Qms apy Geology and Volcanic wh > M g; Petrology of the Lava ** Mountains, San Bernardino County, California GEOLOGICAL SURVEY PROFESSIONAL PAPER 457 Prepared partly in cooperation with the State of California, Department of Conservation, Division of Mines and Geology Geology and Volcanic Petrology of the Lava Mountains, San Bernardino County, California By GEORGE I. SMITH GEOLOGICAL SURVEY PROFESSIONAL PAPER 457 Prepared partly in cooperation with the State of California, Department of Conservation, Division of Mines and Geology A study of late Cenozoic volcanic rocks and a reconstruction of their probable origin in the light of chemical data and the geologic history of the area UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library catalog card for this publication appears after index. For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS 20 ..on le an coe auc o a ned man n o ale in aie ale m n alee cloe lye rset trices Purpose and methods of investigation ____________. Terminology sons ~ Acknowledgments and cooperation-____________ ___ Previous gcologic Geologic setting of the Lava Mountains-___________-__- Pre-Cenozolc Metamorphic Atolia Quartz Monzonite..._._..__.....__-_. Northwestern area-...............-..... Southeastern Origin of intrusive rocks.....:....._...._. Age of intrusive rocks-__..-...:-....___. CenorOolc rl neonl ee Conglomeratic and tuffaceous sandstone older than the Bedrock Spring formation...------ Volcanic rocks older than the Bedrock Spring sae ned Age and relation to other formations---.-. Bedrock Spring Formation............_...._.. Descriptions of lithologic map units__----. Epiclastic Volcanic ban atie s Distribution of rock types and environment of deposition. Age and relation to other formations.-.... Almond Mountain Volcanies._:..._..__._...._. Descriptions of lithologic map units Rocks of the eastern facies_________.. Volcanic intrusives-_______._____. Volcanic breccia ............... Tull... on s- ant ne Rocks of the western as bek Volcanic breccla.......~........ eaten, Sandstone...>..%.-_.=a.s-c=s5% Volcanic source and distribution of rock l eer enn ores ce Environment of deposition-____-___--_--_. Age and relation to other formations -__- Lava Mountains Andesite-..........:.....-... Descriptions of lithologic map units-_----. Andesite Flow Flow conglomerate.....:...._....~:. Volcanic source and distribution of rock cand ass Environment of -_- Age and relation to other formations -__- Page 13 14 15 15 17 17 18 19 19 21 283 24 24 24 25 25 27 27 27 28 29 30 30 31 31 32 32 32 32 34 34 34 36 37 Stratigraphy-Continued Cenozoic rocks-Continued Other upper Pliocene(?) volcanics________:-_. Tuff _L... Int{UsiYO8- 2-2 acne cire Volcanic Christmas Canyon Formation-_----_---_-__-- Description of rock types _______________ Distribution of rock types. .______________ Age and relation to other formations ___: __ Sediment source and environment of deposi- ion.: ... cnl eee eee nen a ae nd o Quaternary Quaternary andesite. Older gravels".___L..!L OO LIN LOSE, -D Tufa and S . _ .. s. - 202 n eve rre a ius = o aa ae ae a ale -. 2.22 tien nene au- ue cers cer Garlock faulb. 22. .L. cen orer ere n eee eee Faults parallel to the Garlock fault. Brown's Ranch fault zone..-..........-..-.,. Faults parallel to the Brown's Ranch fault zone. Blackwater fault- .... anm ane Faults parallel to the Blackwater fault -_---.-- Thrust Folds. cc:.o . c tl te Lon t a Origin of structural 200. cre a s Pediments..- _> c AGA. .-- atas cns -as Economic eel lire in cane Metals .. ->, -~ onn eink ate a Radioactive deposits... nes span reolite and __". Gravel. cloe em nne anne a Favorable areas for future prospecting____--------- Volcanic petrology... ee sada can ens os a Petrography ;. .... ..o an La cise a aun - =s aie nace s Origin and significance of volcanic lithologies. Volcanic rock Descriptions and properties of rock constituents . Biotite. lns uy cl leas a ege aaa Amphibole... . ... .cn 22s ree c oue 2 ule - ae ae Orthopyroxenc........_...0........ .k. ane n Opaque material-.:_..:....0..~._.c..... Other minerals III Page 37 37 38 38 39 40 40 40 41 41 IV CONTENTS Page Volcanic petrology-Continued Volcanic petrology-Continued Petrography-Continued Petrochemistry-Continued Modal composition of the suite______________. 65 Chemical changes with age___________________ Modal changes with age _.. "...... _. 65 Volcanic _.... 67 Mechanics of eruption....-.._-L..-...._._..__.- Chemical composition of the suite______------- 67 Processes of crystallization_...______________. Major element and normative compositions . 67 Source of magma.... Minor element composition _- "1 | References Relations between chemical and modal com- Supplementary eee 4 74 | Index: 20 t [ No.l LCIE ca a uas aa tein es ILLUSTRATIONS [Plates are in pocket] PraAtE 1. Geologic map of the Lava Mountains. 2. Geologic sections across the Lava Mountains. FIGURE 1. Index map of California showing area of figure 2 and plate k bok bek bestest peut fest pent 18. 19. 20. 21. 22. 283. 24. 25. 28. 29. 30. 31. 32. 33. Generalized geologic map of the Lava Mountains and vicinity Panoramic photograph and sketch of the . Diagram showing classification of fragmental volcanic rocks used in this Phase diagram of the system Q-or-ab-an at 5,000 bars pressure showing composition of Atolia Quartz Monzonite _ Type section of the Bedrock Spring . Photograph of lower part of the Bedrock Spring . Photographs showing typical lithology of the Bedrock Spring . Map showing the inferred limits of the basin of sedimentation in Bedrock Spring (middle Pliocene) time-_---... . Sections of the Bedrock Spring Formation showing relative stratigraphic positions of fossil localities-___._.-... . Type section of the Almond Mountain . Vertical aerial photograph of area east of Almond Mountain . Photograph showing typical outcrop of upper Pliocene(?) felsite _ . Photograph showing typical exposure of sandstone facies of the Christmas Canyon Formation-...._....._.... Type section of the Christmas Canyon Photographs showing Quaternary andesite flOW Oblique aerial photograph showing the Garlock fault and Searles Lake shorelines exposed near the northeast l a o i aas Vertical aerial photograph of part of Garlock fault Map of southern California showing location and sense of slip of major Iateral faults-..__:. _C: Profile view of the dissected pediment cut on Atolia Quartz Aerial photograph showing gravel-covered pediment surface cut on Bedrock Spring Formation, and Quaternary andesite flows deposited on the SAME SUIfAC@L _ Photomicrographs showing selected mineral Diagram showing relation between SiO; content and refractive index of groundmass :> Diagrams showing variation in mode of superposed rocks of the Almond Mountain Volcanics and Lava Moun- fain'g Andesite 22.2222 2 l ion ALI IIL ALE E o av a - a B ue ne ne i ar Be ning i e it n n s ie ie ie t hein i n p it te on in mice Walle in Aelia poe ac wore Diagram comparing average component percentages of the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite . Variation diagram showing relations between oxides of the major elements and silic@___-____---__------------ . Variation diagrams showing selected interrelations between Na,0 + K;,0, CaO, FeO +Fe;03, and MgO in volcanic rocks:: 22 seee s a mea de pn Pg Ean aos aie aed a Ha aa e a ha ma ie e he hn m he ma e ps he Siad ao re ml ot on in n an s i n osa a a e alee cate on on a ae a e oer Phase diagram of the system Q-or-ab-an, showing normative compositions of selected volcanic rocks _____._.--- Variation diagram showing the relations between the minor elements and _ ool clogs. Diagrams showing variation between SiO; and modal and normative plagioclase_________-_-_-___------------ Diagrams showing variation in percentage of major element oxides in superposed rocks of the Almond Moun- tain Volcanics and Lava Mountains Diagram comparing averages and confidence limits for chemical components of the Almond Mountain Vol- canies, Lava Mountains Andesite, and Quaternary andesite Diagram showing variation in the amounts of minor elements in superposed rocks of the Almond Mountain Volcanics and Lava Mountains Page 74 78 81 81 86 87 91 95 48 49 52 54 54 61 65 67 68 69 70 72 73 el 75 77 79 FrGurRE Figure TABLE CONTENTS v Page 34. Diagram comparing averages and confidence limits for the minor elements in the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary Andesite. sys 80 35. Phase diagrams for the system Q-or-ab-an-H;0, showing the compositions of the volcanic magma at four stages of 82 TABLES Page t. Major rock units in the Lava Mountaing, anns 10 2. Modal, chemical, normative, and spectrochemical analyses of four samples of Atolia Quartz Monzonite_____..-. 11 3. Normative, chemical, and spectrochemical analyses for oue sample of the volcanic rocks older than the Bedrock Spring Formation. - sL nen sine Ln an alanine o bings alis 15 4. Modal, normative, chemical, and spectrochemical analyses for one sample of the volcanic breccias in the Bed- rock Spring tand: anal ao os bamake ons k mew s so aes sis 19 5. Modal, normative, chemical, and spectrochemical analyses of nine samples of the stratified lithologic members of the Almond: Mountain YoIcanIits. . . - _._ _ . .o 20 oe eo ale o a's a n a a ae a ik aind ile min al mle teed al ie al in n ie in t i ae me on an de e nti an 26 6. Modal, normative, chemical, and spectrochemical analyses tor one sample of propylite from the western facies of She Almond: Mountain. Volcanics... . _...... L_... nus ue ul as bul ba an auld ake aln ale al eal a ae o o aln ano m 28 7. Modal, chemical, and normative analyses of one sample of the subpropylite of the Almond Mountain Volcanics. 29 8. Modal, chemical, normative, and spectrochemical analyses of nine samples of the Lava Mountains Andesite flows . . 35 9. Modal, normative, chemical, and spectrochemical analyses for one sample of the upper Pliocene(?) felsite.._..._. 39 10. Modal, normative, chemical, and spectrochemical analyses for two samples of the Quaternary andesite_.________. 43 11. Averaged modal compositions of volcanic rocks from the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite 0D nels /_ sen am ans es -s meck 66 12. List of alkali index values (Peacock, 1931) of 14 volcanic areas in western North America________________-_---.- 71 13. Averaged major oxide compositions, by chemical analysis, of volcanic rocks from the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary 76 14. Averaged normative compositions of volcanic rocks from the Bedrock Spring Formation, Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite 76 15. Averaged minor element compositions of volcanic rocks from the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite. no Leave ene en bo aa a o aan te ae ince 78 16. Variations in major element and minor element compositions of volcanic rocks in the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary 85 17. Reproducibility of lun lll c anceecss -s an 92 18. Mean value, mean deviation, and percent error of values reported by replicate spectrochemical analyses of five -cell: l LLL LIL L- E saner ss rence cena ane a a a teo a aat 92 19. Table showing spectrochemical values (unrounded) for standard samples G-1 and W-1-_____________________-- 93 20. Identifications of the Lava Mountains vertebrate fauna by locality. 93 21, Locations of rock samples described in this report. s bw 94 GEOLOGY AND VOLCANIC PETROLOGY OF THE LAVA MOUNTAINS SAN BERNARDINO COUNTY, CALIFORNIA By Grorer I. Sure ABSTRACT The Lava Mountains form a north- to northeast-trending range along the north edge of the Mojave Desert, San Bernardino County, Calif. The 140 square miles of mapped area includes all the Lava Mountains and a range of hills to their southeast. The northeastern third of the area consists of low hills formed mostly of middle Pliocene continental sandstones and conglom- erates; the southwestern two-thirds consists of higher hills and low mountains formed by upper Pliocene and lower Pliestocene volcanic rocks that cap the older sandstones. The Garlock fault forms the north edge of the range; the Blackwater fault and Brown's Ranch fault zone converge within it. The oldest rocks in the area are marble, siliceous marble, slate, phyllite, and hornblende amphibolite that form a few small outcrops. They are intruded by Atolia Quartz Monzonite of probable Cretaceous age, which is exposed throughout several square miles in the northwestern and southeastern sectors of the mapped area ; these rocks range in composition from quartz monzonite to granodiorite and have an average mineral and chemical composition about on the boundary between these two types. Sedimentary and volcanic rocks of middle(?) Tertiary age overlie the metamorphic and plutonic rocks but they are almost entirely covered by younger formations, three of which are named in this report. The most extensive of these younger formations is the Bedrock Spring Formation. It consists of about 5,000 feet of coarse-grained arkosic sandstone and con- glomerate plus subordinate amounts of fine-grained sandstone, siltstone, claystone, volcanic breccia, tuff, and lapilli tuff. It is assigned an age of middle Pliocene; vertebrate fossil collec- tions indicate it to be of this age or possibly late early Pliocene. The Almond Mountain Volcanics, a new formation of late Plio- cene age, unconformably overlies the Bedrock Spring Formation. It is commonly from 500 to 900 feet thick, although more than 2 or 3 miles from the volcanic centers it is much thinner. The lower part of the section includes tuff breccia, tuff, lapilli tuff, sandstone, and conglomerate, which grade upward into massive rubble breccia. These stratified rocks grade laterally into their intrusive equivalents, some of which are hydrothermally altered to propylite. The Lava Mountains Andesite, a new formation of very late Pliocene age, unconformably overlies the Almond Mountain Vol- canics. It consists of porphyritic plagioclase andesite, and is found as tabular flows, 200 to 600 feet thick, and as mounds and domes formed above areas of upwelling lava. During the time that the Almond Mountain Volcanics and the Lava Mountains Andesite were being deposited, small volumes of other upper Pliocene(?) volcanic rocks were also deposited; these are mapped separately but are not formally named. In early Quarternary time, sills, dikes, plugs, and small flows of andesite were deposited in the central part of the range. To groundmass, and one contains K-feldspar. their north, gravels were deposited contemporaneously on north- east-sloping pediments cut on the deformed Bedrock Spring Formation. To their east, a newly defined unit, the Christmas Canyon Formation, was deposited on a northwest-sloping pedi- ment as a gravel veneer 75 to 150 feet thick that graded into finer grained lake deposits. This formation is dated as Pleisto- cene(?). A few dikes of basaltic rocks cut this formation. Since those rocks were deposited, older alluvium, alluvium, and tufa have been formed. Within the Lava Mountains area, three fault systems converge. The Garlock fault trends N. 75° E. along the north side of the range; it is a left-lateral fault. The Blackwater fault trends N. 45° W. in the southeastern part of the region ; it is predom- inantly a right-lateral fault. The Brown's Ranch fault zone and its associated faults trend about N. 55° E. in the central and western parts of the area ; they have sustained both lateral and dip-slip displacements. A small thrust fault is present along the south side of the Garlock fault. The Dome Mountain anti- cline trends parallel to the Garlock fault and about 3 miles south of it. The earliest recorded activities on the Garlock fault and the Brown's Ranch fault zone were in middle and late Pliocene times, respectively, although earlier activities are inferred ; re- corded activity on the Blackwater fault is of early Quaternary age. The order in which the last appreciable displacements took place is: (a) Blackwater fault, (b) Brown's Ranch fault zone, and (c) Garlock fault. The Garlock fault, however, has not moved appreciably in Recent time. This sequence of fault- ing requires the activity along all three fault systems to have been in part contemporaneous. The volcanic rocks are virtually all porphyritic plagioclase andesite. Plagioclase megaphenocrysts (3-23 percent), plagio- clase microphenocrysts (4-20 percent), biotite (0-2 percent), hornblende or oxyhornblende (0-8 percent), orthopyroxene and clinopyroxene (0-3 percent), quartz (0-3 percent), and opaque minerals (1-11 percent) are found as crystals in a cryptocrys- talline to glassy groundmass (60-90 percent).. Most rocks contain cristobalite in the submicroscopic fraction of the Twenty-five major element and 24 minor element analyses are listed. The SiO: content ranges from 60 to 72 percent, but most of the rocks fall in the 63- to 66-percent range. In terms of CIPW norms, the rocks are also similar; most are yellowstonose (1.4.3.4.), six are tonalose (II4.3.4.), and two are lassenose (1.4.2.4.). Even within this small range, however, most major and minor elements show variations that are clearly related; the only exceptions are elements normally included in K-feldspars which, in these rocks, have not crystallized. Some mineral and chemi- cal compositions vary systematically with each other, but neither varies with the age of the rock. An alkali-lime index of 58 indicates that the rocks are calc-alkalic. 4 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY The volcanic magma apparently formed by fractional crystal- lization of a mafic magma prior to its first eruption, probably in middle Pliocene time, and the small amount of compositional variation in all younger volcanic rocks is inherited chiefly from this stage. Between middle and late Pliocene time, a magma of similar composition was added to the partly crystallized magma in the chamber. This caused distinct petrographic changes, but the average chemical composition of this new magma re- mained the same. Diffusion and mixing in the magma chamber throughout late Pliocene and early Pleistocene times progres- sively eliminated the inhomogeneities created during early dif- ferentiation and subsequent magma mixing. Three lithologic properties of the volcanic rocks are correla- tive with their stratigraphic position, and they indicate the process by which the magmas were transported to the surface : (1) all the earlier volcanic rocks were formed by explosive activity, whereas the later products were formed by effusive activity; (2) the frequency of eruption increased with time; and (3) the variety of rock types combined in fragmental vol- canic rocks decreased with time. These indicate an eruptive mechanism that combined the effects of increased faulting of the crust, increased temperatures in the conduit walls, and increased vapor pressures in the magma chamber. As the eruptive sequence progressed, these factors allowed eruptions to occur more and more frequently, and the magma to come pro- gressively nearer the surface before solidifying. In the later stages, magma remained fluid until it reached the surface to form effusive flows. INTRODUCTION The Lava Mountains consist of a range of hills and low mountains that lie along the north edge of the Mojave Desert, Calif. (fig. 1). They are in the north- 41° Eureki 40° 124° re > o 39° m a 38° SAN FRANCISCO 37° 190 . mites 117° 16° 20 o zo FIGURE 1.-Index map of California showing area of generalized geo- logic map of figure 2 (heavy outline) and the Lava Mountains area of plate 1 (solid black). western part of San Bernardino County and lie about 110 miles northeast of Los Angeles and 85 miles east of Bakersfield. The mapped area described in this report covers the Lava Mountains proper and the range of hills to their southeast which includes Almond Moun- tain. The total area is thus bounded on the north by Searles Valley, on the west by parts of the Rand Moun- tains and the Summit Range, on the south by Red Mountain and Cuddeback Lake valley, and on the east by Pilot Knob Valley, which contains the U.S. Navy Randsburg Wash Testing Range (fig. 2). The Lava Mountains occupy parts of the Randsburg, Cuddeback Lake, and Searles Lake 15-minute quadrangles. The area is reached by means of a paved road that connects Red Mountain and Trona. A paved U.S. Navy road extends across the northeast corner of the mapped area, but its use is restricted. Supplemental graded roads extend along the north and south edges of the range, and several rough, ungraded roads and trails ex- tend into the mountains from them. A four-wheel drive vehicle can be driven to within 2 miles of any point in the mapped area. The topographic character of most of the Lava Mountains in that of an irregular older surface that has been dissected (see fig. 3). In the northeastern part of the area, the local relief rarely exceeds 200 feet except where the more resistant volcanic rocks project through the younger gravel-covered surface to form hills. In the remaining areas, as much as 1,000 feet of local relief is provided where lava flows that form the uppermost surface have been breached, exposing to erosion less resistant sandstones that underlie them. - Similar relief occurs where intense faulting has reduced the rocks to an easily eroded gouge. Total relief is about 2,800 feet; the lowest area is along the north edge (about 2,200 ft), and the highest point is Dome Mountain (4,985 ft). The climate of the area is warm and arid. The an- nual precipitation averages between 4 and 6 inches, most of which falls during the winter months, some- times as snow. - The mean annual temperature is about 63° F. The flora consists predominantly of greasewood and sagebrush, although a few areas that are underlain by arkosic and granitic rocks support some Joshua trees. Coyotes, rabbits, small rodents, lizards, quail, snakes, scorpions, tarantulas, and other small animals inhabit the region. In the historical record of this part of the Mojave Desert, the Lava Mountains are rarely mentioned ex- cept in discussions of the Randsburg mining district, to the southwest, where gold was first discovered in 1895, tungsten in 1904, and silver in 1919. Each of these discoveries set off a new wave of prospecting in the Lava Mountains, and almost every square mile con- tains mining claims and evidence of prospector's camps. The well-known mule teams from the borate mines in INTRODUCTION 3 Death Valley used to pass across Cuddeback Lake a few miles south of this area. The Trona Railway, built into Searles Valley in 1914, lies about 3 miles north of the area. However, lack of water and proven mineral deposits has apparently discouraged any extensive human activity within the Lava Mountains. PURPOSE AND METHODS OF INVESTIGATION The initial purpose of this project was to develop a general understanding of the geology in the Lava Mountains. Particular emphasis was placed on the character and extent of the middle Pliocene rocks in order to determine the relative positions in the strati- graphic section of recently discovered vertebrate fossil localities. As mapping progressed, it became evident that much information could also be gathered about the late Cenozoic history of the Garlock and Blackwater faults and their effect on this part of the Mojave Desert. It also became clear that the volanic rocks of this area were exceptionally well exposed and were relatively unaltered. Furthermore, the entire volcanic sequence was preserved and most of the activity could be dated reliably as post-middle Pliocene. This report describes the