_"" *y NAY HART * Ce- SCIENCES Complete Bouguer. Gravity and General Geology of the Cape San Martin, Bi’yson, Piedras Blancas, and San Simeon Quadrangles, California GEOLQGICAL SU RV EY PROFESSIONAL PAPER 646-A Complete Bouguer Gravity and General Geology of the Cape San .Martin, Bryson, Piedras Blancas, and . San Simeon Quadrangles, California By STEPHEN H. BURCH GEOPHYSICAL FIELD PNVESTIGA TIONS GEOLOGICAL SURVEY PROFESSIONAL _ PAPEK 646-A Detailed gravity data define the extent of the Burro Mountain ultramafc body, and regional data outline mayor feaz‘zgres of the Salinian and Franciscan basement blocks UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1971 UNITED STATES DEPARTMENT OF THE INTERIOR ( f FRED J. RUSSELL, Acting Secretary y GEOLOGICAL SURVEY ) William T. Pecora, Director f For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page | General geology-Continued evi "o-- "ess A1 Superbasement sedimentary rocks-Continued Page 1 Pancho Rico FOFMatiOon....-.-.---------------- A4 (General 2 Paso Robles Formation._._------------------ 4 Pascment 2 Surficial deposits. ._.----------------------~-~~ 4 Uitramatic 2 4 Franciscan FormAtion....----------------~~~ a | Gravity 5 Metamorphic FOGK8. _ ___._.._..-_------------- 2 Gfavity § Granitic 2 Gravity interpretatiO®.__________-_--_---------~-- 10 Tertiary intrusive POCKS-__-----------------~ 3 Gravity anomalieg_ . 10 Superbasement sedimentary POGK8.......--.------- 3 Detailed survey of the Burro Mountain ultramafic Paleocene and upper Mesozoic 3 (pment oc tar 10 Middle and lower Miocene deposits....------- 3 Regional 10 Volcanic FOCK®8............_.L--.~.---=---.----- 4 | References cited..................._._-.-==&--~*-~-r-=~ 12 Monterey Shale......--...-------<==s------ 4 ILLUSTRATIONS Page PraAtE 1. Complete Bouguer gravity and generalized geologic map of the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, L... . 0. lue "Co In pocket FiguUurE 1. Index map showing {Ocation of map areas.... noon too 0 Lf A1 2. Aeromagnetic profile through the Burro Mountain onn" 11 T A BLESS Page. TABLE 1. Selected exploratory Welle. . Mill.. [V. gil ec. eof n OC A3 2. Principal facts for gravity haste: . ll cfl Le.. CIOL}. 5 3. Principal facts for stationg. .l laz co D.. i uence scorer n tool 00 0 - 5 II GEOPHYSICAL FIELD INVESTIGATIONS COMPLETE BOUGUER GRAVITY AND GENERAL GEOLOGY OF THE CAPE SAN MARTIN, BRYSON, PIEDRAS BLANCAS, AND SAN SIMEON QUADRANGLES, CALIFORNIA By StEeurxn H. Burck ABSTRACT Complete Bouguer gravity coverage of 390 stations and gen- eral geologic mapping were compiled for the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, Cali- fornia. These quadrangles constitute a 30- by 30-minute rectan- gle covering approximately 600 square miles of land area, most of which is in the rugged Santa Lucia Range. Two distinct basement units underlie the map area. In the northeast part, the granitic-metamorphic Salinian block con- stitutes the basement. The eugeosynclinal Franciscan Forma- tion, however, underlies most of the map area and here con- stitutes all but a small part of the Santa Lucia Range. The Nacimiento fault is commonly believed to separate these major basement blocks. Overlying both basement units is a sequence of Cretaceous and Tertiary marine deposits and the nonmarine Paso Robles Formation. Detailed gravity data indicate that the unserpentinized core of the Burro Mountain ultramafic body has a subsurface volume no greater than 1 to 2 cubic kilometers and extends no deeper than 2,000 to 3,000 feet. Aeromagnetic data seem to preclude a large volume of subsurface serpentinite. The major features defined by regional gravity data in- clude: (1) a rather even gradient of 3 milligals per mile in the entire southern half of the area which probably reflects deep structure of the continental margin, (2) a 10-milligal high coincident with the topographic mass of the Santa Lucia Range which suggests a density of over 2.8 grams per cubic centi- meter for this mass, (3) a broad gravity low associated with Lockwood Valley which suggests that the valley is underlain by as much as 7,000 feet of low density sediments, and (4) a conspicuous gravity gradient of up to 20 milligals per mile which cuts diagonally across the entire Bryson quadrangle and represents a fault which vertically displaces the basement sur- face 5,000 to 10,000 feet. The most significant structural feature of the map area, the contact between the Franciscan and Salinian basement blocks, shows little or no gravity expression. INTRODUCTION This report presents and interprets a 2-mgal (milli- gal) complete Bouguer gravity map of the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, California. Detailed gravity coverage was obtained for the Burro Mountain ultramafic body, and the regional coverage was collected as part of a broader survey of the San Luis Obispo 1:250,000 Army Map Service gravity sheet. The gravity map is overprinted on a generalized geologic map compiled from several sources. The four quadrangles constituting the map area (fig. 1) form a 30- by 30-minute block covering approxi- mately 600 square miles of land area, most of which is in the rugged Santa Lucia Range. The area extends AZ: 121° 120- ~ \ C MonTEREY 4 | \\ < Q" $ $ \. mMonTEREY o a Ne county %. "MAHA 3.3 % es ~ SAN LUIS OBISPO 3 L- county € *, \r / [ (x /@J’ King] Cicy\\ L_ y 7 % a pz < 4 MONTEREY CO San LUI§]OBISPO C0 3 1L 5) P L_, 7,1, 1 % "SAN LUIS 0 10 20 30 MILES OBISPO f E1ELL__L__L_I 35° FIGURE 1.-Location of the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, Cali- fornia. 1, area described by Burch and Durham (1970) ; 2, area described by Hanna, Burch, and Dibblee (1971). A1 AZ from Lucia and Cambria on the coast to Lake Nacimiento, Lockwood, and Jolon on the east and northeast. The author wishes to thank D. L. Durham and B. M. Page, who gave generously of their time in dis- cussing geologic problems and contributed their mapping to the compilation on the geologic map. GENERAL GEOLOGY The geologic map (pl. 1) is a compilation from sev- eral sources which are indexed by area at the bottom of the map. Rock units are lumped to provide con- tinuity with companion maps to the east (Burch and Durham, 1970; Hanna and others, 1971). The map area may be separated into two major base- ment blocks, the Franciscan block of eugeosynclinal rocks in most of the area and the Salinian block of granitic-metamorphic rocks in the northeast part. The boundary between these blocks is generally thought to be the Nacimiento fault. Such is indeed the case near the northwest corner of the map area where the base- ment contact is exposed. In the eastern part of the map area, however, it is doubtful whether the gently dipping contact between the Franciscan Formation and Upper Cretaceous rocks, mapped as the Nacimiento fault zone, is also the contact between the two basement blocks. Overlying both basement blocks is a sequence of upper Mesozoic and Tertiary marine deposits and the nonmarine Paso Robles Formation. BASEMENT ROCKS ULTRAMAFIC ROCKS The ultramafic rocks are emplaced only in Franciscan Formation, and most are serpentinites typical of those found throughout the Franciscan (Bailey and Ever- hart, 1964, p. 47). The smaller bodies are thoroughly serpentinized, and intense shearing has destroyed origi- nal textures in all but a few 2-3-inch remnant blocks. The centers of the larger bodies consist of blocky ser- pentinite, but invariably this grades outward to the usual sheared material. These serpentinite bodies crop out as elongate pods and lenses concordant with the regional structure. They commonly form discontinuous trains which continue many miles along zones of appar- ent slippage. The Burro Mountain mass, by contrast, is a large, nearly equidimensional (1 by 114 miles) block of massive fresh ultramafic rock, only the outer 700- 1,000 feet of which is altered to serpentinite. The pri- mary rock, before serpentinization, consisted of approxi- mately 65 percent periodotite and 35 percent dunite BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA (Burch, 1968). Variations among these lithologic types produce a well-defined internal structure traceable through most of the body. In the interior the primary rock was affected only by incipient serpentinization, and densities approach 3.30 g/cm' (grams per cubic centi- meter). This fresh core, however, grades outward through massive, partially serpentinized rock to blocky and sheared serpentinite at the margins, where the density is 2.50-2.60. Fresh ultramafic rock is nowhere in contact with country rock. FRANCISCAN FORMATION The Franciscan Formation in the map area is part of a 10- by 100-mile tract extending from Point Sur to San Luis Obispo. It consists of moderately to highly deformed eugeosynelinal rocks deposited on an unknown basement. Palynomorphs recently collected by B. M. Page from this area are of Early and possibly Late Cretaceous age (W. R. Evitt, written commun., 1969). The rock types and their estimated percentages are: gray wacke (60), siltstone and shale (25), conglomerate (1), mafic volcanic rocks (10), chert and accompanying shale (2), and glaucophane schist and related meta- morphic rocks (2). Granitic rocks are lacking in this section of the Franciscan. The structure of the Franciscan Formation reflects moderate to extreme deformation, predominantly by faulting. Even in the least deformed areas, gray wacke outcrops are cut by slickensided joints and by seams of sheared and hardened mudstone, and outcrops of lay- ered chert are highly contorted. In the more deformed areas, such as major throughgoing shear zones, the Franciscan Formation is characterized by competent tectonic blocks of all sizes and lithologic types incorpo- rated in a sheared matrix of shale and other less compe- tent rock. The structural complexity of the rocks every- where makes stratigraphic work exceedingly difficult. METAMORPHIC ROCKS Metamorphic rocks, probably correlative with the Sur Series, constitute most of the Salinian basement exposed in the map area. Compton (1966, p. 1364), whose mapping extended into the northwestern part of the area, describes these high-grade metasedimentary rocks as "mainly medium-grained quartzites, quartzo- feldspathic gneisses and granofelses (Goldsmith, 1959), cale-silicate granofelses, amphibolites, pelitic schists, marbles, and metadolomites." GRANITIC ROCKS Granitic rocks are seen only as two small outcrops in northeastern Cape San Martin quadrangle and as two % CEOPHYSICAL FIELD INVE STIGATIONS A83 tiny slivers in western Bryson quadrangle. Well data (table 1), however, indicate that granitic rocks consti- tute much of the unexposed Salinian basement. Comp- ton (1966, p. 1365) describes the lithology to the north as chiefly "adamellite, granodiorite, and tonalite, with some potassic granite and basic-to-ultrabasic rocks." TERTIARY INTRUSIVE ROCKS Tertiary intrusive rocks crop out in the San Simeon quadrangle at Pine Mountain and near Cambria. These rocks are porphyritic and nonporphyritic - felsic alphanites. SUPERBASEMENT SEDIMENTARY ROCKS PALEOCENE AND UPPER MESOZOIC DEPOSITS 'The unit designated as Paleocene and upper Mesozoic deposits includes (1) rocks represented as the Knox- ville Formation and as Lower Cretaceous marine on the San Luis Obispo geologic sheet (Jennings, 1958) and (2) an Upper Cretaceous unit and a Paleocene unit, both part of Taliaferro's (1943, p. 132) Asuncion Group. It crops out in a northwest-trending belt 4 to 5 miles wide immediately east of the Franciscan Formation and as small patches resting on the Fran- ciscan. The unit appears to be absent in the subsurface northeast of the Jolon fault, since well data there indicate that Miocene strata or younger beds directly overlie basement. All the rocks lumped in this unit are lithologically similar to the marine sedimentary rocks of the Great Valley sequence; they consist pre- dominantly of massive-bedded, poorly sorted, medium- to coarse-grained, potassium-feldspar-bearing arkosic sandstones with lesser amounts of interbedded mud- stone and conglomerate. The contact between this unit and the Franciscan Formation appears to be a fault contact throughout the map area. The contact is steep in the Cape San Martin quadrangle, but in the north- east part of the San Simeon quadrangle, dips are shal- low and features resembling klippes and fensters sug- gest that the contact may be a thrust fault (B. M. Page, oral commun., 1968). MIDDLE AND LOWER MIOCENE DEPOSITS The unit designated as middle and lower Miocene sedimentary deposits includes rocks of the Tierra Re- donda and Vaqueros Formations. It forms a parallel belt just east of the previously described older rocks. The belt is narrow in the center but spreads out into a broad syncline to the southeast and envelops the granitic core of an anticline to the northwest. It appears to be in depositional contact with (1) the aforemen- tioned granitic core, (2) subsurface granitic basement east of the Jolon fault, and (3) older rocks in the afore- mentioned syncline. It is in fault contact with older rocks along the narrow central belt. The rocks con- sist chiefly of sandstone, commonly fossiliferous; con- glomerate and mudstone are locally abundant. Well- indurated calcareous sandstones commonly alternate with friable partly calcareous or silty sandstones (Dur- ham, 1965a, p. 9). Tasur 1.-Selected exploratory wells from the Bryson quadrangle, California [Elevation: kb, kelly bushing; gr, ground; t, from topographic map] Map Location Year Total refer- Operator Well be- Elevation depth Reported geologic data (depths in feet) ence Sec. T.S. R.E. gun (feet) (feet) (pl. 1) 1 Humble Oil & Refining Co... __ Meta E. Oberg 19 22 8 1957 1,206 kb 5,770 Basement top at 5,690. 2 Empire State Oil Co....----- _. Empire State-Thorup-Plaskett 1. ..... 20 - 22 8 1967 1,160 t 4,526 None. 3 Texaco, Inc........--- .. Martinus C. H. 1..._........-- % 27 22 8 1966 1,530 gr 1,799 Basement top at 1,748. 4 .. i.e. il- ci- Valdez C. H. 1.. 26 22 8 1966 1,310 kb 653 Basement tog; at 607." 5 General Petroleum Corp.....--------- Hotchkiss 35-29........----- 20 22 9 1952 1,842gr 6,259 Bottom in Miocene rocks. 6 Time Petroleum CoO.......------~----- Roth 1.........!.. 5 23 8 1964 975 gr 5,890 Top of Monterey Shale at 2,305; top of middle and lower Miocene deposits at 3,925; bottom in Miocene de osits. & . (lies U0. scene es $. 48 8 1964 1,100gr 4,907 Top of Pancho Rico Formation at 8 Burke, Jack P..........-.-.- 9 General Petroleum Corp 10 Time Petroleum Co._.-- > 11 Humble Oil & Refining Co..... _. Anna L. Pauson 1... 120 '--... Q.:... .A.. WLL» vu sentient Gladys Stockdale 1... .. Keans (NCT-1) 1 Marport-Parlet-Block 1 Floyd L. Patterson 1.. 16: / .... Nor Erl. A dss I. M Diggs 3 17 Chamberlin,C. H........------------- Shepard o. .s} .. Wright-Texas 25-5....._..__..-.-.-.--- . Helmson .- p 2,595; top of Monterey Shale at 2,685; basement top at 4,830. 1964 1,479 kb 850 'Top of granite at 756. 2,587 Bottom in granite. 1964 970 kb 1,656 Bottom in Pancho Rico Formation. 1956 1,000 gr 4,003 Bottom in Miocene deposits. 986 kb 2,388 Bottom in basement. Do. 1957 995 kb 1,493 Basement top at 1,470. Basement top at 1,123. Reported bottom in pre-Tertiary rocks. Bottom in Monterey Shale. SEBSNHSSao o «o « «o gn ao 00 co t w l- © & = 1963 1,589 kb A4 Also lumped within this unit are three small patches shown as Miocene nonmarine by Jennings (1958). These rocks constitute the spectacular outcrops for which the Palisades were named. VOLCANIC ROCKs Miocene volcanic rocks are shown in the Piedras Blancas and San Simeon quadrangles by Jennings (1958). Those of the Piedras Blancas quadrangle are shown as pyroclastic rocks and those in the San Simeon quadrangle as rhyolite. MONTEREY SHALE The Monterey Shale is widespread on both sides of Lockwood Valley and attains a thickness of at least 2,700 feet on the south side of the valley and 6,600 feet on the north side (Durham, 1965a, p. 13). It conform- ably overlies the Vaqueros Formation in this area. It also conformably overlies and intertongues with the Tierra Redonda Formation. In the San Simeon quad- rangle it is in depositional contact with Franciscan Formation and lower Miocene marine (Jennings, 1958) as well as Upper Cretaceous rocks (B. M. Page, unpub. data, 1966). The rocks are chiefly porcelanite, porcel- ancous mudstone, and mudstone with some chert and dolomitic carbonate beds. The dominantly calcareous beds in the lower part of the Monterey Shale southwest of Lockwood Valley constitute the Sandholdt Member. Forminifera indicative of early and middle Miocene age are found in the Sandholdt Member. The overlying siliceous rocks of the Monterey are generally lacking in fossils useful in age determination, but stratigraphic relationships with the Santa Margarita and Pancho Rico Formations indicate that these siliceous strata are probably of late Miocene age but could include beds of latest middle Miocene and early Pliocene age (Burch and Durham, 1970). PANCHO RICO FORMATION Small areas of the Pancho Rico Formation were mapped by Durham ( 1965a) in the vicinity of Lock- wood Valley but were left unnamed at the time his mapping was published. The Pancho Rico conformably overlies the Monterey and contains fossils characteristic of early Pliocene age. The rocks are characteristically fine-grained thick- or massive-bedded sandstone, but pebbly sandstone and mudstone appear locally. BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA PASO ROBLES FORMATION The Paso Robles Formation crops out in and along the sides of Lockwood Valley. This nonmarine unit con- formably overlies the Pancho Rico and unconformably overlies the Monterey. The unit is generally considered to be Pliocene and possibly Pleistocene in age. The rocks are chiefly conglomerate, conglomeratic sandstone, and sandstone. SURFICIAL DEPOSITS Surficial deposits include older alluvium and al- luvium. Older alluvium covers the floors of Lockwood and Stoney Valleys. Alluvium occurs along the beds of most streams. The older alluvium is mainly semicon- solidated silt, sand, and gravel, and the alluvium is similar but unindurated. The older alluvium and al- luvium combined are probably no thicker than a few score feet in most places, but their thickness is uncer- tain, partly because of difficulty in distinguishing older alluvium from Paso Robles Formation in wells. The older alluvium is considered of Pleistocene and possi- bly Holocene age because it unconformably overlies tha Paso Robles Formation. The alluvium is Holocen» STRUCTURE Structure within the Franciscan block is so charac- teristic of the overall Franciscan Formation that it was described earlier (p. A2) in conjunction with that unit. The character of the Nacimiento fault was noted earlier in the broad overview of geologic relationships and again in discussing the contact of the Paleocene and upper Mesozoic unit with the Franciscan Formation. Structures of interest in the Salinian block include the Jolon and Espinosa fault zones. The J olon fault zone, although its surface expression in the mapped area is only minor and its precise location uncertain, is of major structural significance. According to Durham (1965b), it separates contemporaneous but unlike sequences of Miocene and Pliocene strata and has ex- perienced at least 11 miles of right-lateral strike-slip movement. It extends about 20 miles beyond the map area to the southeast. The Espinosa fault zone, in the northeast corner of the map, is generally marked by a zone of crushed and contorted rock 500 or more feet wide and displaces basement sharply downward on the northeast (Durham, 19652, p. 23). It extends beyond the map area 12 miles to the southeast where it termi- nates against the Jolon fault. E GEOPHYSICAL FIELD INVESTIGATIONS A5 GRAVITY D ATA Tasos 2.-Principal facts for bases used in gravity survey Eleva- Observed GRAVITY sURVEY Base Lat N. Long W. - tion _ gravity ! Description (ft) (mgal) The map area includes 390 gravity stations which are | spuyrB__.. s nm in ide 4058 9ro70%.04 USC& ag N - ® ® a L l n Lu + tied to eight gravity bases. The principal facts for the | BRADB..---- as bist (190 47.78 shao 9r9ra7.30 ”$351“ seg at 3 % $ radley. bases are given in table 2 and those for the 390 stations | PSROB------- 35 37.55 120 41.28 720.0 979717.17 Ugc&%s 3M 124 at k . aso Robles. in table 3. All the data are tied to base 173 (Chapman, JOLNB......- 35 solar dot doar: 979.0. 979728.66 “$313“ 979 at F : 35 M.7B 121 27. : 3 C 1966, p. 36) at the U.S. Geological Survey office in gs dm ange dono USORAGS BNY, | % s SSIMB.......: 35 38.04 121 11.26 17.0 979817.49 USGS BM 19 at San Menlo Park, Calif. The observed gravity at this base, Simeon. $ & . CAMBB._...-- 35 38.550 dol 5.383 81.0 9798012M ussgesbgm A694 & ambria. determined by numerous ties to North American | 35 52.31 121 17.25 1620.0 979738. 2M Eslgablished on Los Gravity Standardization Stations at the San Francisco wire r + 1 The amount of scatter among numerous ties between these bases SU; ests that the Alrport’ 18 taken to be 979 $958'74 mga‘l relative observed gravity of each is known to £0.02 mgal. Ce Table 3.-Principal facts for gravity stations Elevation Observed Terrain Free-air Complete Station Lat N. Long W. (ft) gravity correction anomaly Bouguer anomaly (mgal (mgal) (mgal) (mgal) 35 57. 97 121 . 5.12 1104 979717. 46 1. 55 -6. 57 -43. 12 35 57.98 121 5.92 1046 979718. 44 1. 51 -11,. 06 -45. 65 35 57.98 121 6.99 1009 979720. 49 1.51 -12. 49 -45. 81 35 56. 44 121 7.25 938 979724. 39 1. 65 -13. 07 -43, 80 35 58. 47 121 10. 47 979 979722. 64 1. 91 -13. 86 -45. 74 35 57. 70 121 9.82 945 979724. 22 1. 59 -14. 38 -45. 41 35 57.18 121 8.93 930 979724. 66 1. 54 -14. 61 -45. 17 35 56. 24 121 5.87 951 979727. 42 1.49 -8. 53 -39. 87 35 56. 21 121 3.88 986 979729. 79 1. 66 -2, 82 -35. 35 55. 80 121 3.03 979 979733. 88 1. 53 1.19 -31. 07 35 55.25 121 2.06 1046 979731. 25 1. 52 5. 65 -28. 35 54. 49 121 1.12 979735. 91 1. 61 1. 80 -29. 18 35 54. 48 121 0.05 1061 979715. 60 1. 61 -T. 49 -42. 50 35 50. 32 121 2.20 8 979734. 40 1.92 6, 62 -24. 19 35 49. 33 121 3.37 1584 979699. 09 2. 36 32, 54 -19. 75 35 48. 88 121 6.02 996 979747. 18 2. 01 25. 97 -6. 40 35 54. 28 121 7.77 858 979730. 20 2, 40 -11. 63 -38. 85 35 53. 10 121 9.71 1255 979734. 45 2.19 31. 58 -9. 55 35 52. 20 121 10.18 1055 979751. 47 2.17 31. 07 -3. 18 35 52.48 121 12. 81 1 979756. 15 2. 45 37. 98 3.05 35 47. 60 121 11.07 1902 979705. 12 3.79 70. 81 9. 35 46.98 121 10.77 1950 979701. 91 4. 36 73.13 10. 24 35 45. 53 121 11. 64 1265 979741. 53 4.36 50. 39 11.10 35 41. 49 121 11.98 1193 9797 8. 06 49. 94 16. 82 35 39. 00 121 11.93 74 979814. 39 2. 28 20. 53 20. 25 35 39. 26 121 12. 98 53 979818. 43 2. 26 22, 22 22. 65 35 39. 35 121 14, 79 30 979823. 84 2. 25 25. 34 26. 55 35 39. 92 121 15.78 67 9798: 2.36 27.11 27.15 35 40.13 121 16. 45 98 979822. 34 2. 38 29. 13 28.1 35 40. 61 121 17.06 46 979826. 92 2. 37 28.13 28. 91 35 41. 54 121 17.38 18 979820. 32 2.56 26. 57 28. 51 35 42. 08 121 18.06 35 979828. 42 2.77 26. 50 28.06 35 42. 94 121 18. 30 26 979825. 40 3. 03 21. 41 23. 54 35 44. 30 121 18. 78 979824. 85 3. 59 24, 00 24. 35 45.30 121 18.79 120 979822. 80 4. 60 24. 29 24.15 35 45. 89 121 19. 10 32 979826, 49 6. 95 18. 86 24.71 35 47. 32 121 20.06 364 979806. 56 10. 46 28.13 26. 01 35 47. 84 121 20. 74 531 979795. 06 10. 77 31, 59 24. 35 57. 60 121 11.00 945 979720 1.70 -8. 78 -39. 71 35 58. 75 121 13.05 992 9797832. 49 1.83 -3.19 -35. 60 35 59.17 121 10, 64 1024 979725. 15 2, 00 -7. 52 -40. 35 57.01 121 11.95 1282 979715. 56 3. 58 9, 64 -31, 02 35 55.86 121 13. 52 1405 979737. 31 2. 94 . 60 -. 94 35 55. 50 121 14.03 1189 979751. 54 2.16 39. 03 .16 35 55.99 121 14. 60 1219 979751. 79 2. 62 41, 41 1.95 35 56. 50 121 15. 25 1201 979749. 86 2. 28 45. 52 3. 25 35 58. 20 121 16.16 1358 979745. 53 2. 55 45, 06 .15 35 57.72 121 16.74 1287 979754. 63 2. 61 48. 17 6.37 35 57. 67 121 17. 24 1316 979754. 24 2. 58 50. 58 7.74 35 56.93 121 17.66 1284 979756. 82 3.19 51. 21 10, 35 56.28 121 16.79 1204 979759. 74 2.82 47. 53 8. 35 55.70 121 15.25 1258 979;51.68 2. 38 45. 38 4.34 35 55.77 121 14.83 1232 979751. 97 2. 30 43. 12 2. 90 35 54.40 121 12.29 1307 979738. 00 2, 40 38. 16 -4, 54 25 54.46 121 11.08 1352 979728. 58 2.02 32. 89 -11.75 35 54.70 121 10, 53 1248 979726. 48 2. 09 20. 66 -20. 82 35 55.45 121 11.74 1862 979686. 92 7. 46 37.78 -18. 98 35 56.12 121 12.67 1689 979702. 80 5.77 . 43 -16. 06 35 53.04 121 14. 61 1217 979753. 49 3.12 47.13 8.25 35 55.70 121 17.08 1181 979760. 65 4.23 47.11 10. 57 35 58.18 121 18. 87 1304 979756. 99 3. 04 B1. 47 9. 51 35 57.78 121 18.79 1252 979760. 33 3. 22 50. 56 10. 58 35 57.19 121 18.96 1242 979761. 85 4.35 51.92 13. 40 35 58.74 121 15.09 1075 979750. 51 2. 36 22. 65 -12. 09 35 58. 58 121 14.11 1027 979740. 64 2.06 8. 50 -24. 89 35 50. 14 121 13. 40 1007 979730. 23 1.87 -4. 59 -37. 48 406-158 0O-70--2 S A6 BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA Table 3.-Principal facts for gravity stations-Continued Elevation Observed Terrain Frec-air Complete Station Lat N. Long W. (ft) gravity correction anomaly Bouguer anomaly (mgal) (mgal) (mgal) (mgal) 35 58. 58 121 20.19 1315 979760. 51 4.12 55. 46 14. 28 35 59. 51 121 26.00 3463 979634. 99 19. 74 130. 60 31. 07 35 58. 32 121 26. 50 3431 979633. 01 18. 49 127. 31 27. 63 35 58. 27 121 26.99 3306 979637. 72 21. 33 120. 34 27.19 35 57.88 121 25.65 3415 969636. 08 15. 01 129. 50 26. 90 35 57. 64 121 24.78 3210 979649. 64 11.87 124.13 25. 42 35 57.90 121 24.62 3204 979651. 26 11.38 124. 81 25. 82 35 58. 08 121 24.08 3406 979634. 39 16.19 126. 68 25.56 35 58. 09 121 22.67 3441 979630. 76 17.40 126. 33 25. 22 35 57.79 121 21.89 3179 979643. 18 16.42 114. 54 21. 45 35 57.59 121 21.20 2599 979682, 20 8. 78 99. 40 18. 60 35 56.97 121 20.31 1911 979723. 67 6.75 77.06 17. 87 35 56.86 121 19.13 1664 979737. 07 4.18 67. 30 14. 07 35 52.70 121 16.38 2221 979695. 91 7.47 84. 46 15.35 35 52.31 121 17.25 1620 979738. 24 5.26 70. 82 20.19 35 538.18 121 18.78 2335 979098. 24 7.60 96. 82 23. 92 35 53.23 121 19.27 2408 979692. 20 7. 04 97. 57 21.60 35 53.62 121 19.68 2650 979678. 42 9.32 105. 99 23. 98 35 54.18 121 19.41 2322 979699. 89 6.02 95. 82 21.97 35 54.65 121 20.35 3004 979652, 72 13. 09 112. 11 21. 70 35 54.81 121 21.77 3122 979649. 82 11. 54 120. 08 24. 06 35 55.14 121 22.08 3275 979638. 60 16. 01 122.77 25.97 35 54.52 121 21. 84 3369 979630. 79 18.99 124. 68 27. 64 35 50.89 121 18.26 3373 979618. 87 20. 95 118. 32 23. 10 35 50.84 121 15.80 2535 979676. 80 10. 89 97. 53 21. 04 35 57.97 121 9.98 958 979724. 08 1. 60 -13. 68 -45 15 35 57.98 121 8.60 971 979724. 28 1.53 -12. 27 -44.26 35 57.98 121 7.88 i 979722. 53 1.51 -12. 14 -44, 84 35 58.85 121 5.91 1095 979716. 60 1.83 -9. 53 -45. 50 35 57.13 121 61.4 1074 979718. 92 1.83 -6.73 -41.97 35 57.08 121 5.30 1016 979723. 29 1.41 -7.74 -41.41 35. 57. 07 121 4.33 1027 979724. 57 1.48 -5. 41 -39. 39 35. 56.65 121 2.99 1078 979725. 56 1. 55 .97 -34. 69 35 55.33 121 3.8 924 979736. 31 1.39 -. 88 -31. 39 35 54.92 1251 2.02 979 979735. 98 1.45 4. 55 -27.80 35 53.97 121 1.59 944 979737. 58 1. 50 4.21 -26. 87 35 52.89 121 8.58 1668 979700, 24 5.47 36. 51 -15. 56 35 52.62 121 8.04 1450 979716. 42 2. 30 32. 57 -15. 16 35 52.16 121 7.33 1887 979678. 04 5.79 35. 95 -23.35 35 51.79 121 7.39 2131 979664. 28 9. 25 45. 66 -18. 57 35 51.33 121 5.80 1914 979667. 99 5.74 29. 62 -30. 65 35 51. 01 112 5.33 1879 979671. 04 6.00 29. 83 -28. 97 35 50.92 121 4.52 1693 979681. 62 4.78 23. 05 -30. 57 35 49.86 121 4.17 1630 979695. 98 4.26 33. 00 -18.98 35 50. 53 121 5.17 1590 979701. 70 2.62 34. 00 -18. 24 35 49.64 121 5.45 1459 979712. 62 4. 21 33. 87 -12. 26 35 49.75 121 6.75 1189 979736. 70 2. 05 32. 40 -6. 59 35 49. 51 121 7.% 1232 979735. 38 2.19 35. 47 -4.86 35 49. 81 121 8.87 905 979758. 42 5.00 27.32 1.08 35 50.47 121 9.48 979 979755. 96 3. 51 30. 88 1. 59 35 51.81 121 13. 91 1730 979720. 54 5.02 64. 18 9.52 35 51.35 121 13.90 1856 979712. 92 4.65 69. 07 9. 70 35 49.65 121 12.85 1914 979704. 19 5.76 68. 22 7.96 35 49. 21 121 13. 28 2371 979677. 95 7.36 85. 58 11. 20 35 49.79 121 14.37 2744 979654. 80 12. 14 96. 68 14. 25 35 50.13 121 14. 55 2790 979650, 02 13. 76 95. 74 13.35 35 50.77 121 14.78 2208 979692, 41 5.62 82. 49 11.97 35 48.45 121 12. 50 2319 979678. 58 6. 28 82, 40 8. 73 35 47.76 121 11.93 2527 979662. 92 8. 96 87. 29 9. 14 35 58.12 121 28.94 173 979832. 32 12. 64 20, 51 27. 17 35 57.25 121 28. 82 205 979828. 27 10. 08 20. 71 23.71 35 56. 20 121 28. 13 101 979833. 51 9.14 17. 67 23. 32 35 54.78 121 27.99 195 979823. 27 8. 83 18. 30 20. 39 35 52.55 121 26. 66 344 979807. 01 9.82 19. 24 17.17 35 51. 52 121 24.92 39 979822, 14 16. 02 7.15 21. 82 35 50. 18 121 23.41 559 979790. 52 14.19 26. 35 21. 24 35 54.68 121 26.30 2455 979681. 89 16. 32 89. 61 21. 30 35 55.35 121 25.88 2714 979668. 85 16.15 99. 97 22. 59 35 56.00 121 25.78 3178 979640, 48 20. 24 114. 30 25. 07 35 56.74 121 24.29 3169 979648. 99 13. 32 120. 91 25.06 35 56.47 121 23.41 2920 979664. 51 10.78 114. 25 24. 11 35 55.80 121 23.05 2027 979662. 91 14. 23 113.42 26.79 35 53. 74 121 22. 01 3490 979620. 05 19. 44 126. 43 25. 69 35 53.26 121 23.36 3277 979632. 35 18.45 119. 39 24. 97 35 53.08 121 24.85 2672 979867. 80 16. 32 98. 21 22. 44 35 49. 21 121 22.40 645 979787. 75 11.30 33. 05 22. 08 35 33.87 121 4.46 T7 979797. 74 2. 24 11.46 11. 04 35 32.75 121 5.33 81 979801, 25 2. 24 15. 51 14.95 35 34.30 121 6.67 34 979808. 75 1.93 17.81 18. 56 35 34.87 121 7.04 26 979809. 80 1.91 17. 30 18. 31 35 35.79 121 7.50 26 979810. 47 2.02 16. 66 17.78 35 36.41 121 8.17 64 979809. 96 2. 08 18. 84 18. 71 35 37.02 121 8.87 53 979812. 19 2.16 19.17 19. 50 35 38. 08 121 9.79 46 979813. 25 2. 39 18. 06 18. 86 35 38. 64 121 11.2% 17 979817. 49 2. 39 18. 78 20. 58 35 34.88 121 0.54 259 979774. 45 4. 00 3. 85 -1.10 35 34.47 121 22 197 979783. 17 2. 48 7.32 3. 00 35 34.39 121 2.40 166 979786. 01 2. 33 7.36 3. 95 35 34. 23 121 3.55 109 979792. 38 2. 05 8. 59 6. 88 35 33.19 121 4.30 247 979788. 41 1.92 19. 08 12.47 35 32.27 121 3.35 97 979794. 33 2.35 12. 20 11.20 35 32.44 121 0.60 236 979778. 47 2.11 9. 18 3.13 35 30. 55 121 1.35 185 979785. 23 1.96 13. 82 9. 39 35 31.16 121 1.97 148 979788. 71 1. 85 12. 96 9. 69 35 31.76 121 4.15 692 979758. 74 5.71 33. 30 15.12 35 30.46 121 2.61 715 979752. 67 6. 08 31. 24 12. 64 GEOPHYSICAL FIELD INVE STIGATIONS Table 3.-Principal facts for gravity stations-Continued AT Elevation Observed. Terrain Free-air Complete Station Lat N. Long W. (ft) gravity correction anomaly Bouguer anomaly w 121 12. 81 96 979816. 01 2. 74 22. 45 21. 87 121 13.47 357 979802. 05 3. 09 3 21. 75 121 14.41 344 979804. 61 3. 07 31. 95 23.13 121 15.03 508 979794. 19 3. 06 37.19 22.11 121 14.10 434 979799. 40 2.70 37. 24 24. 95 121 15.41 402 979802. 10 2.71 36. 49 25. 31 121 16.03 462 979797. 75 3. 92 36. 70 24. 66 121 16. 68 504 979789. 70 4. 82 39. 50 23. 80 121 17. 07 362 979804. 27 4, 20 33. 20 24. 98 121 14. 55 1590 979723. 87 10. 34 P 22. 24 121 15.10 1523 979727. 75 10. 67 64. 34 22. 46 121 14.77 2476 979666. 78 15. 89 91. 08 21. 62 121 14. 24 1090 979691. 14 12. 40 78. 06 18. 39 121 15. 56 2610 979653. 30 16. 73 89. 82 16. 59 121 16.76 1966 979699. 63 15. 08 75.52 22.19 121 17.83 984 979766. 86 8. 51 49.93 24. 48 121 10. 37 566 979779. 11 4. 68 30. 15. 58 121 9.38 1349 979729. 83 8. 45 54. 17 16. 07 121 9.34 1790 979698. 13 10. 95 63. 32 12. 52 121 10.85 1243 979737. 51 6. O1 50. 84 13. 95 121 10. 39 2552 979657. 32 11.08 86. 06 9.18 121 9.60 2410 979666. 51 8.17 82. 70 7 79 121 9.13 2705 979644. 16 11. 22 88. 6. 89 121 7.70 2535 979652. 78 9. 08 83. 30 5, 09 121 7.37 2644 979644. 55 10. 37 86. 10 5.35 121 7.07 2465 979658. 31 8.19 83. 44 6. 66 121 3.16 3263 979589. 80 16. 47 93. 60 -2, 24 121 2.72 3137 979594. 19 17.17 86. 71 -4.19 121 4.74 1209 979719. 47 7. 68 34. 71 . 67 121 5.89 938 979744. 86 5. 88 34. 49 7. 98 121 0. T7l 979749. 38 2.02 11.74 -12, 86 121 0.73 T7 979748. 99 2. 63 12. 06 -12, 00 121 2.03 788 979749. 54 2. 45 13. 75 -11. 00 121 2.60 787 979752, 50 3. 46 17. 80 -5. 91 121 3.11 784 979753. 73 2. 94 18. 25 -5. 88 121 3.57 779 979754. 07 3. 85 19. 02 -4. 03 121 4.18 770 979756. 46 4. 32 19. 65 -2, 62 121 0.64 778 979750. 78 8. 22 15, 50 -7.96 121 1.59 2115 979661. 88 4. 83 30. 10 -8,00 121 0.61 2256 979650. 32 5. 43 33. 88 -38. 48 121 0.04 2801 979604. 16 11. 69 40. 96 -43. 87 121 0.15 843 979744. 47 2. 28 12. 54 -14. 28 121 9.50 2289 979675. 65 6. 20 76. 40 3. 68 121 9.97 1410 979729. 93 3. 58 46. 06 . 98 121 10.73 2282 979681. 79 5. 58 76. 62 5. 20 121 10.08 921 979729. 18 1. 50 -10. 86 -41. 07 121 8.74 903 979728. 74 1.55 -11. 60 -41, 32 121 9.33 890 979728. 69 2.12 -12. 04 -40, 64 121 10.38 1114 979719. 95 2. 24 -. 37 -36. 58 121 9.00 1315 979706. 09 3. 64 6. 83 -34, 91 121 8.41 1226 979706. 62 3.12 -1. 09 -40, 29 121 9.05 1516 979705. 13 3.71 25. 80 -22. 79 121 10. 87 1239 979739. 03 2. 28 34. 37 -6.16 121 11.46 1433 979728. 32 4, 02 42, 23 -3. 20 121 12.82 1345 979736. 96 3. 33 41.14 -1. 95 121 12.72 1390 979735. 60 3. 63 45. 21 .87 121 13.17 1271 979742. 69 2, 42 38. 74 -2.71 121 14. 54 1267 979747. 66 3. 00 43. 038 2. 31 121 13. 85 1201 979743. 63 $17 42. 39 1. 00 121 14.93 1390 979740. 70 4. 24 48. 85 5.13 121 13.82 1315 979744. 27 4. 07 46. 08 4.77 121 13. 91 1202 979752. 12 2. 66 44, 46 5. 64 121 14. 53 1271 979749. 24 2.92 48, 56 7. 61 121 15.52 1256 979752. 97 4. O1 50. 11 10.77 121 17.08 2325 979690. 92 8. 48 88. 69 17. 01 121 17. 50 2020 979714. 14 4.97 82, 54 17. 85 121 18. 50 1722 979735. 54 4. 38 74. 69 19. 67 121 19. 30 2401 979692. 01 8. 55 93. 91 19. 69 121 18.10 2303 979696. 40 5.97 89. 14 15.71 121 18. 46 2908 979651. 55 13. 23 99. 92 13.12 121 19. 2617 979672. 09 11.83 92. 92 14. 55 121 16.74 2213 979695. 83 8. 04 82.10 13. 83 121 16. 40 1834 979719. 27 6.98 70. 38 14.10 121 15. 56 1526 979733. 84 5.20 55.10 7.74 121 16. 52 1673 979726. 70 5.71 60. 14 8. 14 121 15.07 1527 979731. 53 4.93 51. 28 3. 53 121 15. 60 1583 979729. 06 4, 54 54. 24 4.16 121 15.35 1207 979722. 01 3. 46 13.73 -27. 57 121 16.03 1374 979739. 63 3.15 30. 44 -4. 82 121 16. 70 1710 979710. 75 4. 55 41. 06 -13. 38 121 17. 64 1866 979712. 22 4. 97 57.36 -2. 03 121 17.43 1405 979742. 68 2. 60 45. 53 -. 35 121 18.73 1783 979726. 03 4. 31 63. 06 5. 87 121 19.52 1505 979742. 86 3. 58 62. 88 11. 43 121 19.18 1534 979745. 47 3. 32 60. 37 10. 76 121 18.19 1770 979724. 49 5.47 62. 33 6. 74 121 17.28 2006 979714. 63 5. 58 83. 09 19. 48 121 18.08 2350 979691. 71 6. 85 92. 99 18. 82 121 19.19 3496 979615. 99 18. 84 126.72 25.17 121 7.23 982 979718. 87 1. 53 -12, 48 -44, 85 121 6.78 1016 979717. 77 1. 61 -9. 67 -43. 13 121 . 5.99 1070 979713. 74 2. 20 -7.T7 -42, 42 121 5.91 1232 979704. 64 2. 80 -8. 21 -42, 98 121 3.89 941 979727. 54 1. 81 -5. 81 -36. 48 121 4.79 1270 979702. 94 3. 22 -. 72 -41. 33 121 2.97 977 979731. 22 1.70 . 57 -31. 45 AS BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA Table 3.-Principal facts for gravity stations-Continued Elevation Observed Terrain Free-air Complete Station Lat N. Long W. (ft) gravity correction anomaly Bouguer anomaly (mgal) (mgal) (mgal) (mgal) ........................................... 35 53. 82 121 3.22 921 979731. 46 1.92 -3. 85 -33. 73 607.-:c.. * 35 52.66 121 0.42 925 979738. 89 1.76 5. 61 -24. 57 . 35 52, 94 121 1.31 744 979748. 19 1.42 -2. 52 -26. 79 s 35 53. 59 121 0.51 941 979737. 02 1. 40 3. 91 ~21.37 t 35 50. 52 121 0.27 1165 979715. 93 1. 49 8. 27 -30. 45 A 35 51.25 121 0.02 1075 979722. 37 2. 49 5. 21 -29. 41 ......... 35 51.63 121 1.06 1098 979720. 17 2. 09 4.63 -31. 18 _______ 35 50. 60 121 1.25 1165 979715. 13 1.79 7.36 -31. 06 _______ 35 50.14 121 18. 18 2067 979647. 31 12. 59 109. 66 20. 02 to. 35 49.37 121 18. 54 2685 979664. 21 10. 78 101. 14 19. 38 Pee 35 53. 02 121 2.56 776 979742. 50 1.45 -5. 31 -30. 65 35 52.00 121 231 1090 979721. 52 « 1.92 4.70 -31. 01 35 51.90 121 3.47 1252 979705. 23 2. 98 3.79 -36. 49 35 51.09 121 2.40 1138 979717. 23 2.16 6. 22 -30. 90 35 52.73 121 4.02 1063 979717. 25 2. 86 -8. 16 -36. 99 35 52.83 121 6.12 1355 979698. 53 3. 33 5. 45 -37. 98 35 52.66 121 4.9 1111 979714. 70 2. 03 -1. 09 -87. 41 35 51.66 121 4.56 1445 979692. 23 4. 25 9. 28 -36. 33 35 51.33 121 19.36 3350 979629. 96 15.12 126. 62 26. 36 35 52.20 121 19. 64 2749 979668. 49 10. 99 107. 40 23. 65 35 52. 50 121 18.98 2215 979703. 02 6. 69 91. 29 21. 60 35 58.77 121 28. 55 1893 979726. 24 17. 47 75. 27 27. 45 35 52.18 121 21. 34 3400 979627. 63 15. 61 127.78 26. 29 35 50.87 121 21. 48 3590 979604. 98 22. 53 124. 86 28.77 35 50.15 121 22.22 979645. 62 21. 61 98. 46 21.31 35 51. 51 121 20.85 3200 979639. 35 13. 06 121. 65 . 48 35 50.67 121 17.64 3040 979642, 17 13. 78 110. 62 19.67 35 50.09 121 16.2% 2471 979677. 85 9. 87 93. 63 18. 32 35 51.84 121 11.92 1157 979750. 12 2.30 39. 83 2.19 35 51.00 121 10.52 1375 979731. 42 4.19 42. 83 -. 43 35 50.25 121 8.02 1230 979736. 38 3. 31 35.22 -8. 92 35 51.39 121 9.22 1434 979723. 84 3. 43 40, 24 -5. 81 35 35.76 121 2.44 . 690 979752. 09 3. 61 20. 78 $ 35 35. 70 121 3.40 128 979749, 02 3. 28 20. 90 "z; 35 36. 00 121 1.12 860 979737. 81 4.23 22.15 -3. 31 35 48. 18 121 19.21 2414 979676. 53 14. 39 89. 67 20, 84 35 49.96 121 19.28 2085 979703. 19 7. 56 82. 85 18. 51 35 47. 24 121 18. 34 2390 979678. 25 13. 70 90, 47 21. 78 35 46. 61 121 17.75 2525 979664, 18 19. 61 90, 00 22.57 35 48.35 121 18.61 1532 979739. 19 5.93 69. 14 22,29 35 48. 03 121 16.97 697 979785. 95 6. 70 37. 83 20. 46 35 41.93 121 8.83 1193 979737. 57 4. 82 44.79 8. 43 35 42. 62 121 9.38 1136 979741. 77 5.29 42.65 8.7 35 43.18 121 10.84 1464 979724. 79 7. 01 55.72 11.21 35 45.73 121 13.22 1764 979711. 91 7. 21 67. 42 13. 78 35 46. 59 121 13.44 1541 979728. 76 4. 98 62. 07 13. 88 35 46. 52 121 15.10 1330 979744. 21 5. 34 57.78 17. 22 35 46. 55 121 16.18 1181 979749. 77 10. 07 49, 28 18. 59 35 47.10 121 9.94 2016 979695. 82 4. 81 "73. 07 8. 35 47. 84 121 9.9 2677 979647. 69 10. 25 86. 05 4.03 35 45. 48 121 6.54 1850 979697. 76 4. 21 61. 71 2.11 35 45.83 121 7.34 2279 979669. 40 7.22 73. 20 1. 84 35 46. 62 121 7.40 2282 979670, 22 5.93 73.17 42 35 38. 47 121 8.12 662 979765. 65 4. 60 27. 85 9. 59 35 39. 60 121 7.64 1492 979709. 99 9. 05 48. 65 6. 22 35 38.78 121 6.76 1180 979729, 19 7. 48 39. 67 6, 42 35 59.23 121 11.90 1423 979693. 96 4. 31 =1.87 -46, 35 57.56 121 13.14 1222 979726. 04 3. 08 13. 70 -25. 40 35 56.85 121 13.78 1856 979896, 22 6. 43 44. 52 -13. 07 35 59.22 121 9.73 1219 979712. 21 1. 94 -2.79 -42, 92 35 59. 58 121 8.38 1308 979705. 14 2.15 -2, 00 -44, 99 35 59.28 121 7.19 1114 979718. 10 1.76 -6. 86 -43. 5, 35 59.77 121 5.36 1705 979686, 11 3. 53 16. 03 -39. 25 35 57.93 121 1.87 1386 979706. 54 2.11 9. 09 -36. 63 35 57. 04 121 0.97 1764 979678. 09 3. 01 17. 46 -40, 35 58. 81 121 3.87 1630 979693. 50 3. 16 17. 74 -35. 33 35 59.29 121 2.61 1505 979699, 42 2. 69 11.22 -38. 35 48. 67 121 2.57 1665 979692. 99 2. 28 35. 00 -20. 21 35 48. 40 121 527 978 979745. 21 1. 97 22. 99 -8. 80 35 48. 04 121 0.07 1479 979702. 16 2.76 27. 57 -20. 70 35 49.02 121 1.38 1538 979698, 84 3.19 28. 40 -21, 47 35 47.46 121 4.07 1413 979713. 60 2. 28 33. 63 -12, 35 47.16 121 2.89 1477 979705. 55 2.79 32. 03 -16. 14 35 47. 44 121 5.65 962 979746, 02 2. 96 23. 66 -6. 59 35 52.53 121 17. 57 1620 979739. 02 5.09 71.29 20, 49 35 49.03 121 9.96 2410 979664. 48 10. 05 76. 04 3. 01 35 58. 59 121 16.10 1724 979718. 59 6. 70 51. 99 =.18 35 58. 52 121 11.66 1035 979726. 63 1. 83 -4. 67 -38. 57 35 59. 57 121 11.30 1445 979698. 84 3. 58 4. 59 -41. 69 35 59.95 121 11. 61 1394 979704. 23 3. 20 4. 65 -40, 25 35 58. 91 121 11.23 1414 979693, 25 5. 09 -2. 97 -46. 67 35 52. 61 121 13. 84 1058 979760. 40 3. 84 39. 69 7. 01 35 52.89 121 14.38 1079 979760, 64 3.11 41. 48 7.36 35 52.55 121 14. 52 1235 979752. 28 2. 94 48. 24 8. 57 35 52.63 121 15.69 1877 979715. 20 6. 20 71.76 12, 94 35 52.66 121 16.02 1916 979714. 76 5. 46 74. 69 14. 06 35 52.79 121 16. 22 1947 979714. 12 5.26 76. 82 14. 91 35 52.46 ° 121 14. 22 1199 979753. 82 2.96 46. 61 8.18 35 52.26 121 14.18 1130 979757. 79 3. 64 44. 37 9. 00 35 51.64 121 12. 2 1000 979759. 48 3. 46 34. 70 3. 64 35 51. 29 121 11.94 983 979759. 80 3.73 33. 91 3.71 35 50. 68 121 11.92 983 979760, 17 4.70 35. 20 5.95 35 51.30 121 13. 04 1423 979737. 67 2. 94 53. 06 6. 89 35 51.06 121 13.93 1487 979734. 92 2. 81 56.75 9. 67 35 52.78 121 16.41 2047 979708. 34 5.82 80. 43 15. 65 35 52.81 121 16. 66 1985 979714. 07 4.80 80. 23 16. 59 CEOPHYSICAL FIELD INVE STIGATIONS f A9 Table 3.-Principal facts for gravity stations-Continued Elevation Observed Terrain Free-air Complete Station Lat N. Long W. (ft) gravity correction anomaly Bouguer anomaly (mgal) (mgal) (mgal) * ___ (mgal) 35 52. 71 121 16.76 1971 979716. 08 4, 64 81. 07 17.75 35 52.78 121 16.91 1884 « 4.78 77.85 17. 59 35 52. 60 121 16.98 1809 4. 66 75. 90 18. 15 35 52. 48 121 17.22 1603 4. 83 72. 43 18. 86 35 52.14 121 17.46 1658 5. 58 70.12 18. 49 35 52.14 121 14.23 1182 3. 87 46, 42 9, 51 35 52. 28 121 17.28 1503 5.73 68. 34 19.10 35 52.17 121 17. 27 1584 5.85 67. 53 18. 72 35 52.10 121 17.28 1584 6. 40 66. 76 18. 52 35 51.96 121 17.25 1575 9. 43 65, 41 20, 49 35 51.88 121 16.98 1567 8. 48 64. 90 19. 32 35 51.93 121 16.90 1554 7.92 63. 92 18. 21 35 52.03 121 16.75 1536 8. 69 62. 33 18. 02 35 52. 04 121 16. 65 1508 10. 63 59. 61 18. 21 35 51.99 121 16. 54 1469 11. 56 58. 72 19. 59 35 51. 88 121 16.52 1438 11.82 58. 41 20. 60 35 51.76 121 16. 50 1423 13. 90 57. 79 22. 58 35 51.78 121 16. 33 1387 13. 77 58, 40 24. 31 35 51.79 121 16.12 1337 13. 4 56. 68 23.97 35 51. 90 121 16.02 1313 13. 08 55. 50 23. 26 35 51.95 121 15.88 1289 12. 83 54. 57 22. 98 35 51.98 121 15.77 1269 11.16 53. 52 20. 88 35 51.86 121 15. 56 1245 12. 02 51. 66 20. 78 35 51.93 121 15.39 1236 6.90 50. 51 14. 76 35 51.83 121 15.16 1223 6. 42 50. 30 14. 52 35 51.95 121 14.62 1147 3. 85 45.19 12.09 35 52. 58 121 16.41 2182 6. 29 83.92 14.97 35 52. 20 121 16. 30 2827 18. 98 96. 37 17.93 35 52.18 121 16.31 2797 18. 82 97. 94 20. 37 35 51.82 121 15.03 1190 7. 55 47.89 14. 37 35 52.04 121 15. 47 1225 9, 40 50. 41 17. 53 35 51.20 121 16.26 1645 9. 30 68. 60 21.15 35 51. 84 121 16.71 2228 9. 68 84. 50 17. 36 35 51. 97 121 17. 47 2047 6.85 82, 26 18. 52 35 52. 31 121 17. 25 1605 5. 60 70. 49 20. 72 35 50.85 121 17.30 2499 7. 50 97. 30 18. 96 The accuracy of the gravity data undoubtedly vary from station to station. The observed gravity measure- ments for the 293 stations read with a LaCoste- Romberg gravity meter were probably accurate to 0.02 mgal after correcting for tidal effects. The 97 stations read with a Worden meter (scale constant about 0.5 mgal) were probably accurate to 0.1-0.2 mgal after correcting for drift. Latitude and longitude were measured to +0.01 minute. Elevation accuracy depends critically on the type of source data. Roughly 20 percent of the stations were read at bench marks, and elevation errors for these should be less than 0.5 feet. Another 10 percent are field-checked spot elevations probably accurate to within 1 foot, and 55 percent are unchecked spot eleva- tions accurate to within 5 feet. Nine stations were es- tablished on the shore of Lake Nacimiento with an estimated elevation accuracy of +1 foot. Forty-one sta- tions in the vicinity of Burro Mountain (identified by the letters "BM"-for example 32BM-on pl. 1 and in table 3) were leveled in with a Zeiss Opton Self-leveling Level and were probably accurate to +2 feet. All gravity data were corrected for terrain effects (at density 2.67 g/cm*) out to a radius of 166.7 km. For the inner zones, terrain corrections were made by hand using Hayford-Bowie templates and dividing each compartment into four subcompartments where cor- rections were large. For the outer zones, the coryections were made by computer using a program developed by Donald Plouff. For most of the stations the boundary between inner- and outer-zonge corrections was either 5.24 or 2.29 km, and i-minute and 3-minute terrain digitization grids were employed. For about 40 stations near Burro Mountain, however, computer corrections were carried in to 0.068 km, and additional 0.05-minute and 0.25-minute terrain grids were used. 'All basic measurements were reduced to anomaly values using a gravity reduction program developed by the author. The basic procedures and formulas of the reduction are as follows: 1. The gravity difference (AG) between the base and a given station is calculated in one of six ways de- pending on the reduction option selected. It is then correcéd for tide and drift. 2. Observed gravity (0G) =G@Gravity Base Value+ AG. 3. Theoretical - gravity (THG) =978049 (1+ 005228 sin20-0.0000059sin®20), where latitude. 4. Free-air anomaly (FAA) =0G-THG + (0.09411549 -0.0001377 89sin20) £ -0.0000000067E*, where Z =elevation. 5. Simple Bouguer anomaly (B4) =FFA-0.012T74pE ,- where p= reduction density. 6. Curvature correction (0C) =0.0004462 X E - 3.28 X10#X £*+ 1.27% 105 E*. 7. Complete Bouguer anomaly (CBA) =BA+ TCO-CC, where 7C=terrain correction. ‘ A10 GRAVITY INTERPRETATION Most quantitative interpretation of gravity anomalies here relies largely on a two-dimensional two-layer base- ment-sediment model. Density contrasts of 0.3 and 0.5 g/em' are commonly used to arrive at maximum and minimum dimensions for various anomalous features. The following estimates are used for unit densities : g/em> Surficial deposits 9.2 Paso Robles Formation fel 2.2 Pancho Rico Formation -= 2/8 Monterey Shale x & - Bg Middle and lower Miocene deposits_._._____LLJ_a-1____ 2. 