R., and Lindsey, D. A., 1977, Uranium- lead apparent ages of uraniferous secondary silica as a guide for describing uranium mobility, in Campbell, J. A., ed., Short papers of the US. Geological Survey Uranium-Thorium Sym- posium, 1977: US. Geological Survey Circular 753, p. 39—40. TABLE 8 [Silica Al,0.. total iron as Fe,0., CaO. K,0, 'I‘iO,. and some MnO by X-ray fluorescence by J. S. Wahlberg: MgO and N110 by atomic absorption by C. A. Gent, V. M. Merritt, and H. G. Neiman. Equivalent uranium (eU) by beta-gamma scaler by H. G. Neixnan, V. M. Merritt. and C. A. Gent. Manganese and boron through zirconium by six-step semiquantitative spectrographic method by R. G. Havens and F. E. Lichte. For samples Sp—O, Sp—l. and Sp—Z. lithium by atomic absorption by V. M. Merritt. Uranium and thorium by delayed neutron method by H. T. Millard, Jr.. Cynthia McFee. C. A. Bliss. C. M. Ellis, and V. C. Smith. N, not detected; L. detected but below the limit of detection; <. less than. leaders (—). not determined. Analyses do not total 100 percent because H0 and volatile constituents were not determined; also. SiO, by X-ray fluorescence is subject to considerable but unmeasured error] 60 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS. UTAH TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains Lower limit of detection U3 U10A U33 U37 U65 Composition in percent Si02--- -- 59 ‘ 62 63 62 61 A1203-- -- 18 14 14 13 15 Fe203—-— —- 3.7 4.2 3.8 4.6 4.2 Mg0---- —- .51 1.65 3.69 2.82 3.49 Ca0---- -— 5.7 4 5 3.4 6.2 4.1 Na20--- —— 3.6 3 11 2.98 2.93 3.43 K20---- -- 3.8 3 3 2.2 2.9 2.8 Ti02--— —— .92 .77 .60 .81 .90 MnO---- 10.05; .0001 <.05 <.05 .090 .077 .0065 Composition in parts per million B ------ 20 N N L N L Ba ----- 1 5 1,500 1,000 1,500 1,500 1,500 Be ----- 1 3 2 3 2 1.5 Ce ----- 150 200 L N L 300 C0 ----- 3 10 10 30 15 20 Cr ----- 1 20 150 200 150 150 Cu ----- 1 10 15 15 7 30 Ga ----- 5 20 20 30 20 30 La ----- 30 150 70 70 70 150 Li ----- 50 N N N N N Mo ----- 3 N N 7 N N Nb ----- 10 30 20 15 15 15 Nd ————— 70 100 70 N 70 150 Ni ————— 5 10 3O 70 30 100 Pb ----- 10 30 20 30 30 20 Sc ————— 5 7 15 15 15 30 Sn ----- 10 N N N N N Sr ----- 1,000 500 700 700 1,000 V —————— 7 100 100 150 150 200 Y ------ 10 30 30 20 30 30 Yb ----- 1 3 3 3 3 3 Zr ————— 10 200 150 200 150 300 eU ————— 10 20 30 3O 29 40 U ------ -- 6 4 4 4 6 Th ----- -- 26 19 19 17 30 Footnotes at end ot'tab|e. TABLE 8 TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued 0233 U57 ‘ U62 20222 Composition in percent s102——— 53 60 59 55 A1203" 14 15 15 13 Fe203—- 6.8 ' 5.7 5.5 5.2 MgO--—— 3.6 3.24 3.49 2.20 Ca0---- 5.1 5.3 5.1 3.9 Na20--- 2.68 2.81 2.90 2.85 K20---- 1.9 2.1 2.3 2.7 1:102"- 1.0 .80 .80 .90 MnO---- .12 .090 .090 .09 Composition in parts per million B ------ N L L N Ba ----- 700 1,000 1,000 1,000 Be ----- N 1. 5 1 5 L Ce ----- N N N N Co ----- 15 20 20 15 Cr ----- 30 70 70 10 Cu ----- 5 30 50 20 Ga ----- 15 30 30 20 La ----- L 70 70 50 L1 ----- N N N N Mo ----- L N N L Nb ----- N 10 10 N Nd ----- N 70 70 N Ni ----- 7 15 15 10 Pb ----- 10 20 70 15 Sc ----- 30 30 30 15 Sn ----- N N N N Sr ----- 700 1,000 1,500 1,000 V ------ 150 200 200 150 Y ------ 15 30 30 15 Yb ----- 2 3 3 1. 5 Zr ----- 100 150 150 100 eU ----- 10 20 30 20 u —————— 2 4 4 4 Th _____ 11 12 13 16 Footnotes at. end of table. 62 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 8,—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Confinued U32 U34 U43 U49 U56 T51-A Composition in percent SiOz--— 74 70 76 73 75 73 A1203-- 11 12 13 13 14 12 Fe203-- .70 1.4 1.3 1.6 1.3 1.4 MgO--—- .33 .39 .46 .96 .46 .31 CaO--—- .77 1.6 1.4 1.7 1.3 1.4 N320--- 3.1 3.18 3.30 2.90 3.30 2.93 K20---- 4.5 4.3 3.6 3.1 3.6 3.9 T102--- .067 .29 .30 .30 .20 .21 Mn0—--- .11 .057 .065 .065 .065 .022 Composition in parts per million B ------ L 20 20 L 30 20 Ba ----- 700 1,000 1,000 700 1,000 700 Be ----- 1.5 2 3 3 3 1.5 Ce ----- L N N N N L Co ----- N L L L L L Cr ----- 1 1.5 7 3 1 Cu ----- 1.5 1.5 3 3 1.5 1.5 Ca ----- 15 3O 20 30 30 15 La ----- 7O 70 L 50 L L Li ----- N N N N N N Mo ----- N N N N N L Nb ----- 15 15 15 15 15 15 Nd ----- N N N N N L Ni ----- N N N L L L Pb ----- 30 30 30 30 30 30 SC ----- L L 5 7 .5 L Sn ----- N N N N N N Sr ----- 200 300 500 500 300 300 V ------ 20 30 30 3O 30 20 Y ------ 20 30 15 20 15 15 Yb ————— 2 3 1.5 2 1.5 1.5 Zr ----- 70 70 100 100 100 50 eU ----- 20 20 4O 20 30 30 U —————— 7 6 7 5 7 7 Th ----- 21 19 25 22 25 22 TABLE 8 63 TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued Sp-O u1413 U141A U155 33p—1 3Sp—2 Composition in percent SiOZ—-- 70 74 71 76 76 66 A1203-- 12 13 13 13 9.9 12 Fe203-— 1.1 1.2 1.3 1.2 .9 1.0 MgO—-—- .37 .47 .51 .49 .42 1.23 CaO-—-- 1.46 1.2 1.7 1.1 1.44 2.10 NaZO—-- 3.03 3.18 2.73 3.06 1.00 1.99 1(20---- 4.80 4.0 4.2 4.4 4.82 4.13 Ti02—-- .20 .20 .30 .30 .20 .20 MnO--—- .039 .039 .039 .039 .026 .013 Composit ion in parts per million B ------ 20 30 30 30 L L Ba ----- 700 1,000 1 ,000 1, 500 500 500 Be ----- 1 2 2 2 1 1 Ce ————— 150 200 L L 150 150 C0 ----- N L L L N 5 Cr ----- 5 10 15 15 7 10 Cu ----- 3 3 5 3 2 2 Ga ----- 20 30 30 20 15 20 La ----- 70 100 70 70 50 70 Li ----- <10 N N N <10 20 Mo ----- L L N N N N Nb ----- L 15 20 20 L L Nd ----- N 70 70 70 N N Ni ----- 5 L L L L 5 Pb ----- 20 50 30 30 15 30 Sc ----- 5 5 7 7 N 5 Sn ----- N N N N N N Sr ----- 200 300 300 300 150 500 V ------ 20 30 30 30 15 20 Y ------ 20 20 15 20 10 10 Yb ----- 1 2 2 2 1 1 Zr ————— 100 100 100 100 100 100 eU ----- 50 50 40 30 40 30 U ------ 8 9 8 8 5 4 Th ————— 27 23 23 24 21 27 Footnotes at end of table. 64 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued -U78 U84 T42-A T54-A Composition in percent Si02--- 77 76 70 71 A1203" 13 12 12 12 Fe203-- 1.3 1.3 1.3 1.3 MgO—-—- .61 .73 1.16 .8 CaO---- .50 1.1 1.1 3.5 Na20-—- .80 2.10 2.13 .79 K20---- 6.4 4.0 3.6 4.9 Ti02--- .20 .20 .25 .23 MnO-—-- .065 .09 -- .096 Composition in parts per million B ------ L L 30 L Ba ----- 1 ,000 500 700 700 Be ----- 3 5 3 3 Ce ————— N N N L Co ----- L N 3 L Cr ————— 10 3 2 2 Cu ----- 1 . 5 1. 5 2 1 . 5 Ga ----- 30 30 20 15 La ----- 50 50 30 50 Li ----- N N N L Mo ----- N N N N Nb-—--- 15 15 20 15 Nd ----- N N N L Ni ----- N N N L Pb ----- 30 30 30 30 Sc ----- 5 L 7 L Sn ----- N N N N Sr ----- 200 200 200 150 V ------ 30 20 20 20 Y ------ 15 15 20 15 Yb ----- 1. 5 1. 5 2 1. 5 Zr ----- 70 70 7O 70 eU ----- 50 50 30 30 U ------ 4 5 2 6 Th ----- 23 21 24 21 TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— TABLE 8 Continued U2A U7A 011A U20A U97 U122 Composition in percent Si02--- 68 64 72 73 72 73 A1203—- 11 12 13 12 15 15 Fe203—- 1.1 .83 .89 1.1 .8 .9 MgO---- .16 .11 11 .10 .20 .14 Ca0—--- .98 .52 .64 .65 .80 .20 Na20—-- 3.8 4.2 3.9 3.8 3.5 4.1 K20--—- 4.7 4.5 4.9 4.8 4.5 4.3 Ti02--- .072 (.05 .050 .079 (.05 (.05 MnO——-— (.05 .053 .053 .063 .039 .065 Composition in parts per million B ------ 50 30 30 30 20 L Ba ----- 100 30 30 150 70 30 Be ----- 10 15 10 15 7 10 Ce ----- L L L L L L Co ----- N N N N N N Cr ----- N N N N 3 N Cu ----- 2 1 N N 3 1 Ga ----- 30 50 30 50 100 150 La ----- 70 70 70 70 50 50 Li ----- 150 300 300 300 150 700 Mo ----- N N N N N N Nb ————— 100 70 100 150 150 200 Nd ----- 70 70 N 100 N N Ni ----- N N N N N N Pb ----- 100 70 30 50 70 70 SC ----- N N N N N L Sn ----- 15 30 50 15 20 30 Sr ----- 30 15 15 20 15 15 V ------ 15 10 N 7 N 30 Y ------ 100 70 70 200 50 70 Yb ----- 15 10 10 20 7 10 Zr ————— 70 70 70 150 100 70 eU ----- 50 40 60 50 70 50 U ------ 12 11 10 14 8 13 Th ----- 79 54 55 75 56 50 65 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 8,—Chemica1 analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued T53-TR-A T53-TR-B U21C U26 U100 Compos it ion in percent Si02--- 76 74 73 72 74 A1203—— 14 13 12 11 12 Fe203-- 1.5 1.5 1.2 1.4 MgO———— .13 .08 .10 .08 .11 Ca0—-—- .54 .47 .72 1.7 1. Na20-—- 3.60 3.73 3.8 4.1 3.98 K20-—-- 4.9 4.7 5.0 4.0 3.5 Ti02--- .053 .055 .072 .25 (.05 MnO---- .034 .036 .050 (.050 .090 Composition in parts per million B —————— 7O 30 3O 20 L Ba ----- 15 20 150 50 50 Be ————— 15 7 10 50 30 Ce ----- L L L N N Co ----- N N N N N Cr ----- L L N N 3 Cu ----- L L 1 N N Ga ————— 30 3O 50 30 70 La ————— 70 70 70 L L Li ----- 300 300 700 300 200 Mo ----- 3 3 N N N Nb ----- 150 150 100 100 70 Nd ----- 100 150 70 N N Ni ----- L L N N N Pb ————— 30 30 50 50 70 SC ----- L L N N L Sn ————— 30 30 20 20 15 Sr ----- 15 15 7O 15 70 V ------ L L 10 N 7 Y ------ 70 100 70 70 70 Yb ----- 15 15 10 10 7 Zr ————— 150 150 70 70 100 eU ————— 60 50 50 40 40 U —————— 17 11 12 13 15 Th ----- 71 72 64 51 53 TABLE 8 TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains- Confinued U15 Ul6 U72 TSO-TR-A TSO-TR-B Compos it ion in percent 3102-" 74 72 78 75 76 A1203" 12 12 11 12 12 Fe203-- 1.1 1.2 .80 .90 .84 Mg0---- .23 .18 .09 .20 .07 Ca0—--- .52 .67 .80 1.4 .66 Na20--— 3.5 3.5 2.68 1.95 3.51 K20---- 4.6 4.8 4.8 6.6 4.6 Ti02--— .071 .056 .090 .069 .073 MnO---- .053 .055 .065 .100 .036 Composition in parts per million 13 ------ 20 20 L L L Ba ————— 30 30 50 15 7 Be ----- 15 15 10 7 7 Ce ----- N N L L L Co ----- N N N N N Cr ----- N N 30 L L Cu ----- 1.5 1 1 L L Ga ————— 3o 30 50 15 20 La ----- N L 70 50 50 Li ————— 150 150 L L L Mo ----- N N 3 L 3 Nb ————— 100 100 70 50 50 Nd ----- N N N L N N1 ----- N N N L L Pb ----- 50 50 7o 30 50 Sc ----- N N L L L Sn ————— 10 15 15 N N Sr ————— 30 3o 70 300 20 V ------ 10 10 L L L Y —————— 7o 70 30 20 20 Yb ----- 7 7 5 5 5 Zr ————— 100 100 150 70 7o eU ————— 60 ‘ 50 60 60 50 U ------ 15 17 18 19 ‘ 17 Th _____ 56 53 68 59 61 68 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH TABLE 8.—Chemica1 analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued T52-TR-A T52-TR-B T13-TR-A T13-TR-B T21-TR-A T21-TR-B Composition in percent Si02--- 75 75 75 77 73 62 A1203—- 12 12 12 12 12 10 Fe203-- .97 1.0 .98 1.00 1.0 .93 Mg0--—- .16 .11 .06 .06 .15 .19 CaO---- 1.4 2.5 1.0 .86 1.3 1.6 Na20—-— 3.25 3.14 3.71 3.58 3.51 3.49 K20-—--— 4.7 4.7 4.7 4.8 4.7 4.9 T102—-— .092 .110 079 .095 .087 .087 MnO-—-- .048 .047 071 .053 .061 .053 Composition in parts per million B ------ L L L L L L Ba ----- 20 70 10 15 10 20 Be ----- 7 7 15 10 7 7 Ce ----- L L L L L L CO ----- N N N N N N Cr ----- L L L L L L Cu ----- L L L L L L Ga ----- 15 15 20 20 20 20 La ----- 70 70 50 7O 70 70 Li ----- 100 100 100 100 100 100 Mo ----- 3 3 5 5 7 3 Nb ----- 50 30 70 70 70 70 Nd ————— L 70 L L L L Ni ----- L L L L L L Pb ----- 50 50 50 50 50 50 SC ----- L L L L L L Sn ----- N N N N N St ----- 50 100 15 15 30 70 V ------ L L N L L 7 Y ------ 20 20 30 30 30 20 Yb ----- 5 3 7 7 7 5 Zr ----- 100 70 100 70 100 100 eU ----- 50 50 60 60 50 50 U —————— 18 17 20 17 19 18 Th ————— 61 65 64 74 73 76 TABLE 8 'TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Continued T40-TR-A T40-TR-B SP-l3 U74 TO3-TR-A T03-TR—B Composition in percent Si02--- 64 62 74 75 71 74 A1203-— 10 11 11 11 12 12 Fe203-- .92 1.0 .68 1.0 .91 .96 Mg0---- .12 .18 .02 .19 .04 .05 CaO-—-- .66 .66 .55 1.3 .46 1.1 Na20--- 3.39 3.34 3.45 2.76 4.07 3.58 K20-—-- 4.6 4.9 4.2 4.4 4.5 4.7 Ti02-—- .090 .100 .087 .10 .050 .070 Mn0--—- .043 .050 .032 .039 .12 .050 Composition in parts per million B ------ L L L L 20 L Ba ----- 5 7 15 200 5 10 Be ----- 7 7 5 5 7 7 Ce ----- L L 150 L N N Co ----- N N N N N N Cr ----- L L L L L L Cu ----- L L 1 15 L Ga ----- 20 15 20 30 20 20 La ----- 70 70 70 70 L Li ----- L L 50 L 100 L Mo ----- 5 3 5 3 3 3 Nb ----- 50 50 15 3O 50 50 Nd ----- L L N N L Ni ----- L L N N L L Pb ----- 50 50 50 70 70 70 Sc ----- L L L N N L Sn ----- N N N N N Sr ----- 50 50 10 70 7 15 V ------ L L L 20 N L Y ------ 30 3O 50 50 70 50 Yb ————— 7 5 5 5 7 7 Zr ————— 100 100 100 100 70 70 eU ----- 50 50 60 40 50 40 U ------ 15 15 13 6 20 16 Th ----- 67 68 63 36 51 59 69 70 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS. UTAH TABLE 8.—-Chemica1 analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Confinued No. Rock type Location U3 ————— Drum Mountains Rhyodacite, flow SE1/4SW1/4, sec. 2, T. 13 S., R. 12 W. U10A—-— --do ——————————————————————————— SE1/45w1/4, sec. 35, T. 12 s , R. 12 w. U33---- --do ——————————————————————————— Nw1/4Nw1/4, sec. 21, T. 13 s , R. 11 w. U37-—-- --do --------------------------- sw1/4nw1/4, sec. 30, T. 13 s , R. 12 w. U65---- --do --------------------------- NWI/QNWl/h, sec. 16, T. 14 S , R. 11 W. U233-—— Intrusive diotite -------------- SWI/ASEl/h, sec. 36, T. 14 S., R. 11 W. U57--—- Mt. Laird Tuff ————————————————— SEl/ANEl/a, sec. 21, T. 14 s., R. 11 w. u62-——— —-do ——————————————————————————— NEI/ASWl/a, sec. 16, T. 14 s., R. 11 w. U222—-- Intrusive porphyry, slightly SW1/48W1/4, sec. 35, T. 14 S., R. 11 W. altered, equivalent to . Mt. Laird Tuff. U32---- Joy Tuff, crystal—tuff member-- NWI/QNWl/A, sec. 22, T. 13 S., R. 11 W. U34--—— --do --------------------------- SEl/ANEl/A, sec. 20, T. 13 s., R. 11 w. U43———— ——do ——————————————————————————— SE1/45w1/4, sec. 17, T. 13 s., R. 11 w. U49———— -~do ——————————————————————————— sw1/4sw1/4, sec. 5, T. 13 s., R. 11 w. U56---- -—do --------------------------- SEl/ANEI/A, sec. 21, T. 14 s., R. 11 w. T51—A—— --do ——————————————————————————— NEI/ANWl/A, sec. 10, T. 13 s., R. 11 w. Sp—0-—— Joy Tuff, black glass tuff SWI/ASEl/a, sec. 25, T. 13 8., R. 11 W. member, basal black welded zone. Ul4lB-- —-do --------------------------- NWl/ANEl/h, sec. 36, T. 13 s., R. 11 w. U141A—- Joy Tuff, black glass tuff NW1/4NE1/4, sec. 36, T. 13 s., R. 11 w. member, middle gray welded zone. U155--- --do --------------------------- NWI/ASWI/A, sec. 6, T. 14 s., R. 11 W. Sp-l--- Joy Tuff, black glass tuff SWI/ASEl/h, sec. 25, T. 13 s., R. 11 W. member, upper unwelded zone. sP—2-—- ——do ——————————————————————————— sw1/43E1/4, sec. 25, T. 13 s., R. 11 w. u78-——- Dell Tuff ---------------------- NE1/4Nw1/4, sec. 26, T. 11 s., R. 11 w. U84---- —-d0 --------------------------- SEl/ANEl/é, sec. 2, T. 13 S., R. 12 W. T42-A-- --do Rw1/4Nw1/4, sec. 36, T. 12 s., R. 12 w. T54-A—- —-do- NEI/hSEl/h, sec. 26, T. 12 s., R. 12 w. U2A--—— Spor Mountain Formation, NW1/4NW1/4, sec. 11, T. 13 s., R. 12 W. porphyritic rhyolite member, flow. U7A---- --do-- -- SE1/45W1/4, Sec. 36, T. 12 S., R. 12 W. U11A--- —-do-- —- SEl/Asw1/4, sec. 35, T. 12 s., R. 12 w. U20A-—— --do Nw1/45E1/4, sec. 9, T. 13 s., R. 12 w. TABLE8 71 TABLE 8.—Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains— Confinued No. Rock type Location U97---- --do --------------------------- NWl/ANEl/A, sec. 25, T. 12 s., R. 12 W. 0122——— -—do ——————————————————————————— sw1/4SE1/4, sec. 25, T. 12 s., R. 12 w. T53—TR—A --do ——————————————————————————— sw1/4NE1/4, sec. 8, T. 13 s., R. 12 w. T53-TR-B --do --------------------------- sw1/4NE1/4, sec. 8, T. 13 s., R. 12 w. UZlC-—-- Spor Mountain Formation, NEl/ANWl/h, sec. 10, T. 13 5., R. 12 W. porphyritic rhyolite member, plug. U26 ----- Spor Mountain Formation, NWIIQSEl/h, sec. 9, T. 12 R 12 W porphyritic rhyolite member, extrusive dome. U100---- —-do —————————————————————————— NEl/ASEl/A, sec. 8, T. 12 R 12 w 015 ----- Topaz Mountain Rhyolite, older SW1/ASW1/4, sec. 14, T. 12 s., R. 12 w flow. U16 ————— —-do -------------------------- Nw1/ASE1/4, sec. 22, T. 12 s., R. 12 w. U72 ----- --do -------------------------- NEl/hNEl/A, sec. 19, T. 12 s., R. 12 w. TSO-TR-A --do —————————————————————————— NE1/4NE1/a, sec. 28, T. 12 s., R. 11 w. TSO-TR-B --do —————————————————————————— NEl/4NE1/4, sec. 28, T. 12 s., R. 11 w. T52-TR-A Topaz Mountain Rhyolite, younger NW1/ASWl/4, sec. 15, T. 13 s., R. 11 w. flow. T52—TR—B —-do -------------------------- NW1/4SW1/4, sec. 15, T. 13 s., R. 11 w. T13—TR-A -—do ———— —— sw1/4NE1/4, sec. 1, T. 13 s., R. 12 w. T13-TR-B ——do -------------------------- sw1/4NE1/4, sec. 1, T. 13 s., R. 12 w. T21—TR-A -—do -------------------------- NEI/4SE1/4, sec. 28, T. 13 s., R. 11 w. T21—TR—B -—do —- NE1/45E1/4, sec. 28, T. 13 s., R. 11 w. TAO—TR—A Topaz Mountain Rhyolite, younger NWI/ANEl/A, sec. 16, T. 13 S., R. 11 W. flow. TAO-TR-B --do -------------------------- NW1/4NE1/4, sec. 16, T. 13 s., R. 11 w. Sp-l3--- --do -------------------------- Nw1/4Nw1/4, sec. 16, T. 13 s., R. 11 w. U74 ————— Topaz Mountain Rhyolite, dome. NE1/4NW1/h, sec. 26, T. 12 s., R. 11 w. T03-TR—A —-do -------------------------- sw1/Asw1/4, sec. 14, T. 13 s., R. 11 w. TO3-TR-B ——do-—--e--—-4 ———————————————— sw1/4sw1/4, sec. 14, T. 13 s., R. 11 w. 1Limits of detection for MnO are 0.05 percent by X-ray fluorescence; 0.0001 by six—step spectrographic method. ' 2Rock has been partly altered by hydrothermal fluids. 3Alkali metals may have been leached by ground water. U.S. GOVERNMENT PRINTING OFFICE: 576-034—1982 ,, t?h:rb§fi»ini& . Upper Cretaceous Subsurface Stratigraphy and Structure of Coastal Georgia and South Carolina By PAGE C. VALENTINE GEOLOGICAL SURVEY PROFESSIONAL PAPER 1222 A study based on 24 wells along transects from the Southeast Georgia Embayment northeastward to the Cape Fear Arch and ofi’shore to the Outer Continental Shelf UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON21982 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Valentine, Page C. Upper Cretaceous subsurface stratigraphy and structure of coastal Georgia and South Carolina. (Geological Survey professional paper ; 1222) “A study based on 24 wells along transects from the Southeast Georgia Embayment northeastward to the Cape Fear Arch and offshore to the Outer Continental Shelf.” Bibliography: p. Supt. of Docs. no.: I 1916:1222 1. Geology, Stratigraphic — Cretaceous. 2. Geology — Georgia. 3. Geology — South Carolina. 4. Coasts - Georgia. 5. Coasts — South Carolina. I. Title. II. Series: United States. Geological Survey. Professional paper ; 1222. QE 688.V35 551 .7’7’09757 80—607868 For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Abstract Pagle Upper Cretaceous stratigraphy—Continued Introduction 1 Clubhouse Crossroads corehole 1, South Carolina ________ Acknowledgments 2 Pznar‘llbeolyal lzlmldI filyrctlle 11§each wells, South Carolina; - a as we , o aro ma ______________________ Regional geology . 2 Georgia Geological Survey wells, southeastern Georgia ___ Upper Cretaceous stratlgraphy """"""""""""" 2 Wells in the axis of the Southeast Georgia Embayment ____ Genomama’n-Turonlan Europe-an Stage boundary --- ————— 5 Ages of subsurface stratigraphic units _________________ COST GE'l well, Outer Continental Shelf Off Georgia ———- 10 Geologic section of northern Florida and coastal Georgia and Age of pollen zone IV 11 South Carolina Fripp Island well, South Carolina _____________________ 13 Summary Parris Island No. 2 well, South Carolina ________________ 15 References cited FIGURE 1. 10. 11. TABLE 1. ILLUSTRATIONS The southeastern Atlantic continental margin showing location of wells, geologic sections, and structural features _____-__ Diagram showing Cretaceous stratigraphic units recognized in previous studies of coastal Georgia and South Carolina and the authors’ correlation of their units with Gulf Coast and European stages Diagram showing correlation of upper Cenomanian and lower Turonian stratigraphic units of parts of Europe and the United States . Diagrams showing stratigraphic correlations of wells based on results of this study and comparisons ‘with previous stratigraphic interpretations: . Fripp Island well, South Carolina . Parris Island No. 2 and Fripp Island wells, South Carolina Clubhouse Crossroads corehole 1, South Carolina Penny Royal and Myrtle Beach wells, South Carolina, and Calabash well, North Carolina _______________________ . Georgia Geological Survey wells 876, 1198, 724, 719, and 1197 in the Southeast Georgia Embayment, Georgia ___--- . Section (B—B') through Georgia Geological Survey wells 144 and 7 24 and COST GE-l along the axis of the South- east Georgia Embayment, Georgia Diagram showing reinterpreted correlation of Cretaceous stratigraphic units recognized in previous studies of coastal Georgia and South Carolina with Gulf Coast and European stages Diagram showing stratigraphic correlation along section (A -A’) through the Southeast Georgia Embayment and the Cape Fear Arch, Florida, Georgia, and South Carolina TABLE Well number, name, location, altitude, and depth of 24 wells drilled in coastal South Carolina and Georgia, northern Florida, and offshore Page Page 14 16 17 20 22 24 26 28 Page III UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY AND STRUCTURE OF COASTAL GEORGIA AND SOUTH CAROLINA By PAGE C. VALENTINE ABSTRACT Upper Cretaceous subsurface stratigraphy and structure of coastal Georgia and South Carolina is based on the study of 24 wells along two transects, one extending across the seaward-dipping sedimentary basin termed the “Southeast Georgia Embayment" northeastward to the crest of the Cape Fear Arch, and the other alined east-west, parallel to the basin axis and including the COST GE—l well on the Outer Continental Shelf. A new biostratigraphic analysis, using calcareous nannofossils, of the Fripp Island, S.C., well and reinter- pretations of the Clubhouse Crossroads corehole 1, South Carolina, and other wells in South Carolina, Georgia, and northernmost Florida have made possible the comparison and reevaluation of stratigraphic interpretations of the region made by G. S. Gohn and others in 1978 and 1980 and by P. M. Brown and others in 1979. The present study in- dicates that within the Upper Cretaceous section the stratigraphic units formerly assigned a Cenomanian (Eaglefordian and Woodbinian) age are Coniacian (Austinian) and Turonian (Eaglefordian) in age. A previously described hiatus encompassing Coniacian and Turonian time is not present. More likely, a hiatus is probably present in the up- per Turonian, and major gaps in the record are present within the Cenomanian and between the Upper Cretaceous and the pre- Cretaceous basement. After an erosional episode in Cenomanian time that affected the sec- tion beneath eastern Georgia and South Carolina, Upper Cretaceous marine clastic and carbonate rocks were deposited on a regionally sub- siding margin that extended to the present Blake Escarpment. In con- trast, during Cenozoic time, especially in the Eocene, subsidence and sedimentation rates were uneven across the margin. A thick prograda- tional sequence of carbonate rocks accumulated in the Southeast Georgia Embayment and also built the present Continental Shelf, whereas farther offshore a much thinner layer of sediments was deposited on the Blake Plateau. There is no general agreement on the exact placement of the Cenomanian-Turonian boundary in Europe or the United States Western Interior, and the widespread Sciponoce'ras gracile ammonite zone represents an interval of equivocal age between accepted Cenomanian and Turonian strata. The extinction of the foraminifer genus Rotalipom took place within the Sciponocems gracile zone; it is used here to identify the Cenomanian-Turonian boundary. Pollen zone IV (ComplexiopollisA tlantopollis assemblage zone) is an important and widespread biostratigraphic unit characterized by a distinctive spore and pollen flora. It is consistently associated with lower Turonian calcareous nannofossils on the Atlantic continental margin; these nannofossil assemblages are also present in pollen zone IV, in strata that encompass the Sciponocems gracile zone and the lower part of the Mytiloides labiatus zone in the Gulf Coastal Plain at Dallas, Tex. INTRODUCTION Numerous wells have been drilled along the coasts of South Carolina and Georgia, and many of these have reached pre-Cretaceous basement. Previous investi- gators of these wells have outlined the subsurface geology of the region. Their studies were based pri- marily on lithologic and electric log correlations supple- mented by paleontological interpretations. However, the results of the most recent and extensive investiga- tions of the Upper Cretaceous stratigraphy in the sub- surface of the coastal region do not agree on the place- ment of hiatuses in the rock record or on the age of the lower part of the sedimentary section that lies above pre-Cretaceous basement (Brown and others, 1979; Gohn and others, 1978a, 1980, and in press). The drilling of the COST GE—l well on the Continental Shelf off southeastern Georgia provided an opportunity to investigate the paleontology of a thick section (2,300 ft) of Upper Cretaceous strata (Scholle, 1979). Bio- stratigraphic studies of the sedimentary strata penetrated by this well revealed a nearly complete sec- tion of Upper Cretaceous limestone that includes strata of Coniacian Age and Turonian Age, and a thin, un- fossiliferous, apparently shallow-marine interval above the Albian that may represent the Cenomanian (V alen- tine, 1979a; Poag and Hall, 1979). In order to compare the Upper Cretaceous sequence at COST GE—l with the onshore section, I made a biostratigraphic analysis of the Cretaceous calcareous nannofossils from a well drilled on Fripp Island, SC. This well is also ideal for comparing recently published stratigraphic interpreta- tions of the subsurface, for it was studied by Gohn and others (1978a) and is only 11 miles from a well on Parris Island, 8.0., studied by Brown and others (1979) and by Gohn and others (1978a). The present study incor- porates new biostratigraphic analyses into a re- evalution of the Upper Cretaceous stratigraphy of coastal Georgia and South Carolina. 2 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA ACKNOWLEDGMENTS Two of my colleagues, R. A. Christopher and C. C. Smith of the US. Geological Survey (USGS), have generously allowed me to use their unpublished reports on the occurrences of spores and pollen, planktic foraminifers, and calcareous nannofossils from the sub- surface of South Carolina and Georgia; these reports form the paleontological basis for the stratigraphic in- terpretations of Gohn and others (1978a, 1980, and in press). I am grateful to W. A. Abbott, South Carolina Geological Survey, for providing me with the samples for the Fripp Island well, and to R. K. Olsson, Rutgers University, for samples from the Toms River well. I wish to thank J. E. Hazel, R. A. Christopher, and Peter Popenoe of the USGS for critically reading the manuscript. REGIONAL GEOLOGY The region of coastal South Carolina and Georgia and northern Florida that is treated in this study includes the southwest flank of the Cape Fear Arch and the Southeast Georgia Embayment (fig. 1). The embayment is a seaward-dipping sedimentary basin bounded on the northeast by the Cape Fear Arch, on the southwest by the Peninsular Arch, and on the west by the Piedmont province. Marine carbonate and elastic strata, chiefly of Late Cretaceous and Cenozoic age, are 4,600 ft thick beneath the southeastern Georgia Coastal Plain. Off- shore, the basin descends dramatically to at least 11,000 ft below sea level at COST GE—l, near the shelf edge, where the section also includes 5,000 feet of Lower Cretaceous rocks. I outlined the stratigraphy and struc- ture of the Southeast Georgia Embayment in a previous paper (Valentine, 1979b). Seaward of the Florida- Hatteras slope, the Southeast Georgia Embayment opens into the much deeper (40,000 ft) Blake Plateau basin (Dillon and others, 1979). In the deepest part of the Southeast Georgia Embay— ment onshore, the basal sedimentary strata in some wells may constitute a thin (possibly 150 ft) sequence of undated nonmarine to marginal-marine clastic rocks. These rocks are overlain by a transgressive sequence of Upper Cretaceous marine clastic strata ( ~ 650 ft), which are surmounted by marine carbonate beds (~ 1,150 ft) of latest Cretaceous age. The marine elastic facies thickens northeastward along the coastline to include the entire Upper Cretaceous and extends over Cape Fear Arch, where it is 1,300 ft thick. The Upper Cretaceous marine carbonate facies is confined to the center of the basin, but it is overlain by a widespread, thick (2,100 ft) car- bonate wedge of Paleocene, Eocene, and Oligocene age that thins to the northeast and is absent over the Cape Fear Arch. UPPER CRETACEOUS STRATIGRAPHY The results of drilling into the Cretaceous deposits along the coasts of Georgia and South Carolina and in northern Florida have been reported by many authors, including Applin (1955), Herrick (1961), Herrick and Vorhis (1963), Applin and Applin (1965, 1967), Marsalis (1970, paleontology by S. M. Herrick and E. R. Applin), Maher (1971, paleontology by E. R. Applin), Cramer (1974), Zupan and Abbott (1976), Brown and others (1979), and Gohn and others (1978a, 1980, and in press). Several of these studies deal with the subsurface stratigraphy on a regional basis. Maher (1971) used a combination of lithostratigraphic units and Gulf Coast stages in his broad-scale study of the Atlantic coastal province. However, he did not in- dicate the presence of major unconformities in the Up- per Cretaceous coastal section of Georgia and South Carolina, nor did he correlate his stratigraphic units with European stages. Brown and others (1979) ex— tended into the South Carolina and Georgia subsurface the Cretaceous stratigraphic framework established by Brown and others (1972) for the Coastal Plain north of Cape Fear, and the stratigraphic units are considered by the authors to be chronostratigraphic in nature. Although Brown and others (1979) did not document their interpretations in Georgia and South Carolina, they presumably used the same guidelines as those used in the earlier study (Brown and others, 1972), and the delineation of stratigraphic units apparently is heavily dependent on lithology and the presence of a limited number of ostracode and foraminifer guide fossils. The type sections of these units (Brown and others, 1972) are in the subsurface of North Carolina, just north of the region under study here. Brown and others (1972, 1979) equated their stratigraphic units primarily with Gulf Coast provincial stages. In the other recent studies of the region, Gohn and others (1978a, 1980) recognized a sequence of Cretaceous lithostratigraphic units in the subsurface of coastal Georgia and South Carolina and correlated them with both European and Gulf Coast stages on the basis of analyses of spores and pollen, planktic foraminfers, and calcareous nannofossils. In the following discussion of the stratigraphic units of Gohn and others (1978a, 1980) and Brown and others (1979), the Gulf Coast and European stage equivalents of each unit are those given by Gohn and others and Brown and others, unless otherwise stated. The stratigraphic interpretations of Brown and others (1979) and Gohn and others (1978a, 1980) differ in several aspects; a comparison of their results and a cor- relation by those authors of their units with Gulf Coast and European stages is shown in figure 2. Brown and others (1979) indicated the presence of a regional hiatus 34° 33° 32° 3I° 30° 29° UPPER CRETACEOUS STRATIGRAPHY 83° 82° 8l° 80° .. 79° , . l 8° BAA/(E IDA/47540 EXPLANATION AREA OF UPPER CRETACEOUS OUTCROP — - STRUCTURE CONTOUR 0F UPPER CRETACEOUS SURFACE -CONTOURS ARE IN FEET N BATHYMETRIC CONTOUR- BATHYMETRY IS IN METERS IO. WELL LOCATION— NUMBER IS WELL NUMBER FROM TABLE I O 100 KILOMETERS I-—-‘—I-'—'-i—;r‘ 0 60 MILES DATUM IS SEA LEVEL I I 83° 82° 8I° 80° 79° _, 30° 29° 28° FIGURE 1.—The southeastern Atlantic continental margin, showing locations of wells, geologic sections, and structural features. See table 1 for names and altitudes of wells. Data used in drawing Upper Cretaceous structure contours include additional wells not shown here (Herrick and Vorhis, 1963; Applin and Applin, 1967; Maher, 1971; Cramer, 1974; Hathaway and others, 1979). UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Gohn and others (I978rol980)' Brown and others (I979) A European Stage Provincial Stage Novorroon Moestrichtion K6 Toyloron Companion Austinion Sonton ion Coniocion $i.§é:i:4 - 99999 Tumn'on {2202920 Eagle- fordion D .6.6.6.0A E K3 K2; ?K"?,. Cenomon io n Woodbinion Woshiton and Fr dericks- urglon Albion FIGURE 2,-Cretaceous stratigraphic units of coastal Georgia and South Carolina recognized by Brown and others (1979), Gohn and others (1978a, 1980), and those authors' correlation of their units with Gulf Coast (Provincial) and European stages. Cross-hatched zones represent hiatuses. Note differences in the placement of hiatuses and in the ages of the oldest rocks above pre-Cretaceous basement. UPPER CRETACEOUS STRATIGRAPHY 5 within the Cenomanian between units D (Eaglefordian) and E (Woodbinian). In contrast, Gohn and others (1978a, 1980) recognized a major hiatus between the Santonian (unit K4, middle to upper Austinian) and the Upper Cenomanian (unit K3, middle Eaglefordian) and indicated that the Turonian and Coniacian are absent in this region. With regard to the lowermost sedimentary beds in the section, Brown and others (1979) recognized, in units E and F, strata as old as middle Cenomanian to Albian (Woodbinian, Washitan, and Fredericksburgian). On the other hand, the oldest beds recognized by Gohn and others (1978a, 1980) are their units K3 and K2 (upper Cenomanian, middle Eaglefordian) and unit K1, which was designated upper Cenomanian in South Carolina and later revised to be Upper(?) Cretaceous in their study of the Georgia subsurface. In the present study of the regional Cretaceous stratigraphy, I have attempted to show the relations between stratigraphic units recognized by other authors and to date these units on the basis of their fossil assemblages. The results of one aspect of this study in- dicate that previous authors have underestimated the thickness of the Santonian-Coniacian (lower and middle Austinian) stages and have overestimated the ages of several of their lower stratigraphic units. I believe that Coniacian and, in particular, Turonian strata are pres- ent beneath the coasts of Georgia and South Carolina and that the Cenomanian, if present, is a relatively thin unit above the pre-Cretaceous basement. These conclu- sions rest on the recognition of Turonian fossil assemblages and ultimately on the placement of the Cenomanian-Turonian Stage boundary in Europe and North America. Unless otherwise indicated, in the stratigraphic descriptions that follow, the depths of samples and stratigraphic boundaries in onshore wells are given in feet as originally designated during drilling, whereas COST GE—l depths are given relative to sea level. Table 1 gives locations and altitudes of the wells. CENOMANIAN-TURONIAN EUROPEAN STAGE BOUNDARY The boundary between the Cenomanian and Turonian Stages is ill defined both faunally and lithologically. D’Orbigny (1842, 1847) erected these stages on the basis of their molluscan assemblages, but he did not designate type sections. However, he stipulated the city of LeMans as the type area for the Cenomanian and, somewhat to the south, the environs of Tours as the type area for the Turonian. In the years since d’Orbigny’s original descriptions, many geologists have studied these areas, and they have established a biostratigraphic zonation based primarily on ammonites and in- oceramids. The stratigraphic interpretations of many of the recent European and North American studies are shown in figure 3. At present, there is no universal agreement either on the physical placement of the Cenomanian-Turonian boundary or on a biostratigraphic definition of it. It has been drawn at various levels within a stratigraphically short interval above the upper Cenomanian strata of the Calycocems naviculare zone (as used by Kennedy and Juignet, 1973; = Eucalycoceras pentagonum- Calycoceras namiculare zone of Kennedy and Hancock, 1976) and below lower Turonian strata of the Mammites nodosoides-Inoceramus labiatus zone. These two molluscan zones are widespread and have been recog- nized in both Europe and North America. The interven- ing stratigraphic units are characterized in the type areas of France by beds of variable lithology that show lateral facies changes, and they are frequently bounded by unconformities and contain relatively poor fossil assemblages. During the time between the deposition of strata of the upper Cenomanian Calycocems navicula're zone and that of the lower Turonian Mammites nodosoides— Inocemmus labiatus zone, a major evolutionary event took place in many groups of organisms, including the ammonites, inoceramids, and foraminifers (Kauffman and others, 1976). Early studies of the European type areas reported a lack of ammonite faunas in this inter- val, but subsequent collecting has revealed the presence of a distinctive assemblage representative of the Sciponocems gracile zone that was recognized first in the Western Interior of the United States (Cobban and Reeside, 1952; Cobban and Scott, 1972) and is now known also in northern France and southern England (Kennedy and Hancock, 1976). A composite fauna] assemblage of North American and European elements characteristic of this zone would include, among others, the belemnite Actinocamax plenus, the ammonites Kanabiceras septemseriatum, Metoicoceras gesl'i- m'anum, M. gourdoni, M. whitei, Sciponoceras gracile, Worthoce’ras vemm'culum, and the bivalve Inocemmus pictus. Strata assigned to the Sciponoce'ras gracile zone in the Cenomanian type area are the Sables a Catopygus ob- tusus and the overlying Craie a Terebratella carantonen— sis (= Horizon A; Kennedy and Juignet, 1973; Kennedy and Hancock, 1976). In the Turonian type area, elements of the Sciponoce’r‘as gracile assemblage have been reported from the Craie marneuse at the base of the Turonian in the Fretevou section (Rawson and others, 1978). In northern France and southeastern England, the lower part of the Sciponoce’ras grac'ile zone includes the Actinocamax plenus marls (Metoicoce'ras geslinianum and M. gourdoni zones of J efferies, 1962), and, in Devon, southwestern England, the upper part of UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA TUR NIAN NORTHERN FRANCE LONDON- SOUTHEAST T PE CENOMANIAN TYPE AREA and AREA BELGIUM PAR'ingAS'N FRANCE Lu HAUTE- CAP CAP ELANC uzz/ BASSIN DE 2 TOURAINE SARTHE MAINE SARTHE NORMANDIE BLANC NEZ HAINAUHFHELGJSEFRANCE L' ESTERON l— Kennedy Juignel, . Kennedy Magné . Robasynski P h II (D Lecoinlre Basse (l959) and Kennedy and ngnel and and Robaszynskl and :"J" “959) Hancock (1959) Juignel Leberl (l976) Juignel Polvéche (I97I,l976) Canon ““9137?" “973) “978) (I973) (I960 (1979) " " Craie o 2 S S = < N N noduleuse g 2 — . a . e ° 3; g 9,955, E 0'9Ie Craie Craie E g ‘E a: 21' 1 u g / a _ a 6 ‘3 Modular w 3 ‘S D secigonevgdse E flail/’5’ Mace/01ml: Mace/MM: g while 3 § E ’- ofl"ly’pe" : Mbm,“ lab/alas MAM/[Is : chalk g N I : a: Turonian) ~§ -& 2 § E k UJ s E u, ° “ E g . v. x E g ‘3 Cycle § § 5. 9 § T T T § T § noduleuse g § § —— - — — — —— as a a. U Q C C C C 9, Meyer. g g E §§ E Idem/115 E 3 g a) I: x Q Q N 1 § ‘ . ‘3 § Q o: ._l g 8 Eg Cvaie q Craie Cvaie Crane < N d I § § S <( 5 ‘6 g . a c a a a = o uar \ Marnes u .b g f; g ,2: harem/em E feral). 