results of work on the general geology and petrology of the volcanic rocks of this area (pl. 1 and 2). Mapping was begun in the fall of 1952 by myself ac- companied by George N. White. It was resumed and nearly completed in the fall of 1954, though some addi- tional work was done sporadically between 1955 and 1959. About 130 days were spent in the field and an area of about 140 square miles was mapped. The re- connaissance map, figure 2, required about 25 additional days. The geologic mapping was done on aerial photo- graphs at a scale of about 1: 20,000. The geology was then transferred to the topographic base by inspection and proportional dividers. The base map is compiled from parts of three 15-minute topographic quadrangles published at a scale of 1: 62,500; the western third is from the Randsburg quadrangle map made in 1900 by planetable methods, whereas the eastern two-thirds is from the newer Searles Lake and Cuddeback Lake quadrangle maps made by photogrammetric methods. So that the topography of the two sets of maps would blend, the contours along the east edge of the older quadrangle have been altered. The large discrepancies between the topography as shown on the Randsburg quadrangle map and on the photographs on which the geology was plotted have made it necessary to distort the actual geologic configuration of many areas to "fit" the topography, although the high-angle faults and contacts have been kept nearly straight to preserve the proportions and the general geometric relations as ac- curately as possible. Most of the petrographic study of the volcanic rocks was done on thin sections. A total of about 225 were carefully examined. All textural studies were made in this way, although some mineral identifications re- quired supporting data obtained by oil-immersion and X-ray methods. All thin sections were first studied in a random order. Modal, spectrochemical, and chemical analyses were made on 25 volcanic rocks and 4 plutonic rocks. The modes were made by the point-counting methods de- scribed by Chayes (1949). Between 1,400 and 1,500 points were counted for each slide. To determine the reproducibility of point counts for this type of volcanic rock by the method used, two rocks were analyzed three times, and one was analyzed twice. For each rock analyzed three times, the first two counts were made within a few days of each other, and the third was made about 10 months later. For the rock that was analyzed twice, the counts were made about 10 months apart. All counts were made with the same microscope and using the same conventions. The results, listed in table 17, show that any figure is probably reproducible to within 1.5 percent of the total rock percentage, although the percentage error of any value with respect to the average amount of that component actually present may be much higher. Extreme variability is limited to those components present in small amounts only; they, of course, have not been adequately sampled. The spectrochemical analyses of all but three elements were made on the facilities of the California Institute of Technology, Division of Geological Sciences. The sam- ples for analysis were collected and prepared in the following manner: At the localities to be sampled, 2 to 3 pounds of rock, free of weathered surfaces, were collected. These were then reduced to fragments one-half to one inch in diam- eter. The individual fragments were air-jetted to remove the dust and other impurities. Enough pieces were selected to weigh about 150 grams and these were then crushed in a "diamond" mortar until the entire sample passed through a 40-mesh cloth screen. The minus-40-mesh material was then split by means of a pure aluminum Jones-type splitter until one sample of about 10 grams was obtained. This was used for the spectrochemical analyses and the balance was retained for chemical analyses. The 10-gram sample was then reduced to a flourlike fineness in an agate mortar. The 25-milligram samples actually analyzed were grab sam- ples from this final product. Contamination of the sample was negligible for most spectrochemically analyzed elements. Some iron and GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY S37TIW OT e i9T.8€ ete 120 J \\“\\\\fi\\“\\\\w\ - suleiunom pueg- ~ 10€ SE (eSuey Sunsoi NSN) ysem Singspuey SIBM N ueipuj Ste LTT INTRODUCTION AdVILH3L-384d 'Sjtujfa pur BA¥T oy; Jo drut o1801098 poztfBJouon-7 @HNDI aouessteuuoda1 'I ') g SUL Z potgipow (6261) [NH T Jo saoimmog auaym payop acoym poysng spaq Jo dip pue ayLng ned Ase t nie. q1oda sty] UI apurfopur acoym pays» poqlosop eae ;o 1op1og jorqu09 sy904 snoaubr pun 'snooaubmjau 'Aunpuawmrpasnam fo aunjzru xopduiod fou syoo4 Rimpuawrpasnaow 'su syoo1 5 sy904 Runpuawrpasnau pun jpsshqudhy 'Ry SNH ur sayrip pournab-ourf fo swaunms 120; 'dvu ay} fo jund a) Ut 8390.4 0M1..01P amos 'sya04 Rpsow 'i1 syoo1 © 7 a A4 AwleFi/fz/ et xlwhfifmvflw/ DI0ZON3D 'dzp suomrisoduoa snorina fo smorf arun2100 'jz) soys»20 id pun smorf awos yj1m syoou opuna;0a amsnigur fo qsrsu0a 'snaun qsou ur zard - woo {parnmua4iaffipun 'sya04 aruna;0@ 'Az) sypoI NOILYNVTIAX3 AdVILH3L A A pun pormuf Apsow 'sy204 arunaror pari0oss» 'A] 'sajn4ow01Bu0a pun sauopspuns 's | uornuio buridg 3904 -pog ay ur squowubvaf s» punof 'smorf h amos Aiqngoud 'samsnugur ommohy4i (11 sypo1 puB JIUEIIO A ole ©5.] x* X * a 9.0, 0: wéd?h>,fl whl} x [nko 16 w Mo ox hk w Apybys o auadouf appr arsoy iD hrforyo fuompuioy Burdg yooupag 'q1 uoda4s fo sy904 orumaj0a (;) aussoug aaddn ay; pun sound -10A uimpunop; puow]y ay) sapnjour somenigur pup smorf 1ay30 awos sapnjour jun 'abn ausd -oud 27) fo surmunop; nant '|1 syo04 otureojoa pure Lrejuaunpag r smojf pun 'shnd 'sjps aqrisapun 'ed smoyf «ning Arforya 'syoou "ao sur -unop; mav7 ay; ur abn (;)2ua2098221] £0 uormpuio,f uohuny souqsuwy) ay; apnjo -ur 'paroassip Buraq mou «ap10 '30 pojoas -sip Buraq mou ijps pup puns apo 'sO sappuurd 'sowop 'spunow s» 'nfng pun '110 aonfins hnjo pagovd-paivy » sqisodap aqnp-ohn1d '00 amos ur poroossip hpybys 'umany» '20 sypo1 oatueojoa pug sqtsodap Arequautpag 2409004 1PPVA-244 2400004] «oddn pud PPA ju0093 pun» 2429098121] GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY 'so|[U pI qnogt st umoys se tole poddet ou} Jo S}IWII UooMjoq dout}SI( 'SIIIH JoSurdg oy} wou; yjnos SI MotA - 'SUIBJUNOJ( BA¥7T oy} Jo optnS yojoys pur #u09L Supe o == _- sC. Y sp.. = gyfiflé {3 maa: ee lg) <- f ,. in nfs E3 {\\||/{\|I C S <2 L SLN S Z y an nou, Can ~ °" pr" se z- --- -x og Leif aa Af +- 2 Gey uawuipad 17NV4 . FWJDDI/xh...‘ — juewuipad | saljo4u1 jawvos 11nv4 gouy ero9ss; 4 7 por 1a FmDIIFd | paroassiq Rienia1 AIMF<>>XO32 mm) (4-32 mm) OTHER TERMS tuff breccia Volcanic breccia = (abl!!! bregcua flow breccia rubble breccia Flow breccia = volcanic rubble in lava matrix Flow conglomerate = rounded volcanic fragments in lava matrix FIGURE 4.-Fragmental volcanic rock terms used in this report. Terms applied on basis of the volume percent of components constituting the rock. to rocks composed chiefly of volcanic ash, lapilli, or rubble regardless of whether they are intrusive or ex- trusive, primary or reworked. - The basis for distinction is the size of the volcanic fragments composing the rocks. Rocks containing a prominent percentage of angular or rounded fragments of volcanic rock in a lava matrix are called flow breccia or flow conglomerate. Volcanic breccia is used as a general term to include rubble breccia, lapilli breccia, flow breccia, and tuff breccia. The names of the fine-grained igneous rocks also fol- low the recommendations of Travis (1955). For rocks 8 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY in which the identifications or relative percentages of the visible minerals are doubtful, terms such as dacitic, andesitic, and basaltic are used. The mineral percent- ages applied to that classification are those of the mode rather than the norm. The volcanic rocks are named on this basis because the majority of published descrip- tions of volcanic rocks in the Mojave Desert area follow this convention, and because names based on the norm or bulk chemical composition do not indicate the actual mineralogy of the rock. In some of these rocks, the average composition of the plagioclase-the criterion separating andesite from basalt in Travis' classification (1955)-is closed to the dividing line (Any,) ; these rocks are here called andesite because the mafic acces- sory minerals are chiefly oxyhornblende and biotite rather than olivine and pyroxene. Fine-grained igneous rocks are commonly named by systems other than this, and some workers may wish to compare the system used in this report with one more familiar to them. The volcanic rocks in the Lava Mountains are so similar that one name describes most of them. - By the modal classification used in this report, they are andesite. If they are classified according to their mode by any of the other common classifications listed by Peterson (1961), the same name applies. The modal classification of Rittmann (1952, p. 90-92) ap- plies the name pheno-andesite. Chemical analyses show that complete crystalliza- tion of. these magmas under equilibrium conditions would produce rocks that also contain significant amounts of quartz and K-feldspar. If the rocks in the Lava Mountains are named according to any of the systems that anticipate this holocrystalline mineral- ogy-for example, according to their norms-they would generally be considered dacite or rhyodacite. According to the purely chemical classification of Ritt- mann (1952, p. 93-94), these rocks are mostly thyo- dacite; according to the chemical (CIPW) classifica- tion of Washington (1917), they are mostly yellow- stonose; and according to the average rock composi- tions compiled by Nockolds (1954), they most resemble rocks classified by him as dacite. In the color descriptions of rocks and minerals, the terminology of the Rock-Color Chart (Goddard, 1948) has been used where experience has shown that an accu- rate color description is important in identification. Color names obtained from this chart are always fol- lowed in parentheses by the symbol (such as 5GY 8/2). In the descriptions of rocks in thin section, the termi- nology, optical properties, and curves given in Winchell and Winchell (1951) have been used with the following four additions or modifications: (a) The borderline be- tween hornblende and oxyhornblende has been drawn at cAZ=10°. (b) Opaque materials have been so called without regard to the properties that might distinguish hematite, magnetite, ilmenite, etc. (c) In describing plagioclase zoning, the term "calcic rim" has been used to denote the outermost zone when it consists of a dis- tinct layer of more calcic plagioclase than the zone on which it rests, and the term "oscillatory-normal zoning" to describe oscillatory zoning that shows an overall normal trend. (d) The terms "megaphenocryst" and "microphenocryst" have been applied to plagioclase phenocrysts to distinguish euhedral crystals more than 0.3 mm long from crystals less than 0.3 mm long. To aid in describing locations, each of the seven town- ships in the mapped area has been assigned a capital letter as shown on plate 1. The sections within each township are numbered in the customary order ; the see- tions in township C are projected from adjoining town- ships. Each section is also subdivided into sixteenths and assigned a small letter as shown on plate 1. For example, "D23-m" refers to the NW 14 of SW 1, sec. 23, T. 29 S., R. 41 E. ACKNOWLEDGMENTS AND COOPERATION Many people have assisted in the work represented here. - George N. White and Donald H. Kupfer, then of the Geological Survey, provided valuable aid during the first few months of fieldwork. Extensive cooperation and assistance by Mr. and Mrs. A. Hunt of Randsburg and Mr. and Mrs. H. Howard of Johannesburg proved invaluable. The chemical analyses by S. D. Botts, P. L. D. Elmore, M. D. Mack, H. F. Phillips, and K. E. White of the U.S. Geological Survey have been essen- tial in interpreting the results of the fieldwork. Spec- trochemical analyses by Elisabeth Godijn and Robert Mays, as well as the supervision by A. A. Chodos of my analytical work are also greatly appreciated. The com- piling of this work has benefited especially from dis- cussions with C. R. Allen, W. Barclay Kamb, R. P. Sharp, and L. T. Silver of the California Institute of Technology, with A. M. Bassett of the University of California at San Diego, with Ian Campbell of the California Division of Mines and Geology, and with James F. McAllister, T. W. Dibblee, Jr., G. P. Eaton, W. C. Smith, and P. J. Smith of the U.S. Geological Survey. Final compilation of this work was done in cooperation with the State of California, Department of Conservation, Division of Mines and Geology. PREVIOUS GEOLOGIC STUDIES The Randsburg mining district, which lies about 5 miles southwest of the Lava Mountains, has been an area of interest to geologists since its discovery in 1895. Several reports and investigations were made of the GEOLOGIC SETTING OF district in the early part of the century. One of the first of these early investigations to include a study of the geology of the surrounding area was that of Hess (1910, p. 24-31), in which he summarized the general geology of the western Lava Mountains as well as the other ranges surrounding the mining district. Among the contributions of that study was the identification and naming of the Garlock fault, which is well exposed near the north edge of the Randsburg quadrangle. No geologic map of the region was published, however, until C. D. Hulin's report on the Randsburg quadrangle in 1925. That report summarized the geology of the entire quadrangle which includes the western third of the Lava Mountains. Later, a very generalized geologic map of a much larger area was published in Thompson's report (1929, pl. 8) on the water resources of the Mojave Desert; the geology around the Randsburg area was based chiefly on a previously unpublished map by Hess. A few years later, Hulin made a geologic reconnaissance of a large area surrounding the Randsburg quadrangle and his map was published at a scale of 1: 500,000 as part of a geologic map of California (Jenkins, 1988). Since that time, no map or geologic description of the Lava Mountains has been published. Hulin's report (1925) of the geology of the area divided the rocks exposed in the western Lava Moun- tains into three units: the basement of Late Jurassic Atolia Quartz Monzonite, the unconsolidated conglom- erates, sandstones, and clays of the middle Miocene "Rosamond Series," and the lavas and agglomerates of the Pliocene "Red Mountain Andesite." His subse- quent reconnaissance map (in Jenkins, 1938) carried these units throughout the rest of the Lava Mountains and surrounding areas. GEOLOGIC SETTING OF THE LAVA MOUNTAINS The Lava Mountains lie along the north edge of the Mojave Desert physiographic province. To the north lie the Basin Ranges. The Garlock fault approximates the boundary between these provinces, and the different character of the Cenozoic structures to the north and south of this structure cause most of the notable physio- graphic differences. The differences between the areas on the north and south sides of the Garlock fault are summarized by Hewett (1954a, 1954b). On the north side, in the Basin Ranges, the Cenozoic faults range in- strike between northeast and northwest, have large vertical displace- ments, and exert a major role in controlling the topog- raphy ; on the south side, many of the Cenozoic faults also have north west- strikes, but they have small vertical displacements and play a minor role in determining the topography. The ages and lithologies of the rocks on THE LAVA MOUNTAINS o the north and south sides of the Garlock fault are also different. The pre-Cenozoic rocks north of the fault include large areas of pre-Mesozoic sedimentary, meta- sedimentary, and metaigneous rocks; south of the fault, Mesozoic plutonic rocks form virtually all of the base- ment. The Cenozoic rocks on the north side of the Gar- lock fault are of early, middle, and late Cenozoic age, whereas those on the south side are all of middle and late Cenozoic age. The Blackwater fault, which enters the Lava Moun- tains range from the southeast (fig. 2), is one of the north west-trending faults so common south of the Gar- lock fault. However, it differs from the others in that it forms a boundary between areas that are structurally different. As is evident on geologic and structural maps of the northern Mojave Desert and southwestern Basin Ranges (Jennings and others, 1962; Hewett, 1954b), on the northeast side of the Blackwater fault, the dominant Cenozoic faults trend northeast or east, whereas on the southwest side they trend northwest. The stratigraphic and. topographic characteristics of the areas on the two sides of this fault are the same. The Garlock and Blackwater faults are major Ceno- zoic structural elements in this part of California. They are predominantly strikeslip, and late Cenozoic vertical displacements along neither have been con- sistent enough to restrict deposition of sediments to one side. Along segments of the- Garlock fault, however, vertical displacements have been consistent enough over appreciable periods of time to form local depositional basins several thousand feet deep. The middle Pliocene sedimentary rocks that crop out in the Lava Mountains represent deposits formed in one of these: basins. . In late Pliocene and Pleistocene time, the sense of vertical displacement was reversed, and the area of deposition was uplifted. Volcanic rocks were then deposited on the deformed and eroded basin fill, and the region has been undergoing erosion ever since. !t The Blackwater fault had some influence on the sedi- mentation pattern in the Lava Mountains area, but the areas of middle Pliocene sedimentation and subsequent volcanism were not bounded by it. e STRATIGRAPHY A summary of the major rock units of the Lava Mountains is shown in table 1. The oldest rocks found in the mapped area (pl. 1) are marble, slate, phyllite, and hornblende amphibolite, which crop out in two small areas. They are intruded by the Cretaceous( ?) Atolia Quartz Monzonite, which is exposed over large areas in the northwestern and southeastern parts of the mapped area. Overlying these are pre-middle Pliocene 10 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY Taus 1.-Major rock units in the Lava Mountains, Calif. Formation Age Aluvium Quaternary basalt Recent and Older Quaternary _-_ _ ______ Pleistocene gravels andesite Christmas Canyon Formation Lava Mountains Andesite Other upper Pliocene(?) volcanics & Late Pliocene Almond Mountain Volcanics Bedrock Spring Formation Middle Pliocene Pre-middle Pliocene Volcanic rocks older than the Bedrock Spring Formation Sedimentary rocks older than the Bedrock Spring Formation Atolia Quartz Monzonite - Pre-Tertiary Metamorphic rocks arkosic sandstones, conglomerates, altered volcanics, and unaltered volcanics that crop out in a few small areas. They are nearly concealed by the Bedrock Spring For- mation which unconformably overlies them. This newly named formation is the most widespread sedi- mentary unit in the mapped area. It consists of about 5,000 feet of coarse-grained arkosic sandstone and con- glomerate with subordinate amounts of fine-grained sandstone, siltstone, claystone, volcanic breccia, tuff, and lappilli tuff. This formation is assigned an age of middle Pliocene. Overlying the Bedrock Spring For- mation with pronounced angular unconformity is the Almond Mountain Volcanics, a new formation of prob- able late Pliocene age. It consists of interbedded tuff, lapilli tuff, volcanic breccia, flow breccia, sandstone, and conglomerate. Small isolated areas of tuff breccia, vol- canic intrusives, felsite, volcanic breccia, and tuff also of probable late Pliocene age, crop out in the mapped area. Unconformably overlying the Almond Mountain Vol- canics are flows of the Lava Mountains Andesite, also a new formation of probable very late Pliocene age, whose rocks are mostly dark gray to dark red porphy- ritic andesites. Small dikes, sills, plugs, and flows of a very dark gray andesite were formed in early Quaternary time. The relatively unaltered volcanic rocks of the Bed- rock Spring Formation, the Almond Mountain Vol- canics, the Lava Mountains Andesite, and the Quater- nary andesite are all petrographically similar. The pre-middle Pliocene volcanic rocks probably also had these characteristics prior to extensive alteration. Plagioclase, which constitutes 10 to 25 percent of most of these rocks, almost invariably occurs as euhedral magaphenocrysts and microphenocrysts. Megascopic biotite and oxyhornblende, or their opaque pseudo- morphs, constitute 2 to 10 percent of the rocks. Mega- scopic crystals of quartz and microscopic crystals of orthopyroxene and clinopyroxene make up less than 2 percent. The groundmass forms 60 to 90 percent of all the rocks and consists of skeleton crystals, microlites, cristobalite, cryptocrystalline material and glass, and a dust of fine opaque material. The Pleistocene(?) Christmas Canyon Formation, a new formation, is exposed along the northeast edge of the Lava Mountains. It consists of about 200 feet of fine-grained arkosic sandstone, claystone, and siltstone, which grade southward into boulder conglomerate. A few dikes of basaltic rocks cut the coarser facies of the Christmas Canyon Formation. Pediment gravels were deposited at about the same time in the western part of the area. During late Pleistocene time, alluvium was deposited, shorelines were formed by Cuddeback and Searles Lakes, and local patches of tufa were formed. In Recent time only minor amounts of alluvium have been deposited. % PRE-CENOZOIC ROCKS METAMORPHIC ROCKS The oldest rocks in the Lava Mountains are met- amorphic rocks which crop out in the northeastern and southeastern parts of the mapped area (pl. 1). Meta- sedimentary rocks crop out in the northeastern area, and amphibolites crop out in the southeastern area. The metasedimentary rocks exposed in the north- eastern area crop out as low hills projecting slightly above a gravel veneer, and as jagged cliffs bordering the shallow canyons. Most of these exposures consist of gray to yellowish-orange impure marble, and gray or gray-green slate and phyllite. The thickness of this section is estimated to be about 2,000 feet. No fossils were found, and the age of these rocks can be described only as older than the Cretaceous(?) Atolia Quartz Monzonite, which intrudes them. This series of metamorphic rocks is not clearly cor- relative with any others exposed in the northern Mojave Desert and southwestern Basin Ranges. The amphibolites in the southeastern part of the area, near the trace of the Blackwater fault, form a pendant in Atolia Quartz Monzonite. Typically they consist of crystals of hornblende, up to 2 inches long, and minor feldspar, arranged in an unoriented pattern. In some areas, hornblende appears to form the entire rock. Dikes and apophyses of aplite or pegmatite penetrate the mass, especially near the edges, and knotty in- clusions of epidote, mostly from 1/4 inch to 1 inch across, occur near the contacts. The hornblende amphibolite is probably equivalent to the hornblende gneiss facies of the Johannesburg Gneiss of Hulin (1925, p. 21-23). STRATIGRAPHY 11 ATOLIA QUARTZ MONZONITE The Atolia Quartz Monzonite, a rock first named and described by Hulin (1925, p. 33-42), is exposed over large areas in the northwestern and southeastern parts of the map. Small outcrops are also exposed in the central part of the area (in sections B26 and B27), and in the northeast corner of the map (in C9). Hulin designated all coarse-grained intrusive rocks in the Randsburg quadrangle as Atolia Quartz Mon- zonite. He distinguished several subtypes, which range is composition from quartz monzonite to quartz diorite. Orthoclase, plagioclase, and quartz occur in all these rocks, and biotite and hornblende occur in most. In- clusions are locally common. One type, composed of feldspar and quartz in almost equal proportions, and virtually devoid of dark minerals, was found to be characteristic of the northwestern Lava Mountains. NORTHWESTERN AREA Along the north and south edges of the northwestern area of the Atolia Quartz Monzonite, the topography is rugged as a result of late Cenozoic displacements along faults; erosion has created badland topography in areas of extreme brecciation. Between these scarps, however, the rocks form nearly flat areas or low hills The weathered rocks in this flat area range from grayish pink to grayish orange. f Quartz monzonite and granodiorite underlie about 75 percent of this northwestern area, although, because of their low resistance to weathering, they are mostly con- cealed by a thin layer of gruss. These rocks are mostly medium grained and equigranular. Orthoclase and microcline are generally anhedral and are slightly al- tered to clay. Plagioclase crystals are either euhedral or subhedral, are slightly altered to sericite, and are moderately twinned and zoned. Quartz is anhedral and contains small crystals of apatite and magnetite. Biotite, commonly altered in part to chlorite, is the most common dark mineral. Apatite is locally common and magnetite is sparse. Table 2 lists modal, chemical, nor- mative, and spectrochemical analyses of two samples of rock from this area (samples 26-2 and 41-34). Quartz makes up about 29 percent of each sample, and plagio- clase makes up about 65 percent of the total feldspar; these two rocks are thus quartz monzonite but their compositions are very close to granodiorite. Relative to Nockolds' (1954) average granodiorite and average adamellite (quartz monzonite), the percentages of A1,0; and Na,0 in Atolia Quartz Monzonite are a little high, and the percentages of CaO and possibly total Fe are a little low. The percentage of MgO is notably low in both samples. The normative compositions of both rocks classify them as toscanose and the normative plagioclase compositions are Anis and Ans. 735-720 0O-64--2 Tass 2.