4 Paleocene and upper Mesozoic deposits_______________ 2.5 Basement rocks € --= Using these values, the density contrasts 0.3 and 0.5 fur- nish good approximations to common sediment-base- ment combinations. A mixed sequence of Cretaceous and Tertiary rocks on basement will have a density contrast close to 0.3. Quaternary deposits on basement, however, have a density difference of close to 0.5. Interpretation of anomalies associated with ultra- mafic rocks requires different assumptions, since these rocks range in density from 2.5 to 3.3 depending on degree of serpentinization. In arriving at subsurface mass distribution, graticule and various simple mathematical calculations and in- terpretations using a U.S. Geological Survey modifica- tion of Bott's (1960) interpretation program were fitted to outcrop and well data. GRAVITY ANOMALIES DETAILED SURVEY OF THE BURRO MOUNTAIN ULTRAMAFIC BODY The gravity high at Burro Mountain was first re- ported by Thompson (1963). On the basis of a prelim- inary survey of about 20 stations collected from an area of roughly 20 square miles in the vicinity of the Burro Mountain body, Thompson defined an 8-mgal high over the ultramafic body. This surprisingly small value led him to conclude that the depth of the fresh ultramafic rock was shallow, perhaps on the order of the topo- graphic relief of the body, and he speculated on an abundance of concealed serpentinite at depth, perhaps essential to the emplacement of the high density mass. In the current survey, considerable effort was spent in refining the Burro Mountain anomaly. In addition to selected stations of Thompson (designated by the let- ters GT-for example, 24G-T-on pl. 1 and in table 3) and readings at nearly all elevations available on the topographic map, 41 stations were surveyed in with a BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA Zeiss Opton Self-leveling Level. In order to improve the accuracy of terrain corrections, the computer pro- gram employed an extremely fine digitization grid in this vicinity. Twelve square miles were digitized on a 0.05-minute grid, and approximately 200 square miles were digitized on a 0.25-minute grid. The results of this detailed investigation confirm the essential correctness of Thompson's preliminary con- clusions. The anomaly was again determined to be about 8 mgal (pl. 1) after removing the regional grad- ient. The total anomalous mass was determined by Gauss' theorem to be roughly 5 X 10 grams. Using the widest range of reasonable density contrasts, one ar- rives at a subsurface volume for the fresh ultramafic rock of between 1 and 2 km. This small volume, com- bined with the size and shape of the anomaly and the mapped ultramafic contacts, suggests a maximum depth of 1 km for the fresh ultramafic rock. Considering the known density distribution in plan view (Burch, 1968, fig. 3), the fresh rock probably extends no deeper than 2,000 feet. The one inference made by Thompson which was not corroborated by subsequent detailed investigation is the abundance of concealed serpentinite at depth. On an aeromagnetic profile (fig. 2) the Burro Mountain body produces an anomaly of only 30-40 gammas. This is in striking contrast to the smaller elongate body 4 miles to the southwest, which produces an anomaly of 300 gammas, and several smaller serpentinites to the south- east, which produce anomalies of 100-200 gammas. Both gravity and aeromagnetic data thus suggest that neither the fresh nor the altered ultramafic rocks of the Burro Mountain body extend to any significant depth. This lends support to the author's earlier conten- tion (Burch, 1968) that the body is an isolated tectoni- cally emplaced block. REGIONAL SURVEY The regional gravity map consists basically of a rather even northeastward gravity gradient of roughly -3 mgal per mile except (1) in the northwestern cor- ner where a broad gravity high is associated with the Santa Lucia Mountain mass, (2) in the northeastern corner where a broad gravity low is associated with Lockwood Valley, (38) south and east of Lockwood Val- ley where several basement irregularities are reflected in the gravity picture, (4) west of Lockwood Valley where a very steep gravity gradient (up to 20 mgals per mile) cuts diagonally across the entire Bryson quad- rangle, and (5) at numerous places in the Franciscan Formation where small irregularities reflect small heterogeneities in this unit or possible small errors in elevation control. GEOPHYSICXL FIELD INVESTIGATIONS it All AI Anomaly _- of small serpentinite body 4 miles southwest of Burro Mountain anomaly ~ 200 GAMMAS Burro Mountain anomaly FIGURE 2. -Aeromagnetic profile (A-A' on pL 1) comparing the Burro Mountain anomaly with the anomaly caused by a smaller serpentinite body 4 miles southwest. The even gradient of roughly -3 mgal per mile ob- tains with only minor interruption over the entire southern half of the area. This value corresponds closely with the value shown by Thompson and Talwani (1964, fig. 4) for the continental margin. Such a broad and even gradient is most likely caused by a very deep- seated density contrast and thus probably reflects deeper structure of the continental margin. The evenness of the gradient also suggests a rather homogeneous density for the Franciscan Formation in this area and seem- ingly precludes large masses of anomalous density. It also requires that the overlying patches of Tertiary sedi- mentary rock be relatively thin. The gravity high in the upper left-hand corner of the map area in the southeastern end of a long northwest- trending high which continues more than 40 miles be- yond the edge of the map area (Bishop and Chapman, 1967). To the north the high is associated with the western metamorphic belt of the Santa Lucia Range, but in the map area it crosses the Nacimiento fault and continues with diminished amplitude for over 12 miles within the Franciscan block. The close coincidence of the gravity and topographic highs indicates that the anomaly results from the high density of the moun- tain mass itself rather than from some buried anomalous body. A simple calculation using the slab formula sug- gests that the true density of the mountain mass in the area of the anomaly is 2.80 to 2.85 g/em*. This density must be considered tentative, however, because it is difficult to distinguish residual and regional anomalies at the western edge of the mountain mass. Offshore data (Burch and others, 1970) indicate a gravity low about 7 miles from the coast, where complete Bouguer values may be as low as -20 to -30 mgal. Several interesting anomalies are found in the Salin- ian block in the Bryson quadrangle. The broad low in Lockwood Valley indicates a relatively large thickness of low-density sedimentary rocks. Analysis using the Bott interpretation program suggest a depth to base- ment of 7,000 feet at the bottom of the low. At the northern end of this valley a sharp linear gravity low extends about 2 miles northwest from J olon, along the Jolon fault. This anomaly could represent disruption of the basement along the Jolon fault, but more likely it is caused by low-density diatomite, also probably faulted in along the same fault. At the lower end of Lockwood Valley, crossing the map boundary, is a northwest-trending 8-mgal high. The Bott program, together with a graticule analysis and well data on the flank, suggests that the source is a basement ridge with approximately 2,500 feet of vertical relief which rises to within 300-400 feet of the surface. The gravity high and presumably the basement ridge continue with di- minished size northward out of the map area. The steep gravity gradient east of the high reflects a large dis- placement of the basement surface across the Espinosa fault zone. 'The existence of a northeast-trending basement fault in the northeast corner of the map area is suggested by the 1- to 2-mile right-lateral offset of several north- west-trending features in this area. The gravity con- tours north of the map area (therefore not shown on pl. 1) indicate the offset of the gravity ridge and the parallel low just northeast of it. The southern end of the Lockwood Valley low also appears to terminate against this fault. The significance of the possible fault is that a similar transverse fault, the Indian Valley fault, appears 19 to 20 miles southeast (Burch and Durham, 1970) on the opposite side of the Espinosa and Jolon faults. If these transverse faults once joined, approximately 16 miles of right-lateral strike-slip move- ment on the Espinosa fault would be required, in addi- tion to 3 or 4 miles on the Jolon fault, in order to explain the current 19- to 20-mile offset of the transverse faults. A12 Probably the most conspicuous feature of the gravity map is the steep northwest-trending gravity gradient (up to 20 mgal per mile) west of Lockwood Valley. Such a gradient probably reflects a major and con- tinuous basement fault. Use of the simple slab formula indicates a vertical displacement of the basement sur- face of at least 5,000 feet and possibly 10,000 feet. The fault cannot be traced at the surface since the entire tract of Monterey Shale appears to be cut by numerous small faults. Many small anomalies, or gravitational irregulari- ties, in the Franciscan are defined by a single station only. While some reflect small anomalous-density masses, others are probably caused by elevation uncer- tainties. Station 516 (4 miles southwest of Burro Mountain), for example, is a gravity low located on ultramafic rock and associated with a strong magnetic high (fig. 2). The ultramafic rock, having a low-den- sity and high-magnetic susceptibility, is thus well ser- pentinized. Station 329, on the other hand, controls a small, sharp positive anomaly which could signify either a small anomalous mass or uncertainty in eleva- tion or field data. The most significant structural feature of the map area, the contact between the Franciscan and Salinian structural blocks, shows little or no gravity expres- sion. It was suggested earlier that the gently dipping contact between Upper Cretaceous rocks and the Fran- ciscan Formation in the eastern part of the map area, mapped as the Nacimiento fault zone, probably does not also mark the basement contact. If this is true, a wide area of unknown basement exists between the east- ernmost outcrops of Franciscan Formation and west- ernmost known occurrences of granitic basement. The gravity data permit little more than a speculative guess regarding the location of the basement contact in this area. Subtle reentrants of the gravity contours at stations 486 and 151 could conceivably represent Salinian basement faulted against younger, lighter rocks on the west. A fault at these locations lines up well with the fault extending southeast from the meta- morphic block composing Chalk Peak and with the thrust fault in the eastern part of Bryson quadrangle. Although the relationship between the basement con- tact and the thrust fault is almost certainly coinci- dental, Burch and Durham (1970) suggest that the basement contact may continue to follow the thrust BOUGUER GRAVITY AND GENERAL GEOLOGY, FOUR QUADRANGLES, CALIFORNIA fault several miles southeast beyond the map boundary, eventually merging with the Jolon and Rinconada faults. REFERENCES CITED Bailey, E. H., and Everhart, D. L., 1964, Geology and quicksilver deposits of the New Almaden district, Santa Clara County, California: U.S. Geol. Survey Prof. Paper 360, 206 p. Bishop, C. C., and Chapman, R. H., 1967, Bouguer gravity map of California, Santa Cruz sheet: California Div. Mines and Geology, scale: 1: 250,000. Bott, M. H. P., 1960, The use of rapid digital computing methods for direct gravity interpretation of sedimentary basins: Royal Astron. Soc. Geophys. Jour. [London], v. 3, no. 1, p. 63-67. Burch, S. H., 1968, Tectonic emplacement of the Burro Moun- tain ultramafic body, Santa Lucia Range, California : Geol. Roc. America Bull., v. 79, no. 5, p. 527-544. Burch, S. H., and Durham, D. L., 1970, Complete Bouguer gravity and general geology of the Bradley, San Miguel, Adelaida, and Paso Robles quadrangles, California: U.S. Geol. Survey Prof. Paper 646-B, 14 p. Burch, S. H., Grannell, R. B., and Hanna, W. F., 1970, Bouguer gravity map of California, San Luis Obispo sheet: Cali- fornia Div. Mines and Geology, scale 1 : 250,000. Chapman, R. H., 1966, The California Division of Mines and Geology gravity base station network: California Div. Mines and Geology Spec. Rept. 90, 49 p. Compton, R. R., 1966, Analysis of Plio-Pleistocene formation and stresses in northern Santa Lucia Range, California : Geol. Soc. America Bull., v. 77, no. 12, p. 1361-1379. Durham, D. L., 1965a, Geology of the Jolon and Williams Hill quadrang'es, Monterey County, California: U.S. Geol. Sur- vey Bull. 1181-Q, 27 p. 1965b, Evidence of large strike-slip displacement along a fault in the southern Salinas Valley, California, in Geological Survey research 1965: U.S. Geol. Survey Prof. Paper 525-D, p. D106-D111. Goldsmith, Richard, 1959, Granofels, a new metamorphic rock name: Jour. Geology, v. 67, no. 1, p. 109-110. Hanna, W. F., Burch, S. H., and Dibble, T. W., Jr., 1971, Grav- ity, magnetics, and geology of the San Andreas fault near Cholame, California : U.S. Geol. Survey Prof. Paper 646-C. (In press.) Jennings, C. W., 1958, Geologic map of California, Olaf P. Jenkins edition, San Luis Obispo sheet: California Div. Mines, scale 1 : 250,000. Taliaferro, N. L., 1943, Geologic history and structure of the central Coast Ranges of California : California Div. Mines Bull. 118, p. 119-163. Thompson, G. A., 1963, Geophysical investigation of the dunite at Twin Sisters, Washington [abs.] : Geol. Soc. America Spec. Paper 76, p. 227-228. Thompson, G. A., and Talwani, Manik, 1964, Crustal structure from Pacific basin to central Nevada : Jour. Geophys. Re- search, v. 69, no. 22, p. 4813-4837. yr U.S. Government PRINTING OFFICE: 1970 O-406-158 WE 127 TPC . v. CMAG-B J DAY Complete Bouguer Gravity and General Geology of the Bradley, San Miguel, Adelaida, and Paso Robles Quadrangles, California GEOLOGICAL SURVEY PROFESSIONAL PAPER 646-B U.S.S. p Cs mio tio Ag Complete Bouguer Gravity and General Geology of the Bradley, San Miguel, Adelaida, and Paso Robles Quadrangles, California By STEPHEN H. BURCH erd DAVID L. DURHAM GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 646-B Gravity data indicate the major structures of the Salinian basement in this region UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402, CoNTENTS Page B1 | General geology-Continued Introduction.... .. 5. .- P 1 anns s General 2 Basement LOCKS. 2 Folds . iE ell cure carey -eancanes Ultralfxafic rocks_-_ """""""""""""" 2 Age of deformation-__..._____--------- Franciscan Formation.._____.________--.------ 2 Gravity da GfahItIC =~ 2 fonts § PARS - =-! tr""~* Superbasement sedimentary rocks-_-.-.__.---------- 2 Gravity -we Paleocene and Upper Cretaceous deposits.... 2 Gravity interpretation-.------------------ Tierra Redonda and Vaqueros Formations.... 4 Gravity anomalies-________________-_.------ YOICRNIG POCKSL .. _._. 4 Area east of Jolon and Rinconada faults.... Monterey Shale.__._.._._.____________-_-------- 4 Area near Jolon and Rinconada Pancho Rico and Santa Margarita Formations. 4 Area west of Jolon and Rinconada faults.. Paso Robles 5 | References cited. gain cg. cr Surficial depOSits.._.__.________________------- 5 ILLUSTRATIONS Page Prats 1. Complete Bouguer gravity and generalized geologic map.-......-.--- In pocket Figur® 1. Index map showing location of quadrangles forming area of study...... B1 2. Gravity and structure profiles through the Hames Valley low and San Ardo oil-field high, Bradley quadrangle__________--------~-------- 12 TABLES Page TABLE 1. Selected exploratory wells between the Jolon and San Andreas fault zones.. B3 2. Principal facts for bases used in gravity SUPVeY-______----------------- 6 3. Principal facts fOr gravity St@tIONS_____________c_--_____----<----~~~~- v4 TH GEOPHYSICAL FIELD INVESTIGATIONS COMPLETE BOUGUER GRAVITY AND GENERAL GEOLOGY OF THE BRADLEY, SAN MIGUEL, ADELAIDA, AND PASO ROBLES QUADRANGLES, CALIFORNIA By Steenuznx H. Burcu and Davin L. Dorwan ABSTRACT Complete Bouguer gravity coverage of 313 stations and gen- eralized geological mapping were compiled for the Bradley, San Miguel, Adelaida, and Paso Robles quadrangles, California. These quadrangles constitute a 30- by 30-minute rectangle cover- ing nearly 1,000 square miles in the southern Salinas Valley, an oil-producing and potential oil-producing area. The granitic and metamorphic Salinian block constitutes the basement of the central, and by far the greatest, part of the map area. 'The Salinian basement block is separated from the adjacent Franciscan basement by the San Andreas fault in the north- east corner of the area and by the Nacimiento fault zone in the southwest corner. Overlying both basement blocks is a sequence of Cretaceous and Tertiary marine deposits and the nonmarine Paso Robles Formation. The major gravity features of the Salinian block include (1) the Hames Valley low, which covers much of the Bradley quad- rangle and represents a structural-depositional basin 10,000- 15,000 feet deep, (2) the San Ardo oil-field high to the north- east, where basement rises to within 2,500 feet of the surface, (3) the Vineyard Canyon low, which covers much of the San Miguel quadrangle and represents a basin approximately 10,000 feet deep, (4) the Cholame Hills high to the northeast, where depth to basement is less than 2,000 feet, and (5) the trans- verse Indian Valley fault, a pre-late Miocene basement fault whose presence is deduced from the mutual truncation of the four previously mentioned gravity features. Inferred basement structures suggest three unexplored areas favorable to oil accumulation. The gravity data suggest that in the area of Paso Robles the Jolon fault, rather than the Nacimiento fault, marks the con- tact of the Franciscan and granitic basement blocks and, furthermore, that the Jolon fault probably joins the Rinconada fault to the south. INTRODUCTION A 2-mgal (milligal) gravity map of the Bradley, San Miguel, Adelaida, and Paso Robles quadrangles, Cali- fornia, was prepared by S. H. Burch as part of a re- gional survey for the San Luis Obispo 1:250,000 gravity sheet. This map is overprinted on a generalized geologic map compiled by D. L. Durham. (See source map on pl. 1.) The geology of most of the area (fig. 1) 122° 1218 37° 120° \ d co - * I a??? \' 045.4: *p MONTEREW L ~ mung“ COUNTY ~ 3 ox SAN LUIS OBIS! ounty f 36° |- 0 20 40 MILES ferme e imm, sind 35° | FicurE 1.-Map showing location of the quadrangles forming area of study. was mapped by Durham on a scale of 1: 24,000 during an investigation of the southern Salinas Valley area. Mutual consultation between the authors on their re- spective fields lead to the joint preparation of this paper. The four quadrangles constituting the map area form a 30- by 30-minute block which covers nearly 1,000 square miles in the oil-producing and potential oil- producing region in the Salinas Valley. The map area extends from the San Andreas fault in the northeast B1 B2 corner, across the entire Salinian basement block, and beyond the Nacimiento fault in the southwest corner. GENERAL GEOLOGY The geologic map (pl. 1) consists principally of D. L. Durham's mapping but is supplemented by data from the San Luis Obispo geologic sheet (Jennings, 1958). Rock units are lumped to provide continuity with com- panion maps to the west and east (Burch, 1969; S. H. Burch, W. F. Hanna, and T. W. Dibblee, Jr., unpub. data, 1969). The map area is divided by the northwest-trending San Andreas and Nacimiento fault zones into three major basement blocks: (1) the Franciscan block of eugeosynclinal rocks southwest of the Nacimiento fault, (2) the granitic and metamorphic Salinian block (Compton, 1966, p. 277 ; also Reed, 1933, p. 12) between the Nacimiento and San Andreas faults, and (3) the Franciscan block of eugeosynclinal rocks northeast of the San Andreas fault. The Salinian block covers by far the greatest area and hence receives greatest attention in this report. The geology of the Franciscan block northeast of the San Andreas fault is not discussed here because of its small area and the complexity of geologic problems involved. BASEMENT ROCKS ULTRAMAFIC ROCKS The ultramafic rocks are emplaced only in the Fran- ciscan Formation, and most are serpentinites typical of those found throughout the Franciscan (Bailey and Everhart, 1964, p. 47). The smaller bodies are thorough- ly serpentinized, and intense shearing has destroyed original textures in all but an occasional 2-3-inch rem- nant block. The centers of the larger bodies consist of blocky serpentinite, but invariably this grades outward to the usual sheared material. These serpentinite bodies crop out as elongate pods and lenses concordant with the regional structure. They commonly form discon- tinuous trains which extend for miles along zones of apparent slippage. FRANCISCAN FORMATION The eugeosynelinal Franciscan Formation crops out in the southwestern and northeastern parts of the map area, southwest of the Nacimiento fault and northeast of the San Andreas fault. Although this unit was not studied in detail in the map area, it is presumed to be similar to Franciscan rocks elsewhere in the Coast Ranges. It consists of graywacke, siltstone and shale, conglomerate, greenstone, chert, and glaucophane schist and related metamorphic rocks. The rocks are moderate- ly to highly deformed, principally by faulting. GEOPHYSICAL FIELD INVESTIGATIONS The Franciscan northeast of the San Andreas pre- sumably ranges in age from Late Jurassic to Late Cre- taceous (Bailey, Irwin, and Jones, 1964, fig. 23). Paly- nomorphs recently collected by B. M. Page from the tract southwest of the Nacimiento are of Early and pos- sibly Late Cretaceous age (W. R. Evitt, written com- mun., 1969). GRANITIC ROCKS Granitic rocks crop out in the map area northeast of the Jolon fault zone near Paso Robles and just west of the San Andreas fault zone in the San Miguel quad- rangle. Many wells drilled between the two zones reach the granitic basement (table 1). Direct evidence of basement lithology between the Jolon and Nacimiento fault zones is lacking. The granitic rocks cropping out near Paso Robles are cut by aplite dikes and are similar in appearance to ada- mellite described by Compton (1966, p. 277-278) from the La Panza Range, just south of the map area. Chris- tensen (1963, p. 111) described coarse-grained biotite granite outcrops in the San Miguel quadrangle. The granitic rocks are presumably of Cretaceous age like other granitic rocks of the Salinian block (Comp- ton, 1966, p. 287). SUPERBASEMENT SEDIMENTARY ROCKS PALEOCENE AND UPPER CRETACEOUS DEPOSITS The unit designated as Paleocene and Upper Cre- taceous deposits which crops out only southwest of the Jolon fault zone, includes all sedimentary rocks older than the lower Miocene Vaquerors Formation, exclu- sive of the Franciscan Formation. It is apparently ab- sent in the subsurface northeast of the Jolon fault zone, for well data there indicate that Miocene or younger beds directly overlie the basement complex. This unit, some of which presumably belongs to Taliaferro's (1943) Asuncion Group, consists mainly of sandstone and conglomerate, but mudstone is locally abundant. Since the base of the unit is concealed in the map area, its thickness and the underlying unit are unknown. The Reserve Oil and Gas DeVries 37-25 (pl. 1, exploratory well 42), drilled in the northeastern part of the Ade- laida quadrangle, reportedly penetrated the unit for 5,200 feet without reaching the base (table 1). This in- formation and nearby surface data suggest that the unit is at least 7,000-10,000 feet thick. Marine fossils of both Late Cretaceous and Paleocene age have been collected from the unit at different loca- tions in the area; however, there appears to be no con- sistent lithologic difference between the Paleocene and Cretaceous beds. GRAVITY AND GEOLOGY OF THE PAST) ROBLES AREA, CALIFORNIA B3 TABLE 1.-Selected exploratory wells between the Jolon and San Andreas fault zones [Elevation: kb, kelly bushing; gr, ground; df, derrick floor; t, from topographic map] Map Location Year _ Elevation - Total Reported geologic data No. Operator Well =-- drilled (ft) depth (depths in feet) (pl. 1) Sec. T.(S.) R.(E.) (ft.) 1 Amerada Petroleum Corp..........--.- Creston Community 1-5......... 4 28 13 1952 1,208 kb 1, 643 Top basement complex, 1,385. 2 HOLA Aes cles ab hades ... McWilliams 1..1.....___.._.__.... 32 27 13 1952 1,157 kb 2,180 'Top basement complex, 2,175. 3 Barnsdall Oil Co........_..............- HAFpéF 27 12 1949 892 4,182 Weatlbered granite, 3,995; hard granite, '..... O... :. .L. oneness te op bes ules nae ne me 1.01... 28 28 12 1948 1,900 gr 2,407 Bottom in basement complex (7). Bilills (16.2.1 .L LILLIE G adden P MW.& P: 1.. ..:. LIA Li 29 23 11 1948 851 kb 4,151 Basement complex, 4,088; hard base- ment complex, 4,139. PYE -L sauna ens bale R.sand W.1.............. 21 23 12 1948 1,050 gr 4,100 Bottom in basement complex (?). 7 Bishop Oil Co.. . Alexander B-1. 8 28 11 1953 751 gr 2,468 Granite, 2462-2,468. 8 Bishop Oil Co. and Cuyama Oil Co 32 22 11 1948 769 kb 2,483 Bottom in basement complex. 9 Buttes Gas and Oil Co.__........- 31 24 12 1957 834 kb 5,050 Bottom in Miocene beds. 10 Canon Drilling Co............-- T 28 13 1951 1,340 t 1,707 Granite, 1,677. Chanslor-Canfield Midway Oil Co 3 25 12 1949 1,600 gr 4, 772 Top Vaqueros Formation, 4,651. ..... d.... coclciccece. 11 27 13 1949 1,000 gr 8,374 Top Vaqueros Formation, 3,146. 6 28 10 1946 1,099 8,062 Bottom in basement complex. 28 24 12 1949 1,030 df 5,134 Vaqueros Formation, 4,715. 26 25 11 1930 1,050 5,095 Vaqueros Formation, 4,629-5,095. 10}. ESO.. A.. wine ode. o. lona Ree obi » Ms Nacuniento 1. 32 24 11. 1929 889 5,955 Vaqueros Formation, 5,746-5,055. 17 CuyamaONCo........................ O'Reilly 2... T+ 28 22 11 1948 855 t 2,962 Top basement complex, 2,928. 18 Davis and Thompson.._._..._........- FirantI MIN _... 2.2 5 24 12 1955 913 kb 4,419 Bottom in granite. 19 Douglas, James M..._........--..-.---- SHIN A. ..o cece 36 22 9 1934 1,380 t 4,158 Bottom in Vaqueros Formation. 20 27 26 13 1938 1,026 6,157 Granitic basement complex, 6,020. 21 Estrella Partnership. ......-..-------- Kirchenmann 1................- 28 25 12 1949 660 gr 5,178 Top basement complex, 5,040. 22 Franco Western Oil Co... _. Kd Preszler 45A-8..._.._._...... 3 26 12 1958 742 kb 5,313 Top basement complex, 5,210. 23 General Petroleum Corp. Garrissere 67X-30. 30 22 10 1953 835 t 3,367 Bottom in basement complex. 24 Hamilton and Sherman.. Lenhof l...... 26 27 12 1953 971 kb 5,003 Bottom in beds of Relizian Stage. 25 Hamilton Dome Oil Co.......- Benjamin 1...-. 9 28 12 1958 1,128 kb 2,448 Bottom in basement complex(?). 2 Humble Oil & Refining Co-... A. Orradre et ux. 1.... 25 22 10 1956 60 t 2,072 Bottom in pre-Cretaceous rocks. I7 22. AOX A2 ICOA Ode Edwin Borchert et al. 1. 2 25 13 1956 1,417 kb 6,230 Bottom in Miocene rocks. 28 Jergins Oil Co. . Lanigan 249-35...... 35 22 10 - 1951 620 t 2,804 Bottom in granite. 0 .. OLLL -. 22 Ivan e un on ik u . USL South 1... 26 22 11 1949 1,200 t 3,302 Granite at 3,293. 30 Jergins Oil Co. h American McCool 1. .... 27 22 10 1948 480 t 2,110 Bottom in granite. solidated Oil Co. $1: Kimble, Joseph Gore. pl .u 17 24 12 1952 812 kb 5,135 Miocene red beds, 5,022-5,135. 32 Macson Oil Co..... . & 6 26 12 1951 775 gr 2,669 Top Vaqueros Formation, 2,615. 83+ M.J.M. & M. Oil Co.. Stanford-Johnson 1. e 11 27 12 1955 860 t 3,455 Bottom in granite. 34 Isgm‘is Stamping and Manufacturing Clark BJ 27 13 1945 1,025 3,336 Bottom in middle Mioccone beds. 0. 35 Ngrthofixgerican Consolidated & Jer- Rosenberg 1...__..-.-~.-------- 34 22 10 1947 500 2,500 'Top granite, 2452. gins Oil Co. $6. Northern . 26 22 10 1962 704 kb 2,234 Basement complex at 2,224. B7 Ohio Ofl Co. QZ Araujo C. H. 1......... 12 27 11 1956 1,200 gr 2,152 Bottom in Cretaceous beds. $8 .so. L . c res seks Ohio-Humble-Smith 1. ...--... 35 25 12 1958 760 5,365 Basement complex at 5,340. 39 Petroleum Securities White 1. c. «zee os 27 23 13 1926 1,844 3,205 Granitic basement complex at 3,135. 40 Porter, B. F., POMONA. e ints «in- =k 36 23 10 1949 509 gr 3,354 Bottom in basement complex. 41 Reid, Gene, Drilling, Inc._...._....... MEHMAD 122-2... 35 27 12 1951 1,018 kb 4,239 Middle Miocene beds, 4,100-4,105. 42 Reserve Oil and Gas Co.. ... DeVries §7-25........_.__._....... 25 25 10 1960 1,130 t 5,200 Bottom in Cretaceous beds. 43 Royalty Service Corp... - FOdQrAl 1... .. secs. anos 21 24 10 1943 1,362 7,673 Bottom in Miocene beds. 44 Shell Oil Co.__-....- . Branch 1. . .e dese A SIC 34 24 10 1937 G61 8,994 Granitic basement complex, 8,974. 45 d . D.G.& J. Unit1...........«.. 19 22 10 1948 740 gr 2,721 Granite, 2,571. 46 APHA L. .o. ere e= 32 24 12 1921 974 4, 665 Vaqueros Formation, 4,450-4,465. 47 . Labarere 27-X.. 21 23 10 1950 1,088 df - 10,957 Bottom in basement complex. 48 .. . Mahoney 1..... 4 25 12 1933 742 5,972 Bottom in granite(?). 49 . Alexander 85-9. » 9 23 11 1951 932 df 2,812 Top granite, 2811. B0 AOL.. Alexander B-6-5................ 5 28 11 1951 1,006 df 3,050 'Top basement complex, 3,043. St ...A 16 .L ETL LDL. DLL ILEC» Lete gi be Ruth Hillman 45-19.........-.- 19 25 14 1955 1,180 df 7,395 Bottom in Miocene beds. 52 Starr, E. G... s NOA Lu -r ones ben 9 27 13 1926 925 3, 576 Vaqueros Formation, 3,440-3,576. 53 Sunray Oil Corp...._..--..- .. Sinclair 47-10 10 25 12 1952 933 kb 6,048 Top basement complex, 6,013. 54 Sunray Oil Corp. and Parkford, E. A... Klever1......... 28 27 12 1951 1,000 gr 2,791 Granodiorite, 2,771-2,791. 55 Taylor, Frank M., and Associates.... Estrella Creek 18-20. 20 25 13 1964 930 kb - 6,943 Basement complex, 6,882. §6 dor: NEL ALE 6. - Estrella Creek 85-30. 30 25 13 1963 860 kb 7,100 Basement complex, 6,930. B7 CTERACOINGL.-. A. loc coeval Adrian Orradre 1 19 22 11 1955 813 kb . Granite, 2,147. do. 5 23 10 1957 758 gr 6,785 Bottom in granite. 33 22 10 1957 720 t 4,018 Basement complex, 4,004. 9 28 10 1952 730 gr 3,749 Granite, 3,741. 1 26 11 1954 2,020 'Top basement complex, 1,988. 26 28 9 1955 1,250 kb 5,940 Weathered granite at 3,012. 19 25 12 1952 1 4,909 Bottom in granite. .............. O01 20 22 10 1960 616 kb 3, 451 Bottom in basement complex. .................. Labarere@-1.. lik nous, ). (5 23 10 1952 - 591 gr . 3,402 Granite at 3,266. ...................... ..Z, Ac.... clues 28 25 12 1953 626 gr 4,587 Granitic basement complex, 4,540. .. M. Garrissere 3.. 29 22 10 1962 810 kb , 534 Granite at 4,480. Nichols 1...... # 13 23 11 1952 1,300 t 3,404 Bottom in granite. - Powell 1... 11 23 11 1964 1,041 gr 2,546 Granite at 2,540. 28 22 10 1958 425 t 2,781. Bottom in granite. § 24 11 1956 772 gr 3, 761 Top basement complex, 3,758. 34 28 11 1964 784 kb 3, 520 Basement complex at bottom. 6 24 10 1956 1,671 gr 11,994 Bottom in Miocene beds. 28 24 10 1952 1,572 kb 10,480 Bottom in lower Miocene beds. ..... 25 23 11 1954 1,140 t 3, 824 Fresh basement complex, 3,780. ..... (lo...... = £ 28 28 11 1949 1,150 gr 3, 178 Top granite at 3,114. Thornbury, Geis, and Robinson. . Konekamp 1-3... # 3 25 10 1959 1,201 kb 8,009 Bottom in Miocene beds. Tresaden and Reynolds.... - Tres-Rey & Assoc. 1.. z 31 22 12 1959 1,280 t 3,000 Bottom in basement complex. 79 Union Oil Co...... . Union-Texaco-Connell 1 e 2 25 10 1961 1,090 gr 6, 869 Bottom in Miocene beds. 80 Wilshire Oil Co., Inc................... Hunter-Dryden 71-26........... 26 23 10 1951 775 gr 5,052 Bottom in basement complex. B4 TIERRA REDONDA AND VAQUEROS FORMATIONS In the map area the unit designated as Tierra Re- donda and Vaqueros Formations crops out southwest of the Jolon fault zone. It has reportedly been pene- trated by numerous exploratory wells northeast of the Jolon fault, but is absent in the subsurface near the San Ardo oil field, where younger beds lie directly on basement complex. Both the Vaqueros and Tierra Redonda Formations are sandy and locally conglomeratic. Although each has considerable variation in lithologic character, the Vaqueros is generally darker colored, finer grained, and less massive than the Tierra Redonda. The Vaqueros Formation unconformably overlies Cretaceous and Paleocene strata in the southwestern part of the map area and reportedly lies on granitic basement in the subsurface northeast of the Jolon fault zone (table 1). The Tierra Redonda Formation lies either conformably on the Vaqueros or unconformably on older strata ; in places it intertongues with the lower part of the Monterey Shale. The Vaqueros Formation is about 1,300 feet thick in the southwest corner of the Bradley quadrangle but apparently thins southeastward and lenses out com- pletely in the northeastern part of the Adelaida quad- rangle. The Vaqueros is probably about 1,650 feet thick in the subsurface northeast of the Jolon fault zone near the San Antonio River, where it was penetrated by the Shell Oil Branch 1 (exploratory well 44, pl. 1 and table 1). The formation presumably thins to the northeast, disappearing near the San Ardo oil field. The Tierra Redonda Formation is probably about 1,650 feet thick in the southwest quarter of the Bradley quadrangle but is only about 700 feet thick to the southeast in the Adelaida quadrangle. The Vaqueros Formation in the map area contains early Miocene marine fossils. The Tierra Redonda For- mation lacks fossils in the map area, but stratigraphic relations with the Vaqueros Formation and Monterey Shale restrict it to an early and middle Miocene age. A marine origin for the Tierra Redonda is probable since it intertongues with the marine Monterey Shale, is over- lain and underlain by marine strata, and lacks features characteristic of nonmarine deposits. VOLCANIC ROCKS Basaltic volcanic rocks associated with the Monterey Shale crop out locally southwest of the Jolon fault zone. At least some are submarine flows contemporaneous with Monterey Shale deposition. Rhyolitic volcanic rocks crop out near the San Andreas fault zone in the north- eastern part of the San Miguel quadrangle (J ennings, 1958). GEOPHYSICAL FIELD INVESTIGATIONS MONTEREY SHALE Monterey Shale forms many of the hills west of the Salinas River and crops out in the upper reaches of larger streams in the San Miguel quadrangle. It also crops out southwest of the Nacimiento fault zone. The Monterey underlies younger units in most areas of their occurrence, except northwest of Paso Robles, where the younger Paso Robles Formation lies directly on granite. The lower part of the Monterey Shale, the Sandholdt Member, is chiefly calcereous mudstone but includes por- celaneous rocks, chert, and dolomitic carbonate rock. The upper part is mainly porcelaneous rocks and non- calcereous mudstone with some chert and dolomitic car- bonate rock. The uppermost part of the Monterey, the Buttle Member, is exposed north of Hames Valley and consists of diatomite and diatomaceous mudstone. The Monterey Shale conformably overlies the Va- queros Formation in most of the area, but to the north- east, in the subsurface, it overlaps the Vaqueros to lie directly on basement rock. It conformably overlies or intertongues with the Tierra Redonda Formation. The Sandholdt Member of the Monterey Shale reaches 4,000 feet in thickness in the southern Bradley quadrangle northeast of the Jolon fault. The upper part of the Monterey, exclusive of the Buttle Member, is about 9,000 feet thick in the same area. The Buttle Mem- ber is about 600-700 feet thick on the flanks of Hames Valley. These figures indicate that the total thickness of the Monterey reaches a maximum of 13,000 feet in the northwestern part of the map area. The unit thins considerably to the northeast, however, and is only about 1,500 feet thick in the San Ardo oil field. It is even thinner near the center of the San Miguel quadrangle, where the Monterey intertongues with the Santa Mar- garita Formation. Foraminifera indicative of middle Miocene age are abundant in the Sandholdt Member of the Monterey Shale. The overlying siliceous rocks of the Monterey are generally lacking in fossils useful in age determina- tion; however, stratigraphic relations with the Santa Margarita and Pancho Rico Formations indicate that these siliceous strata are probably of late Miocene age but could include beds of latest middle Miocene and early Pliocene age. PANCHO RICO AND SANTA MARGARITA FORMATIONS The Santa Margarita Formation crops out in a belt just southwest of the Jolon fault and in the eastern part of the San Miguel quadrangle. The Pancho Rico Formation crops out along the margin of the hills south- west of the Salinas River, around Hames Valley, in patches just northeast of the Jolon fault, and in a broad belt from the northeastern part of the Bradley quad- rangle across the San Miguel quadrangle. Only in the GRAVITY AND GEOLOGY OF THE central San Miguel quadrangle are the two units ex- posed in the same stratigraphic sequence. The Santa Margarita is a light-gray to white, medium- to coarse-grained, massive to thick-bedded calcareous sandstone. Fossils are abundant in most of the unit. The Pancho Rico Formation, although char- acteristically sandstone, also contains mudstone, con- glomerate, and siliceous rocks similar to those in the Monterey Shale. Fine-grained sandstone is the most common lithology west of the Salinas River, whereas coarser grained sandstone and conglomerate are com- mon east of the river. The Santa Margarita Formation conformably over- lies the Monterey Shale southwest of the Jolon fault, and northeast of the fault intertongues with it as well. In most of the map area the Pancho Rico Formation conformably overlies and probably locally intertongues with the Monterey, but in the Vineyard Canyon area, it lie with apparent conformity on the Santa Margarita. The Santa Margarita is about 500 feet thick near the Nacimiento River in the southern part of the Bradley quadrangle and thins northwestward. The Pancho Rico is 450-650 feet thick in the northeastern part of the map area and thins southeastward; it is 100-200 feet thick northeast of Hames Valley and thins southward to 20 feet or less near the Nacimiento River. The Pancho Rico is absent in the map area southwest of the Jolon fault. The Santa Margarita Formation contains marine fossils indicative of late Miocene age. The Pancho Rico Formation contains marine fossils characteristic of Pliocene age, generally early Pliocene. PASO ROBLES FORMATION The Paso Robles Formation blankets much of the low-lying central and southeastern part of the map area. It also caps hills and occupies small structural depres- sions near the San Antonio River in the Bradley quad- rangle. It is a predominantly nonmatine unit consisting chiefly of conglomerate and sandstone in units a few feet to scores of feet thick, but mudstone is also common in the formation. Limestone and, more rarely, lignite cccur sparingly. In some places, northeast of Hames Valley for example, the Paso Robles Formation conformably overlies the Pancho Rico Formation; in others, near the Nacimiento River for example, it conformably over- lies the Santa Margarita Formation. The Paso Robles and Pancho Rico locally intertongue in the northeastern part of the map area. Elsewhere the Paso Robles un- conformably overlies older units with varying degrees of discordance. The total thickness of the Paso Robles Formation in the map area is unknown since the upper part of the for- 367-818-70--2 PASO ROBLES AREA, CALIFORNIA B5 mation is so commonly eroded. The formation is at least 1,000 feet thick east of the San Ardo oil field, however, and is probably even thicker to the south. The Paso Robles Formation is generally considered to be Pliocene and possibly early Pleistocene in age since it overlies and may intertongue with the Pancho Rico Formation and since it unconformably underlies older alluvium of Pleistocene and Holocene( ?) age. SURFICIAL DEPOSITS Surficial deposits are made up of older alluvium and alluvium. Older alluvium covers the floors of the larger valleys and forms terraces along their sides. Alluvium occurs along the beds of most streams. The older allu- vium is mainly semiconsolidated sand and gravel, and the alluvium is similar but unindurated. The older allu- vium lies with angular discordance on older rocks, and the alluvium commonly occurs along streams that cut older alluvium or other units. The older alluvium and alluvium combined are probably no thicker than a few score feet in most places, but their thickness is uncertain, partly because of difficulty in distinguishing older allu- vium from Paso Robles Formation in wells. The older alluvium is considered to be of Pleistocene and possibly Holocene age because it unconformably overlies the Paso Robles Formation of Pliocene and possibly Pleisto- cene age. The alluvium is Holocene. STRUCTURE The gross basement structure and the general struc- tural condition of the Franciscan Formation were de- scribed under "Basement rocks." This section is con- cerned principally with structural features of the Sa- linian block. FAULTS The most obvious structural feature in the map area is the Jolon fault zone (pl. 1), which begins near Paso Robles, continues northwest through the San Antonio River valley, and extends at least 15 miles beyond the map area. It is generally 14-1 mile wide at the surface and comprises several traces. The principal trace is the Jolon fault itself, along which stratigraphic relations give evidence of at least 11 miles of right-lateral strike- slip displacement (Durham, 1965). Another element of the zone is the San Marcos fault, which at the surface separates the middle Miocene from the upper Miocene part of the Monterey Shale along much of its length. The Jolon fault zone northwest of the San Antonio River presumably is concealed by alluvium and rocks thrust over it along the San Antonio fault. The zone is obscure southeast of Paso Robles, but its trend suggests that it continues southward to join the Rinconada fault zone. (See section on "Gravity anomalies".) B6 The Los Lobos fault (Kilkenny and others, 1952) is a thrust fault located southwest of the Salinas River in the northern Bradley quadrangle. The principal fault trace is apparently concealed by alluvium in the map area (pl. 1). Well data (table 1), however, define the fault as a southwest-dipping feature separating severely deformed beds in the upper plate from little-deformed beds in the lower plate. It apparently dies out to the southeast. The San Antonio fault (pl. 1) is a thrust fault ex- posed along the northeast side of the San Antonio River. It dips northeastward and separates deformed upper plate beds from relatively undeformed lower plate beds. The Los Lobos and San Antonio faults are believed to have a common origin. They are on opposite sides of the very thick sedimentary sequence of Hames Valley and dip toward the center of the valley. On both faults, the material of the upper plate is thrust from the valley center toward its margins, and on both faults the under- formed sediments of the lower plate rest on structural basement highs on opposite sides of the thick sedimen- tary sequence. For the Los Lobos fault, the depth to basement northeast of the Salinas River is locally less than 2,500 feet, but southwest of the river the basement surface locally slopes over 3,000 feet per mile toward Hames Valley, where it is deeper than 10,000 feet (Dur- ham, 1966, pl. 5). The southwest edge of the platform- like basement high trends northwest beneath the river and roughly parallels the Los Lobos fault. For the San Antonio fault, subsurface information is lacking in the map area, but 1-5 miles to the northwest, just southeast of the fault, the depth to basement is only 1,000-1,500 feet. To explain the observed relations, a northeast-south- west crustal shortening is postulated. The base- ment highs on opposite sides of the basin thus served as buttresses compressing the thick sedimentary section of Hames Valley, and some of the section was forced out over the basement highs via the thrust faults. The strata riding directly on the basement highs were not subject to the compression and thus were not so highly deformed. The evidence seems to indicate, therefore, that the Los Lobos and San Antonio faults are probably related to (1) crustal shortening and (2) the configura- tion of the basement surface. The Espinosa fault zone (pl. 1) branches north- northwest from the Jolon fault zone in the Bradley quadrangle. It is confined at the surface to the Monterey Shale, where it forms a series of en echelon belts of crushed and contorted rock across which structural features are discontinuous. Well data suggest that the basement surface is considerably deeper on the north- east side of the fault zone than on the southwest side. GEOPHYSICAL FIELD INVESTIGATIONS The feature may be a simple basement offset at depth having a complex expression at the surface because of the intervening thousands of feet of Monterey Shale. FOLDs The Monterey Shale is generally deformed into broad folds where it is thick, but near faults it is commonly tightly folded, contorted, and overturned. Sandy and conglomerate units, in contrast, are in general simply tilted or warped into broad folds. AGE OF DEFORMATION The Paso Robles Formation is, in many places, de- formed to the same extent as the underlying units. This indicates faulting and folding after deposition of the Paso Robles (post-Pliocene and Pleistocene?) ; how- ever, because the Paso Robles also lies unconformably or disconformably on the Monterey Shale, deformation probably also occurred before or during deposition of the Paso Robles (late or post-late Miocene). The Va- queros Formation lies unconformably on Cretaceous and lower Tertiary rocks, and the Tierra Redonda For- mation overlaps the Vaqueros to lie unconformably on these older strata; thus, deformation probably occurred before, and perhaps during, Miocene time. Similarly, Miocene strata in the subsurface near the San Ardo oil field apparently pinch out against and lap up onto a basement-complex surface. This slope in the basement surface must result from deformation that occurred in Miocene or pre-Miocene time. GRAVITY DATA GRAVITY SURVEY The map area contains 3138 gravity stations tied to seven gravity bases. The principal facts for the bases are given in table 2, and those for the 313 stations in table 3. All the data are tied to base 173 (Chapman, 1966, p. 36) at the U.S. Geological Survey office in Menlo Park, Calif. The observed gravity at this base, determined by numerous ties to North American Grav- ity Standardization Stations at the San Francisco Air- port, is taken to be 979,958.74 mgal. Taso 2.