791M. Tale/z E chalk Iverles. : 3 Lu 5 5‘ ,5 u, w: ‘ ’ 3 com”. cam/ll cam/II. 1° 'Dieves') g) a c c a: T 3 ~\ 5 é: -' - T; if» — U >_ 9? E 2 2 § T K a: 8 $ 3 N o o c g f .5 g ‘5 '79, Sables g T _‘ C 2 Z 3 2 3“ E g at Grés 9. C 4] - «I +1 «I 3 = ‘5 m ‘ o a -\ no: ,6 2 g Sables é Sables g ‘3 (ff/[(3% g 2 § g g a) _ Q ‘ 5 g as a g 0 % ‘E M § Chalk fig ’s o *3 0 ‘§ 2 z .5 1° Ca/opygus Q Caro/7]. § 3 ,6 :5 E § 5 § § _\ g S E -‘ E (Jo/”5115 a 05/1/5115 3, s» E g N "‘ Q, $ 3 ‘k 2 s g s a s s 2 s s s s 2 0 <2 m ‘3 3 Sables s. s 2 fi , as vs ‘3 g :5 E s lG‘s s s s s “W s s s 5 »— l T T‘ 5. °- s s a s I C G g 53 Cato/7]. 53 g g argileuse h [Mm ; g g ; <2: E MIMI/s N § § plain/5 g E § 5, E '3‘ 5 S .§ E g 3 8 <: E "E 1‘ § .8 ‘3. § § 2 h "i ‘ £ § V § § 0 HIATUS g HIATUS , 3", Chalk § L' 2 g Crgle § \ Cl \ g a Aci/na— a E. mm; .§ § p/e/ms § s s T _ - - C Q) Ma?“ Mames § Marnes § Mornes a : a = a E 3 [70/3521 3 05/190 .3 05km E 05/Iea g g g “8’ _ 2 S Maw/c. ‘5 Near/Z ° b/fll/I/Z‘. N g 2 3 N z w/a/a o 3 a; N < E 1’ 3 g s u. . S E E s u c \ a Crew Q Q u, E \ \ § - § § v, § § g g .3 g grls 5, .3 a O ‘3 '§ E s ‘ Q ‘ 3 c ' r E. Z 3 § a g g §' 0 g (me E Lu § r: g g 4% filo/45121 masswe .,, 3 o 8 § § E g ‘2 subg/a- g g E ‘ to «y W El § § : '3 § § bog/5 § § § CL S 3 E t \x s \x \g E rs Sables s Sables m Sables u Sables s. h: s K, % 9 du “ du 8 du g du k‘ k” x \ Perche Perche E Perche E Perche LB l3 FIGURE 3. — Correlation of upper Cenomanian and lower Turonian stratigraphic units of France (including the type areas), Belgium, England, and the United States Western Interior and Gulf Coastal Plain. Zonation based on ammonites and bivalves. Ranges of Rotalipom and elements of the Sciponocems gram'le assemblage zone are shown in the Cenomanian-Turonian boundary interval. N0 vertical scale. Formation names at Pueblo, 0010., also apply to the section in western Kansas. At Dallas, Tex., J. D. Powell of Grand Junction, 0010., studied out- crops; R. A. Christopher (USGS) and Valentine studied the same core. Heavy horizontal line indicates Cenomanian-Turonian boundary of various authors; dashed where approximate. UPPER CRETACEOUS STRATIGRAPHY , v. T C m d 5:8; .33 528588 .23: i u . ._. ._ T N m a II 0 um: 2:23;. W Sfifiu q % e A V I « h n m m M. a. w. v. w m m S .W \W on. m .H m w w e C L M m N m m 3:8 32333 M\\E§\§\w-mSgSEREQE E 2.8 5:8 M o m m A E .n .m a e A q. I T T m ( m m m. m m w M M”, w. T c a m m. m u -h w m w m m EESE. 5:3 =2:chch $95 «I. J x 0 I U . F D P ( W 9.3 §E\§\ mmkfifiv‘ ocoN wauS “$3338.“, 7. L 0 WNW W I M U m d ml .murnfi W m 3:8 .23: can BEE: 2:5 825: mm 0. r O u a ' G co. ,m m mPu m a W. SEES coztm 8:35“. 825 .m m A mm m .m ( h )1 I . A T C LS % mm B .nn. 6) W Wu 2.: “SEEQ §|« Whom mEEa m, 1‘ ngkxnk d a AA “aroma. WNW WM HM W Eéaxsxmsi _ e m EMUM 5 m5 % MN“ m%rnwd u _ ”MS( Efiktmusmkmm ¥ X _ ”M m 0H NANH (Immmm H m T 233‘ n _ WSWMM E fl .I r m m CK m E m S 2.3 23.8% 8&33‘3 28 033583 3.8232: 4. 82 §>§S m3m§§$wh _ a I )1 ) 3E3§§ T C <|_E§§§< R N M 2 M m ¥ . £2»; w. E “W 0W NW |«. . . £2 i ¢ « T TN m( e m nfififiq \ :tgtwmsmx‘mm \< |II||J N BM MWMH « :Rfix ¢ I. w m R E m 23 QERQ EEEBQS 9.2 .Sufim Etugaim N v ) mmfieghsx T C . d3 d w . . .52}. § X vl| Efiikk m 0 mm MW k « «\Egm wl|5|¢ T. w m( am $33 ,\ SE \ ¥ . S M m m M m , J $3 .\ ‘ E m 03 E m 22 §§§\ mEEEuQS 28 mEEm mEm§§ABh C W - T c O m) e k M” d m. n 1) mmmn Mumm mmdmm E Mdmn .nr W rrrve/nw ”hmwn U .DMCg BC.W( B m Hans IU P W S U L L 2.3 Q‘ENE §SE$uS 9.8 $.th “Emmgafiuh 3:8 .83 2c mm§tm$§ mfimmggmég S T C . E S n 0! 5 m n m. m, m A .m m mm WIEREEm 0 .I O C U / m m o m H 4r. P m .H R d ( W m 22 Basggn mms§E§~ SON ficfim mfim§§kfiw 2.8 mkxfiig muxuuq§a9\s§§§\§q mfiaaégi ME T C Dam «J. “Am ANann M.“ m 7 LF m a a B G . K H ( mm 9.2 mmhsghg wm§§§§ 88 meS “EEEQSQM 2.2 Sfifiig Exmufi§8\§§§\§§ «Emufixkufiu T C ..I S w RH; m w m m.) C U a). D m maum whom mmmmmm m M mm V n n nls .I C | C I B Ix N WU; m 0 In» m M ( M 0 ( B H A 88 mthmSE: $§EE§§ < 558: 28 .Efixém m38§§$ SB Sfififima m33§§§ ES m§§c§§ “Emufixku .L T C n n G v w km mud r w w k a m d on) 0CD .WCE ,MMflm W0 N mm mm mmw amw Mmmm mm E m u a M“ M M K M 2.2 mfiSm§§ QSSSu§ < :38: 82 .Efisg msfifisxfi TEE §§§§§ mEmuR§m§ 22 «SEE: muxmufixsu G Ir N c c E e k .I-nl .0” W S .P-C U k N mm m a mm mm Wm ‘I h / m m m M c _ Mm p _ L c K ( s M 2:: Q§S§ §§€§S 2.2 .Esfigm max§§§ _ 2.8 Eififimg m33§$§ 2.8 SQEEE mEmufixBu H T C T n _ X 5 W .m fl m k m a M m elfighfiak S e 6 .W M I M k m H mg. a R M F p n ( M 2.2 ‘Essgg uEm§~§m§ TEN Efififig m38§$m§ 82 §§§E§m Etukk FIGURE 3. — Continued. 8 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA TABLE 1. — Well number, name, location, altitude, and depth of 24 wells drilled in coastal South Carolina and Georgia, northern Florida, and offshore [Sources of data: (1) Applin and Applin, 1967; (2) Brown and others, 1972; (3) Brown and others, 1979; (4) Gohn and others, 19783; (5) Scholle, 1979. GGS - Georgia Geological Survey well number] ‘ Altitude Total depth No. 13V ell Location Measuring point Ground level Soarce ame County and State Lat N. Long W. feet (meters) feet (meters) feet (meters) data 1 GGS 144; Sun Oil Co., W. J. Barlow No. 1. Clinch, Ga. 30°55’42” 82°47’53” 177 (54.0) 167 (50.9) 3,848 (1,173) 3 2 Sun Oil Co., Ruth M. Bishop, No. 1. Columbia, Fla. (1) 2174 (53) _________ 2,828 (862) 1 3 Hunt Oil Co., H. L. Hunt No. 1. Baker, Fla. (3) 2134 (40) _________ 3,349 (1,021) 1 4 GGS 876; South Penn Oil Co., 0. C. Mizell No. 1. Charlton, Ga. 30°47’28” 81°59’25" 36 (10.9) 25 (7.6) 4,600 (1,402) 3 5 GGS 1198; Pan-American Petroleum, No. 1-B Union Camp Camden, Ga. 30°50'45” 81°50’30" 28 (8.5) 14 (4.3) 4,710 (1,436) 3 6 GGS 724; Humble State-1 Union Bag Camp. Glynn, Ga. 31°08’20” 81’38’20” 29 (8.8) 14 (4.3) 4,633 (1,412) 3 7 GGS 719; Humble No. 1, W. C. McDonald Estate. _____ do _____ 31°14’42” 81°38'01” 25 (7.6) 15 (4.6) 4,747 (1,447) 3 8 GGS 1197; Pan American Petroleum, Union Camp No. 1. _____ do _____ 31°22’20” 81°33’54” 24 (7.3) 13 (3.9) 4,460 (1,359) 3 9 GGS 363; E. B. La Rue, No 1 Jelks-Rogers. Liberty, Ga. 31°41’31” 81°20’54" 26 (7.9) 16 (4.9) 4,264 (1,300) 3 10 GGS 3194; Savannah Ports Authority. Chatham, Ga. 32°07’01" 81°13’19" 20 (6.1) 20 (6.1) 3,440 (1,049) 3 11 Hilton Head Island _______ Beaufort, S.C. (4) _________ 55 (1.5) 2,900 (884) 4 12 Layne Atlantic, Parris Island Test No. 2. _____ do _____ 32°19’40" 80°41’50” 18 (5.5) 15 (4.6) 3,454 (1,053) 3 13 Fripp Island _________________ do _____ 32°19'39” 80°27’42” _________ 55 (1.5) 3,168 (996) 4 14 Seabrook Development Corp., Test Well No. 1. Charleston, S.C. 32°35’30” 80°08'30" 8 (2.4) 3 (0.9) 2,705 (824) 3 15 Kiawah Island ________________ do _____ 32°35’40" 80°07’10” _________ 10 (3.0) 2,287 (697) 4 16 US. Geological Survey Clubhouse Crossroads Corehole 1. Dorchester, S.C. 32°53’15” 80°21’25” 23 (7.0) 18 (5.5) 2,530 (771) 3 17a Charleston Consolidated Railway and Lighting. Charleston, S.C. (6) _________ 55 (1.5) 2,007 (612) 4 17b Charleston Medical Center- _____ do _____ 32°47'03” 79°56’35” _________ 510 (3.0) 2,078 (633) 4 18 Sydnor Well and Pump Co., Snee Farms Corp. _--_- do _____ 32°51’05” 79°49’45” 20 (6.1) 20 (6.1) 2,130 (649) 3 19 Esterville Plantation _____ Georgetown, S.C. 33°15’08” 79°16’24” 18 (5.5) 18 (5.5) 1,835 (559) 3 20 Georgetown Rural Test Well, Penny Royal Road. _____ do _____ 32°20’17" 79°21'43” 20 (6.1) 20 (6.1) 810 (247) 3 21 Myrtle Beach 10th Ave. ___ Horry, S.C. 33°42’30" 78°54’22" 25 (7 .6) 25 (7 .6) 1,448 (441) 3 22 North Carolina Division Water Resources, Calabash Test No. 1. Brunswick, N.C. 33°53'35” 78°35’20” 48 (14.6) 48 (14.6) 1,335 (407) 3 23 Fort Fisher No. 1 ________ New Hanover, 33°58'25” 77°55’10" _________ 5 (1.5) 1,549 (472) 2 NC. 24 COST No. GE-l, Ocean Production Co. (7) 30°37’08” 80°17'59" 98 (29.8) 3136 (41.4) 13,254 (4,040) 5 1 Sec 10, TA 8., R. 17 E. 5 Approximate. 5 Location not given by source of data. 7 Continental shelf off Georgia and Florida. 5 Water depth. ’ Measuring point and ground level not differentiated. 3 Sec 21. T. 1 N., R. 20 E. ‘ North end, Hilton Head Island. UPPER CRETACEOUS STRATIGRAPHY 9 the Lower Chalk; the upper part of the Sciponoceras gracile zone is equivalent to Horizon A in Haute- Normandie, the lower part of the Melbourne Rock in southeastern England, and the Neocard’ioce'ras pebble bed at the base of the Middle Chalk in Devon (Kennedy and Juignet, 1973; Kennedy and Hancock, 1976). In southeastern France, strata corresponding in age to the Actinocamax plenus marls have also been recognized (Porthault and others, 1967). In the Western Interior United States, the Sciponoce’ras gracile zone is in the lower part of the Bridge Creek Limestone Member of the Greenhorn Limestone in Pueblo County, Colo., and ,Hamilton County, western Kansas (Cobban and Scott, 1972), and in the lower part of the Hartland Shale Member of the same formation in Russell County, cen- tral Kansas (Hattin, 1975). The S. gracile assemblage is, therefore, distinctive and widespread geographically; it is commonly confined to a few feet of section between recognized Cenomanian and Turonian rocks, and it represents an interval of time during which ammonites and other fossil groups underwent a remarkable evolu- tionary change. A review of selected recent papers on the Cenomanian-Turonian boundary reveals that as study of the problem has progressed, the boundary has been raised from the basal part of the present Sciponocems grac'ile zone to the top of that zone. These studies (fig. 3) have been based chiefly on the stratigraphic ranges of ammonites and inoceramids. Lecointre (1959) at- tempted to clarify the stratigraphy in France by propos- ing a Turonian type section in the Cher Valley near the type area originally described by d’Orbigny (1842, 1847). Lecointre placed the Cenomanian-Turonian boundary at the base of the Craie marneuse in the Fretevou section and suggested that those beds were laterally equivalent to other facies in the Cenomanian type area, namely the Marnes a Terebmtella carantonensis, the Marnes a Dit’rupa deformis, and the Sables a Catopygus obtusus. As a result of more recent studies in northern France and southern England, however, the Cenomanian- Turonian boundary has been moved higher in the section (Kennedy and Juignet, 1973; Juignet, 1976; Kennedy and Hancock, 1976; Juignet and others, 1978). In the Cenomanian type area, those authors have drawn the boundary between the Craie a Terebratella carantonen- sis (top of both Horizon A and the Sciponocems gracile zone) and the Craie a Imceramus labiatus (Mamm’ites nodosoides zone); in northern France and southeastern England, the boundary was placed at the top of Horizon A, which overlies the Actinocamax plenus marls. In the Western Interior United States, Cobban and Scott (1972) tentatively recognized the Cenomanian- Turonian boundary at the same position, between the Sciponoce'ras gracile zone below and the Inoce'ramus labiatus zone above, and subsequent studies have draWn the boundary at that level (Kauffman and others, 1976, 1977) Although the many studies of Cenomanian and Turo- nian strata have not resulted in general agreement on the placement of the boundary, they have succeeded in delineating a short, faunally distinctive stratigraphic in- terval, the Sciponoce'ras gracile zone, within which the Cenomanian-Turonian boundary undoubtedly lies. The beds assigned to this zone represent a transitional inter- val between strata of undisputed Cenomanian and un- disputed Turonian Age, a conclusion reached by Magné and Polvéche (1961) in their study of the Actinocamwc plenus zone. The concept of a transitional zone encom- passing a stage boundary may not be philosophically ac- ceptable to all stratigraphers, but it seems to best characterize the stratigraphic relations that typify the beds assigned to the Sciponocems gracile zone. The dif- ficulty in determining the exact placement of the Cenomanian-Turonian boundary will remain unresolved as long as no type sections are recognized for these two stages. Molluscan fauna] assemblages historically have been the basis for the highly successful biostratigraphic zona- tion of Cenomanian and Turonian strata in both Europe and the United States Western Interior. However, as a practical matter, stratigraphers must also be able to recognize the Cenomanian-Turonian boundary by using other biostratigraphically important groups such as planktic foraminifers and calcareous nannofossils. Studies of European and North American sections have shown that a major evolutionary event, the extinction of the planktic foraminifer genus Rotal’ipom, took place within the Cenomanian-Turonian transition zone described above. Planktic foraminifers are poorly preserved in this interval in the Cenomanian and Turo- nian type areas (Marks, 1977), but they are well represented in coeval beds elsewhere in Europe and North America. More precisely, the rotaliporids became extinct during deposition of the Actinoca’max plenus beds and their equivalents in Europe and the Sciponoce'ras gracile zone in the United States Western Interior. Magné and Polvéche (1961) have shown in their study of the Cenomanian-Turonian sections at Blanc-Nez and Sangatte on the north coast of France that Rotalipora became extinct in the upper part of the Actinocamax plenus zone, and they placed the Cenomanian-Turonian boundary at that level. In southern England, the last oc- currence of Rotalipom is also in the upper part of the Actinocamax plenus zone (Jefferies, 1962). Similar stratigraphic relations have been reported in northern France and in the Hainaut region of Belgium and France (Robaszynski, 1971, 1976) and also in 10 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA southeastern France (Porthault and others, 1967). The Rotalipom extinction datum has also been delineated in the United States Western Interior (Eicher and Worstell, 1970); a comparison of the range ofRotalipo'ra and the ranges of ammonites and bivalves from the same sections in Colorado and Kansas (Cobban and Scott, 1972; Hattin, 1975; Smith, 1975) shows that Rotalipom became extinct in the Sciponocems gracile zone. This relation also appears to exist in Oklahoma (Kauffman and others, 1977). The rotaliporid extinction event is widespread and easy to recognize, and I follow the practice of many stratigraphers (Pessagno, 1969; van Hinte, 1976) who have adopted it as a reliable datum for the practical determination of the Cenomanian- Turonian boundary. COST GE—l WELL, OUTER CONTINENTAL SHELF OFF GEORGIA Several investigators have conducted biostratigraphic studies of calcareous nannofossils, planktic foraminifers, and palynomorphs from the COST GE-l well on the Outer Continental Shelf off southeastern Georgia (International Biostratigraphers, Inc., 1977; Amato and Bebout, 1978; Valentine, 3979a; Poag and Hall, 1979). I have determined the ranges of stratigraphically important calcareous nannofossil species in this well by examining cuttings samples (Valentine, 1979a), and the resulting biostratigraphic in- terpretation agrees closely with the conclusions of the other workers (Scholle, 1979, table 1). This well is important because it provides an oppor- tunity to compare the offshore Cretaceous stratigraphy with the onshore section of Georgia and South Carolina; it also allows the age of a distinctive spore and pollen flora in wells beneath the Atlantic Coastal Plain to be established on the basis of its presence in rocks contain- ing calcareous nannofossil and foraminifer assemblages. The Tertiary-Cretaceous boundary is 3,562 ft below sea level (water depth is 136 ft at COST GE—l), and the Upper Cretaceous Series is a 2,140-ft sequence of argillaceous limestones that includes Turonian through Maestrichtian rocks. An undated 150-ft-thick interval of shallow—water limestone and calcareous sandstone bet- ween Turonian and Albian rocks may mark an unconfor- mity of Cenomanian Age. This section contrasts sharply with the onshore stratigraphy determined by Gohn and others (1978a, 1980), who recognized the presence of a major hiatus representing Turonian and Coniacian time as well as recognizing a sequence of Cenomanian strata that ranges in thickness from 300 ft at the Cape Fear Arch to about 600 ft beneath the Georgia Coastal Plain. Turonian beds at COST GE—l contain a rich assemblage of calcareous nannofossils, which includes Ahmwzlle'rella octomdiata, Braa’r‘udosphaem bigelo’wii, Chiastozygus cuneatus, C. plicatus, Corollithion achylosum, C. exiguum, C. signum, Creta'rhabdus con- icus, C. coronadventis, C. crenulatus, Cfibosphaera ehrenbe’rgii, Cylindmlithus coronatus, Eifi’ellithus eximius, E. tuMsmfieli, Gamtnemgo segmentatum, Lithastm'nus floralis, Mamivitella pemmatoidea, Micro'rhabdulus belg’icus, M. decoratus, Micula staurophora, Pa'rhabdolithus angustus, P. embergem', P. splendens, Prediscosphae’ra c'retacea, Tranolithus orionatus, Vagalapilla matalosa, Watznaueria bamsae, Zygodiscus acanthus, Z. diplogmmmus, and Z. fibul’ifomis. The ranges of two of these species, Co'r- ollithion achylosum and E'iflellithus exim’ius, are impor- tant in dating the assemblage, and I believe that their co-occurrence indicates a Turonian age. In a worldwide study of Jurassic and Cretaceous strata involving more than 800 samples, Thierstein (1976) reported the ranges of many calcareous nan- nofossil species. He concluded that Corollith’ion achylosum ranges from the Aptian to the uppermost Turonian (Thierstein, 1976, fig. 7 and pl. 3, figs. 39, 40). I have found this species to be restricted to rocks in- dependently dated as Turonian and older in the COST B—2, B-3, and GE—1 wells drilled on the Atlantic margin (Valentine, 1979a, 1980). A re-evaluation of the stratigraphy of the COST B—2 well (Valentine, 1980) has shown the reference to C. achylosum in that well (V alen- tine, 1977, p. 39) to be incorrect. In the Gulf Coastal Plain of Texas, C. achylosum has not been reliably reported from strata younger than Turonian (Gartner, 1968; Bukry, 1969; Smith, 1981), and in a recent un- published study of calcareous nannofossils in a core from the Eagle Ford and Austin Groups at Dallas, Tex. (see also Christopher, in press), I found that the highest oc- currence of C. achylosum is in the upper part of the Ar- cadia Park Formation (upper Turonian) and that it does not range into the overlying Atco Formation (Coniacian). In contrast, Verbeek (1977a) reported C. achylosum to range from the upper Aptian to the Campanian, but this range is based apparently on information from an early paper by Thierstein (1973) on Lower Cretaceous bio— stratigraphy and does not reflect that author’s later determinations (Thierstein, 197 6). Moreover, Verbeek’s (1976, 1977a) studies in Tunisia and Spain have shown that C. achylosum is restricted to Turonian and older strata, except for a single occurrence in younger beds. At El Kef in Tunisia, C. achylosum occurs in the Cenomanian and Turonian part of the section and in two samples (18 and 19) that Verbeek (197 6) at first placed in the Coniacian; later, he indicated that the lower sample (18) is Turonian in age (Verbeek, 1977a). Sample 19 is now the lowest sample in Verbeek’s (1976) Mar- UPPER CRETACEOUS STRATIGRAPHY 11 thasteritesfwcatus zone (Coniacian), which is defined on the first occurrence of M. fwrcatus, even though sample 18, now Turonian, also contains that species. M. fur- catus actually may appear first in the uppermost Turo- nian, as indicated by Manivit and others (1979), and I have found it associated with Corollithio'n achylosum and Eiffell'ithus eximius in the Turonian of the COST B—3 well (Valentine, 1980). Therefore, Verbeek’s “Conia- cian” samples that contain both C. achylosum and M. fiwcatus probably are Turonian in age. E’ifl‘elithus eximius has been reported to range from the middle Turonian to the Campanian-Maestrichtian boundary (Thierstein, 1976, fig. 7, and pl. 5, figs. 28, 29; Verbeek, 1976, 1977a). Wonders and Verbeek (1977) have shown that the first occurrence of this species in the El Kef section of Tunisia is in the Turonian, above the rotaliporid extinction level; the same relation exists in a section at Javernant, France (V erbeek, 1977b; DeVries, 1977). Manivit and others (1977), in a paper on important middle Cretaceous nannofossil marker species, indicated that E. eximius initially appeared in the upper Turonian. Subsequently, Manivit and others (1979) showed that this species first appeared in the middle part of the Turonian. In COST GE—l, Eiffellithus eximius and Coroll'ithion achylosum are present in the 400-ft interval of Turonian Age from 5,302 to 5,702 ft below sea level. The limestones in this sequence also contain spores and pollen. Cuttings samples from 5,592 to 5,602 and 5,672 to 5,682 ft, examined by R. A. Christopher (unpub. data, 1978), were found to contain a flora representative of pollen zone IV, a biostratigraphic unit established by Doyle (1969a) in his study of the Raritan Formation of New Jersey. AGE OF POLLEN ZONE IV The age of pollen zone IV has an important bearing on the stratigraphic interpretations made by the present study. The zone was described from the Woodbridge Clay Member and the underlying Farrington Sand Member of the Raritan Formation in the Raritan Bay area of New Jersey (Doyle, 1969a and b, 1977; Doyle and Robbins, 1977). Work by Sirkin (1974) on Long Island and by Christopher (1977, 1979) in New Jersey has shown that this zone is present in the Woodbridge Clay and in the overlying Sayreville Sand Members of the Raritan Formation. Pollen zone IV, as used in the pre- sent study, refers to the upper part of the zone that is typical of these two members; in this sense, the zone has been redefined by Christopher (1979) as the Complexiopollis—Atlantopoll'is assemblage zone on the basis of the co-occurrence of the nominate genera and the absence of other Normapolles genera. Christopher (1977 and in press) and earlier workers dated the zone as late Cenomanian in age, but Doyle and Robbins (1977) did not rule out the possibility of an early Turonian age for the Woodbridge part of pollen zone IV. The age of the Woodbridge Clay Member of New Jersey can be ascertained from studies by Perry and others (1975) and by Petters (1976). Perry and others (1975, figs. 11, 12) incorporated palynolog'ic data of Doyle (1969a, b) and Sirkin (1974) into a synthesis of Atlantic Coastal Plain stratigraphy; they used a broader definition of pollen zone IV than that described above. In New Jersey, they showed the coincidence of part of pollen zone IV with the Woodbridge Clay Member in the Toms River Chemical Co. well, and they also delineated pollen zone IV in the USGS Island Beach No. 1 well. Using planktic foraminifers, Petters (1976, p. 93, 96, fig. 6) dated the Woodbridge Clay Member as early Turo- nian in both the Toms River well (1,323 to 1,440 ft below sea level) and the Island Beach well (1,950 to 2,200 ft below sea level). In the Island Beach well, R. A. Christopher (oral com- mun., 1979) found pollen zone IV assemblages in sidewall cores from 2,004 and 2,200 ft; these samples are from the interval dated as early Turonian by Petters (1976) and lie within the part of the section identified as pollen zone IV by Perry and others (1975). I have examined the calcareous nannofossils from these two cores. The sample from 2,004 ft is Turonian in age and contains Corollithion achylosum and Ezfiellithus eximz'us as Well as Biscutum blackii, Chiastozygus mneatus, C. plicatus, Corolh'thicm signum, Creta'rhab- dus comlcus, C. crenulatus, Ezfi’ellithus turrisetfieli, Gartnemgo se'gmentatum, G. striatum, Lithastm'nus floralis, L. grillii, Microrhabdulus decoratus, Parhab- dolithus angustus, P. emberge’ri, P. splendens, Prediscosphae'ra cretacea, Vagalapilla matalosa, Watz- muem'a bamsae, Zygodiscus acanthus, Z. diplog'ram- mus, and Z. fibulifomis. The nannofossil assemblage from 2,200 ft is almost identical with the one from 2,004 ft except for the absence of Eiffellithus eximius and the presence of Lithraphid'ites acutum. I interpret this assemblage to be early Turonian in age. Lithmph’idites acutum Verbeek and Manivit is a newly described species that ranges from middle Cenomanian to middle Turonian (Manivit and others, 1977, pl. 1, figs. 7, 8). This form was previously included in the stratigraphically older L. alatus Thierstein, a species once thought to be restricted to Cenomanian and lowermost Turonian strata (Thier- stein, 1976; compare Roth and Thierstein, 1972, pl. 3, figs. 1—8 and Thierstein 1974, pl. 3, figs. 5-11). Since the separation of L. acutum, L. alatus is reported to range from the upper Aptian to only the upper Cenomanian (Manivit and others, 1977), but its upper limit is not documented thoroughly yet, and it probably ranges into 12 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA the lower Turonian. The last occurrence of L. acutum in the Turonian is not precisely known, as implied by the dashed line representing the upper part of its range in Manivit and others (1977, fig. 1). In figure 1 of Manivit and others (1977), I consider the Cenomanian-Turonian boundary to occur at the extinction of Rotalipo’ra cushmam', not at the lower level they have shown; my in- terpretation does not significantly alter the age of the upper part of the range of L. acutum portrayed in their figure. The lower Turonian sidewall core from 2,200 ft in the Island Beach well is at or near the Cenomanian- Turonian boundary and the base of the Woodbridge Clay Member (Petters, 1976). In the Toms River well, Doyle (1969b; oral commun. 1981) examined spores and pollen from sidewall cores and found floras typical of the Woodbridge Clay Member in samples from 1,298 to 1,300 and 1,369 to 1,371 ft, and a somewhat older assemblage at 1,437 to 1,439 ft. All these samples are above 1,440 ft, where Petters (1976) identified the Rotalipom extinction and the Cenomanian-Turonian boundary. I have examined calcareous nannofossils in six sidewall cores from the section immediately above the Cenomanian-Turonian boundary, and the assemblages are similar to those described above in the Island Beach well samples. The upper four samples, from 1,323 to 1,393 ft, contain Cor- ollith'ion achylosum and Eiflellithus eximius and are of Turonian age. The lower two samples contain Lith’raphidites acutum, Podorhabdus albianus, and Parhabdolithus aspe’r', but not Elfiellithus eximius, and they are early Turonian in age. Biostratigraphic evidence from planktic foraminifers (Petters, 1976) and calcareous nannofossils in the Island Beach and Toms River wells shows that the pollen zone IV (Complexiopoll’is—Atlantopollis assemblage zone) floras are early Turonian in age. In the Fripp Island, 8.0., well, a pollen zone IV assemblage resembling that found in the Woodbridge Clay and Sayreville Sand Members of New Jersey has been identified by R. A. Christopher (unpub. data, 1977) in cuttings samples from 3,097 to 3,107 and 3,127 to 3,137 ft. Calcareous nannofossils in individual rock chips from 3,057 to 3,067, 3,097 to 3,107, and 3,137 to 3,147 ft contain Eiffell’ithus exim’ius and Corrollith’ion achylosum and are of Turonian Age. In the Clubhouse Crossroads corehole 1, also in South Carolina, a pollen zone IV flora is present in a 62-ft sec- tion from 2,342.3 to 2,404.8 ft (R. A. Christopher, un- pub. data, 1976; Hazel and others, 1977). I have ex- amined 67 samples from this interval; 33 of them are fossiliferous and contain calcareous nannofossil assemblages of early Turonian age that included such species as Ahmuellerella octoradiata, Corollithio'n achylosum, Cruciellipsis chiastia, Lithraphidites acutum, Parhabdolithus aspen; and Podo’rhabdus albianus. On the Atlantic continental margin, therefore, spore and pollen assemblages of pollen zone IV are found with Turonian calcareous nannofossils in the Island Beach and Toms River wells in New Jersey, in the Fripp Island well and the Clubhouse Crossroads corehole 1 in South Carolina, and in the COST GE—l well off Georgia. Finally, in the corehole beneath the Gulf Coastal Plain at Dallas, Tex. (fig. 3), pollen zone IV assemblages are present in the middle and upper parts of the Britton For- mation (208.9 to 438.6 ft) and possibly in the lowermost part of the overlying Arcadia Park Formation, both of the Eagle Ford Group (Socony-Mobil Field Research Lab Corehole No. 16; Brown and Pierce, 1962; Pessagno, 1969; Christopher, in press). The zone IV spores and pollen are found in a section of the core that has been dated by Pessagno (1969, pl. 9) by means of planktic foraminifers and determined to include mainly strata of late Cenomanian Age (Britton Formation) separated by a disconformity from a short sequence of late Turonian Age (uppermost part of Britton Forma- tion and lower part of Arcadia Park Formation). Planktic foraminifers are relatively sparse in the core, however, and Cenomanian rotaliporid marker species are represented by a single occurrence of Rotal’ipo'ra cuhsmani at 444 ft (Pessagno, 1969, p1. 39b), just below the base of pollen zone IV delineated by Christopher (in press). On the other hand, recent unpublished studies by J. D. Powell of Grand Junction, Colo. (oral and written com- muns., 1980; fig. 3, this report), on the planktic foraminifers, ammonites, and inoceramids collected from outcrops near the Dallas corehole site have shown that the upper 15 ft of the Britton Formation and possibly the lower 30 ft of the Arcadia Park Formation actually represent the lower Turonian Mytiloides “labiatus” zone (Kauffman and others, 1977) and that the underlying approximately 200 ft of the middle and upper parts of the Britton Formation represents the Sciponoce'ras gracile zone interpreted by him to be of latest Cenomanian Age. Christopher (in press) has sug- gested that pollen zone IV in the Dallas core coincides with the Sciponocems gracile zone and with the base of the Mytiloides “labiatus” zone outlined by Powell in out- crop. In summary, pollen zone IV is present in a part of the Dallas core (middle and upper parts of Britton Forma- tion and lower part of Arcadia Park Formation) that has been dated as late Cenomanian and late Turonian by means of planktic foraminifers, whereas, in nearby out- crops, this interval has been dated as late Cenomanian and early Turonian by means of planktic foraminifers, ammonites, and inoceramids. UPPER CRETACEOUS STRATIGRAPHY 13 I have studied the Dallas core (fig. 3), and nannofossils are common to abundant and well preserved except for an interval in the upper part of the Britton Formation where the assemblages are sparse. E’ifi‘bllithus exim’ius first appears at 210.1 ft, just below the top of the Britton Formation and ranges up through the Arcadia Park Formation; its co-occurrence with Corollithion achylosum indicates a Turonian age for this interval. The basal beds of this sequence are in the upper part of pollen zone IV and are equivalent to Turonian strata containing pollen zone IV assemblages that have been already described from the Island Beach well, the Toms River well, and the COST GE—l well on the Atlantic margin. The middle and upper part of the Britton For- mation from about 224.7 ft down to the base of pollen zone IV (438.6 ft) contains a calcareous nannofossil assemblage that includes the following species: Ahmuellerella octomdiata, Cruciellips'is ch’iastia, Lithraph'idites acutum, Micro'rhabdulus belg'icus, Parhabdol’ithus aspen and Podorhabdus alb'ianus. Similar assemblages have been found in the Toms River, Island Beach, and Clubhouse Crossroads wells. The up- per range of Cmiellips'is chiastia has been extended recently from the Cenomanian into the lower Turonian (Barrier, 1980), and the other species all occur in the Turonian. The studies of the Britton and Arcadia Park Forma- tions in Dallas, Tex., indicate to me that pollen zone IV represents: (a) the lower Turonian, found in the middle and upper part of the Britton Formation within the Sciponoceras gracile zone and containing Corollithion achylosum and Lith’raphidites acutum; (b) the lower Turonian, found in the uppermost part of the Britton Formation and lower part of the Arcadia Park Forma— tion within the Mytilot'des labiatus zone, where Cor- ollithion achylosum and Eiflell'ithus eximius are pres- ent, but not Lith'raph’idites acutum. The spore and pollen flora reported by Christopher (in press) from the middle and upper parts of the Arcadia Park Formation and the lower part of the Austin Group in the Dallas core contains elements of both pollen zone IV and the younger pollen zone V-A (= zone V of Sirkin, 1974; = Complexiopollis exigua-Santalacites minor assemblage zone of Christopher, 1979). Calcareous nan- nofossils from the same interval in the core range in age from Turonian (middle and upper part of the Arcadia Park Formation) to Coniacian (Austin Group). We will see in subsequent discussions that similar post-pollen zone IV—pre-pollen zone V floras have also been iden- tified in some wells on the southeastern Atlantic margin (R. A. Christopher, unpub. data, 1977-1979). FRIPP ISLAND WELL, SOUTH CAROLINA The Fripp Island well (table 1, fig. 1) is a key well for interpreting the Upper Cretaceous stratigraphy in South Carolina. I have studied the section in detail, using calcareous nannofossils; in all, I have examined 279 cuttings samples at 94 levels in the approximately 1,700-ft Upper Cretaceous interval. Cuttings were col- lected over 10-ft intervals, and individual rock chips representative of each lithologic unit in a sample were examined for calcareous nannofossils. There is a 263-ft sampling gap in the Campanian part of the section. The biostratigraphy of the well (fig. 4) is based on the ranges of the following species: Micula mum (1,507 ft); Tetral’ithus aculeus (1,507—1,970 ft); T. tm'fidus (1,527—1,97O ft); Broinsomla parcel (1,527—2,427); Eif- fellithus extmius (1,808—3,147 ft, lowest sample); Lithastm'nus g'rVill'i'i (2,263—2,867 ft); Chiastozygus cuneatus (2,427—3,007 ft); Lithastm'nus floral’is (2,457—3,147 ft, lowest sample); Marthaster’itesfurcatus (2,457-2,857 ft); and Corollithion achylosum (3,057-3,147 ft, lowest sample). The Tertiary-Cretaceous contact is at 1,437 ft, and the Maestrichtian—Campanian boundary is at 1,808 ft. The top of the Santonian is at 2,427 ft, and Santonian and Santonian-Coniacian cuttings are consistently present down to 2,867 ft; the samples from 2,637 ft down to the top of the Turonian at 3,057 ft are dominated by coarse quartz sand, and from 2,877 ft, the cuttings are predominantly Campanian cavings. The unfossiliferous sand from 2,637 to 3,057 ft may in fact be Coniacian in age, but because I could not distinguish the boundary between the Santonian and Coniacian Stages, I have designated the entire interval from 2,427 to 3,057 ft as Santonian-Coniacian. Near the bottom of the Fripp Island well, a Turonian calcareous nannofossil assemblage is present in three samples (3,057 to 3,067, 3,097 to 3,107, and 3,137 to 3,147 ft) that include Ahmuelle'rella octoradiata, Cor- ollithion achylosum, C. exiguum, Et'flell'ithus exi’mt'us, and Lithast'm'nus floralis. Two samples from within this interval (3,097 to 3,107 and 3,127 to 3,137 ft) contain a flora indicative of pollen zone IV (R. A. Christopher, un- pub. data, 1977). A sample from 3,117 to 3,127 ft con- tains a planktic foraminiferal assemblage dated as Turo- nian or Coniacian but not older than Turonian, and rotaliporid species indicative of the Cenomanian are ab- sent (C. C. Smith, unpub. data, 1977). Samples were not available from the lowermost 21 ft of the section, and the well did not penetrate crystalline basement rocks (Gohn and others, 1978a). There is little disagreement between my interpreta- tion of the upper part of the Fripp Island section and 14 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Well No. l3, Fripp lsland,S.C. Gohn and others (l9780)I Valentine (this study) Provincial European European Samples Calcareous Stage Stage Stage nannofossrls l400- C . Cenozoic $333)” “W” —r437rr— _ K6 —|507ft— M. mam Maestrichtlan __ l800— Navarroan- Maestrichtian- Tayloran Companion “_rJ —l808ft— -l808ft-—-1 Ear/mm: E Z _l / _ I 060 we a - — l9701'l—J 7. lr/f/o’us 3 3 K5 5;; 2200_ T | Companion a oran- . E Auystinian Companion LL - —2263fi”—1 l grill/'1' Z . I. _ I '12—. K4 —‘ 1242"?“1 62 armed/us E‘s Companion —2427rr— - 2427n———J 5W” Austinian - — - \ Lf/om/I's 2600- Santonran __ 2457ft—l Mfr/[calm WW — 0 K3 ' — E Santonran- H ‘ Conlacian _ H Eagle- 5 33° 2857ft—‘ Mfr/radius fordian g — N £2867fl—l Lgr/'///'/' E 5 3000— g = 0:? o _3007fl_l 6‘. 