-Modal, chemical, normative, and spectrochemical analyses of four samples of Atolia Quartz Monzonite [Chemical analyses by P. L. D. Elmore, S. D. Botts, and M. D. Mack; spectro- chemical analyses for Li and Rb by Robert Mays, remaining spectrochemical analyses by E. Godijn; modal analyses by G. I. Smith] Modes Norms [Volume percents] [Weight percents] 26-2 41-34 181-29 26-2 41-84, 41-85 181-29 Orthoclase.. 22.1 21. 5 15.7 . IO 22.2 28.5 15.1 Plagioclase. 42.4 _ 41.6 _ 48.1 Quartz ..... 20.3 2.0 13.2 .9 . . . 2 Biotite ..... 3.9 2.9 2.9 [6° L4 83.2 25. 2 Horn- - 6.3 25.3 blende... 0 0 16.1 0 0 6. 6 Muscovite... .8 1.3 0 . 2 3.0 1.3 9.6 Pyroxene... 0 Trace 1.5 .9 2.3 77 4. 4 Opaque .8 1.1 .9 1. 4 minerals.. 1.1 2. 4 1. 4 A . 0 1.0 0 Other .3 .3 .3 p minerals.. /4 1.4 1.1 Sy to - co mbol (C.LP.W.) L423 L423 14.23 IL444 Chemical analyses [Weight percents] 26-2 41-84 41-85 181-29 MIOL ices ec reck see real 69. 9 65. 2 69. 5 58. 2 15.5 16.0 15.2 16. 4 1.8 1.6 1.5 3.0 . 58 1.9 . 58 4.3 47 . 60 . 52 3.3 2.2 2.5 1.3 6.9 4A 3.7 4.5 3.0 4.0 3.8 4.2 2.1 . 40 . 55 .45 . 14 14 18 14 . 24 . 06 .08 .05 19 . 87 1.4 1.5 11 . 38 1.6 +14 . 08 100 99 100 100 Spectrochemical analyses [Parts per million. Sought but not found: Ag, As, Be, Bi, Cd, Ge, In, La, Pt, Sb, Sn, U, Th, Zn} 26-2 41-84 41-85 181-29 284-8 70+10 3547 35+10 30+2 110+10 44410 434+14 3854-5 3454-15 310430 51040 Trace 541 Trace 340 4 4 3 16 Trace Trace Trace Trace 45420 70+15 60430 250430 ............ AU 3 441 240 241 T5+1 1043 842 91 944 1804-0 180420 130+10 240g]? .................................... + 1,300+0 _ 1, 2004100 _ 1, 7004-200 2, 2004-300 1643 2148 1948 1442 2443 22+1 ROHS I 1142 13+2 1242 841 610420 590+20 35012? 500i5g Trace Trace 1242 2240 175440 2304-20 180420 140450 LOCATION OF SAMPLES 26-2. Northwestern outcrop area-sec. A34-b, along road. 41-84. -sec. A35-d, outcrop a few feet north of small prospect along road. 41-85. -sec. A35-r, brecciated outcrop on southeast side of canyon. 181-29. Southeastern outcrop area-sec. F18-a, outcrop along east side of wash. About 20 percent of this northwestern area is under- lain by leucocratic quartz monzonite that is notably seriate and almost devoid of dark minerals. Orthoclase and minor microcline constitute about 25 percent of this rock; they form inequidimensional anhedral erystals that are slightly sericitized and contain inclusions of apatite, quartz, and plagioclase. Plagioclase makes up an estimated 45 percent of the rock and has an apparent composition of sodic oligoclase; it forms both large (1- 4 mm) subhedral crystals and small anhedral crystals; both types show strongly developed albite twinning and 12 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY faint normal to oscillatory-normal zoning. Quartz con- stitutes about 25 percent of the rock; the crystals are small, anhedral, and generally embedded in larger anhedral crystals of K-feldspar; they contain small crystals of apatite and magnetite. Biotite, much of which is altered to chlorite, makes up less than 5 percent. Opaque minerals, mostly grouped along boundaries be- tween larger felsic minerals, are present only in minor amounts. Analyses of this rock are listed in table 2 (sample 41-35). The analyzed sample was too badly crushed and altered to allow a reliable modal analysis, but the chemical and normative analyses support the modal estimate given above. Compared to the more common type of quartz monzonite, this leucocratic rock has similar percentages of normative orthoclase and plagioclase, but the plagioclase is distinctly more sodic (An,). The Fe,0, percentage is also similar, and this constituent must be chiefly in the form of hematitic material dispersed throughout the rock. The remaining 5 percent of the rock found in this area is an aplite. Though less abundant than the other two types, it crops out more commonly because of a greater to weathering. It consists of ortho- clase, plagioclase, and quartz with minor microcline, and a trace of chlorite presumably from altered biotite. All minerals are irregular in shape and size. Some crystals are as large as 1.5 mm, but most of them are smaller. The overall texture is seriate, and myrmekitic structures are not uncommon. The feldspars are slightly altered, especially along the cracks, to a claylike material. SOUTHEASTERN AREA The second large mass of plutonic rocks, in the south- east corner of the map area, is notable for a paucity of the pink leucocratic and aplitic phases found in the northwestern area. The predominant rock types appear to be biotite monzonite, quartz monzonite, and granodio- rite. Dikes and apophyses of aplite and pegmatite, as much as 5 feet wide, are exposed locally, especially around the hornblende amphibolite masses. Colors near gray predominate throughout the area, although iron oxides locally add a tinge of yellow or orange. Most of the southeastern area is deeply weathered and covered by a coarse gruss, so that few outcrops of the bedrock can be found. Where exposed along the Blackwater fault, the rocks are brecciated, altered, and locally silicified. Modal, normative, spectrochemical, and chemical analyses of one sample of granodiorite are listed in table 2 (sample 181-29). This is the most mafic plutonic rock in this area. The normative composition agrees adequately with the modal composition, and shows the plagioclase to have an average composition of abogt An;,. Its chemical composition bears little resem- blance to that of Nockolds' (1954) average granodiorite or any of the other average rocks. OTHER AREAS Intrusive rock crops out in two small areas in the northeastern quarter of the Lava Mountains. One ex- posure is approximately 3,000 feet long in sections B26 and B27, and lies along the axis of the Dome Mountain anticline. Medium- to fine-grained dioritic rocks, which are brecciated, altered, and silicified occupy about 90 percent of this area. Locally, aplitic phases are present. A grayish-red coarse-grained rock, possibly a quartz monzonite which weathers to a very coarse gruss, oc- cupies the remaining 10 percent of the outcrop area. It differs from the pink leucocratic rocks found in the northwestern area in that the average crystal size is larger, and dark minerals make up perhaps 5 to 10 per- cent of the total. a The second small exposure of plutonic rock is in Christmas Canyon. The rock there is a fine-grained diorite or perhaps tonalite, and consists mostly of plagioclase, hornblende, and biotite, with quartz present as interstitial grains and fillings. The plagioclase forms euhedral to subhedral crystals characterized by having numerous albite twins and only a few Carlsbad twins. Normal zones are found in almost all the plagioclase crystals and alteration of this plagioclase to clay (?) is extensive. Green hornblende occurs as subhedral crystals, which commonly have been altered to chlorite. Biotite, present as flakes and shreds, is partly altered to chlorite. In at least one outcrop, this rock is clearly intruded into metamorphic rock. ORIGIN OF INTRUSIVE ROCKS The normative compositions of the analyzed samples of Atolia Quartz Monzonite are plotted in figure 5 on diagrams of the phase system Q-or-ab-an at 5,000 bars water-vapor pressure. The samples that represent the most common type of Atolia Quartz Monzonite (26-2 and 41-34) have compositions that lie approximately on the plagioclase-beta quartz boundary. The leuco- cratic rock (41-35), which is much less abundant, has its composition on the eutectic side of these points. The granodiorite (181-29) is on the mafic differentiate side. Apparently this suite of rocks represents solid prod- ucts of a magma that was partially differentiated during crystallization. The granodiorite represents a small fraction of mafic material that became isolated during crystallization; the leucocratic rock represents the liquid fraction that remained after being depleted in these mafic components. The original composition of the magma probably approximated that of the two representative samples. The positions of the phase STRATIGRAPHY Ma ¢, Beta quartz ab-or feldspar Q (1065°) 1000° Beta quartz \“f> > 2, Beta quartz a & Plagioclase ¢ + or-ab feldspar (748°) 700° PS Plagioclase 1100° feldspar EXPLANATION Representative samples of quartz mon- zonite from the northwestern area (samples 26-2 and 41-34) [O Leucocratic quartz monzonite from the northwestern area (sample 41-35) .¢. Granodiorite from the southeastern area (sample 181-29) an (1235°) Ficurs 5.-Phase diagram of the system Q-or-ab-an at 5,000 bars water-vapor pressure showing composition of the analyzed samples of Atolia Quartz Monzonite. from table 2. boundaries in this system shift with changes in vapor pressure, and their positions at about 5,000 bars satis- factorily relate mineralogies and chemical compositions of the coexisting rocks. AGE OF INTRUSIVE ROCKS The Atolia Quartz Monzonite in the Lava Mountains is younger than the undated metamorphic section, which it intrudes, and older than the pre-middle Pliocene sedimentary rocks, which contain fragments of it. It was assigned a Jurassic age by Hulin (1925, p. 42) on the basis of indirect evidence. A Cretaceous age is now suggested for the batholithic rocks of the southwestern Basin Ranges and Mojave Desert by more than 20 lead-alpha age determinations on similar rocks (Jaffe Phase data and isotherms (in degrees centigrade) from Bateman and others (1963, p. 34). Analyses and others, 1959, p. 87-90). Ages between 85 and 130 million years are reported (op. cit.), and although radiometric dates by other methods may indicate ages that are slightly different, it is reasonable to conclude that the correct age of the Atolia Quartz Monzonite is in this range. In this paper, this rock unit is assigned an age of Cretaceous ( ?). CENOZOIC ROCKS CONGLOMERATIC AND TUFFACEOUS SANDSTONE OLDER THAN THE BEDROCK SPRING FORMATION Four small outcrops of conglomeratic and tuffaceous sandstone are grouped along the axis of the Dome Mountain anticline in the central part of the mapped area (sees. B2T and B33). The rocks are well bedded, 14 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY weather to a very light gray or yellowish gray, and are slightly more indurated than the overlying Bedrock Spring Formation. They consist chiefly of conglom- eratic sandstone and tuffaceous sandstone, but contain a few beds of tuff. The small percentage of cobbles and boulders in the very coarse grained sandstone is the most diagnostic feature of the unit; they are strik- ingly well rounded, are as much as 1 foot in diameter, and are composed of light-colored plutonic rocks or banded rhyolitic rocks. The stratigraphic section in B2T-n is described be- low : Thickness (Jeet) Tuffaceous sandstone, well-bedded, beds 1-10 in. thick, yellowish-gray (5Y¥ 8/1) to very light gray (N 8). Top of section removed by erosion. Arkosic sand forms 20 to 60 percent of rock___________ 33 Conglomeratic sandstone, well-bedded, light-brown (5YR 6-8/4), some very light gray (N 8) _________________ T Conglomeratic and tuffaceous sandstone, interbedded; conglomeratic sandstone, colors similar to those of unit above; tuffaceous white sandstone (N 9) with pale brown streaks (5¥R 6/2). Ratio of epiclastics to pyroclastics about 1 to 1.:1.._._-___L_.__._____._____L__ 18 Conglomeratic sandstone, well-bedded, very light gray (N 8) to yellowish gray (5Y¥ 8/1). Well-rounded frag- ments, up to 1 ft across, of quartz monzonitic rocks, some banded rhyolitic rocks; very coarse grained arkosic sandstone forms 95 percent of unit____________ 11 69 Base of section not exposed. These rocks are younger than the Atolia Quartz Mon- zonite and the rhyolite found southeast of the mapped area (see fig. 2), for both rocks are found as boulders in the conglomerate. In section B27-n, the arkosic con- glomerate and tuff unit underlies the Bedrock Spring Formation with a well-defined 20-degree angular un- conformity. At this contact, the color variations sug- gest a fossil soil about 1 foot thick. No fossils were found in this unit; therefore its age remains unproven except within the limits imposed by the Atolia Quartz Monzonite, the rhyolitic rocks, and the middle Pliocene Bedrock Spring Formation. How- ever, it is probably no older than Miocene; in this part of the Mojave Desert, the oldest rocks resting on the plutonic rock basement are Miocene, and the lithology and induration of the rocks described here suggest a similar age. vOLCANIC ROCKS OLDER THAN THE BEDROCK SPRING FORMATION Several types of volcanic rocks, known only to be pre- middle Pliocene, occur within the mapped area. All crop out along the axis of the Dome Mountain anticline. The easternmost outcrops are located near the exposures of arkosic conglomerate and tuff in the central part of the mapped area (sees. B33-a and B33-b). About 2% miles west of that area (between sees. E6-e and E6-k), a single outcrop about 2,000 feet long exposes additional varieties of volcanic rocks. About 1 mile farther north- west (in sees. A33, A34, A35, and D1), still other types of volcanic rock are exposed over a more extensive area. The rocks in each of these three areas have little in common except for their volcanic affinities and the fact that they underlie the Bedrock Spring Formation. The easternmost outcrop of this group (see. B33) con- sists of bedded pyroclastic rocks that dip north. They weather to various shades of gray, yellow, and red. A composite section of these rocks is as follows : Thickness Top of section removed by erosion. (feet) Tuff breccia, cliff-forming, massive, very light gray (N 8) to light greeenish-gray (5GY 8/1) ___________________ 30 Tuffaceous sandstone, tuff, and lapilli tuff, well-bedded, beds 1 in. to 20 in. thick, weathers to light reddish orange (101 6/2) .: .: 2 oul o e e oo oo me eela ul ene e o eac m to m me ie aoe 15 Similar, locally contains small volcanic fragments, weathers to light gray (N 25 Massive tuff and lapilli tuff, locally bedded; weathers to yellowish gray (BY 20 90 Base of section not exposed. The rocks exposed in the second area of outcrop (see. E6) are of two types. Those in the western half con- sist of light-colored arkosic sandstone, tuff, and lapilli tuff that are severely faulted and brecciated. They rest unconformably or in fault contact on a volcanic complex which forms the eastern half of the outcrop. The rocks of this complex are massive andesites(?) that weather to shades of gray, brown, or buff. They are composed chiefly of plagioclase, secondary opaque minerals, cal- cite, and quartz, all embedded in a groundmass of cyptocrystalline material. The plagioclase crystals are seriate, range in size from microphenocrysts up to 4 mm, and have weak oscillatory or oscillatory-normal zones and well-developed albite twins; the composition ranges from andesine to sodic labradorite. The rock is ex- tensively altered. Opaque minerals, apparently altera- tions of biotite and hornblende, compose about 10 percent of the rock; initially hornblende was more abundant than biotite. Veinlets and blebs of quartz, composing about 10 percent of the rock, fill vesicles and cracks in the groundmass and plagioclase. Calcite partly replaces a few of the plagioclase erystals. A dis- tinctive feature is that some of the vesicles are filled with green opal (?). In the third group of outcrops of this unit (see. A83 and surroundings), two types of volcanic rocks are ex- posed. Both form areas of complexly brecciated re- sistant rock. Their contacts with Atolia Quartz STRATIGRAPHY 15 Monzonite commonly dip at high angles, and the entire unit appears to be a shallow intrusive complex. The first of these two types forms about 10 percent of the outcrop and weathers to white or yellowish gray. The rocks in this unit have an estimated original composi- tion of 20 percent plagioclase, 5 percent amphibole, and 5 percent groundmass. Alteration has changed this rock into a propylite or subpropylite (see page 24). The plagioclase is largely altered to calcite, quartz, chalcedony (?), and a trace of opaque minerals; and the amphibole(?) is altered to opaque minerals and zoisite(?). The groundmass now consists of about 98 percent fine-grained quartz(?) with traces of opaque minerals, serpentine, and calcite. The second type of rock found in this group of out- crops weathers to a grayish green and contains light reddish-brown plagioclase phenocrysts that are a dis- tinctive feature of the rock. Originally this rock prob- ably consisted of plagioclase and amphiboles in a groundmass that made up about 50 percent of the rock. It is also altered to a propylite or subpropylite. The plagioclase has been totally altered to sericite, calcite, and opaque minerals in the approximate ratio of 5: 4: 1; and the amphiboles(?) have been altered to chlorite, opaque substances, and sericite in the approximate ratio of 4: 4:2. Maximum original crystal size of the plagio- clase was 4 mm and that of the amphiboles 2 mm. The groundmass is now holocrystalline and consists mostly of patches of fine-grained quartz, but also contains sev- eral percent of clay and a few percent of sericite, calcite, and serpentine. Chemical, spectrochemical, and normative analyses of a sample of this grayish-green rock are given in table 3. The rock is too badly altered for a modal analysis. The large percentages of CO; and H;0 indicated by the chemical analysis confirm the extensive alteration. If the analysis is recalculated to 100 percent without CO; and H0, the SiO, percentage increases from 60.2 to 64.1, and all other components increase proportionally. This percentage of SiO; is similar to the percentages found in all other volcanic rocks of this area, and the normative composition and name (yellowstonose) con- firm this similarity. The subsequent alteration is re- sponsible for all the notable differences. The CO; reported by the analysis is derived from the calcite veins and replacements in the rock, and the CaO that is combined with it to form calcite accounts for most of the CaO in the altered rock. The total amount of CaO in this rock, however, is similar to the amount reported from relatively fresh rocks of this area having the same percentage of SiO,, and this probably means that the CaO) in calcite was present in the original rock, and that only CO; was introduced. TaBL® 3.-Normative, chemical, and spectrochemical analyses for one sample of the volcanic rocks older than the Bedrock Spring Formation [Sample 41-39 from top of small ridge in south half of see. A35-q. | A dash means that the element was not present in detectable amounts. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White; spectrochemical analyses for Li and Rb by Robert Mays, remaining spectrochemical and modal analyses by G. I. Smith] Chemical analyses [Weight percents) Spectrochemical analyses [Parts per million] Norms [Weight percents] 60. 2 18.0 16. 4 0 1.9 15.0 2.9 28.8 1.4 22.0 4.6 A4 3.4 5.0 2.5 2.8 . 90 2.6 . 29 0 . 04 0 3.2 f 0 2.9 can .3 100 Symbol (CLP.W.)...... L484 AGE AND RELATION TO OTHER FORMATIONS All these volcanic units are probably younger than the conglomeratic and tuffaceous sandstone described in the previous section. The two units are not in contact, but the volcanic rocks are inferred to be younger because no fragments of similar volcanic rock are found in the sandstone. The contacts of Atolia Quartz Monzonite with these volcanic rocks do not furnish conclusive evi- dence, but the more probable interpretations suggest that all the volcanics are younger. These volcanics are clearly overlain by the Bedrock Spring Formation, and fragments of them are found throughout most of the Bedrock Spring Formation in the western and northern parts of the mapped area. BEDROCK SPRING FORMATION The Bedrock Spring Formation, a new formation named here, is the most widespread sedimentary rock unit in the Lava Mountains region. It is named after Bedrock Spring, 2 miles north of Dome Mountain and about 1 mile west of the type section. The location and lithology of the rocks in the type section are shown in figure 6. It is assigned an age of middle Pliocene; ver- tebrate fossil collections indicate it to be of this age or possibly late early Pliocene. The rocks assigned to this formation were included in the Rosamond Formation of Hulin (1925, p. 42-48). That name has not been used in this report because the Rosamond Formation as Hulin mapped it included the Bedrock Spring Formation plus part of the overlying Almond Mountain Volcanics of this report, and because as pointed out by Dibblee (1958, p. 135), the name has been applied so indiscriminately to Tertiary rocks in the Mojave Desert that it has lost its usefulness. The most extensive exposures of this formation are in the northern half of the mapped area. The formation 16 GEOLOGY THICKNESS (IN FEET) 5000 12. Sandstone, contains lenses of breccia and conglomerate and some thin tuff beds; weathers to light green, yellow, or brown; well bedded 11. Siltstone, green 10. Sandstone, with conglomeratic lenses, beds thick to massive 9. Conglomerate; matrix of tuffaceous arkosic sandstone, massive 4000 8. Sandstone, arkosic; lenses of well- rounded dioritic, quartz monzonitic, and rhyolitic pebbles; scattered thin beds of siltstone and mudstone; generally nonresistant to weathering; most bedding thick to massive 3000 2000 7. Sandstone, arkosic; tan to light-brown, generally massive; resistant beds locally present in section 6. Sandstone, arkosic, lenses of pebbles; tan to brown; beds thin to flaggy, resistant 5. Sandstone and conglomerate, tan to 1000 yellow; weathers to smooth hillsides 4. Volcanic breccia, angular fragments of volcanic rock in a lapilli tuff matrix; tan to purple; resistant to weathering; massive 3. Pyroclastic breccia, green to blue- green; resistant to weathering 2. Sandstone, arkosic, and a very few lenses of pebbles; yellow to light brown; massive to thinly bedded 1. Sandstone, arkosic, well-sorted, pale red; beds are as much as 3 ft thick and are well defined 0 FIGURE 6.