-Principal facts for bases used in gravity survey Eleva- Observed Base Lat N. Long W. tion - gravity! Description (ft) (mgal) SLUKB...36 7.73 1211.12 405.8 979793.04 U]S:IC&GS BM G154 at San ucas. BRADB...35 51.81 120 47.73 552.0 979737.30 USGS BM 553 at Bradley. JOLNB....35 58.47 121 10.47 979.0 979722.66 USGS BM 979 at Jolon. CAMBB..35 33.55 121 5.33 81.0 979801.24 USC&GS BM A694 at PSROB....35 37.55 120 41.28 720.0 979717.17 U%%nézb(r}1§.BM 124 at Paso PARKB...35 53.98 120 25.92 1535.0 979687. 69 Ugézlésé BM F79 at SHANB...35 39.33 120 22.71 1038.0 979686.02 UgggigfilidfiM W559 at Shandon. ! The amount of scatter among numerous ties between these bases suggests that the relative observed gravity of each is known to 40.02 mgal. GRAVITY AND GEOLOGY OF. THE PASO ROBLES AREA, CALIFORNIA 3.-Principal facts for gravity stations. Observed Terrain Free air Complete Station Lat N. Long W. Elevation (ft) gravity (mgal) correction anomaly Bouguer anom- (mgal) (mga!) aly (mgal) 20 35 54. 28 120 58. 02 1159 979702. 35 1. 41 -11. 16 -49. 76 21 85 58. 95 120 56. 99 936 979712. 38 2. 51 -21. 68 -51. 48 22 35 58. 87 120 56. 26 851 979711. 16 1. 78 -30. 86 -58. 44 23 35 52. O7 120 51. 08 594 979726. 53 . 88 -37. 04 -58. 68 24 35 52. 88 120 52 17 611 979724. 61 1. 04 -37. 74 -57. 80 25 35 52. 59 120 53. 33 649 979721. 61 1. 13 -37. 583 - 88. 81 26 85 52. 91 120 54. 22 763 979709. 73 1. 12 -39. 15 -64. 37 27 35 58. 28 120 55. 16 786 979708. 24 1. 37 - 38. 93 - 64. 70 28 35 50. 18 120 50. 61 684 979719. O7 . 92 -38. 34 -56. 04 29 35 47.78 120 51. 27 593 979726. 31 1. 28 -31. 24 -50. 44 30 35 47. 42 120 54. 76 796 979734. 48 2. 61 -3. 46 - 28. 34 31 35 47. 23 120 57. 19 1055 979729. 58 1. 43 16. 22 -18. 77 32 35 46. 99 120 58. 49 1199 979719. 71 1. 67 20. 28 -19. 43 33 35 48. 29 120 58. 70 1005 979732. 14 1. 33 12. 61 -20. 75 34 35 48. 89 120 58. 79 914 979736. 20 1. 38 7. 26 - 22. 91 35 35 50. 13 120 58. 68 1056 979723. 98 1. 29 8. 63 - 28. 53 36 85 51. 20 120 58. 85 748 979746. 12 1. 44 -1. 73 -20, 11 228 35 59. 92 120 53. 14 439 979766. 83 1219 -22. 53 -36. 50 229 35 58. 72 120 52. 64 470 979763. 97 1. 23 - 20. 76 -35. 76 230 85 57. 89 120 52. 13 460 979764. 70 1. 28 -19. 79 - 34. 39 231 35 55. 56 120 51. 56 501 979752. 68 1. 54 -24. 62 -40. 39 232 35 54. 90 120 50. 48 494 979750. 98 1. 95 -26. 04 -41. 15 233 35 52. 28 120 48. 79 525 979736. 94 127 - 33. 35 -50. 21 234 35 §1:72 120 46. 14 551 979741. 28 1. 20 -25. 84 -43. 67 BRADB 35 S1. S1 120 47. 73 552 979737. 30 . 98 - 29. 85 -47. 94 236 35 50. 76 120 45. 14 554 979738. 58 1. 20 -26. 89 -44. 82 237 35 49. 54 120 44. 95 558 979736. 63 141 -26. 72 - 44. 88 238 35 47. 33 120 43. 66 584 979735. 08 . 90 -22. 67 -41. 94 239 35 44. 17 120 41. 98 632 979727. 92 . 92 -20. 82 -41. 72 240 35 43. 21 120 41. 69 620 979726. 93 48 -21. 57 -42. 20 241 35 39. 69 120 41. 72 716 979730. 76 1. 13 -3. 70 -271. 29 242 35 40. 54 120 41. 58 680 979724. 29 . 82 -14. 76 -37. 42 248 85 41.75 120 41. 47 675 . 979722. 41 T2 - 18. 88 -Al. 47 PSROB 85 37. 55 120 41. 28 720 979717. 16 . 93 -13. 86 -37. 79 245 35 85. 12 120 41. 38 730 979710. 87 . 87 - 15. 77 -40. 10 246 35 34. 42 120 41. 45 758 979712. 58 . 84 -10. 43 - 85. T6 247 35 35. 57 120 41. 67 766 979719. 81 \ . 86 -1, 24 -26. 83 248 35 33. 02 120 42. 04 T72 979720. 74 . 90 1. 04 - 24, 72 249 35 32. 67 120 42. 39 T79 979717. 68 . 98 -. 87 -26. 78 250 35 35. 02 120 42. 82 872 979711. 75 . 97 -1. 39 -30. 52 251 35 34. 47 120 43. 78 923 979709. O1 1. 03 1. 45 -29. 38 252 35 33. 99 120 44. 75 929 979708. 03 1. 18 1. 12 -29. 18 258 35 33. 08 120 46. 50 894 979720. 21 1. 59 11. 90 -17. 38 254 35 32. 96 120 47. 48 925 979717. 24 1. 97 12. 01 -17. 95 255 35 32. 50 120 48. 05 979 979714. 18 2. 95 14. 69 -16. 16 256 85 32. 55 120 49. 07 1065 979710. 22 2. 63 18. 74 -15. 39 257 35 32. 46 120 49. 69 1620 979683. 29 2. 67 34. 74 -15. 04 258 35 32. 34 120 50.17 1207 979704. 10 2. 06 26. 28 -13. 32 259 35 32. 05 120 50. 78 1189 979705. 22 2. 28 26. 12 -12. 64 260 35 32. 06 120 51. 48 1273 979700. 60 2. 29 29. 88 -12. 26 261 35 30. 64 120 50. 68 1413 979687. 12 2. T5 31. 09 -14. 92 262 35 32. 93 120 52. 63 1386 979697. 46 2. 88 35. 64 -9. 31 263 35 33. 44 120 53. 43 1455 979693. 84 2. 88 87.78 -9. 54 264 35 33. 84 120 54. 37 1853 979668. 90 8. 97 49. 70 -10. 24 265 35 34. 21 120 54. 74 1588 979686. 33 4. 17 41. 68 -8. 93 266 35 34. 15 120 55. 88 1033 979718. 49 b 11 21. 73 -8. 82 267 35 34. 12 120 56. 19 673 979737. 18 7. 45 6. 60 -9. 19 268 35 33. 63 120 57. 28 530 979752. 38 4. 88 9. 00 - 4, 43 269 35 34. 29 120 58. 73 367 979763. 70 5. 23 4. 10 -3. 85 270 35 34. 51 120 59. 47 342 979767. 08 4. 59 4. 81 -2. 41 293 35 34. 58 120 46. 99 1265 979693. 85 1. 97 18. 30 -23. 39 294 35 35. 74 120 48. 10 1550 979676. 63 3. O1 26. 23 - 24. 28 295 25 35. 86 120 49. 68 1288 979698. 30 1. 69 23. 09 -19. 67 206 35 37. 69 120 52. 92 1141 979713. 15 2. 45 21. 51 -15. 42 297 35 39. 48 120 50. 30 2032 979656. 19 5. 84 45. 80 - 18. 43 298 35 40. 12 120 50. 82 1732 979677. 10 3. 03 37. 59 -19. 13 299 35 38. 63 120 47. 59 2323 979632. 99 6. 80 51. 18 -22. 11 300 35 39. 63 120 45. 90 1574 979683. 17 2011 29. 50 -22. 70 301 35 38. 98 120 43. 18 1016 979718. 81 1. 42 13. 58 -20. O7 302 35 40. 70 120 42. 37 916 979717. 36 1. 13 . 28 -30. 22 303 35 40. 37 120 49. 21 1116 979718. 67 2. 18 20. 87 -15. 47 304 35 36. 73 120 55. 08 1235 979711. 41 3. 99 29. 98 \_ -8. 65 305 35 44. 48 120 53. 74 973 979729. 37 2. 08 12. 27 -19. 29 BT B8 GEOPHYSICAL FIELD INVESTIGATIONS TABLE 3.-Principal facts for gravity stations-Continued Observed Terrain Free air Complete Station Lat N. Long W. Elevation (ft) gravity (mgal) correction anomaly Bouguer anom- (mgal) (mgal) aly (mgal) 306 35 43. 14 120 52. 72 965 979731. 66 1. 46 15. 71 -16. 14 307 35 43. 35 120 54. 86 1205 979716. 26 2. 65 22. 59 - 16. 35 308 35 42. 99 120 55. 99 851 979740. 91 1. 58 14. 45 - 13. 40 309 35 40. 56 120 58. 46 1775 979675. 21 6. 48 39. 12 -15. 63 310 35 38. 53 120 56. 54 1905 979663. 18 8. 97 42. 20 -14. 53 311 35 39. 89 120 56. 64 1605 979685. 44 5. 16 34. 31 - 15. 90 343 35 46. 07 120 53. 93 TTY 979739. 88 3. 74 1. 51 ~21..37 344 35 45. 04 120 54. 51 785 979743. 47 1. 38 7. 88 -17. 84 345 35 45. 00 120 55. 24 773 979745. 26 1. 58 8. 60 -16.; 56 346 385 45. 53 120 55. 72 773 979744. 90 2. 30 7. 49 -16. 90 347 35 45. 43 120 56. 43 776 979746. 37 1. 66 9. 38 =-15. 75 348 35 45. 00 120 57. 37 778 979748. 06 1.70 11. 87 ~<18. 20 349 35 44. 66 120 57. 74 T78 979746. 10 1. 68 12. 40 129. 78 350 35 44. 55 120 58. 68 778 979747. 95 1. 64 12. 40 -12. 82 351 35 44. 85 120 59. 25 TTT 979747. 90 1. 88 11. 83 18. 11 360 35 44.18 120 59. 93 T73 979748. 92 2. 55 13. 43 -10. 71 361 35 43. 92 120 58. 93 776 979747. 46 2. 44 12. 62 73 362 35 43. 87 120 57. 25 778 979745. 97 1. 70 11. 39 -=18. 77 363 35 43. 09 120 57. 55 775 979745. 73 1. 75 11. 98 -13. 03 364 85 42. 57 120 58. 38 776 979746. 61 2. 39 13. 70 =-10. 71 365 35 42. 86 120 56. 81 775 979745. 00 1. 78 11. 58 - 13. 45 366 35 41. 96 120 57. 15 810 979741. 95 3. O1 13. 10 -11. 85 367 35 41. 23 120 56. 86 772 979742. 24 2. 80 10. 86 -13. 00 368 35 44. 66 120 56. 09 774 979746. 56 1. 66 10. 48 - 14. 58 369 35 44.35 120 55. 20 772 979745. 91 1.9 10. 09 -15. 16 370 35 42. 95 120 55. 70 T76 979745. 33 1. 82 11. 88 -13. 10 S71 35 43. 88 120 53. 76 TTL 979744. 66 1. 92 9. 41 -15. 29 372 35 45. 38 120 54. 18 TTL 979743. 47 1. 53 6. 08 -19. 01 384 35 59. 05 120 59. 14 1189 979720. 41 3.82 2. 83 - 34. 88 385 35 59. 50 120 57. 56 916 979738. 55 3. 33 -5. 85 -33. 64 387 35 57. 63 120 59. 74 2543 979624. 93 8. 89 36. 72 -42. 05 389 35 55. 85 120 58. 98 1953 979655. 09 4. 94 13. 94 -48. 48 390 35 55. 14 120 57. 64 1107 979700. 11 2. 32 - 19. 59 - 55. 48 391 35 56. 08 120 56. 84 1075 979704. 59 2. 36 - 19. 47 -54. 21 392 35 57. 07 120 56, 53 1866 979655. 70 5. 45 4. 62 -54. 29 393 35 57.18 120 56. 70 1912 979662. 04 5. 74 14. 35 -45. 86 394 85 57. 45 120 55. 63 1488 979688. 29 4. 18 1. 12 -46. 04 395 35 56. 33 120 55. 00 1715 979658. 93 4. 12 -5. 29 -60. 33 396 35 55. 48 120 53. 67 1678 979658. 00 4. 11 - 8. 49 -62. 26 397 35 53. 91 120 50. 83 1055 979700. 94 3. 02 -21. 90 -55. 30 398 35 53.42 120 51. 68 1070 979695. 99 2. 04 -24. 74 -59. 63 399 35 54. 34 120 53. 62 877 979705. 81 1. 56 - 34. 39 -63. 10 400 35 53. 63 120 53. 71 768 979714. 44 1: 07 -34. 99 -60. 44 401 35 46. 08 120 44. 40 796 979718. 93 . 85 -17. 10 -43. T4 402 35 45. 95 120 45. 12 713 979724. 52 294 - 19. 13 -43. 02 403 35 46. 67 120 46. 08 886 979711. 58 197 - 16. 83 -46. 05 404 35 45. 01 120 46. 02 953 979706. 17 1. 07 - 18. 57 -45. 40 405 35 45. 56 120 47. 92 650 979723. 74 1. O1 - 25. 28 -46. 72 406 35 46. 70 120 48. 24 584 979730. 14 Bi -26. 72 -46. O1 407 35 45. 65 120 49. 35 673 979722. 983 £: 05 -24. 06 -46. 25 408 35 46. 43 120 51. 24 1317 979675. 90 4. 34 - 11. 63 -52. 74 409 35 47. 28 120 49. 96 670 979723. 17 115 -26. 42 -48. 41 410 35 47.96 120 48. 91 995 979703. 05 1. 42 - 16. 95 -49. 87 411 35 48. 75 120 49. 93 621 979725. 14 . 98 -31. 16 -$1.62 412 35 48. 78 120 48. 36 973 979704. 91 1. 53 - 82 -50.38 413 35 49. 06 120 47. 12 893 979712. 22 . 86 -- 18. 94 -48. 91 414 35 50. 20 120 48. 22 545 979732. 43 1-01 -33. 09 -50. 90 415 35 50. 79 120 47. 12 827 979716. 64 1. 61 -283. 19 -50. 14 416 35 51. 26 120 47. 67 528 979736. 51 . 99 -82. 12 -49. 36 417 35 49. 85 120 46. 30 992 979706. 14 1:65 -16. 83 -49. 483 418 35 48. 75 120 45. 37 556 979736. 45 . 91 -25. 96 -44. 25 419 35 46. 21 120 42. 89 766 979721. 06 . 89 -17. 98 -43. 54 420 35 44. 48 120 44. 12 889 979709. 56 BA - 15. 45 -45. 19 421 35 43. 60 120 43. 97 1033 979701. 48 1. 68 -8. 73 -42.71 422 35 44. 62 120 45. 76 1064 979698. 34 1:88 -10. 41 -45. 55 423 35 43. 70 120 45. 76 1142 979696. 39 1. 30 -8. 71 -Al. 82 424 35 43. 82 120 47. 21 1063 979702. 29 1. 45 -5. 41 -40. 65 425 35 42. 68 120 46. 30 945 979715. 76 1. 04 -$, 41 -33. 00 426 35 41.79 120 46. 31 1058 979717. 06 1. 47 11. 78 -28. 27 427 35 42. 85 120 47. 50 1219 979693. 19 2. 42 1. 54 -88. 11 428 35 41. 84 120 47. 70 1353 979694. 50 1. 90 16. 90 -27. 89 429 35 42. 28 120 48. 34 1708 979668. 94 5. 07 24. 10 -29. 75 430 35 43. 14 120 49. 62 1643 979675. 96 4. 20 23. 78 -28. 70 431 35 43. 69 120 48. 77 1368 9798578. 43 4. 09 -. 40 -43. 52 GRAVITY AND GEOLOGY OF THE PASO ROBLES AREA, CALIFORNIA TaBLE 3.-Principal facts for gravity stations-Continued Observed Terrain Free air Complete ~ Station Lat N. Long W. Elevation (ft) gravity (mgal) correction anomaly Bouguer anom- (mgal) (mgal) aly (mgal) 432 35 44. 62 120 50. 08 1336 979682. 65 4. 98 -O. 51 -41. 64 433 35 44. 78 120 51. 40 1462 979681. 94 3. 29 10. 40 -36. T7 434 35 51. OL 120 51. 55 685 979718. 52 1. 06 - 34. 98 -57. 58 485 35 49. 69 120 52. 02 1484 979656. 12 4. 05 -20. 85 -67. 50 436 35 49. 24 120 50. 91 1016 979694. 35 2. 66 - 25. 50 -57. 91 437 35 47. 35 120 52. 41 978 979702. 19 1. 93 - 18. 58 -50. 87 438 35 46. 32 120 52. 52 1338 979690. 34 2.44 4. 94 -38. 46 439 35 47. 01 120 55. 75 1344 979704. 57 2. 07 18. 75 - 25. 56 440 35 46. 40 120 54. 99 1576 979689. 56 4. 64 26 43 -23. 80 441 35 47. 46 120 53. 60 1217 979697. 81 2.53 . 59 -40. 06 442 35 46. 78 120 56. 43 1000 979732. O1 1. 78 14 17 -18. 57 443 35 46. 35 120 57. 77 1473 979701. 82 2. 91 29. 08 -18. 84 445 35 46. 88 120 59. 37 1232 979718. 30 1. 63 22. 14 -18. 75 446 35 48. 00 120 57. 30 1212 979714. 81 2. 85 15. 17 - 23. 81 447 35 48. 05 120 55. 79 1134 979711. 62 2. O7 . 4. 57 -32. 50 448 35 48. 02 120 54. 55 1130 979706. 23 1. 94 -1. 15 - 38. 22 449 35 48. 98 120 57. 48 968 979727. 70 1. 49 3.71 - 28. 22 510 85 49. 55 120 59. 83 1197 979715. 44 2. 09 12. 18 -27. 05 517 35 54. 28 120 59. 33 1189 979702. 20 1. 64 -8. 49 -47. 89 518 35 53. 37 120 58. O1 1384 979686. 51 2. 74 -4. 62 - 49. 63 519 35 52. 85 120 58. 50 1018 979720. 85 1. 96 -3. 24 -36. 42 520 35 53. 09 120 59. 18 1139 979713. 75 2. 46 -. 02 -36. 87 521 35 51. 86 120 57. 13 918 979720. 74 1..56 -12. 06 -42. 19 522 35 51. 17 120 57. 85 689 979746. 45 1. 59 -6. 90 -29. 11 528 35 50. 27 120 56. 27 890 979731. 40 2. 04 -1. T7 - 30. 45 524 35 51. 27 120 56. 48 1535 979676. 11 3. 94 2. 18 -46, 84 525 35 50. 96 120 55. 02 1659 979659. 60 3. 59 -2. 22 -55. 87 551 35 36. 44 120 58. 72 1447 979699. 25 4. 31 38. 17 -7. 45 552 35 36. 12 120 57. 22 2930 979600. 27 12. 50 79. 11 -9. 35 592 35 38. 42 120. 39. 41 835 979705. 55 . 65 -15. 91 -44, 08 593 35 38. 00 120 37. 67 768 979707. 37 sA -19. 79 -45. 60 594 35 38. 25 120 35. 55 961 979691. 49 . 69 -17. 87 -50. 36 595 35 38. 25 120 33. 58 1019 979686. 96 . 70 -16. 95 -51. 42 596 35 37. 45 120 32. 73 1026 979687. 97 s T9 -14. 14 - 48. 81 597 35 36. 65 120 33. 04 1200 979677. 23 . 86 -T. 88 -47. 93 598 35 35. 62 120 33. 69 922 979698. 98 . 91 -10. 31 -41. 28 599 35 34. 29 120 32. 68 1014 979693. 31 . 73 -5. 44 -89. 71 600 35 33. 03 120 31. 92 1062 979689. 55 & 74 -2. 89 - 38. 81 601 35 31. O7 120 31. 27 1114 979687. 29 . 78 2. 58 - 35. 14 602 35 31. 99 120 33. 48 1104 979692. 60 . 90 5. 59 -S31. 62 603 35 30. 44 120 37. 25 1478 979658. 73 1. 75 9. 10 -40. 15 604 35 30. 11 120 38. 92 848 979705. 27 1. 05 -8. 15 -S31. 38 605 35 31. 55 120 37. 94 1334 979667. 06 1. 98 2. 81 -41. 75 606 35 32. 44 120 36. 76 1249 979677. 67 1. 18 3. 66 -38. 27 607 35 33. 07 120 35. 38 1129 979687. 44 1. 24 1. 25 - 36. 48 608 35 32. 85 120 33. 58 1052 979693. 50 . 80 . 88 -35. 14 609 35 31. 24 120 33. 11 1327 979679. 76 1. 22 14. 79 -29. 79 610 35 31. 08 120 34. 54 1200 979689. 86 1. 33 13. 17 -26. 92 611 35 32. 50 120 39. 42 1004 979695. 42 1. 21 -1. 72 -85. 17 612 35 32. 10 120 41. 53 922 979708. O1 1. 23 3. T2 - 26. 88 613 35 33. 84 120 40. 04 870 979699. 35 . 87 -12. 30 -41. 47 614 35 34. 49 120 36. 57 915 979698. 75 -9. 59 -40. 41 615 35 35. 80 120 37. 28 925 979695. 82 & 74 -13. 44 -44, 64 616 35 35. 72 120 39. 21 846 979700. 92 . 73 -15. 66 -44. 14 617 35 36. 94 120 39. 48 807 979703. 88 . 70 -18. 10 -45. 27 618 35 36. 48 120 36. 52 820 979704. 22 . 83 -15. 89 - 43. 37 619 35 35. 65 120 34. 83 1063 979689. 24 1. 14 -6. 83 -42. 38 620 35 34. 19 120 31. 19 1211 979674. 19 . 90 -5. 88 -46. 78 621 35 35. 43 120 30. 66 1385 979665. 13 1. 08 -. 84 -47. 06 624 35 36. 52 120 30. 29 1335 979669. 80 1. 03 -1. 983 -46. 97 625 35 39. 26 120 31. 97 998 979687. 23 . 76 -20. 09 -53. 78 626 35 39. 18 120 30. 43 855 979697. 58 . 90 -23. 08 -51. 70 679 35 55. 63 120 30. 12 1861 979671. 46 1.37 21. 97 -40. 85 680 35 55. 31 120 32. 03 2601 979625. 11 3. 14 45. 66 -40. 85 681 35 30. 24 120 43. 68 1636 979665. 74 3. 94 31. 25 -21. 25 682 35 30. 11 120 45. 59 1979 979649. 00 5. T2 46. 95 -15. 58 683 35 30. 92 120 47.11 1880 979654. 23 5. 44 41. 72 -17. 68 684 35 30. 85 120 44. 46 1591 979669. 78 3. 02 30. 19 -21. 68 685 35 32. 30 120 43. 29 871 979707. 33 ll -2. 04 -31. 00 686 35 33. 26 120 483. 53 876 979708. 37 1. 05 -1. 89 -31. 08 687 35 35. 46 120 44. 48 1078 979703. 23 1. 33 8. 84 -27. 04 688 35 36. 77 120 42. 37 1286 979673. 45 2. 69 -8. 24 -44. 983 689 35 36. 72 120 45. 22 1624 979669. 18 2. 63 24. 35 -29. 05 B9 B10 GEOPHYSICAL FIELD INVESTIGATIONS TABLE 8.-Principal facts for gravity stations-Continued Observed Terrain Free air Complete Station Lat N. Long W. Elevation (ft) gravity (mgal) correction anomaly Bouguer anom- (mgal) (mgal) aly (mgal) 690 35 38. 88 120 45. 33 1721 979668. 99 2. 69 30. 21 -26. 47 691 35 38. 23 120 42. 44 1125 979701. 90 1. 94 7.99 -28. 90 692 35 39. 67 120 43. 33 1127 979713. 00 2. 05 17. 23 -19. 62 693 35 40. 44 120 44. 12 1108 979716. 20 1. 96 17. 55 -18. 74 694 35 41. 44 120 48. 23 895 979719. 26 1. 09 =:. 80 -30. 66 695 35 42. 87 120 42. 63 838 979713. 40 1. 35 -14. 11 -41. 69 696 35 44. 14 120 40. 12 677 979725. 31 . 84 -19. 15 -A41. 69 697 35 43. 07 120 38. 36 678 979723. 30 «1 -19. 54 -42, 24 698 35 43. 94 120 36. 23 918 979708. 12 . 69 - 13. 39 -44. 39 699 35 44. 92 120 38. 10 977 979707. 37 . 96 -9. 99 -42. 75 700 35 41. 30 120 38. 38 783 979712. 65 . 60 -17. 80 -44. 23 TOL 35 42. 39 120 40. 24 836 979711. 02 . 88 -15. 99 -43. 98 702 35 39. 56 120 40. 26 768 979714. 48 . 66 - 14. 90 -40. 76 703 35 40. 43 120 39. 45 769 979712. 38 . 61 -18. 14 -44. 09 704 35 39. 46 120 37. 88 783 979707. 39 . 62 -20. 44 - 46. 85 705 35 39. 57 120 36. 28 891 979696. 69 . 68 -21. 14 -51. 22 706 35 40. 45 120 36. 27 832 979702. 99 . 60 -21. 64 -49. 76 707 35 39. 57 120 34. 67 898 979695. 06 . 62 -22. 11 -52. 49 708 35 40. 99 120 32. 98 805 979701. 36 . 69 -26. 58 -53. 68 709 35 41. 80 120 31. 00 1113 979681. 77 . 93 -18. 35 - 55. 84 710 35 43. 26 120 30. 28 1393 979667. 87 1.18 -1. 99 - 54. 88 711 35 45. 74 120 30. 70 1819 979646. 36 3. 30 7. 08 -52. 42 712 35 44. 34 120 31. 89 1511 979662. 77 1.17 -3. 54 -53. 90 713 35 42. 64 120 32. 65 1132 979684. 06 1. 10 -15. 47 -58. 44 714 35 41. 46 120 34. 88 750 979708. 67 . 70 -25. 11 - 50. 30 715 35 42. 21 120 36. 09 728 979714. 55 74 -22. 37 -46. 76 716 85 43. 90 120 34. 12 933 979704. 12 . 95 -15. 92 -47. 18 717 35 53. 25 120 45. 86 798 979730. 34 1. 35 -15. 73 -41. 93 718 35 55. 01 120 46. 90 997 979722. 98 1, 54 - 6. 88 -39. 76 719 35 55.38 120 48. 39 1159 979712. 75 1. 53 -2. 41 - 40. 88 720 35 56.15 120 49. 31 1089 979720. 61 1. 53 -2. 28 -38. 29 721 35 56. 13 120 50. 36 894 979733. 23 1. 08 -71. 92 -37. T1 T19 35 48. 97 120 31. 28 1474 979673. 31 1. 46 -38. 07 -52. 47 782 35 46. 39 120 32. 76 1194 979691. 64 1. 23 -7. 40 -47. 88 783 35 53. 31 120 48. 11 974 979714. 04 1. 10 -15. 56 -48. 08 784 35 57. 68 120 49. 89 607 979754. 74 1. 26 -15. 62 - 35. 82 785 85 56. 56 120 45. 65 1008 979727. 17 1. 76 -3. 87 - 36. 91 786 35 57. 84 120 46. 82 1074 979722. 59 1. 52 -4. 07 -39. 63 787 35 59. 15 120 45. 25 1626 979681. 75 3. 34 5. 13 -47. 63 788 35 58. 85 120 48. 46 1202 979713. 53 1. 92 -2. 54 -42. 10 789 35 59. 18 120 50. 80 672 979752. 28 1. 20 -14. 11 -36. 11 790 35 58. 43 120 54. 61 898 979729. 83 2. 33 -14. 23 -42. 90 791 35 54. 44 120 41. 77 1001 979718. 04 1. 43 - 10. 63 -43. 76 792 35 52. 31 120 42. 04 909 979718. 37 1. 37 -15. 92 - 45. 93 798 35 49. 78 120 42. 23 754 979723. 85 1. 17 -21. 41 -46. 27 794 35 48. 82 120 43. 45 1256 979688. 25 2. 95 -8. 42 -48. 82 795 35 50. 92 120 43. 58 1129 979699. 66 1. 49 -11. 95 -49. 48 796 35 55. 28 120 44. 74 1506 979690. 75 2. 05 8. 37 -41. 54 797 35 56. 69 120 42. 75 1819 979668. 60 2. 67 13. 64 -46. 43 798 35 57. 30 120 40. 86 1961 979659. 84 4. T7 17. 37 - 45. 49 799 35 58. 53 120 41. 15 2254 979636. 59 5. 26 19. 91 -52. 54 800 35 57. 05 120 44. 03 1410 979698. 86 1. 66 4. 93 -42. 07 801 35 58. 19 120 44. 05 1225 979709. 95 1. 74 -8. 01 -43. 55 802 35 56. 16 120 40. 42 1124 979714. 66 2. 35 -4. 90 -41. 35 803 35 57. 42 120 38. 89 1232 979714. 38 1. 74 3. 18 -37. 60 804 35 59. 20 120 38. 10 1754 979681. 87 2. 00 17. 22 -41. 29 805 35 59. 47 120 39. 92 2179 979647. 74 3. 36 22. 67 -49. 11 806 35 53.15 120 39. 50 1476 979678. 90 1:11 -3. 26 -53. 08 807 35 54.98 120 38. 24 2039 979646. 15 3. 08 14. 33 -52. 91 808 35 56. 49 120 36. 87 2079 979659. 41 1:72 29. 19 -40. 78 809 35 59. 94 120 36. 66 1837 979677. 93 1. 49 20. 02 -41. 85 811 35 59. 88 120 34. 03 2285 979647. 74 2. 07 27. 35 -47. 64 812 35 57. 04 120 31. 79 2066 979658. 74 1. 45 26. 51 - 43. 28 813 85 58. 25 120 33. 47 1665 979678. 04 2. 08 13. 51 -41. 85 814 35 51. 36 120 35. 52 1315 979684. 78 1. 52 -9. 97 -53. 83 899 35 46. 14 120 40. 52 1147 979697. 11 1. 69 -5. 99 -43. 89 900 35 46. 98 120 38. 88 1421 979676. 90 1. 44 -1. 63 -49. 23 901 35 48. 290 120 39. 17 1462 979674. 53 1. 64 -2. O1 -50. 82 902 35 48. 25 120 41. 21 1406 979678. 26 3. 29 -3. 49 -48. 72 903 35 50. 07 120 39. 77 1371 979682. 80 1. 65 -4. 84 -50. 50 904 35 51. 41 120 40. 90 1311 979693. 74 1. 25 -1. 45 -45, 45 905 85 51. 42 120 39. 01 1427 979679. 83 1. 36 -4. 47 -52. 35 906 35 49. 59 120 37. 78 1615 979663. 64 2. 46 -i87 -53. 63 GRAVITY AND GEOLOGY OF THE PASO ROBLES AREA, CALIFORNIA Bil Tasos 3.-Principal facts for gravity stations-Continued Observed Terrain Free air Complete Station Lat N. Long W. Elevation (ft) gravity (mgal) correction anomaly Bouguer anom- (mgal) (mgal) aly (mgal) 907 '35 46. 96 120 36. T7 1418 979677. 04 1. 49 -1. 75 -49. 19 908 35 49. 70 120 36. 48 1555 979666. 95 1. 99 -2. 86 -54. 52 909 35 52. Ol 120 36. 87 1510 979673. 59 1. 25 -8. 74 -54. 59 910 35 53. 46 120 36. 15 2305 979621. 35 5. 28 16. 71 -57. 48 911 35 54. 72 120 35. 96 2058 979655. 48 2. 08 25. 34 -43. 38 912 35 55. 84 120 34. 83 2540 979626. 53 4. 38 40. 59 -42. 58 915 35 53. 04 120 30. 12 2652 979614. 69 3. 79 43. 28 - 44. 38 947 35 53. 86 120 44. 15 1025 979719. 97 1. 44 -5. 62 -39. 56 948 35 55. 22 120 42. 56 1654 979678. 04 2. 06 9. 67 -45. 38 951 35 54. 04 120 54. 75 816 979708. 62 1. 13 -36. 88 -63. 93 952 35 54. 46 120 56. 07 862 979707. 45 1. 45 - 34. 33 -62. 64 953 35 55. T0 120 55. 39 1290 979685. 80 2. 06 -17. 49 -59. 95 954 85 55.28 120 56. 47 968 979704. 28 1. 84 - 28. 63 -60. 20 955 35 55.60 120 55.39 1092 979892. 14 1. 60 -29. 76 - 65. 85 956 35 57. 15 120 51. 63 495 979762. 12 1. 32 -18. 02 -33. T9 1033 35 48. 89 120 33. T7 1585 979664. 11 1. 72 -1. 72 -54. 68 1034 35 50. 34 120 33. 67 1653 979661. 22 1. 15 -. 28 -56. 16 1035 35 51. 72 120 33. 06 1785 979652. 69 1. 46 1. 63 -58. 48 1036 35 51. 74 120 52. 91 810 979711. 22 2. 80 -B1. 57 - 56. 74 1037 35 50. 88 120 53. 19 885 979700. 68 1.71 -33. 82 -62. 67 The accuracy of the gravity data undoubtedly varies from station to station. The observed gravity values for the 298 stations read with a LaCoste-Romberg gravity meter are probably accurate to 0.02 mgal after correcting for tidal effects. The values for the 15 sta- tions read with a Worden meter (scale constant about 0.5 mgal) are probably accurate to 0.1-0.2 mgal after correcting for drift. Latitude and longitude were meas- ured to +0.01 minute. Elevation accuracy depends critically on type of source data. Roughly 30 percent of the stations were read at bench marks, and elevation errors for these should be less than 0.5 feet. Ten per- cent are field-checked spot elevations probably accurate to within 1 foot; 55 percent are unchecked spot eleva- tions accurate to within 5 feet. Twenty-four stations were established on the shore of Lake Nacimiento and are estimated to have an elevation accuracy of +1 foot. All gravity data were corrected for terrain effects (at density 2.67 g/cm*) out to a radius of 166.7 km. For the inner zones terrain corrections were made by hand using Hay ford-Bowie templates and dividing each compartment into four subcompartments where correc- tions were large. For the outer zones the corrections were made by computer using a program developed by Don Plouff. The boundary between inner and outer zone corrections was either 5.24 or 2.29 km; 1-minute and 3-minute terrain digitization grids were employed. All basic measurements were reduced to anomaly values using a gravity reduction program developed by S. H. Burch. The basic procedures and formulas of the reduction are as follows: 1. The gravity difference (AG) between the base and a given station is calculated in one of six ways de- pending on the reduction option selected. It is then corrected for tide and drift. : 2. Observed gravity (OG@) = Gravity base value + AG. 3. Theoretical gravity (THG) = 978049 [1 + 0.005228 - 0.0000059sin? (20) ], where 0 = latitude: 4. Free air anomaly (FAA) 0G *- THG + (0.09411549 - 0.000137789sin2g) Z' - 0.000000- 00675, Where E' = elevation. 5. Simple Bouger anomaly (BA) = FAA - 0.01274 pH, where p = reduction density. 6. Curvature correction (CC) = -$B9§XI10O®X EFLST x 10-4 E*. ¢. Complete Bouguer anomaly (CBA) = BA + TCO - CC, where TC = terrain correction. 0.0004462 x E' GRAVITY INTERPRETATION Quantitative interpretation of gravity anomalies here relies largely on a two-dimensional two-layer basement- sediment model. Density contrasts of 0.3 and 0.5 grams per cubic centimeter are commonly used to arrive at maximum and minimum dimensions for various anoma- lous features. The following estimates are used for unit densities: g/om>* Surficial deposits <2. 2 Paso Robles Formation 2.2 Pancho Rico and Santa Margarita Formations _ 2.3 Monterey Shale 2. 8 Tierra Redonda and Vaqueros Formations.... 2. 4 Paleocene and Upper Cretaceous deposits____ 2.5 Basement rocks 2. 6T Based on these values, the density contrasts of 0.3 and 0.5 furnish good approximations to common sediment- B12 basement combinations. A mixed sequence of Cretaceous and Tertiary rocks on basement will have a density con- trast close to 0.3. Quaternary deposits on basement, how- ever, have a density difference of close to 0.5. In arriving at subsurface mass distribution, graticule and various simple mathematical calculations, and inter- pretations using a U.S. Geological Survey modification of Bott's (1960) interpretation program, were fitted to outcrop and well data. GRAVITY ANOMALIES The important features of the gravity map (pl. 1) are associated with basement features east of the Jolon and Rinconada fault zones or with the faults themselves. No major anomalies occur west of the faults, owing in part, perhaps, to the lower station density in this area. Dis- cussion here will treat first the area east of the faults, then the fault zones themselves, and finally the area west of the faults. AREA EAST OF JOLON AND RINCONADA FAULTS The anomalies of the area east of the Jolon and Rin- conada faults are assumed to reflect depth to basement rather than changes in basement density. The largest anomaly is the Hames Valley low, which covers much of the Bradley quadrangle. This feature, representing a deep basement depression, is bounded by the steep gra- dients of the Jolon fault on the west and by the steep basement slope paralleling the Los Lobos fault on the northeast. The data indicate two closed lows 7 miles apart. Analyses of the northern low (fig. 2) using GEOPHYSICAL FIELD INVESTIGATIONS Gauss's theorem and Bott's interpretation program sug- gest a depth to basement at the bottom of the depression of slightly more than 15,000 feet. The sharpness of the southern low may be due to near-surface low-density diatomite deposits as well as to the large depth to base- ment. Just east of the Hames Valley low is a northwest- trending gravity high associated with the San Ardo oil field, where basement rises to within 2,500 feet of the surface (Durham, 1966, pl. 5). § In the San Miguel quadrangle, a similar gravity pat- tern obtains, A broad gravity low, called the Vineyard Canyon low, is separated on the northeast by a steep gradient from another northwest-trending gravity high, called the Cholame Hills high. Here, however, the gra- dient on the northeast is not associated with any known major fault, dips in the surface are gentle, revealing no major syncline, and the gradient on the southwest is ex- tremely gentle. Both the Cholame Hills high and the Vineyard Canyon low extend southeastward into the adjoining Parkfield and Shandon quadrangles (S. H. Burch, W. F. Hanna, and T. W. Dibblee, Jr., unpub. data, 1969). A Bott profile indicates that the depth to basement at the bottom of the Vineyard Canyon low is approximately 10,000 feet and at the top of the Cholame Hills high is less than 2,000 feet. The existence of a large northeast-trending basement fault, here called the Indian Valley fault, is indicated by the mutual truncation of all the previously discussed northwest-trending anomalies along a line extending from upper Indian Valley to the lower San Antonio River valley (pl. 1). The significance of this fault is A EXPLANATION A' 15 - > Observed gravity > ad +30 , oz S [- -30 e Calculated gravity é <3 (on basis of depths pa g given below Tg o n 483] t: 485 i = > E f To -60 - c (6) -75 +78 - % 1000" - r- 1000" a heed Assumed densit r 5; fm: ia... Paas ac,. 0 u E & 10,000" -] wie ra HAMES VALLEY LOW 5. ~185,000; 15,000' o Frours 2.-Gravity and structure profiles through the Hames Valley low and San Ardo ocil-field high, Bradley quadrangle. Structure profile calculated by the U.S. Geological Survey's modified version of Bott's (1960) program. GRAVITY AND GEOLOGY OF THE that a similar transverse fault appears 19-20 miles northwest on the opposite side of the Espinosa fault, and the similarity of the two suggests that the Espinosa is a strike-slip fault with a right-lateral offset of ap- proximately 16 miles (Burch, 1969). Apparently undis- turbed overlying upper Miocene formations date the fault as pre-late Miocene. 'The broad gravity high extending northeast from Wellsona (north of Paso Robles) to Ranchito and Hog Canyons may reflect a slight basement rise in this area. 'The steep gravity nose in the southeast corner of the map is caused by the large granite mass of the La Panza Range that crops out about 1 mile to the south. The gravity low separating the latter two features inter- sects the Vineyard Canyon low near Shandon (S. H. Burch, W. F. Hanna, and T. W. Dibblee, Jr., unpub. data, 1969). Apparent positive gravity anomalies of 2-3 mgal are associated with topographic valleys in the center of the map area. This is illustrated by the high gravity values in the Salinas River valley and its main tributaries (where data are available) from Hames Valley to Es- trella Creek. These highs do not reflect basement fea- tures, but rather are relative highs contrasting with the normally low values caused by the extensive low-density topographic plateaus of Quaternary sedimentary depos- its. Failure to remove these highs before using the Bott program will result in sharp basement ridges coincident with the topographic valleys. The gravity data reveal three unexplored areas where basement structure might result in conditions favorable to accumulation of oil. 1. The steep gravity gradient on the west side of Hames Valley is interrupted by a gently sloping bench about 1 mile wide. The depth to basement sug- gested by the low gravity values requires a base- ment ridge of moderate size. Figure 2 shows a Bott program interpretation of the feature. The half width of the anomaly (regional removed) suggests a maximum depth of 7,000 feet to the center of the ridge. 2. The southeast extension of the San Ardo oil-field high indicates a basement high that has not been fully explored. This high may rise to within 2,500 feet of the surface. 3. The Cholame Hills high indicates a major basement high, also little explored. AREA NEAR JOLON AND RINCONADA FAULTS The alinement of the steep linear gravity gradients northwest and southeast of the Paso Robles granite body PASO ROBLES AREA, CALIFORNIA B13 indicates that the Jolon and Rinconada fault zones prob- ably join. The Rinconada is of major importance since in the San Luis Obispo quadrangle immediately to the south, it probably separates granitic basement on the east from Franciscan basement on the west (E. W. Hart, oral commun., 1967). Thus the Jolon fault may bear the same relation to these major basement units within the map area. Evidence from the Bryson quadrangle to the west (Burch, 1969), however, suggests that the base- ment contact branches westward from the Jolon fault and roughly follows the thrust fault east and north of Lake Nacimiento. Despite the deflection of the gravity gradient around the east side of the Paso Robles granite body, the fault passes west of the body and, at the surface, defines its western contact. The deflection is caused by the greater density contrast against the Paso Robles Formation on the east than against the Miocene strata to the west. AREA WEST OF JOLON AND RINCONADA FAULTS The area west of the Jolon and Rinconada faults apparently contains no major gravity anomalies. Pro- ceeding northeast from the southwest corner of the map, a rather even gravity gradient of about -2 mgal per mile bottoms out in a syncline where Monterey Shale is exposed, then reverses and ascends an anticline of Cretaceous and lower Tertiary beds, and finally drops off over the Jolon fault. Both the syncline and anticline are long, broad features without much gravity relief. Perhaps the most significant aspect of the gravity data in this area is the lack of gravity expression of the Nacimiento fault. This fault is generally considered to be the boundary between the eugeosynclinal Franciscan basement on the southwest and the granitic and meta- morphic basement of the Salinian block on the northeast, and thus it is thought to have a significance similar to that of the San Andreas fault. Yet, the following evi- dence suggests that in the map area the Jolon, rather than the Nacimiento, separates the major basement units: 1. The dips are low and gravity expression negligible on the Nacimiento fault. 2. The dips are steep and gravity expression sharp on the Jolon fault. 3. The steep gravity gradient associated with the Rin- conada fault, which marks the contact of the granitic and Franciscan basement blocks to the south, joins the Jolon fault rather than the Nacimiento fault. B14 ~GEOPHYSICAL FIELD: INVESTIGATIONS REFERENCES CITED Bailey, E. H., and Everhart, D. L., 1964, Geology and quicksilver deposits of the New Almaden district, Santa Clara County, California: U.S. Geol. Survey Prof. Paper 360, 206 p. Bailey, E. H., Irwin, W. P., and J ones, D. L., 1964, Franciscan and related rocks, and their significance in the geology of west- ern California : California Div. Mines and Geology Bull. 183, 177 p. Bott, M. H. P., 1960, The use of rapid digital computing methods "for direct gravity interpretation of sedimentary basins: Royal Astron. Soc. Geophys. Jour. [London], v. 3, no. 1, .. ~. p. 63-67. Burch, S. H., 1969, Complete Bouguer gravity and general geol- ogy of the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, California: U.S. Geol. Survey Prof. Paper 646-A (in press). Chapman, R. H., 1966, The California Division of Mines and Geology gravity base station network: California Div. Mines and Geology Spec. Rept. 90, 49 p. Christensen, F. W., 1963, Petrography of some basement rocks © in the King City area, in Guidebook to the geology of Salinas Valley and the San Andreas fault, Am. Assoc. Petroleum Geologists - Soc. Heon. Paleontologists and Mineralogists, Pacific Sec., Ann. Spring Field Trip, 1963 : p. 110-112. Compton, R. R., 1966, Granitic and metamorphic rocks of the Salinian block, California Coast Ranges: California Div. Mines and Geology Bull. 190, p. 277-287. Durham, D. L., 1965, Evidence of large strike-slip displacement along a fault in the southern Salinas Valley, California, in Geological Survey research, 1965: U.S. Geol. Survey Prof. Paper 525-D, p. D106-D111. 1966, Geology of the Hames Valley, Wunpost, and Valle- ton quadrangles, Monterey County, California: U.S. Geol. Survey Bull. 1221-B, 53 p. Jennings, C. W., 1958, Geologic map of California, Olaf P. Jenkins edition, San Luis Obispo sheet: California Div. Mines, scale 1 : 250,000. . Kilkenny, J. E., chm., and others, 1952, Cenozoic correlation see- tion, Salinas Valley from San Antonio River northerly to San Andreas fault through San Ardo oil field, California : Am. Assoc. Petroleum Geologists, Pacific Sec., scale 1 inch to 1,000 feet. Reed, R. D., 1983, Geology of California : Tulsa, Okla., Am. Assoc. Petroleum Geologists, 855 p. Taliaferro, N. L., 1943, Geologic history and structure of the central Coast Ranges of California: California Div. Mines Bull. 118, p. 119-163. 7 DAY Gravity, Magnetics, and Geology of the San Andreas Fault Area 15 Near Cholame, California ? ’¢é—aGEOLOGICAL s U R V E Y P R O FES SION A L P A PER 646 -C / I On; «& ~$ence ues \ Gravity, Magnetics, and Geology of the San Andreas Fault Area Near Cholame, California By W. F. HANNA, S. H. BURCH, and T. W. DIBBLEE, JR. GEOPHYS§ICAL FIELD INVESTIGATIONS CEOLOGICAL SURVEY PROFESSIONAL P A PER 64 6 -C UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600080 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 2401-2170 CONTENTS Page Abstract C1 Introduction .. ___.... __. Z ci 1 Methods of investigation ____.__-____________._._____. 2 Gravity SUrvey 2 Gravity interpretation _______________________- 7 Magnetic surveys 8 Magnetic interpretation ___________________---- 8 (Geologic SELLING 8 ROCK UNITS 4. ncn a -am 9 Plutonic and metamorphic rocks ___________---- 9 Ultramafic roCk$ 9 Franciscan rocks .-.. 11 Cretaceous and Eocene miogeosynclinal rocks ____ 11 Middle and upper Tertiary rocks ______________ 12 Distribution 12 Volcanic TOCKS 12 Lower Miocene sedimentary rocks _______--- 12 Monterey Shale. 12 Upper Miocene and Pliocene sedimentary rocks e te- 13 Upper Cenozoic valley deposits _____________--- 13 Rock units-Continued Surficial deposits _____________________________ Geologic structure and tectonics __________-__-------- Structural setting Structural history Diablo Range - Parkfield-Turkey Flat area ________________-____ Temblor Range -- < == Gold Hill area .____.__._-- Southwest of San Andreas fault _____________._ Gravity and magnetic features _______________------ General patterns - Gravity trends Magnetic trends __________________________ Anomalies northeast of San Andreas fault ___---- Gravity high at Cholame Valley _________-- Anomalies at Table Mountain _________----- Magnetic high at Palo Prieto Pass _______-- Other gravity features ________________z_-__ Gravity anomalies southwest of San Andreas fault Summary - References ___-- ILLUSTRATIONS [Plates 1-4 are in pocket] PLATE p 5 p p a & FIGURE go po Index map showing area investigated ________---- Complete Bouguer gravity and generalized geologic map along the San Andreas fault near Cholame, Calif. Aeromagnetic and generalized geologic map along the San Andreas fault near Cholame. Vertical-intensity ground magnetic and generalized geologic map along the San Andreas fault near Cholame. Idealized cross sections, gravity profiles, and magnetic profiles across the San Andreas fault near Cholame. Comparison of computed gravity anomaly of model with observed anomaly near Cholame Valley _______- . Comparison of computed magnetic and gravity anomalies of models with observed anomalies at Table Moun- tain Ges niente. 4. Comparison of computed total-intensity magnetic anomalies of models with observed aeromagnetic anomaly at Palo Prieto Pass __________ wes 5. Comparison of computed total-intensity and vertical-intensity magnetic anomalies of models with observed aeromagnetic and ground magnetic anomalies at Palo Prigto PRGS o Map showing sampling sites ___________-_-------- 7. Comparison of computed magnetic and gravity anoma PASE . l _lies Sf a model—zvith—Sbsnged_anom—a_1ies_ at Palo P;ieto TABLES TABLE Principal facts for gravity bases ___ Principal facts for gravity stations _____________-- 1 B 3. Selected exploratory wells 4. Summary of magnetic susceptibility data _ III Page C13 13 13 14 15 15 15 15 16 16 16 16 16 16 16 17 20 25 25 27 27 Page C2 17 18 20 28 26 GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY, MAGNETICS, AND GEOLOGY OF THE SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA By W. F. Hanna, S. H. BurcH, and T. W. DIBBLEE, JR. ABSTRACT Complete Bouguer gravity, aeromagnetic, and ground mag- netic data interpreted in light of geologic mapping and well data within a 30-minute quadrangle centered near Cholame, Calif., express highly contrasting subsurface rock and struc- tural patterns on opposite sides of the San Andreas fault. The geologic terrane northeast of the fault is composed of pervasively sheared Franciscan sedimentary and volcanic rocks of Mesozoic age and ultramafic rocks overlain by a thick series of severely deformed Cretaceous and Tertiary sedimentary rocks and locally deformed Quaternary valley sediments. The terrane southwest of the fault has a base- ment of Mesozoic plutonic and metamorphic rocks overlain by gently folded Tertiary sedimentary rocks and Quaternary valley sediments. Northeast of the San Andreas fault the geophysical data reveal the presence of (1) a major gravity high between Parkfield and Cholame, associated with relatively dense, non- magnetic Franciscan and Cretaceous miogeosynclinal rocks overlain by Cenozoic rocks in the northeastern Diablo Range and in contact with low-density Cenozoic rocks southwest of the fault, (2) an elongate magnetic ridge and gravity trough at Table Mountain, probably generated by a thick tabular body of serpentinite dipping steeply north-northeast, inferred to be the main source of the serpentinite extrusion along this mountain, (3) a major elliptical magnetic high in an area of low density gradient near Palo Prieto Pass, probably associated with a large, deeply buried serpentine-rich body bounded on the southwest by the vertical San Andreas fault and on the northeast by a northeast-dipping contact or fault with nonmagnetic rocks, and (4) a northwest-trending grav- ity low over the Kettleman Plain, probably produced by a thickening of low-density Cenozoic sedimentary rocks near the axis of a major syncline. Southwest of the San Andreas fault the data show (1) a local gravity high north of Red Hills, associated with an uplift of plutonic and metamorphic rocks in these hills on the northeast side of the San Juan fault, (2) a prominent southwest-sloping northwest-trending gravity gradient south- east of Red Hills, which suggests a buried basement fault or a steep subsurface contact between basement and sedimen- tary rocks extending from the San Juan fault southeastward to the San Andreas fault near the southern end of the quad- rangle, (3) a discontinuous northwest-trending gravity trough east and north of Shandon, probably associated with a depression of the basement surface under an area near San Juan Creek, and (4) a broad northeast-trending gravity plateau west-southwest of Shandon, probably associated with a thinning of Cenozoic sedimentary rocks overlying a shallow basement surface that slopes from the extensive basement exposures of the La Panza Range southwest of the quad- rangle. INTRODUCTION This report considers the relationship of Bouguer gravity data, total-intensity aeromagnetic data, and vertical-intensity ground magnetic data to general- ized geology in an area critical to an understanding of the San Andreas fault. The gravity data are pre- sented as a 2-mgal (milligal) complete Bouguer anomaly map overprinted on the geologic map. These data were collected by S. H. Burch as part of a regional survey for the San Luis Obispo 1 :250,000 sheet (Burch and others, 1971). The magnetic data include a 20-gamma reconnaissance aeromagnetic map supported in much of the area by ground mag- netic coverage. These data are part of a broader study of the San Andreas fault by Hanna and oth- ers (1972). The geology was mapped by T. W. Dib- blee as part of a regional geologic mapping project of the southern Coast Ranges adjacent to and near the San Andreas fault. Detailed geologic mapping by Dickinson (19662, 1966b) of part of the area of this report was incorporated. The mapped area, centered near Cholame, Calif. (fig. 1), contains a 42-mile section of the San An- dreas fault. It is a 30- by 30-minute block covering nearly 1,000 square miles in the southern Coast Ranges. It is composed of the Orchard Peak, Park- field, "Reef Ridge," and Shandon 15-minute quad- rangles. "Reef Ridge" is the name here applied to the unpublished 15-minute quadrangle containing C1 C2 GEOPHYSICAL FIELD INVESTIGATIONS 121" 120° FRESNO | OCoalinga /) / KINGS King City 0 MONTEREY Burch, (1970) Burch and| Durham CAPE 1970) SAN BRYsoN | BRADLEY martin | _ - \ SAN PAS FIEDRAS "__ ADELAIDA € BLANCAS SIMEON ROBLES SAN LUIS OBISPO 35%- . Santa Maria 0 15 30 MILES (oor... t_] | | FIGURE 1.-Map showing area investigated. the Garza Peak, Kettleman Plain, Pyramid Hills, and Tent Hills 7!4-minute quadrangles. The authors are indebted to Holly C. Wagner, U.S. Geological Survey, who provided data from ex- ploratory wells essential for much of the subsurface interpretation. We also benefited from discussions with R. D. Brown, Andrew Griscom, D. C. Ross, R. G. Coleman, M. C. Blake, and Ivan Barnes, of the U.S. Geological Survey, and with Professor B. M. Page of Stanford University. TABLE 1.-Principal facts for METHODS OF INVESTIGATION GRAVITY SURVEY The map area includes 268 gravity stations tied to 11 base stations. Principal facts for the base sta- tions are listed in table 1; those for all other sta- tions, in table 2. The data are referenced to Califor- nia Division of Mines and Geology Base Station 173 (Chapman, 1966, p. 36) at the U.S. Geological Survey headquarters in Menlo Park, Calif. The ob- served gravity at this base, determined by numer- ous ties to the North American Gravity Standardi- zation Stations at the San Francisco International Airport, is taken to be 979,958.74 mgal. The precision of the gravity data varies from sta- tion to station. The observed gravity measurements for the 219 stations read with LaCoste-Romberg gravity meters have a precision of about 0.03 mgal after correcting for lunar and solar tidal effects. Observed gravity values of the 49 stations read with a Worden gravity meter (scale constant about 0.5 mgal per scale division) have a precision of about 0.1 to 0.2 mgal after correcting for instru- ment drift. Latitude and longitude were measured to within 0.02 minute by using Geological Survey topographic maps. Elevation accuracies vary consid- erably according to the types of source data. Roughly 60 percent of the stations were read on or near bench marks where elevations are precise to at least 0.5 feet. Another 20 percent are field-checked spot elevations precise to about 2 feet, and 20 per- cent are unchecked photogrammetric elevations pre- cise to about 5 or 10 feet, depending on the map contour interval. Three elevations were established by altimetry with an estimated precision of 10 to 20 feet. All gravity data were corrected for terrain effects (by using the standard density 2.67 g/em*) to a dis- tance of 166.7 km (kilometer) from the gravity sta- tion. For the inner zones (within 2.29 km of the bases used in gravity survey [Observed gravity: The amount of scatter among nfimerous ties between these bases suggests that the relative gravity of each is known to be +0.02 milligals. Description: USC&GS, U.S. Coast and Geodetic Survey; USGS, U.S. Geological Survey; BM, bench mark] Elevation (ft) Observed Base Latitude N. Longitude W. gravity Description SLUKB .._. 