6‘006’0/(16‘ E l K2 —W3057le\ — —3057ft—1 a oc/ry/osm 09 8‘ Turonian — , — fordian l _______ _- '—- 3l68 ft TD * EXPLANATION —-— Single occurrence l Highest occurrence _i Lowest occurrence FIGURE 4. - Stratigraphic interpretations of the Fripp Island well, South Carolina; Gohn and others (1978a) compared with Valentine (this study, based on calcareous nannofossils). In sample column, blacked-in areas indicate complete sequence of samples studied. Pollen zone IV flora, indicated by ruled area, identified by R. A. Christopher (US Geological Survey). Depth scale is relative to sea level; stratigraphic boundaries and fossil occurrences are given in original depths (for depth below sea level, subtract 5 ft). UPPER CRETACEOUS STRATIGRAPHY 15 that of Gohn and others (1978a). I place the Tertiary- Cretaceous contact somewhat higher (at 1,437 ft) than do Gohn and others on the basis of the first appearance of a mixed assemblage of Cretaceous and Paleocene nannofossils. In the lower part of the well, Gohn and others (1978a) interpreted their units K2 and K3 to be Cenomanian in age and postulated a major hiatus above unit K3 that represents Coniacian and Turonian time. In contrast, I believe these strata to be younger, and I in- terpret their unit K3 to represent beds of Santonian- Coniacian Age and unit K2 to be Turonian in age. PARRIS ISLAND NO. 2 WELL, SOUTH CAROLINA The Parris Island No. 2 well is only 11 miles west of the Fripp Island well (table 1, fig. 1) and is useful for comparing the stratigraphic schemes of Gohn and others (1978a, 1980) with those of Brown and others (1979). This comparison and the independent dating, by means of calcareous nannofossils, of the units of Gohn and others (1978a) at Fripp Island are important for determining the subsurface stratigraphy of the region. Gohn and others (1978a) used paleontological analyses and electric logs to date and correlate the Parris Island and Fripp Island wells. The basis for the interpretation of the Parris Island well by Brown and others (1979) is not given but, as mentioned above, presumably includes lithologic characteristics and the occurrences of key species of ostracodes and foraminifers. A comparison of the results of these two studies is shown in figure 5. The stratigraphic interpretations of the Parris Island well by Gohn and others (1978a) and Brown and others (1979) do not differ significantly in the upper part of the sections, except that Brown and others placed the Tertiary-Cretaceous contact about 180 ft shallower than did Gohn and others. However, differences in inter- pretation exist at lower levels, for example, from 2,528 to about 2,650 ft, where unit D (Eaglefordian; Turonian— upper Cenomanian) of Brown and others ( 197 9) overlaps the lower part of an ostensibly younger unit K4 (Aus- tinian; lower Campanian-Santonian) of Gohn and others (1978a). And although both of these studies recognized coincident stratigraphic boundaries lower in the well (at 3,144 and 3,256 ft), Brown and others (1979) assigned these strata to their unit E (W oodbinian; Cenomanian) and unit F (Washitan-Fredericksburgian; lower Cenomanian-Albian), whereas Gohn and others (1978a, 1980) considered the same beds to be younger and placed them, respectively, in their unit K2 (middle Eaglefordian; upper Cenomanian) and unit K1 (Upper? Cretaceous). I have not studied any samples from Parris Island, and paleontological analyses are sparse in the lower part of the well, as only three samples are shown by Gohn and others (1978a, sheet 2) to have been studied between 2,350 ft and the bottom of the well at 3,454 ft. Gohn and others (1978a) showed that in the upper part of the Parris Island to Fripp Island section units K4, K5, and K6 dip eastward, Whereas the older units (K1, K2, and K3) dip westward. However, I interpret the electric logs illustrated by Gohn and others (197 8a) to show that the upper units (K4, K5, and K6) also dip gently westward, and I have reevaluated the stratigraphy of the Parris Island well by using the electric logs and applying the stratigraphy that I established by means of calcareous nannofossils for the nearby Fripp Island well. Figure 5 shows my assessment of the Parris Island stratigraphy and its correlation with that of Fripp Island. The strata in the lower part of the well are younger than previous authors have indicated. The beds attributed by Brown and others (1979) to their unit D (Eaglefordian; Turonian-upper Cenomanian) and unit E (Woodbinian; Cenomanian) and by Gohn and others (1978a) to their units K2 and K3 (middle Eaglefordian; upper Cenoma- nian) are now shown to represent the Santonian- Coniacian and Turonian stages. The age of the lower 200 ft of strata drilled at Parris Island, unit F of Brown and others (1979) and unit K1 of Gohn and others (1978a) is unknown at present. CLUBHOUSE CROSSROADS COREHOLE 1, SOUTH CAROLINA The Clubhouse Crossroads corehole 1 is on the north- ern flank of the Southeast Georgia Embayment, 42 miles north of the Parris Island and Fripp Island wells (table 1, fig. 1). The lithology and paleontology of the core has been extensively studied (Gohn and others, 1977; Hazel and others, 1977), and significantly, both Gohn and others (1978a) and Brown and others (1979) have incorporated this corehole into their respective stratigraphic frameworks (fig. 6). Their interpretations are coincident down to the base of the Austinian Stage. At that level, Gohn and others (1978a) indicated the presence of a major disconformity between their units K4 and K3; below the hiatus, they assigned the entire in- terval down to the basalt in the bottom of the hole to units K3 and K2 (middle Eaglefordian; upper Cenoma- nian) and K1 (Upper? Cretaceous in Gohn and others, 1980). On the other hand, Brown and others (1979) inter- preted this same interval to include strata of their unit D (Eaglefordian; Turonian-upper Cenomanian) separated by a hiatus representing unit E (Woodbinian; Cenoma- nian) from the underlying rocks of unit F (W ashitan- Fredericksburgian; lower Cenomanian-Albian). Gohn and others (1978a) and Brown and others (1979), therefore, did not recognize the same hiatus in the lower part of this section and also disagreed on the age of the beds in the lower ~ 200 ft of the corehole. 16 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Well No. l2 Porris |sland,S.C. Well No. t3 Fri pp |sland,S.C. w '—————H MILr-zs——J E Brown Gohn Gohn Valentine and others and others and others (this study) (I979) (l9780,l980)| 09780) lOOO cen0Z0iC Cenozoic (port) (port) l300— A _ Navarroan K6 145511 _________ |z_t_37_f_t— d l600 K6 E B Novorroon— Moestrichtion Maestrlchtion ‘ < Tayloran Moestrichtion- % t890 [303“- Companion § '900" Tayloran __ __fl ——————————— - '35 K5 K5 #— Toyloron- ' Companion E 2200 C Austinion campoman Companion _ E :2" Austinion K4 ————————————— K4 *- 2427ft— - o. , . —2480fr Companion- g 2500— D Austinion Santonion _. ——————— rvvvvvm wvvvvx —————— K3 = K3 Sontonion- 2800— onnge- Sontonion- E Coniocion _ or Ian 5% Coniocion é 3roo— we 3057““ 0 Kim- rvvvvyvx— K-2-—3|44ft Tur0n|0n__ _ _3l68ft _ _ _ __, Woodbin. E _____ TD W h F KI 3256ft 3400 F- bflfgi’on ngfddébzis VERTICAL EXAGGERATION x46 _ 3454ftTD FIGURE 5. — Stratigraphic correlation of the Parris Island No. 2 and the Fripp Island wells, South Carolina, showing stratigraphic interpretations of Gohn and others (1978a, 1980), Brown and others (1979), and Valentine (this study). The stratigraphic units of Gohn and others (1978a, 1980) are expressed as their European-stage equivalents in the Fripp Island well and as their Gulf Coast-stage equivalents in the Farris Island well. Fredericksburgian is abbreviated F-burgian; Washitan, Wash.; and Woodbinian, Woodbin. Dashed lines are electric-log correla- tions. Depth scale is relative to sea level; stratigraphic boundaries are given in original depths (for depth below sea level, subtract 5 ft for the Fripp Island well and 18 ft for the Parris Island well). Well No. l6, Clubhouse Crossroads,S.C. UPPER CRETACEOUS STRATIGRAPHY 17 Brown Gohn Hazel Valentine and others and others and others Samples (this study) (I979) “9780 , I980) (I977) SEA LEVEL 300 - - Cenozoic Cenozoic 600 - — — 804fi —— “—5 A K6 : M r ' ht' E 900 ‘ Navarroan Maesirichiian E aes rrc ran _ : Navarroan- : l025fr——- a B Tayloran -__—_ § 1200— g — “4 Tayloran : . 2 K5 Companion —- Companion m __ L... Tayloran- = __ “- [500 C Austinian, : 2 E = 2 — Auslinian .=_=. “- — I706fl—- I .— \ E l800— Auslinian K4 Sanlonian ? : :1 __ Q " ' ' ' ' " ' " D K3 2 S E l _ = Santonian- fgigdiaan L g E % Coniacian 2l00- BE 5 r-L - L3 .8 E rvvvvvvvvvx C Uptpefl?) E i 2342“ Washiian F _ re aceous — U ; , WV . m 2400- F-burgiari K2 — V/fl Tgfigi‘g‘n __ . . . Cenomanian(?) Trrassrc - Jurassrc basalt _/ 2530 fl TD 2462" FIGURE 6.—Stratigraphic interpretations of the Clubhouse Crossroads corehole 1, South Carolina, of Hazel and others (1977), Gohn and others (1978a, 1980), Brown and others (1979), and Valentine (this study). Fredericksburg'ian is abbreviated F-burg. Samples shown were, for the most part, discussed by Hazel and others (1977). Pollen zone IV flora, represented by ruled area, identified by R. A. Christopher (USGS). Depth scale is relative to sea level; stratigraphic boundaries are given in original depths (for depth below sea level, subtract 23 ft). 18 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Hazel and others (1977), using panktic foraminifers, have shown that the Tertiary—Maestrichtian contact is at approximately 804 ft, and they reported Maestrichtian foraminifers from 1,025 ft, somewhat above a Campa- nian calcareous nannofossil flora at 1,050 ft (Hattner and Wise, 1979). Campanian strata extend down to ap- proximately 1,706 ft, where spore and pollen assemblages have been reported (Hazel and others, 1977) that are typical of the Magothy Formation of New Jersey. The beds from 1,706 to 2,342 ft are almost barren of fossils, and spore and pollen assemblages are present only in the upper 200 ft of the interval. The Magothy flora is present from 1,706 to 1,804 ft; from 1,804 to 1,906 ft, a flora is present that is characteristic of the upper member (South Amboy Fire Clay Member) of the Raritan Formation of New Jersey (Hazel and others, 1977). The Magothy and the South Amboy spore and pollen assemblages constitute the tripartite pollen zone V of Christopher (1977, 1979, and in press). This stratigraphic interval is chiefly Santonian in age, but it also includes strata of earliest Campanian and possibly Coniacian Age (R. A. Christopher, oral commun., 1979). I have examined two samples from this interval for calcareous nannofossils. The sample from 1,752 ft con- tains a rich Santonian assemblage, but nannofossils are rare in the sample from 1,943 ft, and its age is equivocal; the presence of EWellithus eximius, Tetral'ithus obscurus, and Lithastm'nus grillii point to a Turonian Age or younger, possibly Coniacian. Few planktic foraminifers have been observed in the interval from 1,706 to 2,342 ft. Two samples (1,922.5 and 2,308.8 ft) examined by C. C. Smith (unpub. data, 1976) each con- tained a single species (Glob’ige'rinelloides caseyi and G. sp. cf. G. casey'i, respectively) and were attributed to the Cenomanian, partly because several samples from below 2,308.8 ft were dated as Cenomanian. G. caseyi, however, is not restricted to the Cenomanian; it has been reported from the upper Cenomanian and lower and middle Turonian of Kansas and Colorado (Eicher and Worstell, 1970), and the highest occurrence of this species is also considered to be diagnostic of lower Turo- nian strata beneath the Scotian Shelf of Canada (Ascoli, 1976). Furthermore, these strata from 1,706 to 2,342 ft are characterized by sand and silt, and Gohn and others (1978a, sheet 2) have shown this interval, units K3 (up- per Cenomanian) and K4 (Santonian-lower Campanian) to be correlative with a lithologically similar sequence in the Fripp Island well that I have determined to be Santonian-Coniacian in age. Lower in the Clubhouse Crossroads core, a spore and pollen flora characteristic of pollen zone IV and the Woodbridge Clay Member of the Raritan Formation (New Jersey) is present from 2,342.3 to 2,404.8 ft (R. A. Christopher, unpub. data, 1976; Hazel and others, 1977). Samples from these beds (2,364.4, 2,365.4, 2,369.4, 2,373.7, and 2,399.2 ft) contain planktic foraminifers that have been interpreted as late Cenomanian in age, although Cenomanian rotaliporids are absent (C. C. Smith, unpub. data, 1976; Hazel and others, 1977). These planktic assemblages are composed of one to three species, Guembelitr’ia harmlsi, Hedbergella brit- tonensis, and Heterohelix moremam'. The age deter- mination is based chiefly on the presence of Guembel’itm'a harrisi, which is present in all samples; however, the range of this species is not precisely known, and in Colorado, Wyoming, and South Dakota it has been reported to range higher than the rotaliporids and therefore into the Turonian (Eicher and Worstell, 1970). Additional samples from this interval (2,367.5, 2,370.5, and 2,396 ft) have yielded assemblages of planktic foraminfers containing only two species, Guembelitria harrisi and Heterohelix moremani (C. W. Poag, unpub. data, 1981). Calcareous nannofossils from the Cretaceous of the Clubhouse Crossroads core have been studied by Hatt- ner and Wise (1980). They examined four samples from the pollen zone IV interval and reported the presence of Ahmuelllerella octoradiata, Lithmphidites acutum, and L. alatus in an assemblage from 2,372 ft, which they dated as Cenomanian. The other samples are barren or contain a nondiagnostic flora. I have also studied the calcareous nannofossils from pollen zone IV strata in the core; 33 of 67 samples examined in the 62-ft interval are fossiliferous. The range of biostratigraphically important species are: Ahmuellerella octomd'iata (2,37 0—2,407 ft); Coroll'ithion achylosum (2,363—2,407 ft); Cruciellipsis chiastia (2,364.5-2,406 ft); Eiflellithus ex'imius (rare, 2,364 ft); Lithraphidites acutum (2,372.5—2,406 ft); Lithmphid’ites alatus (rare, 2,394 ft); Microhabdulus belg’icus (rare, 2,373 and 2,406 ft); Micula staurophora (2,370—2,407 ft); Parhabdolithus aspe’r (2,363—2,406 ft); and Podo'rhabdus albianus (2,364—2,406 ft). I interpret this interval to be early Turonian in age. These beds are units K2 of Gohn and others (1978a), and they have shown the beds to be coeval with K2 in the Fripp Island well that also contains a pollen zone IV flora and in which I have identified Turonian calcareous nannofossil assemblages. Brown and others (1979) indicated that their unit E is missing from the Clubhouse Crossroads section, but the evidence presented above and the stratigraphy of the Parris Island well (fig. 5) suggest that it is present and that it is probably coincident here with unit K2 of Gohn and others (1978a). The strata in the core between ~2,416 ft and the basalt encountered at about 2,462 ft are unfossiliferous, and I regard them, with reservation, to be Cenomanian UPPER CRETACEOUS STRATIGRAPHY 19 in age. In subsequent drilling, approximately 3 mi from Clubhouse Crossroads corehole 1, the basalt was penetrated to reveal elastic red beds of Late Triassic to Early Jurassic age (Gohn and others, 1978b). PENNY ROYAL AND MYRTLE BEACH WELLS, SOUTH CAROLINA, CALABASI-I WELL, NORTH CAROLINA The Penny Royal, Myrtle Beach, and Calabash wells are on a transect extending from approximately 60 mi south of the Cape Fear Arch northeastward to its crest (fig. 1). The wells have been studied by Gohn and others (1978a) and by Brown and others (1979). Gohn and others (1978a) did not differentiate their units K5 and K6 in these three northern wells, and I have attempted to separate the two units by using the electric logs pub- lished by the authors. On the same basis, I have raised the top of their unit K3 in the Calabash well. The paleontological analyses of the wells by C. C. Smith and R. A. Christopher (unpub. data, 1977, 1978) are the basis for the age determinations of Gohn and others (1978a) and for my own interpretations, and sam— ple levels are shown in figure 7. In the Penny Royal well, the Maestrichtian-Campanian boundary has been drawn at ~ 550 ft, between two cuttings samples (490—510 and 590—610 ft) containing diagnostic planktic foraminifers and calcareous nannofossils.’ At Myrtle Beach, Maestrichtian foraminifers are present in the upper part of the well, and Santonian-Coniacian(?) spore and pollen assemblages of pollen zone V are present from 953 ft to at least 1,242 ft. In the Calabash well, the Campanian- Santonian boundary is at about 750 ft, between two samples (690—7 00 and 800—810 ft) containing spores and pollen. Santonian-Coniacian(?) floras are present in all the underlying samples down to 1,170 ft. The lower part of the Santonian-Coniacian interval, as I have delineated it in the Myrtle Beach and Calabash wells (fig. 7) is interpreted by Gohn and others (1978a, unit K3) and Brown and others (1979, unit D) to be of Turonian or late Cenomanian age. The age of the strata below the last sample and above basement in the Calabash and Myrtle Beach wells is not known, but Brown and others (1979) indicated that the beds were as old as early Cenomanian and Albian, whereas Gohn and others (197 8a) considered them to be late Cenomanian in age. GEORGIA GEOLOGICAL SURVEY WELLS, SOUTHEASTERN GEORGIA Georgia Geological Survey wells 719, 724, 876, 1197, and 1198 were drilled into the deepest part of the Southeast Georgia Embayment beneath the Coastal Plain (fig. 1). The top of the Cretaceous here lies ~ 2,600 ft below sea level, and it dips very gently seaward. Off- shore, at the COST GE—l site 88 miles to the east, where the Southeast Georgia Embayment is deepest, the Upper Cretaceous surface is ~ 3,450 ft below sea level. In contrast to the Upper Cretaceous surface, the crystalline and metamorphic basement rocks are at ~ 4,600 ft beneath the Coastal Plain and dip more steeply seaward to ~ 11,000 ft at COST GE—l. The off- shore section accommodates a sequence of Lower Cretaceous nonmarine and shallow marine elastic and carbonate rocks approximately 5,000 ft thick that are absent, or at least very poorly represented, beneath the Georgia coast. This series of five wells (fig. 8) has been studied, in part, by Marsalis (1970), Maher (1971), and Gohn and others (1980), and all five wells have been investigated by Brown and others (197 9). A comparison of their inter- pretations of the younger strata shows generally good agreement, but their stage asssignments and terminology differ somewhat in the lower parts of the wells, where Marsalis (1970) and Maher (1971) use the terms Atkinson Formation and Comanchean Provincial Series for strata that are identified by the other authors as representing the Eaglefordian, Woodbinian, Washitan, and Fredericksburgian Stages. The relations already established between the stratigraphic units of Gohn and others (1978a) and Brown and others (1979) in the South Carolina subsurface are also evident in Georgia. Brown and others (1979) showed that strata representative of unit E (W oodbinian) are present throughout the deep part of the Southeast Georgia Em- bayment but apparently are absent north of the Parris Island well in South Carolina. The inclusion of the inter- pretations of Marsalis (1970) and Maher (1971) provides an opportunity to compare the placement of the Atkin- son Formation with regard to the stratigraphic units of Brown and others (1979) and Gohn and others (1980). The GGS 724 and GGS 1198 wells have been documented paleontologically, using calcareous nan- nofossils, planktic foraminifers, and spores and pollen (C. C. Smith and R. A. Christopher, unpub. data, 1978). The upper part of the Maestrichtian section is barren in both wells, but the Maestrichtian-Campanian interval can be differentiated from underlying Santonian strata at ~ 3,750 ft in GGS 1198 and at ~3,900 ft in GGS 724. In both wells, the Campanian—Santonian boundary is near the boundary between units K5 and K4 of Gohn and others (1980) and the boundary between units B and C of Brown and others (1979). The Santonian-Coniacian in- terval is present in GGS 1198 down to ~ 4,300 ft and in GGS 724 down to ~4,450 ft. The age of the upper part of this interval is comparable with age determinations made by Brown and others (197 9) and Gohn and others (1980). However, the lower part of the Santonian- Coniacian in these two wells is equivalent to strata iden- tified by Brown and others (197 9) as unit D (Eagleford- 20 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Well No.20 Well No.2l Well No.22 Penny Royal,S.C. Myrtle Beach, SC. Calabash,N.C. SW ;_37 MILES ‘ 23 MILES ' NE Brown Gohn 3 Brown Gohn 3, Brown Gohn g and others and others 3‘ and others and others 2 and others and others ‘5‘ (l979) (I979a) w ((979) 09780) 8 0979) “9780) <3 SEA LEVEL c . A K6 3 A K6 enozonc / . . . . 0 Maestnchtran Navarroon- . Navarroan Maestnchtlan- N°V°"°°“ Tayloran A K6 . ' Campanlan 0 9 300— : Maestnchtran ' - —‘ Navarroan- Lu . 5 K5 > O 3 "WWW“ “mm" . / B K5 Companion Ta low" “flow". < — 550ft . o y Austinlan g o Tayloran Campanlan 600— —IT5' . _ 5 W 9"?" ' Campaman c ' .1 B Austtman 7 __— 750ft- K4 S ”mm" Austinlan K4 3 c K4 ' 0 ’7 ’ . Austinian '— eto ft TD ’- 7 ’ ‘ C“'"P°"'.°“‘ Austlnian Lu , 7 / Austinian Santonlan “‘ 900— 9 ’ . . _ L; ’ - W3 Santoman- —_ _ K3 Coniacian Eagle- D W . E Santoman— Eagle- D s f°““°" Eagle- 3 Coniacian fordiun E . W fordlan l200- W 2 0/ ’7 -" ? F-burg.I _ ,2 ’ , , Washltan, F 3 I ,’ ’ /- 9 F-burg. ? Basementl l335 ft TD 7—— " ' A [5004 Basement I448 ft TD _ VERTICAL EXAGGERATION xl44 FIGURE 7. — Stratigraphic correlation of the Penny Royal and Myrtle Beach wells, South Carolina, and the Calabash well, North Carolina, based on the results of this study and showing stratigraphic interpretations of Gohn and others (1978a) and Brown and others (1979). Fredericksburgian is abbreviated F-burg. Samples shown were originally part of the study by Gohn and others (1978a). Depth scale is relative to sea level; stratigraphic boundaries are given in original depths (for depth below sea level, subtract 20 ft for the Penny Royal well, 25 ft for the Myrtle Beach well, and 48 ft for the Calabash well). ian; Turonian-upper Cenomanian), by Gohn and others (1980) as unit K2—3 (middle Eaglefordian; upper Cenomanian), and by Maher (1971) as the upper part of the Atkinson Formation. In the GGS 1198 well, three samples (4,340—4,370, 4,500—4,530, and 4,590—4,620 ft) from strata assigned to units E and F by Brown and others (1979) and units K2—3 by Gohn and others (1980) contain spore and pollen assemblages that represent pollen zone IV or a biostratigraphic interval between zone IV and the younger zone V of early Campanian-Santonian- Coniacian age (R. A. Christopher, unpub. data, 1978). I place these beds in the Turonian on the basis of the association of Turonian calcareous nannofossils with pollen zone IV assemblages in the COST GE—l, Fripp Island, Clubhouse Crossroads, Island Beach, and Toms River wells and in the Dallas, Tex. core. As mentioned above, Christopher (in press) has reported post-zone IV—pre-zone V spore and pollen floras from the middle and upper parts bf the Arcadia Park Formation (Turo- nian Age) and the lower part of the Austin Group (Con- iacian Age) in the Dallas core. In contrast, C. C. Smith (unpub. data, 1978) has reported the presence of foraminifers and nannofossils of Cenomanian Age in a sample from 4,530—4,560 ft in GGS 1198, but the nan- nofossil assemblage (from handpicked cuttings) has a decided Turonian aspect (Corollithion achylosum, E'ij2 fellithus eximius, Lithraphidites alatus, Parhabdol'ithus aspen and Podorhabdus albianus). The foraminiferal assemblage from the same sample is composed of species that range across the Turonian-Cenomanian boundary, as well as many species that are restricted to Turonian and younger strata; rotaliporid marker species are absent. Guembel'itm'a harm'si is present, but, as noted above, its range is not precisely known. Similarly, in GGS 724, two samples (4,520 to 4,540 and 4,630—4,640 ft) from the lower 100 ft of the section con- tain spores and pollen characteristic of either pollen zone IV or a post-zone IV—pre-zone V interval, and I in- terpret them to be Turonian in age. A thin interval of somewhat older, undated strata may be present just above basement in these wells. Brown and others (1979) and Gohn and others (1980) have also studied GGS 1197, and the relations among their stratigraphic units that have been recognized in other wells are also evident here. Few samples have UPPER CRETACEOUS STRATIGRAPHY 21 been analyzed paleontologically from this well. Santo- nian and Coniacian(?) strata containing pollen zone V floras are present in samples from 3,640 to 3,650 and 3,830 to 3,840 ft (R. A. Christopher, unpub. data, 1979), and I have observed calcareous nannofossils of Santo- nian Age in samples from 3,800 to 3,810 and 4,000 to 4,010 ft. These beds are part of units C and D of Brown and others (1979) and units K3 and K4 of Gohn and others (1980). A single sample from 4,180 to 4,190 ft, ex- amined by R. A. Christopher (unpub. data, 1979), con- tains a spore and pollen assemblage indicative of either pollen zone IV or a pre-zone V interval, similar to the floras observed in the strata at GGS 724 and GGS 1198 that I place in the Turonian. The remaining two wells studied in this region, GGS 719 and GGS 876, provide a comparison of the stratigraphic interpretations of Marsalis (1970) and Brown and others (1979). In the upper parts of the wells, these authors’ delineation of the Gulf Coast provincial stages is fairly consistent and also is in agreement with the work of both Maher (1971) and Brown and others (1979) on GGS 724. However, the stage assignment of the Atkinson Formation, as recognized in these three wells, varies considerably. The Atkinson Formation, as redefined by Applin (1955), is composed of an upper member of Eaglefordian Age and a lower member of Woodbinian Age. At GGS 719, Brown and others (1979) confined the Atkinson Formation, as delineated by Mar- salis (1970), to unit D (Eaglefordian), whereas at GGS 876, Brown and others (1979) equated the Atkinson For- mation with units D, E, and part of F (Eaglefordian, Woodbinian, and part of the Washitan-Fredericks— burgian). At GGS 7 24, the strata assigned to the Atkin- son Formation (Eaglefordian, Woodbinian) by Maher (1971) have been placed by Brown and others (1979) in units C (part), D, E, and F (part Austinian, Eagleford- ian, Woodbinian, and Washitan-Fredericksburgian). Clearly, it is either very difficult to recognize and cor- relate the lithologic units in this part of the Southeast Georgia Embayment or the Atkinson Formation is diachronous within this small region of the basin. I inter- pret the upper part of the Atkinson Formation of Marx salis (1970) in these wells to be Santonian-Coniacian in age and the lower part to be Turonian and perhaps older. WELLS IN THE AXIS OF THE SOUTHEAST GEORGIA EMBAYMENT The geologic section B-B’ (fig. 9) through GGS 144, GGS 724, and COST GE—l lies along the axis of the Southeast Georgia Embayment (fig. 1). The stratigraphy at GGS 7 24 beneath the Coastal Plain and at COST GE—1 beneath the Outer Continental Shelf has been discussed above. GGS 144 was drilled into the shallow western part of the Southeast Georgia Embay- ment. The Upper-Cretaceous section is twice as thick at GGS 724, which was drilled into the deep part of the basin onshore, as it is at GGS 144, whereas Tertiary strata are equally thick in both wells. Studies of GGS 144 by Applin (1955) and Applin and Applin (1967) revealed that the Upper Cretaceous section is about 900 ft thick and that Navarroan strata are absent. The absence of the Navarroan Stage in this well and to the west on the Suwannee Saddle and in part of the Southwest Georgia Embayment has been interpreted as being due to nondeposition in an uplifted area (Applin and Applin, 1967) or to post-Cretaceous erosion (Toulmin, 1955). Tayloran and Austinian rocks are present in GGS 144, as shown by both Applin and Applin (1967) and Brown and others (1979). Lower in the well, the Atkinson Formation has been divided into an upper member of Eaglefordian Age and a lower member of Woodbinian Age by Applin (1955) and Applin and Applin (1967); Brown and others (1979) interpreted this part of the section similarly. Applin (1955) discovered a foraminifer assemblage, the “Barlow fauna,” in a core (3,709—3,7 19 ft) from the lower member of the Atkinson Formation in GGS 144. The “Barlow fauna” contains the planktonic foraminifer Rotal’ipom cushma’n'i, and Hazel (1969) concluded that the fauna is late Cenomanian in age and that therefore at least part of the lower member of the Atkinson Formation is of Eaglefordian rather than Woodbinian Age. The distinc— tive “Barlow fauna” is present in other wells in Georgia, Alabama, and Florida to the west and south of GGS 144 (Applin, 1955; Applin and Applin, 1967), but it has never been reported from wells drilled into the southeast Georgia Embayment proper or on the Cape Fear Arch to the northwest. Cenomanian strata have not yet been recognized at GGS 724, but offshore at COST GE—l the Cenomanian may be represented by a thin unfossiliferous interval between rocks of Turonian and Albian Age. Turonian strata at COST GE—l contain a pollen zone IV flora; I in- terpret Turonian strata to be present also at the base of GGS 724 in unit K2 (middle Eaglefordian; upper Cenomanian) of Gohn and others (1980), where a spore and pollen flora indicative of pollen zone IV or pre-zone V has been reported (R. A. Christopher, unpub. data, 1978) The pre—Cretaceous basement dips seaward along the section shown in figure 9. Beneath the Coastal Plain, the basement is overlain unconformably by a thin (up to 150-ft) interval of unfossiliferous strata that is possibly Late Cretaceous in age. In contrast, a thick (~ 7,500 ft) sequence of Early Cretaceous and older sedimentary and metasedimentary rocks is present beneath the Outer Continental Shelf; these rocks extend farther 22 UPPER CRETACEOUS SUBSURFACE STRATIGRAPHY, STRUCTURE, GEORGIA, SOUTH CAROLINA Well No.4 Well No.5 668 876 668 H98 W L l0 MILES E ' SW 25 MILES Brown ,, m Gohn Brown an is a and and others 2E 5 others others (l979) 2°“ 5% (I980) (l979) 2300 \ ? \ A \ ' '5‘ ? \ A Cenozoic Cenozmc ‘ 3; \ (part) 53 A ~ ~ ? * ~ ~ '2 _ Q . = A 2700— 5, K6 A g 2 Z _ s s A 2 <3 g A E s c . . 2 ' § ‘5 MaestrIchlIon- d SlOO— = g . > 3 2 Common Lu 0 A B o E‘ <[ . [— LlJ - = a) E g B 3 2 E. ' O >‘ c d I3 '— ' K5 5 ca 3500'- I = :3 F- . =.E >‘ a c 35 '5’ LL 1; ? \ \ . :3 z - .5 s 3750ft — '— c I“ 3 E - S a I— 3 77> K4 '5 CI. = . t: 1: Lu <12 _9 to Q 3900’“ D .5 <=[, L S - 3 . Egg D SantonIon- me ' I , . _ :5 «2-325 ConIacmn o-— . EVE 'VW\ 2‘5 \ ? (fie :(OQd’E 52?, ' \ \7 . ' 5 ~ <‘D._ 4300— '“"‘" “LL ' 4300ftQ-Bfla F LE. o Wb. E Wash., “' F . F-burg. ngetr Wash., TuronIan re. @ F—burg Basement © ? 4600 ft TD 3mm“ 4700) EXPLANATION —'|_ A Barren Sample 47|0 fl TD (9 Pollen zone IE or pre-zoneIfloro VERTICAL EXAGGERATION x70 FIGURE 8.—Stratigraphic correlations of Georgia Geological Survey (GGS) wells 876, 1198, 724, 719, and 1197 in the Southeast (1971), Brown and others (1979), and Gohn and others (1980). Wb, Woodbinian; Wash., Washitan; F—burg., Frederieksburgian; or pre-zone V flora identified by R. A. Christopher (U SGS). Depth scale is relative to sea level; stratigraphic boundaries are given UPPER CRETACEOUS STRATIGRAPHY Well No.6 Well No.7 Well No.8 GGS 724 GGS 7l9 GGS l|97 ' 6MlLEs—J——lo MIL£s——l NE Brown Gohn u) Brown 3A u, Gohn Brown Maher and and i and En? a and and (I9?!) others others 5 others 32 5 others others ([979) 0980) w (I979) 2““ 5’) (I980) ((979) 5‘ . . Cenozoic Cenozorc Cenozorc ( a f) A . 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AREA WITHIN INTENSITY VI CONTOUR, IN SQUARE CENTIMETERS EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 7 EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES SANTA BARBARA EARTHQUAKE OF JUNE 29, 1925 (ROSSI-FOREL INTENSITIES; BYERLY, 1925) Known parameters: (1) k = 1% (epicenter in western California) (2) C = 25 (normal depth for California earth- quakes) (3) Location of faulting (defined by observed in- tensities) Unknown parameter: (4) 2L Location of the fault break and its length are con- strained by the observed intensities. In a region where k = 1%, intensity values of IX or greater extend only a comparatively few kilometers laterally from the fault break, particularly when fault breaks are in the range of 30 km. Therefore, the pattern of published isoseis- mals (fig. 3) requires a break length of 30—40 km and a location of the break near or just onshore. Fault lengths of both 30 km and 40 km were investi- gated; figure 3 illustrates predicted intensities for 2L = 40 km. The observed and reported intensity values are in good agreement, and the reported values are as- sumed to be relevant to saturated alluvium because nearly all reporting localities were on alluvium. The SANTA BARBARA EARTHQUAKE JUNE 29, 1925 120°30' 120°00' 119°30' I l 35°00 ’ 34°30’ PAC/['10 OCEAN 0 50 KILOMETERS ;._1_|__|__] FIGURE 3.—Reported and predicted R/F intensity values for Santa Barbara earthquake, June 29, 1925 (2L = 40, C = 25, k = 1%). Reported values are in Roman numerals. detailed shape of the intensity VIII contour from Byerly (1925) is quite certainly the result of trying to include Within one contour line all VIII observations, even though these are distributed on both sides of the Santa Ynez Mountains. The solid or predicted intensity lines on figure 3 are based on saturated alluvium and do not indicate geologic factors. If these are taken into account, the predicted shape of the intensity 8 contour, if the presence of the Santa Ynez Mountains is ignored, is as reported by Byerly (fig. 4). Figure 5 indicates the detailed pattern of predicted intensities when using a 1/2-minute by 1/.2-minute geologic grid. Byerly (1925) acquired intensity data only along or near the present route of Highway 101, that is, along or near the road extending from Ventura through Santa Barbara to Santa Maria and San Luis Obispo. Thus, no observations were collected for the entire northeast quadrant. The absence of values in this area does not indicate low intensities but lack of data. Note on figure 3 that the predicted area of intensity VI reaches only as far to the northwest as was ob- served. Shortening of the fault by a factor of2 (to 2L = 20) would yield predicted intensities that are too low for stations north of the mountains. A comparison of predicted versus observed intensities versus length of 2L is given in table 5. Even for 2L = 40, predictedrvalues may be slightly too low. Attempts to estimate 2L by size of the reported IX area are complicated by the fact that there may not have been reports at the full range of actual IX—level shaking. The reported length of the IX area is 42 km, but a 2L of 40 km predicts a IX-length of 74 km and a 2L of 30 km predicts a IX—length of 55 km. These val- ues would suggest a 2L of 20 or less, which would markedly disagree with the data of table 5. The seemingly most reasonable conclusion is that the length of the IX region for saturated alluvium is not expressed by Byerly’s (1925) contourng and that the individual station reports at greater distances provide the best basis for estimating the fault length (2L). Thus, we conclude that a 2L of 30 to 40 km is in near agreement with observations, and an estimate of 2L 2 40 is favored. MONTEREY BAY EARTHQUAKE OF OCTOBER 22, 1926 (ROSSI-FOREL INTENSITIES; MITCHELL, 1928) Known parameters: (1) k = 1% (western California) (2) C = 25 (normal depth) (3) Location of fault break (aftershocks-main epicenter) 8 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES (4) 2L = 20—40 (maximum length of 40 by after- shocks) The parameters of this earthquake can be set quite well without recourse to intensity data. However, an apparent discrepancy between 2L values based on af- tershocks and on intensity values can be shown. The epicenter of the main shock is well controlled and is just offshore from Monterey. Aftershock locations, based on S—P times at Berkeley, were placed as far north as the coastline west of Santa Cruz (Mitchell, TABLE 5.—Observed and predicted intensities, Santa Barbara earth— quake Site Observed Predicted intensityl intensity 2L : 40 km 2L : 30 km San Luis Obispo ,,,,,,,,,,,,,, IV 4(6) 4(5—6) Pismo ,,,,,,,,,,,,,,,,,, "VI 5(6) 5(5) Arroyo Grande ,,,,,,,, __;VII 6(6) 6(6) Nipomo _______________ “VII 5—6(6) 5—6(6) Santa Maria ,,,,,,,,,,,,,,,,,,,, VII 6—7(7) 6(6—7) Orcutt ,,,,,,,,,,,,,,,,,,,,,,,, VII 7(7) 6—7(6—7) Los Alamos ,,,,,,,,,,,,,,,,,, VIII 7(7—8) 6—7(7) Lompoc ______________________ iVI 7(7) 6(6) Los Olivos ____________________ VIII 8(8) 8(8) Gaviota ,,,,,,,,,,,,,,,,,,,,,, VIII 7— 8(8—9) 7(8) Goleta ,,,,,,,,,,,,,,,,,,,,,,,, IX 8(9) 8(9) Ventura ______________________ VII 7— 8(7—8) 7(7) Santa Barbara ________________ X 8(9) 8(9) ‘First number incorporates ground condition as on (5-minute by 6-minute grid. Second number is for saturated alluvium. 1928)/(fig. 6). If the aftershock zone is deemed a measure of length of faulting at time of the main earth— quake, a 2L of 44 km results. For comparison, we calcu- lated predicted intensity values for 2L values of 22 and 44 km, both fault breaks extending northward from the epicenter of the main event (see table 6 and fig. 7). Table 6 presents observed and predicted R/F intensities for 2L values of 22 and 44 at various sites. The stations are arranged by increasing latitude, and the two modeled fault breaks are indicated in proper latitudi- nal relation to the stations. Both models show excellent agreement between pre- dicted and observed intensities for stations south of the breaks. For stations at the same latitude or more northerly latitudes, the intensities predicted by a 2L of 44 are clearly too high. Average observed intensity for these 14 stations is 5.2, average predicted intensity for 2L of 22 is 5.6, and average predicted intensity for 2L of 44 is 6.3. Our conclusions are that a fault break of 22 km is an appropriate length to use for modeling the high-frequency source of the main event and that this source was the southern portion of the aftershock zone. One might, of course, suggest use of a 2L of 44 km with the energy density reduced below the normal value (Evernden, 1975). Intensities to the north would still be predicted as too high. SANTA BARBARA EARTHQUAKE JUNE 29,1925 2L=30 km, C=25, k=1% 121° 120° ‘ 119° 0 121° 120° 119° 100 KILOMETERS |____—__1_____l FIGURE 4.—Predicted R/F intensity values, 6-minute by 6-minute grid, Santa Barbara earthquake, June 29, 1925 (2L = 40, C = 25, k = 1%). A, Saturated alluvium. B, 6-minute by 6-minute ground-condition units (table 2). EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 9 A point of possible significance is the prediction of lent physical properties and of appreciable depth with too-high intensities for San Jose, Morgan Hill, and the water table at the surface. This is a gross simplifi- Palo Alto. In the presently used codification of geologic cation that will lead to prediction of excessively high maps, all Quaternary deposits are treated as of equiva- ‘ intensities in regions of thin and (or) unsaturated and 120°50’ 35°00’ SANTA BARBARA EARTHQUAKE JUNE 29, 1925 2L=4O km 120°40’ 120°30’ 120°20’ 120°10’ 120°00’ 3P50 -- 3P4W -- 34°30’ -— L————————4————————J 686666666666666777777 777777 77777977888888 I éébbéobfiébéébbcbe 777777774744555555 55 5 555555 5 4555 ’9: D. 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Computer plot of south half of Santa Maria sheet of Geologic map of California. 2L = 40. 10 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES (or) physically different materials. Thus, for the sand- covered areas of much of western San Francisco, the use of a formulation thatassigns all Quaternary mate- rials the same ground condition leads to predicted in- tensities for the 1906 San Francisco earthquake that are much too high (9+ predicted, 7+ observed). In this case, thin unsaturated sand on bedrock (Franciscan Formation) reacted as essentially bedrock. Without de- tails of local geology, we chose in this paper to predict the worst case while advising everyone of that fact and suggesting more refined work in local areas so that improved estimates of expected intensity values in areas of Quaternary deposits can be made. Palo Alto circa 1926 was nearly entirely on ground now characterized as Older Bay Mud. It is stronger than Young Bay Mud. In Evernden, Hibbard, and Schneider (1973), the Young Bay Mud (F) was assigned relative intensity of (+1/2), while Older Bay Mud (E) was assigned a value of (—1/2). MONTEREY BAY EARTHQUAKE OCTOBER 22, 1926 123° 122° 121° 120° 119° 39° 38° 37° 36° 35° 100 KILOMETERS 3,, I l I l FIGURE 6.