-Type section of the Bedrock Spring Formation. The base of the measured column is located in sec. E5-f ; from there it extends N. 35° W. for 2,500 feet to the base of unit 4, then is offset north- east to sec. B31-a, and from there extends N. 35° W. for 3,000 feet and then S. 80° W. for 6,500 feet. thickness is difficult to establish in many areas, though, because of the lack of widespread stratigraphic markers within it. Several thousand feet of section is exposed AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY on the flanks of the Dome Mountain anticline. Sec- tions that exceed 1,000 feet in thickness have been meas- ured elsewhere in the mapped area and to the southwest of it, near Red Mountain. A thickness of more than 5,000 feet is exposed at the type section. This is prob- ably close to the maximum thickness of this formation in the area. The formation consists chiefly of coarse arkosic con- glomerate, sandstone, siltstone, and claystone, with smaller amounts of limestone, evaporites, tuff, tuff breccia, rubble breccia, and lapilli breccia. On the geo- logic map (pl. 1), the larger lenses of these minor rock types have been differentiated wherever their strati- graphic thickness and extent allowed, but neither a fixed order of succession nor a fixed position within the for- mation can be assigned. Most of the formation is in- cluded within the unit termed "epiclastic rock," which indicates sections composed mostly of conglomerate, sandstone, siltstone, or claystone; the small volumes of limestone and evaporites are also included in this unit because of their intimate association with the finer grained epiclastic rocks. The unit termed "volcanic breccia" indicates those lenses of rock that consist pri- marily of tuff breccia, rubble breccia, or lapilli breccia. The unit termed "tuff" indicates those lenses of rock that consist primarily of tuff or lapilli tuff. The contact between Bedrock Spring Formation and older rocks is exposed in many places. These older rocks were almost certainly parts of hills in the deposi- tional basin, though, and still older sediments were deposited in the areas between these hills. Rocks rep- resenting these older sediments crop out along the axis of the Dome Mountain anticline in the central part of the range, but the oldest part of the formation is prob- ably not exposed. The oldest exposed rocks are well-bedded reddish sandstones, which are well sorted and virtually free of conglomerate and volcanic debris (fig. 7) ; their lithol- ogy suggests that they were deposited in a lake (whereas most of the younger sediments of this formation were probably deposited subaerially). Well-bedded reddish sandstones are also exposed about a mile southeast of the fold axis (in see. E5-r) where an intraformational unconformity separates them from overlying yellowish- tan conglomeratic sandstone, indicating that at least one period of local deformation and erosion occurred very early in Bedrock Spring time. The top of the Bedrock Spring Formation has prob- ably been everywhere removed by erosion. In most places, the formation was strongly deformed and several thousand feet of material removed by erosion prior to deposition of the overlying Almond Mountain Vol- canics. In a few places, the formation is conformably STRATIGRAPHY FicurE 7.-Lower part of the Bedrock Spring Formation, as exposed in see. E5-e near the base of the type section. The beds in this area weather to red brown. Pick in the lower left corner gives the scale. Hill in background consists of Quaternary andesite. overlain by the Almond Mountain Volcanics, but the overlying volcanics were apparently all deposited on a continuation of the same erosional surface, so significant erosion of the underlying formation is likely in these areas as well. DESCRIPTIONS OF LITHOLOGIC MAP UNITS EPICLASTIO ROCK Sandstone and conglomerate.-The best exposed sec- tions of sandstone and conglomerate are at the type section and in sections B32 and D2. Most of the rocks are poorly indurated and weather to form rounded hills in areas of low relief, or to form badland topography. The few beds that are well indurated form conspicuous cliffs, ridges, or ledges on what are otherwise smooth hillsides covered with overburden. Bedding is obscure except in areas with unusually good exposures, where the beds exhibit all variations ranging from flaggy (as in sec. E1) to massive (as in secs. B20 and B29). Some beds are crossbedded and channeled and commonly contain a few larger boulders or cobbles in the channel bottoms. Few beds can be traced more than 2,000 feet, and most cannot be traced more than 500 feet. A well-exposed section and a de- tailed view of the most common type of arkosic sand- stone are shown in figure 8. The lithologies shown in these photographs are representative of more than three- quarters of the Bedrock Spring Formation. In most areas these rocks are a light pinkish tan, but in a few areas the rocks are reddish or greenish. Notably red sandstones are found along the axis of the Dome Mountain anticline (in sees. E5-e, E5-f, E4-e, and E4-f) , and on the north side of Klinker Mountain (in see. D2-g) ; they are most abundant in the lower part of formation. The coloring seems due to iron 7 8.-Upper photograph shows an example of the dominant lith- ology of the Bedrock Spring Formation as exposed in sec. B32-c. The beds dip north at about 10° and weather to a uniform light tan. The bushes on the left side are about 1 foot high. Lower photograph is a detail of the lithology shown on the right side of the upper view. Note the crude graded bedding. oxides, but is not obviously related to grain size, bed- ding, or any other textural variable. Some beds change color laterally over a distance of a few hundred feet, grading from red into light green, yellow, or the normal color for this formation. As noted by Hulin (1925, p. 43), the red coloring is not a surface effect, and rock samples from mine shafts several hundred feet deep show similar coloration. The sandstone in this formation is generally composed of angular fragments showing crude sorting. The most common size range is between coarse and very coarse sand but finer material is not uncommon. This rock is only slightly indurated in most areas, and can be gouged out with a pocketknife. The cement is mostly calcareous. Conglomerate clasts, mostly less than 2 inches in diameter, are well rounded to subrounded and 18 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY show little sorting (fig. 8). They include the following rocks: pink leucocratic quartz monzonite, pink aplite, gray quartz monzonite, pegmatitic rocks, quartz, meta- sedimentary rocks, purple porphyritic volcanics, mas- sive rhyolitic rocks with uniform coloring, and rhyolitic rocks showing prominent color banding. Volcanic rocks constitute 10 to 30 percent of the fragments. Exceptionally well indurated sandstones are found about 2 miles south of Klinker Mountain (in sees. D26 and D27), where they have been silicified as a result of intrusive volcanic activity. They break with a smooth conchoidal fracture and weather to form hills that are smooth and steep. Bedding, where visible, is indicated by linear rows of small rounded fragments. The out- crops are generally light brown, although some are slightly orange. The rock consists of about 60 percent detrital quartz and feldspar, and about 40 percent see- ondary quartz (and possibly some chalcedony or opal) which now cements the grains together. Minor amounts of sericite( ?), serpentine (?), and opaque minerals are also present. Siltstone and claystone.-Siltstone and claystone crop out in several areas along the flanks of the Dome Moun- tain anticline. The best examples of these lithologies are found in the central part of the mapped area (in sees. E3-a and F3-g). Although these rocks are com- posed chiefly of silt- and clay-sized clastic fragments, they consistently contain a small percentage of fine to coarse sand. Both the siltstones and claystones are poorly consolidated and weather to form low smooth hills covered with slumped material. The best fossils found in the Bedrock Spring Formation have come from rocks with these lithologies. The colors of the out- crops range from light green through yellow to gray. Bedding, where visible, is distinct and thin. Chemical precipitates form a small percentage of the rocks of this type in four areas; their presence is important because it helps confirm that the depositional environment was a closed desert basin. In the siltstones in section E3-h, small amounts of halite are concen- trated on the weathered surface as an efflorescence. In sections B26-a, b, and g, thin beds and large crystals of gypsum form several percent of the rock, and beds of greenish or yellowish tuffaceous limestone, 1 to 5 feet thick, occur 200 feet higher in the section. Similar beds of limestone are also found in section B27-h. A distinc- tive variety of limestone is found about 2 miles east of Dome Mountain (in sees. E9-b, E9-j, and E9-k) where it is shown on the geologic map by marker contacts. It is 2 to 4 feet thick, weathers to a light yellowish gray, and contains abundant ostracods and possibly other fos- sils. When the rock is dissolved in acid, an oily film rises to the surface. VOLCANIC BRECCIA Tuff breccia, rubble breccia, and lapilli breccia form thick and extensive lenses at several horizons within the Bedrock Spring Formation, mostly in the central part of the mapped area. The best examples of these lithologies are found in sections B31, E2, and E4. The rocks generally occur as single thick layers that are more resistant to erosion than the overlying and under- lying beds and hence are well exposed. In outcrop, they are hackly and jagged, the fragments being more re- sistant to weathering than the matrix. Colors vary from brownish gray to gray with an occasional purplish tint. - Some layers are as thick as 100 feet, although most have thicknesses between 25 and 50 feet. Many can be traced several thousand feet along the strike and two can be traced nearly a mile. Most of these rocks consist of randomly oriented angular to slightly rounded fragments in a matrix of tuff or lapilli tuff. These fragments, which form more than 50 percent of the rock in most outcrops, consist of grayish, brownish, or purplish volcanic lavas or tuffs. The lava fragments are virtually all andesite and con- tain phenocrysts, several millimeters long, of plagio- clase, biotite, hornblende or oxyhornblende, and pyroxene. One bed of tuff breccia contains fragments of only one lava type, and a sample of these fragments was col- lected and analyzed (table 4). The results are used to represent the petrographic and chemical characteristics of lavas extruded during Bedrock Spring time. The lithology of this bed is not typical of the volcanic rocks in the Bedrock Spring Formation, but the fragments are almost certainly all the product of a contempora- neous volcanic eruption. This bed of tuff breccia consists of light-gray angular fragments of porphyritic andesite as long as 2 inches, in a tuffaceous matrix of the same color. Color band- ing in the fragments results from slight variations in the composition of the groundmass. The euhedral plagioclase phenocrysts form crystals as large as 4 mm ; microphenocrysts do not form a distinct generation although crystals smaller than 0.3 mm make up about 5 percent of the rock. Zones are oscillatory-normal, normal, or mottled; calcic rims are not present. Albite twinning is well developed in about half the erystals. Biotite is found in some samples as euhedral crystals with no reaction rim. Hornblende, which may be in part oxyhornblende, occurs as euhedral crystals with no reaction rim and very few opaque inclusions. Ortho- pyroxene, as clear euhedral prisms, commonly has in- clusions of magnetite. A few clinopyroxene crystals are present. The groundmass consists of about 68 percent glass, 30 percent cryptocrystalline material, and 2 per- STRATIGRAPHY 19 cent needlelike plagioclase(?) crystals. There is very little opaque material, although color banding is caused by minor variations in this small percentage. Vesicles, mostly microscopic but some as large as 1 mm, are abundant and show a faint alinement. Maximum crystal sizes are : plagioclase, 4 mm ; biotite, 3 mm ; horn- blende, 2.5 mm ; and orthopyroxene, 0.3 mm. Modal, normative, chemical, and spectrochemical analyses of a sample of fragments from this rock are listed in table 4. The modal composition shows that these fragments are andesite. The chemical composi- tion is close to Nockold's "average dacite" (1954b, p. 1015), and it shows such low percentages of CO; and excess H0 that significant alteration is improbable. The normative composition shows that on complete crystallization, K-feldspar would have formed about 16 percent of the total feldspar, and this rock would have become granodiorite with about 18 percent quartz and a plagioclase composition of Any;. TaBur 4.-Modal, normative, chemical, and spectrochemical analyses for one sample of the volcanic breccias in the Bedrock Spring Formation [Sample 128-32 from see. E2-e, outcrop on south side of wash. A dash means the element was not present in detectable amounts. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White, spectrochemical and modal analyses by G. I. Smith] Modes Norms [Volume percents] [Weight percents] Plagioclase megaphenocrysts... 15.1 (Q ech 18. 2 Plagioclase microphenocrysts..... 5.1 C.. 0 'Total plagioclase. (20.2) or.... 11.6 FETC SCC hees o on 5 ab.... 38. 2 Hornblende. ...... ws 6.3 an 19.2 Oxyhornblende.. . 0 di... 2.9 Orthopyroxene.... 1.2 hy.. 5.1 Clinopyroxene. ... 0 mt.. 2.1 Quarbs:..0........ 0 ils... 1.2 Opaque minerals.. 1.5 hm.. 0 Other minerals.... A fh... 0 70. 2 ta.... ease 0 5 BD.: Bino nue tsun .3 Chemical analyses Symbol (C.LP.W.)...___._._._.__ 14.3.4 Weigh percents) (1s Spectrochemical analyses 16.6 [Parts per million] 1.4 2/2 Co. 2.0 Mn 4.6 Ni. 4.5 Se.. 2.0 ys . 58 Fe. . 20 Cu. . 08 Ga. 1.2 Ti. 27 Seis 100 Ba. Be. Li.. Pb. Rb. Sr.. Yb. Y.. TM .- irene ne boe TUFF Tuff and lapilli tuff occur as thick lenses within the Bedrock Spring Formation, chiefly in the central and northeastern parts of the mapped area. Most of the rocks included in this unit are lapilli tuff. They con- tain ash-sized fragments of euhedral to subhedral feld- spar, quartz, hornblende or biotite, and lapilli-sized fragments of andesite. In the central part of the area, the best exposures of rocks included in this unit are in sections B31-a and E4-1. They weather light green or light blue green, and form cliffs which have uneven and hackly surfaces. They are most commonly massive. Some beds are as thick as 100 feet, but the average thickness is nearer to 20 or 30 feet. Many can be traced along strike for several thousand feet. In the northeastern part of the area, these rocks are best exposed in section B24-h, where they weather to smooth slopes colored light shades of pink, yellow, and green. Most of them are very well bedded, although individual beds cannot be traced for more than a few hundred feet. DISTRIBUTION OF ROCK TYPES AND ENVIRONMENT OF DEPOSITION The distribution and character of rocks of the Bed- rock Spring Formation show that the middle Pliocene sedimentary basin was a closed valley surrounded by alluvial fans that sloped from all directions toward a central lake or playa. The sandstone and conglomerate (see fig. 8) that represent these alluvial fans are strik- ingly similar to present-day alluvium of areas in the Mojave Desert that have a dominantly quartz-mon- zonitic provenance. The central lake is represented largely by siltstone and claystone but contains small amounts of gypsum, limestone, and evaporites. The soluble salts entrapped in the sediments, along with a small number of diagnostic diatoms, indicate that the lake much have been saline and without an outlet for much of the time. Snails found in these fine sediments, however, show that the central lake was at times peren- nially fresh. Most of the lenses of volcanic breccia, tuff, and lapilli tuff also crop out within this central area, but this is probably a result of the position of contem- poraneous vents rather than the shape of the sedimen- tary basin. The surrounding alluvial fans sloped from all direc- tions toward this central playa as shown by a survey of the rock types found as pebbles in the formation. In the north and northwest exposures of the Bedrock Spring Formation, the fragments are derived from the types of Atolia Quartz Monzonite exposed to the north. In the northeast exposures of the Bedrock Spring For- mation, an abundance of fragments derived from the nearby metamorphic rocks shows that these rocks crop- ped out and persumably formed the northeast edge of the sedimentary basin. In the east and southeast ex- posures of the Bedrock Spring Formation, the pebbles were derived chiefly from the quartz-monzonitic and rhyolitic rocks exposed 2 or 3 miles southeast of the mapped area (fig. 2) ; fragments of the hornblende am- phibolite exposed near the Blackwater fault are not 20 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY found here, however, suggesting that the alluvial sedi- ments extended southeast at least to the area of their present outcrop. To the south and southwest, toward Red Mountain, the full extent of the Bedrock Spring Formation has not been mapped in detail, but recon- naisance suggests that the conglomerates in the south- west extension of the formation on Red Mountain were derived from the plutonic rocks exposed a few miles southwest of that area. Along the west boundary of the map area, fragments in the Bedrock Spring Formation derived from the Rand Schist and Johannesburg Gneiss of Hulin (1925, p. 20-31) indicate a source within a few miles to the west; a few outcrops of the Bedrock Spring(?) Formation on the west end of the Summit Range (fig. 2), however, show that an arm of the middle Pliocene basin probably extended westward to that point. The approximate outlines of this inferred basin of sedimentation and of the area within it containing finer grained sediments are shown in figure 9. The indicated area of sedimentation is about 200 square miles; the area of playa or lake sedimentation is about 15 square miles. A gravity survey of the area west and south of 0 5 L 1 1 i 1 | this region (Mabey, 1960) shows that the bedrock de- pression containing these rocks is continuous with Cenozoic sedimentary basins south of Cuddeback Lake and northwest of the Rand Mountains. Although the gravity data permit the interpretation that middle Pliocene sediments extended into those basins, the ex- posed geology of those regions (Dibblee, 195%, p. 38; 1964) indicates that earlier Cenozoic rocks probably occupy them. Downwarping or faulting between the Garlock fault and Brown's Ranch fault zone is indicated as the cause of a depositional basin in this area. As shown by figure 9, the basin was generally parallel to the Garlock fault, but was slightly elongate to the southwest along a line approximated by the Brown's Ranch fault zone. Al- though this inferred relation requires both faults to have been active, the proximity of the playa sediments in the Bedrock Spring Formation to the trace of the Garlock fault suggests that during middle Pliocene time that fault was more frequently active than the faults included in the Brown's Ranch fault zone. The coexistence of evaporites indicative of a saline EXPLANATION Ts - o{ - Approximate edge of sedimentary basin in middle Pliocene time Boundary of area shown on geologic map, plate 1 Outer limit of finer grained facies within middle Pliocene basin 10 MILES 1 FisurE 9.