86° 7.795 121° 1.12° 405.8 979,798.04 USC&GS BM G154 at San Lucas. BREADB .._..__ 85° 51.81" 120° 47.783" 552.0 979,737.30 USGS BM 553 at Bradley. PSROB ...... 85° 87.55 120° 41.28" 720.0 979,717.17 USC&GS BM L24 at Paso Robles. BLAKB .._... 85° 86.92' 119 ©52.01' 643.2 979,675.00 USC&GS BM R11 at Blackwell's corner. CLNGB ._.:.____. 86° 8.785° 120° 21.18" 667.0 979,740.39 USC&GS BM J156 at Coalinga. .... 85° 59.54' 120° 7.65 754.0 979,714.30 USC&GS BM W11 at Avenal. PARKE ..._____ 35° 58.98' 120° 25.92' 1,585.0 979,687.69 USC&GS BM F79 at Parkfield. SHANB __ 85° 89.82" 120° 22.71" 1,088.0 979,686.02 USC&GS BM W559 at Shandon. CHLMB ._.... 35° 44.05" 120° 17.26" 1,187.0 979,699.65 USC&GS BM A624 at Cholame. CchRZLOB ______ 85° 83.44' 120° 6.38' 1,664.0 979,644.84 USC&GS BM J616 in sec. 23, T. 27 S., R. 17 E. KECKB _._. 85° 40.27' 120° 4.85 845.0 979,7183.49 USC&GS BM N559 at Kecks Corner. SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C3 TABLE 2.-Principal facts for gravity stations [Gravity data is in milligal] Station Latitude Longitude Elevation Observed Terrain Free Complete N. w. (ft) gravity correction air Bouguer yet aes ernie anomaly anomaly Deg Min Deg Min 627 35 35.40 120 28.88 1,353.0 979,666.62 0.93 -1.82 -47.58 85 35.27 120 27.28 1,407.0 979,662.22 15 -.96 -48.176 649 3-.-..:>. 35 59.82 120 28.21 3,506.0 979,564.67 7.93 63.88 -48.93 35 58.81 120 27.22 2,749.0 979,612.84 3.39 42.31 -49.04 651 .... 85 39.54 120 29.21 976.0 979,691.53 .64 -18.26 -B1.81 GPZ 35 39.55 120 27.00 973.0 979,695.84 A2 -14.24 -47.11 35 39.54 120 24.75 1,000.0 979,692.41 .69 -15.12 - 48.95 SHANB _...__. 35 39.33 120 22.71 1,038.0 979,686.00 .65 -17.66 -52.84 35 40.39 120 20.97 1,078.0 979,683.20 1.03 -18.67 -54.68 35 41.98 120 19.39 1,102.0 979,690.53 1.06 -10.88 -47.86 35 42.86 120 18.27 1,164.0 979,689.89 .98 -6.94 -46.14 CHLME ---. 35 44.05 190 17.26 1,137.0 979,699.68 17 -1.89 -39.86 35. 44.27 120 15.85 1,243.0 979,700.64 .87 9.23 -32.80 660.2. 35 44.50 120 13.98 1,677.0 979,673.41 .99 22.49 -84.37 35 44.01 120 12.43 1,518.0 979,682.95 1.37 - 83.28 662 ........-.~« 35 48.16 120 10.71 1,308.0, 979,695.56 2.04 11.84 -31.26 665 °° ---.. 85 42.13 120. 8.81 1,097.0 979,701.43 1.33 -.66 -87.20 661 c--._.__.: 85 41.23 120 - 6.88 917.0 979,710.31 .86 -4.48 - 88.23 KECKE ______ 35 40.27 120 - 4.85 $45.0 979,713.95 54 -9.20 -87.83 6066 35 39.22 190 267 745.0 979,715.57 52 -15.49 -40.69 6672 /-- 35 38.09 120 .36 746.0 979,699.09 .39 -80.27 -55.64 660. 85 41.49 120 - 8.38 750.0 979,720.83 AO -12.99 -88.49 G70. ._...--~--~ 35 42.40 120 2.25 722.0 979,723.08 .35 -14.67 -839.25 6A 35 483.63 120 .92 662.0 979,721.55 .36 -23.60 -46.10 612 85 45.55 120 18.28 1,142.0 979,700.05 178 -2.68 -41.37 610 85 46.71 120 19.74 1,154.0 979,704.45 84 119 -87.80 85 48.35 120 21.27 1,202.0 979,705.05 .81 8.97 -36.71 CTB a...... 35 49.11 120 22.72 1,266.0 979,700.96 1.02 4.81 -37.86 GTO 85 50.79 120 22.72 1,309.0 979,705.51 1.06 11.01 -83.10 6Ti 85 52.45 120 24.71 1,449.0 979,693.71 1.16 10.01 -38.83 PARKE ._...: 35 53.98 120 25.92 1,585.0 979,687.56 1.23 9.76 -41.97 yea sol...". 850 87.81 120 23.93 1,146.0 979,680.19 54 -11415 -50.16 85 85.11 120 25.59 1,159.0 979,677.77 7 -8.50 -47.74 heady ...... 35 33.38 120 28.44 1,533.0 979,652.15 1.04 2.80 -49.05 mobail... 35 32.94 120 29.52 1,173.0 979,676.27 19 -5.60 -45.30 aO 35 31.36 120 29.65 1,150.0 979,678.31 «15 -3.48 -42 42 Tedi 35 830.13 120 28.19 1,318.0 979,665.45 .87 1.21 -43.40 ToS L.... 85 31.89 120 26.90 1,268.0 979,666.21 .84 -5.28 -48.15 d 85 30.58 120 26.03 1,586.0 979,651.70 1.14 7.82 -44.53 (90 «...._._.-__ 85 831.84 120 24.20 1277.0 979,662.51 18 -8.02 -51.81 ToL 85 830.75 120 23.82 1,323.0 979,659.76 .82 -4.89 -49.783 92 Mean ce- 35 33.34 120 24.81 1,252.0 979,666.05 .86 -8.96 -51.31 Nop _..._-L~-. 35 36.91 120 26.08 1,063.0 979,686.62 78 -11.24 -47.16 (O4 _s 85 36.74 120 28.46 1441.0 979,660.36 .95 -1.71 -50.48 TOD. awe. 85 38.04 120 28.95 908.0 979,696.57 1.05 -17 48 -47.178 790 naw. cu.. 35 42.46 120 22.63 1,428.0 979,665.60 1.03 -5.83 -54.08 191. st - 85 41.27 120 23.39 1,241.0 979,675.54 1.09 -11.79 -58.53 OB) 35 44.00 120 24.09 1,760.0 979,647.48 .98 5.08 -54.70 (89 35 44.58 120 25.54 1,864.0° 979,640.60 1.55 7.29 -55.52 "a40>.._s..-._-... 35 43.03 120 25.53 1,426.0 979,666.64 .95 -5.19 -54.05 35 41.79 120 26.08 1462.0 979,662.66 1.21 -4.62 -53.86 T4 35 41.49 120 28.14 1,356.0 > 979,668.91 1,29 -7.91 -53.42 (49 as 85 40.90 120 29.45 1,252.0 979,672.71 1.10 -13.06 -55.16 "4C 35 42.47 120 29.53 1,199.0 979,678.63 .81 -14.86 -54.98 "45 &...._._-,.-.. 90 44.70 120 28.85 1,755.0 979,649.35 1.79 5.48 -58.27 740. lsc Lee 85 48.40 120 28.31 1,541.0 979,658.80 1.03 -8.34 -55.48 T 4Ties.cececess 35 45.60 120 22.09 2,113.0 979,629.27 1.80 17.78 -53.28 "AB: 85 46.71 120 21.56 1,813.0 979,657.81 2.87 16.53 -43.64 49 ...s... 85 45.89 120 20.97 1,681.0 979,664.63 1.24 12.10 -44.65 TOV .-. «-c 85 37.81 120 21.51 1,125.0 979,675.34 40 -17.97 -56.40 NOL 35 36.44 120 20.68 1,130.0 979,670.81 10 -20.08 -58.39 T Oe so 35 35.39 120 18.18 1,194.0 979,664.16 1.01 -18.62 -58.82 458 35 34.28 120 16.46 1,238.0 979,661.25 1.00 - 16.41 -58.14 C4 GEOPHYSICAL FIELD INVESTIGATIONS TABLE 2.-Principal facts for gravity stations-Continued Station Latitude Longitude Elevation Observed Terrain Free Complete N. w. (ft) gravity correction air Bouguer bes" . Nin Des Mip anomaly anomaly 1084 35 33.39 120 15.22 1,269.0 979,658.12 0.86 -15.36 -58.30 700 1-21... save. 35 836.86 120 18.90 1,271.0 979,661.19 1.26 17,04 -59.64 100° 85 37.99 120 15.49 1,826.0 979,647.51 1.39 19.87 -«41.72 TOT 85 37.22 120 18.03 1,383,0 979,657.28 1.45 -10.98 708 _o. 85 34.53 120 20.17 1,163.0 979,665.98 19 -19.09 -58.44 789° 2-......__ 85 33.20 120 19.94 1,208.0 979,661.94 .84 =17.01 -57.86 760 __s.___.__ 85 32.20 120 20.02 1,234.0 979,660.86 92 *14.22 -55.89 701 85 830.31 120 19.94 1,284.0 979,660.26 .95 - 7.48 -50.80 102 L.L.._.3.. 35 31.46 120 18.88 1,717.0 979,626.42 1.53 -~2.19 -59.89 108 - 85 31.51 120 15.97 1,410.0 979,645.78 1.05 -11.82 -5690 42 164 85 30.11 120 16.46 1,703.0 979,627.47 .81 -.53 -58.47 165 _i 85. $5.27 120 17.78 1,679.0 979,629.06 1.14 -5.69 -62.47 760% -__-... 85 30.95 120 21.05 1,651.0 979,635.67 1.12 1.58 -54.26 7607. 35 82.94 120 22.00 1,595.0 979,638.38 1.07 - 83.80 -57.76 7108 35 34.24 120. 22.21 1,470.0 979,647.90 1.16 - 1.89 =57.45 769 85 35.64 120 21.86 1,385.0 979,654.16 87 =11.61 -58.53 140 s. S4 IXL 85 45.94 120 24.25 2,055.0 979,635.47 1.93 18.04 -50.89 ___. 35 48.79 120 25.50 2,316.0 979,625.16 2.76 28.22 -48.87 a t 3 85 47.35 1290 - 25.16 2,319.0 979,619.62 3.36 25.01 -51.58 Tis 835 48.36 120 27.21 2,304.0 979,622.39 3.16 24.93 -51.34 774 _ 35 49.96 120 26.52 2,446.0 979,618.85 8.17 32.46 -48.69 75 ___.. _o t 85 51.37 120 25.29 1,644.0 979,679.23 1.42 15.41 -39.89 16 35 53.18 120 27.88 1,944.0 979,663.07 1.93 24.88 -40.24 T1 ul cue _ 85 52.53 120 28.95 2,508.0 $979,622.75 2.66 88.53 - 45.27 TTB ans 85 51.21 120 29.79 1,837.0 979,663.60 2.24 18.16 -42.96 780. 2... 85 49.20 120 29.11 2,035.0 979,639.84 1.71 15.89 -52.58 Tol &.s_....LL 85 47.88 120 29.88 1,938.0 979,641.76 2.47 11.35 -53.02 sis -:. _ 2 35 32.36 120 18.76 1,264.0 979,657.16 .83 -15.33 -58.12 816. -~. .>: 85 81.44 120 13.04 1,293.0 979,653.73 .85 -14.72 -58.50 sit 85 80.33 120 18.63 1,440.0 979,642.27 74 -10.78 -59.78 sis ___ -__ _"" 85 80.38 120 11.56 1,369.0 979,646.18 .97 -13.62 -59.89 S10 35 80.18 120 - 9.36 1,704.0 979,623.52 97 -4.49 -63.00 $20 L cs-ccelss 35 31.18 120" 7.85 1,950,0 979,612.10 48 5.80 -60.97 sal 85 31.16 120 .- 4.17 1,748.0 979,633.03 92 7.80 =b61.91 S22 _. _ 35 82.15 120 - 5.16 1,681.0 979,639.86 62 6.89 -50.48 CRZOB ______ 35 83.44 120 - 6.38 1,664.0 979,644.80 A8 8.40 -48.53 824 .__.__LL2. 85 33.63 120 . 8.12 1,373.0 979,664.52 67 A8 - 46.23 BaD 35 34.24 120 1.08 1,218.0 979,669.65 .86 -9.84 -51.01 820 L______s.- 85 85.05 120 - 8.07 1,777.0 979,644.68 1.27 16.61 -48.41 B27 35 36.36 120 - 9.48 1,924.0 979,638.77 1.27 22.67 -42.42 828. c 85 35.29 120 10.98 1,927.0 979,642.60 1.18 28.30 - 36.98 889 ___.___L_. 35 39.03 120 10.64 2,196.0 979,632.63 1.48 38.31 - 85.93 850 :-..___/-__ 85 39.82 120 - 9.45 1,861.0 979,654.91 2.01 27.96 -34.22 §a1 -__ l_ 35 40.75 120 - 8.59 1,185.0 979,697.23 1.99 5.38 -32.53 ssa ._ iL I1. 35 41.94 120 16.07 1,526.0 979,668.37 1.05 6.89 -44.71 gas ___ __.. a" 85 40 74 120 1477 1,788.6 979,652.24 1.15 1717 -43.38 §§4.:_ 3:00 " 35 39.83 120 18.48 1,977.0 979,640.58 1.28 24.52 -42.38 __ 35 38.18 120 12.12 1,957.0 979,641.46 97 25.87 -40.65 836 85 38.62 120 13.63 2,213.0 979,623.57 1.48 31.43 -43.40 $37 L...... 85 37.86 120 19.80 1,621.0 979,639.48 1.91 -T.830 -61.82 S98 35 35.69 120 23.98 1,311.0 979,663.83 .69 -8.97 -58.52 889 .._..c..__ 85 34.59 120 28.78 1,398.0 979,656.09 12 -6.97 -54.49 840 __.__I____ 35 89.19 120 20.98 1,648.0 979,639.67 2.84 - 6.42 -60.43 841 35 40.21 120 19.16 1,857.0 979,635.55 1.83 7.67 -55.06 $42 L_._._____ 85 40.82 120 17.26 2,128.0 979,625.58 2.54 22.81 -48.583 848 35 89.71 120 16.88 2,007.0 979,632.54 2.14 19.47 -47.60 844 85 836.89 120 14.36 2,288.0 979,612.81 1.62 30.18 -47.08 845 .....L__.. 35 36.30 120 15.58 2,576.0 979,592.40 5.27 37.69 - 45.83 846 35 34.68 120 14.78 2,107.0 979,613.84 3.45 17.33 =5L87 S47 35 36.82 120 12.26 2,455.0 979,600.61 3.50 38.79 -47.834 848 _L.._______ 85 85.18 120 : 12.57 2,068.0 979,623.25 2.15 22.87 - 46.80 $49 L._.._.<___ 85 833.84 120 13.69 1,787.0 979,629.16 2.12 3.175 =855.77 S50 Ll.s...__. 85 33.27 120 12.09 2,058.0 979,609.62 2.56 10.51 -87.90 S51 Cal- 85 34.29 120 10.63 2,212.0 979,611.56 1.56 25.48 - 49.283 SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C5 TABLE 2.-Principal facts for gravity stations-Continued Station Latitude Longitude Elevation Observed Terrain Free Complete N. w. (ft) gravity correction air Bouguer sn memers: anomaly anomaly Deg Min Deg Min 892 85 82.26 120 10.95 1,822.0 979,622.32 1.06 2.45 -59.33 _._ 85° 82.258 120 | 8.98 2,027.0 979,611.11 1.383 10.49 -58.08 B54 ...like, 35 ~ $2.59 120 " 1.090 2,284.0 979,596.98 2.32 20.09 -56.34 _. 35 30.24 120 - 6.48 2,827.0 979,583.92 1.47 14.41 - 64.34 856. 85 33.56 120 _ 8.24 2,270.0 979,603.90 2.49 24.32 -51.46 85. 48.25 120 16.47 1,225.0 979,693.50 .95 1.85 -839.48 C58 __.. 35 44.65 120 18.36 1,550.0 979,668.71 2.21 5.63 -45.64 630. ..... _ 85 . 42.24 120 21.49 1,812.0 979,638.52 2.18 3.51 -56.81 $60 :_=.....~~« 35 43.59 120 21.98 1,895.0 979,638.41 1.68 9.29 -54.39 85 44.74 120 - 21.02 2,136.0 979,628.21 2.58 20.11 -50.96 62 85. 48.57 120 19.49 1,913.0 979,641.40 2.20 14.00 -49.78 B63 !. 35 43.03 120 14.75 1,812.0 979,661.80 1.65 25.67 -35.18 864 ........-.; 85 43.03 120 13.38 2,052.0 979,644.62 1.68 31.06 - 838.03 B65 _...... 85. 41.90 120 13.96 1,803.0 979,659.16 .94 283.19 -87. 46 866... 35 40.59 120. 12.10 2,009.0 979,648.33 1.14 34.20 - 83.94 BOT 85 41.16 120 10.62 2,512.0 979,610.64 5.40 43.00 -88.19 C68: 85. 42.20 120 11.92 2,288.0 979,627.98 4.01 87.19 -37.08 $69 :.....-.--.. 35 44.176 120 11.58 1,708.0 979,673.50 1.175 24.65 -82.85 870 i_... 85 45.15 120 - 10.70 2,152.0 979,607.283 7 Al 56.47 -830.96 Bil 35 44.65 120 - 9.35 2,118.0 979,609.54 7.55 55.83 -80.12 85 44.20 120 1.97 3,125.0 979,575.41 13.84 60.95 -32.86 S10 35 44.36 120 - 6.60 1,886.0 979,660.22 2.90 29.15 -33.00 874%......-_.-.-- 85 . 44.18 120 ©2.68 725.0 979,717.21 .35 -283.58 -48.26 BiB 85 48.08 120 - 4.45 1,009.0 979,707.11 .98 -5.83 -839.173 876 35 48.46 120 8.00 1,064.0 979,697.97 1.05 -9,12 - 44.80 BTT 85 42.52 120 - 1,070.0 979,704.19 .90 -1.00 -37.03 878 85 41.15 120 57 940.0 979,701.48 1.65 -14.84 -45.64 B19 .._. 35 40.36 120 56 642.0 979,719.86 .28 -22.51 -44.45 880 :...... 35 39.44 120 .08 659.0 979,711.85 24 -27.61 -50.13 SBT 2. 85 39.34 120 - 1.38 673.0 979,714.18 .36 -28.87 -46.175 S82 35 40.37 120 - 2.68 726.0 979,718.36 .34 -16.12 -40.85 sss ___._.__-- 35 39.62 120 - 7A7 1,172.0 979,696.10 .95 4.63 - 34.87 bsd :...... 35 38.65 120 ©5902 1,065.0 979,695.66 .90 -4.49 -40.35 885. 85. 87.99 120 4.28 1,268.0 979,680.16 91 -, 48 -48.11 S86 85 87.59 120 ©:1,02 1,387.0 979,661.89 2.61 -6.18 -51.48 S87 35 36.87 120 .56 893.0 979,692.15 .63 -21.65 -51.85 S88 ..._.-.«..- 35. 35.71 120 - 2.49 1,140.0 979,674.04 1.02 -14.87 -53.20 BBQ 35 36.86 120 . 2.98 1,071.0 979,689.24 S1 -7.80 -43.96 ©90°.......:_.- 35 36.31 120 5.26 2,031.0 979,626.85 2.00 20.88 -47.07 891:..__...... 35 36.34 120 ._ 7.12 2,021.0 979,630.84 1.35 23.89 -44. 46 892 2... «--. 85 87.88 120 - 6.74 1,754.0 979,646.32 8.27 12.14 -45.10 898 .._... 85. 37.07 120 . 8.97 2,1837.0 979,627.26 2.82 29.32 -41.55 $94 ......_-_-.- 85 84.170 120 - 8.54 2,014.0 979,620.45 3.47 15.17 -50.82 695 -...... 85 35.06 120 5.97 2,036.0 979,625.28 1.69 21.01 -47.02 890: 85 30.78 120 .93 2,621.0 979,579.10 2.94 36.47 -50.92 807 85 32.50 120 - 8.20 2,106.0 979,615.40 2.15 21.90 -47.97 R98 .__:______ 85. 82.45 120 .58 2,245.0 979,600.11 5.21 19.175 -52.44 919 ......-_.~. 85 54.77 120 28.22 1,1779.0 979,670.96 1.36 14.98 -45.02 O14: ~__.___._. 85 54.00 120 29.77 2,145.0 979,652.17 2.61 31.58 -89.87 916 85 56.91 120 28.39 1,757.0 979,666.08 1.88 4.98 -58.15 917 _...._._... 35 58.47 120 29.48 2,088.0 979,647.81 2.85 15.61 -53.54 O18 85 56.22 120 - 25.00 1,661.0 979,675.38 2.18 6.24 -48.28 919 .: .-..... 35 57.76 120 25.77 3,467.0 979,560.04 11.18 58.52 -49.70 920k.._......... 85 46.17 120 14.06 1,021.0 979,664.45 2.07 33.24 -830.94 gal 35 47.33 120 - 6.69 849.0 979,724.38 .89 -8.45 -36.87 D22 .- 35 49.06 120 - 6.52 847.0 979,714.37 .66 -21.11 -49.70 028:......_.._._.~ 35 50.92 120 - 6.85 784.0 979,711.57 A9 -32.49 -59.07 924 .......... 85 52.00 120 - 9.48 1,110.0 979,700.48 1.52 -14.59 -51.38 929 --. 85 52.00 120 11.80 1,149.0 979,706.72 2.89 - 4.98 -41.75 926 85 50.59 120 - 5.04 746.0 979,709.45 29 -37.12 -63.18 827. 85 | 49.95 120 - 8.43 686.0 979,712.25 AT -89.65 -683.16 85 52.16 120 - 2.03 535.0 979,717.55 .09 -51.70 -70.09 929 35 50.84 120 - 1.24 527.0 979,717.28 07 -50.89 -69.02 C6 GEOPHYSICAL FIELD INVESTIGATIONS TABLE 2.-Principal facts for gravity stations-Continued Station Latitude Longitude Elevation Observed Terrain Free Complete N. w. (ft) gravity correction air Bouguer Deg - Min Deg Min anomaly anomaly 930 -~.._..... 85 49.10 120 _ 0.21 528.0 979,714.91 0.06 -50.64 - 68.81 Jof 85 48.92 120 - 1.69 932.0 979,691.89 .93 -85.40 - 66.64 992 85 47.74 120 .94 869.0 979,695.69 2.51 - 35.84 -63.33 989 85 46.45 120 2.69 612.0 979,716.14 22 =-87.18 -58.64 994 .....-_.-. 85 45.87 120 - 4.69 806.0 979,719.41 .68 -15.38 -42.53 AVNLB _...... 85 59.54 120 . 7.65 754.0 979,714.36 «27 -44.83 -70.59 936 85 59.58 120 - 9.82 824.0 979,711.87 Al -40.79 -68.83 997 85 59.57 120 11.98 931.0 979,710.06 T2 -82.53 -63.95 998 85 59.70 120 14.83© 1,301.0 979,694.69 1.47 -13.28 -56.71 989 ;...... 85 58.07 120 15.23 1,554.0 979,684.78 8.16 2.88 -46.97 910 -....--_.. 85 59.17 120 16.83 1,785.0 979,674.28 2.93 7.88 -49.04 o41":__2.___-1_._ 35 57.81 120 9.80 1,017.0 979,701.54- «74 -830.44 - 64.81 942 85 54.92 120 : 11.29 1,915.0 979,659.58 1.98 16.18 -47.94 948 85 54.25 120 183.72 1,429.0 979,691.69 8.48 3.54 -42.29 944 85 54.48 120 ~ 8.92 1,280.0 979,693.01 1.11 -9.48 -52.55 45 .-..... 35 56.48 120 10.68 1,756.0 979,661.93 8.23 1.42 -55.92 9140 ...._._._._. 85 57.87 120 7.69 813.0 979,710.70 50 -40.56 -68.13 904 85 55.09 120 24.27 2,249.0 979,638.61 2.61 26.38 -48.56 965. ...... 35 58.79 120 283.88 1,683.0 979,677.78 1.48 14.18 -42.40 906. -...... 85 52.71 120 22.73 1,581.0 979,690.52 1.25 14.16 -37. A1 907" scl. 85 52.47 120 19.76 1,637.0 979,684.75 1.74 18.70 -36.03 968 35 51.38 120 16.62 1,657.0 979,687.97 1.44 25.36 -830.37 969 85 50.22 120 17.76 1,489.0 979,700.87 .98 24.11 -26.38 970 35 48.53 120 16.96 1,515.0 979,701.27 .89 29.87 -22.01 Ori 85 47.27 120 17.62 1,313.0 979,713.65 .89 24.55 -19.87 970 (...en... 85 45.38 120 15.85 1,375.0 979,696.47 .85 15.89 -80.71 JTG - 85 46.42 120 10.46 1,482.0 979,692.16 1.983 20.16 -29.04 P4 '...... 85 47.10 120 10.88 1,575.0 979,685.62 1.49 21.40 -81.45 UTO 385 48.91 120 12.183 1,741.0 979,677.39 1.44 26.20 -82.42 970 -__. 35 50.29 120 12.00 1,441.0 979,692.24 2.54 10.87 -86.31 OTI ___.. 85 51.21 120. 12.11 1,260.0 979,702.18 8.51 2.42 -87.55 919% 35 51.96 120 14.14 8,052.0 979,586.35 10.08 54.09 -40.98 PTV Faes nse. 35 48.12 120 13.08 2,535.0 979,629.60 5.03 54.21 -28.14 980 85 49.40 120 13.81 2,290.0 979,647.12 2.31 46.86 -29.78 981 ...... 85 50.05 120 15.02 2,440.0 979,637.60 3.88 50.52 -29.71 982 <...... 85 53.38 120. 2.77 559.0 979,719.47 16 -49.27 -68.41 1)-1..._...__-. 85 45.84 120 18.16 1,141.0 979,701.84 «70 -1.40 -40.08 D2 -...... 35 46.08 120 18.07 1,142.0 979,708.72 .68 .28 -88.51 PB ..:....... 85 46.33 120 17.98 1,146.0 979,707.32 .68 8.85 - 85.02 85 46.59 120 17.88 1,154.0 979,711.44 18 8.35 - 80.76 DS _...... 35 46.97 120 1,196.0 979,717.65 «79 17.97 -22.52 [r6 ...s _...... 85 46.76 120 17.81 1,170.0 979,717.29 «78 15.46 -24.19 083 ._.... 35 46.91 120 18.15 1,171.0 979,716.91 19 14.97 -24.66 984 _.-..___.. 35 45.98 120 16.84 1,186.0 979,710.72 16 11.51 -28.66 995 85 47.56 120 15.73 1,792.0 979,679.01 2.48 34.54 -24.19 980 85 49.22 120 18.68 1,715.0 979,687.18 1.79 33.10 -24.27 ; yemers loon 85 49.85 120 20.95 1,660.0 979,695.98 8.54 35.83 -17.89 088 85 54.57 120 20.20 3,007.0 979,598.17 4.72 52.96 -45.93 989. .-. 85 54.39 120 18.86 8,177.0 979,580.42 5.179 56.45 -47.21 990 85 55.26 120 21.65 3,172.0 979,581.11 4.84 55.48 -49.01 991 85 57.65 120 27.06 2,705.0 979,605.76 5.29 32.15 -55.18 &_. 35 56.34 120 20.36 4,339.0 979,500.51 17.33 83.00 -48.97 9985 «c=... 85 58.50 120 24.98 3,426.0 979,568.88 7.83 62.45 - 48.21 994 .-..... 35 58.07 120 283.56 8,764.0 979,589.31 15.16 65.27 -49.16 995 85 48.01 120 _ 9.21 2,658.0 979,608.19 13.75 44.52 -33.34 906 35 50.38 120 10.75 2,640.0 979,608.66 10.85 39.92 -40.22 997 ° 85 53.06 120 - 6.87 976.0 979,702.32 AT -26.74 -59.96 998. _~.-s.._-. 85 52.64 120 - 4.49 762.0 979,711.66 26 -36.93 -62.98 999 '-. 85 54.39 120 - 5.55 9183.0 979,700.84 .56 -36.04 -67.00 1000 ---......__. 85 55.75 120 - 6.68 872.0 979,706.71 .51 - 85.97 - 65.56 1001 _...... 35 52.63 120 18 466.0 979,720.96 .04 -55.45 -71.51 1002 85 54.25 120 A2 701.0 979,708.18 .38 -48.44 -72.82 10083 _-._L.-_._. 85 54.86 120 . 1.81 521.0 979,719.90 .05 -54.52 -12.47 1004 ...-... 85 55.29 120 14.53 1,642.0 979,679.18 8.85 9.58 -48.22 1005 .._..._..._: 85 55.76 120 15.25 1,740.0 979,6783.34 4.53 12.28 -48.21 SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA avi TABLE 2.-Principal facts for gravity stations-Continued Station Latitude Longitude Elevation Observed Terrain Free Complete N. w. (ft) gravity correction air Bouguer anomaly anomaly Deg Min Deg Min 1006 -...... 85 57.10 120 15.81 2,670.0 979,618.44 4.56 42.98 -44,54 1007 ...... 35 56.82 120 14.82 3,078.0 979,583.48 12.01 46.73 -47.30 1008. 85 57.15 120 16.67 2,951.0 979,600.56 5.46 51.40 -44.82 +009 .-.-..-~~.« 35 57.63 120 17.89 2,719.0 979,616.79 4.33 45.183 -44.25 1010 _._.--____ 35 54.36 120 16.50 ~3,473.0 979,559.53 8.59 63.43 -47.58 1011 85 53.08 120 14.86 2,811.0 979,606.12 5.27 49.67 -41.92 85 59.95 120 19.07 1,334.0 979,702.383 4.19 -2.90 -44.74 1015 ...%.....-- 35 58.94 120 19.17 1,667.0 979,682.32 6.68 9.86 -40.97 1016... -...... 85 58.22 120 19.39 1,937.0 979,665.28 6.10 19.24 -41.47 1017 -...... 35 56.99 120 - 5.55 710.0 979,712.48 25 -47.26 -7T1.52 1018 ...... 35 56.11 120 8.89 576.0 979,717.89 .09 -53.64 -78.45 1019°......_..___ 35 56.41 120 .69 744.0 979,711.28 «87 -44,48 -69.75 1020 ...... 35 57.46 120 187 754.0 979,715.14 .26 -41.08 -66.85 1021" ._..s..___- 35 58.68 120 -- 1.25 1,054.0 979,697.74 80 -32.01 -67.59 8b ©590.57 120 18 767.0 979,717.42 A2 -40.59 -66.65 1028 ...... 85 59.76 120 .- 2.14 1,062.0 979,698.15 .98 -82.99 -68.06 1020 85 59.87 120 - 4.64 1,109.0 979,692.68 1.38 -83.59 -70.49 1020 ...... 35 57.85 120 - 4.48 671.0 979,715.12 i17 -49.46 -72 46 1027. °: ....sc.. 85 59.60 120 - 6.08 790.0 979,713.19 .26 -42.70 -69.72 station) terrain corrections were made manually by | 7. Complete Bouguer anomaly (CBA) equals using Hayford-Bowie templates. (See Swick, 1942.) BA+TC-CC, where TC equals terrain correc- Where corrections were unusually large, individual tion. compartments of these templates were divided into subcompartments according to a scheme developed by H. W. Oliver of the Geological Survey. For the outer zones the corrections were made by a digital computer program developed by Plouff (1966), which employed 1- and 3-minute terrain digitiza- tion grids on topographic maps. Observed gravity values were converted to anom- aly values using a gravity-reduction program devel- oped by S. H. Burch. The basic procedures and formulas of the reduction are as follows, where gravity values are in milligals: 1. The gravity difference (AG) between the base and a given station is calculated and corrected for lunar and solar tide and instrument drift. Observed gravity (OG) equals gravity base value + AG. 3. Theoretical N gravity (THG) equals 978,049 [1+0.005228 0@-0.0000059 sin't (20)], where 0 equals latitude in degrees. 4. Free air anomaly (FAA) equals 0G-THG + (0.09411549-0.000137789 sin? 0) where E equals ele- vation in feet above sea level. 5. Simple Bouguer anomaly - (BA) _ equals FAA-0.012774pE, where p equals reduction density, in grams per cubic centimeter. 6. Curvature correction (CC) 0.000446E -3.28 x 1.27 x10 "E*. equals GRAVITY INTERPRETATION Quantitative interpretation of gravity data here is based largely on two-dimensional crustal models constructed with the use of computer programs of Bott (1960) and Talwani. (See Talwani and others, 1959.) Corrections for finite strike length of these models were made using graticule methods of Hen- derson and Zietz (1957) and a computer program of Talwani and Ewing (1960) for three-dimensional bodies. Preliminary calculations of anomalous sub- surface mass were made by using a program based on the two-dimensional program of Bott (1960). Density contrasts used to analyze anomalous gravity features are average or median values based in part on data of Bailey and others (1964), Burch (1965), Byerly (1966), Clement (1965), and D. C. Ross (oral commun., 1969). Estimates of av- erage densities used for various lithologic units con- form generally to those used in nearby areas (Burch and Durham, 1970; Burch, 1971; Burch and others, 1971), and the following ranges of values, except where they apply to "surficial deposits," may be interpreted as wet bulk densities : Average density Lithologic unit (g/em) Surficial deposits _ 1.4-2.0 Paso Robles and Tulare Formations ________-__- £ 2.2 Upper Miocene and Pliocene rocks _________________ 2.2-2.8 Monterey Shale - wet 2. Lower and middle Miocene rocks ___________________ 2, Eocene and Upper Cretaceous rocks _______________ 2.5-2. Franciscan rocks Granitic rocks _ 3 4 6 Serpentinized ultramafic rocks _____=______________ ~2.5 2 8 es 2.6-2.9 o C8 In first approximations of subsurface mass distributions, a density contrast of 0.5 g/cem' was as- sumed between either Franciscan or Salinian base- ment rocks and overlying post-Eocene rocks, and a density contrast of 0.3 g/em>* was assumed between either Franciscan or Salinian basement rocks and overlying pre-Eocene rocks. Effective models for subsurface distributions of mass were ultimately based on "lumped" average densities for Quater- nary sedimentary rocks and deposits and for most Tertiary sedimentary rocks. The lithologic break- down of densities used in this report conforms to similar density units used in gravity studies to the west. (See Burch, 1971; Burch and Durham, 1970.) MAGNETIC SURVEYS Aeromagnetic data within the map area are part of a reconnaissance survey of part of the San An- dreas fault. (See Hanna, 1968a.) The flight pattern of the reconnaissance survey consists of seven trav- erses 4 miles apart parallel to the San Andreas fault between San Francisco and San Benardino. The central flight line follows the surface trace of the fault. A single transverse line was flown about 5 miles southeast of Parkfield within the map area. The total intensity contoured data (pl. 2) consist of digitally recorded output from an ASQ-10 fluxgate magnetometer flown at 6,500 feet barometric eleva- tion. Flight paths were recovered using an APN-147 Doppler system and were checked with a strip-film camera. A regional magnetic field of 9 gammas per mile in the direction N. 16° E. was re- moved from the aeromagnetic map. Vertical-intensity ground magnetic data were col- lected over the southern two-thirds of the map area and are presented as a 50-gamma contour map. (See pl. 3.) Measurements were made with a Sharpe model MF-1R-100 fluxgate magnetometer having a maximum sensitivity of about +2 gam- mas. Approximately 150 stations were read and tied to six secondary base stations that were adjusted relative to a primary base station near Shandon. Multiple readings of stations within the network in- dicate a precision of about +15 gammas for most of the survey, The azimuth of the instrument was kept at a fixed angle relative to magnetic north in order to avoid "heading" errors. Local magnetic fields generated by an extensive network of pipe- lines and a number of well casings south of Cho- lame were carefully avoided during the ground magnetic work. A regional magnetic field of 15 gammas per mile in the direction N. 16° E. was re- moved from the map. GEOPHYSICAL FIELD INVESTIGATIONS MAGNETIC INTERPRETATION Magnetic data were analyzed with the assistance of digital computer programs for calculating total- and vertical-intensity magnetic fields over two- and three-dimensional bodies. Regional magnetic gra- dients of the vertical and total fields, due primarily to the dipolar nature of the field produced by the earth's core, were subtracted from the observed data using U.S. Coast and Geodetic Survey mag- netic charts. An induction-type apparatus (Hanna, 1968b) was used to measure magnetic susceptibili- ties of selected rock specimens. Magnetization contrasts used in analyzing anoma- lous magnetic features are estimated average values partly based on data of Burch (1965), Saad (1968), and DuBois (1963) and partly based on data pre- sented in this report. In the present study, total magnetization (induced plus remanent) contrasts of 0.0005 to 0.0050 emu/em:* (electromagnetic units per cubic centimeter) were assumed between serpentin- ites and surrounding rocks. Although Salinian base- ment rocks may have magnetizations of as much as 0.001 to 0.005 emu/em:* locally within the map area, a probable average magnetization value for most of these rocks is 0.0001 emu/cm* or less, considerably lower than values for magnetic rocks near Table Mountain and Palo Prieto Pass. GEOLOGIC SETTING The mapped area is separated into two major tee- tonic units by the northwest-trending San Andreas fault (pl. 1), probably California's most spectacular and widely known structural feature. Tens, and per- haps hundreds, of miles of right-lateral strike-slip movement have taken place on the San Andreas in Cenozoic time (Hill and Dibblee, 1958; Crowell, 1962; Dickinson and Grantz, 1968), and it has been the source of major and recurring earthquakes in recent historic time. Within the mapped area, active right-lateral strike slip on the San Andreas fault is indicated by (1) prominent offset of stream chan- nels and other features, (2) measurable offset of triangulation networks (Burford, 1966; Howard, 1968), and (3) offset of manmade features and sur- face ruptures formed during the 1966 Parkfield- Cholame earthquake. (See Brown and others, 1967.) The tectonic unit northeast of the San Andreas fault includes the southeastern Diablo Range and northwestern Temblor Range. Pervasively sheared rocks of Mesozoic age composed of Franciscan rocks and associated ultramafic rocks constitute the "base- ment" of this terrane. Owing to its pervasively sheared condition, this "basement" is plastic and is e SAN ANDREAS FAULT AREA structurally overlain by approximately 18,000 feet of marine sedimentary rocks of Tertiary and Creta- ceous age, which in turn is overlain by nearly 3,000 feet of Quaternary valley sediments. The region southwest of the San Andreas fault is an area of low hills that are transected by and drain into Cholame and San Juan Creeks, which empty into the Salinas River. It is part of the "Sali- nia" block of Reed (1988, p. 21) or Salinian block of Compton (1966), of which the basement is com- posed of crystalline rocks made up of plutonic rocks of Mesozoic age and local pendants of older meta- sedimentary rocks. Because of its erystalline texture, this basement is moderately rigid. It is overlain by roughly 4,500 feet of Tertiary sedimentary rocks, which in turn are unconformably overlain by Qua- ternary valley sediments as thick as 3,000 feet. Rock units are lumped to conform with similar units of the generalized geologic maps used for gravity interpretation to the west. (See Burch and Durham, 1970; Burch, 1971.) The main purpose of combining rock units in this fashion is to facilitate geophysical interpretation. Because the lumping of units was in places arbitrary and because these units appear On similar generalized geologic maps to the west, it is necessary to describe in some de- tail the lithologic and structural features of the units as they occur within the map area. ROCK UNITS PLUTONIC AND METAMORPHIC ROCKS Within the mapped area the basement of the Sa- linian block is composed mainly of plutonic rocks and a few small lenses, pendants, and xenoliths of gneiss, schist, and marble. These basement rocks crop out in the Red Hills and at several places near Parkfield; they were penetrated by 11 exploratory wrells (Nos. 1, 11, 18, 19, 20, 31, 89, 40, 42, 44, and 47 on pl. 1 and in table 3). The most prominent ex- posures are in the Red Hills, where the rock is a gray, medium-grained, somewhat gneissoid quartz diorite containing roughly 15 percent biotite and lesser amounts of hornblende. Outerops of grano- diorite with less biotite occur about 4 miles west of Parkfield. The granitic rock penetrated in the six exploratory wells ranges from granodiorite to horn- blende-rich diorite. Granodiorite is exposed exten- sively in the LaPanza Range southwest of the map area. In addition to their occurrence west of the San Andreas fault, plutonic rocks are found in the fault block between the San Andreas and Gold Hill i NEAR CHOLAME, CALIFORNIA C9 faults. The small exposures near Middle Mountain northwest of Parkfield are composed of granitic rocks and pendants of marble and schist that are typical of basement rocks west of the San Andreas fault. However, the outcrops at Gold Hill differ in that they are mainly hornblende quartz gabbro com- posed of hornblende and calcic plagioclase in nearly equal amounts, a small percentage of quartz (Ross, 1970), and a very small mass of marble. The age of the gabbro was estimated by the potassium-argon method to be about 143 million years. (See Hay, 1963, p. 11g-115.) This age is similar to ages ob- tained for plutonic rocks of the foothills of the Sierra Nevada, yet it differs greatly from the age of 80.5 million years obtained for areas of basement rocks in the LaPanza Range to the west. (See Hay, 1963, p. 113.) Densities of plutonic rocks of the Salinian block are assumed to average 2.6 to 2.7 g/em*; those of as- sociated metamorphic rocks may be as much as 3.2 g/em' if the rocks are comparable to schists from the Salinian block outside of the map areca. (See Compton, 1964, p. 281.) The hornblende quartz gab- bro of Gold Hill averages 2.90 g/em*, based on den- sities for nine samples of D. C. Ross (oral commun.,, 1969). The magnetizations of Salinian plutonic rocks in the map areca probably average about 0.0005 emu/cem* or less. Metamorphic rocks are generally nonmagnetic relative to the plutonic rocks. Meas- ured susceptibilities of hornblende quartz gabbro from a single outcrop at Gold Hill average 0.0001 emu/em* or less, although the value of one sample exceeds 0.0010 emu/cem*. ULTRAMAFIC ROCKS The ultramafic rocks are commonly emplaced along faults and within Franciscan rocks. Most are serpentinites typical of those found throughout Franciscan terrane. (See Bailey and Everhart, 1964, p. 47.) The smaller bodies are thoroughly ser- pentinized, and intense shearing has destroyed orig- inal textures in all but a few small remnant blocks. The centers of the larger bodies consist of blocky serpentinite, but invariably this material grades outward to the usual sheared serpentinite. These serpentinite bodies crop out as elongate pods, lenses, or sheets generally concordant with the regional structure. They commonly form discontinuous trains that extend many miles, probably along major fault zones. In a few places the serpentinite contains small masses of medium- to coarse-grained diabase composed mainly of hornblende and plagio- S C10 GEOPHYSICAL FIELD INVESTIGATIONS TABLE 3.-Selected exploratory wells [Compilation by H. C. Wagner. Elevation measured from the following: KB, Kelly bushing; GR, ground; DF, derrick floor. T, data obtained from topographic map] No. Operator Well Location Year _ Elevation Total <1an drilled (feet) depth Reported geologic data pl. (feet) (depths in feet) Section Township south Range east 1-- Cholame Valley Oil and I 9 i i ODzveloplmglt Co. at. mi lg h Pre-1900 2,000 T 950 Bottom in granite. 2-- Occidental Petroleum Corp _._... Kreyenh 4 28 $2 i i §: Kitleman North Bome yenhagen 8 22 16 1963 1,214 KB 6,350 Bottom in Eocene. Top of Eocene, 55. ssociatiqn. ____________________ T6 wee due, 22 18 1931 807 GR 9,206 Bottom in lower Miocene. 4-- Slayton Drilling Co ma Cglgrove-Taylor 8 --- 15 23 14 1948 1,600 T 4,996 Bottom in Cretaceous. 5... Future; Success Co __ - Livermore 1 _ 18 .28 . 16 __ 2,600 T 1,815 Bottom in Franciscan serpentine. -- Canadian Co ._ _______ 1 =--... 19 28 is 22 2,700 T 250 Do. 7-- Pacific Inland Oil Co _ 1 -- 19 23 15 1936 2,800 T 8,100 Do. §__ Shell Oil Co Crow 41-2 2 28 16 1944 1,189 DF 7,825 Bottom in Eocene or Cretaceous. 9-- Sunray DX Oil Co. ... Lynch-Moure --- 9 28 17 1950 1,089 KB 11,962 Bottom in lower Miocene. 10-_ Knudsen-Schmidt Co . Avenal 1% _________ --- 86 23 17 1932 800 T 6,854 Bottom in Eocene. 11... :A. A. Angierson Millman A: 15 24 14 1938 2,400 T 4,006 Bottom in granite. Top of granite, 3,970. 12-- Humble Oil & Refining Co Avenal Land & Oil Go, 5 -- 8 24 17 1961 861 KB 5,838 Bottom in upper Cretaceous (Panoche). Iew 0 Orchard a a arage 12 24 17 1959 789 KB 5,866 Do. 14-. Ancora Corp ..___L___.___ Capital 2 _ - 16 23 18 1953 673 DF (?) 6,148 Bottom in Oligocene. 15-_ Amerada Petroleum Corp Malley 1 __ - 22 23 18 1958 660 T 6,301 Bottom in upper Miocene. 16. Bell Petr-elem]? CO -- Orchard 37-30 _______ -- 80 23 18 1958 690 KB 6,461 Bottom in upper Cretaceous. 17... Havenstnte. Oil Co .; ies Havenstrite-Cleary 1..... 8 25 14 1953 1,690 KB 6,236 Bottom in Oligocene red beds. 18-- Socony-Mobil -- Hillman-Darnall 22-34 --- 84 25 14 1951 1,385 KB 7,716 Bottom in granite. Top of granite, 7,660. 19... Cholame Syndicate ___ ne dack 2 cell. 25 15 1934 1,650 T 3,867 Bottom in granite. Top of granite, 3,650. 20-- Continental Oil Co a= - Taylor 1 __ 25 15 1949 1,720 T 6,559 Bottom in granite. Top of grante, 6,523. R!-- CmW. Colgrove _-.____..__.___.__- Jack 18-8 8 25 16 1949 1,570 KB 2,737 Bottom in Cretaceous. Top of Creta- ceous, 2,010. 25-- Franco Western ..___..______c.__ RCK A nye eee nl den.. 8 25 16 1949 1,185 KB 1,490 Bottom in Franciscan. Top of Francis- can, 1,297, O. -W, Colgrove _._.___.._____ SackA7-10 __.____._-____ 16 25 16 1950 1,360 T 1,850 Bottom in Franciscan. Top of Francis- can, 1,370. lns N0 guck 88-17 .___________._ 17 25 16 1950 1,384 KB 2,810 Bottom in Franciscan. Top of Francis. can, 2,802. e: felon Mie aces concn lull.... Teller 1 oue. 17 25 16 1949 1,180 GR 2,025 Bottom in schist. Top of schist, 1,987. 26-. Buass Development CO Sack I 0 19 25 16 1946 1,146 DF 3,405 Bottomgin Franciscan. Top of Francis- can, 980. Bi-~ 0.- W. Colgrove _._ Jack 83-21 21 25 16 1954 1,400 T 4,997 Bottom in Franciscan. Top of Francis- can, 4,997. 28.,..D. C. :Basdo, Jr Jack 1-9 l ___ 2 25 17 1959 859 KB 3,156 Bottom in Cretaceous, Top of Creta- ceous, 200+. 'C. He BeNF __. D.B. 7 25 18 1954 675 T 2,516 Bottom i2n4 gretaceous. Top of Creta- ceous, 2,440, 8$0-._ Tidewater Oil Co Levis acc.. 15 25 18 1952 619 KB 4,423 Bottom anngretaceous. Top of Creta- ceous, 4,278, 31... General Petroleum Corp __________ Clarke 84-25 ___ 25 26 14 1950 1,127 KB 5,687 Bottom in granite. Top of granite, 5,657. 32-- Humble Oil & Refining Co -«- Hillman 1 ___ -- 82 26 14 1957 1,400 T 5,068 Bottom in lower Miocene. 33-- Tutin and Twisselman ___ =~ White T ___ -- 5 26 15 1945 1,300 T 5,509 Bottom in upper Miocene. 34._ Amerada Petroleum Corp __ -- Still B-1 - 8 26 17 1950 1,084 KB 4,199 Bottom in Cretaceous. 85... W. H. Davis I node ono arnd nage L 13 26 17 1927 900 T 3,250 Bottom iln3 gretaceous. Top of Creta- ceous, 1,350. 36-. Garner-Jones Operator ___________ SAH 1 80 26 17 1962 2,280 T 1,815 Bottorg in Franciscan. Top of schist, 37.. Richard O. Mase ________________ K0. 3 26 18 1958 708 KB 4,636 Bottom in Cretaceous. 'Top of Greta- ceous, 2,850. 38... Humble Oil & Refining Co ________ Newsom 1. 20 26 18 1961 837 KB 5,082 Do. Ses cen do -s we Clarke 1 _______Z 2 10 27) 14. Toss 1,465 KB 4,505 Bottom in granite. Top of granite, 4,820. 10. Texas Co ..________ Clarke NCT-1 1 -15 27 14 1985 1,435 KB 4,333 Bottom in granite. Top of granite, 4,078. Deo N0 -s ean O'Donovan 21 27 14 1949 1,512 KB 2,803 Bittgm 2in 0Oligoct-me(?). Top of red eds, 2,470. 17~- Vanguard Oil Co -..___.________._ Clarke 1 26 27 14 1937 1,685 T 5,000 Bottom in granite. Top of granite, 4,949. 43-- Latin American Oil Upton 1 81 27 15 1925(7) 1,325 T 4,618 Bolt/[tom in lower Miocene, Top of lower jocene, 4,335. $EXAS MXO anc .c 15 27 16 1956 1,738 KB 2,426 Bottom in granite. Top of granite, 2,416. 45-. Brookline Oil (C0 -- McCornack-Viecki Issel. 16 27 16 1956 1,511 KB 6,016 Bottgm in Oligocene(?). Top of red beds, 5,026. 40-. Texas Co z....___________. - Cammatta Ranch 2 ______ 29 27 16 1955 1,316 KB 6,015 Bottom in lower Miocene. 47-. Shell Oil Co eax - Grant Estate C.H. 87-17 __ 17 27 17 1949 2,030 T 2,993 Bottom in granite, Top of granite, 2,608. 48... T. V. Bereman Bereman 1 } *. 5 27 18 1949 1,392 KB 7,141 Bottom in Cretaceous. Top of Creta- ceous, 7,129, 49-_ Sunset Internat. Petroleum Corp __ Shell USL L Los 17 27 18 1957 1,709 KB 2,897 Bottom in 1Cretaceous. Top of Creta- ceous, 1,610. clase. The largest of these is in a quarry one-half of | sumed for anomaly-producing serpentinites within a mile west of the Antelope pumping station near the mapped area. Polonio Pass. Measured magnetic susceptibility values of ser- Densities of serpentinized ultramafic rocks vary | pentinized rocks within the mapped area fall into considerably within the mapped area, the lowest two distinct groups: one of about 1 to 4x 10- emu/ values occurring in highly sheared rocks at Table | em* and another of about 5 x 10- emu/cem *. The most Mountain. For purposes of gravity interpretation, a highly sheared, mechanically incompetent serpentin- wet bulk density range of 2.3 to 2.5 g/em* was as- | ites of Table Mountain possess the strongest immense induced magnetizations. Remanent magnetization was not measured, mainly because of the difficulty of finding material sufficiently coherent for collec- tion an subsequent drilling. For interpretation purposes, serpentinites were assumed to have ¥ total magnetization intensity range of 0.002 to 0.005 emu/cm * Because magnetic intensities of ultramafic rocks increase with degree of serpentinization (Burch, 1965; Saad, 1968), the average magnetiza- tions of serpentinite bodies may be strongly a func- tion of depth below ground surface or distance from the center of the body. FRANCISCAN ROCKS Euge synclinal Franciscan rocks lie structurally below younger Cretaceous rocks in the Diablo and Temblor Ranges. In this area Franciscan rocks con- sist of a pervasively sheared assemblage of eugeo- synclinal sedimentary and mafic volcanic rocks. They are composed of large and small shattered len- ticular masses Or monoliths of graywacke, green- stone, chert, and a few small blocks of glaucophane schist in a matrix of pervasively sheared argillaceous shale or claystone. The lenticular masses are gener- ally oriented parallel to the shear planes of the ma- trix, which generally strike northwest and dip northeast, mainly at high angles. In the mapped area Franciscan rocks were described by Dickinson (1966a, p. 462), as| an| intensively deformed "tectonic breccia." Owing to the pervasively sheared condition of the breccia, the thickness of Franciscan rocks here is difficult to estimate. Its age is not definitely known but is presumed to be Late Jurassic to Late Cretaceous on the basic of fossils reported from Franciscan rocks elsewhere. (See Bailey and others, 1964, fig. 23.) Densities of various Franciscan lithologies are eS- timated to range from about 2.0 to 3.2 g/em ®. Meas- ents of Bailey, Irwin, and Jones (1964, pl. 2) on 16 graywacke samples clustered 9 miles north- northwest and 7 miles east of Parkfield (10 samples are within the mapped area, 7 are just north of the area) give an average density of 2.65 g/em * exclud- ing an anomalously low value of 2.34 g/cm ® for a single sample north of the map area. Greenstones are assumed to average about 2.8 g/em * on the of Clement's data (1965); chert, about 2.65 g/cm *, the average density of quartz. Small blocks of glaucophane schist have densities that may aver- age 3.0 to 3.2 g/em *} on the basis of data of Borg (1956) and Bloxam (1959, 1960). Argillaceous shale i SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C11 and claystone are assumed to have densities that range from about 2.0 to 2.5 g/cm ®. It is clear that no single density can adequately characterize the Franciscan rock assemblage in the mapped area. For purposes of gravity interpreta- tion, however, an average density of 2.65 to 2.40 g/cm} was found to be reasonable over much of the area. Except for local occurrences of greenstone that have average total magnetizations probably less than 0.0005 emu/em ®, Franciscan rocks are believed to be relatively nonmagnetic in the mapped area. CRETACEOUS AND EOCENE MIOGEOSYNCLINAL ROCKS Northeast of the San Andreas fault pervasively sheared Franciscan rocks and ultramafic rocks are overlain, either unconformably or tectonically, by a thick series of miogeosynclinal marine clastic sedi- mentary rocks of Cretaceous and Eocene age. This series as defined by Dibblee (1972) was divided into two major sequences, one of Cretaceous age and one of early Tertiary (Eocene) age. Because these se- quences are of about the same density, they are grouped on the map as one unit (pl. 1). The lower sequence, defined by Dibblee (1972) as the Cretaceous marine sedimentary sequence and also known as the "Great Valley sequence" (Bailey and others, 1964, p. 123), consists of as much as 10,500 feet of strata. It is composed of two distinct units, in ascending order: (1) Badger Shale of Marsh (1960), dark argillaceous shale and thin in- terbeds of graywacke, as thick as 3,900 feet, of Early Cretaceous (?) age, and (2) Panoche Forma- tion, interbedded argillaceous shale and sandstone, as thick as 6,800 feet, of Late Cretaceous age. The upper sequence, defined as the lower Tertiary marine sedimentary sequence, unconformably over- lies the Cretaceous sequence, is mainly of Eocene age, and is composed of two units, in ascending order: (1) Avenal Sandstone, including Acebedo Sandstone of Dickinson (19662), about 300 feet thick, middle Eocene, and (2) Kreyenhagen Shale and Point of Rocks Sandstone (probably strati- graphic equivalents Or facies) totaling about 3,000 feet thick, middle and upper Eoceng, and in part Oligocene. In the strip between the San Andreas and Gold Hill faults, about 2,000 feet of massive marine sandstone of probable Eocene age overlies horn- blende quartz gabbro. Densities of the Cretaceous and Eocene shales and sandstones probably range from about 2.1 to 2.7 g/em * on the basis of data of Byerly (1966) for e C12 GEOPHYSICAL FIELD INVESTIGATIONS lies unconformably on formations ranging from Eo- cene to Franciscan, with increasing discordance westward toward the San Andreas fault. In most places it is a few hundred feet thick, although in parts of the Temblor Range it may be either absent or as much as 4,000 feet thick. Southwest of the San Andreas fault, this unit is represented by ter- restrial. red conglomerate exposed only in the Red Hills. It is as thick as 3,000 feet and is composed of unsorted granitic cobbles and boulders in a matrix of red to gray arkosic sandstone. It here rests di- rectly on granodiorite. About 3,200 feet of similar an area to the north. In the present investigation, an average density range of 2.5 to 2.65 g/cm ® was assumed for these rocks. f Magnetizations of the Cretaceous and Eocene sed- imentary rocks are negligibly small. MIDDLE AND UPPER TERTIARY ROCKS DISTRIBUTION Sedimentary rocks of middle and late Tertiary age occur extensively on both sides of the San An- dreas fault and are composed of two sedimentary sequences as defined by Dibblee (1972). The lower one, of Oligocene and Miocene age, is composed of clastic sedimentary rocks and a low-density unit of siliceous shale. The upper unit, mainly of Pliocene age, is composed of clastic sedimentary rocks. In this report this series of middle and upper Tertiary sedimentary rocks is divided into two units of clas- tic sedimentary rocks separated by the siliceous shale unit. These units are,, in ascending order: lower Miocene deposits, Monterey Shale, and upper Miocene and Pliocene deposits. In addition to these sedimentary rocks, there is a local volcanic unit of Miocene age which is not shown on the map. Densities of middle and upper Tertiary rocks fall within the range 1.4 to 2.6 g/cm , although average density values used in gravity interpretation are re- stricted to the range 2.2 to 2.4 g/cm ®. All middle and upper Tertiary rocks in the mapped area, including volcanic rocks, may be con- sidered nonmagnetic for purposes of interpretation. sandstone of the Vaqueros Formation and ter- restrial red beds, then into granitic basement. The average density assumed for the lower Mio- cene rocks within the map area is 2.4 g/cm *, al- though local variations from this value are common. MONTEREY SHALE The Monterey Shale is a distinctive unit of marine siliceous shale of early, middle, and late Miocene age. It is present on both sides of the San Andreas fault, but the stratigraphy of the northeast side differs from that of the southwest. Northeast of the fault, it is exposed in the Temblor and Diablo Ranges. It is as thick as 4,000 feet in the Temblor Range near Bitterwater Creek but thins northward to only 600 feet in McLure Valley. In the Bitterwa- ter Creek area, the Monterey Shale is composed of three members, in ascending order : (1) Gould Shale Member, siliceous shale, about 600 feet thick, lower and middle Miocene, (2) Devilwater Shale Member, clay shale and siltstone, about 1,000 feet thick, mid- dle Miocene, and (3) MclLure Shale Member, sili- VOLCANIC ROCKS Silicie volcanic rocks, radiometrically dated by Turner (1970) as being about 28.5 million years old, or of Oligocene age, crop out west of the San Andreas fault near Parkfield. These rocks are com- posed of perlitic obsidian and rhyolite that are probably of both intrusive and extrusive origin. They are intruded through, and extruded onto,. granitic basement and are unconformably overlain by Santa Margarita Formation. A small dike of basalt crops out east of the fault near Franciscan Creek at the northern edge of Miller flats, south of Barrel Valley. and MclLure Valley, these two lower members are absent. The McLure Shale Member persists throughout the Temblor and Diablo Ranges but is only about 600 feet thick in the Diablo Range. Southwest of the San Andreas fault the Monterey Shale is not exposed within the mapped area but is present in the subsurface. Exploratory wells drilled LOWER MIOCENE SEDIMENTARY ROCKS Clastic sedimentary rocks of early Miocene age are present on both sides of the San Andreas fault. Northeast of the fault they are the marine Temblor Formation of Dickinson (1966, 1966b), which is composed of semifriable arkosic sandstone, gray claystone, and local basal conglomerate. The unit imm e SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA Cis quin Valley region east of the Diablo and Temblor Ranges are known as the Tulare Formation. They lie conformably on Pliocene marine deposits north of McLure Valley, and unconformably on older rock units south of that valley. They were derived largely from the Diablo and Temblor Range uplifts. The sediments deposited in the upper Salinas Valley region west of the Diablo and Temblor Ranges are known as the Paso Robles Formation, which lies un- conformably: on Pliocene and Miocene formations. The Paso Robles Formation was derived largely from the Santa Lucia and La Panza Ranges to the southwest and the Temblor and Diablo Ranges to the northeast. On the basis of a heavy-mineral study in the Paso Robles Formation, Galehouse (1967) suggested right-lateral movement on the San Andreas fault of approximately 25 miles since Paso Robles deposition. .