—Location of main shock, aftershocks, and isoseismals, Monterey Bay earthquake, October 22, 1926. Morgan Hill and San Jose are located in the Santa Clara Valley on thick valley alluvium. If the ground were saturated, these areas would be expected to reach nearly the intensity values predicted for our stand- ardized Quaternary (J of table 2). However, by the time of the 1926 earthquake, the water table in the San Jose area had been lowered by several tens of meters. It may be relevant to point out that predicted intensities for San Jose resulting from the San Francisco 1906 earthquake were correct but those for 1926 were too high. The predictions for San Jose and Morgan Hill for the Monterey Bay earthquake are probably too high because the present model fails to incorporate depth to water table in its predictions. Thus, any predictions for a coming earthquake based on our present modeling of Quaternary deposits should be lowered by at least one intensity unit for parts of the Santa Clara Valley in which the water table has been lowered 10 m or more. One of the most effective ways to protect a community from high intensities (VIII+) might be to lower the water table several tens of me- ters. Shaking intensities of greater than R/F VII—VIII are probably impossible under such conditions. SAN JOSE EARTHQUAKE OF JULY 1, 1911 (ROSSI-FOREL INTENSITIES; TEMPLETON, 1911) Known parameters: (1) k = 1% (western California) (2) C = 25 (normal depth) (3) Location (Wood, 1911) (4) 2L = 5—11 (aftershocks) TABLE 6.—0bserved and gredicteré R/F}; intensity values, Monterey ay eart qua e Site Observed Predicted intensity Intensity 2L¢22 2L=44 Santa Maria __________________ III 3—4 4 Lompoc ______________________ II— III 3 3 San Luis Obispo ______________ IV 3 3 Paso Robles __________________ IV—V 4 5 King City ____________________ V— VI 5—6 6 Carmel ______________________ VI 6 6 Monterey ____________________ VI + 6— 7 6— 8 Salinas ______________________ VI—VII 7 8 Hollister ______________________ IV—V 6 7 Watsonville __________________ V— VI 7 8 Soquel ________________________ VI— VII 7 8 Santa Cruz ____________________ VII+ 7-8 8 Saratoga ______________________ V 5 7 San Jose ______________________ V 6 7 Morgan Hill __________________ V 6 6 Palo Alto ______________________ V 6 7 San Leandro __________________ V 5 5 Berkeley ______________________ V 5 6 Walnut Creek ________________ V 4—5 5 Novato ________________________ V 5 5 Petaluma ____________________ V 4— 5 4— 5 Santa Rosa ____________________ IV 4 5 Martinez-Concord ______________ V/IV—V 5 5 Stockton ________________ 5 5 Merced __________________ 4 + 5 San Francisco (downtown) ____________________ 5 5 EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 11 This earthquake is of interest because it is one of the very few earthquakes of approximately magnitude 6 that has occurred in central California since 1906. The location of this earthquake is as suggested by Wood (1911). To quote from him (p. 39) “Examination of the map and its explanatory table shows clearly that the circles of origin-distance for a majority of the shocks (and also, in the main, those most reliably determined having Mt. Hamilton at center) intersect the projected course of the Hayward fault at frequent intervals all the way from a point due south of Mt. Hamilton to a point due north of Gilroy, a distance of about 12 km along the course of the fault.” Though more recent geologic mapping has led to the interpretation that the region considered by Wood to contain an extension of the Hayward fault actually contains an extension of the Calaveras fault, his basic mode of estimating fault length is still valid because the Calaveras fault in this part of its course lies exactly where Wood considered the Hayward fault to be. We modeled this earthquake MONTEREY BAY EARTHQUAKE OCTOBER 22, 1926 2L=22 km, C=25, k:1% 123° 122° 121° 120° 123° 122° 121° 120° 0 100 KILOMETERS ;__;_l FIGURE 7.—Predicted R/F intensities for Monterey Bay earthquake of October 22, 1926 (2L = 22, C = 25, k = 1%). A Saturated alluvium. B, 6-minute by 6-minute ground condition. y 12 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES using two different values of 2L, a value of 11 to agree with the aftershock zone, and a value of 51/2 as half of that zone; we chose to use the south half of the after- shock zone. Table 7 gives a comparison of observed and predicted intensity values for 2L values of 11 and 51/2. The major unexplainable discrepancy between re- ported and predicted values is the area of San Fran- cisco, for which Templeton gives a value of VI— VII; the model predicts only V on saturated alluvium. The re- ported San Francisco intensity is inconsistent with the entire pattern of other observed values. In order for intensity VII to be predicted at San Francisco, this earthquake would have required a 2L of 75, a totally inadmissable value in View of the absence of surface breakage and of the inconsistency of such a length with all other intensity data. In addition, McAdie (1911) re- ported “There was * * * no damage of any consequence in San Francisco.” “Few objects were overturned * * *”. The VI-VII value assigned by Templeton must be in error. Gilroy, Watsonville, and Santa Cruz are all pre- dicted to have experienced higher intensities than re- ported, the predicted intensities (table 7) having been derived from the assumption that these communities are situated on saturated alluvium. The explanation for these too-high predictions may be that the appro- priate ground condition for these sites is less sensitive than saturated alluvium. Both San Jose and Morgan Hill experienced inten- sities as predicted for saturated alluvium, in contrast TABLE 7.—Observed and predicted R/F intensities, San Jose earth— quake Site Observed M intensity 2L=5‘/2 2L=11 Modesto ______________________ 4 5 6 Sacramento __________________ 4 4 4 Santa Rosa ____________________ 4 3—4 4 Monterey ____________________ 4 3— 5 3— 5 Berkeley ______________________ 5 5 5 Hayward ______________________ 5 5 5— 6 Stockton ______________________ 5 5 5 Watsonville __________________ 5 16—7(A1) 7(A1) Santa Cruz ____________________ 6 6—7(A1) 7—8(Al) Belmont ______________________ 5 5 6 Pleasanton ____________________ 5— 6 5 5 Livermore ____________________ 5 5 5—6 Oakland ______________________ 6 5 5— 6 Redwood City ________________ 6 5 6 Palo Alto ______________________ 6 6 6 Calaveras Valley ______________ 6 6 6 San Martin __________________ 6 6 7 Gilroy ________________________ 6 7— 8(Al) 8 Boulder Creek ________________ 6 5-6 6 Pescadero ____________________ 6— 7 5 6 San Francisco ________________ 6—7 5(Al) 5(Al) Morgan Hill __________________ 7 7 8 Los Gatos ____________________ 7 7 7 Saratoga ______________________ 7 6—7 7 Santa Clara __________________ 7 6—7 7 San Jose ______________________ 7 7 7 Coyote ________________________ 8+ 8 8 ‘(All signifies that predicted intensity values entered in table are based on saturated alluvium, Discussmn of the discrepancies between observation and prediction for these stations is included in the text. to the prediction for the Monterey Bay earthquake, thus supporting the conjecture made earlier about the effect of lowering of the water table in the Santa Clara Valley between 1906 and 1926. The observed data appear to agree better with a 2L of 51/2 than with one of 11, and averaging of observed and predicted values would suggest a 2Lenearer 51/2 than 11. FORT TEJON EARTHQUAKE OF JANUARY 9, 1857 (MODIFIED MERCALLI INTENSITIES; AGNEW AND SIEH, 1978) Known parameters: (1) k = 1% (region of concern south and west Of San Andreas fault) (2) C = 25 (normal depth) (3) Location of faulting (San Andreas fault) (4) 2L = 320 (surface breakage—Cholame to Cajon Pass) This earthquake has been reanalyzed (Evernden and others, 1973) using the model of Evernden (1975) and using a k value of 1%. The most interesting aspect of this study is that, thanks to the labors of Agnew and Sieh (1978), there is now available a compilation of numerous intensity observations for this earthquake. Intensity values experienced at numerous sites in California can now be compared with predicted values. Table 8 indicates Rossi-Forel intensity values esti- mated by us using the data of Agnew and Sieh (they chose to use M/M intensities) and R/F intensity values predicted on saturated alluvium for a fault break ex— TABLE 8.—0bserved and predicted R/F intensities, Fort Tejon earth- quake Site Observed Predicted intensity intensity San Diego __________________________ V— VI 5 San Bernardino __________________ VII— VIII 8 San Gabriel Valley ____________________ VIII 8 Los Angeles (downtown) ______________ VII+ 7 (high) San Fernando Valley ________________ VIII— 8 34.6°N. 117.4°W. ____________________ VIII— 8 34.1°N. 119.0°W. ____________________ VIII— 7 Ventura ____________________________ VIII + 8 Santa Barbara ________________________ VII 7 San Andreas fault ____________________ = IX 2 9 Fort Tejon ________________________ VIII—IX 9 34.0°N. 118.7°W. ______________ VIII+ 9 35.4°N. 119.0°W. ____________ VIII+ 9 35.9°N. 119.3°W. __________ __VI-VII 7 36.2°N. 119.3°W ______________________ VII+ 7 Visalia __________________________ VII— VIII 6 36.7°N. 121.3°W. ______________________ VII 6 Monterey __________________________ IV— V+ 4— 6 Santa Cruz ________________________ III—V + 3— 5 San Francisco ________________________ V+ 4 Stockton ______________________________ IV 5 Sacramento __________________________ V + 41 5— 62 ‘All path k: 1%. 2Part ofpath k=11/2) EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 13 tending along the San Andreas fault from lat 34°18.3’ N., long 117°3.15’ W. to lat 35°55.0’ N., long 120°27.9’ W. Agreement between prediction and observation is excellent, there being virtually no sites at which ob- served and predicted R/F intensities differ by as much as one (1) intensity unit. The data for the 1857 earthquake substantiate the prediction that the peak intensity to be expected in the Los Angeles area from a great earthquake on the San Andreas fault is R/F VII/VIII, San Fernando Valley and eastern Los Angeles experiencing possibly VII. These intensities presume a zero depth to water table. The marked lowering of the water table in much of this area in the intervening 120 years should result in peak intensities VI/VII in most alluviated areas of the San Fernando Valley and Los Angeles. We have modeled this earthquake in detail using the 1/2-minute by l/a-minute grid. Plate 1 indicates geology of the area and also indicates predicted R/F intensities with ground-condition corrections of table 2 applied and assuming zero depth to water table in alluviated areas (J regions of plate 1). This plate is as published by Blume and others (1978); they used our predictions in constructing it. As pointed out by Algermissen (1973) and substan- tiated by Blume and others (1978) and this study, a repeat of the great 1857 Fort Tejon earthquake will not be a disaster of the magnitude sometimes imagined. San Fernando Valley will suffer less from an 1857 re- peat than it did from the San Fernando 1971 earth- quake. The remoteness of the San Andreas fault from heavily urbanized areas in southern California and the high rates of attenuation in the region will result in a repeat of the 1857 earthquake having a surprisingly small impact on the area as a whole. This conclusion is supported by results given in a later section in which predictions of losses for numerous potential California earthquakes are given. A fact worth mentioning here is a basic disagree- .ment of the general near-fault patterns of predicted intensity shown on plate 1 and previously presented for the San Francisco 1906 earthquake (Evernden and others, 1973) with the reported pattern shown by Law- son (1908, maps 21—23) for the San Francisco earth- quake. Lawson shows narrow zones of all intensities as the fault is approached, irrespective of ground condi- tion and any ideas of attenuation as linked with depth of focus. No model incorporating legitimate values of attenuation and depth of focus can predict such pat- terns as shown by Lawson. In addition, there is total absence of data within Lawson (1908) to support the near-fault intensity contouring on his maps. In fact, the data of his study specifically refute his contouring. Apparently, Lawson worked under the assumption that all intensities must occur between VI and X—XI, even though intensities VI through IX are largely de- fined by shaking criteria whereas X and IX are defined by ground rupture. It is perfectly possible to have sever- feet of displacement associated with shaking inten- sities on only VII—VIII. It is our belief that Lawson’s near-fault contouring is almost totally a derivative of misconception and is quite erroneous. No modeling of expected intensities for a repeat of 1906 or any other earthquake should incorporate near-fault patterns of intensity as shown by Lawson. LONG BEACH EARTHQUAKE OF MARCH 10, 1933 (MODIFIED MERCALLI INTENSITIES; NEUMANN, 1935) Known parameters: (1) k = 1% (western California) (2) C = 25 (normal depth) (3) Location of faulting (aftershocks) (4) 2L = 22-44 (S and P travel times, aftershocks) This is an interesting earthquake for a variety of reasons, a principal one being that it was the first major earthquake to be reported by the U.S.G.S. in Modified Mercalli units ofintensity. Thus, it is the first significant earthquake without reports of intensity IX in the epicentral region. It is certain that, if this earth- quake had been reported in units of Rossi-Forel inten- sity, a clearly defined region ofintensity IX would have been defined, and this earthquake would have been accorded greater status in the hierarchy of historical California earthquakes. Two possible models for this earthquake seem ap- propriate. The first is to make 2L equal to the after- shock zone, that is, about 40 km, as reported in Hile- man, Allen, and Nordquist (1973). The second is to fol- low Benioff (1938) and use a 2L of about 27 km, an estimate based on comparison of S-P arrivals at south- ern California stations, the solution being restrained to lie along the Newport-Inglewood fault as indicated by the aftershocks. In our initial studies, we used 2L val- ues of 22 and 44 km with the south end of both models being at the epicenter (k of 1.750). Table 9 gives ob- served and predicted Modified Mercalli intensities for both 2L values. Assuming that all sites in alluvial plains or on beaches behaved as for saturated al- luvium, a 2L value of 22 is indicated as appropriate. If the 6-minute by 6-minute ground condition is used, the values in square brackets are predicted for San‘ Clemente, El Toro, and San Diego; these values are in better agreement with observed values. As to the ap- propriateness of the assumption of saturated ground near Long Beach, Wood (1933) points to the correlation between “bad natural ground” and "deep water-soaked 14 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES alluvium.” The general over-prediction of intensities in the V11 zone when using a 2L of 44 would seem to imply too great a 2L because most of these sites were on a1- luvium. However, if pumping had lowered the water table to a 10—m depth, a 2L of 44 would be a better value. We conclude, on the basis of information in hand, that a 2L of 22 is more likely than a 2L of 44. Estimating the losses to be expected from a repeat of this earthquake depends strongly on knowledge of the depth to the water table in 1931 and today. Wood (1933) comments on the absence of intensity IX values as being an indication of the small size of this earthquake. However, figure 8 makes clear that the change in definition of intensity units as of 1931 was the real reason for the absence of reported intensity IX values for the Long Beach earthquake. TABLE 9.—Observed and predicted Modified M ercalli intensities, Long Beach earthquake [All values for saturated alluviuml Site Observed Predicted intensities intensity if 2L:22 2L:44 Anaheim ________________________ 8 7/8 8 Bellflower ______________________ 8 7 8 Costa Mesa ____________________ 8 8 8 Cypress ________________________ 8 7 8 Garden Grove __________________ 8 8 8 Huntington Beach ______________ 8 8 8 Newport Beach __________________ 8 8 8 Santa Ana ______________________ 8 8 8 Seal Beach ______________________ 8 8 8 Signal Hill ______________________ 8 8 8 South Gate ______________________ 8 7 8 Willowbrook ____________________ 8 7 8 Torrance __________ 7 8 Redondo Beach "A- 7 8 Norwalk ............ 7 8 Manhattan Beach 7 7 East Los Angeles ________________ 7 7 8 Lomita ___________________________ 7 7 8 Laguna Beach __________________ 7 7 8 Huntington Beach ______________ 7 7 8 Artesia ________________________ 7 7 8 Fullerton _______________________ 7 7 8 Alhambra ______________________ 6 6 7/6 Beverly Hills ____________________ 6 6 6 Covina ___________________________ 6 6 7 Culver City ____________________ 6 6 6 Fillmore ________________________ 6 5 5/6 Gardena ________________________ 6 7/6 8/7 Glendale ________________________ 6 6 6 Montebello ______________________ 6 7/6 8 Oxnard _________________________ 6 5 5/6 Pasadena _______________________ 6 6 7 Placentia _- 7 7 . Pomona _____ 6 7 Santa Monica 6 7 Simi _____________________________ 6 5 6 Ventura ________________________ 6 5 5 Whittier ________________________ 6 7 8 San Clemente __________________ 5 7/6 [5]' 7 Escondido _______________________ 5 5 — Moreno _________________________ 5 5 — El Toro _________________________ 5 7/6 [6]‘ — Cardiff-by-the-Sea ______________ 4 5 — Carlsbad ________________________ 4 5 — Santa Maria ____________________ 3 3/4 — San Diego _______________________ 3 5 [31‘ — 1[ ] 6-minute by 6-minute ground condition As an example of the further refinement in estima— tion of event parameters apparently possible by use of the statistical model described earlier, we analyze the data of the 45 reporting stations via the several criteria mentioned earlier. We consider the earthquake to have been on the Newport-Inglewood fault, so the only pa- rameters evaluated are length of break (2L), position on the fault line (S), and the appropriate k value, there being the possibility that actual k values in any given area of western California are slightly different from the 1% value routinely used for this region. Table 10 presents the results of these calculations (A through D are for k = 1.750, and E, F, and G are for k = 1.825). Table 10A, presenting values of (H-L) and |H-L1, indicates (H-L) to have a zero value at about 2L = 22 for S = —8 and 2L = 25 km for S = —12. We include within the dashed line the most likely 2L/S values. Table 10B presents (Obs-Cale) values, the minimum value being at 2L = 22, S = —8. The s.d.(0bs_ Cale, is such that a great range of 2L/S values are per- missible at 95 percent confidence (area within dashed lines). Table 100 gives lObs-Calcl values, the minimum value being at 2L = 22, S = —12, with a large range of 2L/S values permissible at 95 percent confidence. Table 10D presents CP values, the minimum being at 2L = 22, S = —4 with 2L = 22 and 4 2 S 2 —8 as well as 2L = 24, —4 2 S 2 —12 acceptable at 95 percent confidence. The only area of overlap of all criteria at 95 percent confidence is 2L = 24, —8 2 S 2 —12. The solution 2L = 24, S = -10 has its south and north termini at latitudes 33°38.9' N. and 33°48.8’ N., respectively. These are to be compared with the re— ported latitude 33°37’ N. of the Long Beach earthquake with the aftershocks extending srom 33°35’—37’ N. to 33°51’—53’ N. With the best solution for k = 1.750, there was a slight tendency for mean (O-C) values in each intensity band to be function of O values and thus of distance, implying a slightly incorrect value of k. Therefore, we redid the analysis using a k value of 1.825; tables 10E and F show these results. The best solution via (H - L) and |H — L| (table 10E) is 2L = 34—38 and s = —8. Table 10F, based on CP values, gives as a best solution 2L = 34, S = —4. Given the calculated CP and s.d.Cp values for these coordinates (CP=.079, s.d.Cp=.012), all so- lutions based on k = 1.750 are rejected at 99 percent probability. At k = 1.825, solutions within the 95 per- cent confidence area have 2L = 32—36 and 4 2 S 2 —12. Table 10G gives an abbreviated listing of R.M.S., CP, and (H - L)/|H - L i values, indicating essential agreement as to the best event parameters. The follow- ing table compares predicted location of fault break for 2L = 34, S = —4 and —6 (CP solution and average of H - L and CP solutions) and observational data. EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 15 Latitude south end Latitude north and Solution 2L = 34, S = —4 33°39.3’ N 33°53.4' N Solution 2L = 34, S = —6 33°38.5’ N 33°52.5’ N Main shock epicenter 33°37’ N — Aftershocks 33°35'—37’ N 33°51’—53’ N The marked decrease in GP value, the better fit to the aftershock zone, and the elimination of the depen- dency of residuals upon distance suggest that the solu- tion based on k=1.825 is superior to that based on k=1.750. BRYSON EARTHQUAKE OF 21 NOVEMBER 1952 (MODIFIED MERCALLI INTENSITIES; MURPHY AND CLOUD, 1954) Known parameters: (1) k = 1% (western California) (2) C = 25 (normal depth) Unknown parameters: (3) Location of epicenter (4) 2L This earthquake is of interest for two reasons. First, it is the only historical earthquake of significant mag- nitude that has occurred between the Lompoc and Monterey Bay earthquakes, though it was located on a different fault than either of these. Second, the re- ported intensities are apparently anomalous at first glance because intensities of VI and VII were reported to significant distances but no intensity VIII was re- ported. Even though the reported values were Modified Mercalli rather than Rossi-Forel, the reported inten- sity pattern appears anomalous. However, there is a logical explanation. Table 11 presents observed and predicted M/M intensities for two different models (2L = 20 and 40 km) of this earthquake. The fault break is distributed equally on either side of the calculated epi- center. The NOAA epicenter (Murphy and Cloud, 1954) is lat 35.8° N., long 121.2° W. Recently, the loca- tion of this event was recalculated (W. V. Savage, oral commun., 1979), the result being lat 35°47.9’ N., long 121°11.4’ W., while Bolt and Miller (1975) gave the location as lat 35°44’ N., long 121°12’ W. and assigned it a “b” quality. These locations agree with the NOAA epicenter but have an uncertainty of 10—15 km in the northeast-southwest direction. All predicted intensity values in table 11 are for ground condition according to the 6-minute by 6-minute California grid. Some entries show the esti- mated alluvium value included in square brackets. En- tries designated (Al) indicate either that the assumed ground condition is saturated alluvium on the 6-minute by 6-minute grid, although this assumption may be inappropriate (predicted greater than observed; San Simeon, for example), or that the peak value pre- LONG BEACH EARTHQUAKE MARCH 10, 1933 2L=22 km, C=25, k=1% 0121“ 120° 119° 118° 117° 1 A M/M lntensities— Saturated alluvium R/F lntensities— Saturated alluvium 50 100 KILOMETERS FIGURE 8.—-—Predicted M/M (A) and R/F (B) intensities for the Long Beach, Calif, earthquake of March 10, 1933 (2L = 22, C = 25, k = 1%). 16 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES TABLE 10.—Calculated parameters for the Long Beach earthquake A Reference Fault: Newport-Inglewood Reference Coords: 33°48.0’ N 118°10.7’W k=1.750 T=0 C=25 Bands Used: VIII, VII, VI, V, IV, 111 Upper Values: (H - L) Lower Values: IH - LI 2L 8 12 16 20 24 28 32 8 ' —14 —7 —1 2 6 9 16 17 17 16 16 19 4 —16 —7 —1 3 7 10 16 17 13 15 17 16 0 —20 —8 —1 3 7 10 20 16 13 16 15 15 —4 —22 —11 —2 4 9 13 22 15 14 14 11 13 —3 ~21 —14 —6 2 8 12 21 18 16 12 10 12 —12 —22 +15 —11 —2 6 11 22 19 15 12 10 11 —16 -22 —15 —12 —7 1 7 24 17 16 15 13 13 2L = 22, s z —8: (H-L) = 0,1141]: 12 B Reference Fault: Newport-Inglewood Reference Coords: 33°48.0’ N 118” 10.7' W k=1.750 T=0 C=25 Bands Used: VIII, VII, VI, V IV, 111 Upper Values: (0 — C) Lower Values: s.d. m _ 0 2L S 12 16 20 24 28 32 8 .402 .209 .067 —.068 —.176 —.275 (.100) 4 .400 .206 .062 —.073 —.178 —.276 (.092) (.092) 0 .410 .216 0.69 —.066 ~.173 -.272 (.085) (.085) —4 .432 .238 .090 —.047 —.154 —.253 (.080) (.080) —8 , .468 .272 .124 —.015 —.127 —.225 (.076) (.077) —12 .512 .316 .167 .028 —.087 —.187 —16 .565 .370 .219 .079 —.033 —.139 2L = 23, S : —8: (O —- C) = O, s.d.“) 1“, = .076 C Reference Fault: Newport—Inglewood Reference Coords: 33°48.0' N 118° 10.7’ W k=1.750 T=0 C=25 Bands Used: VIII, VII, VI, V, IV, III Upper Values: I O — C ILower Values: s.d. I0 1 r I 821‘ I 12 I16 I 20 24 I 28 I 32 8 .644 .578 .549 .537 .547 .574 4 .602 .542 .513 .503 .512 .536 0 .570 .509 .483 .472 .481 .505 -4 .550 .478 .456 .452 .455 .473 —8 .549 .463 .432 .430 .442 .460 ~12 .566 .462 .422 .415 .435 .459 ____1 (.048) (.043) —16 .609 .483 .433 .417 .428 .461 2L = 22, S = —12:0 — C = .412, s.d.” , (- = .041 TABLE 10.—Calculated parameters for the Long Beach earth- quake ——Continued D Reference Fault: Newport-Inglewood Reference Coords: 33°48.0' N 118° 10.7' W k=1.750 T=0 C=25 Bands Used: VIII, VII, VI, V, IV, 111 Upper Values: CP Lower Values: s.d.cp 2L S 12 I 16 I 20 I 24 28 I 32 8 .292 .180 .154 .191 .264 .355 (.041) (.013) (.048) 4 .298 .163 .130 .167 .252 .343 (.046) (.012) (.052) 0 .317 .161 .118 .148 .237 .329 (.051) (.013) (.049) —4 .340 .173 .114 .130 .216 .308 __ (.017) —8 .373 .197 .121 .124 .191 .283 —12 .405 .228 .145 .126 .170 .256 —16 .439 .267 .180 .149 .167 .226 2L = 22, S = -—4: CP = .110, s.d.“. = .013 E Reference Fault: Newport-Inglewood Reference Coords: 33°48.0’ N 118° 10.7’ W k=1.825 T=0 C=25 Bands Used: VIII, VII, VI, V, IV, III Upper Values: (H - L) Lower Values: IH - LI 2L S I 20 I 24 I 28 I 32 I 36 I 40 12 —16 —8 -6 —3 O 3 18 16 18 17 16 17 8 -—15 —10 —4 —2 —1 2 17 16 18 16 15 14 4 —15 —10 —5 —1 0 4 17 16 17 15 14 16 0 —20 —10 -—7 0 3 4 20 16 15 14 15 14 —-4 —22 ~14 —7 —3 4 6 22 16 15 15 12 12 —8 -21 —17 —11 —2 1 5 21 19 17 14 11 11 —12 —21 —18 -13 —9 —1 1 21 18 17 15 13 11 F Reference Fault: Newport-Inglewood Reference Center: 33°48.0'N 118°10.7’W k=1.825 T=O C=25 Bands Used: VIII, VII, VI, V, IV, III Upper Values: CP Lower Values: s.d.cp 2L 7 S I 20 24 I 28 I 32 I 36 I 40 8 .312 .214 163 143 147 175 4 .327 .200 .124 .093 .104 .156 (.020) 0 .345 .217 .124 .085 .091 .146 (.031) (.024) -4 .370 .240 .139 .085 .083 .130 . (.024) (.016) —8 .399 .268 .162 .099 .086 .118 . (.039) (.010) (.044) —12 .430 .299 .192 .125 .099 .115 (.042) (.016) (.026) 2L = 34, S = —4: CP = .079, s.d.cp = .012 1 EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES TABLE 10.—Calculated parameters for the Long Beach earth- quake—Continued G Reference Fault: Newport-Inglewood Reference Coords: 33°48.0’ N 118°10.7’ W Bands Used: VIII, VII, VI, V, IV, 111 A. k = 1.7500 2L 3 CP RMS (H-L)/J H-L] 20 —4 114:.017 .537 -2 14 20 —8 .121 .521 —6/16 20 —12 .145 .527 20 —16 .180 .557 24 —4 .130 .533 24 —8 .124 .510 2/12 24 -12 .126 .504 —2/12 B. k = 1.8125 . 2L s CP RMS (H-L)/H-L 32 0 .085 .578 0/14 32 —4 .085 .549 —3/15 32 —8 .095 .537 32 —12 .125 .541 36 0 .091 .572 36 —4 .083 .544 4/12 36 —8 .086 .527 1/12 36 —12 .099 .528 0/13 TABLE 11.—Obserued and predicted M/M intensities, Bryson earth- quake [Values for 6-minute by 6-minute ground condition] Site Observed Predicted intensity intensity 2L:20 2L=40 Bradley ____________________________ 7 7 7 10 miles NW of Bradley ____________ 7 7 7 Bryson ____________________________ 7 7 7 Arroyo Grande ______________________ 6 5 6 Atascadero ________________________ 6 5 6 Cambria __________________________ 6 6(Al) 7(Al) Carmel Valley ______________________ 6 5 5[6(Al)] Cayucos ____________________________ 6 5/6 6 Chualar ____________________________ 6 5 6 Guadalupe ________________________ 6 5 5 Harmony __________________________ 6 5 6 King City __________________________ 6 6 7 Lockwood __________________________ 6 6 7 Morro Bay __________________________ 6 6 6 Oceano ____________________________ 6 5 6 Parkfield __________________________ 6 5 6 Paso Robles ________________________ 6 6 6 Pismo Beach ________________________ 6 5 5 Salinas ____________________________ 6 5 5 San Ardo __________________________ 6 5/6 6 San Luis Obispo ____________________ 6 5[6(Al)] 5[6(Al)] . San Simeon ________________________ 6 » 7(Al) 7(Al) Santa Margarita ____________________ 6 5 5[6(Al)] Templeton __________________________ 6 6 7 Avenal ____________________________ 5 5 6 Ben Lomond ________________________ 5 4 4 Big Sur ____________________________ 5 5 5 Buellton ____________________________ 5 4 5 Buttonwillow ______________________ 5 4 5 Casmalia __________________________ 5 5 5 Cholame __________________________ 5 5 5 Coalinga __________________________ 5 5 5 Corcoran __________________________ 5 4 5 Dos Palos __________________________ 5 4 5 Hollister __________________________ 5 5 5 Kettleman City ____________________ 5 4 5 Lompoc ____________________________ 5 4 4[5(A1)] Foot Hills __________________________ 5 4 5 Maricopa __________________________ 5 4 4(A1) Monterey __________________________ 5 5 5(A1) Nipomo ____________________________ 5 4/4 5 Orcutt ____________________________ 5 5 5 San Miguel ________________________ 5 6(Al) 7(Al) Santa Cruz ________________________ 5 4/5 5 Santa Maria ________________________ 5 4/5 5 17 dicted by model is below the reported value (predicted less than observed; Maricopa, for example). It appears from the table that 2L values of 20 and 40 bracket the best estimate of 2L, the suggestion being that 40 is somewhat too long because a few sites for which intensity 7 was predicted actually reported 6. Av- eraging all the predicted values indicates a 2L of 40 km to be nearly correct. In addition to the studies above, we investigated the Bryson intensity data by use of our statistical pro- grams. Though there were a greater number of report- ing stations, their nonuniform distribution created a problem in use of CP. The Virtual absence of stations in a 180° quadrant centered northwestward from Bryson for a distance of 75 km (that is, to intensity of about 5.5) resulted in very poor control on S when solving for CP values. Tables 12 A—C indicate marked disagree- ment between R.M.S. and CP estimates of the event’s parameters, the CP criterion actually having its TABLE 12.—Calculated parameers for the Bryson earthquake A Reference Fault: Nacimiento Reference Center: 35°47.9’N 121°11.4’W k = 1.750 2L = 30 C = 25 Bands Used: VIII, VII, VI, V, IV, 111 Upper Values: CP Lower Values: RMS T s —15 —10 —5 I 0 l 5 10 15 .457 .408 .364 .328 .298 .377 .266 10 .411 .361 .316 .279 .249 .227 .218 .366 .313 .268 .231 .201 .184 .176 0 .324 .271 .225 .189 .159 .141 .135 —5 [.466] [.447] [.439 [.455] .284 .230 .186 .147 .121 .105 .102 — 10 [.461] [.441] [.432] [.437] [.455] .246 .192 .148 .112 .084 .071 .073 —15 [.468] [.449] [.439] [.442] [.459] [.489] [.530] 7—“ .211 .157 .114 .078 .055 .047 .055 -20 [.487] [.471] [.462] [.464] [.478] [.504] [.540] Values in box are for parameter values having CPT minima. See table 13 and text. B Reference Fault: Nacimiento Reference Center: 35°47.9’N 121°11.4’W k = 1.750 2L = 35 C = 25 Bands Used: VIII, VII, VI, V IV, 111 Upper Values: CP Lower Values: RMS T s —15 —10 —5 0 5 10 ‘15 5 —‘—“ .268 .218 .175 .144 .123 .113 .116 0 [.433] .422 .420 [.434] .225 . . .103 .087 .081 .087 —5 [.422] [.409] [.409] [.423] .185 . .w .067 .054 .057 .069 ~10 [.425] [.414] [.412] [.423] .147 .098 .059 .041 .042 .055 .067 — 15 [.431] [.429] [.440] [.463] .115 .066 .037 .037 .063 .080 .089 —20 [.461] [.460] [.469] [.488] Values in box are for parameter values having CPT minima. See table 13 and text. 18 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES TABLE 12.—Calculated paramaters for the Bryson earthquake— Continued C Reference Fault: Nacimiento Reference Center: 35°47.9’N 121°11.4’W k = 1.750 2L = 40 C = 25 Bands Used: VIII, VII, VI, V, IV, III Upper Values: CP Lower Values: RMS T s —15 —1o —5 0 5 10 15 .293 .248 .209 .182 .165 .158 .160 10 .247 .202 .165 .138 .125 .122 .129 5 [.424] [.418] [.424] .204 .157 .123 .103 .095 .096 .106 0 [.407] .401] [.407 [.429] .162 . .074 .075 .085 .099 —5 [.398] [.455] .126 . .. .066 .086 .107 .118 —10 [.415] [.410] [.415] .096 .062 .061 .095 .124 .143 .153 —15 [.439] [.436] [.441] [.457] .074 .058 .094 .129 .157 .175 .182 —20 [.473] [.473] [.479] Values in box are for parameter values having CPT minima. See table 13 and text. minimum on tables B and C at S S —20, an unaccept- able location because the seismological data place a constraint of a very few kilometers on the northwest— southeast position of the epicenter. The problem with the estimate of S is that data to the north effectively cover such a short range of intensity that, given the noise level in the data, a CP minimum is found (that is, artifically low mean (Obs — Calc) value as a function of O) by moving the calculated fault south of its correct location. A technique for suppressing this effect is to multiply calculated Obs — Calc values by the cosines of the angles between the north- ward direction of the fault and the radials from the center of the fault to the stations before calculating a CP-type number. The quantity derived from this oper- ation, called CPS, is used to find the best S value as a function of 2L and k. The same effect that yields a poor value for S may also yield a poor estimate of T. Therefore, we calculated CPT values after the CPS values, CPT differing from CP in that all (Obs — Calc) values for stations west of the fault are multiplied by (—1) prior to calculation of a CP-type number. We did not use a sine function be- cause such a procedure suppresses the influence of near-station intensity values on the 2L estimate and thus on T. Table 13 gives CPT and R.M.S. values for 2L, S pairs having small CPS values for T=0. R.M.S. values, of course, are as they were in tables 12 A—C. Use of CPS and CPT does effectively suppress the effects of poor station distribution in the intensity data, giving small CP values for k values of 1.6825 and 1.7500 over small ranges of event parameter values while also giving best estimates very near those suggested by the R.M.S. values. We interpret the great range of equally accept- able 2L, S, T sets for k = 1.8125 as an indication of smearing of the analysis by use of an incorrect parame- ter value. We conclude that the event’s location probably was near the Nacimiento fault as given by all published locations, but we cannot certainly rule out a location 15 km to the southwest, in the zone of active faults cross— ing San Simeon Point southeast to northwest. Therefore, the intensity data indicate that this earthquake was as large or larger than the Santa Barbara or Long Beach earthquakes, both of which had significant areas experiencing Modified Mercalli inten- sity VIII. The explanation, of course, lies in the facts of TABLE 13.—Calculated parameters for the Bryson earthquake Reference Fault: Nacimiento Reference Center: 35°47.9’N. 121°11.4’W. Bands Used: VIII, VII, VI, V, IV, 111 A. k = 1.6875 2L S T CPS CPT RMS 12 -15 0 .037 .044 .558 16 —15 5 .008 .088 .470 16 —15 0 .001 .008:.O56 .473 20 —10 —5 .027 .037:.017 .434 20 —10 0 .030 .075 .444 24 —10 —-5 .013 .054i.008 .420 24 —10 —10 .015 .103 .412 28 -5 —5 .027 .085 .432 28 —5 —10 .028 .100 .416 28 —15 -15 .035 .155 .411 B. k = 1.7500 2L S T CPS CPT RMS 30 —10 0 .029 .051 .451 30 —10 —5 .026 .046 .443 35 —10 —5 .015 0291.019 .413 40 —5 —5 .022 .042 .409 40 —5 —10 .023 .081 .401 45 —5 —10 .015 .079 .398 45 —5 —15 .004 .120 .395 50 0 —15 .034 .120 .398 C. k = 1.8125 Minimum CPVr at 55, —5, —5 of 0141.049. There results a great range of2L values that give CPT values of less than .014 + .049(1.64) : .105. Table is for low RMS values. 2L S T CPS CPT RMS 60 — 5 —5 .019 .396 60 -— 5 — 10 .071 .391 65 0 —5 .042 .396 65 0 —10 .060 .388 65 0 — 15 .1 19 .394 70 0 -5 .056 .400 70 0 —10 .058 .385 70 0 —15 .096 .385 75 0 — 10 .061 .391 75 0 —15 .082 .385 75 0 —20 .129 .391 80 5 —15 .086 .393 80 5 —20 .129 .395 Wide range of equally acceptable parameters interpreted as meaning incorrect k value with consequent smearing of analysis. EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 19 geology. The epicentral region of the Bryson earth- quake is in a remote, nearly unpopulated region with only very small stream valleys and occasional ranches. Although an area of 2,700 km2 was predicted to experi- ence M/M VIII on saturated alluvium, the nearly total absence of such material in the epicentral region led to peak reported intensities of VII. The contrasting M/M intensities for the Bryson earthquake as predicted for saturated alluvium and as predicted when incorporat- ing the 6—minute by 6-minute ground condition are shown in figure 9. According to table 1 of Evernden (1975), a 2L of 40 km in western California leads to a predicted mag- nitude value of 6.85 as compared to a reported value of 6: for this earthquake (Coffman and van Hake, 1973). KERN COUNTY EARTHQUAKE OF 1952, (MODIFIED MERCALLI INTENSITIES; MURPHY AND CLOUD, 1954) Known parameters: (1) Location (main shock and aftershocks) (2) 2L = .30—60 (observed fracturing and after- shocks) BRYSON EARTHQUAKE NOVEMBER 21,1952 2L=40 km, (3:25, k=1% 123° 122° 121° 120° 38" 37° 36° 35" 0 12 3. £13 (T2 1 2 . 122° 121" 120” H: ls 2 I 50 100 KILOMETERS l—__L___l FIGURE 9.—Predicted M/M intensities, Bryson, Calif., earthquake (2L = 40, C = 25, k = 1%). A, Saturated alluvium. B, 6—minute by 6-minute ground condition. 