-Diagrammatic map showing the infe‘rred limits of the basin of sedimentation in Bedrock Spring (middle Pliocene) time and the approximate limit of the fine-grained facies within the basin. STRATIGRAPHY 21 or dry lake, and snails indicative of a perennial fresh lake, seems contradictory; a probable explanation is that the basin center was occupied by a playa during most of middle Pliocene time, but that occasional wet cycles produced a small lake that could maintain a fresh-water snail population for a few years until evaporation raised the salinity to an unfavorable level; these wet cycles must have also created a connection with a perennial lake in which such snails lived. The middle Pliocene climate in this area may thus have been similar to that of today's climate in northern Nevada and southeastern Oregon. Many of the closed basins in those areas now contain wet playas or small lakes, and it is evident that small changes in the present climate would convert one type of lake into the other. The precipitation in those areas presently is two to three times that of the Mojave Desert. AGE AND RELATION TO OTHER FORMATIONS A large collection of fossils from the Bedrock Spring Formation was made in 1952 by R. H. Tedford and Robert Shultz, Jr. Some additional material was col- lected by T. W. Dibblee, Jr., G. N. White, and me. (G. E. Lewis writes of the vertebrate fossils as follows (written commun., 1956) : I have made comparisons with other collections in several museums. My best guess is that the age of most of the fossil vertebrates in your collection from the Lava Mountains area corresponds to that of the middle to upper parts of the Ogallala group of Nebraska. The Lava Mountains fossil vertebrates are comparable to those found in the Ash Hollow formation of the Nebraska Survey, and seem to be intermediate between those of the Chanac and Kern River formations of California. This age may be as young as middle Pliocene (assuming the age of the Villafranchian fauna of Europe, and that of the Equus (Plesippus) fauna, to be earliest Pleistocene in conformity with interpretations given at the 1948 International Geological Con- gress), but may be as old as late early Pliocene. Significant determinations are : Order PERISSODACTYLA Family Equidae Pliohippus cf. P. leardi, from localities LM-6, LM-13, LM-20, and LM-21. Pliohippus sp., from localities LM-10 and LM-15. Order ARTIODACTYLA Family Camelidae ?Pliauchenia sp., from localities LM-6, LM-8, LM-11, LM-13, LM-15, LM-16, LM-20, and LM-26. @Megatylopus sp., from localities LM-10, LM-16, and LM-21. Order LAGOMORPHA Family Leporidae cf. Hypolagus sp., from localities LM-15 and LM-16. Order RODENTIA Family Cricetidae cf. Neotomodon sp., from localities LM-15 and LM-16. A complete list of determinations is attached. [See table 20.] Although these fossil data show that the Bedrock Spring may be as old as late early Pliocene, for simplic- ity the formation is referred to throughout most of this report as being of middle Pliocene age. One fossil locality also contained abundant snails of which D. W. Taylor writes the following (written commun., 1960) : The fossils from locality LM-6 (U.S. Geological Survey Cenozoic locality 22324), represent two kinds of snails. One is a small low-spired or planispiral form not surely identi- fiable. It may be a land snail, or one of the fresh-water family Planorbidae. The other species is a relatively large fresh-water snail of the genus Bulimnea. Perhaps it is B. megasoma (Say), recorded from late Miocene deposits in the Barstow syncline to the south (Taylor, 1954), but the material is not well enough preserved for certain identification. The known range of Bulimnea@a is from the middle to late Miocene Mascall formation, Oregon, to Recent. West of the Rocky Mountains, however, none of the occurrences is younger than middle Pliocene. * * * The fossils may not belong to the living species Bulimnea megasoma (Say). Nevertheless the relatively large size of the Lava Mountains specimens, and their similarity in proportions of shell to the living Bulimnea, justify some general inferences about habitat. By analogy with the recent occurrences of B. megasoma, the fossils from the Lava Mountains indicate a perennial body of fresh water. This may have been a lake, marshy pond, or swamp, but probably had little current action. No detailed interpretation of local habitat or of climate is war- ranted, but from the occurrence of a perennial water body in what is now arid desert one may suppose the annual rainfall was formerly somewhat greater. Only one of 38 samples examined for diatoms by K. E. Lohman contained an identifiable assemblage. Of it, he writes (written commun., 1962) : The following meager assemblage was obtained from field sample No. LM-119 [same locality as LM-8, shown on plate 1] ; USGS diatom locality No. 5451 : Caloneis cf. C. schumanniana (Grunow) Cleve Caloneis sp. Navicula spp. Nitzschia spp. Rhopalodia gibberula (Ehrenberg) Muller Surirella sp. Of the two diatoms that could be identified specifically, Caloneis cf. C. schumanniana has a known geologic range of late middle Miocene to Recent, and the identification is a doubt- ful one as indicated. Rhopalodia gibberula was the only diatom found in a state of preservation adequate for definite specific identification. It has a known geologic range of middle Pliocene to Recent and is living today in very saline lakes. This assemblage was probably deposited in a shallow, very saline lake. The only age assignment that can be made with the meager data at hand is middle Pliocene or younger. The approximate stratigraphic positions of the fossil localities and the probable correlation between beds in three of the fossil-bearing sections are plotted in figure 10. No other Pliocene rocks in the northern Mojave Desert or in the southwestern Basin Ranges are known to con- 22 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY Type section THICKNESS (IN FEET) 5000 Section extending 5000 feet S. 50° E. from section E3-a THICKNESS (IN FEET) 4000 - 4000 - sq) LM-12 LM-13 LM-24 3000 7 3000 - 2000 - 2000 - Section extending 1100 feet N. 20° W. from section B-23-j LM-2 0 THICKNESS LM-15 (IN FEET) LM-16 500 - LM-17 666 LM-21 LM-18 ak LM-22 Lm-8 1000 - LM-23 o - LM-Z | LM-3 C. LM-20 LM-4 LM-5 0 LM-11 FIGURE 10.-Measured sections of the Bedrock Spring Formation showing the stratigraphic position of fossil localities shown on plate 1. Localities that can be traced directly into the measured sections are underlined and those approximately correlated are not. Localities LM-9 and LM-10 are east of the type section and probably are several hundred feet stratigraphically below its base. Correlated beds are connected by lines. See table 20 for fossil lists. STRATIGRAPHY 23 tain fossils of exactly the same age as those in the Lava Mountains. Within the Mojave Desert, three other Pliocene faunas are known-all only a few miles south of the Garlock fault. In the Avawatz Mountains (Henshaw, 1939), rocks containing lower Pliocene fos- sils are exposed. Rocks about 4 miles northwest of Mojave contain fossils of middle Pliocene age (identi- fications by Lewis, Stock, and Tedford, as quoted by Hewett, 1955). A limestone from Castle Butte, east of Rosamond, yields lower Pliocene diatoms (determined by K. E. Lohman, quoted by Hewett, 1954a, p. 16). All the other fossil collections from areas south of the Garlock fault, within the Mojave Desert, are of Miocene or Quaternary age. - North of the Garlock fault, in the southwestern Basin Ranges, dated lower Pliocene rocks are exposed in the Ricardo Formation of the El Paso Mountains (Dibblee, 1952, p. 25-30) and in Death Val- ley (Axelrod, 1940; Curry, 1941; Noble and Wright, 1954, p. 147-149), and very late Pliocene or Pleistocene rocks containing E'guus (Plesippus) are exposed in the Coso Mountains (Schultz, 1937). ALMOND MOUNTAIN YOLCANICS The Almond Mountain Volcanics is a new formation named here. It includes both intrusive and extrusive volcanic rocks that are exposed in the southern half of the mapped area. -The formation is named for Almond Mountain, located in the southern part of the map area. The type section is designated as the section exposed about a third of a mile northwest of the summit, but neither this nor any other one section is representative of the formation over a large area. The type section diagrammed in figure 11 is about 900 feet thick but in- cludes only the lower half of the formation exposed in this vicinity; (see pl. 2, section D-D") ; the rocks that form the upper half crop out east of Almond Mountain, but they are not as well exposed as those at the type section and cannot be stratigraphically tied to that see- tion with the desired accuracy. This formation includes rocks that were divided by Hulin (1925, p. 42-58) between the "Rosamond Forma- tion" and the "Red Mountain Andesite." He reported that tuffs and vocanic breccias were represented in both formations, but did not indicate either that they were markedly concentrated along the top of his Rosamond Formation and the base of his Red Mountain Andesite, or that they were mostly separated from the lavas and sandstones by an unconformity. By comparing his ge- ologic map with the one accompanying this report, though, it is evident that the rocks here assigned to the Almond Mountain Volcanics were divided between those two formations. Rocks of the Almond Mountain Volcanics are in- truded into, or rest with an angular unconformity on, the middle Pliocene Bedrock Spring Formation and THICKNESS (IN FEET) 900 -I 8. Rubble breccia; weathers to light gray (N7) and olive gray (5Y 4/1); lithology similar to unit below; massive; grades into the unit below 800 - a P, P 7 Eso %. b/s. ca 7. Rubble breccia; weathers to medium light gray (N6) and pale red purple (5RP 6/2); andesite fragments as much as 5 ft wide make up more than 98 percent of the unit; fragments are angular with large conchoidal cracks; massive 700 - 600 -I 6. (Lava Mountains Andesite sill; contorted color bands of dark gray (N3) and pale reddish brown (10R 5/4); upper half slightly brecciated; not part of Almond Mountain Volcanics type section) 500 - 5. Rubble breccia; fragments mostly one type of porpyritic andesite; andesite fragments make up about 98 percent of the total; medium gray (N5) in top two-thirds; light brownish gray (SYR 6/1) in lower third; I massive 4. Tuff, with a small percentage of andesite frag ments; very light gray (N8), yellowish gray (5Y 7/1), and dusky yellow (5Y 6-7/4-6); yellow more pronounced in top and bottom thirds; grades into unit above 3. Rubble breccia; fragments mostly one type of porphyritic andesite; weathers from medium light gray (N6) to reddish brown (10R 4/4); angular fragments as much as 200 - F 6 ft across; massive 2. - Lapilli tuff; weathers to light gray (N7), pale red (5R 6/1), and grayish red purple (SRP 4/2); about 30 percent subrounded to subangular fragments of lava, lapilli tuff, and pumice in a tuff matrix; fragments are as large as 18 in., but average 1 in. across; locally well bedded 100 -I 1. Tuff breccia; weathers to light olive gray (5Y 6/1) and brownish gray (5YR 5/1); about 30 percent angular fragments of andesite and lapilli tuff as much as 10 in. in diameter in a tuffaceous matrix; massive o - FIGURE 11.-'Iype section of the Almond Mountain Volcanics. The measured column extends 1,700 feet S. 70° E. from see. E34-c. The letters show the relative positions of the 124-22 series of samples described in table 5. Unit 6, part of the Lava Mountains Andesite, is not part of the type section. older rocks. The other upper Pliocene(?) volcanics shown on the geologic map are in most places demon- strably younger than the Almond Mountain Volcanics, but the exposures are inadequate to rule out some of them as contemporaneous or a little older. The Almond Mountain Volcanics are intruded or unconformably overlain by the Lava Mountains Andesite. 24 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY (On the geologic map (pl. 1), rocks of the Almond Mountain Volcanics have been divided into nine lith- ologic map units, of which four make up the eastern facies of the formation and five the western facies. The rocks of the two facies are areally separated by the valley containing the Brown's Ranch fault zone except at the northeast end of the valley where they are inter- bedded. Though the rocks constituting the eastern and western facies are similar, they can be distinguished in the field by their lithology or lateral distribution. The distinction made in the field between facies is re- tained on the geologic map (pl. 1) and in this descrip- tion because it clarifies the depositional pattern, namely, the simultaneous eruption of volcanic material from an eastern and a western center to form a continuous layer of debris over 75 to 100 square miles. Both eruptive centers apparently consisted of several irregularly shaped vents that brought lavas to the surface to form domes immediately above the vents, and a much larger area of fragmental volcanic debris around them. The rocks closest to the western eruptive center were subse- quently altered. Some interbedded lenses of arkosic sandstone are in- cluded in the Almond Mountain Volcanics. Though they might well be considered part of another formation because of their distinctly different lithologies and origin, they are included in this formation because they constitute too small a unit to be separated, and their stratigraphic position relative to the Bedrock Spring Formation is too different to allow their inclusion in it. The eastern facies of the Almond Mountain Volcanics includes the type section and consists of a variety of rock types that have been divided on the geologic map among four lithologic map units. These units have varying stratigraphic relations within the formation; they are therefore considered as a group to be con- temporaneous. One of these lithologic map units con- sists of unstratified volcanic rocks that are mostly intrusive, although the field relations show that they also grade laterally into the stratified members. The other lithologic map units are stratified. These layered rocks have been combined into three categories: (a) "volcanic breccia," which includes all flow breccias, rub- ble breccias, lapilli breccias, and tuff breccias; (b) "tuff," which includes all tuffs and lapilli tuffs, and (c) "sandstone," which includes all sandstones and a few conglomerates. - The western facies consists of five lithologic map units.~ Two of them constitute a hydrothermally altered volcanic complex; the other three are composed of stratified rocks. The rocks characterized by the more intense hydrothermal alteration are propylites* In this report, the rocks that are less intensely altered are called "subpropylite." The subpropylite appears to overlie the propylite, but this reflects a boundary in the intensity of hydrothermal alteration rather than a strat- igraphic or intrusive succession. Reconnaissance map- ping suggests that this alteration zone extends another 2 or 3 miles southwest of the mapped area. The strati- fied rocks plunge into or overlie the altered volcanic complex in many places, but probably this too reflects the outer and upper limits of alteration rather than an order of deposition. Most of the rocks of the hydro- thermally altered volcanic complex are intrusive necks or dikes; the bedded volcanic rocks are apparently ex- trusive equivalents. The thickness of each of these lithologic map units varies markedly from place to place. The number of separable units in any given stratigraphic section also varies. Both their thickness and number depend chiefly on the distance between the stratigraphic section being considered and the center of the contemporaneous vol- canic activity. The stratigraphic order of units is also variable, and even the lithology of the basal unit varies to include all the layered rock types represented in the formation. - Because of this variability, individual stratigraphic sections have only local significance, and the sequence of lithologic map units in one area cannot generally be duplicated in another. DESCRIPTIONS OF LITHOLOGIC MAP UNITS ROCKS OF THE EASTERN FACIES Volcanic intrusives The lithologic map unit designated "volcanic in- trusives" includes the porphyritic andesites of the east- ern facies that are not bedded. Most of the outcrops of this unit are inferred to be shallow volcanic intrusives on the basis of crosscutting relations or steeply dipping flow structures, although some are probably volcanic domes or very thick flows. The largest outcrops of this unit are on the west and south sides of Almond Mountain, and on the large hill 11/, miles northeast of it. The rocks form steep-sided hills whose lower slopes are almost entirely sheathed by talus. On weathered surfaces, the color varies from the dark brown of desert varnish to that of the fresh rock, which is light brownish gray. The petrography of the massive volcanic intrusives is similar to that of the fragments in the stratified breccias. Most of the rocks contain plagioclase, biotite, oxyhornblende, clinopyroxene, and orthopyroxene; a few contain quartz. The biotite and oxyhornblende crystals are heavily altered, but the plagioclase and ' Propylite is a term that was first used by von Richthofen (1868) for rocks in the Comstock Lode that are now known to be hydrother- mally altered andesites. The thoroughly altered rocks at the original locality are characteristically greenish and contain secondary epidote, chlorite, albite, calcite, sericite, and quartz (Coats, 1940, p. 11-12). STRATIGRAPHY 20 pyroxene crystals are almost fresh. The groundmass consists of 5 to 95 percent glass or eryptocrystalline material, the balance being skeletal crystals and opaque minerals. The maximum size of the plagioclase, biotite, and oxyhornblende crystals is about 3 mm; the orthopyroxene and clinopyroxene crystals are mostly less than 1 mm. Volcanic breccia Volcanic breccia forms the bulk of the Almond Mountain Volcanics in the area around and south of Almond Mountain. The best exposures are found at the type section and in sections F19 and E25. In outcrop, these rocks form resistant beds and ledges, the flow and rubble breccias being somewhat more resistant than those containing a large percentage of tuff or lapilli tuff. The colors of the breccia fragments are distinc- tive, and provide one means of distinguishing rocks of this formation from those of the overlying Lava Moun- tains Andesite; these distinctive colors vary from me- dium light gray (N 5-7), through light olive gray (5¥ 6/1) and brownish gray (5Y2 4-8/1), to pale red (102 6-7/2) and pale red purple (GRP 6/2). Most of the mapped layers are 100 feet or more thick; one may exceed 1,000 feet in thickness. Within these layers, beds defined by textural changes are rare, but strati- graphic successions of distinctively colored breccias are common. Along the strike, rocks assigned to this map unit may be continuous for a mile or more, but the tex- tural detail of any given volcanic breccia normally changes within a distance of a few hundred feet. Most of the rocks of this unit are rubble breccias, and fragments form more than 90 percent of the total volume. Flow breccias, lapilli breccias, and tuff brec- cias are subordinates. The fragments are without ex- ception volcanic rocks. All are blocky and angular; neither bombs nor fragments having bread-crust struc- tures were observed. The matrix in the rubble breccias generally is lapilli tuff. The matrix of the flow breccias is lava, which in some places is identical with the frag- ments and in others is different in color though of about the same mineralogy. Results of thin-section study emphasize the similarity between the fragments contained in widely separated outcrops of these rocks. All contain plagioclase pheno- crysts, and three-quarters of the rocks also contain biotite, oxyhornblende, and orthopyroxene. Only about one-third contain quartz. The plagioclase is found as megaphenocrysts and microphenocrysts in about equal proportions; zone and twin development is variable, and a calcic rim is present on the megaphenocrysts in about half the rocks. Many of the biotite and oxyhorn- blende crystals show partial or complete deuteric alteration to opaque materials and pyroxene; plagio- clase grains in the crystals may be either alteration products or inclusions. The orthopyroxenes are vir- tually unaltered. Clinopyroxene is found in about two- thirds of the rocks and shows partial alteration in well over half. f The results of modal, normative, chemical, and spectrochemical analyses of fragments from four units exposed at the type section are presented in table 5. These four (samples 124-22 F, I, M, and Q) represent a stratigraphic thickness of about 600 feet. Although their modal percentage of plagioclase ranges from 15 to 27 percent, and their groundmass percentage ranges from 67 to 78 percent, their chemical and normative compositions show very little difference. The S10; per- centage ranges from 64.8 to 66.9, and most other constit- uents are similarly constant. Their normative com- positions are also similar, and the normative feldspar compositions are all between An,, and Ang,. Most of these rocks are slightly vesicular and have a pronounced lineation resulting from the alinement of the plagioclase microphenocrysts and the oxyhornblende crystals. The plagioclase megaphenocrysts are very commonly clustered together. The euhedral shape of some of these crystals shows that clustering occurred after they had grown to nearly normal shape and size, but more commonly it occurred at an earlier time so that there was a mutual interference of growth. Micro- phenocrysts of plagioclase rarely appear as attached pairs. In most rocks, the plagioclase, oxyhornblende, and biotite crystals range in size from about 14 to 3 mm. The orthopyroxene and clinopyroxene crystals are gen- erally less than 1 mm long. Tuff The rocks constituting the tuff map unit of the eastern facies are mostly lapilli tuff ; tuff is present but definitely subordinate. The best exposures of these rocks are found in the type section and in sections E24, F6, and F19. The rocks are well exposed and form cliffs where associated with soft sandstone or conglomerate, and are poorly exposed in areas where associated with rubble breccia, flow breccia, or flows of the overlying Lava Mountains Andesite. Most of the rocks weather to sub- dued shades of yellow, orange, red, or purple. Typi- cally, they form mappable units that are several tens of feet thick, the units composed of lapilli tuff being thicker than those composed of tuff. Rocks containing an appreciable amount of sedimentary epiclastic mate- rial or pumice are locally well bedded and even graded ; rocks composed entirely of other volcanic materials are generally massive. Along strike, individual beds are lenticular and rarely can be traced more than a mile. The tuff and lapilli tuff included in this lithologic map unit consist chiefly of fine-grained glassy or eryptocrys- talline material, although individual grains of euhedral biotite, feldspar, and hornblende crystals and small 26 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY TaBus 5.-Modal, normative, chemical, and spectrochemical analyses of nine samples of the stratified lithologic members of the Almond Mountain Volcanics [ A dash means that the element was not present in amounts greater than the limit of detection. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White; spectrochemical analyses for Li and Rb by Robert Mays, remaining spectrochemical and modal analyses by G. I. Smith] Western facies Eastern facies 97-381 97-38K 97-381 97-38 F 128-33 194,-28F - 194-981 - 19;-92M - 194-22Q Modes [Volume percents] Plagioclase megaphenocrysts.............__...__.._.. % 9.0 9.6 8.9 10.7 5.9 22.9 3.3 6.7 11.2 Plagioclase microphenocrysts........_....._..___._... 9.3 13.9 13.1 1.7 7.4 4.5 12.0 11.6 13.0 (Total 0.000 (18.3) (23.5) (22.0) (2.4) (13.3) (27.4) (15.3) (18.3) (24.2) Biotite.................- 1:2 1.0 4 7 0 2.2 . 4 by Hornblende. . 0 0 0 0 0 0 0 0 0 Oxyhornblende .6 .8 y .6 Trace 0 JA 4 1.0 Orthopyroxene.............- .9 .9 1.3 14 Trace 1.7 2.4 2.4 1.0 Clinopyroxene. 18 .6 .6 .4 . 0 .6 .3 $7 Quartz......... 4 §" 11 .6 {1 0 t 0 Trace OpaQUC MINCrAIS. ... 4.1 8.1 3.3 3.6 10.2 1.2 1.6 1.8 3.1 Other minerals. 111-1 ~-. 0-92 ents 1.0 4.7 i .4 .2 A .8 A .5 20. - . co or o -L hue sonst soled axial s ae aln olo alle un 73.0 60.2 70.8 70.2 75.9 67.4 78.8 76.9 68.8 Chemical analyses [Weight percents] 62.2 61.0 63.6 62.5 67.7 65. 4 66.8 66. 9 64.8 16.5 17.0 16.9 16.6 16.5 16.5 16.2 16.2 16. 2 2.4 2.3 1.5 1.8 $: 1.2 1.1 1.0 1.9 1.6 1.9 2.6 2. 4 14 2.0 1.9 2.0 1.4 3.1 2.8 2.8 2% . 57 1.2 1.2 1.8 1.2 4.6 5.0 4.4 5.0 4.0 3.9 3.8 3.5 5.0 3.8 3.6 3.9 3.8 4.3 4.2 3.9 3.8 3.6 2:2 2:2 2.8 2.7 2.8 2.6 2.7 2.9 2.8 . 65 . 69 . 68 . 68 . 54 . 55 . 50 . 50 . 50 . 24 .26 .25 . 24 19 .18 16 16 AT . 06 .06 . 06 . 06 . 02 05 .05 . 04 .05 3.1 3.2 1.1 1.8 . 38 2A 2.0 1.9 $2 .05 . 05 . 05 . 05 <.05 .05 .05 . 05 11 is L0 19.000, 1.1. bbl s dee U bed eect a 100 100 101 100 100 100 100 101 100 Norms [Weight percents] 4 17.4 15.7 18.9 28. 2 20.1 23.0 22.6 21:7 to 0 0 0 0 0 4 1 .8 12.8 16.7 16.1 16.7 15.6 16.1 15:2 16.7 .0 30. 4 33.0 32.0 36.2 35.6 33.0 32.0 30. 4 27, 23.1 20.3 20. 3 17.5 18. 4 18.6 17.5 19.7 .9 0 1.3 3.6 17 .9 .2 0 2.6 4 7.4 8.7 5.7 .6 4.3 4.6 6.6 2.7 ¥ 8:2 2.1 2.6 2.1 1.9 1.6 1.4 2.8 $s 1.4 1.4 1.4 1.1 1.1 1.4 .9 .9 .g 0 0 0 1.9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 v8 f [9 .3 .3 .3 .3 48 .3 [A HAB. A- ILA49.4.. | I: 1.4.3.4 1.4.3. 4 1.4.3.4 1.4.3.4 1.4.3.4 Spectrochemical analyses [Parts per million] 100 80 60 50 400 380 60 50 5 5 80 70 26, 000 25, 000 30 20 10 10 2, 700 2, 000 40 30 1, 800 1, 400 2 24 10 10 40 60 1, 700 1, 300 1 1 5 5? 120 80 LOCATION OF SAMPLES 97-38L. See. E 6-, from section trending S. 35 W. on face of cliff. Sample collected about 5 feet below top of section. 97-38K. Same section, collected about 20 feet below top of section. 97-381. Same section, collected about 50 feet below top of section. 97-38F. Same section, collected about 100 feet below top of section. 128-33. Sec. E 10-d, sample from top of hill. 124-220. Type section of Almond Mountain Volcanics, 14 mile northwest of Almond Mountain; see fig. 11 for location of sample. 