- for oil or gas southwest and west of Shandon (Nos. 82, 89, 40, 41, and 42 on pl. 1 and in table 3) passed through more than 1,000 feet of Monterey Shale be- tween sandstones of the Santa Margarita and Va- queros Formations. Northeastward this shale unit thins out at or near the San Juan fault. The Mont- erey Shale of this area is of middle Miocene age and is composed of a lower unit of argillaceous shale and an upper unit of siliceous shale. The wet bulk density assumed for the Monterey Shale is 2.8 g/cm ' in conformity with Clement's data (1965) from an areca to the north. Dry bulk densities for this shale may be as low as 1.4 g/cm ®, as suggested by Byerly's data (1966). UPPER MIOCENE AND PLIOCENE SEDIMENT ARY ROCKS Sedimentary rocks of late Miocene and Pliocene age are of different stratigraphy on opposite sides of the San Andreas fault. Northeast of the fault the sequence, in ascending order, consists of (1) the Reef Ridge Shale, 0 to 500 feet thick, uppermost Miocene, and (2) the Etchegoin Formation, in- terbedded marine sandstone, siltstone, and clay shale, 7,000 feet thick. In the Kettleman Hills the equivalent of the upper 2,000 feet of the Etchegoin Formation is mainly marine and brakish-water silt, stone known as the San Joaquin Formation, upper Pliocene. (See Woodring and others, 1940.) Southwest of the San Andreas fault, the sequence, in ascending order, consists of (1) the Santa Mar- garita Formation, a marine friable sandstone about 1,200 feet thick, upper Miocene and lower Pli- ocene (?), exposed in the Red Hills and northwest of Cholame, (2) Pancho Rico Formation, diatomaceous mudstone, as thick as 1,200 feet, lower Pliocene, ex- posed west of Cholame Valley only, (8) unnamed sandstone and siltstone, Pliocene (?), as much as 2,000 feet thick in wells northwest and southwest of Shandon only, and (4) Morales (?) Formation, ter- restrial gravel and sand, 0 to 700 feet thick, upper Pliocene (?). Average bulk densities used for gravity interpre- tation range from 22 to 2.3 g/cm ' slightly less than the assumed average wet bulk density of the Monterey Shale. Bulk densities of the valley deposits assume a broad spectrum of values depending upon whether or not the materials are water-saturated. In the present study, an average wet bulk density of 2.2 g/cm * was assumed. SURFICIAL DEPOSITS Surficial deposits include alluvium of Pleistocene and Holocene age and landslide debris. The alluvium is composed of unconsolidated gravel, sand, and clay. Much of the older alluvium covers the floors of major valleys to depths of perhaps 200 feet. Much of the more recent alluvium is found along streambeds and as flood-plain deposits. Landslide debris is very common and covers large areas, Par- ticularly on the west flank of Table Mountain. Sur- ficial deposits are not shown on the geologic map at places where they are small in extent and the ident- ity of the underlying rock unit is relatively certain. Assumed wet and dry bulk densities for these various surficial deposits range from. 1.4 to 2:0 g/cm ®. GEOLOGIC STRUCTURE AND TECTONICS STRUCTURAL SETTING By far the most significant structural feature in the area is the San Andreas fault. The style of de- formation throughout the area in fact may be re- lated to the fault, with the intensity of deformation decreasing away from the fault. The same general structural pattern prevails on both sides of the fault, although deformation was considerably less intense in rocks overlying the relatively competent plutonic and metamorphic basement to the south- UPPER CENOZOIC VALLEY DEPOSITS Alluvial and lacustrine sedimentary deposits of late Pliocene and Pleistocene age are extensive on both sides of the San Andreas fault. The sediments are weakly consolidated gravels, sands, clays, and thin marl beds having an aggregate thickness of as much as 3,000 feet. Those deposited in the San Joa- a69-845 O - 73 - 2 _____—_ — C14 GEOPHYSICAL FIELD INVESTIGATIONS west and more intense in rocks overlying more plas- tic Franciscan and ultramafic rocks to the north- east. The area northeast of the San Andreas fault was intensely squeezed and the Diablo and Temblor were deposited, that is, presumably in early Creta- ceous or very late Jurassic time. The unconformity at the base of the Eocene ma- forces. The prevasively sheared and brecciated Franciscan and ultramafic rocks reacted to stress as a plastic mass would, and the overlying sedimentary leocene or early Eocene time. Severe diastrophism probably affected the whole area during Oligocene time. In the area northeast of during which uplift the major folds that had formed in Franciscan rocks, Cretaceous rocks, and Eocene rocks during the Oligocene diastrophism were further compressed and perhaps faulted and many new folds in the Miocene and Pliocene sedi- mentary rocks were formed. The diastrophism that began at the end of the Pliocene, if not locally continuous through Pleisto- cene time, was rejuvenated in late Pleistocene time Andreas fault are sedimentary rocks mildly de- formed into minor folds, which, like folds northeast of the faults, have axes that trend about N . 50° W. STRUCTURAL HISTORY The mapped area was affected by diastrophism during the following times: (1) early Cretaceous (2), (2) Paleocene, (8) Oligocene, (4) late Pliocene to early Pleistocene, and (5) late Pleistocene. The granitic rocks of the Salinian block were em- placed in the metamorphic rocks during Cretaceous time according to the age determination of Evernden and others ( 1964). Other plutonic rocks, such as the hornblende quartz gabbro of Gold Hill, were em- placed earlier. It is not definitely known where or how the Fran- ciscan rocks became pervasively sheared and brec- ciated. The ultramafic rocks were emplaced during or after their deposition and are themselves sheared wand brecciated. The lack of such deformation in the overlying Cretaceous and Tertiary rocks sug- gests that the shearing took place before these rocks north- to northwest—trending faults show evidence of right-lateral displacement, The structural pattern of this region strongly suggests that the folds and imm —_———7 SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C15 faults are genetically related to, and possibly sub- sidiary to, right-lateral movement on the San An- dreas fault and were formed by the same stress O stresses, as postulated by Hill and Dibblee (1953). If this is so, the San Andreas fault must have been active during all the Cenozolt diastrophic events, if not continuously during Cenozoic time. Southeastward, the pervasively sheared Francis- can and ultramafic rocks are covered by the Creta- ceous marine sedimentary sequence, and the anticlinal structure terminates in a syncline in Cot- tonwood Canyon. The northeastward-dipping Cretaceous section of Orchard Peak was elevated on the northeast-dip- ping reverse fault along which the southern Diablo Range was elevated. Seattered well data in the ad- jacent part of Antelope Valley suggest that the Cre- taceous rocks of the foothills were elevated on an- other reverse fault against Miocene rocks to the south. Thus nearly all structural features of the area may be directly or indirectly related to movement on the San Andreas fault. Abundant evidence within the mapped area and elsewhere indicates many tens of miles of right-lateral movement along the fault in Cenozoic time, as noted by Hill and Dib blee (1953) and Dibblee (1966) < PARKFIELD-TURKEY FLAT AREA The structure of the foothills southwest of the Diablo Range uplift is essentially that of a syncline that trends northwest through Turkey Flat. This structural feature involves Pliocene and Miocene sediments that overlie Lower (?) Cretaceous and Franciscan rocks, and to the southeast it becomes one of several small folds, cut and displaced right- laterally by many north-trending faults. DIABLO RANGE The Diablo Range within the mapped area is composed of two southeastward-plunging anticlines separated by a syncline. The southwestern margin of the range is essentially an uplift on a major northeast-dipping reverse fault. The core of perva- sively sheared Franciscan and ultramafic rocks ex- posed along the anticlinal uplift north of Castle Mountain and along Table Mountain is the south- eastward continuation of the 30-mile-long exposure of these rocks along the northeast side of the San Andreas fault. (See Jennings and Strand, 1958.) The northeast anticline, referred to here as the Pyramid Hills anticline because it plunges south- eastward into the Pyramid Hills, has a steep north- east flank, part of which has been displaced south- eastward relative to the southwest flank, along & fault near the fold axis. TEMBLOR RANGE The northwestern Temblor Range within the mapped area is structurally complex. The Miocene rocks, which lie unconformably on Cretaceous and Franciscan rocks, are compressed into many north- west-trending folds. The Cretaceous rocks dip re- gionally to the northeast and are locally folded. The Franciscan and ultramafic rocks, with some mix- tures of Cretaceous rocks, appear to be present ad- jacent to the San Andreas fault for an extent of possibly 23 miles but are in large part concealed be- neath a thin cover of the Paso Robles Formation. The exposed parts of these pervasively sheared rocks include large masses of serpentinite, and an unknown amount of this rock may be concealed by the Paso Robles Formation. The Castle Mountain syncline, which forms the prominent feature of Castle Mountain, plunges east- ward and is offset northward on a fault from the McLure Valley syncline, its southeastward continua- tion. The Franciscan rocks of Table Mountain are partly overlain by remnants of a thrust (?) plate of Cretaceous sedimentary rocks, aS mapped by Dick- inson (19662, 1966b). Along the crest of this moun- tain both of these rocks are partly covered by a body of serpentinite that was mapped and described by Dickinson (1966a) as a subbhorizontal sheet about 12 miles long which was extruded by plastic flowage from narrow, steeply-dipping dikes of ser- pentine breccia. Dickinson postulated that these dikes were fissure feeders, possibly along faults, that extended upward from a subterranean mass of serpentinite mobilized into cold plastic material by stresses during the Pliocene and Pleistocene oroge- nesis of the Diablo Range. COLD HILL AREA Exposures of crystalline basement rocks at and near Gold Hill and northwest of Parkfield are un- conformably overlain by Focene(?) rocks. All these rocks form a fault slice as wide as 2 miles on the northeast side of the San Andreas fault, dissimilar to any other rocks in the mapped area northeast of the San Andreas fault. The basement rocks are generally similar to those southwest of the San An- dreas fault, but rocks of Eocene age are not known to occur southwest of the fault within the mapped area. As the Gold Hill fault that bounds this slice on the northeast is exposed at few places, its char- e C16 GEOPHYSICAL FIELD INVESTIGATIONS acter is not definitely known. East of Gold Hill, it ward decrease in average density of the lower conti- appears to be a near-vertical zone of gouge as wide anomalous concentrations of high- or low-density m within t} i & soUTHwEsT or san FAbET asses within the upper 10 miles of the crust The area southwest of the San Andreas fault is deformed within a few miles of the fault and near the San Juan fault. Northwest of Cholame the Mio- cene rocks are gently compressed into folds whose axes|itrend, about N. 50° W. -The unconformably overlying Paso Robles Formation is, within 3 miles of the San Andreas fault, deformed into many minor folds with axes of similar trend. All these folds must have formed by right-lateral drag on the San Andreas fault. The only major fault within the Salinian block in the mapped area is the San Juan fault which, in the Red Hills uplift on the east side of the fault, ex- poses granitic basement and overlying Miocene rocks. This fault extends southward for many miles. Northward it disappears below the Paso Robles Formation, but an en echelon alinement of north- west-trending minor folds in that formation sug- gests a continuation of the fault below the Paso Ro- bles. These and other minor folds associated with and (2) a broad elliptical high that spans the San Andreas fault near Palo Prieto Pass (pl. 2). Each of the positive anomalies has an associated polariza- tion low developed over relatively nonmagnetic ter- rane to the north. The anomalies are superposed on a regional field that increases uniformly toward the north-northeast. Coast and Geodetic Survey charts indicate that the gradient of this regional field as estimated from the aeromagnetic map is greater than that attributable to the dipolar component of the earth's field by about 5 gammas per mile in a northeast direction. This residual gradient, if due to interpolation uncertainty resulting from use of small-scale charts, may reflect an average north- eastward increase in magnetization of that part of the earth's lower crust above the Curie isotherm. The dipolar regional field of 9 gammas per mile in- creasing north-northeastward has been removed from plate 2. ANOMALIES NORTHEAST OF SAN ANDREAS FAULT GRAVITY HIGH AT CHOLAME VALLEY The salient feature of the gravity map is a broad high northeast of the San Andreas fault that peaks about 7 miles north-northwest of Cholame. This fea- ture consists of a 10-mgal doublet covering some 40 GRAVITY AND MAGNETIC FEATURES GENERAL PATTERNS GRAVITY TRENDS rupted by a broad gravity high northeast of the San Andreas fault near Cholame. Smaller perturbations of the regional field occur northeast of the fault as anomalies near Table Mountain, the Kettleman Plain, Palo Prieto Pass, and Bitterwater Valley and southwest of the fault near Red Hills, San Juan Valley, and the Estrella River valley. The north- eastward decrease of the regional field may be caused partly by a northeastward thickening of the continental erust, perhaps coupled with a northeast- covered in places by sedimentary rocks. Although the principal source of the gravity high appears to be a combination of Franciscan and mio- density sedimentary rocks to the northeast and southwest, the configuration of the anomaly is prob- ably much influenced by subsurface serpentinites mmm o i SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C17 of the anomaly north-northwest of Cholame is asso- | an average density 0.05 g/cm ® greater than that of ciated primarily with serpentine—deficient Francis- | Salinian basement rocks. can rocks similar to those associated with a major The configuration of the gravity high appears to positive gravity anomaly northwest of Panoche Val- | be strongly controlled on the north and south by ley in the Diablo Range, about 60 miles northwest | two major subsurface accumulations of serpentinite, of the mapped area. (See Bishop and Chapman, | as interpreted on the basis of positive magnetic 1967.) The decrease in gravity at the northeastern anomalies. At the northern flank of the high near flank of the anomaly is largely caused by a thick Table Mountain, an inferred steeply dipping tabular northeast-dipping sequence of miogeosynclinal Cre- | serpentinite mass below exposed serpentinites and taceous rocks and overlying lower density Cenozoic | tectonic breccia produces a shallow negative gravity rocks. The steep southwestern flank of the anomaly | trough. Southeast of Cholame the southeastern is produced by a combination of Franciscan rocks flank of the Cholame high is depressed, though northeast of the San Andreas fault in contact with | somewhat less conspicuously, about 4 miles north of granitic rocks of similar density overlain by low- | Palo Prieto Pass, where magnetic data suggest a density rocks of the Paso Robles Formation south- deeply buried magnetic mass. In the absence of west of the fault. these lower density subsurface masses, the Cholame One possible subsurface density profile across the high would appear as an asymmetrical northwest- gravity high at Cholame Valley is shown in general- trending gravity ridge, approximately parallel to ized form ! figure 2. The computed anomaly the outcrop pattern of Franciscan and miogeosyn- associated with the idealized density section com- clinal Cretaceous rocks in the area. pares favorably, though approximately, in shape The doublet configuration of the highest part of and amplitude with the observed anomaly. It will be the gravity anomaly may be attributed in part to noted that density units used in figure 2 and else- | the gabbro of Gold Hill and to the serpentinite where in the report need not correspond uniquely to | along the Gold Hill fault. The plutonic rock at Gold particular stratigraphic units. For example, in | Hill is as much as 0.2 to 0.5 g/cm # denser than most figure 2, Franciscan rocks and Salinian basement | surrounding rocks, and this density contrast proba- rocks adjacent to the San Andreas fault are inter- | bly contributes significantly to the local gravity preted to have identical average densities; farther | peak at Gold Hill. The intervening gravity saddle to southeast, Franciscan rocks are interpreted to have | the southeast may be caused in part by the serpen- tinized ultramafic rocks cropping out along the Gold EXPLANATION Hill fault between the local gravity peaks. Both gravity and magnetic data indicate that the subsur- face volume of these ultramafic rocks is small. The southeastern local peak may be interpreted as a rel- V 3 ative high associated with Franciscan rocks in con- ----- * ® tact with serpentinized ultramafic rocks and folded Cenozoic sedimentary rocks. Other relative highs associated with Franciscan OY miogeosynclinal Cre- taceous rocks, in places partly covered by Cenozoic sedimentary rocks, are the segmented gravity ridge immediately northeast of Table Mountain, the local positive gravity anomalies west and northwest of Antelope Valley, and the gravity ridges north and west of Bitterwater Valley. Observed anomaly 30 s s Computed anomaly MILLIGALS SW NE . SAN ANDREA FAULT An idealized cross section {through the center of U’JO mices the gravity high at Cholame Valley is shown as pro- 5 -s _ io KitomErers file A-A' on plate 4. __ ANOMALIES AT TABLE MOUNTAIN FIGURE 2. --Comparison of computed gravity anomaly of a A relatively narrow, arcuate magnetic ridge that two-dimensional model with observed Bouguer gravity f f rends parallel t and n ast of the San A dreas anomaly near Cholame Valley. Strike of model is N. 40° W. J; 9 his E Th el 9th { ortheast z th S 3 Gravity datum is arbitrary. Line of section corresponds aullt a e northwest corner 0 e mapped area approximately to profile A-A' of plate 4. (pl. 2) bends east-southeastward northeast of Park- ii — C18 GEOPHYSICAL FIELD INVESTIGATIONS field to follow the trend of Table Mountain. This anomaly, about 150 gammas in amplitude at a level 0.6 mile above mean topography, has a symmetry axis slightly south of the Table Mountain ridge line, and interpreting this regional field in terms of lat- eral thickening of the earth's lower crust and lat- eral uniform changes in density in the earth's lower crust or upper mantle. Part of the regional field of remanent magnetization on the direction of total magnetization is unknown, interpretation of the magnetic source relies significantly on the Bouguer gravity data (bl. 1). Observed aeromagnetic and gravity profiles ex- tending from Gold Hill northward to lat 36° 00' N., long 120° 20 W. across the magnetic high and gravity low at Table Mountain are shown in figure 8A, B, and C with computed anomalies of various models. Both the magnetic and gravity anomalies of Gold Hill, Computed anomalies for models should therefore have a southern gradient somewhat lower than that of the observed anomaly, as is the case for models in figure 3A, B, and C. As additional information in interpreting the anomaly source at Table Mountain, aeromagnetic source along this profile is a tabular body to 2 to 2% miles thick that crops out at ground surface and dips 75° NF, to a depth of about 10 miles, with Fraurg 3.-Comparison of computed total-intensity magnetic parallel to magnetic north; strike of model D is N. 45° w. and gravity anomalies of two-dimensional models A, B, and Assumed magnetization inclination is 60°. Positive num- C with observed aeromagnetic and Bouguer gravity anoma- bers refer to total magnetization intensity contrasts in emu/em 3; negative numbers in parentheses, to density lies at Table Mountain, and comparison of computed total- contrasts in g/em ®. Datum levels of anomalies sre aFb)- intensity anomaly of two-dimensional model D with aero- magnetic anomaly along the single transverse flight line directly from magnetic tape, independent of contoured map. across Table Mountain, Strike of models A, B, and C is (See facing page.) mmm, e 0 SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C19 200 100 GAMMAS P _ < 9 a 2 2 MILLIGALS EXPLANATION Flight elevation t 0.0005 Mean topographic elevation --= "1 (-0.20) Observed anomaly a k 060+ ® Computed anomaly M Total-intensity magnetic anomaly $ C 5 10 MILES D Bouguer gravity l pe anomaly 0 5 10 KILOMETERS 4—— — C20 GEOPHYSICAL FIELD INVESTIGATIONS Mountain is about 2% miles thick over most of its extent, dips 45° to 75° northward to northeastward inclination of 60° generate magnetic anomalies that are centered south of the plan-view centers of the models. in figure 44 the southwest-dipping dike bounded by a hypothetical southwest-dipping San Andreas fault and in gure 4B the vertical prism bounded by a hypothetical vertical fault may be tentatively dis- missed as probable sources on the basis of anomaly symmetry. The northeast-dipping dike of figure 4C and the trapezoidal cross-section models of 4D, E, F, G, and H generate anomalies similar in shape to the observed anomaly, MAGNETIC HIGH AT PALO PRIETO PASS The largest known magnetic anomaly along the San Andreas fault (in terms of amplitude and areal alies of total- and vertical-field intensity of given models shown in figures 5 and 7 and discussed below. The remainder of the anomaly shift is an ex- pression of the more accurate location of the verti- cal-field anomaly, which was contoured on the basis of data more densely and uniformly distributed one another in the subsurface. As an aid to under- standing possible locations of the buried magnetic source rocks relative to their associated anomaly, total-field anomalies of eight magnetic models are A more detailed inspection of plate 3 suggests Frgur® 4.-Comparison of computed total-intensity magnetic anomalies of three-dimensional models A through H with observed aeromagnetic anomaly at Palo Prieto Pass. Horizontal lines of model cross sections represent flight-line eleva- tion. Models are composed of rectangular parallelepipeds shown as dashed lines. Assumed magnetization inclination is 60°. Numbers refer to total magnetization intensity contrasts in emu/em 3, C21 SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA NE SW NE SW SW SW fl possess a uniform average total magnetization in- tensity contrast of 0.002 emu/em*. The roof of the northeastward. source is taken to be 6 miles wide and 3 miles below Because of the inherent lack of precision in the aeromagnetic contouring due to the wide spacing of flight lines and to the possible bias of the central flight line on the contouring, it is instructive to compare both total. and vertical-field anomalies of selected models with the observed anomalies. It Total- and vertical-field anomalies of eight models striking 45 ° Ww. of magnetic north and dipping single magnetic source. The source material for the anomaly at Palo Prieto Pass is not definitely known, Geologic map- ping within the study area indicates that three rock types might be capable of possessing the requisite magnetization of 0.002 emu,/em *: (1) plutonic rocks, proximately 45° N E. Analyses of the models in B, C, and D and other relatively complex models not o o SAN ANDREAS FAULT AREA NEAR CHOLAME, CALIFORNIA C28 400 GAMMAS ro o 6 400 GAMMAS ro o S 0.0030 0.0030 0.0040 E F G H EX PLA NA TIO N Lo nter f Observed anomaly filgntselevation." . =_. out. _. Oni tol 0 5 10 MILES t Computed anomaly ler—J Mean topographic elevation T. 0 5 10 KILOM ETERS Total-intensity anomaly l’L—J SAF San Andreas fault, V relative to observed anomalies Vertical-intensity anomaly FIGURE 5.-Comparison of computed total- and vertical-intensity magnetic anomalies of three-dimensional models in A through H with observed aeromagnetic and ground magnetic anomalies at Palo Prieto Pass. Models strike N. 45° W. and have magnetization inclinations of 60°. Numbers refer to total magnetization intensity contrasts in emu/cm3. Datum levels of anomalies are arbitrary. pods of limited outcrop extent northeast of the: enough to produce the anomaly, and found in Fran- fault. Other rock types that might be magnetic ciscan terrane to the north, are gabbro and spilitic ii 1 ©24 GEOPHYSICAL FIELD INVESTIGATIONS basalt. (See Grommé and Gluskoter, 1965; Griscom, 1966; Wentworth, 1968.) Because the source of the anomaly is believed to TABLE 4.—Summary of magnetic susceptibility data Number of samples with magnetic susceptibility (emu/em?) of- Sampling General Total number be confined to the northeast side of the fault, sam- site rock type of samples <104 10-4to ig- >10-3 ples of potentially magnetic rock types were col- N1 Serpentinite ______ 20 -+ 9 11 te tht Ua anns 27 T 18 lected for magnetic measurement along, and north- 8. , Toal: de. ______~ 16 e 8 8 4 Quartz gabbro 13 12 12 Ceologic history ..> . cocco 0 _. 3 Relation of aeromagnetic patterns to tectonic features 12 Rock Units .._..__------ 4 'The Hope fadlt ___ _n n, a a a,. 12 Structure .-- 5 Fault control of emplacement of granodiorite ...-- 13 Ore deposits _------- g Aeromagnetic data and the mining districts _---- 16 Metamorphism ...-- ss ces co \. og | Conclusions a a es coc oo 16 Geophysical fasta . .s nse ec ( n 0 . 7 | References gited ' ...-- 16 E porro ie iman a ILLUSTRATIONS Page PLATE 1. Conibined geologic and aeromagnetic map, aeromagnetic profiles, magnetic models, and geologic sections of part of the Pend Oreille area, Tasho. and Montana 00" In pocket FIGURE 1. Index map showing the location of the aeromagnetic survey --- 2 Geologic map of northern Idaho and northwestern MONEANA® | 3. - Graph showing magnetization of rocks from the Pend Oreille area o 4 Map showing aeromagnetic anomalies, major faults, and principal mining districts . 5. Diagrams showing postulated origin of the Magee fault sone .... es ou vas Gy pee nee in TABLE Page TABLE 1. Magnetic properties of rocks from the Pend Oreille area, {dando _s. an rans ette toi. D8 III o GEOPHYSICAL FIELD INVESTIGATIONS S ann r ine mae see- GEOLOGIC IMPLICATIONS OF AERO MAGNETIC DATA IN THE PEND OREILLE AREA, IDAHO AND MONTANA By ELIZABETH R. Kinc, Jack E. HARRISON, and ArraN B. GRIGGS ___ __ the geologic mapping and study of ore deposits bY Anderson (1930, 1947) in the Clark Fork district and (Anderson, 1940) in Kootenai County, and by Sampson (1928) in the Pend Oreille district. Gillson (1927) reported on the granodiorites of the area. More recent work includes aeromagnetic mapping bY Meuschke, McCaslin, and others (1962), geologic mapping by Harrison and J obin (1963, 1965), a com- bination of new mapping and compilation of previ- ous work by Savage (1967), and geologic mapping by Griggs (1968). The aeromagnetic survey discussed in this report was made in 1959 and covered an areca of about 1,000 square miles. About 200 square miles of the survey that covers the Elmira 15-minute quadrangle, which is directly north of Pend Oreille Lake, is ex- cluded from this report. Interpretation of that part of the aeromagnetic data must await completion of a modern geologic map, which is now in progress. A preliminary interpretation of part of the aero- magnetic data was given by Harrison, Jobin, and King (1961). After the preliminary aeromagnetic map was open filed in 1962, Savage (1962) published an interpretation of the data. Unfortunately, his in- terpretation was made without the benefit of modern geologic maps OT measurements of magnetic proper- ties of the rocks. For this report, King is largely responsible for the various calculations using the magnetic data and for the geophysical interpretation of the aeromag- netic map. Harrison and Griggs share the responsi- bility for the compilation and interpretation of the geology—Harrison for the north half of the area and Griggs for the south half (fig. 1). The advice and assistance of 'J. W. Allingham in the use of computer techniques for calculating suitable models to fit the observed magnetic anom- alies are gratefully acknowledged. ABSTRACT An aeromagnetic survey of about 1,000 square miles of northern Idaho was made to outline intrusive bodies to help provide three-dimensional control for a geochemical study of contact and regional metamorphism in the northern half of the surveyed area. Most of the host rocks in the area are a part of the Belt Supergroup. This report discusses inter- pretations of the southern 800 square miles of the survey and is confined to tectonic and ore deposit problems. Positive aeromagnetic anomalies reflect both exposed and buried Cretaceous granodiorite cupolas and stocks. The reticularity of some aeromagnetic contours is related to major block faults that were reactivated during tectonic swelling and emplacement of the granodiorite. Subsequent collapse created a block mosaic of minor faults within the older blocks; geologic maps of the area have a shattered-glass appearance. One zone of closely spaced high-angle faults (The Magee fault zone), which is about 20 miles long, Was caused by collapse of a monoclinal flexure; a long narrow pluton raised and tilted a block bounded on three sides by old major faults. The fourth side formed the monocline which eventually stretched and collapsed. The aeromagnetic survey does not show any relation be- tween the principal mining areas and the aeromagnetic anomalies. INTRODUCTION The U.S. Geological Survey, aS part of its program of regional investigations, has conducted geologic studies in the northeastern Washington—northern TIdaho-northwestern Montana area for several years. Reasons for the studies include the need for informa- tion on the complex tectonics of the area, the need for information Oon mineral resources including the relation of the mining districts to the broad geology of the area, and the opportunity for examination in detail of some of the less well understood geologi¢ processes displayed in the rocks of the area. This report presents some of the results of these continu- ing studies. Among the previous geologic work in the area is D1 i e D2 GEOPHYSICAL FIELD INVESTIGATIONS ©Clark Fork MONTANA e It: < A - Area of report FiGUrRE 1.-Location of the aeromagnetic survey (pl. 1). GEOLOGIC SETTING contains Belt rocks; (2) a structurally complex area REGIONAL SETTING south of the Hope fault and east of the Purcell trench paragneisses and granitic intrusive rocks, as well as the west (fig. 2+. These major structures were Used Precambrian and Paleozoic sedimentary and meta- by Yates, Becraft, Campbell, and Pearson (1966, | sedimentary rocks. The area of this report is virtu- fig. 38-1) to divide the region into tectonic subdivi-| &UY all within the Coeur d'Alene Subprovince and sions: (1) a structurally simple area north of the | almost fills the apex of an inverted Y formed by the Hope fault and east of the Purcell trench (the | intersection of the Hope fault and the Purcell trench Kootenay-Flathead Subprovince), which principally (fig. 2). mmm ooo GEOLOGIC TMPLICATIONS, AEROMAGNETIC pATA, PEND OREILLE AREA, IDAHO AND MONTANA D3 117 116° 1158 EXPLANATION BRITISH COLUMBIA fs TV" F \ X \ \ ® p. A 49° yg! TERT! ARY Columbia River Basalt w 4 ~" Az 52 Granitic rocks CRETA~ cEoUus Mostly granodioriie and quartz monzonite =-- C-- } C----3 | Sedimentary rocks Include Lakeview Limestone, Rennie Shale, Gold Creek Quartzite. and correlative rocks $ __ CAMBRIAN | g < Low-grade metasedimentary rocks ¢ Mostly rocks of the Belt Supergroup m 2 < 0 C , WW High-grade metamorphic rocks E Probably include both Belt Supergroup and older rocks 48° $3 Stg: > a Contact 2 gt Fault Dotted where inferred Strike-slip fault Showing direction of relative movement _____ +__.___ Anticline Showing trace of axial plane _____ +_____ Syneline Showing trace of axial plane 0 10 20 30 40 50 MILES Approximate limit of area covered by aeromagnetic survey FIGURE 2.-Northern Idaho and northwestern Montana (from Yates and others, 1966, fig. 3-2). GEOLOGIC HISTORY lateral strike-slip faults was intermittent through- out much of Phanerozoic time. In the Paleozoic Era, only Cambrian time is rep- resented by dated rocks in the area. The depositional sequence during early Middle Cambrian time is represented by quartzite, shale, and limestone and dolomite. Another possible representative of Paleo- zoic time is found just north of the map areca in the Purcell trench. This rock of possible Paleozoic age is a poorly sorted conglomerate that is confined to the trench and that contains cobbles and pebbles of only Belt rocks and Purcell sills. The deposit was inferred by Anderson (1930, p. 20-22) to be a fan- glomerate of Paleozoic age. This inference seems sound to us, and it requires further tectonism, prob- ably faulting, in and near the trench sometime dur- ing the Paleozoic. The geologic history displayed by the rocks of the Pend Oreille area (pl. 1) began with Precambrian deposition of at least 38,000 feet of fine-grained sedimentary rocks (the Belt Supergroup) . During Precambrian time, these rocks were intruded in their lower part by quartz diorite (the Purcell sills) and then were gently folded into broad synclines and anticlines that trended northward. Some faulting during or after the folding disrupted the beds before erosion at the end of Precambrian time had leveled the area and had formed the vast surface of uncon- formity on which the Cambrian rocks were depos- ited. As postulated by Hobbs, Griggs, Wallace, and Campbell (1965, p. 125-128), the first movement on the Osburn and Hope faults probably began in Pre- cambrian time. Movement on these major right- e e D4 The Mesozoic Era is represented in the area by Cretaceous intrusive rocks and mineral deposits. One A final surge of right-lateral movement along the Hope fault in and near the Purcell trench is here tentatively assigned to the early Tertiary. The event area. These porphyry dikes occur only in the area north and northwest of Pend Oreille Lake. South of on the Hope fault was accompanied by dilatancy to make room for the porphyry dikes and completed an apparent right-lateral slip of 16 miles, which compares exactly with the maximum amount of apparent right-lateral slip on the Osburn fault (Hobbs and others, 1965, P. 74-83). This event may have been a final phase of the complex intrusion process that accompanied emplacement of the Cre- taceous batholith, but it resulted in such a clear-cut and definable geologic event that we prefer to em- phasize its importance in the geologic history by identifying it separately and by assigning it a slightly younger age. A radiometric age-determin- ation program now in progress will verify or deny the tentative age assigned here. The remainder of Tertiary to Holocene time is represented by (upper (?) Tertiary) gravels, a wide variety of glacial and glaciofluvial deposits of Pleis- tocene age, and alluvium, talus, and other surficial rock debris of Holocene age. ROCK UNITs Rocks of the area have been described in detail GEOPHYSICAL FIELD INVESTIGATIONS in several reports. Only brief descriptions will be given here, but the interested reader is referred to Anderson ( 1980) for petrography of the intrusive rocks in general, to Gillson (1927 ) for petrography of the granodiorite, to Sampson ( 1928) for descrip- tion of the Cambrian sedimentary rocks, to Harrison and Jobin ( 1963) for descriptions of the Belt rocks, and to Savage (1967) for descriptions of the surficial deposits. The oldest rocks in the area are low-grade meta- sedimentary rocks of the Belt Supergroup. These consist principally of quartzites, argillites, and siltites with significant amounts of carbonate-bearing layers in the upper half of the supergroup. The coarsest detrital grain size of these rocks was fine sand, and most of the original sediments were silt or clay. At least 38,000 feet of the supergroup is exposed in the area, and these rocks form by far the largest amount of bedrock in the area (pl. 1). The Belt Supergroup is unconformably overlain by the Gold Creek Quartzite of Cambrian age. Where exposed, the unconformity shows an angular dis- cordance of about 7 °, and Gold Creek Quartzite rests on the middle to upper part of the Wallace Forma- tion; the Wallace rocks are reddish for a few feet below the unconformity, and the Gold Creek Quartz- ite is conglomeratic in its lower few feet. Overlying the Gold Creek Quartzite is an olive fissile shale- the Rennie Shale-which contains fossils identified by A. R. Palmer, U.S. Geological Survey, as trilobites The principal intrusive rocks of the area are dioritic to gabbroic sills of Precambian age (the Purcell sills) and stocks, plugs, and dikes of grano- dioritic rocks of Cretaceous age. The various bodies of granodiorite shown on plate 1 are remarkably similar in outcrop and generally show only slight differences in texture from body to body. Minor amounts of diorite, lamprophyre, and granodiorite porphyry of Tertiary age form dikes or small plugs, most of which are too small to be shown at the scale of plate 1. —'-—_———f GEOLOGIC IMPLICATIONS, AEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D5 This lead was followed by Anderson (1930) for the Clark Fork district and (Anderson, 1940) for Koote- nai County with the modification that the major right-lateral strike-slip faults (the Hope and Os- burn) were added to the postintrusion fault group. In a later report on the Clark Fork mining district, Anderson (1947) confined his comments to faults within the district but recognized that low-angle thrust faults and minor strike-slip faults also 0¢- curred in the area and that the thrusts were cut by high-angle faults. For the Clark Fork quadrangle, Harrison and Jobin (1963) proposed a general classification scheme of Hope fault, block faults, and mineralized faults. They suggested (p. K28) that this sequence was also probably chronological, from oldest to youngest. Savage (1967) accepted this scheme and applied it to his studies of Bonner County. Several lines of evidence point to a more complex history of faulting than previously supposed. The horizontal persistence of Purcell sills has been mapped by several geologists. (See Anderson, 1930, pl. 14; Kirkham and Ellis, 1926, pl. 3.) A Precam- brian age of faulting is implied by the lack of con- tinuity of Purcell sills across the Hope fault. For example, the various segments of quartz diorite shown in the northwestern part of plate 1, south of the Hope fault, are parts of a single persistent gill that occurs stratigraphically about 4,500 feet below the top of the Prichard Formation. This sill, how- ever, is missing from an equivalent (or even a higher) stratigraphic position on the north side of the Hope fault (north and slightly east of the town of Clark Fork). Direct evidence of faulting prior to deposition of the Cambrian sedimentary rocks can be seen in the Packsaddle Mountain quadrangle (Harrison and Jobin, 1965) in the area of Packsad- dle Mountain. Here, on the northwestern slope of the mountain, the Gold Creek Quartzite overlies the middle part of the Wallace Formation with angular unconformity of about 7°. About 2 miles to the south- east across the Packsaddle fault, not only are middle and upper rocks of the Wallace present but also more than 1,000 feet of the Striped Peak Formation which overlies them. Thus, the Packsaddle fault must have been active before the Gold Creek Quartzite was deposited, and the middle and upper parts of the Wallace and the Striped Peak rocks are preserved only because they were below the surface of uncon- formity on which the Gold Creek was laid down. Al- though other direct evidence of such old faulting has not yet been found, it seems reasonable to believe that the Packsaddle fault was not unique when Plate 1 shows the geology of the area simplified primarily through grouping of the rock units into units larger than on the original, more detailed maps. We also have deleted several minor faults not re- quired to show the fault pattern of the area or to justify offset contacts between rock units. Plate 1 shows a high degree of correlation between positive magnetic anomalies and exposures of the Cretaceous and Tertiary intrusive rocks. Positive anomalies not directly correlatable with such ex- posures are found over various rock units and ob- viously correlate with some quality other than sim- ple stratigraphy. Interpretation of these anomalies will be discussed later in this report. STRUCTURE The mapped area is one of simple folds but com- plex and intricate faults. The only major fold is a broad, ill-defined syncline whose axis trends approxi- mately N. 15° E, and plunges gently northeast across the southeastern part of the mapped area (pl. 1). All the area south of the Hope fault and northwest of this fold axis is on the northwestern flank of this syncline. The continuity of beds on this structure is not readily apparent because of the abundant faults and repetition of beds. Other folds do occur in the area, some with wavelengths as much as a mile, but they are all local in extent and are related to drag- ging and flexing along and between faults or, less commonly, to shouldering aside during emplacement of a few of the plutons. The fault pattern is complex, and at first glance presents a shattered-glass appearance (pl: 1). The gross pattern can be described briefly as a block- mosaic fault system in which the faults generally step down from east to west. Other important ele- ments include the Hope fault, a major right-lateral strike-slip fault, and the Magee fault zone, 2a belt of closely spaced high-angle faults that separates an area of more intense block faulting in the south- western part of the area from an area of less intense block faulting in the southwestern part of the area from an area of less intense block faulting in the southeastern part of the area. Less common elements in the fault pattern include high-angle strike-slip faults and low-angle thrust faults. Determination of the age and sequence of faulting has been a problem in the area for many years. Sampson (1928) was the first to recognize and map the fault mosaic. He attributed the pattern to crustal breakage and foundering of blocks during intrusion by the granodiorite, and he classed the mosaic faults as "intrusion faults" and "postintrusion faults." ii S D6 GEOPHYSICAL FIELD INVESTIGATIONS Mountain and the community of Granite is also prob- ably Cretaceous in age. This fault is best explained as a bedding-plane thrust that resulted from pres- sures exerted during forceful intrusion of the Pack- saddle Mountain Granodiorite body. Faulting younger than the granodiorite is repre- sented in the mapped area south of the Hope fault by fractures of minor displacement that contain dike rocks (mostly diorites and lamprophyres) and weakly mineralized quartz veins, which at places also fill fractures in some of the granodiorite bodies. formed and that some of the other faults now in the mosaic were also first formed in Precambrian (pre- unconformity) time. appear to represent generally late minor adjustments in various blocks of the shattered rocks, but they are of economic significance because they were formed granodiorite age for the conglomerate. Pending fur- i ther study, we have tentatively accepted Anderson's inferred Paleozoic age of tectonism and deposition of the Sandpoint Conglomerate. Block faulting in Cretaceous time during emplace- ment of the granodiorite and related rocks was dis- cussed in some detail by Sampson (1928) and by Harrison, Jobin, and King ( 1961). The basic obser- p. 28-25; Anderson, 1930, p. 37-39; Harrison and Jobin, 1963, p. K32-K33). Faults of a still younger surge of movement on the Hope fault are those filled by Tertiary (?) granodi- orite porphyry. These are known only in the north- ernmost part of the mapped area, where they are common in and near the Purcell trench. One major group of faults, the Magee fault zone, has not been included in the summary given above. The relation of these closely spaced high-angle faults o the regional fault pattern will be discussed under "Fault Control of Emplacement of Granodiorite." marized briefly as follows : 1. The broad gentle folding of the region can ac- count for only a few thousand of the many thousands of feet of structural rise from east to west across the area. 2. The structural rise is accompanied by stepping down of the blocks in the same direction as the rise. 3. Local folding within some of the keystone blocks must have been in response to an upward push because some of the keystones are bounded by faults that converge upward. 