20 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES Unknown parameters: (3) k (4) C This earthquake was studied in the first paper of this series (Evernden and others, 1973) with unsatisfactory results. In particular, the model used failed to predict the northward extent of intensities V and VI along the east side of the Sierra Nevada. When that paper was written, the existence of gross regional differences in attenuation had not yet been appreciated. The location of this earthquake near the boundary between regions of k = 1% and k = 11/2 should lead to pronounced per- turbation of observations from predictions based on a uniform k model. In order to construct a predicted intensity map for this earthquake, the following steps were taken: (1) Define the line through California that separates regions having k values of 11/2 and 1%. Through the Central Valley, the boundary (shown as a heavy solid line in figures 10 and 11) is assumed to be along the contact between granite and the Franciscan as- semblage buried under the Tertiary sedimentary rocks in the middle of the valley. By trying several models, it was concluded that the White Wolf fault, focus of the Kern County earthquake, is in the region of k = 11/2 and the k boundary is to the west of the fault (see fig. 10). The boundary is assumed to then swing sharply eastward, essentially paralleling the Garlock fault. It is assumed that a k value of 11/2 applies all the way to Needles, Calif, that nearly all the path to San Diego has a k value of 1%, and that the position of the k boundary is uncertain along some intermediate south- east azimuths. (2) Calculate expected intensities for different fault lengths (30 and 60 km) and a k value of 11/2. Compare with observations in regions having k values of 11/2. Select appropriate 2L value. (3) From the boundary between 11/2 and 1%, propa- gate intensities predicted for k of 11/2 into regions of k of 1% according to predictions for attenuation in k of 1%. This was actually done by: (a) noting that predicted I values along the k boundary near the epicenter were on the average about 11/2 intensity units lower for k of 1% than for k of 11/2 when assuming uniform models of 1% and 11/2; (b) thus, increasing all I values predicted by uniform k 1% model by 11/2 units in regions of k = 1%; (c) adjusting the misjoin of the predicted inten- sities in the two k regions by assuming that values in k regions of 11/2 were correct, intensity values in k re- gions of 1% were correct if ray directions made large angles with the k boundary, and I values in other k regions of 1% were obtained by interpolation. Figure 10 indicates the result of these several steps and the resultant predicted M/M intensities on saturated al- luvium for 2L : 60 and C = 25. The figure also indi- cates the high intensity values reported in each region of predicted intensities, it being assumed that these reports of high intensities are correlative with pres- ence of saturated alluvium or equivalent ground condi- tion. There is excellent agreement between prediction and observation in both k regions. The much further northward extent of intensity V values east of the Sierra Nevada than along the coast of California is clearly predicted by this model. The predicted extent of intensities VII and VIII in regions of k of 11/2 and 1% is confirmed by observations. As pointed out above, the model used for figures 10 and 11 assumes a length of fault break of 60 km. A fault break of 30 km predicts too small an areal extent for intensity values of V through VIII. Figure 11 indicates the difference in intensity be- tween published contours (Murphy and Cloud, 1954) and those predicted when adjusted for 6-minute by 6-minute ground condition. The difference is small up the Central Valley. However, there are large differ- ences throughout the Sierra Nevada. The published isoseismals of Murphy and Cloud (1954) ignore the granite and are based solely on scattered sedimentary sites in and east of the mountains. On the other hand, the predicted values based on the 6-minute by 6-minute grid give great regions of low intensity throughout the mountains. Thus, figure 10 predicts in- tensity IV for sites on saturated alluvial ground north and west of Lake Tahoe as observed, while figure 11 shows all ofthis as intensity II because the 6-minute by 6-minute grid sees only volcanic rocks and granite. Though a model with parametrs of 2L = 60, C = 25, k = 11/2 satisfactorily explains intensity values of VIII and less, it does seem to predict too-high intensities in the epicentral region. Thus, as shown in figure 12A, a C value of 25 causes a prediction of a large area of intensity X (shaking intensity) for saturated ground condition. Figure 128 indicates that a C value of 40 results in total elimination of predicted X and halving 0f the area of IX values, while figure 12C shows near elimination of IX values when incorporating 6—minute by 6-minute ground condition. We conclude that this earthquake occurred at significantly greater depth than is typical of western California earthquakes. The absence of intensity IX values in the observed intensities for this earthquake, even though 8 feet of displacement was measured in a Southern Pacific Rail- road tunnel, was simply a quirk of observation com- bined with the odd definitions of intensities VIII and IX on the Modified Mercalli scale. If M/M values are con- verted to R/F values, nearly all M/M VIII values be- come intensity IX, numerous M/M values of VII be- come VII to VIII, the result being to give a clearly EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 124° 122° 120° 118° 116° 114° 42° — I I I I I I I I _ 40° — _ 38° — 6 36° — _ _ 5 _ 6 34° — _ 5 0 50 100 KILOMETERS I—_1___l 5 6 5 5 5 I I I I I ' I I 1 I I FIGURE 10. -—-Observed (spot values) and predicted (contours) M/M intensities for Kern County Calif. earthquake of July 21,1952(2L= 60 C = 25). 22 SEISMIC INTENSITIES OF EARTHQUAKES 0F CONTERMINOUS UNITED STATES 124° 122° 120° 118° , 116° 114° I I I l l l I - -1—. d— .— KERN COUNTY EARTHQUAKE 40° _ 2L=60 km,C=25,k=1’/2 0 50 100 KILOMETERS L.__l—_1 slip“ 38° — EXPLANATION 35° _ Difference between observed and predicted Intensities --3 340 —- +1 +2 E Will _ NC Not contoured FIGURE 11.—Comparison of predicted and observed M/M intensities for Kern County earthquake of July 21, 1952 (2L = 60, C = 25, k = 11/2). Contours of observed values from Murphy and Cloud (1954). Patterned areas indicate difference between observed and predicted intensity values. EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES defined area of intensity R/F IX for this earthquake. Figure 13 indicates the contrast in intensity maps that results when using Rossi-Forel and Modified Mercalli units. As noted earlier, there are sound reasons for abandoning the Modified Mercalli scale and reverting to the Rossi-Forel scale. Our conclusion is that a model based on juxtaposi— tion of zones of k equal to 1% and 11/2 can satisfactorily explain the observations of intensity of the Kern County earthquake of 1952. In fact, we have been un- able to explain the observations in any other way. The data of this earthquake constitute a beautiful confir- mation of the existence of regions of varying k value, that is, of varying attenuation. Another point that can be emphasized at this time is that these data are explained only by a model assum- ing a regional k value, combined with local ground condition responding to the energy delivered by the basement rocks. A model such as that used by Blume and associates (Blume and others, 1978) cannot accu- rately predict published intensity values from IX through IV. Their model must fail because it incorpo- rates local ground condition as the ground condition 23 controlling attenuation along the entire propagation path. ' Finally, it should be noted that there is marked dis- crepancy between the magnitude value (7.1) associated- with a 2L of 60 km for western California and the magnitude value (7.7) observed for this earthquake (Richter, 1955). This discrepancy between the observed magnitude and that predicted for such an earthquake in western California serves as confirmation of re- gional changes in attenuation and of the location of this earthquake in a region having a k value of 11/2. This matter of interregional discrepancy between magnitude values and energy release was discussed in some detail in Evernden (1975, 1976) and is discussed further in a following section. SEATTLE EARTHQUAKE OF 13 APRIL 1949 (MODIFIED MERCALLI INTENSITIES; MURPHY AND ULRICH, 1951) Known parameters: (1) k = 11/2 (Evernden, 1975) Unknown parameters: (2) C value (3) Location of epicenter KERN COUNTY EARTHQUAKE JULY 21, 1952 2L=60 km, C=25, k=11/2 a120° 119° 118° 117° 119° 2L:60 km, (3:40, k:1V2 2L=60 km, C=40, k=11/2 119° 118° 118° 37 35° 34° ’ 100 KILOMETERS FIGURE 12.—Predicted M/M intensities for Kern County earthquake of July 21, 1952 (2L = 60, C = 25, k = 11/2). A, C = 25 (saturated alluvium). B, C = 40 (saturated alluvium). C, C = 40 (6-minute by 6-minute ground condition). 24 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES (4) 2L value This earthquake is the largest histo ical earthquake ‘in the Seattle area. It is important [to try to decide whether this is the maximum potential earthquake in the area. If not, can we estimate how large the maximum earthquake may be? As for the Long Beach and Kern county earthquakes, use of the Modified Mercalli scale precluded reports of intensity IX for the Seattle earthquake. As so many communities reported intensities for this earthquake, table 14 is limited to communities having population of 2,000 or greater in the 1960 census (Na- tional Atlas). It can be shown that a k value of 1.45 and a 2L 0f 40 km or a k value of 1.55 and a 2L of 100 km give very similar predictions and are nearly indistin— guishable on the basis of available data. A recent study by Milne (1977), using observed accelerations in the Georgia Strait-Juan de Fuca Strait area for earth— quakes of the region, found the appropriate attenua- tion factor (k value of the paper) to be 1.4, that is, in essential agreement with our analysis of the data of the Seattle earthquake. All calculations for table 14 were based on a k value of 1.50. All predicted intensity values not in parentheses are predicted for saturated alluvium. Intensity values in parentheses are pre- dicted using the ground condition of the 25—km by 25-km grid of the United States map. Figures 14 and 15 present maps of predicted intensities (2L = 40 km and 2L = 100 km) for the northwestern United States for saturated alluvium and for the 25—km by 25-km KERN COUNTY EARTHQUAKE JULY 21, 1952 2L=60 km.C=25,k=11/2 120° 119° 118° 117° 37 L1 46 4 4 l 6 6 65 4 36—— 5 4 E <17 8 a? 6 4 8 9 5 l0 7 35°— 9 59 8 7 “ 16 6 9 0 120° 119° 37" 36° 35° 34° 100 KILOMETERS |—._l__—l FIGURE 13.—Predicted Rossi-Forel and Modified Mercalli intensities for Kern County earthquake of July 21, 1952 (2L = 60, C = 25, k = 11/2, 6-minute by 6-minute ground condition). A, Modified Mercalli. B, Rossi-Forel. EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 25 TABLE 14.—Observed and predicted M/M intensities, Seattle earth- quake [Population 2 2,000] Predicted intensity Observed Sites intensity 2L: 40 2L: 100 Aberdeen ______________________ 8 7 7 Auburn ________________________ 8 7 8 Berkeley ________________________ 8 7 8 Centralia ______________________ 8 27/8 8 Chehalis ________________________ 8 8 8 Hoquiam ________________________ 8 7 7 Kelso __________________________ 8 7 8 Longview ______________________ 8 7 8 Olympia ________________________ 8 7 8 Puyallup ________________________ 8 7 8 Richmond Beach ________________ 8 7 8 Seattle __________________________ 8 7 8 Shelton ________________________ 8 7 7 Tacoma ________________________ 8 7/8 8 Tumwater ______________________ 8 7 8 Arlington ______________________ 7 7 7 Bremerton ______________________ 7 7 7/8 Camas __________________________ 7 7/6 7 Des Moines ____________________ 7 7 7/8 Enumclaw ______________________ 7 7 7/8 Everett ________________________ 7 6/7 7 Kirkland ________________________ 7 7 7 Seahurst ________________________ 7 7 7/8 Vancouver ______________________ 7 6/7 7 Astoria, Oreg. __________________ 7 7 7 Hillsboro, Oreg. ________________ 7 6 7 North Portland, Oreg _____________ 7 6 7 Oregon City, Oreg. ______________ 7 6 7 Portland, Oreg ___________________ 7 6 7 Seaside, Oreg. __________________ 7 6 7 Anacortes ______________________ 6 6 6 Bellingham ____________________ 6 7 8(6) Bryn Mawr ____________________ 6 7 7(8/5) Chelan __________________________ 6 5 . Mercer Island __________________ 6 7 8(8/5) Montesano ______________________ 6 7 7(6) Omak __________________________ 6 5 5/6 Port Townsend __________________ 6 6 7(5) Prosser ________________________ 6 6 6 Snohomish ______________________ 6 6 7(7/4) Spanaway ______________________ 6 7 8 Baker, Oreg _____________________ 6 4 5 Beaverton, Oreg. ________________ 6 6 7(6) Corvallis, Oreg. ________________ 6 5 6 Dallas, Oreg _____________________ 6 6 6 Forest Grove, Oreg _______________ 6 6 7(6) Gresham, Oreg. ________________ 6 6 7(6) Lebanon, Oreg ___________________ 6 5 6 McMinnville, Oreg _______________ 6 6 6/7(6) Monmouth, Oreg _________________ 6 6 6 Newberg, Oreg ___________________ 6 6 7(6) Newport, Oreg ___________________ 6 5 6 Prineville, Oreg. ________________ 6 5 5/6 Redmond, Oreg. ________________ 6 5 6 Salem, Oreg _____________________ 6 6 6/7(5) Silverton, Oreg. ________________ 6 6 6/7(5) Tillamook, Oreg. ________________ 6 6 6/7(4/6)7.0 Toledo, Oreg _____________________ 6 5 6 Woodburn, Oreg. ________________ 6 6 7(6) Bellevue ________________________ 5 7 7(5/7) Colfax __________________________ 5 5 5 Colville ________________________ 5 5 5 Ellensburg ______________________ 5 6 6(4) Marysville ______________________ 5 6 6/7(4/7) Pomeroy ________________________ 5 5 5 Port Angeles ____________________ 5 6 6(5) Sedro-Woolley __________________ 5 6 6(5) Spokane ________________________ 5 4 5 Walla Walla ____________________ 5 5 5 Wenatchee ______________________ 5 6 6(4) Yakima ________________________ 5 6 6(4) Albany, Oreg. __________________ 5 5 6(5) Gresham, Oreg. ________________ 5 6 7(6) TABLE 14.-—-0bserued and predicted M/M intensities, Seattle earth- quake—Continued Predicted intensity Observed Sites intensity 2L: 40 2L: 100 Hood River, Oreg. ______________ 5 6 7(4) La Grande, Oreg ,,,,,,,,,,,,,,,,, 5 5 5 Milwaukie, Oreg ,,,,,,,,,,,,,,,,, 5 6 7(4/6) North Bend, Oreg. ______________ 5 5 5 The Dalles, Oreg ,,,,,,,,,,,,,,,,, 5 6 6(4) Eugene, Oreg. __________________ 4 5 5(5/3) Saint Maries, Idaho ,,,,,,,,,,,,,, 4 4 4/5 Sandpoint, Idaho ________________ 4 4 4 'Saturated alluvium—no parenthesis. 2Ground condition applied~parenthesis First valuevcorrect square. Second value—adjacent square. ground condition. See the plates for correlation of latitude and longitude and of units of US. grid. Note first that a 2L value of 40 km predicts intensity 7 at numerous sites where 8 was observed, 6 at 7, and 5 at 6, indicating that a 2L of 40 is certainly too short with a k of 1.50. For a 2L of 100 km, the suggestion is that too-high values are being predicted. Though the intensity values given in parentheses are invariably as low or lower than observed (implying that ground con— dition may be the explanation for the observed values being lower than the intensity values predicted for saturated alluvium), it still seems that too many pre— dictions are high. We conclude that a 2L value of about 75 km is appropriate for this earthquake when using a k value of 1.50. To obtain the intensity values of table 14 required use of a C value of 60, a larger value than used for any other US. earthquake. As C is linked to depth of focus, becoming larger as depth increases, the requirement for a value of 60 for C means that intensity data are sensitive to depth of focus in the range 10— 70 km. Nutt- li (1951) reported a depth of focus of 70 km for this earthquake. The data of his paper do not allow evaluaton of the accuracy of that depth estimate, and there are no short-range S-P data for establishment of time of origin and thus of depth. There seems to be no doubt, however, that the depth was in the range 40—70 km, so an unusual C value for a US. earthquake is in agreement with an unusual depth for a US. earth- quake. A few reported intensity values are not explainable, such as the one at Baker, Oreg., where a VI was re- ported even though a 2L of 100 predicts only a 5.1. In addition, the paucity of reports of intensity IV is sur- prising. Where IV was reported, it was predicted. How- ever, there is a vast area where IV is predicted, extend- ing well into Montana and the northern Sacramento Valley of California, and from which there are no re- ports in Murphy and Ulrich (1951). It may be that dis- tance from the epicenter was so great (800—900 km) that people were not canvassed or that they failed to 26 associate shaking at intensity IV level with the Seattle earthquake. The 2L of about 75 found when using k of 11/2, when considered in light of the discussion in Evernden (1975), implies that this earthquake is essentially the largest that can occur in the Seattle area. The only possibility for more severe shaking is to have a compa- rable earthquake occur at a shallower depth. For il- lustration, we present below predicted intensities for an earthquake of 2L = 75 km at various depths (C values), the shallowest event having a C value equal to that found appropriate for earthquakes of western California. Intensities are as predicted for saturated alluvium, Y 2 distance parallel to fault from center of fault, X = distance from line of fault. The column headed “1(X = 0, Y = 0)” indicates predicted R/F and SEATTLE EARTHQUAKE APRIL 13, 1949 (M/M—SATURATED ALLUVIUM) 110 100 \ l 1 o 10 20 B. 2L=4O km, C=60, k=1V2 0 100 200 300 400 KILOMETERS l_1_.|_L._L..—J FIGURE 14.—Predicted M/M intensities for Seattle, Wash, earth- quake of April 13, 1949, saturated alluvium.A, 2L = 100, C = 60, k = 1V2. B, 2L = 40, C = 60, k = 11/2. NC, not contoured. SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES M/M intensities at the center of the break. The col- umns headed “X(km)(I = 8.5)” and “X(km) (I = 7.5)” indicate the perpendicular distances in kilometers from the center of the break to intensities 8.5 and 7.5 on both the R/F and M/M scales. [IX : Y 2 0) X(km)(l : 8.5) Xlkm)(l : 7.5; C RIF M/M R/F M/M R/F M/M 25 10.5 10.5 74 48 132 68 40 9.7 9.4 62 27 128 60 60 9.0 8.3 43 -- 120 51 It is clear that occurrence of an event like the one of April 13, 1949 at a depth of 5 to 10 km would be a drastically different experience for the Puget Sound area than was the actual event. The item for serious research in the Seattle area is determination of SEATTLE EARTHQUAKE APRIL 13, 1949 (M/M—25-km by 25-km ground condition) EEEEEEE 557% r—H—Sfii . 10 20 30 B. 2L=40 km, 0:60, k=1V2 0 100 200 300 400 KILOMETERS l_1.__L_|__...L_—_| FIGURE 15.—Predicted M/M intensities for Seattle earthquake of April 13, 1949, 25-km by 25-km ground condition. A, 2L = 100, C = 60, k = 11/2. B, 2L = 40, C = 60, k = 11/2. NC, not contoured. EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES Whether earthquakes comparable to 1949 can occur at shallower depths. LOMPOC EARTHQUAKE OF NOVEMBER 4, 1927 (ROSSI-FOREL INTENSITIES; BYERLY, 1930) Assumed parameters: (1) k = 1% (western California) (2) C = 25 (normal depth) Unknown parameters: (3) Location (4) 2L The location of this earthquake published by Byerly (1930) is far offshore. The purpose of the initial inves- tigation of this earthquake was to ascertain whether the observed isoseismals were consistent with such an epicenter. Figure 16A gives observed intensities along with intensities predicted for a fault passing through Byerly’s epicenter with a fault break of 600 km oriented parallel to the coast (that is, along the struc- tural trend in this part of California). Figure 163 shows the results for a fault passing through Byerly’s epicenter with a fault break of 600 km oriented east- west and reaching within 5 km of Point Arguello (no onshore faulting was observed). Figure 16A shows what is certainly an excessively long break, but it was 27 Used to illustrate the impossibility of reaching the ob- served intensities for such a location and orientation of faulting no matter what the length of break. Figure 163 illustrates that one way to attain high predicted onshore intensities is to have the end of a long fault near Point Arguello. However, this specific model has no credibility when considered in terms of the tectonics of the region. The predicted and observed intensities for this model have many similarities, but other models achieve better agreement with isoseismals and tectonic style. Hanks (1978) calculated the epicenter on the basis of S and P data from stations in southern California (fig. 17). Three different fault models were put through this epicenter. The first (fig. 17A; 2L = 300 km parallel to shoreline) was to illustrate the inability of any fault through this epicenter and parallel to the San Andreas fault to explain the observed isoseismals. This model does not predict any onshore IX values. It gives VIII values in much of the region in which VIII was ob- served but, in so doing, it predicts VIII, VII, and VI values far north of where they were observed. The second model based on Hanks’ epicenter (fig. 173) hypothesizes a 2L of 300 km oriented east-west with the fault break reaching within 5 km of shore. Though this fault does predict IX values as observed, it LOMPOC EARTHQUAKE NOVEMBER 4, 1927 118° I 37° 36° 35° 34° ‘ 100 KILOMETERS ' “ / l | 122° 121° 119° 118° 37" — 36° '- 35" — 34° -— 100 KILOMEI'ERS l l l 2L=600 km, C225, k=1% Byerly’s epicenter Strike of fault: Parallel to regional structure 2L=600 km, C=25, k=l% Byerly’s epicenter Strike of fault: East-west, east end of break very near shore FIGURE 16.—Predicted (arabic numerals) and observed (roman numerals)R/F intensities for Lompoc, Calif, earthquake of November 4, 1927. A, Based on hypothetical fault through Byerly’s epicenter (Byerly, 1930) and parallel to shoreline (2L = 600, C = 25, k = 1%).B, Based on hypothetical fault through Byerly’s epicenter and oriented east-west (2L = 600, C = 25, k = 1%). ' 28 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES badly fails to predict VIII, VII and VI values. The third model based on Hanks’ epicenter (fig. 18) has a 2L of 80 km and an orientation as shown and as suggested by Hanks. Even when all observed VI values are treated as having been at sites on saturated al- luvium, the predicted VI area is less than half that observed. There are no onshore IX values predicted, and the predicted VIII area is less than half that ob- served. The basic failing of these models is placement of the fault too far offshore. Any tectonically credible orienta- tion at such locations fails to generate sufficiently high intensities onshore. Even the tectonically incredible east-west faults fail in detail to predict observations. The actual fault break must have been near shore and must have been nearly parallel to the shoreline (no onshore fracturing), while the size of isoseismals re- quires a break length of several tens of kilometers. Figure 19 presents the first effort to place the fault break so as to satisfy the isoseismals (2L = 125 km). A major point of this model and of all others that attempt to explain observations is placement of the south end of the break near Point Arguello in order to explain the observed IX values in this area. The main difference between this model and the one described below is its more northwesterly strike. The result is greater sep- aration of faulting and shoreline northward and the resultant need for a greater fault length to explain on- shore intensities. Though Figure 19 does indicate satis- factory agreement of observed and predicted VI values, the predicted area of VIII may be too small. Predicted areas of IV and V show great disagreement with re- ported observations. Next, we model the fault break as suggested by Gawthrop (1978) along the Hosgri fault. The 2L length of 80 km was arrived at by trying several lengths be- tween 50 and 125 km. Figure 20A shows intensities predicted for saturated alluvium, while Figure 20B shows intensities as predicted using the 6-minute by 6—minute ground-condition units. Figure 20A shows excellent agreement between observation and predic— tion for intensities VI and VIII. Figure 203 indicates marked shrinkage of the area of predicted intensity VIII, probably because some areas of saturated alluvium were ignored by the 6-minute by 6-minute grid. All intensity IX values have disappeared for similar reasons. When the 1/2—minute by 1/2-minute grid is used, predicted intensity IX values extend from Point Arguello northward along the coast as far as they do on Figure 20A. An apparent failing of the last two models is that they predict too large an area of intensity IX. Many of LOMPOC EARTHQUAKE NOVEMBER 4, 1927 118° 122° 121° 120° 119° 118° 37° 35° 34° 100 KlLOMElERS - 1 I I I 37" 36° 35° 34° 100 KlLOMElERS - ‘ | I , . . 2L=300 km, C=25, k=1% Hanks' epicenter Strike of fault: Parallel to coast 2L=300 km, 0:25, k=1% Hanks' epicenter Strike-of fault: East—west FIGURE 17.—Predicted and observed R/F intensities for Lompoc earthquake of November 4, 1927. A, Based on hypothetical fault through Hanks’ epicenter and parallel to shoreline (2L = 300, C = 25, k = 1%). B , Based on hypothetical fault through Hanks’ epicenter and oriented east-west (2L = 300, C = 25, k = 1%). EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 29 the IX values are predicted along the beach and in sand-dune areas where there probably were no people at the time of the earthquake and where our use of the J category for all alluvium is in error. The other areas of predicted IX at some distances from the shore are along streams and rivers. Flowing water is seldom seen in these rivers, and some may have no surface runoff for years at a time. Building sites have not been devel— oped in these river courses, however, because when there is enough rain to produce surface runoff, flooding is common. The absence of dwellings suggests the likelihood that no basis for observations existed. Also, low water saturation and the physical characteristics of the alluvial materials (cobbles in sand) would imply intensities below those expected for saturated al- luvium. Therefore, we do not believe that differences between observed and predicted IX values are a basis for rejecting either of the last two models. This conclusion requires that at such stations as Bet- teravia (VIII vs. 8.9), Casmalia (VIII vs. 9.0), Lompoc (VIII vs. 8.8), and Oceano (VIII vs. 8.9), for all of which Byerly reported intensity VIII, and all of which are shown as alluvium on the 1/2-minute by 1/2-minute grid and thus are treated as on saturated alluvium in the LOMPOC EARTHQUAKE NOVEMBER 4, 1927 122° 121° 120° 119° 118° 36° L‘ 35° - - 34° —— 0 100 KlLOMETERS '— |__.|_1 l I l l EXPLANATION _ _ /v_ — Observed — 5 — Predicted —— Fault FIGURE 18.———Predicted and observed R/F intensities for Lompoc earthquake of November 4, 1927, based on hypothetical fault through Hanks’ epicenter with length and orientation as sug- gested by Hanks (2L = 80, C = 25, k = 1%). calculations, predicted values were too high because the ground at these sites was somewhat less sensitive than saturated alluvium. The major remaining task relative to our program for predicting intensities is to identify and properly characterize various types of al- luvium. Hanks, on the basis of seismological arguments about S-P intervals and the consequent restraints on potential epicenters, suggested the shortening of the fault shown in figure 20C. The resultant predictions are in serious disagreement with observations, the predicted area of VI being half that observed, and the predicted area of VIII being a third or less of that ob- served. As we did for several other earthquakes, we investi- gated the Lompoc earthquake by using site-intensity values. Table 15 lists sites for which Byerly reported Rossi-Forel intensities of VI or greater. The faults modeled (shown in figure 21) can be described as fol- lows: (A) Hosgri fault—70-km break LOMPOC EARTHQUAKE NOVEMBER 4, 1927 122° 121° 120° 119° 118° 100 KILOMETERS 4 I L l ': 13331; EXPLANATION —-/V—— Observed __ 5 — Predicted — Fault 2L=125 km, C=25, k=1% Epicenter selected so intensity IX could be predicted Strike of fault: Parallel to coast FIGURE 19,—Predicted and observed R/F intensities for Lompoc earthquake of November 4, 1927, based on hypothetical fault placed so as to yield isoseismals in agreement with observations (2L = 125, C = 25, k = 1%). Intensities as predicted on saturated alluvium. 30 (B) Hosgri fault—52-km break (9 km off each end of (A) ) (C) Hosgri fault—25-km break (center third of (A) ) (D) Location suggested by Hanks—80-km break The observed and predicted intensities for these sev- eral models are given in table 15. Two modes of analysis seem justified, the one chosen depending upon one’s point of View: (A) The first is to select the model for which the average predicted intensity for stations within a given intensity bandwidth is equal to the cen— tral intensity value of that bandwidth. Under ideal conditions, the same model will achieve such agree— ment or near agreement for all bandwidths; (B) The second is to select as small an earthquake as possible such that no (or nearly no) observed intensities are greater than predicted intensities on saturated al- luvium. A model in which more than a very few ob- served intensities are greater than those predicted on saturated alluvium is inadmissible because no pertur- bation of ground condition permissible within the model could explain such stations. For analysis of mode A, consider table 16A. The headings of the last three columns indicate observed intensity and center intensity of each bandwidth. S/A and G/C indicate whether calculations of intensity were based on saturated alluvium (S/A) or ground con- dition (G/C) as on 1/2-minute by 1/2-minute ground- condition data. Using the latter values, table 16A indicates that fault D is systematically predicting av- SEISMIC INTENSITIES OF EARTHQUAKES 0F CONTERMINOUS UNITED STATES TABLE 15.—Predicted and observed intensity values at specific sites, Lompoc earthquake of November 4, 1927 Site Population Fault A Fault B Fault C Fault. D 1977 S/A G/C S/A G/C S/A G/C S/A G/C Intensity IX Surf ,,,,,,,,,,,,,,,,, .._ . 9.3 8.3 9.0 8.0 8.2 7.2 8.4 7.4 Honda ,,,,,,,,,,,,,,,, . . 9.2 9.2 8.8 8.8 7.9 7.9 8.5 8.5 Intensity VIII Arlight ,,,,,,,,,,,,, __, 9.1 9.1 8.6 6 7.7 7.7 8.5 8.5 Arroyo Grande. 7,500 8.7 .7 8.6 6 8.1 7.1 7.9 6.9 Betteravia . 400 8.9 .9 8.7 .7 8.3 8.3 8.0 8.0 Cambria. 1,000 7.5 .5 7.1 .1 6.4 6.4 7.7 7.7 Casmalia 250 9.0 .0 8.9 .9 8.4 8.4 8.1 8.1 Cayucos _ .. 1,000 8.2 .0 7.7 5 6.9 4.7 8.0 4.8 Conception . W 8.3 8 7.8 3 7.0 5.5 7.8 6.3 Guadalupe... 3,100 9.0 0 8.9 9 8.5 8.5 8.1 8.1 Halcyon _.. d. 8.7 7 8.6 6 8.1 7.1 7.9 6.9 Harriston . ._ 8.9 9 8.7 7 8.0 7.0 7.9 6.9 Huasna H. _... 8.8 3 7.8 3 7.4 5.9 7.3 5.8 Lompoc ..... 25,300 8.8 8 8.5 5 7.7 7.7 7.9 7.9 Los Alamos , 800 8.1 6 7.9 4 7.3 5.8 7.4 5.9 L05 Olivos ..... 200 7.6 6 7.3 .3 6.7 6.7 6.9 6.9 Morro Bay..- 7,100 8.6 6 8.1 1 7.3 7.3 8.2 8.2 Nipomo ..... 3,600 8.5 5 8.4 .4 8.0 8.0 7.1 7.1 Pismo Beach. 4,000 8.9 4 8.7 2 8.2 6.7 8.1 6.6 Oceano ....... 2,600 8.9 9 8.7 7 8.3 8.3 8.1 8.1 San Luis Obispo 34,500 8.6 6 8.3 3 7.7 6.7 8.0 7.0 Santa Maria... .. . 32,700 8.5 5 8.4 .4 8.0 8.0 7.7 7.7 es and V H B lev- ('D '3 m n. n- .— ,wvwawc:samuwwewwweeeewsmewsmaesmwaemssspequmq 9w«n«axiomsmuwewewwewwswwwimswsmwsemesseswmumsm Adelaida ............ .. 7.3 .8 6.9 .4 6.3 4.8 7.2 5.7 Atascadero _ . . 10,300 7.8 3 7.4 .9 6.8 5.3 7.5 6.0 Bakersfield . ..... 69,500 5.3 3 5.2 2 4.7 4.7 5.1 5.1 Buellton ........ . 250 7.8 8 7.5 5 6.9 6.9 7.1 7.1 Buttonwillow . . . 950 5.9 9 5.7 .7 5.2 5.2 5.6 5.6 Car interia ......... 7,000 6.0 0 5.8 8 5.3 5.3 5.8 5.8 Cho ame ....... 15 6.6 6 6.4 4 5.8 5.8 6.4 6.4 Creston ......... . 7.5 5 7.2 2 6.6 5.6 7.1 6.1 Gaviota . ...... 75 7.7 2 7.3 8 6.6 5.1 7.1 5.6 Goleta ........... 5,000 6.7 7 6.4 4 5.8 5.8 6.3 6.3 Harmony . 5 7.8 6 7.3 1 6.6 4.4 7.9 5.7 King City . 3,400 5.8 8 5.5 5 4.9 4.9 5.9 5.9 Las Cruces 25 7.8 3 7.4 9 6.7 5.2 7.2 5.7 Naples fl. ..-. 7.0 5 6.7 2 6.1 4.6 6.6 5.1 Oxnard H. . .. 85,000 5.4 4 5.2 2 4.7 4.7 5.3 5.3 Paso Robles ......... 7,200 7.3 .3 7.0 .0 6.3 6.3 7.1 7.1 Reward .............. ..-. 6.2 .2 6.1 1 5.6 5.6 5.9 5.9 Santa Barbara ........ 70,200 6.4 .4 6.1 .1 5.6 5.6 6.1 6.1 Santa Ynez .......... 350 7.4 4 7.2 2 6.6 6.6 6.9 6.9 Santa Margarita ...... 1,000 8.1 6 7.8 3 7.1 5.6 7.6 6.1 Solvang .............. 1,500 7.6 6 7.3 3 6.7 6.7 7.0 7.0 Taft. . .... . 4,300 6.0 .0 5.8 .8 5.4 5.4 5.7 5.7 Templeton . . 900 7.6 6 7.2 2 6.6 6.6 7.4 7.4 Ventura W. ....... 58,000 5.7 7 5.4 4 4.9 4.9 5.4 5.4 Wasioja ............. ”H 7.0 2 6.9 1 6.4 4.6 6.6 4.8 LOMPOC EARTHQUAKE NOVEMBER 4,1927 122° 121° 120° 119° 1 18° 121° 120° 119° 118° 122° 121° 120° 119° 118° IV —— Observed B EXPLANATION —- 5 — Predicted —— Fau|t FIGURE 20,—Predicted and observed R/F intensities for Lompoc earthquake of November 4, 1927. A, Based on location of Hosgri fault with placement and length of break chosen so as to predict isoseismals in agreement with observations (2L = 80, C = 25, k = 1%). Intensities as predicted on saturated alluvium. B, Same as A, but intensities as predicted using ground condition of 6-minute by 6-minute grid. C, Based on location of Hosg'ri fault with northward extent of break controlled by S—P arguments of Hanks (1978). EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 31 121°00’ 120°40’ 0 50 100 KILOMFIERS gJ..__.J 35°10' ' ~ 35°00’ ‘ 34°50’ 34°40’ 34°30’ FIGURE 21,—Fault models used in calculations of site-intensity values (table 15) for Lompoc earthquake of November 4, 1927. erage intensities that are too low. Any shorter fault break along this same line would show greater dis- agreement. As in the previous analysis, the intensity data appear to reject any location for the Lompoc earthquake as far at sea as that suggested by Hanks. Fault A gives the best agreement of average pre— dicted and central intensity value in each bandwidth; B and C yield values that are too low. From these data, too, a break of almost 70 km along the Hosgri fault is suggested. For analysis of mode B, consider table 16B. The number following the intensity value in the headings is the number of reporting stations in each bandwidth. High predictions under S/A are deemed permissible, low predictions under S/A are not permissible. We TABLE 16.—Average predicted intensities for saturated alluvium and l/2-minute by 1/2-minute ground-condition units, Lompoc earth— quake, using two sets of intensity data Fault 2L IX (90! VIII (8.0! VliVIll (6.57 (km I S.’A QC 8/ A G/ S/A G/C A 70 9.25 8.75 8.58 7.95 6.83 6.27 B . .. _._ 52 8.90 8.40 8.31 7.68 5.49 6.03 C . ,.. . 25 8.05 7.55 7.70 7.07 . ., W. D _ H, . 80 8.45 7.95 7.86 6.23 6,55 5.99 B Fault 2L 1X12) VIII (20) VI—Vll (25: SA G/C /A G/C S/A GO A 70 OH/OL Ol'I/lL 13H/OL 7H/4L 7H/2L 3Hr’4L B ........ 52 OH/OL OH/lL 9H/2L 5H/8L 1H/2L OH/7L C . , . ,. 25 2L 2L OH/7L 0H/12L D , h. H 80 OH/lL OH/lL OH/3L OH/lOL 2H/3L OH/4L H : Prediction above bandwidth L : Prediction below bandwidth might expect a nearly equal number of low and high predictions under G/C. With less certainty, fault D is again rejected because 1 of 2 and 3 of 20 stations were predicted low even when assuming S/A conditions. As in other modes of analysis, fault A satisfies the mode of interpretation best. There are no L values for VIII and IX and only 2 of 25 for VI—VII under S/A conditions, while there are similar H and L values under G/C con- ditions. Finally, we analyze the intensity data of the Lompoc earthquake via the previously described statistical model and present calculations of CP, S.d.Cp, and R.M.S. Table 17 presents calculations based on a k of 1750,. 2L values of 50, 60, 70, 80, 90, and 100 km, and S and T values of —30, —20, —10, 0, 10, and 20 km. For “ob- served” intensity values in calculations for these ta- bles, we used the midpoint of each band defined by Byerly (1930)(IX, VIII, VII—VI, and V—IV.) As a matter of fact, nearly every point in band V—IV is certainly within the area of intensity V. Therefore, we redid all the calculations using an “observed” value in the (V—IV) band of 5.0 rather than 4.5 and obtained the results in table 18. Because of the possibility that a k value of 1.750 might be slightly in error, and because we were uncertain of the impact of such an error on the predicted fault parameters, we redid most of the calcu- lations using a k value of 1.6750. The results of this procedure are shown in tables 19 and 20. On all of these tables, we have marked the zone of geologically “acceptable” solutions, the definition of ac- ceptability being that the fault break does not intersect land nor does the fault line extend into the Santa Barbara Channel. We consider any offshore position of the fault to be “acceptable.” The reference fault line used is as shovVn on figure 21 with the following changes: (a) northward, the fault is extended accord- ing to published maps; (b) southward, it is extended arbitrarily along its general strike into the Santa Barbara Channel. This is done simply to provide the basis for calculations. The reference coordinates (S = O, 32 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES T 2 0) are at lat 34°55.0’N., long 120°44.3’W., that is, at the center of the Hosgri fault as shown on figure 21. Table 17 indicates that a broad range of solutions yielding low values of CP and s.d.cp are found when using k = 1.750. All 2L values are found to yield CP values of around 0.10 at some combination of 2L, T, and S, all minima on all tables being geologically un- acceptable. The minimum CP at an acceptable site is 2L = 70, S = 0, T = 0 with an s.d.cp = 0.019. In a partial search for a true minimum, a CP of 0.147 with s.d.(;p = 0.011 was found for 2L = 70, S = 0, T = 4. Thus, even 2L = 70, S = 0, T = ~10 is strongly rejected (d = (0.329—0.177)/0.019=8.0), while a location 30 km offshore is clearly impossible. The only other accept- able coordinates having a low CP value are 2L = 60, S = 0, T = 0. Table 18 (V—IV band treated as V band and termed MODIFIED on tables), shows a great reduction in GP values, implying that the change made in treatment of the (V—IV) band was appropriate. No low CP values are found for 2L = 50, but a long band of low CP values are found for all other 2L values, nearly all of them being geologically inadmissible. Geologically admissi- ble low CP values are found for 2L = 90, S = 10, T = 0, and 2L = 80, S = 10, T = 0. The actual approximate minimum for 2L = 80 is at S = 10, T = 2, with CP = 0.029 and s.d.cp = 0.038. Values of CP as high as 0.10 are rejected. Table 19 gives calculations for k = 1.675 and band (V—IV) treated as (V—IV). The most significant result is that essentially all CP values are higher than for simi- lar calculations when using k = 1.750. No CP values as low as 0.20 are found, while values below 0.10 were found in table 18. As expected, minima at each 2L move westward and the length of fault with the minimum CP gets shorter. However, the coordinates having minimum CP values are still near T = 0. Thus, the smallest CP values at 2L = 60 are all at T = O. The smallest CP (0.206) at 2L = 50 is as S = —10, T = 0 while slightly lower values (0.186 and 0.163) are at geologically unacceptable locations (S : —, T = 10; S = —20, T = 10). Again, any location more than a few kilometers west of the minimum is rejected. Table 20 shows calculations for k=1.675 and band (V—IV) treated as band (V). An interesting phenome- non here is the disappearance of the long line of similar CP minima that characterized nearly all values of 2L when using k = 1.750. No 10w CP minima are found for 2L = 80 and 70, while there is only a small region of CP minimaforZL: 60(S= 0,T= 0;S= —10,T = O;S= —20, T = 0), only points S = 0,T = 0 and S = —10, T = 0 being geologically acceptable. Low minima are found for 2L = 50 at geologically unacceptable coordinates. This phenomenon, a line of minima on S versus T TABLE 17.