124-22 M, 122-221, 124;-22F. Same section, see fig. 11. STRATIGRAPHY rock fragments are locally common enough to be con- spicuous. These components are generally angular and unsorted. In a few beds, up to a quarter of the mate- rial is sand-sized epiclastic material which normally shows some sorting and bedding. Most beds also con- tain a small percentage of angular to subrounded frag- ments, 1 to 3 inches in diameter, of purple, tan, or gray porphyritic andesite, or light-colored well-indurated lapilli tuff all embedded in the softer matrix of tuff or lapilli tuff. A distinctive lapilli tuff bed is found northeast of Almond Mountain, especially in sections E23-h, E24-f, E25-k, and E35-f. One-third to two-thirds of it con- sists of light-gray or white pumice as well-rounded fragments up to an inch in diameter; the matrix is pale yellow-orange tuff or tuffaceous sandstone. Pumice is present but rare in other parts of the eastern facies of this formation; it is virtually absent in the western facies. As is evident from the position of this distinc- tive unit in the geologic sections (pl. 2), the bed lies a short distance above the top of the type section. More detailed mapping would fix the position of the bed relative to the type section, and it could probably then be used to relate the rest of the layered volcanic rocks east of Almond Mountain. Sandstone Sandstone in the eastern facies of the Almond Moun- tain Volcanics is well exposed in only one area, about 21/, miles northeast of Almond Mountain (in see. E24- g). Here, it crops out in a canyon as a 50-foot bed of medium- to very coarse grained faintly bedded arkosic sandstone containing very few pebbles. It is well ce- mented by calcium carbonate. It weathers to yellowish gray (5Y 8/1), a color that is distinctly different from the pinkish-tan hues characteristic of the sandstone in the Bedrock Spring Formation. At this outcrop, the bed is overlain and underlain by lapilli tuff. East and south of here, it becomes relatively flat lying and its presence can only be inferred from the arkosic float. ROCKS OF THE WESTERN FACIES Propylite The propylites of the Almond Mountain Volcanics were formed by the hydrothermal alteration of three different volcanic rock types : (a) brecciated porphyritic andesite, apparently intrusive in most places, which forms over 90 percent of the outcrop area, (b) thinly bedded lapilli tuff, and (c) porphyritic andesite dikes which are less brecciated than the first variety. The dikes cut both the lapilli tuff and brecciated porphyritic andesite. These rocks form steep-sided and hummocky hills, although the dike rocks are generally more resist- ant to erosion than the others. 735-720 0O-64--3 27 The common characteristic of these rocks is the type of hydrothermal alteration, and the contacts shown on the map convey information about the upward or out- ward extent of this alteration, not about the original stratigraphic or intrusive succession of the altered volcanic rocks. Although the term "propylite" was originally applied to altered volcanic rocks with slightly different secondary mineral content (see Coats, 1940, p. 11-15), its application to these rocks is thought to be within the intended scope of the term. The propylites of this area are megascopically char- acterized by distinctive colors, and though the contacts shown on the map are based primarily on these colors, they seem to represent very closely the actual limits of propylitization. These colors include olive gray (5¥ 5/1), pale olive (10¥ 6/2), and greenish gray (5GY 6-8/1). Unweathered samples collected from the dumps around exploratory shafts are very light blue green, green, or gray. Small patches of unaltered rock show that prior to propylitization, the andesite consisted of plagioclase, green hornblende, and a very few biotite phenocrysts, all in a fine-grained or glassy groundmass. Plagioclase was present as both megaphenocrysts and micropheno- erysts, and had an average composition of about An,. In the megaphenocrysts, twinning was only moderately developed ; oscillatory and mottled zoning were present, but calceic rims were rare. Plagioclase crystals were as long as 5 mm, but averaged 1 or 2 mm ; biotite and horn- blende crystals averaged 1 to 3 mm in length. Hydro- thermal alteration has resulted in perhaps a third of the plagioclase being replaced with calcite (or some other carbonate), sericite, and a small amount of albite. In most specimens, the hornblende and biotite have been entirely altered to opaque minerals, chlorite, calcite, chalcedony, epidote, and serpentine (?). The ground- mass is now a nonvesicular fine-grained mottled mate- rial with undulatory extinction ; it appears to be mostly plagioclase, clay, and secondary quartz. Unweathered samples from the dumps of small mines also contain a small percentage of chlorite and pyrite. These two minerals are generally lacking in weathered rocks, ap- parently because surface weathering oxidizes the pyrite to iron oxides and sulfuric acid, and the acid attacks the chlorite to form an unidentified yellowish-brown mineral which gives the rocks of this unit their distinc- tive color. The groundmass of some specimens of this rock includes other silica minerals or zeolites, some of which are laumontite or its alteration product leon- hardite. The modal, normative, chemical, and spectrochemical compositions of one sample of propylite from one of the porphyritic andesite dikes are listed in table 6. These 28 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY rocks are generally less altered than other types from this unit, and therefore allow a more accurate estimate of the original modal composition of the rock. Prior to alteration, this rock probably contained about 16 per- cent plagioclase (approximately the present percentage of plagioclase plus calcite and other nonopaque min- erals) and about 6 percent oxyhornblende, biotite, and opaque minerals (approximately the present percentage of chlorite and opaque minerals). - The plagioclase per- centage is a little lower than in most volcanic rocks from this area (see table 11) ; the percentage of dark minerals is close to normal. The chemical analysis confirms the small degree of alteration by indicating only 1.8 percent water and 0.64 percent CO,. _ Comparison of this chem- ical analysis with those of other volcanic rocks in the Lava Mountains (fig. 26) shows that even though slightly altered, the present chemical composition of the rock resembles that of the others; the only signifi- cant differences are a notably higher percentage of CO,, and slightly higher percentages of CaO, H0, and possibly Na,0. Tasur 6.-Modal, normative, chemical, and spectrochemical analyses for one sample of propylite from the western facies of the Almond Mountain Volcanics [Sample 29-22 from outcrop along south side of canyon in see. D24-f. A dash means the element was not present in detectable amounts. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White; spectrochemical analyses for Li and Rb by Robert Mays, remaining spectrochemical and modal analyses by G. I. Smith] Modes [Volume percents] Chemical analyses [Weight percents] Plagioclase megaphenocrysts.. . 4.2 65.8 Plagioclase microphenocrysts. .. 7.3 16. 2 'Total plagioclase........... (11. 5) 1.4 cr 0 1.8 Hornblende......... 0 1.2 Oxyhornblende.... 0 3.9 Orthopyroxené....... 0 4.6 Clinopyroxene........ 0 2.3 Quartz phenocrysts. .. .4 . 51 Opaque minerals.... 3.2 .16 14.2 . 05 as 2.3 1.8 Other mineralg................. .2 . 64 Groundmass (mostly quartz)... _ 78.1 100 Norms Spectrochemical analyses [Weight percents] [Parts per million] 20. 5 50 0 80 13.3 360 38. 8 10 17.0 3 2.0 40 3.0 18, 000 2.1 10 .9 10 0 2, 600 0 10 sew 0 1, 900 AD recess cb ewer eevee bee MB D rece del es cbc cess i ie bee Symbol (CLP.W.)......__._.l 1.4.3.4 90 110 ' Calcite percentage is higher than in most samples of dike rocks from this unit. Modal average probably between 1 and 2 percent. Subpropylite The subpropylites of the Almond Mountain Vol- canics were formed by hydrothermal alteration of the same varieties of volcanic rocks as propylite, namely, brecciated intrusive porphyritic andesite, thin-bedded lapilli tuff, and crosscutting andesite dikes. The out- crop terrain is also similar; only the color and second- ary minerals are distinctive. The colors vary from medium gray (N 5), through purplish blue (10P2 6-7/2) and very pale purple (5P 7-8/1-2), to pale reddish purple (5RP 4-6/2). The secondary minerals are less abundant than in the propylite, and chlorite is not found. Inasmuch as the type of hydrothermal alteration in these rocks is obviously related and very similar to that in the propylite, but is less intense, the term "subpropylite" is adopted. These rocks are characterized by white plagioclase phenocrysts and well-developed hexagonal flakes of biotite. The plagioclase is euhedral, sometimes forming erystals up to 8 mm long but more commonly 1 to 3 mm long. Most of the plagioclase is andesine. Zones range from weak to strong and the amount of twinning is variable. Many of the plagioclase crystals have been altered to calcite and sericite. Some of the biotite erys- tals, as large as 3 mm across, are unaltered, although most have been changed to iron oxide. Hornblende or oxyhornblende, which is much less abundant than biotite, is almost invariably altered to sericite, calcite, serpentine( ?), epidote, or opaque materials. Clinopy- roxenes and quartz occur locally but are generally not altered. The groundmass consists of impure glass, cryptocrystalline material, quartz, K-feldspar, and montmorillonite. In one outcrop, in section D25-m, stilbite(?) forms several percent of the rock. Like the propylites, these rocks generally contain no vesicles. Chemical, normative, and modal analyses of one sample are presented in table 7. The modal analysis of this sample lists no original dark minerals, and the 5.7 percent of opaque minerals represents their altered resi- due. The plagioclase content of the original rock was a few percent higher than in the altered rock, but a large part of the 11.4 percent of calcite reported in this modal analysis is in the form of vesicle or vein fillings rather than a plagioclase replacement. The chemical analysis indicates 5.1 percent HO and 1 percent CO,; calcite furnishes the CO;, and montmorillonite probably fur- nishes much of the HO. Otherwise, this analysis resembles those of the other volcanic rocks of this area (fig. 26) except that the subpropylite is depleted in CaO and Na,0, and possibly in total Fe as In gross aspect, the relations between the propylite and subpropylite are simple, but in detail they are com- plex. On most hillsides where both types are exposed, the subpropylite is above the propylite; in schematic cross section, the subpropylite would be projected to form an arch over the propylite. - In detail, however, it is not uncommon to find irregularly shaped fragments STRATIGRAPHY 29 TaBus® 7.-Modal, chemical, and normative analyses of one sample of the subpropylzte of the Almond Mountain Volcanics [Sample 20-36 from dump of city well in sec. D22-r. Chemical analyses by P. L. D. Elmore, S. D. Botts, I. H. Barlow, and Gillison Chloe; spectrochemical analyses for Li and Rb by Robert Mays; modal analyses by G. L. Smith] Modes [Volume percents] Chemical analysis [Weight percents] Norms [Weight percents} Plagioclase megaphenocrysts - 6.8 Plagioclase micro- phenocrysts. .._. 14.5 Total plagioclase . . (21 3) Blofite..:_./l..... ...... 0 Oxyhornblende.... 0 Orthopyroxene.... 0 Clinopyroxene.... 0 1: 5. lemans nouao ongca & co , com, Quarts..___:_.... .8 4 Opaque minerals.. 7 Symbol Other minerals.. .6 ...... T42A4 b as Groundmass. . 60. 2 ___100 ! Analysis of similar material gives H;O+=2.5 percent and H20‘=2 4 percent; analysis by Sarah Neil. w 5 MSs . or "dikes" of one material in the other, but the contacts between the two rocks are rarely sharp. The color change is gradational over a zone of several millimeters or a few centimeters. The original mineralogy of both types was similar. 'In most respects, the products of hydrothermal alter- ation are also similar-calcite, albite, or sericite replac- ing the plagioclase; and calcite, sericite, serpentine( ?), zeolite, epidote, pyrite, or opaque minerals replacing the groundmass, biotite, and hornblende. The present differences between the two types lie chiefly in the al- teration intensity and in the formation of chlorite in the propylite. These reflect differences in the degree of alteration but not in its types. Presumably, the altera- tion resulted from the postdepositional introduction of hot waters containing CO;, HS, and perhaps other components. The more intense alteration, in the lower and central parts of the volcanic complex, produced chlorite in addition to the other alteration products. But the presence of these other products in the overly- ing rocks shows that the hydrothermal solutions also reached the upper and outer parts of the complex. A drop in the temperature of the solutions may have been chiefly responsible for the disappearance of the chlorite phase. Volcanic breccia The volcanic breccia of the western facies is mostly rubble breccia but includes some flow breccia, lapilli breccia, and tuff breccia. It resembles the volcanic breccia of the eastern facies, but is distinguished from it by slight differences in color, its virtual lack of pumice fragments, and by its areal distribution. Good ex- posures of rocks included in this unit of the western facies are in sections D11, D23, and E6. The volcanic breccia fragments are mostly either pale red (102 6/2) or light olive gray (5¥ 5/1) ; the matrix material is gen- erally a lighter shade of the same hue. The units weather to form jagged surfaces on which the frag- ments stand out in relief from the less resistant matrix. Some of the volcanic breccia beds shown on the map (pl. 1) are massive, others consist of two to five distinct beds. Although the lithologies of these units vary greatly along strike, most can be traced several thousand feet or a few miles. Most of the volcanic breccias of this map unit are ex- trusive; they rest conformably on each other or under- lying units of the same formation, and show no cross- cutting relations. Along the northwest side of the Brown's Ranch fault zone, though, intrusive breccias included in this unit are also exposed. The lithologies of both intrusive and extrusive breccias in this area are similar; some are tuff breccias, others are flow breccias. A grayish-purple color is characteristic of both types. The composition of a sample of extrusive flow breccia differs in detail from other breccias of this facies (see table 5, sample 128-33), but the petrographic properties of these rocks are similar, and they all occupy the same position in the stratigraphic section. The fragments in the volcanic breccias of this facies are mostly angular, although some units contain some rounded material. The average fragment size ranges from 1 to 6 inches; the maximum is most commonly between 1 and 2 feet, but in some rocks it is as much as 6 feet. There is a crude direct correlation between the percentage of large fragments in the rock and the size of these fragments. The small percentage of matrix in the rubble breccia is most commonly lapilli tuff. In the flow breccias, the matrix is lava of about the same composition as the fragments. The fragments are gray or purplish porphyritic andesite in which minerals make up one-fourth to one- third of the rock and groundmass the rest. Plagioclase crystals are divisible into megaphenocrysts and micro- phenocrysts in about one-third of the samples; zones in most large crystals are medium to weak, and a calcic rim is found on about one-quarter ; twin development is not pronounced. Biotite, present in about 90 percent of the rocks, shows nearly complete alteration in about half. Hornblende or oxyhornblende, present in all the rocks studied, is almost entirely altered in over half of them. Orthopyroxene, present in about two-thirds of the rocks, is nearly fresh; and clinopyroxene, present in three- quarters of the rocks, is most generally unaltered. Quartz is found in about 80 percent of the samples as rounded and embayed crystals, some showing alteration halos of fine clinopyroxene. The groundmass composi- tion is variable, but on an average consists of one-half cryptocrystalline material, one-fourth small acicular crystals or crystallites, and one-fourth glass ; it also con- 30 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY tains a light to medium dust of reddish or black opaque materials. Many of the individual volcanic fragments have flow lineation defined by the acicular hornblende or oxyhornblende crystals, and by the microphenocrysts of plagioclase; some have elongated vesicles. The plagio- clase crystals have an average length of 14 to 1 mm and are as much as 4 to 8 mm long; hornblende, oxy- hornblende, and biotite have maximum dimensions of about 1 to 2 mm; orthopyroxene and clinopyroxene rarely are larger than 0.5 mm. Quartz is usually 1 or 2 mm in the maximum dimension. Chemical, spectrochemical, normative, and modal compositions of five samples are given in table 5. Four samples (97-38 L, K, I, and F) were fragments that came from four distinct breccia beds within a single mapping unit in the upper part of the formation ex- posed three-quarters of a mile northeast of Dome Moun- tain; one sample (128-33) was a fragment from a re- sistant layer that crops out on the northwest side of the Brown's Ranch fault zone. The modal compositions of the four samples from the same section are similar (except that one sample has an uncommonly high percentage of opaque minerals be- cause the fragments in this bed were very heavily al- tered). They also have nearly identical percentages of oxides, but all are lower in silica and are more mafic than most of the volcanic rocks in the Lava Mountains area. The norms confirm this difference, and classify these four rocks by the C.IL.P.W. system (Washington, 1917) as tonalose. Their normative plagioclase ranges from An, to An,. The remaining sample (128-33) has distinctly lower percentages of plagioclase and dark minerals, and a higher percentage of groundmass. It is also higher in silica and is more felsic than most vol- canic rocks of the area. Its norm confirms this tend- ency, although it classifies the rock in the same group as most of the other volcanic rocks of the area (yellow- stonose). The normative plagioclase is Anys:. Tuff The tuff of the western facies includes tuff, lapilli tuff, and a few beds of tuff breccia too thin to separate on the map. The best exposures are found on the north sides of Klinker and Dome Mountains, especially in sections E5-p, D11-f, D11-m, and D12-d. The rocks exposed there resemble the tuffs included in the eastern facies, but tend to be a little better sorted, more persist- ent along the strike, perhaps more intensely colored, and are virtually free of pumice. Some of the tuffs in the western facies form cliffs whereas others form soft slopes. The weathered surfaces are usually uneven and some are cavernous. The rocks weather to light or bright shades of gray, yellowish gray, green, pinkish gray, and red. In general, the finer tuffs are lighter colored than the coarser ones. The layers range from 5 to 50 feet in thickness, and thicknesses of 10 to 20 feet are commonest. Within these layers, the rocks are generally massive, but there is some local stratification. Along the strike, the individual layers can be traced 2,000 or 3,000 feet and a few can be traced a mile. Most of these rocks are lapilli tuff. The individual lapilli are composed of lava, indurated tuff, or rarely pumice. Ash-sized euhedral crystals of biotite are com- mon; euhedral crystals of hornblende and plagioclase are less common. Many beds also contain a small per- centage of epiclastic sand. A small percentage of larger angular to subrounded fragments of purple or gray porphyritic andesite or lapilli tuff is commonly found in these rocks. Most are a few inches across although some measure several feet. In the vicinity of the Brown's Ranch fault zone, the rocks included in this lithologic map unit are poorly exposed. They weather as if very soft, but the crush- ing that occurred during movement on the fault may be largely responsible for this lack of resistance. Most of them are probably tuff, but local patches of lapilli tuff, tuff breccia, and lapilli breccia are included. The colors are conspicuously variegated, but tend toward shades of purple. Some hydrothermal alteration and bleaching is suggested in the vicinity of the large sheared zone (in sees. E17T and E20); this may be re- sponsible in part for the variegated color. Sandstone Conglomerate, sandstone, and some tuffaceous sand- stone, siltstone, and claystone assigned to this formation are exposed at several places in the vicinity of Dome and Klinker Mountains. The best exposures are located in sections D11-f, D12-k, and E6-j. In general, these rocks are not resistant to weathering and form soft slopes that are largely concealed by debris; beds that contain an appreciable percentage of tuff are somewhat more resistant. Most of the rocks weather to yellowish gray ; the conglomeratic facies tend to be more brown, and the finer grained rocks more red or green. In many areas, layers of these rocks exceed 100 feet in thickness and in one area apparently exceed 1,000 feet. Within these thick sections, bedding is faint, and in many out- crops is marked only by stringers of pebbles or by beds of fine-grained material. The sandstone is in most places a poorly sorted arkose with fragments that usually range in size from very coarse to medium sand, but in some areas are of fine-sand size. It also includes beds of siltstone and claystone, al- though they are not common. Some of the detritus ap- pears to have been derived from the pink leucocratic STRATIGRAPHY 31 Atolia Quartz Monzonite, for many fragments are freshly broken pink feldspar or clear quartz. Some parts of this member contain an appreciable percentage of pyroclastic material. The pebbles in the sandstone and conglomerate are subrounded to subangular and rarely exceed 3 inches in diameter. Most of the volcanic pebbles are purplish or gray andesite, apparently de- rived from other members of the Almond Mountain Volcanics. A few banded rhyolitic rocks were found that probably were reworked from the Bedrock Spring Formation. Nonvolcanic pebbles include fragments of quartz, schist, and Atolia Quartz Monzonite. Rocks of this map unit are distinguishable from the sandstone and conglomerate of the Bedrock Spring Formation by the higher percentage of volcanic rocks among the pebbles and cobbles. In these sandstones, the prevailing ratio of volcanic fragments to others is about 5 or 10 to 1; in those of the Bedrock Spring Formation, the reverse ratio is more general. The sandstone in the Almond Mountain Volcanics also tends to be yellower than that in the Bedrock Spring Forma- tion, but this characteristic is not consistent enough to serve as a basis for distinguishing the rocks of the two formations. VOLCANIC SOURCE AND DISTRIBUTION OF ROCK TYPES The distribution of the bedded volcanic rock facies of the Almond Mountain Volcanics indicates that vol- canic rocks erupted chiefly from two centers. Today, volcanic necks and dissected domes mark the positions of these centers; coarse volcanic breccias surround them and finer fragmental volcanic rocks predominate some distance away. The petrographic and chemical sim- ilarity between the intrusive rocks and the fragments in bedded volcanic rocks supports the conclusion that they represent the same volcanic episode. These eruptive centers