4. Both preintrusion and postintrusion faults are documented in the area. 5. Magma invaded faulted ground; the upward pressure caused a general tectonic uplift in the west and additional uplift of some pre- existing fault blocks; a final phase of the process involved collapse in the area of general tectonic uplift. ORE DEPOSITS Descriptions of the mines and ores of the area were given by Sampson (1928), Anderson (19830, 1947), and Savage (1967). The brief comments made here are largely summaries from those reports. Ore deposits of the area are primarily narrow fissure veins, which contain ores of silver, lead, cop- per, and zinc and minor amounts of gold. Zones of altered wallrock rarely extend more than a few feet on each side of the vein. These small districts are near Clark Fork, Talache, Granite, and Lakeview (fig. 4). Neither these small districts nor any other parts of the area show the vast "bleaching" and loss of pigmented minerals (largely iron-bearing ones) so characteristic of the Coeur d'Alene district about 20 miles to the south. (See section by P. L. Weis in Fryklund, 1964.) An expansion of this concept is presented in this report in the section on the relation of aeromagnetic patterns to tectonic features. The low-angle thrust fault between Packsaddle o oi GEOLOGIC IMPLICATIONS, AEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D7 Antelope Mountain (P). 1), the outer part of the aureole is marked by conspicuous flakes of secondary biotite. This identification of the outer limit of the contact aureole cannot be applied to rocks lower in the Belt Supergroup because the regional metamor- phism had already prograded those rocks into the biotite zone. Thus, the ability to recognize the outer limit of contact metamorphism in the field becomes progressively more difficult with depth in the strati- graphic section. METAMORPHISM Several types of metamorphic effects can be seen in the rocks of the area. The most widespread effect is a regional metamorphism of the Belt Supergroup into the greenschist facies. The upper part of the Belt rocks is in the chlorite-sericite zone; secondary biotite first appears sparsely but consistently in the middle part of the Wallace Formation ; and all rocks below the middle part of the Wallace are in the biotite zone of metamorphism. The amount of biotite in rocks of appropriate composition increases pro- gressively with depth in the stratigraphic section, and the Prichard Formation commonly has at least 10 percent of secondary biotite. The Purcell sills have commonly been altered. The original intrusive rock consisted principally of quartz, calcic plagioclase, hornblende, and pyroxeng ; it contained minor amounts of magnetite, sulfide minerals, and other common accessories. At many places, particularly near faults, the present rock is largely chlorite, sericite, epidote, sodic plagioclase, quartz, and calcite with some relict hornblende and traces of magnetite, pyrite, sphene, hematite, and leucoxene. Contact metamorphism is identifiable around many of the intrusive bodies. A zone of hornfels a few feet wide was formed in the Prichard Forma- tion adjacent to the Purcell sills. Younger dike rocks commonly show chill margins against the intruded rocks; and a hornfels zone, if present, is only a few inches wide. The Cretaceous granodiorite bodies caused extensive metamorphism in the Belt Super- group and in Cambrian rocks adjacent to the in- trusive bodies. These effects were described in min- eralogic detail by Gillson (1929). Recrystallization of the rock in contact zones has at places increased the magnetite and (or) pyrrhotite content of the rocks, which results in local magnetic effects notice- able in the field through erratic behavior of the com- pass needle. Aureoles of contact metamorphism are readily identified within about 2,000 feet of an exposed in- trusive body. Limestones and dolomites are marmatr- ized at the outer edge of the aureole, and garnet, diopside, and epidote are conspicuous at the inner edge. The fine-grained argillites and siltites are prO- gressively more gneissoid from the outer to the inner part of the aureole, where clots of microcline, mus- covite, and biotite are common and crystals of anda- lusite are conspicuous in rocks of appropriate com- position. Where rocks of the Belt Supergroup above the middle part of the Wallace Formation are near an exposed intrusive, such as the small body near GEOPHYSICAL DATA THE AEROMAGNETIC SURVEY The aeromagnetic data were recorded with a flux- gate AN/ASQ-3A magnetometer towed by the U.S. Geological Survey DC-3 aircraft. The flight paths were east-west and spaced 2 miles apart in the south- ern half of the area and half a mile apart in the northern half of the area, except for a 5-mile strip at the northern end where the flight spacing Was 1 mile. The location of flight lines was shown by Meuschke, McCaslin, and others (1962). Flight ele- vation was 6,000 feet above sea level, except over Packsaddle Mountain where it was increased to 7,000 feet to clear the 6,400-foot summit. The distance of the magnetic detector from the surface varied greatly over short distances (pl. 1; 48, B3). Pend Oreille Lake is 2,048 feet above sea level, and the lake floor slopes steeply from the shore to depths of over 1,000 feet. The topographic relief of the land surface is rough; many of the higher ridge crests and peaks are at elevations between 4,500 and 5,500 feet. North of the Hope fault, the crest of the mountains ranges from 6,000 to 7,000 feet, and only a small part of this higher area was covered by the survey. The air- craft was equipped with a gyrostabilized continuous- strip camera to provide location control. The data were compiled on the U.S. Geological Survey topo- graphic quadrangle maps of the area at scales of 1:62,500 and 1 :125,000; all data were reduced to a scale of 1:125,000 on plate 1. The contour interval is 10 gammas, and the datum is arbitrary. The sur- veying techniques, instrumentation, and compilation procedures were described by Balsley (1952). MAGNETIC PROPERTIES The interpretation of the aeromagnetic data was supported by laboratory measurement of the mag- netic properties of rocks in the surveyed area. Sedi- mentary rocks rarely produce significant positive anomalies, and the flatness of the magnetic map Over the larger expanses of Belt rocks shows that these rocks have little or no anomalous character. For this ei — D8 GEOPHYSICAL FIELD INVESTIGATIONS reason, most of the samples were collected from vari- measurements were made in the U.s. Geological ous bodies and kinds of igneous rocks to evaluate Survey's magnetic properties laboratory in Silver them as sources of anomalies. Spring, Md., by William Huff. An inductance bridge was used for determining the susceptibility, and a The properties measured were magnetic suscepti- motor-driven "spinner" magnetometer (Doell and bility and magnitude and, for some specimens, direc- Cox, 1965) was used for measuring the remanent tion of the remanent magnetization (table 1). All the magnetization. TABLE 1.-Magnetic properties of rocks from the Pend Oreille area, Idaho [Data obtained from cores of samples; two sets of numbers shown if two cores from the same sample were measured. Magnetic measurements made by William Huff] Magnetic Remanent Bearing of Inclination Kiim'gsberger Sample® susceptibility magnetization declination of J ratio Rock type (k), in egs units (J), in emu/em? of J» (positive below (Q) No. x 10-s x 10-# horizontal)» Cretaceous %...... Granodiorite 0.83 0.78 349 +68 0.16 +59 16 -84 .21 -B51 57 9.4 -69 £13 -72 +77 06 +78 .05 27 .20 13 +75 .09 -+-63 17 4-80 11 +66 26 +44 35 4-48 27 +78 16 +87 14 .22 .31 §" .09 E .03 .07 .38 g 13 .03 f .20 at sin _._ ._ "_ "~~ 11 .04 .06 11 .06 .09 Bam : 12. feel amen olo, .07 .01 .03 .07 .02 .05 J . xis l. 12 do e t e ae u .09 .02 .04 10 .36 62 Ad ...... ; $o y c a ys .09 .63 Rec e- 1.2 ab .ll _ [. do .ms .... Pao en ideo en oll 11 .06 .09 26 :...... Prichard Formation, siltite speckled .97 1.18 21 with magnetite, 27. Prichard Formation pyrrhotitic argillite ___ .06 .05 16 28 a...... Burke Formation, siltite speckled .06 A1 F with magnetite. \ * Sample locality shown on plate 1, ® Declination and inclination of J shown only for oriented samples,. ° In Pend Oreille area, Q =- oser * Core showed unstable remanent magnetism, imm e GEOLOGIC IMPLICATIONS, AEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D9 Measurement of the magnetic susceptibility allows | quired largely at the time of crystallization of the comparison of the magnitude of induced magnetiza- igneous rocks, although it may be subsequently modi- tion in the present earth's field with the magnitude | fied in various ways including exposure to lightning of remanent magnetization to determine the relative | strikes or chemical alteration. The ratio of remanent importance of each. Remanent magnetization is ac- | to induced magnetization is called the Konigsberger ROCK TYPE AND SAMPLE LOCATION NO. W gx Cl _- Packsaddle Mountain 2< Granodiorite A L__ | | | | | | | | | a x | | | s * | =a | | Gran_odiori_te j 5<fi = t#---% \ \ (Granite Point) \ 6< pere ct \ \ l Granite Creek CW. l‘ 3 l \ Granodiorite as. \ ‘ l Cg JI' | --A ‘ | Ar 1 Ca ds. | 10 j . o Granodiorite P * El $1 sid E a (Whiskey Rock) | < lll Ii JFK | y | %.] | | &" } #,] | ~ =< $. a ta aer t +S | | | | f ‘Grangdiogte f l ‘ \ Lightning Creek 15< | | | Lle— | | | | Biorite: . 17- | i | 5 l Quartz monzonite 18=- l 1 l \ Lamprophyre 19- | | ‘| ll =- Granophyric silt: - 20- il i‘ ll \\ ich § i § § B& 1 3 C a #] | # | fe a [ itll. l als Prichard Formation< si‘lti-te "> |\ * t l l argillite _ 27- \ l l l Burke Formation, siltite 28- 0 1 2 3 4 5 6 7 8 9 10 12 X'10-4 emu/cm? FIGURE 3.-Magnetization of rocks from the Pend Oreille area. @, remanent magnetization; 'X, induced magnetization (in earth's field of 0.58 oersted) ; k, magnetic susceptibility. Sample numbers refer to those given in table 1 and shown on plate 1. i fl D10 ratio, or Q, which is defined as Q =- fi, where J is the remanent magnetization, H is the earth's field, and k is the susceptibility. Q is independent of the direction of the remanent magnetization and was calculated for all samples (table 1). The direction of remanent magnetization was determined for sev- eral of the granodiorites, for which oriented samples were collected in the field. The locations of the sam- ples are shown on plate 1. Figure 3 shows remanent and induced magnetizations for each sample, plotted to the same scale. More than one core was obtained from many of the samples, resulting in the paired measurements. The results of the laboratory measurements con- firm what can be deduced from the aeromagnetic map-that is, the bulk of the magnetic anomalies are produced by the bodies of granodiorite, with which they show an excellent correlation. Other rocks show an appreciable magnetization but occur only in bodies too small to affect the aeromagnetic map. The Purcell sills in the northern part of the area all have a negligible susceptibility and a remanent magnetization below the detection limit of the equip- ment. Diorites are usually quite magnetic, but, as previously noted, these Precambrian quartz diorite sills have been subjected to alteration which may have destroyed any initial magnetization. The rocks of the Belt Supergroup are generally poor in magnetite (8-5 percent combined FeQ and FeO; for most of the rocks), and the regional meta- morphism has not augmented their magnetite con- tent significantly. Samples of a few Belt rocks in which magnetite or pyrrhotite was present in ap- preciable amounts were measured ; only one of these, a Prichard siltite from a tectonically disrupted zone over an intrusive cupola about 5 miles north-north- east of Packsaddle Mountain, was magnetic. A local- ized increase in magnetization is shown in the field by erratic behavior of the compass needle in areas where some contact metamorphic aureoles are suf- ficiently high in magnetite or pyrrhotite content. In such places, the anomaly caused by the granodiorite might have been augmented slightly. Representative samples were measured for most of the bodies of granodiorite in the area. The meas- urements show that the granodiorite south of the Hope fault tends to have a susceptibility that aver- ages 0.001 egs (centimeter-gram-second) units. The remanent magnetization is also fairly uniform, with a small Q of not more than 0.2, with only a few ex- ceptions. One unoriented sample from Packsaddle GEOPHYSICAL FIELD INVESTIGATIONS Mountain had a susceptibility in the normal range but a remanent magnetization of nearly 40 (electromagnetic units per cubic centimeter). An- other oriented sample from the top of Packsaddle Mountain had a slightly higher than normal Q and a remanence in the negative direction-that is, point- ing upward. These abnormal magnetizations are the effects of lightning, which is most prevalent on peaks and other exposed locations. These magnetizations are local and random, so that for the body as a whole, the susceptibility is the dominant factor in produc- ing the observed anomaly. The remanent magnetiza- tions are small and, although the azimuths show a wide range, they dip at fairly steep angles and probably slightly enhance the induced magnetiza- tion. There were some differences in susceptibility for the various bodies sampled. The Packsaddle Mountain body shows the least scatter in values, which average 0.0009 egs units. The granodiorite of Granite Point tends to have a slightly higher value, although a sample from the block immediately to the south of Packsaddle Mountain had an abnormally low susceptibility. The Whiskey Rock group of sam- ples shows more scatter in that values of suscepti- bility range from 0.0003 to 0.0017 egs units, but the average is 0.001 egs units. In general, magnetic properties of the granodiorites south of the Hope fault are similar enough to suggest that they have a common source. North of the Hope fault, the granodiorite of Lightning Creek has a relatively low susceptibility and negligible remanent magnet- ization. The remaining measurements were of samples from several dikes and sills. A narrow diorite dike from Warren Island was intensely magnetic but was not crossed by a flight line. T' wo lamprophyre dikes of the ore-stage age also had a high susceptibility and, although too small to have any effect on the aeromagnetic survey, perhaps could be mapped by detailed ground magnetic methods. INTERPRETATION OF THE AEROMAGNETIC DATA The contoured aeromagnetic data shown on plate 1 portray large areas of remarkably low gradient upon which a preponderance of positive anomalies with small to moderate amplitudes are superimposed. The pronounced slope of the magnetic surface to the southwest is almost entirely the effect of the earth's main magnetic field, which decreases in this area at a rate of 10 gammas per mile in a direction 36° west of south. Most of the magnetic anomalies are sharp enough so that they are not obscured by this gradi- mmm GEOLOGIC 1TMPLICATIONS, AFEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D11 ent, but a few very gentle variations are brought out east, as well as emphasizes some broad low areas by removing this gradient, as shown in figure 4.| such as the one south of the Hope fault and those Removal of the regional gradient brings out a feW | along the Cascade fault and the Magee fault zone. more small positive magnetic closures in the south- der 116°00' 15 |_e0 0° ® BL ) 0G A he EXPLANATIO N w 0 _L g‘ oClark Fork < Fault A Dotted where inferred. Bar and @ 0 UP 0 ball on downthrown side \ < w- 900 --- Aeromagnetic contour Earth's main magnetic field has OC ; abinet been removed from contoured magnetic data. Interval is 50 gammas; supplementary 10- gamma contours shown only alies or clarify trends. - Ha- chures indicate closed area of lower magnetic intensity. X. position of high or low within closed contours 48° | 900 . 50 47" 45 10 MILES Socata FIGURE 4, -Aeromagnetic anomalies, major faults, and principal mining districts. where needed to identify anom- , — D12 ber of small circular anomalies in a random pattern and which includes the Magee fault zone, and (3) the western half, which has a strongly reticulate pattern of higher amplitude anomalies. rison and others, 1961) presents discussions on cor- relations in the northwestern third of the area. That report documents in some detail the close association between positive anomalies and exposures of Cre- taceous granodiorite or zones of increased metamor- phic grade in the metasedimentary rocks that indi- cate near-surface bodies of granodiorite. The most prominent of these anomalies is the high associated with the Packsaddle Mountain Granodiorite body. granodiorite. The other small highs in the north- eastern part of the surveyed area coincide with local zones of higher grade metamorphic rocks. The series of highs north-northeast of Packsaddle Mountain are over an area where the rocks are not only higher in metamorphic grade but tectonically disrupted as well; a sample of Prichard siltite from one of these localities was found to have a relatively high magnetic susceptibility. These same correlations are recognizable in the remainder of the area south of 48° N. latitude. The only two exposures of granodiorite mapped are marked by well-defined magnetic highs, one north- east of Lakeview and the other southeast of Pack- saddle Mountain (pl. 1). When the regional gradient was removed, two positive magnetic closures were isolated in the area east of the (fig. 4), but they are not as clear cut as those to the north. Perhaps a closer spacing of flight lines in the southern half of the area would have provided de- tailed data that would have outlined other small anomalies similar to those in the northeastern quar- ter of the area. The mapping of metamorphic grade netic high. The relatively high magnetic susceptibility of the granodiorite (table 1) and the obvious correlation between positive: anomalies and exposed granodi- orite bodies (pl. 1) give a high degree of assurance to the interpretation of all positive anomalies in the area as reflections of granodiorite masses. This is particularly true when one considers that ( 1) other RELATION OF AEROMAGNETIC PATTERNS TO TECTONIC FEATURES The aeromagnetic map pattern also shows some direct relations to the fault pattern (pl 1; fig. 4). In figure 4, only those faults that have an apparent vertical throw of at least 2,000 feet are shown, with the exception of the Magee fault zone, which has a cumulative stratigraphic throw of several thousand feet on many small faults and which is indicated by delineation of the bounding faults only. THE HOPE FAULT The Hope fault is clearly delineated by the con- toured magnetic data, which show a very smooth flat plane on the south side juxtaposed with a variable pattern of magnetic anomalies on the northeast (pl. 1). These anomalies are cut off abruptly at the fault. Although magnetic data are limited on the northeast side of the fault, some of the anomalies can be interpreted together with the known geology. The granodiorite along Lightning Creek was found to be less magnetic than the granodiorites south of of the Purcell sills, but they were found to have negligible magnetic susceptibility (table 1), which suggests that a near-surface body of granodiorite may be responsible. In fact, a granodiorite dike mapped just west of long W. is coincident with a smaller magnetic high. The 1,350+ gamma anomaly at the east edge of the surveyed area is probably a buried intrusive whose surface indica- tions include a fine-grained granodiorite sill and a E GEOLOGIC IMPLICATIONS, AEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D13 "bleaching" to pale green of the normally purple | if the anomaly is two dimensional and has a lin St. Regis Formation exposed in the area of the large anomaly. The Hope fault is paralleled on the south- west by a broad low which is roughly coincident with the surficial deposits of the Clark Fork river valley and the adjacent portion of Pend Oreille Lake. FAULT CONTROL OF EMPLACEMENT OF GRANODIORITE The correlation of the reticulate pattern expressed | body will be too small. in the aeromagnetic map with the complex fault syS- tem shown on plate 1 and in figure 4 is striking. Such angularity of contours is unusual and on the basis of that characteristic alone is suggestive of faulted blocks. The excellent correlation of most magnetic highs with exposed granodiorite and the uniformity of magnetic properties for the grano- diorite as a whole support the conclusion that the anomaly pattern is primarily influenced by the shape and relative depth to fault-bounded blocks of grano- diorite. Both the geologic and the geophysical evi- dence suggests that granodiorite underlies the entire area and that as the granodiorite pushed upward in a rather viscous state, it elevated some blocks along preexisting faults, and later some of the blocks col- lapsed into the cooling melt. Subsequent erosion following the structural adjustment has exposed, or nearly exposed, granodiorite in the fault blocks that now give high magnetic anomalies. The magnetic data facilitate recognition of several of these blocks. A nearly square low that surrounds Packsaddle Mountain indicates a tectonically low block, and this indication from the magnetic data is supported geologically by the preservation of Cam- brian sedimentary rocks in this dropped block. Tec- tonically higher blocks occur to the northwest at Granite Point, to the southeast between the Cascade and Magee faults, to the west at Whiskey Rock Bay, and to the northeast, although this block is more complex than any of the others. The depth of burial of the granodiorite masses was estimated for individual anomalies by means of the observed profiles and methods described by Vacquier, Steenland, Henderson, and Zietz (1951). These methods assumed that the mass of rock caus- ing the anomaly has a uniform magnetization, is flat 'topped, and extends downward vertically for a great distance. Under such conditions, the vertical dis- tance from the detector to the top of the body is a function of the horizontal extent of the steepest gradient on the side of the anomaly. The best esti- mates are made from traverses across the anomaly normal to the gradient, but a correction can be made most of the length of the ridge. ear gradient extending for a considerable distance On the map. The granodiorite blocks are almost uni- form in magnetization and are nearly vertically sided. However, many blocks do not have flat tops, and the calculated downward vertical extent may be too small if granodiorite underlies the entire region. For bodies that show a small anomaly, estimates of distance from the detector down to the top of the Error in the estimates of distance from the de- tector to the top of any particular granodiorite body in the Pend Oreille area ranges from about 10 per- cent too large an estimate, made from sharp high- amplitude anomalies, to about 35 percent too small an estimate, made on small, low-amplitude anoma- lies. Percentage of error is based on comparison of calculated elevations and actual elevations of ex- posed granodiorite bodies. When we take into at- count these limits of error, we find that with the exception of the Packsaddle Mountain body, all cupolas top out between elevations of 2,000 and 4,500 feet. The ridge of granodiorite between the Cascade fault and the Magee fault zone (pl. 1) is only about 1,000 feet below the present topographic surface for The shapes and relative depths of the granodiorite masses were further studied by selecting two repre- sentative geologic sections and calculating a theoreti- cal profile to fit the observed data along these sec- tions (pl. 1). Locations of the sections are also shown on plate 1. A-A' crosses the Whiskey Rock and Packsaddle Mountain bodies and the small cir- cular anomaly south of Cabinet. B-B' crosses the Cascade fault and the Magee fault zone in the south- ern half of the area. The gradient caused by the earth's main magnetic field has been removed from the second profiles (A2, B2) shown on plate 1. The theoretical anomaly profiles were computed by use of a FORTRAN program based on a method gimilar to that used by Talwani and Heirtzler (1964). The calculated magnetic profile fits the positive anoma- lies quite well when a susceptibility of 0.001 egs units is used in the calculations. The Packsaddle Mountain Granodiorite (pl. 1) is found to be a nar- row body. The fit was improved by allowing for the effect of the sharp crest and steep west wall. The Whiskey Rock block at the west end of section A-A' is a nearly flat-topped body about 2 miles wide. The body causing the small anomaly near the east end of section A-A' is cautiously estimated to be 3,000 feet wide and 3,500 feet above sea level. The depth of the granodiorite underlying most of the mapped — D14 area is difficult to determine. The fit of the calculated anomalies is improved by considering that a floor of granodiorite lies under the area, but the calculations are not very sensitive to changes in the level of this floor, which may be from sea level to several thou- sand feet below sea level. A representative magnetic profile B-B' across the Magee fault zone and the ridge of granodiorite to the west is shown on plate 1. The granodiorite is probably bounded on the west by the Cascade fault, but it does not extend east as far as the Magee fault zone. A deeper, wider block of granodiorite indicated by the broad low anomaly at the west end of the profile is probably a continuation of the granodiorite at Whiskey Rock. The Magee fault zone, although formed under a similar environment and during the same period of crustal adjustment as the faults making up the mosaic system, has quite a different structural pat- tern. It consists of a series of steeply dipping to vertical fault wedges within a distinct structural band 3 to 5 miles wide extending about 20 miles in a north-northeasterly direction. At the north it ends abruptly at the intersection of a group of northwest- and northeast-trending faults, and at the south (just south of the mapped area) it terminates against a series of west-northwest-trending faults. To the east it is bounded by Belt rocks of the same formational units as those in the fault zone, but which are rela- tively undisturbed and lie within the western limb of the broad gentle north-northeast-trending syn- cline. To the west the zone is bounded by a tilted fault block in which Belt rocks underlying those in the fault zone dip moderately to steeply to the east. An incomplete section of Belt rocks about 3,500 feet thick, which includes the uppermost part of the middle Belt map unit and most of the upper Belt map unit, is involved within the fault zone. In general, the strike of these rocks parallels the trace of the faults, which in turn parallels the trend of the zone, although individual fault wedges are discontinuous. From a few hundred to a few thousand feet of sec- tion make up each fault block. Typically, from west to east, one finds older to younger rocks interrupted at each fault and then a complete or partial repeti- tion of the same section in the next fault block, with a progression upward through the 3,500 feet of section (pl. 1). A positive magnetic anomaly of unusual linear extent coincides with the extent of the tilted block that bounds the Magee fault zone on the west. This anomaly has a prominent maximum over a small GEOPHYSICAL FIELD INVESTIGATIONS granodiorite pluton just west of the north end of the fault zone, and tails off gradually to the south. The juxtaposition of the magnetic anomaly with the tilted fault block and the parallelism of the unique fault zone that bounds them on the east are strongly indicative that these phenomena are related in origin. The anomaly undoubtedly shows the pres- ence of a buried intrusive whose upper surface is a ridge trending slightly east of north. The ridge either slopes gradually to the south or is downfaulted in two steps from north to south. The small pluton at the north is a cupola of this intrusive body and the only exposure of it at the surface. During its em- placement, this granitic mass shouldered up a part of the section of Belt rocks forming the tilted block. The Cascade fault is the major break bordering this tilted block on the west, and the apparent vertical throw along it amounts to as much as 12,000 feet. The Cascade fault nearly parallels the Packsaddle fault and has the same relative direction of move- ment as does the Packsaddle. The Cascade fault may have had its beginning in Precambrian time also, but much of the movement is most logically related to the intrusive period. The Magee fault zone on the east is most plausibly explained as a block that foun- dered in relation to the tilted block. A sharp mono- clinal flexure formed along the Magee fault zone during emplacement; and as tilting of the block to the west continued, successive wedges foundered from the east margin of the tilted block. The postulated origin of the Magee fault zone is shown in figure 5. At the inception of the tilting of the block and the formation of the monocline, the forces were most logically compressional (fig. 5B, C), but during the period of collapse and the founder- ing of the wedges they were tensional (fig. 5D). Scattered through the zone are small fault blocks made up of rocks younger than those in the wedges that bound them. In some blocks, the strata dip at moderate to low angles in contrast to the near- vertical attitude of the beds in the bounding wedges. One of the more unusual of the blocks, which lie along the east margin of the zone, is an isolated ex- posure of Cambrian Gold Creek Quartzite at the sur- face, which there probably lies on the upper part of the Striped Peak Formation or on the lower part of the Libby Formation. The process of formation of the Magee fault zone represents in miniature the process believed responsible for the block-mosaic pattern of the entire western half of the area (com- pare fig. 5 of this report and fig. 67.3 of Harrison and others, 1961). GEOLOGIC TMPLICATIONS, AEROMAGNETIC DATA, PEND OREILLE AREA, IDAHO AND MONTANA D15 u o < 0 w < 0 Cig p€em pCl Present p€m pEl EXPLANATION T +K + T ) 3 z P §, < < : @ Lakeview Limestone, n 2 3 Rennie Shale, and 3 #2 Middle part 0 Gold Creek Quartz- U 4 Includes Wallace, St. Regis, Revett, and Burke o ite m Formations a. pEl Lower part Prichard Formation anne Fault Incipient fracture Ep. l e in s Contact FIGURE 5.-Postulated origin of the Magee fault zone. e D16 GEOPHYSICAL FIELD IN VESTIGATIONS AEROMAGNETIC DATA AND THE MINING DISTRICTs was cool and viscous, as suggested by the small contact-metamorphic zones and the almost total lack of pegmatite or aplite in the bodies, (2) the re- gional stresses at the time of magma upwelling may have been compressional and thus aided forceful intrusion and uplift of the blocks, and (8) collapse of the blocks may have followed relaxation of re- gional stress and may have occurred after partial cooling of the melt. Ore deposits are probably related in time to the intrusive process, but the known deposits are not reflected by the aeromagnetic data. Because the wide- spread buried granodiorite masses are reflected by the aeromagnetic data and because the intrusive process took advantage of preexisting faults, aero- magnetic surveys of the faulted Belt terrane to the east of the Purcell trench might reveal areas of buried granodiorite associated with old fractures that could be broad target areas worthy of explora- tion for ore deposits. The principal mines of the area are shown in figure 4. These generally are grouped into four small mining areas near Clark Fork, Talache, Granite, and Lakeview. Many small prospects exist in the area, but the mines indicated in figure 4 are those described by Savage (1967, p. 81-98) as having had significant amounts of ore shipped from them. We can find no detailed and specific relation between either positive or negative. anomalies and the min- ing areas. This is disappointing but not suprising. None of the ores contain sufficient magnetic minerals to have caused a positive anomaly in the airborne survey, and the altered zones are too small to have caused a negative anomaly recognizable in the survey. CONCLUSIONS Aeromagnetic anomalies in the Pend Oreille area are principally a reflection of the greater magnetic intensities of surface and near-surface granodiorite masses. Positive anomalies reflect exposed or buried cupolas and stocks from a larger mass inferred to extend under most of the area. The location of the cupolas and stocks is related to major faults, most of which are believed to have been in existence at the time of the intrusion. These major faults are part of a north-trending zone of weakness that corresponds in part with the Purcell trench, and the zone has served to help localize intrusion as well as later erosion that exposed some of the intrusives. Many of these major faults are reflected in the magnetic pattern by elongate lows. The intrusive process included tectonic raising and forceful invasion of magma into a previously faulted segment of the earth's crust. The western part of the area rose considerably more than the eastern. Tilting and gross flexing accompanied in- trusion, and a mosaic of minor block faults formed as the tectonically raised area collapsed into the underlying magma. Among the more spectacular features that were formed was the Magee fault zone. It was caused by an elongate cupola that lifted and tilted a block bounded on the west by the Cascade fault. As the block continued to rise, a monoclinal flexure formed a few miles to the east beyond the edge of the cupola. This flexure eventually stretched, shattered, and collapsed to form the Magee fault zone. The complete lack of granodiorite dikes along the faults indicates that the whole process of uplift, in- trusion, and collapse did not cause sufficient ten- sional stresses to allow dike intrusion. Several fac- tors were probably involved : (1) the granodiorite REFERENCES CITED Anderson, A. L. 1980, Geology and ore deposits of the Clark Fork district, Idaho : Idaho Bur. Mines and Geology Bull. 12, 182 p. 1940, Geology and metalliferous desposits of Koote- nai County, Idaho: Idaho Bur. Mines and Geology Pamph. 53, 67 p. 1947, Geology of the lead-silver deposits of the Clark Fork district, Bonner County, Idaho: U.S. Geol. Survey Bull. 944-B, p. 37-117. Balsley, J. R., Jr., 1952, Aeromagnetic surveying, in Lands- berg, H. E., ed., Advances in geophysics: New York, Academic Press, v. 1, p. 813-349. Doell, R. R., and Cox, Allan, 1965, Measurement of the remanent magnetization of igneous rocks: U.S. Geol. Survey Bull. 1203-A, p. A1-A32. Fryklund, V. C., Jr., 1964, Ore deposits of the Coeur d'Alene district, Shoshone County, Idaho, with a section on The bleached rock in the Couer d'Alene district, by P. L. Weis: U.S. Geol. Survey Prof. Paper 445, 103 p. Gillson, J. L., 1927, Granodiorites in the Pend Oreille district of northern Idaho: Jour. Geology, v. 35, no. 1, p. 1-31. 1929, Contact metamorphism of the rocks in the Pend Oreille district, northern Idaho: U.S. Geol. Survey Prof. Paper 158-F, p. 111-121. Griggs, A. B., 1968, Geologic map of the northeast part of the Spokane 2° quadrangle, Idaho and Montana: U.S. Geol. Survey open-file map. Harrison, J. E., and Jobin, D. A., 1963, Geology of the Clark Fork quadrangle, Idaho-Montana: U.S. Geol. Survey Bull. 1141-K, p. K1-K38. 1965, Geologic map of the Packsaddle Mountain quadrangle, Idaho: U.S, Geol. Survey Geol. Quad. Map GQ-375, with 4 p. text. Harrison, J. E., Jobin, D. A., and King, Elizabeth, 1961, Struc- ture of the Clark Fork area, Idaho-Montana, in Short papers in the geologic and hydrologic sciences: U.S. Geol, Survey Prof. Paper 424-B, p. B159-B163. GEOLOGIC IMPLICATIONS, Hobbs, S. W., Griggs, A. B., Wallace, R. E., and Campbell, A. B., 1965, Geology of the Coeur d'Alene district, Sho- shone County, Idaho : U.S. Geol. Survey Prof. Paper 478, 139 p. Kirkham, V. R. D» and Ellis, E. W., 1926, Geology and ore deposits of Boundary County, Idaho: Idaho Bur. Mines and Geology Bull. 10, 78 p. Meuschke, J. L McCaslin, W. E., and others, 1962, Aero- magnetic map of part of the Pend Oreille area, Idaho : U.S. Geol, Survey open-file map, 2 sheets. Sampson, Edward, 1928, Geology and silver ore deposits of the Pend Oreille district, Idaho: Idaho Bur. Mines and Geology Pamph. 31, 25 p. Savage, C. N. 1962, Geomagnetics and geologic interpreta- tion of a map of eastern Bonner County: Idaho Bur. Mines and Geology Inf. Cire. 15, 16 p. AEROMAGNETIC DATA, e s PEND OREILLE AREA, IDAHO AND MONTANA D17 1967, Geology and mineral resources of Bonner County: Idaho Bur. Mines and Geology County Rept. 6, 131 p. Talwani, Manik, and Heirtzler, J. R. 1964, Computation of magnetic anomalies caused by two dimensional structures of arbitrary shape, int Computers in the mineral indus- tries, Pt. 1: Stanford Univ. Pubs. Geol. Sci., v. 9, no. 1, p. 464-479. Vacquier, V. V. Steenland, N. C. Henderson, R. G. and Zietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geol. Soc. America Mem. 47, 151 p. Yates, R. G., Becraft, G. E., Campbell, A. B., and Pearson, R. C., 1966, Tectonic framework of northeastern Wash- ington, northern Idaho, and northwestern Montana : Canadian Inst. Mining and Metallurgy Spec. Volume 8, p. 47-59. f # U.S. GOVERNMENT PRINTING OFFICE: 1970 O-383-350 @ \ Gravity and Magnetic Anomalies © in the Soda Springs Region, Southeastern Idaho (~ Jun 8- 1970 * if} > wo \ T/ io Q ¢ ¥ ! Oniraunt aC 24 \ YZ SCciEnct US Gravity and Magnetic Anomalies in the Soda Springs Region, Southeastern Idaho By DON R. MABEY and STEVEN S. ORIEL GEOPHYSICAL FIE L D INV ES TIG AT IONS GEOLOGICAL sURVE Y PROFESSIONAL PAPER 616- FP Geop/zysz'ca/ data indicate the thickness of Cenozoic sedimentary rocks and the distribution, and locally the thickness, of ig neous rocks UNITED STATES GOVERNMENT PRINTING» OFFICE, wWASHINGTON :; 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page lec. e. conn "f E1 | Geophysical E4 o > 1 Gravity survey and 4 eneral gediogy. .. l..... ...-. Aeromggnetic survey and map.----- -- 5 Physi Fell. eep set > 3 Interpretation of local geophysical anomalies.----------- 5 ysiography Gem Yall z em Valley:...... ci- _o" 5 Stratigraphy - oal . __ 2 Blackfoot lava field-....-~.----«----=:--->----~~ 8 = 3 Upper Blackfoot River 11 Pertinent rock physical properties..._.___..----------~ 3 Local anomalies in the 11 ices ao" 3 | Regional gravity 12 _ 4 | "~" 15 o ILLUSTRATIONS Page Prats 1. Geologic and gravity-anomaly map and section of the Soda Springs region, southeastern Idaho...--------- --- In pocket 2. Aeromagnetic map of the Soda Springs region, southeastern 00 __ In pocket fiabre i Rion ep .... cn nang on n heme dr Tae. El 2. Density of major ro amine ns. onne rac ene nar rse ren on 2 00 aon 4 3. Profiles across southern ern cl,. cel-. capedec scena n ttl 6 4. Aeromagnetic profile in Appin ium aP o onl r 1". 11 5. Regional gravity wyt meto oo oci un- 13 GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY AND MAGNETIC ANOMALIES IN THE SODA SPRINGS REGION, soUTHEASTERN IDAHO By Dor R. Marex and Srevex S. Ortet ABSTRACT Two major gravity lows indicate deeply filled troughs in Gem and Bear Lake Valleys in the western, block-faulted part of the Soda Springs region. A third large and compound gravity low at China Hat may record an igneous collapse structure, but the evidence is equivocal. In contrast, the relatively uniform gray- ity values in the eastern, mainly folded, part of the region indi- cate little detrital fill in strike valleys in the folded and thrust belt. Gravity data also indicate that variations in regional topography in the surveyed area are isostatically compensated but that the region in general is undercompensated. All but two of the aeromagnetic anomalies are attributable to surface volcanic features and to structures inferred there- from. The magnetic anomalies indicate the distribution and thicknesses of basaltic rocks. The largest anomalies, north of Niter, west of Grace, and north-northwest of China Hat, are associated with vents and craters, and these may be guides to concealed related intrusive bodies. The magnetic survey also confirms the presence of extensive areas of inversely mag- netized, slightly older basalt east of the Blackfoot River Reservoir. Two positive magnetic anomalies of undetermined origin at Eighteenmile Creek in the Chesterfield Range may reflect dikes or sills of Cenozoic age within Paleozoic sedi- mentary rocks, lava flows within Pliocene strata, or secondarily iron-enriched Carboniferous beds. INTRODUCTION Gravity and aeromagnetic surveys were made in the Soda Springs region as a part of the more extensive geophysical studies undertaken in conjunction with con- current geologic investigations in the Idaho-Wyoming thrust belt. The gravity measurements are useful in esti- mating depths of fill in intermontane basins and in delimiting partly concealed bounding faults. Aeromag- netic data are particularly helpful in outlining partly concealed bodies of basalt which are abundant in the re- gion and, indirectly, in locating ancient stream valleys. Because none of the local magnetic anomalies seem to be related to basement rock, the basement must be either nonmagnetic or deeply buried. We have been unable to apply the geophysical data directly to the recognition of thrust-fault relations at depth. The surveys cover about 1,200 square miles in the Portneut, Henry, Bancroft, Soda Springs, Lanes Creek, and Slug Creek 15-minute quadrangles, which are all in the northwest quarter of the Preston 1° by 2° sheet. Major topographic features in the region include, from west to east, a part of the Portneuft Range, Gem Valley, the Chesterfield Range, the north end of the Bear River Range, the Blackfoot lava field, the valley of Bear River near Soda Springs, and small ranges and basins in the upper drainage of the Blackfoot River (fig. 1). Gravity observations were made by Mabey during several brief periods from 1961 to 1964. The aeromag- netic surveys were flown in 1960 (Meuschke and Long, 1965) and 1963 (Mitchell and others, 1965). Geologic material was assembled by Oriel from published reports 111° 112" $ > e go* 43° j | \ Pocatello vo \ | P I Ortneyy 3 1 | A l i Z 1 C + | § *+ River) (( 1 | * 1 | 13), Grace [L5 m f; O- T & ZI p S o >. 5 2 V1 AREA OF o C INDEX MAP io. <. l| 3) C as Malad a; £ Preston m |I o tase ise __ _ _ ... e x UTAH 0 10 20 30 40 MILES FIGURE 1.