—-Calculated parameters for the Lompoc earthquake using midpoint of Byerly’s (1930) "observed” intensity bands A Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 50 C = 25 Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: S.d.(‘p T S —30 —20 l —10 l 0 l 10 20 30 30 .978 — .660 .517 l .420 .402 .470 .920 — .577 .421 i .312 .293 .366 20 g (.029) (.022) ' .888 — .524 .354 : .230 .204 .281 10 l (.030) (.021) .884 _ .510 .329 . .185 .140 .216 0 i (.038) (.022) .908 — .537 .356 i .198 .115 .181 —10 _____ ,u (.099) ( 058) (.028) .957 _ .594 .T .412 .253 .145 .185 —20 ___ ____~_ ____ _ _ _ _j (.106) (.085) (.039) 1 026 .6 2 .495 .336 .219 .231 —30 B Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3'W k : 1.750 2L = 60 C x 25 Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: S.d.(‘p T s . -30 —20 —10 l 0 10 20 30 .885 _ .574 .444 : .368 .376 .457 30 i (.021) (.012) (.012) .823 — .484 .342 ; .264 .280 .367 20 . .784 — .420 .262 i .179 .205 .294 10 (.037) . .772 _ .390 .214 l .116 .150 .239 0 @148)"; (.014) (.036) (.027) .787 — .399 {.211 .085 .111 .203 — 10 :(067) (.029) (.041) (.030) .827 — 446 : .256 .105 .091 .184 —20 _______________ j (.063) (.015) (.021) 887 — .520 337 .177 .119 .194 —30 (.037) (.014) C Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 70 C = 25 Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: s.d.Cp T s —30 —20 -10 I 0 10 20 30 30 .824 _ .521 .405 : .354 .386 .474 ' .760 _ .431 .306 i .268 .319 .407 20 (.033) (.017) . (.020) .720 _ .364 .228 1 .203 .280 .358 10 (.035) (.016) : (.035) .704 _ .329 .177 . .159 .262 .323 0 (.042) _£-_Q1_Q)_l L052) .713 — .328 1.159 .118 .230 .292 —10 ____________(._51)_l(.031) (.054) (.096) .744 _ .359 .177 .087 .171 .252 —20 (.051) (.020) (.101) (.064) .795 — .419 .234 .105 .122 .219 —30 (.036) (.035) (.037) 4 EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES 33 TABLE 17. —Calculated parameters for the Lompoc earthquake using midpoint of Byerly’ s (1930) "observed” intensity bands—Continued D Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 80 C = 25 Bands Used: IX, VIII, VII—VI, V— IV Upper Values: CP ,Lower Values: s.d.(-.. TABLE 18.—Calculated parameters for the Lompoc earthquake using modified "observed” intensities Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 50 C = 25 Bands Used: IX, VIII, VII—VI, V—IV (MODIFIED) Upper Values: CP Lower Values: s.d.(v,. T T s —30 —20 —10 0 10 20 30 —30 —20 —10 0 10 20 30 s .777 _ .487 .389 g .370 .428 1.154 _ .756 .562 :398 .317 .360 30 (.024) (.011) : (.027) 30 : (.096) (.073) .716 — .400 .302 : .313 .400 1.108 _ .696 .498 : .332 .233 .258 20 (.022) (.011) : (.046) 20 . (.116) (.122) (.081) - '___.675 _ .337 .236 i .287 .407 1.084 _ .661 .458 . .290 .192 .190 10 (.023) (_Ql_5_) : (.075) 10 l (.112) (.118) (.105) ——.657 _ .302 ,F 191 .271 .397 1.085 4 .653 .445 1 .274 .173 .166 0 . (.027) . (.016) (.087) 0 H.112) (.118) (.120) " ——.661 .. .293 : .162 .228 .358 1.111 _ .676 .464 . .288 .183 .167 —10 ___________ ( ._034_)__!(011) (.090) —:0 _____: (.117) (.122) (.125) —‘ 684 4 .309 .154 .161 .292 1.162 _ .732 1' .5 1 .343 .231 .201 —20 (.045) (.019) (.090) (098) —20 ________ _ _ _ _ _ _ _j (.125) (.130) (.133) — .725 _ .351 .179 .110 .204 —30 —30 (.041) (.031) (.105) E Reference fault: Hosgri Reference Center: 34°55. 0 N 120°44 3 W k=1.750 2L=90 C: 25 Bands Used: IX, VIII, VII—VI, V— IV Upper Values. CP Lower Values: 5. d. .Cp T -30 —20 —10 0 10 20 ! 30 S 1 .744 _ .469 .393 E .411 .495 30 (.016) (.015) 1 .686 — .391 .324 l .393 .513 20 (.013) (.025) I .648 —— .335 .276 I .402 .532 10 (.013) _(;Q3_4_) _,I .629 — .301 E2 . 383 .518 0 ___________ _.(g15)_ j(.038) .627 — .201 .336 .472 —10 . ( 020) (.032) (.087) .641 — .286 .172 .265 .401 —20 (.030) (.017) (.092) .672 — .309 .164 .178 .311 —30 (.042) (.015) (.090) F Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 100 C = 25 Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: $.d.cp T s —30 l —20 —10 0 10 20 30 .720 .581 .465 .417 l .479 30 _ (.021) (.010) (.041) . .667 .519 .398 .370: .501 20 (.021) (.010) (.941): .631 .476 .352 I .3 .512 10 , (.021) (.010) :(053) .610 .450 3.18 l .309 .491 0 _______(._0_2§l_ 1.0921' .1 (- 057) .603 438 .295 .265 .440 —10 , (.027) (.010) (.053) .610 .439 .283 .218 .368 -20 (.033) (.016) (.040) .630 .456 .287 .182 .281 —30 (.041) (.027) (.021) (.096) Reference fault: Hosgri Reference Center: 34°55. O’N 120°44.3’W k=1.750 2L=60 C=25 Bands Used. IX, VIII, VII— VI, V—IV (MODIFIED) Upper Values: CP Lower Values. s. d. .Cp T s —30 J —20 —10 0 10 20 30 1.003 — .589 .391 i .244 .209 .284 30 1 (.048) (.035) .958 — .530 .322 I .148 .101 .186 20 l (.039) (.025) .937 — .497 .284 g .107 .016 .107 10 I (.108) (.024) (.014) .940 -— .493 275 : .097 .044 .052 0 _____l (.085) (.027) (.019) .968 — .521 f . 0 .128 .076 .021 -10 : (.068) (.013) (.051) 1.019 — 579 : .361 .175 .087 .023 —20 ______________ _: (.119) (.038) (.114) 1.090 — .663 .451 .269 .147 .102 —30 (.130) (.135) (.139) c Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1750 2L = 70 C = 25 Bands Used: Ix, VIII, VII-VI, V—IV (MODIFIED) Upper Values: CP Lower Values: s.d.cp T s —30 -20 —10 0 10 20 30 .886 — .469 .280 : .168 .178 .267 30 i (.036) (.012) .843 — .398 .187 : .068 $271 .199 20 (.083) (.013) (. 1)‘ .824 — .368 .144 E .041 .151 .168 10 (.100) | (.105) (.110) —‘ "“ .830 — .369 .142 i .070 .161 .176 0 (101) _1 (.049) (.110) .859 — 402 F .71"4 .088 .138 .163 —10 ______ ________: (.106) (.023) (.111) .909 — 461.237 .095 .098 .117 —20 (.113) (.022) (.073) .978 — .543 .326 .138 .059 .040 —30 (.124) (.012) 34 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES TABLE 18.—~Calculated parameters for the Lompoc earthquake using modified "observed” intensities—Continued D Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.750 2L = 80 C = 25 Bands Used: IX, VIII, VII—VI, V—IV (MODIFIED) Upper Values: CP Lower Values: S.d.(‘p TABLE 19.—Calculated parameters for the Lompoc earthquake using midpoint of Byerly’s (1930) "observed” intensity bands A Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.675 2L = 50 C = 25 Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: s.d.(~p T T —30 —20 —10 0 10 20 30 8 ~30 —20 —10 I 0 l 10 20 t 30 s I .791 __ .379 .211 ‘ .153 .212 ——$fll——- : 30 0069) 0043)§ 0020) 0060) 20 -745 ~607 ~480 -387 5‘354 ~389 .744 _. .298 .113 . .137 .251 ———————— ““““‘.725 .— .256 .043 g 173 .290 ._1Q___. 0025) “011); (028) 10 (090) I_0936)1 (.102) 0107) -664 '504 ~351 -241 I ~226 290 —— .733 _ 216 173 294 0 (.028) (.011) l (.040) 0 : ( (0)99) ( 103) (2108) .661 .495 331.206 I .186 .262 '.763 __ .297 :.063 .138 .263 ~:lfl——— (035) i-9141J (-046> _:19_____ ________________ 1 0103) 0094) (112) .684 .516 .347 - .163 .226 .812 _ .356 .127 .100 .201 i-_--___-_______1911§l_' (- 026) 0029) -20 (110) 0057) 0119) “30 “‘ "“‘.877 __ .435 .213 .075 .110 —30 (117) 0013) 0114) B Reference fault: Hosgri Referlence Center: C34°55.0'N 120°44.3'W k = 1675 2 = 60 = 25 . E o , a , Bands Used: IX, VIII, VII—VI, V—IV Reference fault: Igosgllu750 Refglfnfeggenteé: 5425550 N 120 44.3 W Upper Values: 013 Lower Values: S.d.cp Bands Used: IX, VIII, VII—VI, V—IV MODIFIED T Upper Values: CP Lower Values: S.d.Cp s _30 _20 -10 0 10 20 30 T 30 ' : -30 ‘20 -10 0 10 20 30 .695 .567 .457 .395 i .409 .477 S . _ 20 0022) 0012) 0017): .717 _ .316 .179 g .199 .306 _352 .643 .504 .385 .327 E .371 .462 30 p 0052) 0014); 0077) __1Q___, 0023) 0011) 0024)l .668 __ .235 .108 g .257 .377 .392 .613 .464 .334 .274 : .353 .458 20 0052) 0023): (104) ___11___ 0026) 0012) _(jgg) ' 0091) "“"““3646 __ .186 086 . .290 .415 .435 .605 .450 .309 {.237 .324 .434 10 (059) _(.997): 0100) _:19___ 0016) :0030) .650 _— .174 g 076 .283 .413 .441 .620 .461.312 1.217 .267 .384 0 __________ 0015.) J (.098) (.104) _—_20.___ __ _ _ 195:7.) _ -0023) - (019) (.095) .679 __ .205 .035 .242 .375 .411 ~30 L. (.098) (.101) (.104) .726 __ .263 .029 .173 .307 .349 ——20 0102) 0091) 0111) (116) ' 788 __ .339 .112 .098 .216 .263 C —30 (.108) (.114) (.075) (.124) Reference faultzkfloslgri Reference Center: 34552550 N 120°44 3’ W F Reference fault: Hosgri Reference Center. 34°55. 0’ N 120°44.3’W k=1750 2L=100 C=25 Bands Used: IX, VIII VII— VI V— IV MODIFIED Upper Values. CP Lower Values: 5. (1. .cp T \\\\\\\\ —30 -20 —10 0 10 20 30 S .654 .455 .275 .186 l .307 30 0034) 0026): 0109) .608 .396 .203 .167 a .371 20 , 0058) 0030) 00§ZLE "".584 .362 .158 F 186 .399 10 0061) 0030) £0097) .582 .354 .136 i .173 .388 0 ______ 0068)__003§)J 0096) .601 .371 .138 .130 .342 —10 0079) 0058) 0099) .642 .413 .091 .067 .273 —20 0094) 0091) 0104) (107) “.700 .478 .244 .014 .187 ‘30 0105) 0068) 0115) T s —30 ~20 —10 0 10 20 l 30 30 i .670 .551 .459 .431 l .498 .595 __29___ 0015) 0011) 0034). _ .620 .492 .395 .384 l .502 .611 10 0015) 0012) 0046): 590 .454 .350 .347 l .492 .605 0 0016) 0013) (.957)J .579 .437 322 ..3 .457 .574 —10 _____ _11@9L_(011)J(.058 .586 .439 .269 .396 .518 —20 0025) 0011) (.047) —30 675 2L— — 70 C Bands Used: IX, VIII, VII—VI, V—IV Upper Values: CP Lower Values: s. (1. .CP EXAMPLES OF OBSERVED VERSUS PREDICTED INTENSITIES TABLE 19. —Calculated parameters for the Lompoc earthquake using midpoint ofoerly s 1930 observed” intensity bands— Continued D Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W —1. 675 2L- — 80 C: 25 Bands Used: IX, VIII, VII— VI, V— IV Upper Values. CP Lower Values. s. d. .fl, 35 TABLE 20.——Calculated parameters for the Lompoc earthquake using modified "observed” intensities—Continued C Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.675 2L = 70 C = 25 Bands Used: IX, VIII, VII—VI, V—IV (MODIFIED) Upper Values: CP Lower Values: s.d.(v,, T T s —30 —20 —100‘10 20 30 S —30 —20 —10 i 0 l 10 I 20 30 30 1 30 i .658 .550 .478 4.91 : .616 .729 .568 .396 .251 .204 ; .335 .439 20 (.010) (.018) (.051): 20 (.044) (.020) (.043) : .611 .497 .426 .468 i .630 .747 .526 .341 .185 .194 . .372 .478 10 (.010) (.023) (_067_)_} 10 (.046) (.016) (.098) I .582 .462 .386 f. .616 .737 .509 .314 .144 .197 i .378 .488 0 (.020) (.010) (.024) : (.079) 0 , (.053) (.018) (_100) _. — .568 .441 .356 g .403 .575 .700 ‘ .519 .317 .129 .166 .351 .466 —10 (_.023)__ (_012)__(.021)_ I (.081) —10 ______ 00133;)- _(.03_1) i(.105) ‘—'.568 .334 .347 .511 .637 . .553 .349 .146 .106 .391 .410 _38 (.016) (015) (.071) _38 (.056) (.111) TABLE 20. —Calculated parameters for the Lompoc earthquake using mo dified "’observed intensities A Reference fault: Hosgri Reference Center: 34°55. 0’N 120°44.3’W 1. 675 2L =50 C— — 25 Bands Used: IX, VIII, VII—VI, V— IV (MODIFIED) Upper Values: C? Lower Values: s. d. .Cp T S —30 I —20 ~10 0 10 20 30 30 : 20 .752 .564 .380 .224 I .148 174 .719 .519 .319 .143 .' .073 141 10 (.059) . (.032) .717 .512 .302 .102 l .063 .159 __0__ (.100) l (.110) .743 .536 .324 .120 l .051 .150 —10 r _____ (.090) (.120) .792 .588 .378 g .175 .031 .104 —§0 ________________ _1 (.118) (.011) (.120) — O B Reference fault: Hosgri Reference Center: 34°55. 0 N 120°44. 3 W k— - 1. 675 2L— — 60 C: 25 Bands Used: IX, VIII VII—VI, V—IV (MODIFIED) Upper Values. CP Lower Values: s. d. .(p T S —30 I —20 —10 0 10 20 I 30 30 l .638 .457 .290 .179 l .199 288 20 _ (.042) (.010) l (.075) _ .598 .403 .218 .110 I .227 .326 _L (.045) (.021) i (.106) .586 .380 .178 .070 I .238 .341 0 (.057) 1.996).} (.107) .604 .394 .180 l .041 .218 .326 —10 (.079) l (.107) .649 .442 .226 .024 .162 .276 ‘38 ____________ ( 10611 (054) T s —30 —20 —10 0 10 20 30 30 : 20 .520 .360 .246 .285 g .467 .576 (.031) (.018) (.097) : .478 .309 .196 .316 :502 .615 10 (.031) (.018) (.997) _- _“ " .460 .282 .161 {.312 .501 .619 0 (.035) (.021) {(098) .463 .277 .137 ..276 .468 .589 —10 _____ _(_.O_43)_(.Ql_4)_ '(102) .487 .294 .128 .215 .405 .529 —20 (.015) (.108) —30 ‘ D Reference fault: Hosgri Reference Center: 34°55.0’N 120°44.3’W k = 1.675 2L = 80 C = 25 Bands Used: IX, VIII, VII—VI, V—IV (MODIFIED) Upper Values: CP Lower Values: s.d.(vp plots or equivalent minima for several 2L values, re- sults from the limited azimuth of observation of inten- sity values for this quake. Little more than 120° is subtended by all stations from the center of the fault. Thus, given somewhat noisy observations, several statistically equivalent solutions are possible, particu- larly when any model parameters are incorrectly set. This is the identical phenomenon observed when try- ing to locate earthquake epicenters with data from too limited range and azimuth and a slightly incorrect traveltime curve. For the parameters used in table 12B (k = 1.6875, 2L = 60), the lowest determined CP and R.M.S. values are as follows: S T RMS CP 10 20 .690 .402 10 - 10 .626 .218 10 0 .632 .110 36 10 —10 .692 .178 0 0 .637 .070 0 10 .695 .238 —10 — 10 .658 .180 —10 0 .667 .041 Thus, minimum CP and RMS values are associated with nearly the same parameter values. We should point out that RMS values as low as those given above are found for 2L values of 70 and 80 km. However, these RMS values are associated with higher CP val- ues (tables 120 and D). Since we consider the GP to be a more critical estimator of proper event parameters, we regard the solution based on a 2L of 60 km as superior to those based on 70 and 80 km. The conclusion seems clear that, if we accept the general applicability of the model, the intensity data for the Lompoc earthquake require a location on or very near the Hosgri fault. Any location even a few kilometers farther west is rejected at high confidence. The tendency of the analysis based on k=1.675 to achieve a sharper minimum is interpreted to mean a more correct estimate of the k value. Thus, we conclude that the most probable parameters for this earthquake are a 2L of about 60 km centered at or a bit south of the lat 34°55.0’ N., long 120°44.3' W. Solutions as long as 75 km or so cannot be rejected. However, if such lengths are correct, k = 1.750 is more appropriate. Whatever the k value or 2L, the model requires that the fault break was very near the Hosgri fault. It seems to us that there is little doubt that the intensity and geologic data together require a location on the Hosgri fault. An issue engendering much heated debate in recent years has been the seismic risk associated with the Diablo Canyon reactor (approximate coordinates lat 35°13.5’ N., long 120°22’ W.). The site is within a few miles of the trace of the Hosgri fault opposite a part of the fault that probably broke in 1927 (fig. 23). If that be true, the site experienced in 1927 the maximum inten- sity that it will have to endure, because there is no evidence of a major fault nearer the site. We predict that the site would experience an intensity of 9.2 (R/F) if it were on thick saturated alluvium. However, the site is actually on Miocene shale of the Monterey For- mation, a formation for which the predicted intensity would be 1.5 units less than that for saturated a1- luvium. Therefore, we predict that the site would expe- rience a maximum intensity of 7.5—8.0 (R/F) or 7(M/M) for a repeat of the Lompoc earthquake. An even longer break would cause only a small increase in predicted intensity at the reactor site. According to the seismic- gap theory, the next earthquake on the Hosgri fault would not include the 1927 break but would break SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES northward from the end of the 1927 break. Such an earthquake with 2L comparable to that of 1927 would give essentially the same predicted intensity at the site as predicted for the 1927 earthquake. EARTHQUAKES OF EASTERN UNITED STATES (EUS, K=l AND 1%) Several earthquakes in eastern North America have been studied via their published intensity contours and the graphic technique described on pages 4 and 5. Of particular interest is the Timiskaming earthquake of November 1, 1935, which is important because it is the largest earthquake (felt in much of eastern Canada and the US.) for which there are S-P data from several near stations. These data allow us to determine the earthquake’s origin time unambiguously and thus to obtain a close estimation of depth of focus. Average estimated O.T. from use of S-P data of 5 stations is 06 h 03 m 37.4 s G.m.t. Analysis of the teleseismic (A 2 22°) data of the ISC with this restrained O.T. gives a depth of 10 km. The recalculated epicenter coordinates are lat 46.98° N., long 78.990 W., D = 10 km. Therefore, there is no doubt that the wide spacing of isoseismals is an attenuation phenomenon. The estimated 2L and A“ values for this and a few other Eastern United States earthquakes are given in table 21. It is of interest to point out that, when radii of intensity zones for the Cornwall/Massena quake were measured along the St. Lawrence River, the solution required a k value of 1% if essential agreement be— tween all data was to be obtained. This is just another example of the reality of the k=11fl; zone shown along the St. Lawrence River on plate 2. The M0 values published by Herrmann, Cheng, and Nuttli (1978) for the Eastern United States earth- quakes are included in table 21. The seemingly un- usual pairing of calculated 2L and M0 values are dis- cussed in a following section entitled "Length of Break Versus Moment Versus k Value Throughout the United States and Suggested Interpretation.” FAULT LENGTH VERSUS MOMENT, MAGNITUDE, AND ENERGY RELEASE VERSUS K REGION It was pointed out previously (Evernden, 1975) that there is no direct correlation between size of intensity contours and energy release for earthquakes distrib- uted throughout the US. The impact of differing rates of attenuation is so severe that totally erroneous con— clusions have been drawn when this factor has been unappreciated or ignored. We will illustrate this fact in two ways. FAULT LENGTH VERSUS MOMENT, MAGNITUDE, AND ENERGY RELEASE VERSUS K REGION 37 TABLE 21—0bserued and estimated parameters for selected earthquakes in the Eastern United States Earthquake Date Latitude Longitude M I(MX) 2L A“ ‘MU YR.MO.DY km 10" cm2 1024 dyne-cm Grand Banks2 1929.11.18 44.5°N 55° W "q ____ 1.3 (1) 20 63 East Missouri2 1965.10.21 37.9°N 91.1°W 5.2 VI .03—.04 (1) 56 0.1 Cornwall/Massens 1944.09.05 45.0°N 74.8°W -1“ VIII 1.0 (1%)3 6.4 2.5 Illinois2 1968.11.09 38.0°N 88.5°W 5.3 VII .08 (1) 4.8 1.0 New Hampshire4 1940.12.20 43.8°N 71.3°W ”,1 VII .016 (1) 0.78 1.0 .16 (1%) 1.2 1.0 Timiskaming2 1935.11.01 46.8°N 79.2°W __-_ VII .11 (1) 6.8 3.2 Missouri2 1963.03.03 36.7°N 90.1°W 4.5 VI .022 (1) 1.1 0.1 ‘Hernnann, Chen , and Nuttli (1978). “Earthquakes use in figures 23 and 24. 5k=1 not permitted by data. Result in agreement with Evemden (1975) and plate 2. ‘k=l% solution in agreement with local magnitude but earthquake in k 1 region of plate 2. Uncertain interpretation. First, it is frequently assumed that energy release in earthquakes in the Eastern United States (EUS) is comparable to that in California, a conclusion based on the occurrence of three great historical earthquakes in EUS (Cape Ann, Mass, Charleston, S. Car., and New Madrid, M0.,) and three in California (Fort Tejon, San Francisco, and Owens Valley, the last sometimes de- scribed as larger than the 1906 San Francisco quake). Table 22 lists calculated 2L and implied approximate E0 values (total energy released) for numerous earth- quakes studied in one or more papers of this series (energy vs. 2L as in Everden, 1975, p. 1290). The earthquake often considered the largest and greatest U.S. earthquake—December 16, 1811, New Madrid—was in fact the smallest earthquake studied if energy released in intensity-relative frequencies is the primary measure of size. The earthquake classed by Wood (1933) as simply a large local earthquake— March 10, 1933, Long Beach—released approximately 100 times as much elastic energy at such frequencies as did the New Madrid earthquake. As the Cape Ann earthquake was no larger than the Charleston earth- quake, the total energy released in these “great” EUS earthquakes was of the order of 5 x 1021 ergs, while the three great California earthquakes released about 3 X 1024 ergs, the Owens Valley earthquake (March 26, 1872) providing less than 1/100 of this energy. There- fore, the intraplate region of EUS has released no more than about one thousandth the energy released in the three cited California earthquakes. According to the historical record, there have been other great Califor- nia earthquakes since the Cape Ann event, so the con- trast in energy release between EUS and California is even greater than here calculated. If these numbers are converted to ergs/kmz/year, the results are: for US east of long 100° W.3.9 X 1012 ergs/ka/yr, for California __________ 6.2 X 1016 ergs/kmz/yr, a contrast in energy-release rates of 1/15,000. There simply is no comparison between release rates of elas- tic energy (at frequencies relative to intensity data) by earthquakes in intraplate areas of US. and in Califor- nia. The second point we wish to emphasize is the clear TABLE 22.—2L, "EU, and "M” values for selected earthquakes in the United States Earthquake ........................ k 2L log"E.."l "M"2 San Francisco 1906 ________ 1% 400 24.2 8.25 Fort Tejon 1857 ____________ 1% 320 24.0 7.98 Long Beach 1933 __________ 1% 22 21.4 648 Seattle 1949 ______________ 11/2 75 22.6 7.98 Owens Valley 1872 ________ 11/2 60 22.4 7.85 Kern County 1952 ________ 11/2 60 22.4 7.85 Charleston 1887 __________ 11/; 20 21.4 7.92 New Madrid 1811— 12 ,,,,,, 1% 20 21.4 7.92 1 5 20.2 7.83 1log "E,” I 18.7 + 2.11(log 2L), (p. 51—54 and Evernden, 1975). 2"M" by formulas of page 41. correlation between observed seismic moments and the regional k factor, a relation indicating either correla- tion of stress drop and attenuation factor or the influ- ence of regional characteristics subsumed under our k factor on observed seismic moments. To begin, we illus- trate the correlation of calculated and observed 2L val- ues and observed and calculated seismic moments in regions ofk 11/2 and k 1%. Hanks, Hileman, and Thatcher (1975) illustrated the general correlation between observed seismic moment and area included within the intensity VI contour (AV-1) by either MM or RF intensities and in either k 11/2 or k 13/"; regions. They found that use of such a mix of data types still yielded A“ vs. M0 data points that showed a general correlation over a large range of MO values. Because AV] values are strongly influenced by k value and intensity scale, we have reanalyzed the data used by Hanks, Hileman, and Thatcher (1975) while adding a few additional events, the intent being to normalize all intensity data to the same scale and to separate data from different k regions. In addition, we have compared observed and calculated 2L values and plotted observed M0 against calculated/observed 2L values rather than against AVI values. All observed and calculated quantities are given in table 23. The “observed” 2L values are those actually observed or calculated on the basis of high-frequency spectral data or short-period seismograms. All calcu- lated 2L values are obtained by use of observed inten- sity data and formulas of this and the previous report (Evernden, 1975). 38 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES A few comments on the data of table 23 and the cal- culated values used in the subsequent discussion are required. In addition to use of Am values for estimating 2L and M0, some events were analyzed by using the full set of intensity contours and our graphic technique. If the graphic technique was used, we always chose to accept the 2L values obtained from it, but we applied it only when the 2L and M0 estimates differed markedly from those obtained from the A“ data. Such discrepan- cies were found for only a few events for which Aw values were in the 1013 cm2 range, that is, small A“ areas. An aspect of the total intensity data included in table 23 is the maximum shaking intensity. Note that agreement between observed maximum shaking in-. tensity and calculated I(MX) is much better for graphic estimates of 2L than for several A“ values, again sup— porting use of the 2L and M0 values calculated from total intensity data. The values of 2L in table 23 that are used in figure 22 and the subsequent discussion are followed by an asterisk. Table 23 illustrates that the mode of analysis fol- lowed here and originally presented in Evernden (1975) leads to estimates of 2L that are in essential agreement with observed breakage or with 2L values estimated by use of short-period seismograms 0r strong-motion records. This agreement is independent of whether the earthquake is in a region of k 11/2 or k 1%, the 2L calculations for earthquakes in k 1M2 assum- ing an energy density equivalent to an earthquake of equal 2L in k 1%. As an additional test of whether the 2L values de- termined for k 11/2 earthquakes are meaningful, we analyzed the published intensity data on all events of the region for which there is documentary evidence of length of surficial cracking or displacement (data pro- vided by M.G. Bonilla of the US. Geological Survey). The graphic technique described earlier was used to make the analysis. Table 24 shows reported and calculated 2L values for the earthquakes studied. The only additional comments required are: (a) The 2L value calculated for the Manix earth- quake seems to be in serious disagreement with observed values of I (MX) and ML but in excellent agreement with the MO values of 1.4 X 1026 reported by Hanks and Thatcher (1972). (See figure 24 and note that, in k 1%, 2L of 10 implies an M0 of 1.25 X 1026 dyne- cm.) Perhaps the low value for I (MX) is to be explained by a low water table, and the low ML value by the fact that it was measured at Pasadena (that is, although the earthquake occurred in a region where k=11/2, the path to the Pasadena station was mostly through a region in which k=1%). (b) The intensity VI (5.5) dimension for the TABLE 23.—Observed Earthquake No. In. Reg. Date YR.MO.DY Hemet 01 MM 7 1963.09.23 Lytle Creek 02 MM 6 1970.12.09 Coyote Mountain 03 MM 7 1969.04.28 Parkfield 04 MM 7 1966.06.28 Desert Hot Springs 05 MM 7 1948.04.12 Long Beach 06 MM 7 1933.03.11 Santa Rosa Mt. 07 MM 7 1934.03.19 San Fernando 08 MM 7 1971.02.09 Borrego Mt. 09 MM 7 1964.04.08 Imperial Valley 10 MM 7 1940.05.18 San Francisco 11 RF 7 1906.04.18 Santa Barbara 12 RF 7 1925.06.29 Lompoc 13 RF 7 1927.11.04 Fort Tejon 14 RF 7 1857.01.09 Wheeler Ridge 15 MM 6 1954.01.12 Truckee 16 MM 6 1966.09.12 Bakersfield 17 MM 6 1952.08.22 Fairview Peak 18 MM 6 1954.12.16 Kern County 19 MM 6 1952.07.21 Oroville 20 MM 6 1975.08.01 Pocatello Valley 21 MM 6 1975.03.28 Oroville earthquake was not used because it is so small (see discussion above). The ground condition in the Oroville area prob- ably accounts for the small area mapped as intensity VI. (c) The intensity data for the Hebgen Lake earth- quake indicate that the attenuation region surrounding the epicenter is not uniform. They imply (via our model) an attenuation factor of k=11/2 in the area to the south (to- ward a region in which we know k to be 11/2 on the basis of several earthquakes) but be— tween k 11/2 and k 1% to the north and east. If a k value of 1.35 is used to the north and east, we obtain a 2L value similar to that for the data from the area to the south and k 11/2. (d) The Fort Sage Mountains and Galway Lake intensity data are grossly inconsistent with reported lengths of fracture. All Fort Sage Mountains data agree on a 2L of 1.0 km (versus reported 8.8 km), while all the Gal- way Lake data imply a 2L of well under 1 km (versus reported 6.8 km of surficial breakage). Because the 2L values we calcu- late are for the equivalent k 1% earthquake to provide the energy required to develop the observed k 11/2 intensity pattern, these very short calculated 2L values suggest one of three conditions: stress drops were abnor- mally low, high-frequency energy was re- leased from a short piece of the break (as at Parkfield), or observed surficial fracturing , was influenced by factors other than rupture length at depth. Whatever the condition, the short calculated 2L values imply, if any- thing anomalous, that less energy is re- FAULT LENGTH VERSUS MOMENT, MAGNITUDE, AND ENERGY RELEASE VERSUS K REGION 39 and calculated parameters of earthquakes in regions of k =1% and k=1 V2, California and Idaho ________________ Observed Values_____--___-“.-- .1----1__-____---_______"Av1 Calculations--__.1___-__-____1___._,-_ ,___._______Intensity calculations__._________ Mag. Mom. Av. 2L I(MX) Mom. A" 2L Mom. Morn. ML I(MX) 2L Mom. Mom. ML I(MX) SHKG N,7 RF,7 RF,7 MM,6 7 RF,7 MM,6 7 , 5.3 0.02 0.24 VI 0.48 1.2 .018 4.7 6.2 5.4 0.10 0.22 VII 0.44 1.1 .015 4.7 6.2 1.1 0.09 5.6 7.1 5.9 0.50 0.61 VII 1.2 3.6 0.19 5.4 6.9 5.5 1.3 0.54 3 VII 1.0 3.1 0.12 5.3 7.4 6.5 1.0 2.6 VII 4.7 27 6 6.6 8.2 10 1 6.0 7.6 6.3 2.0 1.2 VIII 2.1 9 0.8 5.9 7.5 22 3.4 6.5 8.1 6.2 4 2.3 V1 3.9 23 4 6.5 8.1 6.4 4.7 2.3 16 IX 3.9 23 6.5 8.7 19 3 6.4 8.6 6.5 6 3.4 VII 5.6 38 10 6.8 8.5 7.1 20 3.3 IX 5.4 37 6.8 8.5 60 20 7.1 8.8 8.2 850 16 400 IX 130 150 400 1500 8.3 9+ 6.3 20 4.4 IX 40 11 6.9 9 7.3 6.5 IX 70 31 7.2 9 8 900 320 IX 320 900 8.1 9+ 6.6 0.33 1.75 VII+ .026 0.65 1.8 0.04 0.5 4.9 7.5 6.4 0.5 1.61 VII .04 0.60 1.6 0.03 0.41 4.9 7.4 5.8 0.55 0.32 VIII .023 0.11 0.33 .000 .007 3.6 6.1 2.1 0.54 5.1 7.6 7.0 90 14 40 VIII 6.0 4.8 31 6.7 100 6.7 10 7.7 170 17 60 IX 11 5.8 40 11 170 6.9 8.8 60 23 241 7.1 9.1 5.9 0 2 0 48 1.5 VII .017 0.18 0.36 .002 .018 4.0 6.4 1. 5 O. 35 4.8 7.4 6.0 0. 65 1. 4 3 VIII .053 0.52 1.4 .023 0.28 4.8 7.3 3 0 1. 13 5.3 7.8 Notes: "In." — -Intensity type of published data. ‘MM” =Modified Mercalli. '“RF —Rossi Forel' Reg" - k region. ""6 — 1/2. 7” 1% "."Mom —Seismic Moment in 1025 dyne- -cm. “”N = Moment of Region 6 (1V2) earthquake normalized to Region 7 (1%)“AH” — Area in 102‘ cm2 included in intensity VI contour “Observed AH“ values are in intensity and regional units of columns 3 and 4. ML" = Local Magnitude. “’0bserved magnitude values in all regions are calculated with Richter formula for southern California (Reg. 7). “Calculated I(MX)” is maximum predicted shaking intensity and is in units (MM or RF) ofobservations.‘ In. Calculations" are based on full attern of intensity observations. “”2L values followed by an asterisk are those used in figure 22. For event 0 17, Mom (RF 7) entry under A” Calculations is 0.0004. AREA WITHIN INTENSITY VI CONTOUR, IN SQUARE CENTIMETERS 10‘3 1014 15 re 1026 27 28 1000 : I lcl) 10 I 1:) HI) : I 11 : 3 - 14/ — 1.11 E - 1 g Mo=2.56logAV.(R/F,1%)»1176 E 100 .— _: E E E g _ / _ < - _ Lu - E -18- ~ .3 _ _ D E Mo=2.56 log Av,(M/M.1V2)-11.76 u. 10 r— _ O ’1 : I : _. 5 _ EXPLANATION : Z - '11 5 Earthquakes in k 1%, Mo (observed) ‘ ' /4 —17— Earthquakes in k 11/2, M0 (observed) 4 ' —15 ‘17 1_7 Earthquakes in k 1V2, Mo (observed) 1 —20--162- normalized to k 1% (see text) I I l—II-III l J IllllII I I I II|||I | lJJIIIII 11013 10” 1023 102‘ ' 1025 1026 1027 1028 SEISMIC MOMENT, IN DYNE CENTIMETERS FIGURE 22.—Length of fault break (2L) as a function of seismic moment (Mu; right) and area within R/F and M/M intensity VI contours (Aw; left) for k = 1% and k = 1V2. 40 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES leased by the k 11/2 quakes than by k 1% earthquakes of equivalent 2L. Thus all available data seem to indicate that 2L val- ues for k 11/2 earthquakes as calculated by means of our model are quite accurate estimates of actual break lengths and, thus, that energy released in the intensity pass—band of k 11/2 earthquakes is essentially identical to that of k 1% earthquakes of the same 2L. For our discussion of seismic moment (M0), we first consider events occurring in regions where k= 1%. The left side of figure 24 indicates the theoretical relations between A“ and 2L for both M/M and R/F and k 11/2 and k 1%. The equivalent areas for the same 2L are deter- minable and are shown to increase by about a factor of ten from AVI (M/M, 1.75) to AVI (R/F, 1.50). In order to relate A“ or 2L to M0, an empirical relation must be established as done by Hanks, Hileman, and Thatcher (1975). On separating data of different k regions and intensity scales, we find a different scaling law of AVI and M0 than found by them. The relation found appro- priate to k 1% data (normalizing Avr (M/M, 1.75) values to A“ (R/F, 1.75) values by figure 22) is: log M,,=2.56 log A\v1(R/F, 1.75) - 11.76 This curve, drawn on the right side of figure 22, is expressed as a relation between 2L and M0 via the Aw (R/F, 1.75) vs 2L curve of the left side of figure 22. The non—underlined numbers plotted along the curve are calculated or observed 2L vs observed M“ values for k 1% earthquakes. In most cases, there is excellent agreement with the empirical curve over the entire range of M0 values. This agreement implies that, for most k 1% earthquakes, both short- and long-period energy derive from the same fault length according to a single spectral scaling law. Two events show marked disagreement with the curve. Event No. 4, Parkfield, is of particular interest because it has been described extensively in the litera- ture. Numerous investigations of long-period data of this earthquake find an M0 value of 1025 and observed surface breakage of 30 km or more. However, the strong-motion data indicate that the high-frequency energy came from a 3-4 km length of the fault (Lindh and Boore, 1981). In addition, the intensity VII contour is only 22 km long, a value that is totally anomalous for a normal California earthquake with a 2L of 20 km or more. As shown in table 22, the intensity data imply a 2L of 3 km. The Parkfield quake clearly was abnor- mal for California because the high-frequency energy was derived dominantly from a short piece of the fault at normal California stress levels, while the long- period energy was derived from failure of a much longer, slowly breaking fault segment. On the basis of other analyses, discordance of the Parkfield datum point with the empirical curve of figure 22 should have been expected. Inadequate analysis has been done on event No. 3 (Coyote Mountain) to ascertain whether a similar explanation applies to that datum point. As regards event No. 5 (Desert Hot Springs), the agreement with the curve shown on figure 22 would not have resulted from use of the A“ value reported by Hanks, Hileman, and Thatcher (1975). The reported intensity data for that quake show some remarkable inconsistencies in distribution of M/M VI values. These were reported as far away as Los Angeles but were interspersed with many much lower values. The 2L value of 27 km deriving from the A“ area used by Hanks and his coworkers predicts epicentral inten- sities that are too high and a felt area much too large. The “Limit of Detection” (L.O.D.) boundary drawn in “US. Earthquakes” is usually near intensity 3.0. A 2L value of 10 km for the Desert Hot Springs earthquake places the 3.0 boundary inside the outer lobes of the L.O.D. In addition, a 2L of 10 km or less is required to predict intensities of only VII at Desert Hot Springs and only IV at Death Valley and Needles. Without ig- noring the anomalous M/M VI values reported for this earthquake, but for purposes of making a 2L estimate most consistent with intensity observations, we use a TABLE 24.