are recognized by the abun- dance in them of massive volcanic rocks that have steeply dipping planar flow structures, and the lack of internal or basal contacts. The center for the western facies is approximated by the outcrops of propylite and subpropylite southwest of Dome Mountain ; a small area of such rocks also crops out southwest of Klinker Mountain, but this is probably an outlier of the same center. The eruptive center for the eastern facies is in the vicinity of Almond Mountain. The marked zonation of fragmental volcanic rocks around both eruptive centers is evident on the geologic map (pl. 1). In the western facies, volcanic breccias form most of the sections exposed within a mile of the center; the tuffs and sandstones form an appreciable percentage of this facies only outside of this zone. The sandstones are thickest near the north and east edges of areas containing this formation because they were derived by erosion of the Atolia Quartz Monzonite and Bedrock Spring Formation, which cropped out to the north and east. Comparison of stratigraphic sections in three areas illustrates these changes. In sections D23-d and D14-n, adjacent to the western eruptive cen- ter, the formation is about 550 feet thick and consists of 85 percent volcanic breccia and 15 percent tuff. In section D11-f, located about 1 mile from the edge of this center, the formation is 600 feet thick and consists of 60 percent volcanic breccia, 12 percent tuff, and 28 percent sandstone. In section E6G-j, located 114 miles north of the center, the formation is 500 feet thick and consists of 47 percent volcanic breccia, 3 percent tuff, and 50 percent sandstone. The same distribution of the stratified rocks can be discerned around the eruptive center in the eastern part of the area. At the type section, located within the eruptive center, the formation is 900 feet thick and con- sists of 93 percent volcanic breccia and 7 percent tuff. In sections E24-h, -J, on the northeast edge of the center, the formation is 475 feet thick and consists of 55 percent volcanic breccia, 35 percent tuff, and 10 percent sand- stone. -In section E6-q, approximately 24/4 miles north- east of the nearest part of the center, it is about 150 feet thick and consists of 20 percent volcanic breccia, and 80 percent well-indurated tuff and lapilli tuff. ENVIRONMENT OF DEPOSITION Prior to the first eruption of the Almond Mountain Volcanics, the Lava Mountains was an area of low hills. The Atolia Quartz Monzonite and Bedrock Spring For- mation, which underlay these low hills, had been folded and probably faulted. Along the axis of the Dome Mountain anticline, the Bedrock Spring Formation was tilted at angles ranging from 20° to nearly vertical, and, as is evident from sections of the area (pl. 2, espe- cially section B-2'), several thousand feet of the for- mation had been removed by erosion. Simultaneous faulting may have created a northeast- trending topographic grain in these low hills. The composition of the basal layer of the Almond Mountain Volcanics varies on the two sides of several northeast- trending faults exposed on the northeast side of Dome Mountain; this lithologic variation suggests that topo- graphic breaks existed along these lines by the time the first volcanic debris was deposited, and the most likely sources of such breaks are earlier vertical dis- placement along these faults. Volcanic activity originated in the two eruptive centers almost contemporaneously as shown by inter- bedding of the eastern and western facies near the northeast end of the Brown's Ranch fault zone. A 32 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY southwest-trending valley apparently formed between these two volcanic centers, either by graben-forming fault activity along the Brown's Ranch fault zone, or simply as a result of the relatively greater growth of the volcanic hills on both sides. To the north of these centers, volcanic debris vied with the arkosic sediments from the north in locating the central axis of the east- west valley. When volcanic activity was most continu- ous or intense, the size of the volcanic pile increased rapidly and the valley axis was shifted to the north; when such activity was reduced, the arkosic alluvial fan north of the valley was built up, and the toe mi- grated southward over the volcanic pile. The average position of this valley is approximated by the township line separating T. 28 S. from T. 29 S. There is some evidence that the largest of these alluvial fans grew toward the southwest across the area now occupied by Dome Mountain, for the southernmost outcrops of sand- stone in this formation lie in this area. On all other sides of the eruptive centers, volcanic debris extended to the edge or outside of the mapped area. The lack of arkosic sediments in the outermost exposures of this debris shows that either the limits of the volcanic debris extended well beyond the area mapped, or that these outside areas did not slops toward the Lava Mountains. AGE AND RELATION TO OTHER FORMATIONS The Almond Mountain Volcanics lie with a marked angular unconformity on the Bedrock Spring Forma- tion; typical examples of this relation can be seen in sections D2-f, E3-c, E3-k, and F7. The Almond Moun- tain Volcanics are unconformably overlain by the Lava Mountains Andesite, and typical examples of this rela- tion can be seen in sections ET-, D2-m, and D11-n. The regional characteristics of the unconformities at the base and top of this formation, however, are different. In the interval between the deposition of the Bedrock Spring Formation and the deposition of the Almond Mountain Volcanics, much of the deformation consisted of folding, so most of these contacts are angular uncon- formities with several thousand feet of the underlying formation removed by erosion ; in the interval between the Almond Mountain Volcanics and the Lava Moun- tain Andesite, much of the deformation consisted of faulting without appreciable tilt of the fault blocks, so most of these contacts are disconformities. No fossils were found in the Almond Mountain Vol- canics. Inasmuch as it rests on the Bedrock Spring Formation, it is at least as young as middle Pilocene, and the intensity of tilting and amount of erosion asso- ciated with the unconformity suggests that an apprecia- ble period of time had elapsed. The Almond Mountain Volcanics are overlain, in turn, by the Lava Mountains Andesite and several deformed and eroded Pleistocene rock units. As appreciable time was also required to form this succession of rock units, an age of late Pliocene seems probable for the Almond Mountain Volcanics. LAVA MOUNTAINS ANDESITE The Lava Mountains Andesite, a new formation named here, consists mostly of dark-gray porphyritic andesite. It is found as flows, large domes, and small moundlike intrusives throughout the southern two- thirds of the mapped area. Typically, it forms the caps of rolling or flat-topped hills, cropping out to form soft or boulder-strewn areas on the top and blocky cliffs along the edges. The type locality is on the southeast side of Dome Mountain (in see. E8-m ; see sample 98-7 in table 8) ; the name is derived from the Lava Mountains of which it forms an important part. This formation is approxi- mately equivalent to Hulin's (1925, p. 55-58) "Red Mountain Andesite." Although that formational name has been widely used in subsequent geologic studies of the Mojave Desert area, a new name is used here because the prefix "Red Mountain" was already in use for another group of rocks at the time of Hulin's publication. The Lava Mountains Andesite rests unconformably on the Almond Mountain Volcanics and its predeces- sors. Four of the five rock units mapped as "other up- per Pliocene( ?) volcanics" could be either contempora- neous with or slightly older than the Lava Mountains Andesite ; the fifth, the tuff breccia, is equivalent in age to the lower part of the andesite. Although no rocks other than a few patches of gravel rest on top of the Lava Mountains Andesite, five Quaternary rock units are inferred to be younger on the basis of indirect evidence. Three lithologic map units of the Lava Mountains Andesite are distinguished on the geologic map. "Andesite flows," consisting of dark gray, red, or brown plagioclase andesite porphyry, constitute the most abun- dant and widespread rock type in the formation. "Flow breccia," consisting of angular fragments of the Lava Mountains Andesite in a matrix of the same material, occurs as bodies large enough to be distinguished on the map. A single layer of "flow conglomerate," con- taining rounded fragments of light-gray Lava Moun- tains Andesite in a matrix of red-brown andesite, is prominent on the west side of the range. DESCRIPTIONS OF LITHOLOGIC MAP UNITS ANDESITE FLOWS Flows of Lava Mountains Andesite form most of the formation, although some outcrops included in this unit, STRATIGRAPHY 33 especially near Dome Mountain and Almond Mountain, are probably the upper parts of volcanic necks or domes. Small patches of volcanic breccia are also included in the unit, but they are of very limited extent. On the tops of the mesalike hills, these rocks crop out as a field of boulders resting on fine-grained chocolate-brown regolith. Around the edges of these areas, especially in sections B31, D1, E1, and E34, sections of the ande- site flows are well exposed except where sheathed in their own talus. Near the basal contacts of these flows, the rock commonly weathers to flaggy slabs an inch or two thick that are parallel to the contact at the base and gradually change toward vertical upward from it. Above this flaggy zone, perhaps 20 or 30 feet thick, the rock either is irregularly blocky or forms columnar joints a foot or two across. Most of the flows are uniformly dark reddish brown or very dark gray in outcrop. On freshly broken sur- faces, the groundmass very commonly exhibits the same two colors, and this characteristic is one of the most reliable means of distinguishing these volcanic rocks from others in the area. Generally, medium gray (NV 4-5) is predominant, and pale red (102 4-6/2-4) forms streaks about 1 mm wide and 10 mm long. In many out- crops, however, the colors are the same, but their rela- tive abundance is reversed. In most samples from this unit, the plagioclase crystals are very faintly yellow or orange; this seems to be a characteristic independent of the groundmass color but restricted to the formation. The rocks of this map unit locally are as thick as 600 feet, although only 200 to 400 feet of section is gen- erally present. Only along the west edge of the area, west of Klinker Mountain, are thicknesses less than 200 feet characteristic. In most places, the formation ap- pears to consist of a single flow ; in a few places, though, it consists of several superposed flows separated by a thin zone of volcanic breccia. Some of the outcrops thickest in appearance-for example, the hills southeast of Almond Mountain and the hill east and south of Bedrock Spring-are massive, have steeply dipping nearly concentric flow structure, and apparently cut nearby layered rocks; therefore, they are believed to be volcanic necks or domes. About 45 samples of the andesite flows were carefully selected to give a proportionate sampling of the entire unit, and thin-section studies of them furnish a semi- quantitative estimate of its petrographic characteristics. In 95 percent of the samples, plagioclase is present both as megaphenocrysts and microphenocrysts; their com- bined volume percentages range from about 10 to 30. The average composition of the feldspar in both size groups is close to An,, ; it commonly ranges from Ans, to An;;, although some crystals have zones with An percentages as low as 20 or as high as 60. Most of the megaphenocrysts have well-developed zones, and twin- ning in them is conspicuous. In over half the samples, a majority of megaphenocrysts have calcic rims, but in no samples do all megaphenocrysts have calcic rims. The last major resorption of the plagioclase mega- phenocrysts occurred after the crystals had clustered to form a glomeroporphyritic texture. In every in- stance noted, the resorbed area, and the subsequent calcic rim, were formed only on the outward faces of the crystal group (as shown in fig. 224). Small in- clusions of yellowish glass(?) throughout the mega- phenocrysts give the crystals a yellowish color in hand specimens. Biotite is present in 80 percent of the samples, and strongly altered in more than half. Oxy- hornblende, present in 90 percent, shows a similar de- gree of alteration. Clinopyroxene and orthopyroxens are each present in about 75 percent of the rocks, and are nearly unaltered in two-thirds of them. Quartz, though present in 70 percent of the samples, is sparse in all but a few of those rocks. Generally, the groundmass consists of about 30 per- cent microscopic crystals and crystallites, 45 percent cryptocrystalline material, and 25 percent glass, al- though the percentages vary greatly. In part, the un- identified crystalline material consists of small percent- ages of submicroscopic cristobalite and clay that can be detected by X-ray; K-feldspar is not detected. The concentration of opaque materials is variable, ranging from a few small fragments in some rocks to virtually the entire groundmass in others. The reddish-brown rocks contain the highest percentage of opaque miner- als. The nature of these opaque materials varies; some are small euhedral crystals that appear black, whereas others are featherlike, hairlike, or fibrous objects that appear black in plane light and red in conoscopic light. Texturally, these rocks are characterized by the porphyritic plagioclase, biotite, hornblende, and pyrox- ene. Plagioclase crystals may be as long as 5 or 6 mm ; biotite and oxyhornblende crystals are mostly less than 3 mm long, and pyroxene crystals are mostly less than 1 mm long. The average lengths are about one-third of these maximums. Quartz is generally present as crystals 1 to 3 mm across. Many rocks also have small but numerous vesicles, some forming a quarter to a third of the volume. A few of these have linings of opal, chalcedony, tridymite, or a yellowish or greenish fibrous anisotropic material. Deuteric alteration has affected virtually all these rocks, although it is extreme in some and light in others. A. well-exposed flow in section EQ9-h provides good evi- dence that such alteration takes place very late in the cooling history of a rock. The upper part of this flow 34 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY consists of red-brown lava, and this grades downward over a distance of a few inches into an unaltered gray microperlite (fig. 225), which apparently was chilled so quickly after reaching the surface that normal altera- tion was prevented. Comparison of these rocks shows that during the normal period of cooling, deuteric al- teration changed all light-colored clear hornblende (eAZ=14°) to oxyhornblende (¢cAZ=10°), all light- colored biotite (y'=1.69) to darker colored biotite (y'=1.72), all clear orthopyroxene crystals to discolored or opaque crystals, and most opaque minerals and much of the groundmass to reddish opaque minerals. The plagioclase crystals were not affected. Original textures are also well preserved in this un- altered flow, and they provide evidence that there are few xenocryst minerals included in these rocks. The crystal faces are not altered, rounded, or resorbed, and the phenocrysts in this rock clearly are not fragments broken from a larger crystal and have not grown from a nucleus consisting of a broken crystal. Modal, normative, chemical, and spectrochemical analyses of nine samples are given in table 8. These samples include the full range of rock types found in the Lava Mountains Andesite flows, and modal analyses of them indicate the amount of mineral variation nor- mally found in these rocks. Plagioclase megapheno- crysts constitute 4 to 12 percent of these rocks and microphenocrysts 7 to 20 percent; mafic minerals make up 1 to 3 percent of most of them, and quartz generally makes up less than 1 percent. The chemical analyses show even more uniformity. The SiO, percentages mostly lie between 63 and 66, the Al;O;, percentages between 16 and 17, the CaO +Na.0 percentages between 6.2 and 7.0, and the CaO) percentages between 3.9 and 5.0. Elements detected by spectrochemical analysis are also present in similar amounts in these rocks. The normative compositions reflect the uniformity evident in the chemical analyses; by Washington's terminology (1917) (which draws fairly fine distinctions between analyzed rocks) seven are yellowstonose and two are tonalose. Normative orthoclase makes up between 18 and 24 percent of the total feldspar, and the norma- tive plagioclase has compositions ranging from Ans; to Ang,. The chemical and normative compositions show that if these rocks were crystallized to a more advanced degree, they would be classified as dacite or rhyodacite; and if crystallized completely, they would be granodiorite or quartz diorite. The analyzed rocks of this formation are compared with other volcanic rocks of the Lava Mountains area in a later section. FLOW BRECCIA Flow breccias as much as 150 feet thick form a dis- tinctive unit of the Lava Mountains Andesite in two areas, one on the southwest side of Dome Mountain and the other on the south end of Klinker Mountain. They crop out as jagged resistant rocks ranging in color on the weathered surfaces from grayish red to brownish gray. On fresh surfaces, they are lighter colored. The fragments are not sorted and range in size from sand size up to a foot across; exceptionally, 2-foot frag- ments are found. Generally, the smaller fragments are angular to subangular, and the larger ones are subangular to subrounded. In most exposures they form half to three-fourths of the total rock, the lava matrix forming the balance. These fragments are mostly blocks of Lava Mountains Andesite. The matrix lava appears to be identical with that constituting the andesite flows of this formation. FLOW CONGLOMERATE Flow conglomerate forms a distinctive bed of the Lava Mountains Andesite that crops out on the crest and on the west side of Klinker Mountain. The origin of the rounded fragments is not known. In most places, the bed is several tens of feet thick. The rock consists of very well rounded fragments, several inches across, of medium-gray andesite embedded in a matrix of pale- brown andesite. The outcrops shown on the map are apparently all part of the same flow, suggesting that the western part of the Lava Mountains represents an area that was part of one slope at the time of extrusion. Presumably this slope was toward the west, although the variation in flow thickness is inadequate to confirm this direction. VOLCANIC SOURCE AND DISTRIBUTION OF ROCK TYPES The Lava Mountains Andesite covers most of the higher hills in the southern two-thirds of the mapped area. The andesite flows are most abundant in the vicinity of Klinker Mountain, Dome Mountain, and Almond Mountain, and in the large area 3 to 5 miles northeast of Almond Mourtain. The flow conglomer- ates and flow breccias are restricted to the southwestern part of the mapped area. The relatively constant thickness of the Lava Moun- tains Andesite flows suggests that a number of sources existed. If it is assumed that the underlying surface was flat, and that a slope at the flow surface greater than about 200 feet per mile was necessary to maintain flow (this is about 2° and approximately the flattest slope formed by basalt flows in Hawaii ; see Wentworth, 1954, p. 430), it follows that a flow of Lava Mountains Andesite that has a maximum thickness of 600 feet, and a minimum thickness of 200 feet, is all within 2 miles of its source. Although the preexisting surface was generally not flat, the viscosity of the andesite was undoubtedly greater than that of basalt, and this would STRATIGRAPHY 35 TaBus 8.-Modal, chemical, normative, and spectrochemical analyses of nine samples of the Lava Mountains Andesite flows [ A dash means the element was not present in detectable amounts. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White; spectrochemical analyses for Li and Rb by Robert Mays, remaining spectrochemical and modal analyses by G. I. Smith] 27-21 27-28 98-7 97-380 124-28J 12,;-23F 12,;-23B 181-28 Modes [Volume percents] Plagioclase megaphenocrysis...._.....___.._.o_....__......_... 13.8 8.5 10.9 10.6 12. 4 12.5 10.5 4.3 7.9 Plagioclase microphenocrysts.... .. Rees 10. 20. 2 10.0 12.2 9. 4 6.8 6.8 11.6 8.3 (Total plagioclase)......_....... (24. 5) (28.7) (20.9) (22.8) (21.8) (19. 3) (17.3) (15.9) (16. 2) ioe 2s s eous e ere ece un aa - £ . 5 .3 A .3 0 f 0 0 .2 0 0 0 0 0 0 0 4 0 2A :A 3.1 1.6 £97, .% Trace Orthopyroxene. 1.4 .* .2 x A .9 VA 2.5 v7 C URODyrORCRE -. : 2. 22200000000 odc Lodon o een nece ne, .8 2.0 .8 2 0 4 0 3 .9 Quartz.._... .6 *. a hy 0 .3 11 0 3.2 Opaque minerals 5.4 2.5 6.7 6.0 9.5 2.2 11.4 1.4 8.2 Other minerals .5 .6 7 2.9 .2 0 5 v4 Groundmass.. 66. 2 65. 6 68. 5 67.3 65.3 69. 2 70. 5 78. 9 69. 9 Chemical analyses [Weight percents] 64. 5 63.3 63. 8 65.0 64. 2 64.1 64. 9 66. 1 60. 6 16. 4 16. 4 16.7 16.7 16.8 16.7 16.9 16.2 16. 2 2.1 1.4 3.8 3. 8 3.7 1.8 4.1 2.3 2.8 1.9 2.8 14 . 34 . 44 2.1 .02 . 88 2. 4 2.3 2.4 2.3 15 2.3 2.1 2.0 1.6 3.5 4. 4 5.0 4.7 4.2 4. 4 4.1 4.3 3.9 5.8 4.2 4.2 4.1 4.4 4.5 4.0 4.5 4.0 3.8 2.3 2.3 2.1 2.6 2.5 2.7 2.5 2.7 2. 4 . 66 . 68 . 58 . 68 . 66 . 64 . 66 . 50 76 .23 . 28 . 29 . 24 . 28 .26 . 30 .18 . 28 . 06 . 06 . 06 .02 . 06 . 06 . 06 . 04 . 07 1.0 f .2 1.8 AT 1.4 . 20 1.4 1.2 <.05 5 <.05 <.05 <.05 <.05 .21 .74 100 101 100 100 100 100 101 Norms [Weight percents) 18. 4 15.9 19.0 21.0 17.6 18. 5 17.7 21.7 14. 0 0 0 0 0 0 0.3 0 0 0 13.3 13.3 12.2 15.6 15.0 16.1 15.0 16.1 13.3 35.6 35.6 34.6 37.2 37.2 34.1 38.2 34.1 32. 0 19.2 19.2 21.1 18.1 18.6 18. 6 18. 4 18.1 20.3 2.2 4.5 4 1.3 .9 0 0 11 5.4 5.3 6.6 5.6 1.3 5.1 6.5 5.0 3.5 6.9 3.0 2.1 0 0 0 2.6 0 1.6 4.2 1.4 1. 4 .8 .8 .9 1.2 0 .9 1.5 0 0 3.8 3.8 $7 0 4.2 1.1 0 0 0 1.2 .8 .6 0 1.0 0 0 0 0 0 0 0 0 3 0 0 .3 T +1 .8 17, 27 v4 $7 T 1. 4.3.4 II. 4.3.4 1.4.3.4 1.4.3.4 1.4.3.4 1.4.3.4 1.4.3.4 I. 4.3.4 IL 4.3. 4 Spectrochemical analyses [Parts per million} 80 80 70 60 40 40 40 40 707 60 80 30 20 80 60 40 50 60 3900 420 390 170 420 420 380 280 470 50 60 607 50 30 20 30 10 50 67 5 10? 5 5 5 5 5 5? 60 70 60 70 60 60 40 90? 22, 000 22,000 28, 000 24, 000 28, 000 22, 000 25, 000 16, 000 31, 000 20 20 20 30 20 10 20 10 30 10 1 10 20 20 10 10 10 2, 600 2, 400 2, 100 2, 800 2, 500 2, 100 2, 600 2, 100 2, 400 40 4 20 20 30 30 20 40 20 1, 700 1, 600 2, 000 1, 800 2, 000 1, 400 2,000 2, 000 1, 900 19 2 40 6 27 16 21 27 36 10 10 10 10 20 10 10 10 10 30 30 30 40 50 60 30 80 40 1, 400 2, 000? 1, 600 1, 400 1, 500 1, 200 1, 800 1, 000 1, 600 <1 <1 <1 #1 <1 <1 1 <1 1 5 5 10 5 10 5? 10 10 5? 120 110? 100 110 110 80 120 110 90 LOCATION OF SAMPLES 27-21. Sec. D 2-r, sample from top of ridge. 27-23. Sec. D 3-d, sample from southwest side of hill. 98-7. Sec. E 8-n, sample from about 50 feet above marker contact shown on pl. 1. 97-8380. Sec. E 6-j, sample from about 100 feet above base of formation. 12%;-28J. Sec. E 34-k, on southwest side of Almond Mountain, sample collected about 375 feet above base of formation. 12}-23F. Same section, sample collected about 210 feet above base of formation. 124-23B, Same section, sample collected about 50 feet above base of formation. 12}-22K. Sill intruded into Almond Mountain Volcanics type section; see fig. 11 for position. 