-Index map of Soda Springs region, southeastern Idaho, showing area of gravity and magnetic surveys (shaded). E1 E2 by Mansfield (1927, 1929), unpublished maps of the Soda Springs quadrangle by Frank C. Armstrong (1969) and of the Bancroft quadrangle by Oriel (1968), and observations made during reconnaissance mapping of the Preston 1° by 2° sheet. Although the reconnais- sance geologic mapping underway in the Preston 1° by 2° quadrangle has not been synthesized, publication of the geophysical results seems desirable, despite the pos- sibility that the work now in progress may result in future reinterpretations. We gratefully acknowledge Mr. Armstrong's gen- erosity in making his map available to us before publication. We are also indebted to Mr. Armstrong, H. J. Prostka, L. B. Platt, and M. D. Kleinkopt for their many helpful suggestions during review of this manuscript. GENERAL GEOLOGY PHYSIOGRAPHY The Soda Springs region lies in both the Basin and Range and the Middle Rocky Mountains physiographic provinces. The boundary between the two provinces commonly has been placed along the west edge of the Wasatch and Bear River Ranges (pl. 1) and at various places north of Soda Point. The boundary was first placed along the course of the Blackfoot River (Fen- neman, 1917, p. 82), was later shifted to the valley of Meadow Creek (Mansfield, 1927, p. 11), and still later was placed along the east edge of the Blackfoot and Willow Creek lava fields (Fenneman, 1931, footnote 2, P. 170). Although some of the criteria used to recognize the boundary * would support placing it along the east side of the Bear Lake and Blackfoot River Reservoir valley (along the west edge of the Aspen Range), no attempt is made here to redefine the boundary. The western, or Basin and Range, part of the region consists of wide, deeply filled flat basins which separate broken and tilted ranges formed by block faulting. Local relief is moderate, with altitudes ranging from 9,167 feet at Sedgwick Peak in the Portneuf Range to 4,950 feet along Bear River 7 miles to the east. The eastern, or Rocky Mountain, part of the region consists of numerous narrow subparallel ridges and the east the closely crowded mountains whose forms are determined chiefly by close folding, thrust-faulting, and erosion, and on the west what seem to be block mountains due perhaps to normal faulting, more GEOPHYSICAL FIELD INVESTIGATIONS thinly filled valleys of the folded and thrust belt. Local relief, which is moderate, decreases northeastward as the altitudes of valley floors increase. The region lies in the Columbia River and Great Basin drainage systems. The northern and westernmost parts of the region are drained by the Blackfoot and Portneuf Rivers, which flow into the Snake River, whereas the southern part is drained by the Bear River, which flows into Great Salt Lake. Drainage divides in Gem Valley and on the Blackfoot lava field are low and barely perceptible. Anomalous features characterize all three of the major rivers in the western part of the region and are products of considerable late Cenozoic deposition, vol- canism, and recurrent block faulting. The Portneuf River heads near the Blackfoot River, flows southward through Gem Valley, leaves the valley through a canyon on the west, flows south and west before turning north to enter American Falls Reservoir 13 miles from the mouth of the Blackfoot River. The Bear River heads in the Uinta Mountains and flows generally northward for about 150 miles before turning sharply, at the bend near Soda Springs, to flow southward toward Great Salt Lake. The Blackfoot River flows west-southwest before turning to the northwest in channels cut in basalt flows. The divide between the Blackfoot and the Bear Rivers is about 3 miles south of, and less than 100 feet higher than, the Blackfoot River at the bend. The divide between the Portneuf and Bear Rivers in Gem Valley is within 1 mile of, and about 100 feet higher than, the Bear River but is less than 50 feet higher than the top of the channel cut by the river in the basalt flows. Quaternary basalt flows are thick enough to have diverted any of the three major rivers from their pre- basalt courses. Drainage in the eastern part of the region is much more regular; a few of the main streams cut across strike ridges, but almost all the tributaries occupy strike valleys which parallel fold axes and faults. Erosion of old strata predominates over recent deposition. A few anomalous, wide, continuous valleys, such as Dry Val- ley, are undoubtedly related to stream capture. STRATIGRAPHY Stratigraphic units recognized in southeastern Idaho are summarized in the explanation on plate 1. Thick- nesses and facies of all Paleozoic and Mesozoic rock units in the region (Mansfield, 1927 ; Armstrong, 1953; Armstrong and Oriel, 1965) indicate that the rocks formed within the Cordilleran miogeosyneline. Both the oldest and the youngest rocks of the region are extensively exposed in the Basin and Range part of the region; rocks of intermediate age are exposed only e GRAVITY AND MAGNETIC ANOMALIES, sparsely but may be more abundant at depth. The Cam- brian and Precambrian Brigham Quartzite and over- lying Cambrian units are abundantly exposed in the Bear River Range and are predominant in the Portneuf Range and the several ranges to the west. Underlying Precambrian metamorphic rocks are exposed no closer than the Ogden, Utah, area, 60 miles to the south. Or- dovician through Triassic sedimentary rocks, mainly carbonates, underlie the mountain ranges. Upper Terti- ary sediments assigned to the Salt Lake Formation bury parts of the ranges and are thousands of feet thick in the intervening basins. Upper Cenozoic volcanic rocks are especially extensive in the basins and join northward with similar rocks of the Snake River Plain. Most of the volcanic rocks are of Pleistocene age, although some are as old as Pliocene, and others, as young aS Holocene. Basalt predominates overwhelmingly, but rhyolite is also present, primarily in the cones in and south of the Blackfoot River Reservoir. Water wells indicate that the basalt is, in places, at least several hundred feet thick. Upper Paleozoic and Mesozoic rock units underlie most of the Middle Rocky Mountains province; the province has no exposed rocks older than the Devonian in the region discussed, although some are present far- ther east. The units present extend from the Devonian Jefferson Dolomite to the Jurassic Twin Creek Lime- stone. The most extensively exposed units, however, are of Pennsylvanian, Permian, and Triassic age. The province includes comparatively little upper Tertiary Salt Lake Formation and Quaternary basalt. No Cretaceous or lower or middle Tertiary rocks are in the region, except for Lower Cretaceous strata that crop out in the Caribou Range, in the northeast corner of the mapped area. Strata in the southeastern part of the region, formerly assigned to the Wasatch Forma- tion (Mansfield, 1927, pl. 6), are regarded on the basis of recent observations as considerably younger. STRUCTURE The western part of the Soda Springs region is characterized mainly by homoclinal strata cut by sev- eral sets of block faults of late Cenozoic age. Although broad broken folds are also present, pre-Mesozoic strata in most ranges dip 20°-30° east-northeastward. Only one thrust fault, the Paris fault of the Bannock zone (Armstrong and Cressman, 1963, p. J8), is present and well documented in this part of the region. The fault ac- counts for the juxtaposition of the Brigham Quartzite and the Thaynes Limestone near Cavanaugh Siding (plate 1, southern part), but its trace is covered north- ward. Several generations of steeply dipping faults strike north-northwestward, northward, and northeast- sODpA SPRINGS REGION, soOUTHEASTERN IDAHO E3 ward. Many cut the Salt Lake Formation. Young range- front faults strike north-northwestward and northward. They clearly cut the Salt Lake Formation, and a few offset post-Pliocene basalt flows. Fairly recent recur- rent movement apparently has occurred along some faults, but data are insufficient to date all the times of movement along the various faults. A better understand- ing of the stratigraphy and structure of the Salt Lake Formation would help date many events, for the strata of the Salt Lake Formation are involved in at least some of the deformation. The eastern part of the Soda Springs region is char- acterized by thrust faults (Rubey and Hubbert, 1959, p. 186-200) and tight asymmetrical folds. Block faults are also present, but they are not so dominant as they are farther west. The major thrust of this region is the Meade fault (Armstrong and Cressman, 1963, p. J8; Cressman, 1964, p. 68-80), which places Mississippian limestone and, locally, Devonian dolomite over Upper Jurassic and Lower Cretaceous strata. This fault, which is a single surface in its westernmost exposures, splays into a thrust zone of several slices eastward along Crow Creek valley, Sage Valley, and Stump Creek valley. Folds above and below the fault are asymmetrical and have axial planes that dip moderately to steeply west. The folds and the Meade fault were formed at about the same time. Movement along the Meade fault prob- ably occurred late in Early Cretaceous time, somewhat later than the first major movement along the Paris fault, which occurred in latest Jurassic and earliest Cretaceous time (Armstrong and Cressman, 1963, p. J14-J15; Oriel and Armstrong, 1966). Block faults and some transverse faults cut the Meade fault and, there- fore, are considerably younger, although some of these faults were formerly mistaken for the Bannock fault (Mansfield, 1927). The block faults are mainly normal faults, downthrown to the west, and strike mainly north- northwestward. Some of them cut Salt Lake strata and are clearly post-Pliocene, but others cannot be dated more closely than postthrust. The transverse faults strike due east and apparently are strike-slip faults. They offset minor thrust slices and adjoining strata. Their ages are not established, but the offset would sug- gest that they may be either tear faults formed in a late stage of thrusting or strike-slip faults formed later. PERTINENT ROCK PHYSICAL PROPERTIES DENSITY Densities measured on several hundred samples of major rock units are summarized in figure 2. The den- sities of the Quaternary and Tertiary sediments are estimates based on measurements of a few samples of Salt Lake Formation and on measurements made on h E4 GEOPHYSICAL FIELD INVESTIGATIONS similar sediments in other areas. The composition of the Salt Lake varies so markedly that an estimate is necessary. Because no crystalline basement is exposed in the area, the density of the basement is assumed to be similar to that of the Farmington Canyon Complex of Eardley (1939) in Utah. Density, in general, increases with increase in age of the strata down to Middle Cambrian carbonate rocks. Lower Cambrian and upper Precambrian quartzites, however, are less dense than the overlying carbonate rocks. The largest density contrast is between Cenozoic detrital strata and the Paleozoic carbonate rocks. In the analysis of the gravity lows produced by Cenozoic basin fill, a density contrast of 0.45 g per em® (gram per cubic centimeter) between the fill and the enclosing rock is assumed. If all Cenozoic material in a basin were uncon- solidated sediments, the density contrast could be as great as 0.6 g per em?, and the actual thickness would be about 75 percent of that computed with an assumed tive to Paleozoic rocks, DENSITY, IN GRAMS PER CUBIC CENTIMETER 20 21 22 331. 324 28: 26. 21 2s 2_i9 Quaternary * BasaI’t _-_ Tertiary __| f Magnetic properties were FEET 0 Cretaceous 5000 10,000 Whore ences. 2 l. at has l_ Jurassic Jy where all the basalt appears Permian cm was assumed. Mississippian GEOPHYSICAL Devonian ilurian m Ordovician e e dias Cambrian rangle maps. Altitudes of ad , of low relief were determined Upper Precambrian estimated to be about 20 feet, Lower Precambrian Froure 2.-Density of major rock units in the Soda Springs region. Thickness of units based mostly on data from Mansfield (1927). Asterisk (*) indicates estimated density. | variations. density contrast of 0.45 g per cm*. A density contrast of less than 0.45 g per em' is possible in a basin filled with dense volcanic units or well-indurated sediments. Under these conditions the actual depth of fill could be several times the computed depth. Thick accumulations of Mesozoic strata, and perhaps thick sequences of Cambrian and Precambrian quartzite, could also pro- duce gravity anomalies of sufficient amplitude and ex- tent to be apparent in the gravity survey reported here. The average density of dry samples of basalt in the region is slightly less than that for dry samples of Pale- ozoic rocks in the region. If the rocks are saturated with water, the density of the basalt is equal to, or slightly greater than, that of Paleozoic rocks. However, scoria, abundant unfilled joints, and inclusions of Cenozoic sediments reduce the average density of volcanic-rock sequences to below that of Paleozoic rock. Thick se- quences of volcanic material are positive mass anomalies relative to Cenozoic sediments, but are negative, rela- MAGNETISM not measured systemati- cally for various rocks in the region, but they were measured on six oriented samples of basalt from four localities in Wooley Valley. The total in-situ magmnet- ization of these magnetized rock samples is about 5x10° emu per em (electromagnetic unit per cubic centimeter) in a direction approximately opposite to that of the earth's present magnetic field (reversely magnetized). A magnetic anomaly produced by a known thickness of normally magnetized basalt in Gem Valley suggests a total magnetization of about 5.7 x 10 emu per cm' Quantitative analyses of the magnetic anomalies produced by the basalt were attempted only to be magnetized in the direction of the earth's magnetic field, and a total mag- netization (induced and remanent) of 5.7 X 10 emu per SURVEYS GRAVITY SURVEY AND MAP Most gravity observations were made where the alti- tude could be obtained directly from bench marks or spot elevations shown on U.S. Geological Survey quad- ditional stations in areas by altimeter surveys. The maximum error in altitude of the gravity stations is and the altitudes of most stations are known to within 5 feet. Although most stations are in the valleys, enough observations were made in the ranges to define the larger regional GRAVITY AND MAGNETIC ANOMALIES, SODA SPRINGS REGION, soUTHEASTERN IDAHO Observed gravity values were referenced to base station WU29 in Salt Lake City, Utah (Behrendt and Woollard, 1961). Theoretical gravity was computed from the International Formula. Bouguer anomaly values were computed by using an assumed density of 2.67 g per em' for material above sea level. Terrain corrections in excess of 1 mgal (milligal) through Hayford zone O have been applied (Swick, 1942). Terrain corrections range from less than 1 mgal in the larger vaileys to 24 mgal for a station on Sedgwick Peak. Bouguer anomaly values (pl. 1) range from -164 mgal for stations in the Portneuf Range to -210 mgal for a station in Bear River valley. The largest local gravity anomalies are in Gem Valley, over the Blackfoot lava field south of Henry, and in Bear River valley southeast of Soda Springs. These anomalies occur at exposures of Cenozoic sedimentary and volcanic rocks and presumably reflect depressions filled with these low-density rocks. Small, but significant, local gravity variations occur over pre-Tertiary rocks in several of the mountain ranges. The local anomalies are super- imposed on a regional southeastward decrease in gravity values. , AEROMAGNETIC SURVEY AND MAP The aeromagnetic survey was flown in two parts. The first was a survey of a small area east of Henry (Meuschke and Long, 1965). Flight lines were approxi- mately N. 45° E., half a mile apart and 7,000 feet above sea level. The data were obtained with a modified ASQ- 3 fluxgate magnetometer towed from a DC-3 aircraft. Data for the more extensive survey (Mitchell and others, 1965) were obtained with an ASQ-8 fluxgate magnetometer in the tail boom of a Convair aircraft flown along east-west flight lines 1 mile apart and 9,000 feet above sea level. On both surveys, flight path of the aircraft was recorded by a gyrostabilized continuous- strip-film camera; magnetic data were compiled rela- tive to an arbitrary, but approximately common, datum. Magnetic contours are shown on plate 2 as an overlay for plate 1. Because the flight lines are at different alti- tudes and have different spacing and orientation, mag- netic contours do not exactly match across the boundary between the surveys. Although the magnetic field is complex within the region it is, in general, adequately defined at the flight altitudes. Complex. magnetic anomalies prevail in the central and northeastern parts of the mapped area. These anomalies reflect Cenozoic volcanic rocks that are ex- tensively exposed in the basins and, locally, in the ranges. The dominant magnetic anomalies are highs flanked on the east or northeast by correlative lows. However, in the northeastern part of the mapped area 359-233-T70--2 E es E5 several discrete magnetic lows are apparent. Magnetic relief is low where Cenozoic volcanic rocks are absent; notable exceptions are a pair of north-trending highs in the Chesterfield Range. INTERPRETATION OF LOCAL GEOPHYSICAL ANOMALIES GEM VALLEY More gravity observations were made in Gem Valley * than in any other area included in the survey. Prelim- inary results of the gravity survey and seven low-level aeromagnetic profiles were described earlier by Mabey and Armstrong (1962). Additional gravity observa- tions made in the valley since preparation of the pre- liminary report do not substantially change the earlier map. The 35-mgal gravity low in Gem Valley, which is larger than any other measured in this survey, is com- parable in magnitude to the larger anomalies associated with other intermontane basins in the Western United States. The residual amplitude of the gravity low in Gem Valley is largest in the northern part of the valley, de- creases over a gravity saddle northwest of Grace, and increases again toward the south end of the valley. In the north end of the valley, the lowest gravity values are east of the center of the valley ; in the southern part, the lowest values are near the center of the valley. The gravity anomaly in Gem Valley indicates a deep trough of low-density material bounded by steep sides (pl. 1, section A-4'). An earlier interpretation (Mabey, 1964) that the trough may have formed as a pull-apart gap at the rear of a major overthrust sheet (Rubey and Hubbert, 1959, p. 194) is now regarded as improb- able, on the basis of stratigraphic data from bounding ranges (Oriel and others, 1965). The trough is probably a sediment-filled graben. If all the gravity anomaly in Gem Valley is interpreted to reflect low-density sedi- ments, the sediments are thickest north of Bancroft, thinner near the old settlement of Central, and thicker southward. The trough narrows at the north end, but its floor apparently remains relatively flat as far north as the north end of the Portneuf Reservoir, where it rises abruptly. Because no deep holes have been drilled in the valley, | the nature of the low-density material producing the large gravity low is not known. The gravity low may reflect poorly indurated detrital strata of three possible ages: Quaternary, Tertiary, and Mesozoic. Quaternary sediments and lavas cover the area of the gravity low everywhere except on the southwest side of the Port- neuf Reservoir, where an exposure mapped as Salt Lake Formation (Mansfield, 1929, pl. 1) extends to the 2 Gem Valley, as used in this report, includes Portneuf Valley and the north end of Gentile Valley. h E6 center of the low. If the indicated exposure is, indeed, Salt Lake, the gravity anomaly must be produced largely by low-density rocks of Salt Lake age or older. Moreover, post-Salt Lake sediments are probably too thin in the region to account for an anomaly of this magnitude. Triassic rocks, such as those exposed in the northern part of the Chesterfield Range, and other Mesozoic rocks could be preserved under Tertiary rocks in Gem Valley. However, the density contrast between Mesozoic and Paleozoic rocks is only about 0.15 g per cnr ; a trough 5 miles wide containing 20,000 feet of lower Mesozoic rocks (their maximum probable thickness) would produce only about 25 mgal of the 35-mgal anom- aly. Although Mesozoic rocks may be preserved under Gem Valley, they cannot account for the entire gravity B 1600 - 1500 - 1400 - IN GAMMAS 1300 - TOTAL INTENSITY MAGNETIC FIELD, =170 -I =-180- —190J 10,000'- GRAVITY, IN MILLIGALS GEM VALLEY 5000'7 SEA Aa=-0.45 g per cm? GEOPHYSICAL FIELD INVE STIGATIONS anomaly, unless they are unusually thick, or unless a thick prism of low-density Cretaceous detritus (un- known this far west) is present. The latter possibility is unlikely. Basalt flows in Gem Valley do not seem to affect the gravity anomaly significantly. An exception may be north of Grace; there, the amplitude of the gravity low is reduced where magnetic data indicate a thick- ening of the basalt along the east side of the valley (fig. 3). Steep gravity gradients along or near the margins of Gem Valley are interpreted as indicating high-angle faults. Places where these inferred faults coincide with the range front are as follows: 2 miles northwest of Bancroft; an area several miles long north of Monroe Canyon; at Buckskin Mountain; and in the north end Bl - Ac=-0.22 g per cm3 LEVEL SCALE .1:125,000 0 AGAR 1 i 3. -Gravity, section shown on plate 1. Ao, ‘15 MILES 1 1 aeromagnetic, and interpreted geologic section across southern Gem Valley. Line of assumed density contrast. GRAVITY AND MAGNETIC : ANOMALIES, SODA SPRINGS REGION? soUTHEASTERN IDAHO of Gem Valley. Elsewhere, the major inferred faults lie within the valley and separate a belt of fill of inter- mediate thickness adjacent to the range front from a thick prism of fill along the axis of the valley. An area of several square miles located about 5 miles northwest of Bancroft and west of the inferred fault along the west side of the valley probably is underlain by Paleo- zoic rock at relatively shallow depths. Major features of the gravity anomaly in Gem Valley parallel the axis of the valley; however, the linearity of some of these features is locally disrupted. These disruptions reflect variations in the thickness of fill which are controlled either by structures transverse to the valley or by a buried erosion surface on the older rocks. About 1 mile southeast of Bancroft, the gravity relief decreases along a probable transverse fault, such as the fault east of Gem Valley, 2 miles south of Ten- mile Pass, where it strikes east-northeast, and the faults west of Gem Valley just south of Red House Canyon, where they are about 1 mile south of the locus suggested by the gravity data. 'The relation of these faults to those mapped on the east side of Gem Valley, just north of Upper Valley, is undetermined. The gravity data sug- gest that the embayment in the range front at Lund may be bounded by faults which parallel probably related faults mapped in bedrock on the north, west, and south sides of the embayment. Faults mapped in the bedrock west of Turner can be inferred from the gravity data to continue into the valley. South of Tur- ner the gravity data suggest either complex structure along the range front or an intricate buried topographic surface. The magnetic survey, which was flown 3,500-4,000 feet above the land surface, shows extensive anomalies that indicate the thickness and distribution of basalt flows and related intrusive masses. Craters near Niter and Grace coincide with large-amplitude magnetic highs (fig. 3). The coincidence suggests that these anomalies are partly produced by basalt necks and dikes that fed the basalt flows. The magnetic highs near Grace and Niter are probably composite features produced by both intrusive units and local, thick sequences of flows. In areas away from the basalt sources, the magnetic anomalies probably reflect the relative thickness of flow sequences and the proportion of interbedded sediments. Little is known about the relief on the prebasalt floor of Gem Valley. Undoubtedly, there was some relief re- lated to the ancestral major drainage systems of the Portneut and Bear Rivers (Bright, 1963), but the ab- sence of any older rocks protruding through the basalt suggests either deep burial or shallow relief. The basalt came from vents in the Grace-Niter area and from vents in the Blackfoot lava field via Tenmile Pass and E ET perhaps the gap at Soda Point. The thickest accumu- lations of basalt are probably in prebasalt topographic lows, such as river channels, and near-source areas. Con- versely, the basalt is thinner over prebasalt topographic highs and where waterborne sediments were deposited in the valley during volcanism. The magnetic high in the north end of Gem Valley is along the west edge of a deep trough indicated by the gravity data. The location suggests that this mag- netic belt reflects a prebasalt topographic low that now contains a greater thickness of basalt flows than do the adjoining areas. If an intensity of magnetization of 5.7X10% emu per em? is assumed for the basalt sequence, the base of the thickest accumulation of basalt is about 5,000 feet above sea level. Northward, in the Chesterfield Reservoir area, where basalt is absent both on the surface and in shallow wells, the magnetic data .do not indicate that basalt is present at depth, either. Although basalt seems to be thin over much of the area of relatively shallow fill northwest of Bancroft, local thickenings are indicated by magnetic highs extending westward into the canyon of the Portneuf River, and also along the northeast front of the Fish Creek Range. These elongate magnetic anomalies suggest ancient river channels filled with basalt. The magnetic anomaly at the head of the canyon of the Portneut River is paral- lel to the flight lines of the magnetic survey; hence, it is not well defined, but the indicated amplitude of this anomaly suggests that the basalt is about 300 feet thick. A detailed gravity profile across the head of the canyon did not reveal a gravity low, such as would be expected if several hundred feet of sediments underlay the basalt. A magnetic low at Hatch suggests that the basalt is relatively thin over an area of several square miles. Abundant prebasalt travertine now exposed in this area may be the reason. The magnetic high in, and west of, Tenmile Pass reflects both higher elevation and greater thickness of basalt than in adjoining areas, for lava entered Gem Valley from the Blackfoot field via the pass. The zone of relatively low magnetic intensity north of Talmage Siding coincides with a slight topographic low; this probably reflects an area of basalt thinner than that in the areas to the north and south, which were near the sources of flows. A drill hole 295 feet deep in this area about 1 mile from the east edge of the valley penetrated only T5 feet of volcanic material, and this, within 90 feet of the surface. The northwest-trending zone of higher magnetic intensity northeast of Talmage Siding may reflect a local prism of more abundant basalt. Along the west side of the valley near Bancroft, an elongate magnetic high, which is continuous with the o E8 GEOPHYSICAL FIELD INVESTIGATIONS high to the north, can be explained best as a local basalt- filled channel largely concealed by Holocene sediments. Basalt and interbedded sediments to depths of 270 and 283 feet were found in two water wells along the axis of the high about 1-2 miles south of Bancroft, where the surface elevation is about 5,450 feet above sea level. The measured magnetic anomaly in the area of these wells could be produced by a prism of basalts and sediments 300 feet thick and 3,000 feet wide and with an average intensity of magnetization of 4% 10~ emu per cms. ® Basalt must be absent west of the prism and thinner east of the prism. The former was confirmed by drilling, but no test drilling has been done to verify the latter. The large magnetic high in the south end of Gem Valley could be caused almost entirely by a thick ac- cumulation of basalt flows or by a combination of basalt flows and intrusive bodies related to the cones and craters. About 1,500 feet of basalt with an intensity of magnetization of 5.7 X 10-* emu per cm® would produce the observed anomaly. However, the very close coinci- dence of the crest of the magnetic anomalies with large craters suggests that at least part of the anomaly is related to basaltic intrusive bodies. Along the west side of the valley from Lund south- ward, the magnetic pattern is relatively complex. The magnetic nose west of Niter coincides with several craters and with the west edge of the basalt flows. The magnetic high east of Buckskin Mountain, along North Extension Canal, coincides with extensive exposures of basalt, whereas the low to the north coincides with moderately thick deposits of silt southwest of Central. The magnetic anomalies probably also reflect the proportion of Quaternary lacustrine sediment that in- tertongues eastward with basalt flows. These inter- tonguing relations are well displayed for the higher flows in the valley of Bear River a few miles to the south. The closed high northeast of Lund appears to be produced by a widening of the inferred basalt prism which is to the north. Alexander Crater and adjacent craters to the south lie along a zone of moderately steep gravity gradients on the east side of Gem Valley. If allowance is made for the gravity effect of thick flows or of intrusive units in this area, a fault zone inferred from the gravity could control crater locations (fig. 3). However, the craters north and northwest of Niter are not along this inferred fault but are near local, irregular gravity anomalies along the west side of the valley. These latter craters, which lie along the projected trace of a minor castward-trending fault mapped in Ordovician rocks ® The drill holes indicate that about 70 percent of the material is basalt. Therefore, the indicated intensity of magnetization of ' the basalt is 5.7 X 10-% emu per ems. in the east end of Beaver Basin, may well indicate local structure transverse to Gem Valley. In summary, the gravity and magnetic data for Gem Valley provide several clues to the geologic history of the valley. The data suggest that subsidence of the cen- tral trough, at least in the northern part of the valley, occurred during deposition of the Pliocene Salt Lake Formation. Although Quaternary sediments and basalt are exposed in most of the valley, the major part of the low-density material, estimated to be about 10,000 feet thick, is Tertiary. Mesozoic strata may be preserved under the Tertiary fill, but this possibility is unlikely. The magnetic data in Gem Valley suggest a subbasalt surface of moderate relief. In the northern part of the valley, the basalt does not appear to extend below an altitude of about 5,100 feet and is locally thin or absent. A belt of thin basalt is indicated between the area of the Tenmile Pass flows, at the north end of the valley, and the area of locally derived flows to the south. This inferred deficiency of basalt may reflect either a sub- basalt topographic high, possibly breached by a channel north of Talmage Siding, or more abundant sediments interlayered with the basalt. Over the southern part of the valley, the magnetic anomalies are complex because of the effects of numerous source areas. The basalt in the Grace-Niter area, indicated by magnetic data to be much thicker than anywhere else in the valley, prob- ably was ponded in a depression related to the eruption of the basalt. The magnetic data suggest two basalt- filled channels : one that extends east from the head of the canyon of the Portneuft River, and another, along the front of the Fish Creek Range near Bancroft. A third, not as well defined, may be north of Talmage Siding. These basalt-filled channels, presumed to be the courses of the major streams prior to diversion by the basalt flows, should be excellent sources of ground water. BLACKFOOT LAVA FIELD The Blackfoot lava field (Mansfield, 1927, p. 86) is the area of basalt that extends from a few miles south- east of Soda Springs northward to a few miles beyond the north border of the mapped area (pl. 1). The sur- face of the lava field ranges in altitude from about 6,700 feet east of the Blackfoot River Reservoir to about 5,800 feet south of Soda Springs. Numerous cliffs, col- lapse depressions, and fissures make the surface very irregular. Several basalt and cinder cones and craters and three large and two small hills of rhyolite rise above the lava field. Quaternary alluvium covers the basalt in most of the valley of Corral Creek and in sey- eral smaller areas along the mountain front, adjacent to the lava field. GRAVITY AND MAGNETIC ANOMALIES, A large gravity low southeast of Soda Springs centers over the valley that is the northern extension of Bear Lake Valley. This anomaly, which includes the ex- posures of the Salt Lake Formation in the Bear River Range, could be produced by 5,000 feet of material that is 0.45 g per cm ® less dense than the enclosing rock. The gravity data indicate a high-angle fault along the front of the Aspen Range; normal faults with the down- dropped Salt Lake Formation to the southwest have been mapped in reconnaissance along the range front. The gravity data suggest a fault along the east margin of the Bear River Range, where several mapped north- westerly trending faults cut the Salt Lake at the surface. West of these faults the low-density material appears to gradually thin over several miles. Cambrian rocks are exposed 3 miles south of Soda Springs in an area where gravity data suggest about 1,000 feet of low-density material; these Cambrian rocks may be large slide blocks mixed with the Salt Lake Formation, or they may be parts of a narrow horst of bedrock bounded by a Salt Lake-filled graben. The gravity data are too sparse to distinguish between the alternatives. The gravity low in Bear River valley probably re- flects a thick prism of Tertiary sediments. A 3,520-foot- deep hole drilled near the axis of the anomaly penetrated more than 2,500 feet of Salt Lake Formation and may have bottomed in Triassic strata that are un- conformably below the Salt Lake. Triassic rocks exposed in Bear River valley at the south edge of the mapped area and at Threemile Knoll, north of Soda Springs, may be part of a continuous sheet beneath the intervening Cenozoic cover. However, a gravity profile across the valley 2 miles south of the mapped area did not indicate a negative anomaly where Triassic rocks are exposed ; hence they may not be a major cause of the gravity low southeast of Soda Springs, although Triassic rocks underlie part of the valley. A magnetic high, which occurs along Bear River valley from Soda Point to about 5 miles southeast of Soda Springs, coincides with extensively exposed and partly covered basalt. This magnetic anomaly could be produced by 750 feet of basalt. If concealed basalt feeders contribute to the anomaly, the flows may be thinner than 750 feet. The Bear River valley gravity low ends abruptly near Soda Springs. Gravity data indicate that the depth to pre-Tertiary rock is less than 1,000 feet at the west side of Rabbit Mountain. Sedimentary fill is probably thin or absent at the Soda Point Reservoir. Threemile Knoll consists of Triassic rocks surrounded by basalt flows, and the absence of gravity lows adjoining the knoll suggests that the basalt is not underlain by thick Cenozoic sediments. sODA SPRINGS REGION, SOUTHEASTERN IDAHO Eo Gravity relief over the Blackfoot lava field, north of Threemile Knoll, is relatively complex. The largest fea- ture is a compound dow, which has two areas of closure. Cenozoic basalt is extensively exposed in this region, three rhyolite hills rise above the basalt, and Quater- nary travertine and detrital sediments occupy several square miles adjacent to mountain fronts. North of Conda, the gravity low extends over Tertiary and Qua- ternary sediments that are older than the youngest basalt ; no other Cenozoic material beneath the basalt is exposed. The gravity anomalies could be produced by about 5,000 feet of material that is 0.45 g per cm® less dense than the enclosing pre-Tertiary rocks (pl. 1, see- tion A-A'). Westward projection of the Blackfoot fault (Mans- field, 1927) extends through Middle Cone and is be- tween the two areas of closure in the compound negative anomaly. The apparent strike-slip movement along the fault is left lateral. The relation of the anomaly to the fault is not understood. Rocks responsible for the anom- aly probably postdate fault movement, and the anomaly may bear only an indirect relation to the fault. The gravity low is bounded on the southwest by a steep gravity gradient, indicating a north-north west- trending high-angle fault. Elsewhere along the margins of the anomaly, the gradients suggest (but do not re- quire) high-angle faults, such as those mapped along the Aspen Range front to the east. Southwest of the anomaly, gravity data suggest local relief on the surface of the pre-Tertiary rock at shallow depths. The magnetic field over the main part of the Black- foot lava field is complex. Southwest of North Cone and west of the Blackfoot River Reservoir, high-amplitude positive anomalies occur, whereas east of the reservoir, smaller amplitude negative anomalies predominate. East of Meadow Creek a high-amplitude magnetic low coincides with exposed basalt. Most of the basalt craters and cones are near the crests of magnetic highs or are on magnetic noses. Exceptions are the cones along the east front of Reservoir Mountain and the small cone 1 mile north of Broken Crater. These occur midway between flight lines; hence, small anomalies may not have been detected. Magnetic highs, which reflect basalt flows, flank Threemile Knoll on the west and the east. The high on the west is part of an elongate north west-trending anomaly. Amplitudes northwest and south of Threemile Knoll are 3007 (gamma) and 2007; however, the ampli- tude southwest of the Knoll is 1407, the smallest ampli- tude along the anomaly. Northwest and south of the knoll, about 1,000 feet of basalt would be required to produce the measured anomaly, whereas near the knoll, only about half this thickness would be required. The E10 lower amplitude of the anomaly at the knoll could also be explained if a basalt prism of uniform thickness were assumed to lie from north to south but to have marked narrowing near the knoll. Magnetic anomalies north of Threemile Knoll, as in Gem Valley, are believed to reflect both the variations in thickness of basalt flows and the presence of intrusives related to vents. Data along flight lines directly over the rhyolite hills reveal small magnetic highs which seem to reflect topography. A zone of low magnetic intensity and high gravity, which extends north-northwest across the lava field from Threemile Knoll to a spur of the Chesterfield Range, is interpreted as indicating a bedrock ridge be- neath relatively thin basalt. This inferred ridge may be partially breached northwest of Fivemile Meadows. Another magnetic low 3 miles northeast of Tenmile Pass suggests a buried connection between the south spur of the Chesterfield Range and the N inety Percent Range to the south, both of which are underlain by De- vonian and Mississippian carbonate rocks. Magnetic intensity is very high near China Hat and east of the inferred fault zone, which bounds the gravity low along its southwest side. This magnetic high is in- terpreted as reflecting thick basalt flows ponded in a depression and intrusive rocks related to known basalt vents. Magnetic intensity is high east and north of Reser- voir Mountain and up the valley of Corral Creek. The magnetic intensity increases eastward toward a maxi- mum at Crater Mountain which suggests that the flows thicken toward a vent in that area. The magnetic data indicate that basalt underlies alluvium at the north end of the valley of Corral Creek for about 2 miles south of the exposed basalt. In the central part of this valley, basalt is either absent or thin. The gravity low in the valley of Corral Creek could be produced by about 1,200 feet of fill. The negative magnetic anomalies east of the Black- foot River Reservoir are probably produced by re- versely magnetized basalt that is older than the nor- mally magnetized basalt to the southwest. This older basalt, which is at a higher elevation than the normally magnetized flows, presumably was a topographic high when the younger flows were extruded. The reversely magnetized basalt differs from the normal basalt in sev- eral ways: it is much lighter gray, more finely crystal- line, and more altered. The slightly greater degree of alteration of the reversely magnetized basalt may reflect its somewhat greater age. Two cycles of volcanism and associated deformation in the central part of the Blackfoot lava field are sug- gested by the gravity and magnetic data. First, basalt GEOPHYSICAL FIELD INVE STIGATIONS flows accumulated in a local depression east of the pres- ent site of the Blackfoot River Reservoir. Evidence of their former extent is now masked by younger flows. The reverse remanent magnetization of these flows sug- gests an age greater than 0.7 million. years. During the second phase of volcanism a thick sequence of normally magnetized basalt flows accumulated in depressions south and west of the Blackfoot River Res- ervoir, Some of these flows extended south into Bear River valley, west into Gem Valley, northwest along the Blackfoot River, and over a surface of considerable relief east of Tenmile Pass. Several lines of evidence suggest that the large com- pound negative gravity anomaly may indicate a volcanic collapse structure related to the withdrawal of magma at depth. The anomaly lacks the pronounced elongation that characterizes troughs in this region and in other parts of the Basin and Range province. The general area abounds in craters, cones, and large magnetic anomalies, and the spatial relations between the basalt flows and the gravity depressions suggest that subsidence occurred during, or shortly before, eruption of the basalt. Local magnetic anomalies obviously related to exposed rocks are superimposed on an extensive area of high intensity. The gravity and magnetic anomalies could be produced entirely by exposed basalt flows, pumiceous rhyolite and sediments filling a collapse structure; however, an alternate interpretation is that a part of each anomaly is produced by an underlying granitic intrusive body. Both interpretations are sug- gested by the rhyolite hills that occur only in this area. The presence of inversely magnetized basalt in the Blackfoot River Reservoir area precludes calculations of the thickness of the basalt, as in the Bancroft-Soda Springs area. However, adjacent to the Chesterfield Range and the Soda Springs Hills, where there is no evidence of inversely magnetized rock, the magnetic anomaly indicates a prism of basalt, about 1,000 feet thick, extending northwest from the west side of Three- mile Knoll to the Tenmile Pass area. Because the nar- row gap at Tenmile Pass is between flight lines, the maximum amplitude of the magnetic anomaly is not known. The basalt could be 1,000 feet thick at the pass, but this would require a higher amplitude positive anomaly than was inferred during preparation of the magnetic-contour map. The magnetic anomaly in the Canyon of the Blackfoot River northwest of Corral Creek suggests that the basalt is less than 400 feet thick there. *The reverse remanent magnetization is interpreted as indicating that the flows cooled through the Curie temperature when the polarity of the earth's magnetic field was the reverse of the present polarity. The most recent major reversed period ended about 0.7 million years ago (Cox and others, 1967). GRAVITY AND MAGNETIC ANOMALIES, SODA SPRINGS REGION, Magnetic anomalies produced by the basalt west of Threemile Knoll, in range gaps at the head of Portneuf River canyon, and at Soda Point and Tenmile Pass are all consistent with an elevation of about 5,000 feet above sea level for the prebasalt surface. A higher elevation is suggested, however, along the Blackfoot River near the mouth of Corral Creek. Several possible prebasalt drainage systems, involving interconnections of the Portneut, Bear, and Blackfoot Rivers near Soda Point, Threemile Knoll, and Tenmile Pass, are consistent with the magnetic data. An earlier connection between the Bear River and the present Blackfoot River drainage into the Snake River seems unlikely unless recent re- gional deformation has raised the northern part of the region several hundred feet relative to the central part. UPPER BLACKFOOT RIVER DRAINAGE East of the Blackfoot lava field are several narrow, generally north trending valleys. In Upper Valley a gravity low of about 5 mgal indicates that about 1,000 feet of fill is present. In the other valleys the gravity anomalies are smaller or absent, although some small gravity variations may have been overlooked because of insufficient control. In the upper Blackfoot River drainage area the mag- netic map is dominated by local magnetic anomalies produced by basalt in the valleys. Negative anomalies in Lower, Wooley, and Upper Valleys, the south end of Enoch Valley, and Pelican Slough are produced by reversely magnetized basalt. Reverse magnetization is confirmed by laboratory measurements on samples col- lected in Wooley Valley. Positive anomalies are pro- duced by normally magnetized basalt flows in the north end of Enoch Valley and along Meadow Creek, north- west of Pelican Slough. The magnetic data indicate that basalt underlies alluvium in the north end of Upper Valley, the north end of Enoch Valley, and along the Blackfoot River south of Fox Ranch. LOCAL ANOMALIES IN THE RANGES Local gravity variations in the ranges are small. The lows in the Portneuf Range, near the southwest corner of the mapped area, and in the southeastern part of the Chesterfield Range coincide with Salt Lake Formation exposures and probably reflect local thickenings of the Tertiary sediments. The high in the Monroe Canyon area coincides with exposed