-—Observed and calculated 2L values for selected earthquakes Earthquake YR MO DY LAT(N) LONG(W) M(OB) 2L(OB) 2L(PRED) Cedar Mountain, Nev. ______________ 32 12 20 38.8 118.0 7.2 61 66 Excelsior Mountain, Nev ,,,,,, 01 30 38.0 118.5 6.3 1.4 3.5 Hansel Valley, Utah ,,,,,,,,, 03 12 41.5 112.5 6.6 8 10 Manix, Calif. _______________ 04 10 35.0 116.6 6.4 4 10 Fort Sage Mountains, Calif ___________ 50 12 14 40.1 120.1 5.6 8.8 1 Kern County, Calif. ________________ 52 07 21 35.0 119.0 7.7 160 60 Rainbow Mountain, Nev _____________ 54 07 06 39.4 118.5 6.6 18 18 Rainbow Mountain, Nev ,,,,,,,,,,,,, 54 08 23 39.6 118.4 6.8 31 36 Fairview Peak, Nev. ,,,,,,,,,,,,,,,, 54 12 16 39.3 118.2 7.1 48 40 Hebgen Lake, Mont. ________________ 59 08 17 44.8 111.1 7.1 24 228 Galway Lake, Calif. ________________ 75 05 31 34.5 116.5 5.2 7 1 Pocatello Valley, Idaho ______________ 75 03 28 42.1 112.6 6.0 33 3 Oroville, Calif _______________________ 75 08 01 39.4 121.5 5.7 34(1.5) 1 5 ‘33 kilometers of fracture in bedrock. However, epicenter was about 30 kilometers away under Quaternary deposits. See text. 2By use of data to south of epicenter. See text. 32L values as determined from short-period seismograms. See text. FAULT LENGTH VERSUS MOMENT, MAGNITUDE, AND ENERGY RELEASE VERSUS K REGION 41 2L value of 10 km, which yields a calculated M0 (1%) of 1.0 X 1025 dyne cm, ML of 6.0, and I(MX) of 7.6. Note again that a formula relating AVI and Mo serves very well to predict most observations (fig. 22 and table 23). The point of greatest relevance here is one men- tioned only in passing above. If observed M0 values for earthquakes in k 11/2 regions are “normalized” to k 1% by the ratio of moments predicted via Av, (R/F, 1%) and AV] (M/M, 11/2) values calculated for the 2L value found by observation or analysis of intensities, one obtains normalized M0 values that agree with predicted MO values for earthquakes of this 2L (underlined numbers in fig. 22) in k 1% regions. This correlation strongly suggests that observed MO values of k 1% earthquakes are not directly comparable with observed M0 values of k 1% earthquakes insofar as implying relative levels of long-period energy release but are probably correlated with details of the relevant relaxation and radiation phenomena. The explanation of the high M0 values for k 11/2 earthquakes certainly is not a simple regional difference in Q attenuation. The absence of such re— gional differences in Q is demonstrated by the fact that for nuclear explosions, and thus for point sources, the MS versus yield curve is independent of k region (Evernden and Filson, 1971). Now note the reported and calculated M1, values in table 23, wherein all calculated values are for equiva- lent 2L earthquakes in k 1% regions. In Evernden (1975), the relation: ML: (log 2L + 3.2667)/0.711 was empirically developed on the basis of data from k 1% earthquakes. The validity of this relation with re- gard to the earthquake studied is shown by the fact that the average observed ML value for k 1% earth- quakes of table 23 is equal to the average ML calculated via the above formula when 2L is either observed or obtained from analysis of intensity data. The next point to note is the marked disagreement between reported ML values for k 1% earthquakes and the ML for an earthquake of equivalent 2L in k 1%. The average reported ML of studied k 11/2 earthquakes is 6.5, while the average calculated ML for the equivalent 2L earthquakes in k 1% is 5.5, a difference of 1.0. This difference is an average measure of the inconsistencies routinely occurring in ML estimates of k 1% earth- quakes. If magnitude estimates were used only as orig- inally intended by Richter (1955), that is, as a scheme for ordering earthquakes of a region according to a generalized size parameter, the only error in the pres- ent calculations would be an incorrect attenuation formula for k 11/2 earthquakes. However, since ML es- timates are considered by many as measures of energy release, moment, and other parameters, a major incon- sistency in present practice is to apply a formula devel- oped in k 1% regions to earthquakes in all regions. We should be using a set of formulas of the following type: Region Formula k 1% M.,=log A + 1.75 log (D/lOO) k 11/2 =log A + 1.50 log (D/100) —a1 k 1% =log A + 1.25 log (D/lOO) -a2 k 1 =log A + 1.00 log (D/lOO) —a;; The formula for k 1% is inconsistent with Richter (1958, p. 342), because it predicts a value for M1, at 600 km that is 0.5 lower than would be predicted from the data in Richter’s table. It is interesting to note that the University of California at Berkeley often reports ML values about 0.5 ML greater than does California Institute of Technology, Pasadena, for southern California earthquakes (T. V. McEvilly, oral commun., 1979). A possible explanation for the differing ML val- ues is that the data used to establish Richter’s curve may have been contaminated by multiprovince paths. Using the formulae of Evernden (1975) and nor- malizing the set of equations so that earthquakes of the same high-frequency energy, and thus fault length, are given the same ML, it follows that a1=0.5, a2=1.0, and a3: 1.5. Universal use of the k 1% formula for estimat— ing ML leads to errors for stations at about 200 km of 0.6, 1.2, and 1.8 in k 11/2, k 1%, and k 1 regions, re- spectively. There are other possible schemes of scaling, and there are reasons why the scaling used in Evernden (1975) may not be directly convertible to maximum amplitudes versus distance. The disagree- ment between 1.0 and 0.6 suggests that additional fac- tors may be influencing maximum amplitude. The important point is that the high ML values pres- ently assigned to earthquakes on faults with small 2L in k 11/2 areas indicate that an invalid formula was used for calculating M1,, not that stress drop was higher for these quakes than for k 1% earthquakes of equiva- lent 2L. We suggest that the following formulae be used until more detailed work has been done: Region Formula for ML 1% log A + 1.75 log (D/lOO) 11/2 log A + 1.5010g(D/100) — 0.75 1% log A + 1.25 log (D/lOO) — 1.50 1 log A = 1.0010g(D/100) — 2.25, where A is amplitude in micrometers, D is epicentral distance in kilometers, and amplitudes are as meas- ured on standard Wood-Anderson seismometers. A suggested pattern of k values for the United States is to be found on plate 2. Corrections for multiregional paths must be made. 42 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES CRUSTAL CALIBRATION AS FUNCTION OF REGION To elaborate further on the theme of the last para— graphs, one of the major continuing errors in estimat- ing seismological parameters is the use of invalid calibration formulas. The routine mb values presently published by USGS use a B(A) term in the distance range 0°—20° that was demonstrated to be invalid years ago (Evernden, 1969). The ML B(A) term (in M1, = log A + B(A)) used universally is the one empirically deter- mined by Richter (1955) as being appropriate to south- ern California. It is certainly invalid (that is, does not yield consistent estimates of M1,) in regions of different attenuation. A recent example of the use of such methods to calcu- late m1, and ML is seen in the data for the earthquake of March 28, 1975, in Pocatello Valley on the Idaho-Utah border. The published parameters for this quake (Arabasz and others, 1981) are An (M/M, 1.50) = 1.4 X 10H cm2 ML 2 6.0 I(MX) = VIII M/M 2L = 3.0 km (analysis of short-period seismograms) M0 = 6.5 X 1024 dyne-cm. The m,, values are as calculated and published by the USGS. One anomaly in these data is that values of 6.6 to 7.0 are listed in the 8°—9° range; that is, an incorrect calibration curve for In), is still being used in this dis- tance range. In Evernden (1969), the incorrect shape of the B(A) curve routinely used in the Western United States was extensively discussed and the resultant errors in estimation of mb were illustrated. The mb values reported for stations in the distance range 0°—20° for this earthquake should be ignored. Secondly, the mean In,J value reported for stations in the range 20°—54° is 5.8, while the mean mb value re- ported for stations in the range 65°—83° is 6.1. It was pointed out in Evernden and Clark (1969) that the Gutenberg B(A) curve for P waves is inconsistent with modern observational data, as this curve yields mh values that are 0.4 higher in the range 65°—83° than values in the 20°—54° range. If calibration is made against the nearer stations, the reported mh for this earthquake becomes 5.8, and all reported values within this range fall at or between 5.5 and 6.1 Finally, how does this magnitude compare with that expected for the same event occurring in western California and recorded at low-amplitude stations (Evernden and Clark, 1969; Evernden, 1977)? The source calibration is assessed to be in the neighborhood of 0.25 mb unit (Evernden, 1969). Nearly all stations in the 20°—54° range are in shieldlike areas, that is, EUS— type crustal structure, a condition that leads to an ex- pected difference of 0.5 mb unit relative to low- amplitude stations (Evernden and Clark, 1969). There— fore, when compared with the magnitudes used in other reports by this author, the m., value for this event would have been mb 5.0 if it had occurred in western California and been recorded at low-amplitude sta- tions. Next, consider the reported ML value of 6.0. The B(A) curve used by the USGS and by Arabasz, Richins, and Langer (1978) in converting Wood-Anderson data to estimates of M1, is the curve established by Richter (1955) for southern California. In other words, a cali- bration curve appropriate for a region of k 1% is being used in a region of k 11/2, a procedure that is almost certain to lead to gross errors if ML values in k 11/2 are to be both independent of distance and correlative in some way with M], values of events in k 1% regions. Ignoring all USGS M1, values because of the great distance of the stations used, consider only the two ML values reported by Arabasz, Richins, and Langer (1981), that is, 5.9 at 210 km and 5.8 at 310 km. We assume that these values indicate an average value of 5.8-5.9 in the range 200—300 km. If the value for k is assumed to be 1% while it is actually 11/2, magnitudes in the 200—300 km range may be overestimated by 0.6—0.9 M], unit. Therefore, a calibrated ML value for this event would have been about 5.0—5.3. The reported AV] (M/M, 1.50) value of 1.4 X 1010 cm2 leads to a predicted 2L value of 1.35 km and a predicted M0 (1.50) of 2.8 X 1024 dyne cm, while our graphic technique predicts a 2L of3 km and an M0 of 1.1 X 1025 dyne cm. Both values of M0 are to be compared with the reported value of 6.5 X 1024 dyne cm, which would predict a 2L of 2.0 for an earthquake in a region of k 11/2. All of these values are probably indistinguishable from the 2L of 3 km calculated on the basis of short— period seismograms. A 2L of 3.0 implies an earthquake of M1, 5.3 for a region f0 k 1%. Thus, the intensity data, M1, data, and m1, data imply that this event was much smaller than usually considered (about ML 5% if in k 1%), and all the data agree with the 2L estimate based on high-frequency data. In addition, the 2L value of 3.0 km for a k 11/2 earthquake is predicted to be associated with an IMAX (M/M) of 7.8. This value is consistent with the observed value of VIII in a very small area. All these observations appear to establish that this earth- quake was of small dimension (2L of 1.5 to 3.0 km) and that its energy output in the pass—band typical of in— tensity values and ML values (approximately 0.5—3 Hz) was equivalent to that of an earthquake of equivalent 2L in western California. LENGTH OF BREAK VERSUS MOMENT VERSUS K VALUE 43 LENGTH OF BREAK VERSUS MOMENT VERSUS K VALUE ’ THROUGHOUT THE UNITED STATES AND SUGGESTED INTERPRETATION A point that probably requires reemphasis is just how meaningful, in a physical sense, are the 2L values we calculate from the intensity data. The procedure for calculating these 2L values must be reiterated. Given the appropriate k value for a region, the intensity data are then used to calculate the energy required at the focus to create the observed quantitative pattern of in- tensities. Then, we determine the 2L value required in a region where k=1%, that is the region of calibration for 2L versus energy, to supply the calculated energy requirements. To evaluate whether these calculated 2L values in each region are meaningful, we compare them with other data that establish actual lengths of break for earthquakes for which we have such esti— mates. Where k= 1%, we should and do get nearly cor- rect values because this is the region of calibration. The data in tables 23 and 24 prove that the procedure described above serves successfully to estimate the 2L values of earthquakes in regions where k=11/2. All k 11/; events with known or presumably known 2L values are included in these tables, which show that the agreement between observed and calculated 2L values extends over at least the 2L range from 1 to 60 km. As for 2L estimates in regions further east, we stress again (as was done in Evernden, 1975) the agreement the calculated 2L (20 km) of the 1886 Charleston earthquake and the size of the high-intensity isoseis- mal for that quake. In addition, we point out that loca- tions of presently occurring small earthquakes in the Charleston area cluster along a 20-km zone exactly placed to fit within the high-intensity contour of the 1886 earthquake (Arthur C. Tarr, written commun., 1979). Finally, master-event locations by James Dewey (oral commun., 1979) of all historical and instrumen- tally locatable earthquakes in the Charleston area are along this same 20-km zone. Several earthquakes originally placed offshore can be proven to have oc- curred in this 20—km zone. Thus, all seismic activity in the Charleston area for the past several decades has consisted of aftershocks of the 1886 earthquake. In other words, there is only one seismic locus in the re- gion, and it seems certain to have been the locus of the 1886 earthquake. There are no other earthquakes in the Eastern United States for which unequivocal demonstrations of length of break exist. On the basis of geologic and seismologic evidence, Frank McKeown (written com- mun., 1979) has concluded that there are no fault seg- ments longer than 10 to 20 km in the New Madrid area. To quote from his letter addressing this point: I believe that source dimensions of so—called New Madrid earth- quakes are small, e.g., not more than 10—20 km in length. The basis for this opinion is that faults in the postulated New Madrid fault zone of Heyl and Brock, as mapped in the Illinois-Kentucky fluorspar district, are of such dimensions (see Heyl and McKeown, 1978). Also, about that time, I started thinking about an apparent relationship of mafic instrusives to earthquake source zones in eastern US. This resulted in a speculative paper (McKeown, 1978) which would be different if I were to write it today, but short fault lengths were postulated based upon some circuitous reasoning. I still don’t think the ideas in the paper are all wrong, but more emphasis should have been made of the evidence of intraplate rifts and associated rocks. As you know, evidence of a rift and associated structures and intrusives has been accumulating. Hildenbrand, Kane and Stauder (1977) show pretty clearly a rift-like structure that appears to be terminated by northwest-trending structure of some kind near New Madrid. Prior to the aeromagnetic and gravity data, the presence of alkalic mafic intrusives in the subsurface of the embayment and surface around the embayment was indicative of rifting. Evidence for short faults in the New Madrid area can be inferred from the seismicity pattern, focal mechanisms, and reflection profile data. The subsurface struc- ture near New Madrid must be very complex with no apparent through-going long faults, as indicated by diverse epicenter trends and differing focal mechanisms. South of Caruthersville, Mo., the seismicity trend appears to be in the center of the riftlike structure. One can infer that it is related to a fault or fault zone about 100 km long. If, however, a typical east-African type of intraplate rift is present in the subsurface, I cannot believe that the seismicity is along a single long fault. Rifts do not have such faults according to the maps and literature that I have examined. A rift contains numerous parallel-to-subparallel normal faults that apparently re- sult from the tensional stress across a broad arch that preceded for- mation of the rift. Perhaps if the seismicity were confined to one side of the rift, a long fault zone could be postulated, but even the bound- ing faults of rifts are commonly en echelon. Our estimate of a 21/2- to 5-km 2L value for this earthquake (k= 1, Evernden, 1975) may or may not be consistent with these field observations. Because we do not know the correct k value (anything between 1 and 1.25 being permissible on the basis of the available data), we cannot deny the possibility that the fault is as much as 10 km long. There is positive evidence that our technique for es- timating 2L values yields accurate results in several k regions (k 1% to k 14), there is no reason why the tech- nique should fail in regions of k=1, and available data in the New Madrid area indicate that, indeed, it does not fail but yields accurate 2L estimates. The conclusion that one must draw from these re- sults is that the fault zones in all regions of the United States are very similar. Asperities must be of essen- tially equal strength and equal mean distribution on fault surfaces in all regions. Whatever the effective stress conditions are in one region, they are duplicated in all others. The inhomogeneity in stress on fault sur- faces in k 1% regions (San Andreas fault), expressed by asperities with stress drops of several hundred bars while average stress drop is a few tens of bars, is now common knowledge. What is new here is that this pat- tern is probably similar in all regions of the United 44 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES States. The mean stress can rise somewhat without affecting high-frequency energy but cannot rise an order of magnitude. Given the validity of our estimates of 2L and its as— sociated implications, the next point to note is the strong dependence of the 2L vs M0 relation on k value. All of the data of tables 22 and 23 are plotted on figure 23. It is obvious that the relation between moment (M0) and 2L is not fixed across the United States. Moment actually increased by a factor of about a thousand for the same 2L from k 1% to k 1. Because 2L is estimated from high frequencies (~1 — 4 Hz), while M0 is meas- ured on the basis of long periods (20 seconds to infin— ity), figure 23 can be considered as a plot of short- period energy versus long-period energy, and the plot implies a thousandfold increase in long-period energy relative to short-period energy from k 1% to k 1. As a point of interest, the correlative A“ versus M0 data for the earthquake of figure 23 (plotted in fig. 24) show a similarity between the M0 versus A“ relations for regions k 1% and k 11/2, while the relations of 2L versus M0 for these two regions are markedly different. This insensitivity of Aw versus M0 to change from k 1% to k 11/2 is why Hanks, Hileman, and Thatcher (1975) were able to get a common curve for AVI versus MO when they mixed the data of earthquakes from regions of 1% and k 11/2. The question, then, is how to explain the 2L versus M0 relation of figure 25. To be specific, how is it possible for an M0 value of 1026 dyne—cm to be associated with a 2L value of 1 km, the paired values on the k= 1 curve of figure 23‘? No permissible association of ,u, L, D, and H in a uniform half—space could possibly explain these values. There must be another operative in- homogeneity. We suggest that earthquakes of the Eastern United States are along fault zones that con— stitute soft inclusions in an otherwise highly rigid and strong crust. Following Eshelby (1957), we consider the following situations: (a) uniform elastic medium of shear modulus [10 (b) soft inclusion (sphere) of shear modulus ,ul im- 1000 l : llllllll l I IllIlll I I lllllll l llllllll l I Illlll‘ l llllllll 7 llllll : — El ' EXPLANATION 100 _— o k earthquakes m : o k 1V2 earthquakes 35 u _ ,_ - LLJ 2 g lO :- u k 1% earthquakes 1 x — I E 3 : g i _ < .— 32 ‘ Parkfield m — 5 _ 2 t: 1.0 _— : O : _‘ I — Z l— ._ 0 _ _ Z LL] " .1 _J 0.1 _—— --:J 001 i I 1111”] llllllll l illllll I ulllllll 1 llllHll l llllllll l lllllll ' 102' 1022 1023 1024 1025 1026 1027 1028 SEISMIC MOMENT, IN DYNE CENTIMETERS FIGURE 23.—Length of fault break (2L) as a function of seismic moment (M1,) for all k regions of conterminous United States. LENGTH OF BREAK VERSUS MOMENT VERSUS K VALUE bedded in an otherwise uniform elastic medium of shear modulus ,uo. Presuming shear stress to be applied at distances that are large compared with dimensions of the soft inclusion, we investigate shear stress, 7-,, at the center of the space and at the center of the sphere in terms of the remotely applied shear stress, TA. In case (a), T, = TA In case (b), 71 TAMI/[Mo — But” — #1)] where [30 = 0.1333 (4 — 5vo)/(1 — v0), and v0 = Poisson’s ratio outside the inclusion. For 12,, = 0.25, ,8 = 22/45 5 1/2 Then, we find the following relations: 0.05 0 0.10 0 0.50 0.67 0.10 0.18 [Ll/I440 1 TI/TA 1 Thus, the stress within the soft inclusion is less than the distantly applied stress. Or, in order to achieve a given level of shear stress on a fault within the inclu- 45 sion, it will be necessary to apply a greater distant shear stress. If ,u, is one-tenth of ,u”, the externally applied stress must be 5.5 times the stress required on the fault surface. If ,u, is one-twentieth of Mu, the exter— nally applied stress must be ten times that required on the fault surface. Thus, if mean shear stress required for failure is 100 bars, regional stresses outside the inclusion must be 550 to 1,000 bars, that is, a low- stress—drop quake in a highly stressed regional envi- ronment. Now, consider the comparative changes in strain en- ergy associated with fault-zone failure at shear stress 7,. Since strain energy change is a measure of moment, this analysis will be relevant to figure 23. Consider the following situations, assuming total stress drop on the fault: (a) As before, that is, uniform space; A E, = 8r37-12/7p.” where r=radius of circular fault patch. 1013 : I I IIIIIII l Illlllll I lllllll] l llllllll I I lllllll I llllllll IIIIH_ " EXPLANATION ‘ g 1017 _— o klearthquakes —_ ,_ _ _ E : 2L—5km E l— 3 _ o k11/2 earthquakes LIJ — a: _ < a . u: IO‘SL— El k1% earthquakes T: 2 : : n:- : : 3 _ o _ I— _. z ._ o Q _ S t (—5 1015 :— —: z _ _ ,_,_, _ _ l— .. E - : g _ _ I l; __ .. 3 < E 14 < 10 E— —: 1013 1 ll Illll I llllllll l llLlllIl l lJllllll I lllllll J llllllli I 111111: 102‘ 1022 1023 1024 1025 1026 1027 1028 SEISMIC MOMENT, IN DYNE CENTIMETERS FIGURE 24,—Area within intensity VI contour (Aw) as a function of seismic moment (M0)for fault lengths (2L) of 5, 50, and 400 km in all k regions of conterminous United States. 46 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES « up. Mudumof‘“ 200 KILOMETERS , .5." ...om.‘,q§ P» Conn-gnaw M‘“ 5.» my.» :55, s...“ Rnla \ 5.". am". ‘ a %\ s—m emu-m 5 XS... Chm-M: 1 \ \3 s." weal» \ FIGURE 25.—Fault breaks used in models for estimating replacement value of damaged wood-frame construction. MAPS OF PREDICTED INTENSITY PATTERNS 47 (b) there exists a soft region with radius R and shear modulus M1; A E, = 8r3712/7u1 ,r << R: (c) same conditions as (b) except that total stress drop is presumed to take place within the entire vol- ume of the soft inclusion; A Ec = 7TR3(p.0 + M1)712/3p.0,u.1 , for B = 1/2. We then obtain the following values: 111/11,, 1 0.5 0.10 0.05 0 Eb/Ea -- 1 2 10 20 cc EC/Ea R/r= 2.5 29 43 157 300 cc Ec/EaR/r= 5 229 343 1250 2405 cc E413a R/r=10 1832 2749 10079 19242 as It is apparent that nearly any desired ratio of 2L and M0 is possible in concept. The asperities on the fault surface provide nearly all the high-frequency energy, while inhomogeneous relaxation of much lower aver- age stress level can provide the long-period energy. The questions that arise are: 1—Why should one hypothesize total volume re- laxation?, and 2—What are reasonable values of R (given a satis— factory answer to 1)? The basis for a total-volume—relaxation hypothesis is founded on: (a) The conclusion, based on intensity and 2L data, that all fault zones are similar and thus strongly conditioned and weakened. (b) The suggestion in observations that dilatancy may occur in the Eastern United States, such dilatancy implying extensive fracturing and weakening of the volume surrounding the fault. (c) The fact of high values of measured ambient stress in many Eastern United States rocks along with the fact of pervasive fracturing of rocks in the epicentral region of the New Mad- rid earthquake (Frank McKeown, oral com- mun., 1977). This highly fractured mass may then relax partially or entirely with release of the fault surface (and may keep on relaxing for decades, as at Charleston?) We suggest that the difference between the envi- ronments of earthquakes of the Western United States and Eastern United States is mostly the differences away from the fault zone, everything being “sof ” in regions where k=1%, while only the inclusions are “sof ” where k=1. We must hypothesize that the rele- vant ,ul is not that associated with propagation of shear waves through the inclusion but a ,u] related to stress storage, that is, a pseudo-p. related to nonlinear defor- mation of a highly fractured mass. An increase in M0 of probably no more than a factor of 100 is required to overcome the limitations of standard models for estimation of M0. Thus, given a Ml/Mo ratio of 0.1, only very limited volumes of total relaxation are required, or only partial relaxation in a , larger volume is needed. The required dimensions do not seem to be denied by any available data. A point that we address only qualitatively is that of the corner-frequency effects noted by investigators of Eastern United States earthquakes and the implica- tion of these effects regarding 2L values within con- ventional models. We suggest that rapid fault break- age followed by a slower rate of relaxation in an appre- ciable volume of soft inclusion leads to a spectral shape uninterpretable by homogeneous models. Some interesting relations are suggested. A major implication is that one should seek sites of potentially damaging Eastern United States earthquakes by seek— ing zones of low ambient stress. High stress implies high rigidity and little or no chance of fault failure. ' Low ambient stress and extensive fracturing, possibly associated with evidence of fluid movement from depth, should typify seismic zones in the Eastern United States. Another interesting possibility is that high deforma- tion associated with relaxation in a finite volume (radius of 1 to several kilometers) would provide the environment in which detectable small strains could be seen many kilometers from the epicenter. If an inclu- sion deformed premonitorily before an earthquake with small 2L, there should be concomitantly detecta— ble deformation in the strong region surrounding the weak inclusion. Thus, there may be a mechanism for effecting measureable strains at distances inconceiv- able under a model based upon a homogeneous elastic model. MAPS OF PREDICTED INTENSITY PATTERNS As illustrations of the use of our programs for pre- dicting expected intensity patterns for earthquakes anywhere in the conterminous U.S., we include several plates, all of which are in the pocket on the rear cover. ' Plate 2 consists of two maps: Digitized geology of the United States (see table 3 for correlation of geologic and ground-condition units and table 4 for designated relative intensities for ground- condition units of plate 2) and the pattern of 4k values presently in our program. 48 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES Plate 3 consists of two maps: Composite Predicted Intensities on Saturated Al- luvium: (1) San Francisco 2L = 400 km, k = 1%, C = 25 (2) Wasatch fault 2L 2 60 km, k = 11/2, C = 25 (3) Cape Ann 2L = 10 km, k = 1%, C = 40 Composite Predicted Intensities Corrected for Ground Condition: (1) San Francisco 2L = 400 km, k = 1%, C = 25 (2) Wasatch fault 2L = 60 km, k = 11/2, C = 25 (3) Cape Ann 2L = 10 km, k = 1%, C = 40 Plate 4 consists of two maps: Composite Predicted Intensities on Saturated Al- luvium: (1) Charleston 2L = 15 km, k = 1%, C = 40 (2) Owens Valley 2L = 60 km, k = 11/2, C = 25 Composite Predicted Intensities Corrected for Ground Condition: (1) Charleston 2L : 15 km, k = 1%, C = 40 (2) Owens Valley 2L = 60 km, k = 11/2, C = 25 Plate 5 consists of 2 maps: Composite Predicted Intensities on Saturated A1- luvium: (1) New Madrid 2L = 20 km, k = 1%, C = 40 or(2L= 5km,k= 1,C = 40) (2) Seattle 2L = 40 km, k = 11/2, C = 65 (3) Fort Tejon 2L = 320 km, k = 1%, C = 25 Composite Predicted Intensities Corrected for Ground Condition (1) New Madrid 2L 2 20 km, k = 1%, C = 40 or(2L= 5km,k= 1,C=40) (2) Seattle 2L = 40 km, k = 11/2, C = 65 (3) Fort Tejon 2L = 320 km, k = 1%, C = 25 We estimate that the earthquake models used to produce these maps represent almost the maximum credible earthquakes in each area. It should be remembered that the grid size on the US. map is 25 km by 25 km. Therefore, nearly all river beds, that is, sites of saturated poor ground, will not constitute the dominant ground condition in hardly any grid element and will thus not be sensed on the maps corrected for ground condition. Those maps are useful, therefore, only for indicating intensities to be expected on bedrock. The New Madrid earthquake was modeled as if it were in a region ofk = 1% rather than one ofk = 1. It was pointed out in Evernden (1975) that this earth- quake can be modeled either way because only in the epicentral region (50 km) at most does k : 1%. The southwestward projection of intensity V on plate 3 for the Cape Ann earthquake results from calcula- tional problems concerned with the grid size and prox- imity of the irregular boundary between k z 1 and k = 1% to the hypothetical epicenter. The implied pattern is probably erroneous. For all k regions except k = 1%, the program takes account of k boundaries and changes attenuation rates in accordance with plate 2. Adequate documentation to justify all details of plate 2 does not now exist. The regional contrasts in length of break (energy release) and size of felt areas are apparent on all of the maps. ESTIMATE OF DOLLAR LOSS FOR INDIVIDUAL POTENTIAL EARTHQUAKES As a guide to relative risks associated with different faults and potential earthquakes on these faults, we have developed a simple program to estimate expected replacement value for wood-frame construction. We know of most of the inherent dangers in such esti- mates, but we believe that it is important to have the capability to make rapid estimates of relative potential damage from different potential earthquakes, even if the estimates are too large or too small by a factor of two. . The procedure followed closely parallels that used in Blume and others (1978). Because we have made some changes from their procedures and because their report is not in the hand of many potential readers of this report, we will briefly outline the technique. It has been implemented for cities in California, and it could be implemented for any region. Data sets required: (a) List of all California cities and unincorporated areas with populations of greater than 950 in 1977, including county, population, latitude and longitude, and ground condi-~ tion; (b) List of all 1/2’ by 1/2’ latitude and longitude points in California; (c) Estimated dollar value of wood-frame con— struction in a city of 75,000 in California (1977)—$1.06 billion (population/75,000) (Blume and others, 1978); ((1) Table of percentage of damage (P) expected to wood-frame construction versus Rossi—Forel intensity (RFI), based on values given in Freeman (1932), Association of Bay Area Governments (1978), and Blume and others (1978); (i) if RFI < 5.9 P=O (ii) 5.90 S RFI < 6.00 P=(RFI—5.90) (iii) 6.00 S RFI < 6.80 P:(RFI—6.00)><0.25+0.1 (iv) 6.80 S RFI < 7.40 P=(RFI-6.80)><50+O.3 (v) 7.40 S RFI < 7.85 P=(RFI—7.40)><1.11+0.6 (vi) 7.85 S RFI < 8.25 P=(RFI—-7.85)><2.25+1.1 (vii) 8.25 S RFI < 8.70 P=(RFI—8.25)><3.33+2.0 (viii) 8.70 S RFI < 9.05 P=(RFI—8.70)><7.14+3.5 (ix) 9.05 S RFI < 9.50 P=(RFI—9.05)X6.67+6.0 (X) 9.50 S RFI < 1000 P=(RFI—9.50)><6.00+9.0 (xi) 10.00 S RFI P=12; ESTIMATE OF DOLLAR LOSS FOR INDIVIDUAL POTENTIAL EARTHQUAKES 49 (e) Parameters of each hypothesized earthquake — 2L and coordinates of points on fault, k value, and C value. We virtually ignore character of fault motion (see below). For the time being, we assume that ground condi- tions at all sites are one intensity unit less than appro- priate for saturated alluvium. Most communities are in alluviated valleys, and in most of these valleys, the water table is at least 10 m deep today. The program can use any or all of three specified ground-condition values. We now calculate and list loss estimates for J (saturated alluvium), J-l, and A (granite) for all sites. For this paper, we tabulate relative losses based on J -1. The procedure of calculation is as follows (given earthquake parameters): (a) For each community: i~—calculate expected RFI (J, J-l, and A) by normal formulas; ii—calculate expected percentage of damage, P, to wood-frame construction (table under ((1) above); iii—calculate expected replacement value for wood-frame construction for J, J-1, and A (population/75,000) X 1.06 X P; an- swer in billions of dollars. (b) For each county: Sum expected replacement values for all com- munities in county. (0) For state: Sum expected replacement value for all coun- ties. (d) For each l/2° by 1/2° grid point and for center and ends of fault, calculate expected RFI (J, J-l, A) and expected percentage of damage. The results of such calculations (through step c above) for several potential earthquakes are given in table 25. Figure 25 and table 25 show all fault breaks modeled ‘for estimates of replacement value of damaged wood- frame construction. The numbers on figure 25 refer to equivalently numbered earthquakes in table 25. Because the San Fernando earthquake is the only one for which we have relevant damage data, a few comments on the models used for that earthquake are in order. Most California earthquakes modeled would occur on vertical strike-slip faults. For these, we placed our line source along the surface trace and used a C value of 25, such a C value having been appropriate for such earthquakes by study of the 1906 San Francisco quake (Evernden and others, 1971). Within our simple model, radiation pattern as well as depth of focus are subsumed under a “best fit” C value. For the San Fernando earthquake, we used a 2L of 19 km (appropriate for M 6.4), placed the epilocus of the hypothetical line source 6 km in the downdip direc- tion from the surface trace of the fault (halfway be- tween the surface trace and the epicenter), and applied C values of 25 and 20. A C value of 25 predicted inten- sities that were too low very near the fault (maximum predicted value on saturated alluvium of 8.6), while a C value of 20 gave satisfactory near-field intensities (9.2) but did not affect far-field values. Calculation of damage to wood-frame construction in southern California changed from $190 million with C=25 to $260 million with C220, and most of this increase was, of course, in San Fernando and nearby parts of Los Angeles. Following Blume and others (1978), and using the data from the San Fernando earthquake as a basis, we doubled this figure to obtain an estimate of total replacement value and obtained values of $380 million and $520 million. These values are to be com- pared with the reported value of $498 million (Stein- brugge and Schader, 1973). Thus, both peak intensities and dollar damage suggest that a C value of 20 is more appropriate than one of 25 for this thrust-generated earthquake. Therefore, for the several thrust faults modeled, we placed the epilocus of the line source 6 km in the downdip direction from the surface trace of the fault and give the dollar-loss estimates relative to C values of 20 and 25. All strike-slip earthquakes are modeled with the epilocus along the surface trace and a C value of 25. , The first earthquake for which estimates are given in table 25 is for a repeat of the 1906 San Francisco earthquake on the San Andreas fault. Evernden and others (1971) showed the necessity of extending the 1906 break to Cape Mendocino in order to explain the observed isoseismals in northern California. The sec- ond earthquake of the table is for a fault break from Richmond to San Jose on the Hayward fault. The ground condition for all communities on the east side of San Francisco Bay is treated as J -1 but several of them include areas in which the water table is very near the surface. Dollar losses rise by a factor of three (R/F 8.5 to R/F 9.5 in most cases) if the J ground condition is used. However, the ground condition in parts of the East Bay may be J=1.5 or more, and intensities in areas east of Highway 101 on the San Francisco penin- sula will probably be appropriate to the J ground con- dition (9.8—10). Losses there would be higher than assumed in the calculations. Thus, we consider it prob- ably true that losses to wood-frame construction will be greater for a repeat of 1906 than for an M7 earth- quake on the Hayward fault. The shorter hypothesized breaks shown for the Hayward fault represent the cen- tral part of the big break and a 30-km break opposite San Jose. Losses from the former are predicted to be twice as high as those from the San Fernando earth- quake, while damage from the latter is predicted to 50 equal about half that caused by the San Fernando earthquake. We disagree with estimates by Wesson and others (1975) that a potential M7+ earthquake might occur on the “Zayante fault,” because the character of the mapped surface trace of that “fault” (fortuitous coalesc- ing of short, apparently separate failure zones) seems to deny the possibility of a simultaneous break along all the features that they assumed to be part of it. The 4-km Santa Barbara break is intended to simu- late the recent earthquake (1972); the east end of the break was placed at the epicenter of the main shocks and the length was constrained to predict no greater than an M 5.4 quake. All losses are predicted to be in Santa Barbara County, and almost all the damage would be in Santa Barbara. The aftershock zone is more than 4 km long (Lee and others, 1978), and whether a 2L of more than 4 for the main earthquake is appropriate is unknown. The 320-km break on the central San Andreas fault is for a repeat of Fort Tejon 1857. The predicted losses are well below those for the two large earthquakes in the San Francisco Bay area, yet this earthquake is commonly considered to pose the greatest threat of SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES damage to southern California. As seen in table 25, our estimate of the replacement value of damaged struc- tures is $1.2 billion (2 x 0.61), a value in essential agreement with the $1.3 billion estimated by Blume and others (1978). The high attenuation rates in south- ern California and the distance between the fault and the most heavily urbanized areas will lead to far lower losses than normally assumed (J-l intensities pre- dicted for the city of Los Angeles range from R/F 7.5 to 6.3 or M/M 6.8 to 5.7). Table 25 clearly illustrates the existence of potential threats to southern California nearly as great or greater than that posed by the San Andreas fault. A break (13B of table 25) 0f 22 km (M 6.5) immediately north of the part of the Newport-Inglewood fault that broke in 1933 will cause losses comparable to those of an 1857 repeat (M 8.1), while a long break of 42 km (M 6.9) (13D of table 25) along the same fault is predicted to cause shaking damage 12/3 times greater than a re- peat of 1857. A 20-km break on the Whittier fault (M 6.4) will cause greater losses than did San Fernando and about half the losses to be expected from an 1857 repeat. Even a 31-km (M 6.7) break on the Malibu Coast fault will cause losses amounting to half as much TABLE 25.—Predicted replacement value of wood-frame construction (and all construction) damaged by potential earthquakes in California Predicted replacement value (J-l) No. Fault 2L M Lat1 Long1 Lat2 Long2 Wood-frame N W N W construction Total 1 San Andreas (1906) ________ 400 8.3 36°51.00' 121°33.10’ 40°15.90' 124°27.20’ $1.72B $3.44B 2A Hayward (1836?) __________ 100 7.4 38°00.61' 122°22.92' 37°11.76' 121°44.62’ 1.14 2.28 2B Hayward __________________ 50 7.0 37°48.89’ 122°13.73' 37°24.46' 121°54.58' .54 1.08 2C Hayward __________________ 20 6.4 37°27.53' 121°52.28' 37°11.76' 121°44.62' .11 .22 3 Calaveras (1911) __________ 11 6.1 37°09.10' 121°34.90' 37°14.20' 121°39.90' .02 .04 4 Palo Colorado— San Gregorio ____________ 30 6.7 37°05.68’ 122°18.31' 36°50.47' 122°10.47’ .04 .08 5 Zayante _______________ __- 15 6.3 37°02.43' 121°53.64' 36'56.55' 121°47.03' .03 06 6 "Hos 1” __________________ 80 7.3 34°29.90' 120°54.50' 35°12.20' 121°05.90' .01 .02 7 Hosgri (1927) ______________ 70 7.2 34°36.00' 120°38.80' 35°11.90' 120°54.50' .02 .04 8 Nac1miento (1952) __________ 20 6.4 35°44.20' 121°07.70' 35°52.10' 121°16.90' <.001 ,1__ 9A Santa Barbara (1925?) ______ 40 6.9 34°28.20' 120°04.80' 34°25.70" 119°39.10' .023 .046 QB Santa Barbara (1925?) ...... 