Devonian and Silurian dolomites, which are among the densest rocks of the region. The magnetic field of the ranges is generally feature- less, with only minor deviations from a general north- eastward increase related to the primary field. Notable exceptions occur in the Chesterfield, Bear River, and Grays Ranges. Positive anomalies in the northern Ches- soOUTHEASTERN IDAHO Ell terfield Range and in the Bear River Range are clearly related to exposed volcanic rocks. In the Grays Range the anomaly may be related to concealed igneous rock, for the known distribution of volcanic rock could not produce the anomaly indicated on the contour map. The anomalies along Eighteenmile Creek in the Ches- terfield Range do not correlate with known geology. Along Eighteenmile Creek in the Chesterfield Range, two parallel positive magnetic anomalies, about 2 miles apart, trend northward in areas covered mainly by Salt Lake Formation. These anomalies persist, with greatly diminished amplitude, across exposures of up- per Palezoic rock. The anomalies parallel the regional strikes of bedrock units beneath the Salt Lake and, also, the strikes of some previously unmapped block faults. Rocks in the area of maximum intensity were examined, but no explanation of the anomaly was apparent. Magnetism of basalt adjoining the Chesterfield Range partially masks the configuration of these anom- alies; however, enough of their configuration is evident to justify an analysis (fig. 4). North of Eighteenmile Creek the amplitudes of both anomalies are 25 y or less, but at the creek the amplitude of the eastern one exceeds 50 y and remains high for about 2 miles south of the creek. The western anomaly terminates between Eighteenmile Creek and Little Flat Canyon. Depth analysis of the eastern anomaly near Eighteenmile Creek suggests the source to be within 500 feet of the bottom of the canyon. C C yz 1400 ww s ag W <2 U < Computed i * Z intensity e 3 hel [- =< 12 z 5 < gs | L 1300 10,0007 f- 50004 K=2.5%10-3 SEA LEVEL | K=2.5%10-3 5000 SCALE 1:125,000 o 4 MILES ‘ 1 1 1 4] FrGurE 4. -Aeromagnetic profile and interpretive section in the Chesterfield Range. Line of section shown on plate 1. K is magnetic susceptibility, in electromagnetic units. E12 The eastern anomaly at Eighteenmile Creek suggests a north-trending sheet of magnetic rock dipping steeply toward the west. The anomaly on the west is more diffi- cult to analyze because the amplitude is low, and the west side may be distorted by nearby valley basalt; however, it does not appear to be caused by a westward- dipping sheet. The anomalies could be produced by a tabular mass either folded into a syncline or faulted, with opposing dips (fig. 4). The magnetic data do not yield a minimum thickness for the mass that produces the anomaly; the relative amplitudes of the computed anomalies could result either from eastward thickening of the magnetic mass or from eastward increase in susceptibility. The available data are insufficient to explain the anomalies along Eighteenmile Creek. However, two sug- gestions are offered: The anomalies may be produced by Cenozoic lavas, dikes, or sills in the Paleozoic or Tertiary rocks or by iron-enriched strata in the Amsden Formation. Four miles north of the eastern magnetic anomaly and on the projection of this anomaly, a thin dike of weathered andesitic material found in a prospect in Phosphoria Formation was reported by Mansfield (1929, p. 40) to closely resemble larger bodies of hornblende andesite porphyry exposed at Sugar Loaf Mountain 20 miles to the northeast. However, a test flight over Sugar Loaf Mountain revealed an anomaly of only a few gam- mas-much smaller than the anomaly at Eighteenmile Creek. If dikes or sills are producing the anomalies at Eighteenmile Creek, they are more likely to be basalt than andesite porphyry. No tabular bodies of basalt are exposed in the Chesterfield Range, but basalt vents are present in the Bear River Range, along strike to the south, and a small remnant of basaltic lava is exposed near the center of the NEL see. 24, T. 9 S., R. 40 E., in the Soda Springs Hills (F. C. Armstrong, written com- mun., May 10, 1968). These basalts, though possibly somewhat older than the valley basalts, are still presum- ably of Quaternary (possibly early Pleistocene) age; however, conclusive data on age are lacking. A third alternative for the igneous origin of the magnetic ano- malies is suggested by the presence of both basalt and rhyolite in the Salt Lake Formation sequence to the north, as in sec. 8, T. 5 S., R. 39 E. (Mansfield, 1929, p. 41-45, pl. 1). A folded or broken layer of basaltic lava of presumably Pliocene age in the Salt Lake For- mation at Eighteenmile Creek could best account for the magnetic anomalies (with the imprecisely known local geology). However, this interpretation does not adequately explain the apparent northward extension of the eastern anomaly across an area near the crest of the range, where the Salt Lake Formation is absent. GEOPHYSICAL FIELD INVESTIGATIONS No magnetic rocks are known in the Paleozoic se- quence within this or adjoining regions. The Amsden Formation is reported to include pisolitic iron oxide farther east in Wyoming in the Bedford quadrangle (Rubey, 1958) and the Little Flat Canyon Formation of the Chesterfield Range contains ferruginous beds (Dutro and Sando, 1963, p. 1970). Although the Ams- den Formation has not commonly been recognized and mapped in the Soda Springs region, equivalent rocks may be present near the contact between. Mississippian limestones and Pennsylvanian quartzites in the Ches- terfield Range. Secondary iron enrichment of Paleozoic units underlying the Salt Lake Formation in the Eight- eenmile Creek area may have occurred, but this is not regarded as a likely cause of the magnetic anomalies. None of the suggestions offered here is sufficiently documented to adequately explain the magnetic anoma- lies in the Chesterfield Range. More intensive geophys- ical surveys or shallow drilling is required to resolve the questions. The accompanying maps do not indicate any local geophysical anomalies of possible economic interest. Small deposits of manganese oxide, and copper, lead, zinc, and iron sulfide minerals, some of which have been mined on a small scale, are present in the Portneuf Range near the southwest corner of the survey area. These deposits are in the area of an extensive magnetic low, but neither the gravity maps nor the magnetic maps show any local features related to them, possibly because of insufficient data. REGIONAL GRAVITY ANOMALIES Bouguer anomaly values unaffected by local valley anomalies decrease eastward and southeastward from about -165 mgal over lower Paleozoic sedimentary rocks on the west side of Gem Valley to less than -200 mgal over upper Paleozoic rocks in the ranges at the east edge of the mapped region (fig. 5). The region lies between the topographically low Snake River Plain on the northwest and the Lake Bonneville basin on the south, and the rugged Wyoming and Salt River Ranges to the east. Although elevations of the range crests generally do not increase eastward, the elevations of the valleys increase eastward by about 1,000 feet to 6,500 feet above sea level in Upper Valley. In the east the ranges occupy a greater percentage of the area, and this, combined with higher valleys, produces an eastward rise in regional elevation. Quantifications of average rise in regional elevation depends on the method used in measuring regional elevation. Regional elevations were computed here by averaging elevations in circular areas around the gravity station. Averaged over areas 32 and 64 kilometers in radius, the increase in elevation in this GRAVITY AND MAGNETIC ANOMALIES, SODA ——_—— SPRINGS REGION, SOUTHEASTERN IDAHO E13 {11} EXPLANATION ---200 -- Regional Bouguer gravity contours Interval 5 milligals 6800-- Average elevation contours (32 km) Interval 200 feet --6800-- Average elevation contours (64 km) Interval 200 feet 42°30' 15 20 MILES | asd FicUurE 5.-Regional gravity and topographic contours. Gravity contours are based on stations on pre-Tertiary rock. Topographic contours are elevations averaged over circular areas with radii of 32 and 64 kilometers. region is about 1,000 feet (fig. 5). The accompanying decrease in Bouguer anomaly values of about 35 mgal is about that expected if the higher topography is isostat- ically compensated (Mabey, 1966). Tsostatic anomalies have been computed at the seven U.S. Coast and Geodetic Survey pendulum stations in and adjoining the Soda Springs region (Duerksen, 1949). Pratt-Hayford isostatic anomalies at all these stations are positive, ranging from +13 to +31 mgal.s The average isostatic anomalies at the seven stations for different depths of compensation are: +26.1 mgal (56.9 km), +19.5 mgal (96 km), +17.0 mgal (113.7 km), +24. mgal (96 km, corrected for indirect effect). Free-air anomalies for the stations occupied in this survey ranged from -22 mgal for a station in Gentile Valley, where both elevation and Bouguer anomaly value are low, to a high of +119 for the station at the highest elevation. The average free-air anomaly for all 5 The isostatic anomaly values were adjusted for differences between original pendulum gravity observations and data obtained in the more accurate gravity-meter survey reported here. stations is about +14 mgal. However, this average is heavily weighted toward the valleys, where stations are more abundant and the free-air anomaly is relatively low because of local negative anomalies and low eleva- tions. The average free-air anomaly for the entire region is more positive, probably about +35 mgal. The positive free-air and isostatic anomalies and the correlation between Bouguer anomaly values and re- gional elevation indicate that variations in regional topography are approximately compensated but that the entire region is undercompensated. Gravity data from surrounding regions indicate that the zone of positive free-air and isostatic anomalies extends to the north and east, but not southwestward into the main part of the Basin and Range province. Although eastward decrease in Bouguer anomaly values across the Soda Springs region clearly correlates with increase in regional elevations and does not require any mass anomalies in the upper crust, the southward component observed in the southern part of the mapped e E14 area does not have an obvious correlation with regional topography. North of Fish Creek Basin, between the Portneuf and Fish Creek Ranges, anomaly values of -162 to -165 are indicated for exposed Paleozoic rock. Between Fish Creek Basin and the south edge of the mapped area, the values decrease about 10 mgal in a distance of 6 miles. Along the Chesterfield Range, the anomaly values on or near Paleozoic rock have a varia- tion of about 3 mgal; the values decrease slightly over the Soda Springs Hills and then more rapidly over the Bear River Range. A similar southward decrease is evident in the Aspen Range near Sulphur Canyon, and a similar, but smaller, decrease is suggested in the ranges to the east. The cause of the southward decrease in Bouguer anomaly values observed in the eastern part of the Portneuf Range and in the Bear River and Aspen Ranges is not obvious. Southward along the Chester- field Range-Soda Springs Hill-Bear River Range chain, progressively older rocks are exposed, suggesting that the basement surface may rise as the gravity values decrease. Although the nature of the basement rock in the region is unknown, the density of the basement is not likely to be less than the upper Paleozoic rocks, and, therefore, a basement high should not produce a gravity low,. A southeastward thickening of the Cam- brian and Precambrian quartzites would be consistent with the gravity data, but geologic evidence does not support this possibility. Structural thickening of the quartzites is a possibility, but this cannot be evaluated without additional geologic mapping. GEOPHYSICAL FIELD INVE STIGATIONS Bouguer values on the east side of the north end of Gem Valley are not much lower than those on the west side. If a linear eastward decrease in regional gravity is assumed across the mapped area, data in the Chester- field Range suggest a residual gravity high relative to other ranges. Such an anomaly would be expected if basement were relatively high beneath the range. East of the Chesterfield Range, in the vicinity of the Black- foot lava field, the Bouguer anomaly values unaffected by low-density basin fill decrease eastward more ab- ruptly than in areas to the east and west. A similar abrupt decrease occurs across the valley of the Bear River south of Soda Springs. East of this zone of abrupt decrease in regional Bouguer anomaly values, no rocks older than Mississippian are exposed ; west of this zone, older rocks are abundant. The gravity data suggest a greater thickness of younger Paleozoic rocks east of this zone. Although gravity and magnetic surveys provide much information for the interpretation of subsurface struc- ture in areas underlain by Cenozoic sediments and lavas, they fail to yield unequivocal clues to structures within pre-Tertiary rock. Only in the Chesterfield Range are magnetic anomalies possibly related to pre-Cenozoic rock, and even here these anomalies could be caused by Cenozoic igneous rock. The relatively high gravity values in the Chesterfield Range and the southward decrease of gravity values probably reflect mass anoma- lies in the upper crust. Geological interpretations in- volving the upper few kilometers of the crust should explain these anomalies. GRAVITY AND MAGNETIC ANOMALIES, sODA SPRINGS REGION, soUTHEASTERN IDAHO ———— E15 REFERENCES Armstrong, F. C., 1953, Generalized composite stratigraphic see- tion for the Soda Springs quadrangle and adjacent areas in southeastern Idaho, i» Intermountain Assoc. Petroleum: Geologists 4th Ann. Field Conf., 1953: chart in pocket. 1969, Geologic map of the Soda Springs quadrangle, southeastern Idaho : U.S. Geol. Survey Misc. Geol. Inv. Map I-557. Armstrong, F. C., and Cressman, E. R., 1963, The Bannock thrust zone, southeastern Idaho: U.S. Geol. Survey Prof. Paper 374-J, 22 p. Armstrong, F. C., and Oriel, S. S., 1965, Tectonic development of Idaho-Wyoming - thrust belt: Am. Assoc. Petroleum Geologists Bull., v. 49, no. 11, p. 1847-1866. Behrendt, J. C., and Woollard, G. P., 1961, An evaluation of the gravity control network in North America: Geophysics, v. 26, no. 1, p. 57-76. Bright, R. C., 1963, Pleistocene Lakes Thatcher and Bonneville, southeastern Idaho: Minnesota Univ. unpub. Ph. D. thesis. 1967, Late Pleistocene stratigraphy in Thatcher basin, southeastern Idaho: Tebiwa, v. 10, no. 1, p. 1-1. Cox, Allan, Dalrymple, G. B., and Doell, R. R., 1967, Reversals of the Earth's magnetic field: Sci. American, v. 216, no. 2, p. 44-54. Cressman, E. R., 1964, Geology of the Georgetown Canyon-Snow- drift Mountain area, southeastern Idaho: U.S. Geol. Survey Bull. 1153, 105 p. Cressman, B. W., and Gulbrandsen, Dry Valley quadrangle, Idaho : 1015-I, p. 257-270. Duerksen, J. A., 1949, Pendulum gravity data in the United States: U.S. Coast and Geodetic Survey Spec. Pub. 244, 218 p. Dutro, J. T., and Sando, W. J., 1963, New tions and faunal zones in Chesterfield Range, Portneuf quadrangle, southeast Idaho: Am. Assoc. Petroleum Geologists Bull., v. 47, no. 11, p. 1963-1986. Eardley, A. J.. 1989, Structure of the Wasatch-Great Basin region: Geol. Soc. America Bull., v. 50, no. 8, p. 1277-1310. Fenneman, N. M., 1917, Physiographic division of the United States: Assoc. Am. Geographers Annals 6, p. 19-98. 1931, Physiography of Western United States : New York, McGraw-Hill Book Co., 534 p. Gulbrandsen, R. A., McLaughlin, K. P., Honkala, F. S., and Clabaugh, S. E., 1956, Geology of the Johnson Creek quad- rangle, Caribou County, Idaho: U.S. Geol. Survey Bull 1042-A, p. 1-23. Mabey, D. R., 1964, Regional gravity and magnetic anomalies in southeastern Idaho and western Wyoming, in Abstracts for 1963: Geol. Soc. America Spec. Paper 76, p. 212. R. A., 1955, Geology of the U.S. Geol. Survey Bull Mississippian forma- O 1966, Relation between Bouguer gravity anomalies and regional topography in Nevada and the eastern Snake River Plain, Idaho, in Geological Survey research 1966: U.S. Geol. Survey Prof. Paper 550-B, p. B108-B110. Mabey, D. R., and Armstrong F. C., 1962, Gravity and magnetic anomalies in Gem Valley, Caribou County, Idaho, in Short papers in geology, hydrology, and topography : U.S. Geol. Survey Prof. Paper 450-D, p. D73-D75. Mansfield, G. R., 1927, Geography, geology, and mineral resources of part of southeastern Idaho, with description of Carbon- iferous and Triassic fossils, by G. H. Girty: U.S. Geol. Survey Prof. Paper 152, 453 p. . 1929, Geography, geology, and mineral resources of ghe Portneuf quadrangle, Idaho : U.S. Geol. Survey Bull. 803, 110 p. Meuschke, J. L., and Long, C. L., 1965, Aeromagnetic map of part of the Lanes Creek Quadrangle, Caribou County, Idaho : U.S. Geol. Survey Geophys. Inv. Map. GP-490. Mitchell, C. M., Knowles, F. F., and Petrafeso, F. A., 1965, Aero- magnetic map of the Pocatello-Soda Springs area, Bannock and Caribou Counties, Idaho : U.S. Geol. Survey Geophys. Inv. Map GP-521. Montgomery, K. M., and Cheney, Stewart Flat quadrangle, Caribou County, Idaho : Survey Bull. 1217, 63 p. Oriel, S. S., 1968, Preliminary geologic map of Bancroft quad- rangle, Caribou and Bannock Counties, Idaho : U.S. Geol. Survey open-file map. Oriel, S. S., and Armstrong, F. C., 1966, Times of thrusting in the Idaho-Wyoming thrusts belt-Reply [to discussion of 1965 paper by E. W. Mountjoy, 1966] : Am. Assoc. Petroleum Geologists Bull., v. 50, no. 12, p. 2614-2621. Oriel, S. S., Mabey, D. R., and Armstrong, F. C., 1965, Strati- graphic data bearing on inferred pull-apart origin of Gem Valley, Idaho, in Geological Survey research 1965: U.S. Geol. Survey Prof. Paper 525-C, p. C1-C4. Rioux, L. L., Hite, R. J., Dyni, J. R., and Gere, Willard, 1966, Geologic map of the Upper Valley quadrangle, Caribou County, Idaho : U.S. Geol. Survey open-file map. Rubey, W. W., 1958, Geology of the Bedford quadrangle, Wyoming: U.S. Geol. Survey Geol. Quad. Map GQ-109. Rubey, W. W., and Hubbert, M. K., 1959, Overthrust belt in geo- synclinal area of western Wyoming in light of fluid-pres- sure hypothesis, pt. 2 of Role of fluid pressure in mechanics of overthrust faulting: Geol. Soc. America Bull., v. 70, no. 2, p. 167-205. Swick, C. H., 1942, Pendulum gravity measurements and iso- static reductions: U.S. Coast and Geodetic Survey Spec. Pub. 232, 82 p. T. M., 1967, Geology of the U.S. Geol. #; UNITED STATES DEPARTMENT OF THE INTERIOR yy, GEOLOGICAL SURVEY e AB E. | (JUNIPEROISERRA) e (NH "of PROFESSIONAL PAPER 646-A PLATE | Too Locelafbood:""" (BRADLEY) (ADELAIDA) } a Ragged Point\_ # s (‘éz 35°45! LOCATION OF GRAVITY STATIONS IN THE VICINITY OF BURRO MOUNTAIN 40' |- EX -P :E A_ N A "T | .O N SUPERBASEMENT SEDI- MENTARY ROCKS < 14 ery: § & wx Confct 9% $ Qs - & A : s § s 4 T Dashed where apprzimately located, -$ S E . 5 Z dotted wher concealed wil Surficial deposits 9 .. = mesa . o C ; 5g; g TE Fait g S ot tC SE Dashed where approximately located; dotted $ é $ T S Pz where concealed; queried where doubtful C Ts . C C IK S. € Paso Robles Formation C Syneli it yncline Monterey Shale > < Showing troughline l— x 17 § Tv L ¢ S & Exploratory well C S Volcanic rocks Number refers to that in table in text 3 2 a mes. Lines of equal Bouguer anomaly, Middle and lower Miocene 9p 4 231111135215 * H y ontour imierval Z mulgals deposits U E 57. € 0:03 < s* g § $ TK) J F Gravity station and number tees & § S n o C §§ ¢ §4L SC ~ Paleocene and upper Mesozoic | K- f (s s $ ~<3 g deposits 9, 10111 o Gravity base and designation 5 Z o U 1 BASEMENT ROCKS s A A k---------- os Location of gravity and structure 6 E E profiles in figure 2 LW < § Intrusive rocks t- ( U £53 LW O saa s & LJ Granitic rocks J G 1 D 4 U z <4 D tr < <2]: - 2 *% £ > - . 50 ® w Franciscan Formation 2 1 E i So 80 409 5 < 7 W s Metamerdis rocks i 03% N soURCES OF GEOLOGIC DATA Ne N 1. Durham (1965a; unpub. data, 1966) 2. Stanford Geological Survey (unpub. data, 1953) 3. B. M. Page (unpub. data, 1966) a 4. Burch (1968, p. 530; unpub. data, 1967) Ultramafic rocks 5. Jennings (1958) 750 000 Feet sar | ' 1 s 321°3o' (960000 FEET t 35" 20" I | 121°15 1 04 EE ; \ INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C.-1971-G69443 RoE Base from U.S. Geological Survey, 1948 and 1949 SCALE 1:62 500 10,000-foot grid based on California coordinate system, zone 6 $ 1 Va 0 1 2 3 4 5 MILES E EMH -H- -H- ro f 1 $2%"O 1 2 3 4 5 KILOMETERs & e e APPROXIMATE MEAN CONTOUR INTERVAL‘S 40 AND 80 FEET DECLINATION, 1971 DASHED LINES REPRESENT 20-FOOT CONTOURS DATUM IS MEAN SEA LEVEL DEPTH CURVES AND SOUNDING IN FEET-DATUM IS MEAN LOWER LOW WATER SHORELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER THE MEAN RANGE OF TIDE IS APPROXIMATELY 5 FEET COMPLETE BOUGUER GRAVITY AND GENERALIZED GEOLOGIC MAP OF THE CAPE SAN MARTIN BRYSON, PIEDRAS BLANCAS, AND SAN SIMEON QUADRANGLES, CALIFORNIA < UNITED STATES DEPARTMENT OF INTERIOR ¥ GEOLOGICAL SURYEY on _ ' , (SAN ARDO} 5" R kn £,. 45°" . 43" (PRIEST VALLEY] $5" R:13 F. R. 14 F T. 225 (PARKFIELD 1:24 000) EXPLANATION Qs Pleistocene and Surficial deposits oT Pliocene and Paso Robles Formation Pleistocene(?) Holocene(?) Miocene e i ao a soi ae o oo Miocene and Pliocene Pancho Rica and Santa Margarita Formation Im: Monterey Shale B on meni 9) I Tv > & & Tierra Redonda and Vaqueros Formation to 3 $ ® S: § ..... s NS g .de 5 5 Paleocene and Upper Cretaceous 5 deposits Cg BASEMENT ROCKS - ND b ia ia e gr tn © Granitic rocks Franciscan Formation Ultramafic rocks (CHOLAME HILLS 1:31680) Contact Gradational or approximately located Fault Approximately located, dotted where concealed moc, nfm me mome cf Thrust fault Sawteeth on upper plate _¢—67 Exploratory well $% ( > % rer As ({CC- $ 5 ATC f § Mel 1 \ Number refers to table 1 Oil field - 30 T 298. deers cane onit ine tne enemee Lines of equal Bouquer anomaly in milligals Contour interval, 2 milligals 237 Ld Gravity station and number [ Psros Gravity base and designation A ----- A' Location of gravity and structure Profiles in figure 2 (SHANDON 1:24 000) 40 T. 2065: (SAN SIMEON) sOURCE OF GEOLOGIC DATA 1. Mapped by D. L. Durham, 1964-67 2. Adapted from Jennings (1958) T. 27.8. Als] o 6 L)’ s P 30 ; if), 4 £1 J f x , \ 48 Geology by Durham 1964-67, with supplemental es Base from U.S. Geological Survey, 1947- data from Jennings (1958) & COMPLETE BOUGUER GRAVITY AND GENERALIZED GEOLOGIC MAP OF BRADLEY SAN MIGUEL, ADELAIDA, AND PASO ROBLES QUADRANGLES, CALIFORNIA SCALE 1:62 500 4 H Va 0 1 2 3 4 SMILES $ ETE m peee ae s é { I5 0 1 2 2 4 5 KILOMETERS 2 ECH HHH --- r-- r-- s oes CONTOUR INTERVALS 20 AND 40 FEET DASHED LINES REPRESENT 10-FOOT CONTOURS DATUM IS MEAN SEA LEVEL TERTIARY. CRETACEOUS JURASSIC CRETA- TERTIARY QUATER- SUPERBASEMENT SEDIMENTARY ROCKS AND QUATERNARY(?) AND TERTIARY CEOUS AND CRETACEOUS NARY PROFESSIONAL PAPER 646-B PEATE | 2 ; M Q0 D. G - £9) l 0 0 s I atio PAPER 646-C °iz_ _ UNITED STATES DEPARTMENT OF THE INTERIOR } h" “017133310133“ 1 E £ g \ e GEOLOGICAL SURVEY $ 120° 30° op 6 E ‘ (PorvaDpero aar 1:62 00) : B E. ar 36°00. pmsome ee tm , $. S. EXPLANATION POST-FRANCISCAN SEDIMENTARY AND VOLCANIC ROCKS OS y i g $ Surficial deposits 7a $t$ & r .% § é L <>. E R - § £ + = ... & QT bs $ s 2 I < t u) m 3 < ius fans R SGS lu 0 Z |- " ~ 3 mu Ad " _ (k , 2 U Coy "x >] LOX RA \ eva p a # > sy a.m. eN ~ fa. M $i gi %L _ < | foe: Ultramafic rocks Sighs) ANX AS xX shee a oB : PDA y ak youl s( c C2, ; &_ foy R A1 AX z \ : JN ok } Largely serpentinite Contact Dashed where approximately located; short dashed where gradational or inferred; dotted where concealed T, T j recive sere tes wes aces a's a's Fault Dashed where approximately located; short dashed where inferred; dotted where concealed. Arrows in- dicate relative horizontal movement watts. codice n cs a + cs Thrust fault Dashed where approximately located; dotted where concealed. Sawteeth on upper plate Anticline Showing crestline and direction of plunge. Dashed where approximately located Syncline Showing troughline and direction of plunge. Dashed where approximately located -50 Bouguer gravity contours Hachures indicate closed areas of lower gravity. Con- tour interval 2 milligals Terrain corrections made to a distance of 166.7 km, using a standard density of 2.67 g/em' e 729 Gravity station Numbered or lettered stations refer to table 2 $. C SHANB T Gravity base station and designation See table 1 O 25 $ =p < Drill hole or exploratory well T Number refers to table 3 [ - | Z < | & my G © (cam [if [era yan ilt 3 o eol W " llor‘gy compiled by'IIth; Dabbéeer” 1968-69 (£3174? 0%o Parkfield, Shandon, and Orchard Peak 1:62,500, 1961; 5 a F5 f $ 3 f MILES Orn) hole data compiled by N-C. Washer YA. $9160 Garza Peak, Kettleman Plain and Pyramid Hills & t 4 -E H El + n ; ] p % $vvb 1:24,000, 1953; Tent Hills 1:24,000, 1942 0 & 2 1 's ' o 1 2 § 4 KILOMETERS ECH-HHH I ] CONTOUR INTERVALS 25, 40 AND 80 FEET mate mean DASHED LINES REPRESENT 5-FOOT CONTOURS Prenton tone DOTTED LINES REPRESENT 20 AND 40-FOoOT contours DATUM IS MEAN SEA LEVEL See plate 4 for cross sections COMPLETE BOUGUER GRAVITY AND GENERALIZED GEOLOGIC MAP ALONG THE SAN ANDREAS FAULT NEAR CHOLAME, CALIFORNIA ¢ UNITED STATES DEPARTMENT OF THE INTERIOR éeiqfio PROFESSIOESAATEAQPER 646-C 2. \. GEOLOGICAL SURVEY RleE noire. 5. f pian ; sane, | s 120° 30" YE _. pr. B ~ p erea a Cn _. (ex's Pent. s. A_ l xe 36°00 --- =- % f >= 45 \ {Ifa g > Broa - $3 > SCL > yas" ~a Kf aire > eat [3% (ex [ buce Aivj—jr‘gl—‘f‘fiiifi < 2; H EXPLANATION POST-FRANCISCAN SEDIMENTARY AND VOLCANIC ROCKS Qs Surficial deposits Holocene 8 QUATERNARY Pleistocene and Q Landslide deposits *] Pliocene and Pleistocene TERTIARY AND QUATERNARY ( of i Paso Robles and Tulare Formations Mainly velley deposits s { f Sedimentary rocks m 3 ass ns aun oen oa Ae ey TERTIARY =S gM Sc Vaquero "Spring Miocene and Pliocene r. & $ j psy) [Pa VM'é‘éfQo/EON\\_\7 Bh 1444 2s Monterey Shale ar -_- {1 Amey . § yey ; _ | ors, .- o saut i o ‘TK ‘rjr\,5”/\ 6 f 6 (a' p S ax, rot ans yr a's \ \ Yos m- Sen IP ac sgn: 30% -' 4 ig on ; v * \a sels) .." ; f “i, ema a *--> a _,_..(__”i_4_'I_/_}__ * jm / l; \! j welt -g : 4 Y an ‘ : ". & T Xl" " Mit ase" ys --t CTH X 24), e mo s Fes o To ("l Huytey , 7 hh _ 1 am 3 f « ag { G= T 4 \ 3p Ag _i i(. 4 ) "op G , f \ { \ k f te ns ¥. 2A NM s yx! /f € $ C | ax ~ \ "T 8. B r sin's Sedimertary rocks a B Ne aran Ca- CRETA. Oligocene Miocene E Volcanic rocks (LA RAMBLA 1:62 500) >_ % F D C § < § TK 2 0 < 5 § § O 2 w 8,0 § "m (U3, < (¢ L? gp roads O Miogeosynelinal rocks E \ P Syria 7 'D" t gr If 2 Q LW W O Z o < t LW < -! Plutonic and metamorphic rocks jJ.0 0 o f Y opr & a UO Franciscan rocks | in O g pels | < "4 < TK Qs! " m <5 4“ Spring um wa L + ~ 3 O Ultramafic rocks Largely serpentinite yun 4A & % h Re (3% I BMA1271(1943) BM M : % >] Contact Dashed where approximately located; short dashed where gradational or inferred; dotted where concealed Fault Dashed iwhere approximately located; short dashed where inferred; dotted where concealed. Arrows in- dicate relative horizontal movement Thrust fault Dashed where approximately located; dotted where concealed. Sawteeth on upper plate -4-----L__-_-- Anticline Showing crestline and direction of plunge. Dashed where approximately located Exe f coe Syncline Showing troughline and direction of plunge. Dashed where approximately located & & 200 Aeromagnetic contours Showing total intensity magnetic field of the earth, in gammas, relative to arbitrary datum. - Hachures in- dicate closed areas of lower magnetic intensity. Con- tour interval 20 gammas Regional magnetic gradient of 9 gammas per mile in the direction N. 16° E. has been removed from the original data. =, p Spring Flight path Showing location and spacing of data. Flight elevation 18 6,500 feet above sea level $30 Drill hole or exploratory well Number refers to table 3 3 (EMIGRANT HILL 1:24 000) Koxkg Corne (SHALE POINT 1:24 000) _- F iB l/ N p P E ARC Peon < «h elo \ "ath Cave Chats 1 \. & se" bos) ="} lif |B - ) ole 2 > L3 SH -X 9. & he b ? rei L - oice t. \ d : s cl. x1 S a*30' 25"R. 14 E.R. 15 E. t 2 ¢ 5 15 R.17°E: (LA PANZA 1:62 500) ; % 120°00' A f 1X“ 357 Base from U.S. Geological Survey SCALE 1:62 500 Geology compiled by T. W. Dibblee, Jr., 1968-69 - Ag {$50 Parkfield, Shandon, and Orchard Peak 1:62,500, 1961; { Drill hole data compiled by H. C. Wagner 3 % G 0 j j 1 Va 0 1 2 3 4 MILES 3°C. «2.9 Garza Peak, Kettleman Plain and Pyramid Hills z EEB H - -+- - BRD, \>\.6” 1:24,000, 1953; Tent Hills 1:24,000, 1947 $ P6 é 1 15 0 1 2 3 4 KILOMETERS a ECHCEHE e-- Fe- ] CONTOUR INTERVALS 25, 40 AND 80 FEET APPROXIMATE MEAN DASHED LINES REPRESENT 5-FOOT CONTOURS SHINee DOTTED LINES REPRESENT 20 AND 40-FOOT CONTOURS DATUM IS MEAN SEA LEVEL See plate 4 for cross sections AEROMAGNETIC AND GENERALIZED GEOLOGIC MAP ALONG THE SAN ANDREAS FAULT NEAR CHOLAME, CALIFORNIA 459-845 O - 73 (In pocket) UNITED STATES DEPARTMENT OF THE INTERIOR $0 PROFESSIONAL PAPER 646-C GEOLOGICAL SURVEY 12m; \ PLATE «3 ~ 36°00 120° 30° 36°00 -- EXPLANATION POST-FRANZISCAN SEDIMENTARY AND VOLCANIC ROCKS - >- C § ' ¢ os R I4 § _ .g Surficial deposits a HBe & = § S a s m C R, < D O Lardslide deposits t > $" 4§ t_ $ $ § § pat :s 6.2 Faw R § Paso Robles and Tulare Formations L 2 Mainly valley deposits F:#3 O § A% $ 3 § Tp s §.8 € '% § Sedimentary rocks 7 ta G s 11.4, \ flex) ;) 4 ¥ ~ § Monterey Shale L S < es - C. g LU | Ts {- L | Sedmentary rocks y % $ oy w € ° 3 $ Tv ~G $ a a 3) 3| ; H es Volcanic rocks Siz 4" (Ls 5: W & TPM m >- z! :< h | < & 'in - C& “XS/(Q ‘cl Sfi§ TK [EDDif ~ TB 3 s s $ W-0 Z{ $741 O Miogeosynelinal rocks 0 0 E "W PU ~ gr W E 2 Oo u LAY EON NFojird al 1. C WU < - Plutonic and metamorphic rocks o o nv@ 1 ggt.‘ f W U <2 f Franciscan rocks 7 O LU (s #20 I i 48 | um O : W m - C Harlay fi sp, T... g Hr 7 % Ultramafic rocks C Largely serpentinite f ( Py 27 vii? e RG i and mle devas ~) x Contact _g1 Dashed where approximately located; short dashed @ 33 where gradational or iiferred; dotted where concealed MONTEREY Fault Dashed where approximately located; short dashed where inferred; dotted where concealed. Arrows in- dicate relative horizontal movement _._ | . MONTERE (U <> ‘ SaN.LUIS OBISHO 1CO! @ 7 } Al _Al oA lA ..A Thrust fault Dashed where apgroximately located; dotted where concealed. Sawteeth on upper plate _<_—_E_ e ree aie vse Anticline Showing crestline and direction of plunge. Dashed where approximately located Syncline Showing troughlinme and direction of plunge. Dashed where approximately located <4 - Lf -~ Qrcharg- oof $sh 100 Magnetic contours Showing vertical intensity magnetic field of the earth, Je ] 4 f . in gammas, relative to arbitrary datum. - Hachures x as 1 oe {ti} a | . 3 > 7 @ y 6960053 ; 3 j ) ~ Net ! 1", f N\ol a & mat st. a ; T R Alp 5 a Fras 7 : & R 7 ¢ i indicate closed areas of lower magnetic intensity. ey Spr ok s (oA p f A 5 ? \ 2 tal" a c \ io C s % Cp %, \ f a BTE c 2 y +A dvz % 2 $ 160 § 4 9, ‘ - g: j | ; & R Apr [! si? N id - af XL 6 P \& V sm, NG .; "seem \ge ot 20 "a= ip / - St : C / S XTO TZ ¥ ) Contour interval 50 gammas es | -s I | Regional magnetic gradient of 15 gammas per mile in the direction N. 16° E. has been removed from the original data. e meals Station of ground magnetic survey be Drill hole or exploratory well Number refers to table 3 -i ho ® uw (EMIGRANT HILL 1:24 000) Keoks Corner / @ pocacyou ;- "to o XC : [# Lam © ffi‘lflfinl)‘ 195. ‘I‘NZFT" poZ! $ @) M \ / *s Blue "Z Sprifih/{f 2 1 X vi? 1 wI Wess B7. o/ I xjvr (SHALE PoINT 1:24 000) 3 P I if‘fl) 109 y bat esa bla AP a . t_ 5 Linke ; ? M Sch ath oB hic Pecs % - i rac. moans F C a : 5 2 35°30 120°00' e «y/ Geology compiled by T. W. Dibblee, Jr., 1968-69 +, Am ¥ Base from U.S. Geological Survey SCALE 1:62 500 e 0°, Parkfield, Shandon, and Orchard Peak 1:62,500, 1961; a Drill hole data compiled by H. C. Wagner Prs. d c Pesk f p ; 1 V2 0 1 2 3 4 MILES age iy 2 arza Peak, Kettleman Elam and Pyramid Hills £ B M T T me perreo & 7 ] o T SW" 6" 1:24,000, 1953; Tent Hills 1:24,000, 1942 ? p of P 1 5 0 1 2 m 4 KILOMETERS & a ECHCH- HHC T I ] CONTOUR INTERVALS 25, 40 AND 80 FEET APPROXIMATE MEAN DASHED LINES REPRESENT 5-FOOT CONTOURS DECLINATION, 1972 DOTTED LINES REPRESENT 20 AND 40-FOOT CONTOURS DATUM IS MEAN SEA LEVEL See plate 4 for cross sections VERTICAL-INTENSITY GROUND MAGNETIC AND GENERALIZED GEOLOGIC MAP ALONG THE SAN ANDREAS FAULT NEAR CHOLAME, CALIFORNIA 1 469-845 O - 73 (In pocket) UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GEOLOGIC SECTION Surficial deposits not shown A) A 1200 1200 2 2 ps3 3 3 5 1100 é fo] 1100 1000 1000 A1.-AEROMAGNETIC PROFILE FROM ORIGINAL FLIGHT LINE 200 200 a 2 fs 5 = S 3 100 < < to] 100 & o 0 A2.-AEROMAGNETIC PROFILE ADJUSTED TO ARBITRARY DATUM AFTER REMOVAL OF REGIONAL GRADIENT Approximate flight elevation 5000' 5000' SEA LEVEL SEA:LEVEL 5000' 5000' A43 -COMPUTER MODEL OF MAGNETIC ANOMALIES Dashed where inferred ZZ 5 aP 4 ~ z ally an R zz @ <3 3 ~lO ZO A' A PACKSADDLE I < < olg og 218 eig _ MOUNTAIN Tha iy 210 25 6000 —W ge 's. Fe p a" Hu 6000 4000' -| PEND OREILLE aG | 4000" - LAKE aly Cig 2000 - 7 1 [- 2000" A4 -GEOLOGIC SECTION Surficial deposits not shown B B/ 2 1100 1100 2 is =S 3 3 < O fo] 1000 1000 B1.-AEROMAGNETIC PROFILE FROM ORIGINAL FLIGHT LINE Dashed where inferred ZOO—<1 200 7 ex w S S <2): 100 100 A 3 3 0 0 B2.-AEROMAGNETIC PROFILE ADJUSTED TO ARBITRARY DATUM AFTER REMOVAL OF REGIONAL GRADIENT Approximate flight elevation Approximate flight elevation 5000' 5000' Topographic surface SEA LEVEL SEA LEVEL 5000' 5000' B3.-COMPUTER MODEL OF MAGNETIC ANOMALIES Dashed where inferred - * 8 3 l $ § s '< & & 7 g & s 6000' 3 a § 5 6000' g 0 MAGEE FAULT ZONE "§ § 4000' f < 6] 4000" 2000" mum 2000" £ / Base from U.S. Geological Survey, 1:250,000: Sandpoint, 1958, and Spokane, 1955 TRUE NORTH APPROXIMATE MEAN DECLINATION, 1970 6 80 ( A ,,,,, 22 FOREST AVER PE © Lookout ~\§1TT AL C okout tower THEDRAL Pexty / _ vad | 10 INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C. -1970-669380 8 10 MILES y 10 KILOMETERS y hs & a 5 Bxift ookout / ® I NC, , o c LJ Mfi‘flé *6 f= > B/PE kont tower 3 fy sugphN peak \ € At 1 191 kout to M- ~ [ ¥ "| X } & / \ kop peu #2" y S2 £ y- © / ien \ HAIFWAY P -f © 18 \ \ 1 \\ Geology compiled by J. E. Harrison and A. B. Griggs, 1968 Aeromagnetic survey flown at 6,000 feet barometric where topography permits, 1959 2 to. 116° OC 5 ye 5 {VS ...... T. 28 N a* { pCul 3 i . QTs aut, % f | T. y 8er [GS. \\ L. & f \\\\ \\\ C+ \ #. a > A AX T (@: \ 4 . MNE £ 0 N 19 p€u r N3 s R P- -t- ___ ___ ___ l_ __ nnn 222 __ p> COMBINED GEOLOGIC AND AEROMAGNETIC MAP, AEROMAGNETIC PROFILES, MAGNETIC MODELS, AND GEOLOGIC SECTIONS OF PART OF THE PEND OREILLE AREA, IDAHO AND MONTANA Belt Supergroup PMe. PROFESSIONAL PAPER 646-D PLATE 1 EXPLANATION SEDIMENTARY AND METASEDIMENTARY Rocks QTs Surficial deposits Include mass-wasting deposits, allu- vium, glacial and glaciofluvial deposits, and high-level gravel TERTIARY AND QUATERNARY Lakeview Limestone, Rennie Shale, and Gold Creek Quartzite CAMBRIAN peu - Upper part Comprises Libby and Striped Peak Formations Middle part Comprises Wallace, St. Regis, Revett, and Burke Formations T x PRECAMBRIAN Lower part Prichard Formation J IGNEOUS ROCKS Granitic to dioritic intrusive rocks AND TERTIARY PRECAM- CRETACEOUS BRIAN Dioritic to gabbroic intrusive rocks (Purcell sills) Contact Dashed where approximately located meee cece a r 4145 + Fault Dashed where approximately located; dotted where inferred. Arrows show direction of apparent move- ment; bar and ball on downthrown side malice mutts muttice a n a t i a as a Thrust fault Sawteeth on upper plate. Dotted where concealed ___._+__ ........ Syncline, showing approximate trace of axial plane Dotted where concealed 25 -- Inclined Vertical 70 -g ® Overturned Horizontal Strike and dip of bedding Total intensity aeromagnetic contour Dashed where inferred. Contour interval 10 and 50 gammas. Ha- chures indicate closed area of lo- wer magnetic intensity. X , position of high or low within closed con- tour. Datum is arbitrary A15 Sample locality and number UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY R.39 E. QTr 52.495, 45’ 3 | T ay ty -) c . QTr t . o f G j F: 2 d \I €1 C Osp Qal C??? % Of % Osp < r Of A 1° Osp 70. 16, ¢ 168.2 os - R38 E. €bl OQ _i y | \ A ' = | "ral _ \| \_ \% "RCX" CRATBR (Gs: ~ . tenn we ue a nnn 178.8 a se- kk nn a ak u o 10 Qal. "A - ¢ a> a Rig AIK NA TSS, d .+ NRVZ / R o f A a $ . ¢ Qc R.46 E. R.A19 w. 111°00 i al 43°00 34 y* 3: T.35 N Pugin 1 ‘ Q,_» 0 R.119 W. T.34 N. T. 33 N. 45' Ts 0 6 w Kgu ch T.31 N. . An C 1.30 N T 90 N 1.29 N # } Base from U.S. Geological Survey 47, SCALE 1:125 000 0 f 1:250,000, Preston, 1955 $ o 2 4 6 3 10 MILES % . g O <1" F 1 ro- r=: -L F--- r «i /f F. o 2 4 6 8 10 KILOMETERS £ é" E- pe- --- --- & MATE MEAN CONTOUR INTERVAL 200 FEET \ 1 he. WITH SUPPLEMENTARY CONTOURS AT 100-FOOT INTERVALS 4 A DATUM IS MEAN SEA LEVEL f A\ R.46 E Oy N: if)” ; Ui Geology compiled by S. S. Oriel, 1965-67 ff“ 3 Gravity data by D. R. Mabey gathered i* /"§.) : " from 1961 to 1964. a 15! RA5E. R43 E R.44 E. 22 A' A ale 1700 1700 - alo r M [y 6 |- 1600 a- 1600 4 a 0 z o z .C i 5 noes dne mae eate ae ear nk" - 1500 € 1500 - 9 f E s Fm Regicgflfl;@jfl—fl~ip//—/_/\/d a ai er ea en rok a M ti iS 6 M [-: 1406 2Z 1400 - Lu f- Z E we. o e heeling e sore og" ~ AEROMAGNETIC PROFILE gate" £ 1500 rn ie seen eee 5 P ~: ~~ m -~ 1200 1200 - -160 160 A [— - -170 ~170 -I a S t" .=180 a 3 a3: -, G ssi s 9 tl,. . Z § Regional -. <~190 . -R. gradi £:>-180 - a CC 5 |. <200 £200 4 GRAVITY PROFILE 4 +319 —210J -10,000" 10,0005 BLACKFOOT LAVA FIELD Noch UPPER CHINA, HAT I VALLEY CHESTERFIELD RANGE VALLEY GEM VALLEY I seng 5000'- N Ao=-0.45 g per cm' Ae =- 0.45 g per cms I- SEA LEVEL SEA LEvEL - INTERPRETED GEOLOGIC SECTION 5000' ( e. INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C. -1969-G63129 Note:A sassumed density contrast GEOLOGIC AND GRAVITY-ANOMALY MAP AND SECTION OF THE SODA SPRINGS REGION, SOUTHEASTERN IDAHO spats Section B-B' shown in figure 3; section C-C' shown in figure 4 Holocene Pleistocene Ad Pliocene A Monroe Canyon Formation Mainly fine- to coarse-grained bioclastic and finely to medium crystalline limestone. ward out of the mapped area into red mudstone assigned to Amsden Formation. Little Flat Formation Calcareous sandstone, and silty limestone. 400- J Alluvium Unconsolidated gravel, sand, and mud Colluvium Includes hillwash, talus, loess, and alluvium in tributary valleys Tufa Porous to spongy white, buff, and yellow calcium carbonate Basaltic cinders Basalt Da'k finely to medium crystalline vesicular, porphy- tic, and coarsely crystalline massive olivine basalt. Maximum observed thickness about 300 feet Qm Main Canyon Formation of Bright (1967) Poorly consolidated to unconsolidated silt and marl grading into sand and gravel near valley margins. Maximum thickness exceeds 600 feet Rhyolite Tsl Salt Lake Formation Very light gray to white poorly consolidated tuffaceous calcareous siltstone, claystone, sandstone, and con- glomerate. About 10,000 feet thick Kw Wayan Formation Red calcareous mudstone and red, buff, and white sandstone and conglomerate. mined) thousand feet thick Kgu Kel Several (undeter- Gannett Group Kgu, upper part. Includes an upper unnamed red mudstone and siltstone unit, a middle thin- to me- dium-bedded aphanitic gray limestone (Draney Limestone) and a lower conglomeratic unit (Bechler Conglomerate) of red and white conglomerate and sandstone and red mudstone. feet thick Kgl, lower part. About 2,000-3,000 Includes medium to massive beds of pale-gray and pastel aphanitic limestone (Peter- son Limestone) and red conglomerate, red and tan sundstone, and purple, red, and green mudstone (Ephraim - Conglomerate). feet thick About 1,200-2,000 Stump Sandstone and Preuss Redbeds Gray, white, and pale-green calcareous sandstone, quartzite, and sandy limestone, underlain by pur- plish-red and maroon siltstone and fine-grained sandstone. About 1,000-2,000 feet thick [« Twin Creek Limestone Gray to dark-gray thin- to medium-bedded silty to shaly limestone. About 2,500-5,000 feet thick JRn Nugget Sandstone Tan, pink, and red quartzitic and partly calcareous sandstone; weathers mainly to brown manganese- oxide-coated talus. About 900-1,600 feet thick Fa Ankareh Formation As mapped includes Wood Shale Tongue of Ankareh Formation, Deadman Limestone, and Higham Grit. Consists mainly of red siltstone and mudstone. About 200-500 feet thick Thaynes Limestone ty, Timothy Sandstone Member; gray to olive-gray fine- to coarse-grained calcareous sandstone. About 250-400 feet thick Tt, argillaceous and silty limestone, calcareous muddy siltstone, and calcareous sandstone. 3,200 feet thick About 2,500- Dinwoody Formation Light-grayish-brown to olive-brown claystone, gray limestone, and brown- to black-weathering siltstone. About 1,000-2,000 feet thick Pp Phosphoria Formation and associated rocks Mainly cherty mudstone, phosphatic mudstone, chert, phosphorite, limestone, and dolomite. About 450-550 feet thick PPw Wells Formation Sandstone, mainly quartzitic but partly calcareous, fine to very fine grained; limestone and dolomite; includes some red mudstone near base assigned to the Amsden Formation farther east. 1,500-2,000 feet thick thick- to - massive-bedded Upper beds grade east- 900-1,300 feet thick siltstone, _ fine-grained 1,000 feet thick Lodgepole Limestone Thin- to medium-bedded cherty fine- grained to coarsely bioclastic lime- stone. 700-1,000 feet thick Brazer Limestone of Mansfield (1927) 1,600-2,000 feet thick Madison Limestone Similar to Lodgepole Limestone y QUATERNARY 5 CRETACEOUS 4 rng o Me art et Genna l Merron aime ns Y TRIASSIC Y MISSISSIPPIAN EX PLA NA TIO N TERTIARY JURASSIC TRIASSIC(?) AND JURASSIC (?) PERMIAN t a CARBONIFEROUS Beirdneau Formation Gray, tan, and pink silty and sandy limestone and calcareous sand- stone, and gray to dark-gray lime- stone and dolomite. 400-850 feet thick Hyrum Dolomite Dark-blue-gray, brown-weathering, €bn, upper limestone unit. €lb, middle shale unit. petroliferous-smelling _ laminated dolomite and light-gray dolomite with interbeds of limestone. 400- 1,650 feet thick PEATE 1 x Beirdneau Formation and Hyrum Dolomite undif- ferentiated .\\ Laketown Dolomite Very light to medium gray, white-weathering, medi- um- to thick-bedded dolomite. 5300-1,000 feet thick Fish Haven Dolomite Dark-gray, brown-weathering, fetid thin- to medium- bedded dolomite. 500-1,000 feet thick Swan Peak Quartzite Well-sorted very fine to fine grained medium-bedded to massive quartzite. 500-1,200 feet thick Garden City Limestone Medium-gray dolomite and dark-gray chert in upper part, grading downward to dark-gray thin- to me- dium-bedded limestone with bioclastic, oolitic, and intraformational conglomeratic beds. feet thick. 1,150-1,350 St. Charles Limestone €sc, upper part. dolomite and dark-gray limestone. thick €sw, Worm Creek Quartzite Member. Light- to medium-gray and brown 700-900 feet Light-gray vitreous quartzite and white to pink quartzitic arkose with interbedded light- to medium-gray dolomite. 200-900 feet thick Nounan Limestone Medium- to light-gray dolomite and dark-gray silty limestone and intraformational limestone conglom-- erate. 700-1,050 feet thick Bloomington Formation Shaly micaceous green mudstone and claystone with interbeds and nodules of pastel aphanitic limestone and oolitic and intraformational conglomeratic lime- stone. About 1,000 feet thick Blacksmith Limestone Medium-gray to buff medium- to thick-bedded lime- stone, partly oolitic and partly silty. thick €bn €lb €tk Rocks in the Bancroft area Thin- to medium-bedded medium-gray partly silty and mottled limestone. 5300 feet thick Mainly green, partly dark-gray to black, mudstone and light- to medium- gray limestone. 400 feet thick €tk, lower limestone and quartzite unit. Medium- to dark-gray me- dium- to thick-bedded oolite and Girvanella-bearing limestone, and brown-weathering porous - sand- stone and green quartzite inter- bedded with limestone in lower part. 600 feet thick 750-950 feet Cu €1 Ute Limestone and Langston Formation €u, Ute Limestone. - Thin-bedded light- to bluish-gray argillaceous partly oolitic limestone; partings and interbeds of green shaly clay- stone. 750 feet thick €1, Langston Formation. Light- gray coarsely crystalline dolomite, weathering brown to reddish brown, that grades laterally into units of dark-gray partly oolitic limestone and green and dark- gray claystone. 250-600 feet thick Brigham Quartzite Mainly light-gray to tan and green very poorly sorted partly conglomeratic quartzite with units of tan, green, and brown argillite; grades downward into purple quartzite. not exposed in area Contact More than 2,500 feet thick; base Dotted where concealed ssg X Dotted where concealed bx, breccia along fault Thrust fault Dotted where concealed. Sawteeth on upper plate _ (s Cones and collapse depressions in basalt Gravity station All values are negative milligals _J_l_x__2oo_b_x_g 200,050... 00000... .cc ges... Gravity contours Dashed where approximately located. Hachured contours indicate closed gravity lows. ligals. Contour interval 2 mil- Contours represent complete Bouguer for a reduction density of 2.67 g per cm 112° 1118 43° MANSFIELD (1927) PP-152 MANSFIELD (1929) B-803 RIOUX AND] | OTHERS (1966) or GULBRAND- | CRESsMAN] montcom- SEN AND | _ AND | ERY AND OTHERS |GULBRAND-| CHENEY (1956) | |SEN (1955)] - (1967) B-1042-A | B-1015-1 | B-1217 ORIEL (1968) ARMSTRONG (1969) or a 1-557 MANSFIELD | _ CRESSMAN (1964) (1927) B-1153 PP<152 42°30" B, bulletin; 1, miscellaneous geologic investigations; OF, open file; PP, professional paper INDEX TO SOURCES OF GEOLOGIC MAP DATA DEVONIAN PROFESSIONAL PAPER 646-E -- CAMBRIAN C Yy ORDOVICIAN SILURIAN PRECAMBRIAN