29 6.7 34°27.70' 120°00.10' 34°25.90' 119°41.40' .015 .030 10 Santa Barbara (1978?) ______ 4 5.4 34°22.20' 119°43.00' 34°23.34' 119°45.44' .003 .006 11 San Andreas (1957) ________ 320 8.1 34°18.30' 117°31.50' 35°45.10' 120°17.80' .61 1.22 12 Laguna Beach-New Clemente 40 6.9 33°34.05’ 117°56.10' 33°18.65' 117°36.92' .16 .32 13A Newport-Inglewood ________ 45 6.9 33°54.50' 118°17.40' 33°36.30' 117°58.80’ .90 1.80 133 Newport-Inglewood ________ 22 6.5 33°54.50’ 118°17.40' 33°45.50' 118'08.10' .53 1.06 13C Newport-Inglewood (1933) _, 22 6.5 33°45.40' 118°08.10’ 33°36.30' 117°58.80' .33 .66 13D Newport-Inglewood ________ 42 6.9 34°02.69' 118°25.77' 33°45.50’ 113308.10, 1.05 2.10 14 Whittier __________________ 20 6.4 33°58.99’ 118°00.00' 33°55.14' 117°47.80' .35 .70 15 Raymond Hill ______________ 10(8=20) 6.0 34°12.18' 118°01.95’ 34°10.35’ 118°07.80' .2? .32 ( =25) .1 . 16A Elsinore (South) ____________ 70 7.2 33°25.80' 117°00.00' 33°00.00' 116°27.00' .04 .08 163 Elsinore (north) ____________ 30 6.7 33°40.54’ 117°22.93' 33°29.19' 117°09.27' .03 .06 17 San Fernando (1972) ________ 19(C=20) 6.4 34°22.78' 118°30.31’ 34°18.62' 118°16.55’ .26 .32 (C=25) .19 . 8 18A San Jacinto ________________ 30 6.7 34°03.04' 117°11.80' 33°51.89’ 117°02.44' .06 .12 1813 San Jacinto ________________ 38 6.8 34°08.96' 117°16.76' 33°51.89’ 117°02.44' .09 .18 19 Malibu Coast ______________ 31((é=20) 6.7 34°06.10’ 118°56.44' 34°05.10' 118°33.17’ .33 .76 ( =25) .2 .56 20 Santa Monica ______________ 32(C=20) 6.7 34°06.10' 118°33.17' 34°10.01' 118°09.27' 1.14 2.28 (C=25) .71 1.42 21A Rose Canyon ______________ 51 7.0 32°53.11' 117°18.05' 32°30.41' 116°58.79' 28 (J-l) .56 (J-l) .07 (J-2) .14 (J—2) 2113 Rose Canyon ______________ 32 6.7 32°48.85' 117°14.44’ 32°34.67' 117°02.40' .23 (J-l) 46 (J-l) .06 (J-2) ‘Coordinates at one end of break. 2Coordinates at other end of break. MATHEMATICAL DETAILS OF MODEL FOR PREDICTING INTENSITIES 51 or more than the damage associated with an 1857 re- peat. The greatest apparent threat in southern California, however, is the Santa Monica fault, for which we predict a total replacement value of $1.5—2.3 billion (2 X (0.7—1.l4) ) as the result of an M 6.7 (2L = 32 km) earthquake. However, this calculation illus— trates the danger of calculating dollar loss without considering recurrence time. Evidence in hand, from marine terraces along the southern California coast and from geodetic measurements in the general region, indicates that significant displacements on the Santa Monica fault have been extremely rare in recent mil- lenia and that the major active thrusts today are much farther north. Therefore, return times for an earth— quake such as we have modeled may well be many hundreds to thousands of years. Combining a very long expected return time with an estimate of potential damage as high as we have calculated leads to a pre- dicted annual loss that is very low. Though the San J acinto fault is the most active fault in southern California today, its location and the ap- parently limited size of earthquakes that occur on it render it a minimal regional threat, although it cer- tainly is of great significance to San Bernardino and environs (18A and 18B). Because the earthquakes hypothesized for the San Jacinto fault are assumed to be related to breakage along the northern 30 or 39 km of the fault, even doubling of that length by extension southward would not greatly increase the predicted losses. The two modeled breaks on the Elsinore fault are probably larger than expectable. The 70-km break on the Elsinore fault was hypothesized to be the greatest earthquake that can have impact on San Diego. Pre- dicted replacement value of wood-frame construction from such an M 7.2 earthquake is only $40 million. We did not know the appropriate dimensions to attach to a potential earthquake on the Rose Canyon fault. If a break of a few tens of kilometers can develop on this fault, damage in San Diego would greatly exceed $40 million. Because San Diego is built largely on bedrock or marine terraces, and because reported intensities are frequently two units less than on saturated a1- luvium, we included replacement values of the earth- quakes on the Rose Canyon fault (21A and 21B) for both J~1 and J-2. Table 25 clearly indicates that the California earth- quakes with comparatively short return times that will cause the greatest damage are maximum expectable earthquakes on the Hayward fault and the northern section of the San Andreas fault. A repeat of the San Francisco 1906 is predicted to cause damage for which the replacement value would be nearly three times that of a repeat of Fort Tejon 1857. Even a 50-km break on the central Hayward fault is predicted to cause losses comparable to those of a Fort Tejon repeat. Although other fault breaks could have been modeled, the examples that we have cited illustrate that numerous possibilities exist for extensive damage from earthquakes in southern California. Our calcula- tions indicate, however, that losses to wood-frame con- struction amounting to more than 5 times the damage caused by the San Fernando earthquake are not likely. An important point to keep in mind is that all damage estimates given above are based on damage due to shaking. Potential losses from dam failure and con- sequent inundation, rupturing of dikes, extensive fires, and other hazards are not included, nor are indirect costs resulting from disruption of a variety of services and industries. MATHEMATICAL DETAILS OF MODEL FOR PREDICTING INTENSITIES A long curved fault (k= 1%) is assumed to be a series of uniform point sources as closely spaced as desired. The formula used is 1011.8+1.5M l/y n 1/‘Y (1) —‘—— 2 (R1 + CV“ ’n i=1 a=A (effectively, equation (7) of Evernden, Hibbard, and Schneider (1973) with y = 4 and the coefficient of M = 0.864 rather than 0.80) and I = 3(0.5 + log a) (Richter, 1958) (2) where a = “acceleration” I = intensity (Rossi-Forel) = I(R/F) M = local magnitude = ML '5 MS 11 = number of equally spaced subevents used in the model to achieve nearly uniform release of energy along the fault break s = 1011-8”5M = energy (ergs) released by earthquake of magnitude M (Richter, 1958, p. 366). R, = distance, in kilometers, from point i of n points on fault to point of observation C = pseudo-depth term chosen so as to give proper near-range die-off of intensities. Intensity values beyond 50 to 100 km are nearly insensitive to var- iation in expected values of C for earthquakes in the United States. k = term controlling rate of die-off of a (a 0: A‘k) and thus effectively of I y = log [energy arriving at point]/a or a = [energy arriving at point]l/7 A = 0.779 = arbitrary leading coefficient selected so 52 as to give correct intensity values at a uniform ground condition for a particular earthquake. Once set for the normalizing earthquake, it cannot be changed. The value given above is set to give identical short-range I values as given by equation (7) of Evernden, Hibbard, and Schneider (1973) with y = 4. The points of the "fault” are distributed over a length (2L) appropriate to the M value (or an M value is used appropriate to the length of break 2L). See below. When shorter faults in other regions are considered, equation (1) is slightly altered in order to simplify analysis and manipulation and to escape the multi- point aspect of energy release. Replace (R, + C) by (R,2 + C2)”2 and convert equation (1) into an integral ex- pression of uniform energy concentration along the break (using “energy concentration along the break” as a semantic device while recognizing that the source of energy is not the fault but the strained volume of rock). 1011.8+1.5M +L dl 1/4 a = 0.779 (3) 2L _L (R2+Comz where definitions are as in the sketch below. is = point of observation I X, Y are coordinates of point of observation relative to the center of break with the Y axis oriented along the line of break. W=W+W—W 1011.8+1.5M 1 0N 14 a = 0.779 —— ______ ‘ 2L ((X2 + CZ)(4k‘1)/2 I COS4k_20 d0 (4) 9s where Y + L Y — L p = (X2 + C2)‘/2, 6N = tan‘1 , GS = tan—1 p 9 Now, we discuss the role of the various parameters in controlling predictions. (a) L and M .—As equation (4) stands, L and M are entered as separate quantities. A one-unit change in M with no change in L results in a 1.1-unit change in I for y = 4 or 0.8 for y = 6, while a tenfold change in L with no change in M causes a 0.75-unit change in I(y = 4) or SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES 0.50 (y = 6) plus an effect from the integral, the influ- ence of this term being dependent upon position. As a matter of fact, it appears from empirical California data that there is a general correlation be- tween L and M. In all that follows, M is eliminated from the equation by assuming an L, M relation of a form designed to agree approximately with California L, M data 2L 2 1001 >< 5M so that 2L = 10 kmforM= 6, and 2L = 400 km for M = 8.25 leading to 2L : 100.711M — 3.2667 M = (3.2667 + log 2L)/0.711. If 1011.8 + 1.5 M ED = (ergs per kilometer of break), 2L we obtain the results shown in table 26. TABLE 26—Magnitude (M) Relative to length of break (2L) and energy density (61)) 2M 2L loge“ -M 2L loge.) 2M 2L loge” 4 0.4 18.2 6 10 19.8 8 265 21.4 4% 0.9 18.6 61/2 23 20.2 8% 400 21.6 5 2 19.0 7 50 20.6 81/2 600 21.8 5V2 4.5 19.4 71/2 116 21.0 9 1350 22.2 ‘Laws relating L and M are given in text. 3M, Local magnitude of California earthquakes. (b) The C parameter—The C parameter plays the role of depth in calculations. Although C must have a definite relation to depth of focus, the exact value does not agree with depths expected from travel—time analysis. Influence of uncertainty in C on estimation of k values can be largely eliminated by considering I values observed at distances of 50 km or more from the epicenter. Observations indicate that C values less than 25 do not seem to be appropriate for large US. earthquakes, and values greater than 60 or so seem irrelevant. Failure to achieve agreement between C value and probable depth of focus may well arise from the fact that the dominant energy influencing intensity is in trapped modes, whereas amplitudes of such modes show a complicated relation to depth of focus. Calculated intensity values from equations (4) and (2) (1 values reduced by 1.05 for reasons to be described below) are given in table 27. It appears that uncer- tainty as to whether C should be 25 or 50 will cause confusion in the'estimation of k by less than 1%; when using I data from 50 km. If the earthquakes used are large enough to give valid estimates of intensities to distances of more than 400 km, estimates of k can be based on intensity data from 100 km and more, thus eliminating any confusion in k estimates arising from uncertainties in C. If appropriate values of C are sug- MATHEMATICAL DETAILS OF MODEL FOR PREDICTING INTENSITIES 53 TABLE 27—Influence of variations in L and C on predicted intensity values (7 = 4, Y = 0) TABLE 28—Influence of variations in 'y and k on predicted intensity values (C = 25, Y = 0) A (km) L y k A (km) 61100 k L C 0 50 100 400 800 0 100 400 800 800 1% 50 25 9.5 7.9 6.6 3.6 2.0 200 4 11/2 11.1 8.8 6.4 5.1 3.7 1% 50 40 8.6 7.6 6.5 3.6 2.0 6 11/2 11.1 9.0 6.8 5.6 3.3 1% 50 50 8.1 7.4 6.4 3.6 2.0 4 1% 12.2 10.4 8.4 7.3 3.0 11/2 50 25 10.6 9.3 8.1 5.5 4.2 10 4 1% 9.8 7.2 4.4 3.1 4.1 1V2 50 40 9.8 9.0 8.1 5.5 4.2 6 11/2 10.3 7.9 5.5 4.3 3.6 1% 50 50 9.4 8.8 8.0 5.5 4.2 4 1% 10.9 8.6 6.4 5.3 3.4 1% 10 25 8.8 7.0 5.6 2.5 0.9 1 4 11/2 8.3 5.6 2.9 1.5 4.1 1% 10 40 7.7 6.7 5.5 2.5 0.9 6 11/2 9.3 6.8 4.4 3.2 3.6 1% 10 50 7.2 6.4 5.4 2.5 0.9 4 1% 9.3 7.1 5.0 3.7 3.4 11/2 10 25 9.8 8.3 7.1 4.4 3.1 11/2 10 40 7.9 8.0 7.0 4.4 3.1 11/2 10 50 8.5 7.8 6.9 4.4 3.1 gested by other data, confusion is eliminated. Also, peak intensities and estimates of L from field observa- tions place very severe limits on permissible k values. Therefore, uncertainties in C have no serious impact on estimates of k from moderate or larger earthquakes. Table 27 shows that the behavior of intensity values at short ranges will lead to clear predictions of appro— priate C values, subsequent to setting of k and L val- ues. (c) k and y. Equation (4) would suggest that k and 'y values might be strongly correlated. For a fixed k value, however, the y value influences primarily the 6,) factor (a factor not a function of k) and has minimal effect on (X2 + C2)‘k7‘“/2 for )1 variation from 4 to 6 (the probable range requiring consideration); in addition the change in value of the integral is small for such changes in y. Table 28 illustrates these points. The I values for k = 11/2, y = 6 are normalized to give the sameralue at L = 200, A = 0 as fork = 1%), 'y = 4. The (y = 6, k = 11/2) I values are between those of (y = 4, K = 11/2) and ('y = 4, k = 11/2), and they yield a rate of decay with distance more similar to (y = 4, k = 1%) than (7 = 4, k = 11/2). The actual predicted I values do not fall as rapidly with decreasing L for 'y = 6 as for y = 4. Study of US. intensity data as reported in Evernden (1975) has shown the appropriateness of using a 31 value of 4. ((1) Leading coefficient.—This coefficient is com— pletely arbitrary and can be set once for a given ground condition. The coefficient used will be considered as that appropriate for predicting I values on saturated alluvium, that is, the ground condition with the zero correction factor in Evernden, Hibbard, and Schneider (1973), and has been chosen to give the correct I values for the San Francisco earthquake of 1906. (6) Problem of energy to be summed for very long fault breaks—For long fault breaks such as the one produced by the San Francisco earthquake of 1906, a question can arise as to whether the integration in equation (4) should include energy from the entire length of the break or energy arising in a time window around the time of arrival from the nearest part of the fault. Since the intent is. to have one formula that is applicable over the scale from long breaks to short ones, this factor should be considered. The mode of analysis is to set the time window, then calculate from it which part of the fault would produce arrivals at the point of observation within the time window (assuming velocity of break of 3.5 km/s and velocity of wave prop— agation of 3.5 km/s). The next problem is how to select the appropriate time window. One second is certainly too short, and 100 s seems certainly too long. The Win- dows considered are :5 and :10 s around the time of arrival of the peak increment (from the nearest point of the break). Throughout the study, it is assumed that the phasing of arrivals is not critical because the gen— eral prevalence of earth inhomogeneities is adequate to confuse phasings. Because all intensity values reported for the San Francisco 1906 earthquake were shown by Evernden, Hibbard, and Schneider (1973) to be ex- plainable on the basis of short—period data, this as- sumption does seem appropriate. Table 29A gives predicted I values for IT (total en- ergy independent of window length), I,—, and 110 (energy in time window :5 and : 10 s around peak arrival) for the San Francisco 1906 event. It is seen that 15(7) and 1110(7) (I5('7)NORM and I,0(7)NORM also) are indistinguish- able. I5(7) and I5(8) are I values for ky of4 X 1.75 and 4 x 2.0, respectively. The superscript NORM signifies I values normalized to give IT(8) at an epicentral dis- tance of zero. 15(7) has essentially the same rate of decay with A as does IT(8). The formula for IT(8) is the exact one used for the San Francisco 1906 event by Evernden, Hibbard, and Schneider (1973). The A ver- sus I data for the same earthquake (Y = 0) are as in table 30. Thus, the I (observed), IT(8), and I5/1,,(7)N°RM are nearly indistinguishable. Therefore, the rule of using windows 5 to 10 s on either side of peak arrival will be followed. This means that the k factor for the West Coast will be 1% instead of 2 as used in Evernden, Hibbard, and Schneider (1973). Data for other smaller California earthquakes where the time-window correc- tion is not required support this change in k value. Since the fundamental setting of equation parameters 54 SEISMIC INTENSITIES OF EARTHQUAKES OF CONTERMINOUS UNITED STATES TABLE 29.—Effect of length of time window on predicted intensity [Fault Breaks From Center: V”, : V,- : 3.5 kin/sec] (AIL : 200, C : 25, Y = 0 (San Francisco. 1906) X 1;,(7) I“.(7) 11(8) I;(7)“‘"'“ I..,(7)“‘““ [5(8) 0 11.0 11.1 10.0 10.0 10.0 9.9 45 9.5 9.6 8.4 8.5 8.5 8.2 100 7.9 8.2 6.8 6.9 7.1 6.5 200 6.4 6.6 5.2 5.4 5.5 4.7 400 4.8 5.1 3.6 3.8 4.0 2.9 800 3.2 3.4 1.8 2.2 2.3 1.1 (B)L : 30, C : 25; 1(6) : I Planimetric base, digitized geology, and predicted intensities from Blume, Scholl, Somerville, and Honda (1978) NOTE: See table 1 for correlation of ground-condition and geologic units and table 2 for relative intensities of these units as compared to saturated alluvium ROSSI'FOREL INTENSITIES - - :r INTERIOR AGEO-LdGICALSURVE-V,-RESTON,VA—198-l— 68:613 0 GROUND-CONDITION UNITS 0 25 50 100 KILOMETERS l l I J l l 0 25 50 MILES GROUND—CONDITION UNITS DIGITIZED ON 1/2-MINUTE BY 1/2-MINUTE GRID AND PREDICTED ROSSI-FOREL IN TENSITIES FOR THE 1857 FORT TEJON EARTHQUAKE, SOUTHERN CALIFORNIA 110.0 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 180.0 180.0 PLATE2 180.0 55555 5 \ 555555555 55555 5 555 5555555 170.0 170.0 170.0 \ PROFESSIONAL PAPER 1223 5 5 5 55 \ 5 555555555555 160.0 5 5 \ 5 5 160.0 160.0 555555 555555 555 5555 5 \ 5555555555555555 1 1 1 55555 5555555555 555 5 55555555 5555555 555 5 \ 555555555555555555 5, 555555 5555555555 150.0 5 5 5 5 5 5 5 5 5 5 150.0 150.0 \— 5555555\ 5555555555555555555555555555‘ 5555555555555555555555555 55555555 5555 5 55 55555555555555 555555‘55 55555555 ‘5555555555555555555555 5555555 55555555 5555555555 5555555555555555555 5555555 55 5 5 5555555555 55555555 55 555555555 140.0 140.0 55555 55555 555555555 555 5 5 5 140.0 5555555555 5 5555555555555 555555555555 555555 5155555555555 55555555555 5 5555555555555 5 555555555555 555555 555 5555555555555555 5 55555 55555555555 55 55 55555 ‘5 5 [5555 1 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BE BB 1555 JIJBK B B B B B B [9195 [9 5 [95 [919191919 1919191919 19551919 519 77 1 B) [9 [91919519 5 777777777‘777 BBBKKKKKLKI]1111111111IIDDDDDDDD’DDDDDDDDDDDDDDDDDDCCGCCCCCGDCIDDDDDDDDDDDDJJ K‘E~K,KKLKKEELLL1,.ILILLBKKKKDCFDCCCDDDCCCDDDCCDDC B K I I - I KKKKKFBEFEBBEEBBBBBCCDDEBCCDDDCDCCFFFFJJCICCCDDB BLLBBD mo 77 7777777777 7777 7 '77 51919191919519 20.0 20.0 51951933195 7 {IQKKKKKKKKKKEKLLLL’LLLLDKKKKDCIFDDDDDDUDDDDDCCDCC KBBKKBKBBB’FEBBEEBBBBBBDDDDCICDDCCGEOFFFFJJJDF‘C BK 5.1.1.IBBBRJJJJJKJJKKFFFF'FFIFIBBE HBKK BBKK 7777 KKBBKKKKKKBBLLLL’LKKFFFFFKKKKKKKKIKKDCCDFFDCCI-CFU‘B‘ KKKKKKKKKKBKKLLUKF’FKFF‘KKKBKKKDDK‘KKDDDODFJCCCCCC KKKKKKKKKKKKKKKKLLLILLLIIDCDKUCIFI‘F KKKKKKBKKKKKKKKKJKKKBKBBBBFFFDUCCIJJFDDCCCDDDDCCCCIDDDFFDCCCCCCDDDIDDDDDDDI1IIIDDDD FF FF LLLLBBBBKBKBBBKBBJBBKKIIKKKKKKKKBBKKBFFFFFFFFFEEBBBBBBBBBFFFF‘FIFFFFFFFFFFGHFF‘FFF [91919519191919 1919191919 7777 777 ,KKKKBKKKKKKKKKKBKKKKKKKKBFHDFDFCCC BLLLBBBBB/KKKKKBBBJBBBJJI1F‘F'FKKKKKKBBBBFFFFFFFBBEEBBBBBBBBBFFF‘FF BLLLBBBLLKBDDBBJJBKJJJJIIFF‘KKEKKKKK'KDFBBFFJFFfBEEBBBBBBBBBEE LLLLLBLLBfFBBBBBBJJKKKFFFFEEEEDDKKKDEEBB BLLLL {BLBBEBEBBKKKFKKFIBBFJJBBBFBBHEDDC B 19 [9 [919 [91919195 [919 51919555 [9 195 555 [9 1919519195 19519335 [91955 [9 19191919519 5191919 777777777 FIBLLLLJBBLBBXKBJBJKJJJFFF EXPLANATION HBBBBLLBBLBBB’BJJBJBKJF‘F‘FK KHLLJLLBLKBKB 1919 [9195 55 51919 [9555519 5555519 [919 5519195 519 ‘19 [919 [9191919 55 B) [9 [9 [9 7 7777777 10.0 HHLLBBBBLLBBBFKKJBFIFKFFFFFFEEDEDDIEEEDDDBBBBDKKBKKF ‘fiufl‘LLBLKBKLBBHFKIKJBFFFFFFFFFEEDEDDIEEEDDDBBBBDKKBKK n L L L L L H L L L L L units and ground-condition units H H [9 [9 I 10.0 10.0 I I HHLBBBLLBBBE~RBLLBJKJBFFFKFFFFEDDDD ~HBBJLBBLLLBKBB‘ R R B/HBHBLLLLLBHBBHHBHHIBHBBEFEEEEEDDEFEFFEEDDDEFIKKKKBBFD BIBHHHLLLBKBHHHBBBHH,’ E E H R B H [91955519555555 [95191919191955 [9191951919 77 I I I I I l CDDBBBKKKKKKKKKKCDKKKKKKKILL1'1IIIII1[KDDDDDDI'DDDDDDDDDDDDCCE} E‘QBBBKKKKKKKRBBBKKKKKKKKKLLLLLLIIFI[IF‘DFFFDDDDDDDDDCDDDCCC EEKKKBKKBBKAKBBBKKKKKKBBLLLLLKKFJJF‘FBBBJKKKK, GCDB B/BDBLLBL.‘B‘B.BBBBBBLBK/JKBBFFFFFFFFEEDDDDEFBDDBBBBBDDBKKJB See table 3 in textfor correlation between geologic Rossi-Forel intensity increment to be subtracted H R F1 F1 D D B B I F1 H B D B B 77777 77 77777719519191.55191951919 [9 [9 1919 1919191955583 [9191919191919 I CCCCCKKKKBKKKKKEEEKKKKEEKKKLLLIIT‘I‘LJJBFBJJDKKII ‘E E E.CCEEKKKKBKKKKKIKKKKKKKKKBKKLLLJLKKJBFDB‘BEJKKKJJ O. O m o. m m m o. o. o. o. o. o. o 5 m w m w / A 3 3 ,G / / ‘ HLL , HHH4H 5 , I G HHHH GGGGLH \\\ HHHHGGGGGG/ G \\\\\ GGGGGGGGGG GHG \\\ B t\1 GGGGGGLGGG GBGG \\\\\ BH \\\\\\ 8988868968 85351an G LFF \\ GGGGGGGGGGI GHGGLBGG GLLFFF BGBWWVGNUPJ BBLLBGG LBLLLLFL \\\\. BL HGBLGBBGBEBBLLLL \\\\\ HHLLGGGBLGGGGEHG B W \ ,GHLLGGBBGGBGGGBEE BR / m GGGHHHHGHGGGEEEEJ fl .. xGGGGGGLJHHHGJEHHJHH , /HHHHHHLHJHHHHHHHHEEE « ,HHHHHHGHHHHHHHHJJJJEDBFBBH WHHHHHJJHHHGGHGHHHJJEDDBBH\1\\BBR fi HHHJJJHJJHHGGGGGGGGJEELDxnmmU HR BB ,HJJJJHJJJHGGGGGGEIQQNLEETDBH Ban fififi ,HJJJJJJJJHGGEFDAwQMFFFHEEFIDB BB fiBfiBB muJJJJJJ\I[_‘H\G\GGGGGGFFFHHEIJ_JIXDD 8858839888 R HJU‘IH.\HJHGGGGGGGGFFEHEHIII BBBBBBBBBBBB Bun B \,\HJJJJHHGGBBBEBBBFGEFHH,IIIDDBBBBBBBBBBBI DDDC ,HJJJBQBBGBGBGBFFGBFHEXHII LDBBBBIIBL11 DDDD \\ \1\\l\~ J HGEGGGBGGFFGBGGHJLEIIIEELLJJIILLII DUUD 111‘1DHHIIDHDH \1.\.\ H GEGBGGGGFFGGGGGUMHHJJEEEILLLLLIII Dunn 1.11 \\,.\ HHBBHH H BGGGGGGFFFGGGGQHHHHJJIIXIIILIIL BDnD HR BHHHBBBBBB GGGGGGGGFFFGGEFGGGGHHJJJIIIIILL DDnD CC H HBRH BBBB GGGGGGFFFFFGGGEGGGGGHEHJJIIILE DDDD CD, 1H1\ HHHHRHBBBBBB GGGGGG FFFFFFGGGGGGGEHHJJIIEI FIHIU NHL B ,, HHHHHRRRBBBBBB GGGGGGFFFFFFFFPFFFGGGGGGHHIJII .FHIHD CC BR , HHHBBBBBBBBBBHB GGGGGGFFFFFFFFEFFFFFGGGGGGJMerD LITL CG BBBR RHHHHBBBBBBBBBBB GG GGGGFFFFFFF FFGGIWMFFtnuWiGHHJJ 1,,l.L CC CBBHBR H fiflflfiHBBBBBBBBBBB GGGFFFFFFFFWFFAHWMIFFFFFFHHJJ LTL L DC CBBBHH BBBRBLBBBBBBBBBBBBB GGFWVFFANHMIFFFFFFFFFFFFHHHHJ L.L 1 DC BBBBBB BBBRB.BBCCCBBBB 1x111aV|mFFFFFFerFFFFFFFFFFHHHHJJ 1L ,. DC BBBBBB BBBRB,BC CCBBBB 1\\.\ GGFFFFFFFEFFFFFFFFFFFFHHHJ [LLt L 11 BBBBBB 8858830 B BBFFFFFFEFFFFFFFFFFFFHHHH ILLLLL II CBBBBB BBBBBmmD H GFFFFFFFBFFFFFFFFFFFFFHHH JJLLfLI II CBBBBB BBBRBBE I \\\t FFFFFFFFFFFFFFFFFFFFHHH JJJLLI II DCDBBB BBBBBQB w \1131 FFFFFFFIFFFFFFDIFFFFFFHH HJJJJI II ICCDCB 538858 000 FFFFFF FFFFFFFFFFFFHH HIJJTJ II IIDCCB BBBBBB FFFFFF FFFFFFFFFFFFFH HIIJTJ LL 110000 CBBRBB G GGGGGG GGFFFFFFFFFFFH HIIJWJ LL lllCCC CCBBBB FFFFGG GGGGGG GGGGGGGGFFFFFF HHIIJl 11 111000 8008881 \\\\\\\ \1 I FFFFFFG GGGGGGE GGHHHHHHCFFFFF HHHHLI II 111000 CCCAn-B.mu FFFFFF GGGGGGGG HHHHHHHHHHFFFF FHHHHNH Hl \KxK 100. 000088 FFF FFFFFF GGGGGGGB HHHHHHHHHHFFFF FFHIH..HLH M\HI IKKIDD CCGCBH FFFFFFFFFF FFGBGGGG HHHHHHHHHH FF MrIFFFFrHHHHH [[KLDD CCCCBH . FFFFFFFFFF FFFGGGGD HHMniHIMHI.HHHFFFF FFFFFHFFHHH [[[LDD 00388 4 G FFFFFFFFFF FFFGLIKnVGIGI HHHHHH HGGFFFF HFFFFFFFHH [IKIDD CCBBB HE FFFFFFFIF‘FLN FFF 5888 983888 BGGFFFF HFFFFMWFFFF HHIDDD CCBBB , HEB .FIHAIIFIWFFFFF FFFGGBGBBfiQBGBFF FFFFFFF HHHHHPFFFF FFIDDD CBBBB 1V1! [[[[[[[ l: H “BFFFFFFCIFF FGGGGBGBGQFPFFFF DIFFPIDIFFI HHHHHAFFPF PFDDDO DBBRBH _ H TGGFFFFF FGGGGGGGGFFFFFFF FFFFFFF FFHHFEFFFF DDDDDD CBBRBfi ” H , FFF GGGGGGFFFFFFFF FFPFFFF FFFFFF.FFDD DDDBDD CBRHH H , GGGGFFFEFFFFF FFFFFFF FFFFFDFFDD DDDDDC CBBRBB » ,_ GGGGFFFEFFFFF FFFFTFF GGGGGDDDDD DDDDCC BBBRBR , H “G GGGGGGFFFPFFFFF FFFFFFF. DDDDHD.DDDD DDDDCC BBBRBH ,, HHH. 8888 G. GGGGHGGGEGFFFFFF FFFFFFD CCUCDDMUUDD DCCCCC BBBBBH , JJHHHHHGGGBGGG GHHGHFFFFFFFFFFF FFFFFFD CCCCCCUCCC CCCCCC BBBBBHH H .W J JJJHDHHHHHHQHH GHHHHFFFFFEFFFFF FFFFFFC CCCCCCVCCC CCCCCC BBBBBHHHHHH _ J JJJJ.JJJHHHHHH HHHHHFFFFFLFFFFF FFFFFGC HHHCCCGVBCC CBCCCC BBBRBfinlnbuLnIinH [[[[[[[ 111 . J JJJJPJJJHHHHHH HHHHGFFFFFLFFFFF FFFFFGH fiBHCCCBanm 05:18.qu anbeIRBBHHrnHH J. 1 JJJ JJHHHHHH HHHHGFFFFFF.FFFFF FFFWHLHIPP BBdHWlfiflWflCBRHHBBBCD BBBBBBHHTBB . I JJJJHJJJHHHHHH HHGIPHWIFLFWFEF HEFFFFFFHHHH HHHBHHBHHRHHHHBHBD BBBBBBHBHHR _ I JJJJIUUIIFHHIHIHIh IIIIIIIII BFFFFF FFFFFFHFHJJH BHCBDHRHRBHBHBHHHB BBBBBBfin—umnnnfi 111“ IIIIIIIII LII I J JJJUHHHHHHH GFFF FFFFFHHFHHHH HHHUBH%HBHHHBHHBHH BBBBBBHHWWH IIIII _ I I J JJJAJHHHHHHH GFFF FFFFHHHHHHHH HHHBHHflBBRHHBfififiHH BBBHBHBHRHB _ II I J JJJdHHHHHHH FFFF FFFFHHHHHHHH HHHHHH BHHHRCHHHB BBBHHHHHWHH H III I J JJJIjHHHHHHH FFFF FFFFHHHHHHHH HHHFFF flaunnnnnncanknaufi Hfianauaunnnnnnnwaufl . JJII 1. JJJuflnHHHHHHH FFFF FFFFHHHHHHHH HHHFFF BBCBCHBRCH CBBRBBHBBH JJJI l HHHAHHHHHHH FFFF FFFFHHHHHHHH HHHFFF CCCBCBHCCC CBfiBBBBHan JJJJ l lHHHHHHHHHG FFFF FFFFHHHHHHHH HHFFFF HHCBRBCCCC CBBBBBHHBB JJJJJ J H HHHHHGG FFFF FFFFFHHHHHHH HFFFFF HFCHBBCCCC CfiBRBBBHBH JJJJJ IIIHH HHHHHBG FFFF FFFFBHHHHHHaF .HFFFFF HFFDHBCCCC CCCCBBBBLB JJJJJ JIIHH HHHHHGG FFFF FFFF.FFFHHHFF HHFFFF FHFDDHCCCC CCCCBBBHBH JJJJJ I’JIIH HHHHHGG FFFF FFFFFFFHFFFF HH.FFFF FHFDDflfifififi CCDCBBBBBH _, JJJJJ JJJJH HHHHHHG FFFF FFFFFFFHFFFF FFFF.FF FHFDHHCCCC CCCCBBBBBHB _ JJJJJ JJJJH HHHHDDH UFFF FFFFFFFFFFFF FFFFFF FFFDRHCCCC CCCCCBBBEHH _ JJJJJ JJJJD DDDBDDD DUFF FFFFFFFFFFFF FFFFFF FFFDHBDCCD CCDCDBBBEHH IIIII ._PIIII1IIJIJIHIJIHHLIIIIIilwljilhwhu WIDIWHIDIWWIIIIIIIIIIWIF...NWIIIII FFFFFFFFFF .vIF. _ r1 IFFipflD.¢RDIGI 505505.5B‘finifimnlfiiliilllli " JJ JJJJJJJJJD JJDD D FFF F FFFFFFFF FFFFFF F FDDDCCCC CCCCCBBBB_BHHBH _ JJ JJJJJJJJJD JDDD D FLHF F FFFFFFFF FFFFFF F FDDDDDCC CCCDCCBBBuflfiHHH _ J DD JJJJJJJJDD DDDD D Fer P. FFFFFFFF FFFFFF F FBDDDDDC CCCCCCCBQBBHHRB _ H DD DDUDDDDDDD DDDD D EFF. F FFFFFFFF FFFFFF F FDDDDDDD CCCCCCCBB_BHHRBB H H DD DUDUDDDUDD DDDD D Fr“.nr _..r FFFFFFFF FFFFFF F IDDDDDDD CDCDCBCCBWBBHHHHH _ H DU DUUUUDDDUD.DDUU D D1nmnr F FFFFFFFF FFFFFF F IDUDUDDD DDDDCCCCC_BBBHHBHR H H DD DDDDDDDDDD DDDD D. .DBD D FFFFFFFF FFFFFF F FDDDDDDD DDDDDDCCQCCBBRRRHH 0 0 DD DDDDDDDDDD DDDD D D1DD D FFFFBFFF FFFFFF F FFDDDDDD DUDnDDDCCHCCCBBBHflRHB on _ D DD DDDDDDDDDDDTUDDD D DBD nu DDHFBFFF FFFFFF F FFFDDDDD DDDDDDDDQCCDCBBBHBBHHHHRBH . D DD DDDDDDDDDDDDDDD 0 D30 0 DDBFBFFF FFFFFF F FFFDFFFF DDDDDDDDLDCCGCBBBBRBHBBBBB D DD DDDDDDDBDDJDBDD D DDD D DDBBHBBP FFFFFF F FFFFFFFF DDDDDHHDDDDCCCBBBBHBBHRBHH 0 DD DDDDDDDDDDDDDDD D DBD D DDHBBBBF FFFFF.F F FFFFFFFF DDDFHIIDQDDDBDDCBBBBBBHBB W I. D DD DDDDDDDDDDDDGDD B DWDD nU DDDHHHBF FFFFFF F FLFFFFFFF DUFFHIIan—DDUCBCCCBBBBQUBBH m IIIIIIIIII D I010! UUDDBUIDIDDDKUUDD 3 Damon. B DDDBHHBF FFFFF.F F FFFFFFFF DFFFHIHDDUDDCCCDBCCBBBDCB CD CCDDDDDDDIDlmwflquB IB. IIII BBD B DDDDBHBF FFFFFF F FFFFFFFF FFFFFHHDQDDDCCCCCCCCCCCC H CO CCDDDDDDDDQD DD B H H BWIBInwulimnIfi [BI DIUIEBFBPF BFFFFF F FFFFFFPF FFFFDDDDUmDDDCCCCCCCCC,C 5 CC CDDDDDDDDDHVD DD B R B BBBB . B DBDDBB BFBFFF yI .PIFIHE.FIH.FIF. .FFTFIDIDDDDBDUDCCCCCCC . CC CCUDDDDDUDED DD B R H BBBB B DBBDHB BFBBBF FFFF FFDFFFFn.DIUIWWIDIWWIDImUiGIGdIlII II _. CC CCCDDDDDDDBD DD 8 H H BBBB B DBBBHB BBBBBB FFFF FFDDFFF.DDDDDBDDDDDDC _ CC CCCCCDDDDDWD DD B H B 8858 B DBBBHBB BBBBRB FFFF HEEFFFDDDDDBDDDDD _ CC CCGCCCDDDDDD DD B fi fl 5858 B BBBBBBBB BBBFBBB BBBE EEDDDDDDDDTDDD .. BC CCCCCCDDDDUD DD B on H BBBEB_B B BBBBBBBB BBBFRBL. BBBE EDDDDDDDDDUD _ CC CCCCCCDDDDVDD DD 8 B B BBBBB~ B BBBDBBBB BBBBFBB BBBB BBDDDDDDDDU L DC CCCCCCDDDD.DD DD C B H BBBBB.B B BDDDBBBB BBBBFBE BBBB BBDDDDDDDDV M CC CUCCCDDDDDLDD DD 0 B B CBBBRWB B DBDDBBBB BBBBFB? BBBB BBDCDDDDDDD ~ BC CBEDDDDDDDDD Fl. 0 B C 808838 B DBDDDBBD BBBBFBB BBBB BBDEDDDDDDED _~ BC BCDDDDDDD E FF 0 D 80003 D BBDDEEDD BBBBEEB BBBB BDEEEEDDDDD IIIII. IIIII DD DDDDDDDDU E FD D hi... 8000*. D BDUDEEDD DUBBEET BEBE BEEEERDDUDUD ~ 6 BvBDDDDDDD E DD PM U CUUDfi B BDDDDDDD HUBER—Lyra“ BEBE BEEEEDD\DFFDDD M DDIDD. 5100.. C B CDDDD C BBDDDDDD DDDDEEmE BBBB EEEEE.DD\DFF_.DDUD _ CCDD CC 5 ..Bi DDDD. C DBDDDDDD DDDEEE EEEE EEFEDDKKKKDDDD ‘ CCCC C 10.5550 G DDDDDDBB BDDEEE EEEE FFFDDDKKKLDDD H CCCC C DD I._I IDIDJID. balm DDDDFEm FFFF FFFFDDKKKKKK L DCCC C JJ J JJJ F KQDFE FFFF FFFFFFKBKKKK 0080 C K F FE E FFFFHHDK a 00.. D F B FF h. AWN. Hand... . DD. F J B JF F FF DD DI F J K KF P F B FJ» D D K KB “F F Fr. 0 K K BB B FF F D K DD BF F D K 0D Dc 0 D D DK J D D D BB K J D DF E D EF E D DD E D DB ML E EB E 85 E IEE a 55 E E? E EE 0 D E E E E E E E F B B F wwwmmw 22.2333. 555555 from calculated intensity on saturated alluvium F1 D BB EB E E BDDRFIEEILLLLKLKKBBBBKBFFB’FBKBEBBKKKKKDEEED fiDDHfiHELLLLKKBKKKKBBKKEB LHLEHHHHLLLLLFFFBKKBBBKHKIBBKKKBKKKKKKEDEEF EELBBfififiLLLLLFI-‘FKBKKKK/KH B B B B H E E E 777775 191919 755 [9 7777 5 55519 [9 [91955191951911.5555 [95 [91919551919519 5519195 DFI EE EHEEEEBKKKKKKBKK I EEEEFF‘KKKKKKKBK I EELLFKKKKKKKKKBKKKBKKKBKKKBBBBKKKKKKKKKBHDFFFCBCCFFDDCDFDCCCC EEEFFFKKKKKKKK E B B 7777 UNITED STATES DEPARTMENT 'OF THE INTERIOR GEOLOGICAL SURVEY 0.0 I I l . I BB BBBB 1 BEDS KK L 8888 EH L BBBB BG L 1111111 L L L FFFF HHBB LLllLllll K L L K K 1K1 HHHB HHHH LlI...JILK11 K L L K K KK B.FIB.HB BKF.FF FLIIIKIKKK K KNK L L K K KK KBBBB IB;H»HIFF FHHIIKKKKK K K.K K K B K KK KBBJBFmFFKB/W LFFLLKKKKK K «‘K B K K K KK KBBG.B£FBFFK FKLLLLKKKKK K KKK K K K K KK KBBHHBBFFKK FLLLKKKKKKK K WK K K K K KK KKBBBWHHHKK LFLKKKKKKKK K LK K K B K KK KKBBBHBBBKK LlL/LKKKKCBKK K KK K K B K KK BBFKKBKKoKKK I . DLLIKIK’KKCBKB K EK K K K K EE BFFBBKKFBKK FDLLK.KIK.K.K/KK K KK K K B K BB BBBBBKKFFBK FLFLFFKKKK/WIB, K .KKKK K B K K BB BEBBFKKKKKK FFFFLLKKKKBK ,vw. IKKK K K K B BE EBE BKBBKK FFFFCCKKK.KKK K KMHK K K B K KB EBB BKKBKCKK HFLLLLKKKKKK K .KKKK 5 1K K K KB LBB BBKBKKKK HHBLLLIKKKKKK K KKKK K tuln K K KK LBF .BBBBBKKK HHHHKKKKKK KxKKKK K K K/K KK BKK KBKKKKKK HHHRCKKKK KKKKKK K B K K ”MW «BE. KKLKKK K KKfiHfifiCBKKK K.‘KKKKK K K K K KK K15 KKKKKLLL DKKBKKCHBCBC KKKKKKK K K K K KK K kn MIE..L.LLLLL DDKCBBBBBBBB KKKKKK K K K K KK L 5K KKLLLIUILF DBBCBBBKBBBB KKKKKK K K K K KK F F FFFFFFFE DCCHBBKBBBKC BKKKK K K K K KK K F FFFFFEEE BBKK BBBKK K K K K KK K F FFFF CCCCC E C F F FD B B,_ HRHH 00000 E F. F F FB B H... HHHH CDC E E F FF \U D Bfififi C E E E EE E E DEUH E E E EE E E EKKD E E HE E E EKEE B DD D D D D BDEEEFFFFKKKKKKK DEEEDHB‘FWJ‘LLB D'EJLBDBBKKKKKK 13 0.0 0.0 0.0 1mo woo we mo no wo” wo_ we we” moi mo 1mo 90.0 80.0 no we we we mn‘ 10.0 180.0 fiINTERIOR —GEOLOGICAL SURVEY, RESTON, VA— 1981— 681613 170.0 Ground-condition units based on geology from Geological Map of the United States, National Atlas of the United States of America, p. 74 — 75 120.0 130.0 140.0 150.0 160.0 110.0 400 MILES 600 KILOMETERS 300 500 1mo (GRCHINILCXNNLHTTCHVIJNITS SCALE l:7,500,000 100 200 90.0 100 80.0 HHHHH 100 50.0 60.0 70.0 100 H H H H H 40.0 30.0 20.0 10.0 GROUND-CONDITION UNITS AND ATTENUATION FACTORS DIGITIZED ON 25-KM BY 25-KM GRID FOR THE CONTERMINOUS UNITED STATES 0.0 0.0 PLATE3 PROFESSIONAL PAPER 1223 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY m m 0 0 0 0 0 0 0 0 0 o u m m w m m w m w m m 0 N N N N w m 11 w w 0 8 1 3 . . .I. , a 3 , . . 33 1\ . , 33 1 , , , r 1r \ 333W . . 1 111 33WW \1 3. 111,11 .jWWm0 W 8 w 111 11 . 3W3W 55 90 . \\\11 . 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[‘11 2 1111110 0 3 5333 "I .J 3 353 1 3502911 2 011 21111 221111111112 21113 33333 7 33 33333 333 0 3333 3333033333 5000 3033333 33 333 00003333333003333 05555000000000 33300003333 30 00 0055555000 077 00330000000000000000555555000077 550007 222222222 255355 333 2222222 222555 2 2 2 25555555311‘01111151111133 2 2 23 3 35011311055 E11 5 5 33353333355555555 l111333 35 555533 2 00 0 000005 5 33 7 0 2233 0000000050077000 00 40.0 1 111333333333 3 “41311171111100 0077‘779 3 22 3555 1 5 5'5 1 223 5500 0077006 0 000221 1 2 2222 5 3 1 1 1 1 1 21 2 222 2 5 55000007778 1 1 2022 220 00000 33 33330333335555000 3 333303305500000 333333333055 11111110111311 0 30.0 30.0 0000 0050000000 0 00555 000000050000000000 5555555555507 50555555 55 5 00¢ " 00 555500 0055500 55555500 55555500001 55 000000 05 55000005 50000551 555‘ 333333333305500000 3 333300000000555555 55 5555555 55 .5 20.0 20.0 I 8 ’ 0 EXPLANATION Faint length E111111-11 01000111031055 Locality 0mmvwmcwi 6Mm 50000 X 1 \ 1C.O 0-0 _ 00 0.0 10.0 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00000000005555555 555555555555 55000 0000000000077 1333333000000000000000000000000000000 0000000000055555555555555 55 500 0000 0 0'0 00’ m0 - 3 33 00000000 00000000 000000000000000000000000555555 5 __-- 000000000 200 ’ 3333333 000 0000000000000000000 000000000000 555 5 00000 EmNANNHON 3 000 000000000000000000 000 0000 L000 030 00 000000000000000 00I 00000000001 00000000 00: 0000 0 00000000 0000 0 1 m0 . 0 0000 000 5555 5‘ 100 1 0 00 000 5555555 00000000 55555551 0 0 00 5555‘ 0 0o 00 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 150.0 170.0 180.0 SATURATED ALLUVIUM SCALE 127,500,000 100 100 200 300 400 MILES 1:11—11—11—11—1 100 0 100 200 300 H H H H H r - 1 400 500 600 KILOMETERS 1 PREDICTED ROSSI-FOREL IN TENSITIES ON SATURATED ALLUVIUM AND ON DIGITIZED GROUND-CONDITION UNITS IN THE CON TERMINOUS UNITED STATES FOR THE 1886 CHARLESTON, SOUTH CAROLINA, AND 1872 OWENS VALLEY, CALIFORNIA, EARTHQUAKES 3Ku mm“ 1T Rm WP m L A N O I S S E F O R P UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY B00 1N0 1W0 1W0 M00 B00 1N0 MOO W0 W0 mo W0 W0. “0 W0 W0 mo 00 1mo r20 GROUND—CONDITION UNITS w m m o m o o o o a n o H m 9 W 7 W W w W W w Q 1.11“ N a... -- m... m. ,9...— z . 222 , . 22222 \- x 2 22222 22222222 2222222222 1\\ 2222222222 2 xx 5 1 2222222222 22 55 2222222222 222 2 33 x 1. 2222222222 2222 22 33 . 2222222222 2222 22 2232 \1 \\ 2222 2222 22 2222 x 2 2222 22 2222 1 W \\ 2 2222 22 2223 m 2222 22 3333 1 2222 22 2232 11 1 2222 22 2222 , 2222 22 2222 2222 \x 2222 22 2222 33mm... 5 55 2222 22 2222 3232 55 55555 2222 22 2233 3323 ,5555555 2222 22 3333 3322 22552222 2222 22 3333 3332 5555 25555555 2222 22 2333 3332 5555 55555552 \..222 22 2333 333.2 5555 55535552252. \\1\..\ ,. 22 22 2233 3333 5555 5333333222 . \\\1\ .. 2 2233 3333 3333 3333333222 155 , . 3333 3333 3333 33333333 22 1 55122“. 1 x 2333 3333 3333 33333223 2222 2222 2333 3333 3333 33333332 2222 1- mm . 2333 3333 3233 33323333 252% . 2222 p 2333 3333 3233 33233333 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W 1M 3333 2222 22222222 3333 2555 8, . 3333 2222 22222222 3333 5252 . .11 3333 2222 22222222 3333 5222 5 -1 1111 3333 3333 2222 22222222 2333 5522 5 xxxxx 3333 3333 2222 22222222 2223 2522 25 3333 3333 2222 22222222 2222 2252 5 33 3333 3333 mmmmmmm mmmmmmmmwmwmmw mewmmmx 2.22.2. , 33 3333 3333 222222222fl2222 2222 2222 d 33 3333 23333222222 2 22222255555 222222 33222222 . 3 33 3333 322833 22222 2255555225 222222 222222 5 33 33 33 3338 222333 2222222222222555552 MMTWMWWM 222222 ” 333317w~n3233 3333 222332 22222555555555 52222222... 24222.5 . 11113331 33 3333 22222332 2222 55555555 5522222222222255 H \\\\\\\\ 333 33 3333 232322.. 2222—222me 55555555 5552.422. 2222252125 . 11 333 33 3333 2222 2222 55555555 5552 2252 . 333 u 333 2222 2225 55555555 5555 2255 H 3333 , 222 2255 55555222 5555 2555 . 333 H 222 2255 55552222 2555 5555 . 3333 ,3 22.22. 5555 55552277 2555 5555 _ 3333 . 33 222.2. 5555 55522777 7255 5555 2233 3.3 3333 32.2mm. 5555 55222788 7.222 5555 5 2 2233 33 3333 3222 2555 55222292 7222 5555 22 5 111111111 2 2233 33 3333 32222222222555 55222295 7222 5555 22555. \\\\\\\\\\\\\ c 2233 33 3333 3222 22555 55222990 722211 5553 22555 2233 33 3333 32222222222555 55522299 @227 5555 22555 2233 33 3333 3&21 fl 2355 55552229 @277 2555 22555 2233 55 3533 -1132222222222555 55555222 @777 2255 22555 \\\\\\ 121111 2232 35 3333 32222222222255 55555552 2777 2222 22555 W ‘‘‘‘‘‘‘‘‘‘ 2 2223 33 3333 322222 2255 55555555 7777 2222 22555 9 22 2223 33 3333 3222 2225 55555555 777 2222 22555 222 2223 33 3333 3322 2225 55555555 7777 2222 25555 22222 2223 33 3333 33mm 2222 22555555 5777 5252 2555 22222 2223 33 3333 3322 2222 22255555 5255 5522 2555 22222 2223 33 3333 322mm mmmmmwm. 2222.22.55 52.2mm 552:2u 5555 222222 2222 33 3333 22.2.2... mwmmmmm mwmmwmmmmmwramm mfmmmmmm 52.22. 2.555 222222 2222 33 3333 mmmmmm. mmmmmm 22222552 mummmmm mmwmm .2555 222222 2222 33 3333 222%. 2.2.2.2 2.2222222 2.22.2. 2.2.2.2 2.555 222222 2222 3 3333 33222 222222222222222222222222222 222222 222255 222222 22222333 3333 3322222222222WWWMMMMTMWWMEMEMMMWMWMMMW 22222222222255 222222 22222232 3322 32222 222222222222222 22222222222222222222222255 222222 22222222 2222 222222222222222222W22222 22222222222222222222222255 2222222 22222222 2322 332222222222222222 MTMWMTM W M 2 2 255 xxxxxxxxxxxxxxxxxxxxxx 1: lllll 12222222222222 32 222% 1111339211 mem 2222222222 MWWW 22222 222225 2222222 ~2222 3133mm3mw 23.333 Nymwmmm mmmmmmmmmm mummmm. mmmm mumwmura 2222222 2222 32 2222 2233 2222 22222222 2222 2222 2225 22233322 2222 33. mmmmmmmm 2.2.33 2.2.22 mmmmmmmmmmmmmw mummmmmm mwmmm mmmmmm 22B33333 3333 33 2233 2222 2222 22222222 2222 2223 2222 22333333 3333 33 2.2.33 mwmwmmm mummmmmw 2222.22me 2.1223 2.2.2.3 2.2.2.2 23333333 3333 33. 3223 22:22. 32.2mm. 2222.22.22. mwmmmm3 2.2.23 223mm 23333333 3333 35. 332.3 2.2.2.2. 2222... 2.2.2.2222me 2.22.3 22.233 2333 55 33333333 3333 33 3323 222.222 22.22. 22.222222 M1233 22.23 333.3 5555 33333333 3333 33 3332.. 22‘2222 2.22 23233333 3333 mumi23 3333 5555 33333333 3333 33 3333 5222 222222T22223233333 3333 3323 3333 2555 33333333 3333 33 3333 5255 22A222m222255333 3.333 3333 2333 2.255 . 33333333 3333 33 3333 2255 22222222255333 3333 3333 2333 22.22 W 33333333 3333 33 3333 2255 2.242222222325333 32333 3333 2333 322.2 m 1 ........... 33568363 3333 33 3333 2255 2222 22225333 3333 3333 2333 3332 . 33333322 3333 33 3333 -115565 2555 22525333 3333 3333 2333 3333 33333333 3333 33 3333 5522 lllll 5535 22555583 1333 3333 3333 3333 33333333 3333 33 3333 5555 5555 22555553 3333 3333. .1113333 333 33333333 3333 33 3333 5555 5555 52255553 3333 3333 -332333 3 :::::::::::::: 33333333 3333 33 3332 5552 2555 55255553 @333 3333 3333 33333333 . 3333 33 333m 555m... mummmfmm 55255553 5333 5533 3333 33333333 3333 33 . 33mm. 5552. 222.3 22.555555 5333 5333 3333 33333333 3333 33 3322 5552 2223 22222222 535222 3333 3333 33333333 3333 33 3322 552222222222W222§3222222 22222 3222 333 33333333 3333 3.3 3332. 55222222222222 33212222225 3222222222222222 333 33333333 3333 33 333333222252222222222223 3322223222 3222222222222222 333 33333333 3333 13 223333222222222322222223 33322332222232T2W222222222T2 .333 33333333 3333 33 222333222222222333222233 33333332 3322222222 2223 33 11 33333333 3333 33 2333332222222223 3233 33333332 32222222222223 3333 333.33355353333 33 33333322222422222213 3322. 33333332 322%2m12mlmmmmwmwfiw3 225333 333333333 3333 53 3333 2222 2222 33333322 3222 3333 223333 333333333 3333 33 3333 @222 2222 33333333 3233 3333 223333 .333333333 3333 33 3333 3%22 2223 33332233 3333 3332 22333 333333333 3333 33 3333 2222 2222 23222223 3323 3322 2222 333333333 3333 .33 3333 3332 2222 2333331212 3122.3 3222 2222 3333333333 3333 33 3333 3332 2222 33222223 3333 32221212 _. 2333333333 3333 33 3333 3222 2222 22.22.2222 3333 2255. 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