181-28. Sec, E 1-n, sample from about 40 feet above base of flow. 36 tend to offset the error introduced by assuming a flat surface. The specific locations of the many sources of andesite flows can only be inferred. Steeply dipping flow struc- tures have been mapped in many areas, but their atti- tudes are too inconsistent to be reliable criteria. Fortu- nately, this formation is young enough to have some of the original topographic features remaining on the flow surfaces. Among these are numerous small mounds and small to large domes, quite similar to those de- scribed by Williams (1932, see especially figs. 20¢ and 37) and Coats (1936). Many of these also have steeply dipping flow structures, and it is inferred that most of FIGURE 12.-Vertical aerial photograph of area east of Almond Mountain. GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY these mounds and domes indicate areas that were cen- ters of upwelling lava. The best examples of such features are the mounds and domes of Lava Mountains Andesite that lie to the east and southeast of Almond _ Mountain (fig. 12). Fifteen or twenty smaller and less clear-cut examples are present elsewhere in the mapped area, and they are spread over an area almost as large as the present areal extent of the Lava Mountains Andesite. ENVIRONMENT OF DEPOSITION Flows of Lava Mountains Andesite crop out in the highest part of the range and also near the level of the Outcrop areas of Lava Mountains Andesite enclosed by dashed lines. Some of the topographic mounds and domes believed to indicate areas that were centers of upwelling Lava Mountains Andesite are indicated by C. U.S. Navy aerial photograph PTK-5-®*. STRATIGRAPHY 37 alluvium in the southern part of the area. This repre- sents nearly 2,000 feet of relief. There has not been enough faulting and tilting since deposition to account for this amount of relief, so it is reasonable to conclude that in the southern part of the area, the surface on which the flows were originally deposited provided most of this relief. The contacts shown on the geologic map (pl. 1) and sections (pl. 2), though, show that relative to the pres- ent topography, the depositional surface was gently rolling and nearly undissected. Most of the slopes indi- cated for this preexisting surface are less than 5°, al- though a few dip as steeply as 15°. For example, the large flows in sections D3, D14, E8, E9, and F7 rest on surfaces that are nearly horizontal; the flow con- glomerate rests on a west-dipping slope of about 5° ; and the lava flow in section D1-c, noted by Hulin (1925, p. 56), rests on a surface that dips north about 15°. AGE AND RELATION TO OTHER FORMATIONS The Lava Mountains Andesite is probably of very late Pliocene age although it may in part be Pleistocene. In many places it is clearly unconformable on both the Bedrock Spring Formation of middle Pliocene age and the Almond Mountain Volcanics interpreted to be of late Pliocene age. Most of the rocks grouped with the other upper Pliocene(?) volcanics are either con- temporeaneous with or older than the Lava Mountains Andesite. The relation of this andesite to all the younger units can also be established. The older gravels contain fragments of Lava Mountains Andesite; the Quaternary andesite appears to cut the Lava Moun- tains Andesite (see. D1-e) and is contemporaneous with these older gravels. The Christmas Canyon Formation, dated as Pleistocene(?) on the basis of a single fossil, contains fragments of the Lava Mountains Andesite in fairly high proportions; hence both it and the basalt that cuts it are also younger. Within this part of the Mojave Desert, many volcanic sections have been correlated with these volcanic flows under the name of Red Mountain Andesite, as they were called by Hulin (1925, p. 55). Although many of these correlated flows have a similar lithology and occupy a comparable position with respect to the stratigraphic sequence of the area, there is little reason to think that they are correlative in the sense of being once connected or contemporaneous. The evidence in the Lava Moun- tains area indicates that the flows of the Lava Moun- tains Andesite were very local and did not extend more than a few miles from their source in any direction. There are, to be sure, numerous sources within the Lava Mountains area, but they form a very distinct cluster which is isolated from the other parts of the Mojave Desert by large andesite-free outcrops of older rocks. Nevertheless, many parts of the Mojave Desert do have the same general succession of Cenozoic rocks, namely (a) sandstones and conglomerates, (b) pyro- clastic breccias and tuffs, (c) capping flows of dark- colored andesite, and (d) very young looking basaltic rock. This repetition of the sequence from place to place suggests that some widespread fundamental process controlled Cenozoic deposition. OTHER UPPER PLIOCENE(?) VOLCANICS Five other volcanic rock units have been mapped separately. All are of relatively small areal extent, but have sufficiently diverse lithologies to be mapped and treated separately. Because of their lithology and stratigraphic position, none can be logically included in the more extensive upper Pliocene volcanic formations, although most or all may be genetically related. TUFF BRECCIA Along the west edge of the mapped area, upper Pli- ocene( ?) tuff breccias crop out to form steep debris- covered slopes. The best exposures within the mapped area are found in sections D3-q, D9-j, and D15-b, al- though the most extensive outcrops of these rocks are a few miles to the west. Generally they are a little bet- ter exposed than the underlying Bedrock Spring For- mation, and not as well exposed as the overlying flows of Lava Mountains Andesite. On hillsides, the rocks are light reddish purple or light brownish gray ; where well exposed on canyon walls they are somewhat browner. The tuff breccia ranges from an impure lapilli tuff in which rock fragments less than 1 inch across con- stitute 20 to 50 percent, to a rubble breccia in which fragments up to 4 feet across constitute about 90 percent of the unit. Rocks with an average composition con- tain about 60 percent rock fragments in a matrix of tuff. The fragments are predominantly purplish por- phyritic andesite from the Almond Mountain Volcanics, but up to 30 percent of them are derived from early flows of the Lava Mountains Andesite. Also included is a small percentage of metamorphic fragments, mostly knotty schist. The mapped relations in sections D10-h and D15-a suggest that these breccias lie unconformably on the Almond Mountain Volcanics, but the exposures are not adequate to make this certain. These breccias clearly lie on the Bedrock Spring Formation with a slight unconformity in at least one area (D15-g). The tuff breccias are clearly overlain by late flows and flow conglomerates of the Lava Mountains Andesite, but are apparently contemporaneous with the early flows, for they contain rock types not abundant in the area prior to that formation. Probably they were deposited 38 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY as mud flows that resulted indirectly from the volcanic activity in early Lava Mountains Andesite time. INTRUSIVES Five distinguishable varieties of upper Pliocene( ?) porphyritic or aphanitic intrusive rocks crop out in the central and western part of the mapped area. Each variety forms several individual intrusives that are limited to a discrete area as much as a square mile in size, but the areas characterized by each variety do not overlap. The upper Pliocene(?) intrusives in four of these areas form dikes; in the fifth they form small plugs and necks. In sections D1-k and D1-q, dike rocks form resistant ridges that project above the eroded surface of the Bed- rock Spring Formation. The groundmass is a distinc- tive grayish orange pink (5YR 8/2). The phenocrysts of plagioclase are the same color as the groundmass, whereas those of biotite form relatively unaltered dark- brown euhedral books. Light-brown areas, as much as 20 mm long, may represent altered acicular hornblende. In sections D1-h, E5-e, E6-c, and E6-e, dike rocks also form resistant ridges, but pronounced swirling color patterns and filled vesicles allow them to be easily distinguished from those in the previously described area. The fresh surface is mostly grayish orange pink or very light orange; darker colors form the swirling, contorted patterns. Vesicles are very abundant, com- monly lenticular and alined, and commonly coated with a lighter orange fine-grained material. Phenocrysts of plagioclase, biotite, and hornblende(?) rest in the groundmass of microcrystals of plagioclase(?) and an unidentified anisotropic material. The plagioclase present as megaphenocrysts up to 4 mm long as well as microphenocrysts, is weakly zoned and twinned; the composition ranges up to An;;. The biotite is almost entirely altered to opaque material, plagioclase, and dis- colored calcite. The hornblende(?) is entirely altered to opaque minerals. In section A35, dike rocks intrude Atolia Quartz Monzonite. All these rocks weather to shades of red, red brown, and purple. They are so strongly brecciated and altered, however, that none of the included minerals can be accurately identified in thin section. In section A23, dikes crop out that have weathered to soft, crumbly, dark-grayish rock containing small white phenocrysts of plagioclase as the only visible mineral. These rocks are also very strongly brecciated because of their nearness to the Garlock fault. The color and recognizable plagioclase distinguish them from the dikes in section A23. In sections D13, D14, and E18, several small volcanic plugs and necks form steep-sided round hills Most weather to dark grayish red. In detail, planar frac- tures, color banding, and lineation are sometimes vis- ible; they generally dip at high angles, but have no regional pattern. In hand specimen, these rocks ap- pear somewhat coarser grained than the other volcanic rocks of the area, and also contain a higher percentage of quartz. They range in composition from andesite to dacite and are composed of plagioclase, biotite, oxyhorn- blende, orthopyroxene, clinopyroxene, and quartz, in a groundmass predominantly of microcrystals of plagio- clase and cryptocrystalline material. The plagioclase shows zones that are commonly well developed and often have a calcic rim; twins are not common and the composition ranges from andesine to sodic labradorite. Most of the biotite is partially altered to opaque mate- rials. Usually, the oxyhornblende is strongly or entirely altered. Orthopyroxene and clinopyroxene, present as small euhedral crystals, are not altered. Quartz, which ranges from fairly common to abundant, is present as rounded and embayed crystals, some of which show a reaction rim of clinopyroxene. Most of the samples show a faint alinement of the plagioclase micropheno- erysts and contain a small number of vesicles. Maxi- mum erystal sizes are: plagioclase, 4 mm; quartz, 2 mm; biotite, 2 mm; oxyhornblende, 1.5 mm; orthopy- roxene and clinopyroxene, about 0.75 mm. The average sizes are about two-thirds of these lengths. The four series of dikes described in this unit are known to be intrusive only into the Bedrock Spring Formation and the Atolia Quartz Monzonite. The small plugs and volcanic necks are probably intrusive into the Almond Mountain Volcanics, although they could represent topographic highs that existed prior to the time the Almond Mountain Volcanics were being deposited. The relation between these intrusives and the Lava Mountains Andesite is not known. The rocks are considered late Pliocene(?) in age, but the possi- bility cannot be precluded that they may be in part late middle Pliocene or Pleistocene. FELSITE Felsite of late Pliocene (?) age crops out in two areas in the central part of the map. In one of these areas, it forms a northeast-trending steep hill about 1 mile east of Dome Mountain; in the other, which lies about 3 miles northeast of the first, it forms two large hills and a small dike that lie along the axis of the Dome Mountain anticline. These outcrops are thought to be mostly volcanic plugs, although in a few places a small remnant of the associated flow remains. This rock is characterized by a strong tendency to spall off as flaggy slabs as illustrated in figure 13; these planar fractures are parallel to the thin bands of darker colors and to the alined minerals, and probably to the STRATIGRAPHY original directiou of flow. Normal to this planar frac- ture, it breaks with a smooth conchoidal fracture. The fracture planes dip at high angles either toward the center of the volcanic plug (as in secs. E5 and E8) or away from it (as in see. B27). The weathered sur- faces of these felsites are mostly light brown ; the fresh surfaces are somewhat lighter and slightly more gray or orange. FIGURE 13.-Typical outcrop of the upper Pliocene(?) felsite. background consists entirely of Quaternary andesite. Hill in The felsite consists of a few plagioclase and biotite megaphenocrysts in a groundmass of very fine grained microphenocrysts, crystallites, and cryptocrystalline material. The plagioclase megaphenocrysts are euhe- dral, have very sharp edges, and are faintly zoned and twinned. The composition is variable, ranging from about Ans to An,; the average is perhaps Ans. The plagioclase microphenocrysts have a similar average composition, although some have a composition as high as An;s. Biotite, partially or totally altered to opaque materials, is found as very thin books about 0.02 mm thick. Some oxyhornblende(?) occurs in trace amounts. The groundmass consists predominantly of microscopic crystals, crystallites, and cryptocrystalline material which X-ray diffraction shows to be predomi- nantly cristobalite and sanidine; glass makes up less than 20 percent of the material. The groundmass con- tains a light to medium dust either of equant black opaque or of irregular red translucent fragments. The texture of these rocks is notable for the almost perfect alinement of the plagioclase microphenocrysts, crystallites, and biotite crystals. Some specimens show 39 vesicles, but most have none. Plagioclase and biotite crystals are as much as 3 mm long, but the plagioclase microphenocrysts average about 0.02 mm. The results of modal, chemical, spectrochemical, and normative analyses of one specimen are shown in table 9. _ The modal analysis indicates that about 90 percent of this rock is groundmass and 10 percent identifiable minerals; this proportion is probably close to the aver- age for this unit. A chemical analysis confirms the felsic composition suggested by the rock's color and texture, and shows it to be the most silicic volcanic rock analyzed. Its normative composition shows that on more complete crystallization this rock would be a very light colored rhyodacite, dacite, or quartz latite com- posed almost entirely of plagioclase (An;;), K-feldspar (80 percent of total feldspar), and quartz. Tapur ©9.-Modal, normative, chemical, and spectrochemical analyses for one sample of the upper Pliocene(?) felsite [Sample 97-39, from top of ridge in north half of see. E 5-r. element was not present in detectable amounts. Chemical analyses by P. L. D. Elmore, H. F. Phillips, and K. E. White; spectrochemical analyses for Li and Rb by Robert Mays, remaining spectrochemical and modal analyses by G. I. Smith] Modes [Volume percents] A dash means the Norms [Weight percents] Plagioclase megaphenocrysts..... 1.2 Q. 30.1 Plagioclase microphenocrysts 6.3 C. .8 (Total plagioclase) . __. (7.5) or. 19.5 mack % | ab... 34.1 Hornblende. ... 0 an.. 12.5 Oxyhornblende. di... 0 Orthopyroxene.. .. ./: 0 By c.. c.. 0 Clinopyroxene . .. 20 mb_-...-.- 0 <£0 ils icc .5 Opaque minerals.. Ranges zs O em "G' us > \ \ N & x 0 40 80 MILES FisurE 19.-Map of southern California showing location and sense of slip of major late Cenozoic faults. Abbreviations as follows: (SN) Sierra Nevada fault; (DV) Death Valley fault; (BP) Big Pine fault; (MB) Malibu Beach fault; (SG) San Gabriel fault; (SJ) San Jacinto fault. Data adapted chiefly from Hill and Dibblee (1953, pl. 1), Hill (1954, fig. 1), Allen (1957, fig. 1), and Grose (1959, fig. 3). STRUCTURE 53 approximate mirror image of that at the junction of the San Andreas and Garlock faults Hence, satis- factory explanations of the San Andreas-Garlock fault system should not only provide for those faults and their junction relations, but also for faults like the Blackwater fault. Hill and Dibblee (1953, p. 454-458) suggested that the Garlock and San Andreas faults are simple comple- mentary fractures created by north-south horizontal stresses, though, as noted by Cloos (1955, p. 254), the areal pattern of these faults seems to preclude this pos- sibility. At their junction, as can be seen in figure 19, the blocks bounded by acute angles have been moved away from the apex, and according to theoretical and experimental data (for example, see Anderson, 1951, p. 7-21; Cloos, 1955; Griggs and Handin, 1960), blocks having this displacement during compression of homo- geneous material are invariably bounded by obtuse angles. This anomalous relation between fault-block angles and displacements exists, though, throughout the Mojave Desert and Transverse Range provinces wherever left- and right-lateral faults approach each other. Figure 19 shows the most important of these junctions, but the same relation is found at less impor- tant junctions as well, and may thus be considered a tectonic characteristic of the two provinces. The proper relation between junction angle and dis- placement was created by Cloos (1955, p. 253-255) in a two-stage scale model experiment. He first formed complementary fractures in the experimental material by north-south horizontal compression, then rotated the fractured material until the angles and displacements of the fractures resembled those of the Garlock and San Andreas faults. This produced the correct junc- tion angle, but failed to produce a large displacement along both the Garlock and San Andreas faults. Furthermore, his illustrated result (Cloos, 1955, pl. 8, fig. 1) does not show bends in the fractures analogous to the San Andreas and Garlock faults, or fractures comparable with the fault set represented by the Black- water fault. Moody and Hill (1956, p. 1219) suggested that the Garlock fault was a second order? response to dis- placements on the San Andreas fault. Though the junction angles, relative displacements, and age rela- tions predicted by their theory approximate those of the San Andreas and Garlock faults (and also those of the Blackwater fault if proposed as a third order 3 Second order faults, as used by Moody and Hill (1956), are crustal fractures similar to those predicted on theoretical grounds by McKinstry (1953, p. 405-413) and Anderson (1951, p. 160-173) that form in response to stresses set up in blocks undergoing displacement along first order faults. Third order faults result from stresses set up by dis- placements along second order faults, etc. fault), their explanation has at least one serious short- coming. This is that the bend in the San Andreas fault seems very clearly to express left-lateral crustal drag in response to displacement along the Garlock fault, and the theoretical basis for their conclusion precludes the possibility of the second order Garlock fault deflecting the trace of the first order San Andreas fault that cre- ated it. Similarly, the curvature of the Garlock fault cannot be attributed to drag created by right-lateral displacement of faults like the third order Blackwater fault. The major characteristics of the fault pattern being discussed are interpreted here as indications that the area has been alternately subjected to two horizontal stress patterns that are oriented differently, one pat- tern oriented as if the major stress at the surface was from the north and south so that it formed the San Andreas and Blackwater faults, and a second pattern oriented as if the major stress at the surface was from the southwest and northeast so that it formed the Gar- lock and Transverse Range faults. Stresses with these apparent orientations account satisfactorily for the ob- served fault pattern and displacements. Alternation of these stress patterns accounts for both the history of contemporaneous fault displacements, and the bends in the San Andreas and Garlock faults which are inter- preted as crustal drag related to subsequent activity along the opposing set of faults. This interpretation requires the Blackwater fault to be very young because it parallels the northern part of the San Andreas fault rather than its deflected trace to the south; conse- quently, it must have been formed after most of the deflection was completed. The segments of the Gar- lock and Blackwater faults in the Lava Mountains indicate this age relation, so this interpretation is sup- ported by the evidence presented locally. A possible source for such alternating fracture pat- terns was indicated by Anderson (1951), and applied by Allen (195%) to a complex of strike-slip and thrust faults exposed along one segment of the San Andreas fault. They point out that if nearly equal major and intermediate stresses are acting on a region, small ir- regularities in the intensity of one can temporarily reverse their relative intensities, and that this reversal can create a different fault pattern at the surface. Ideally, the stresses indicated by these two fault pat- terns would be complimentary, yet as noted previously, the stress orientations indicated by the observed fault pattern shown in figure 19 are not complementary. In all likelihood, though, the stresses creating the major faults being discussed are applied at great depth, and the resulting deformation transmitted upward through rocks with different mechanical properties. Further- 54 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY more, the basement rocks of the area probably have marked lines of weakness so that they will break in some directions more easily than others. Both fac- tors will cause some deviation from the ideal comple- mentary fault pattern, and the tectonic interpretation given here depends on such factors to produce the pat- tern that is observed. The faults within and parallel to the Brown's Ranch fault zone differ from the strike-slip faults described above. Not only are the faults of this zone predomi- nantly dip-slip, but their extent is short, their sense of displacement is variable, and their fault zone un- commonly wide. The near parallelism of these faults probably means that they originated as fractures caused by one set of stresses, but have since sustained other kinds of displacements that obscure their original simi- larities. These initial stresses are inferred to have been either expressions of vertical forces related to the incipient Pliocene volcanic activity, or stresses that normally would have produced faults parallel to the Garlock fault but were deflected by a subsurface grain in the concealed rocks. The Dome Mountain anticline is probably a shallow compressional feature related to the Garlock fault which it parallels throughout most of its length. It is probably not related to the Blackwater fault, as the anticlinal axis crosses the projection of that fault with- out any change in average strike or character. The thrust fault along the Garlock fault is probably also a result of local stresses but the direction of thrust- ing is not known. GEOMORPHOLOGY PEDIMENTS Remnants of one or more pediments are found in the northern half of the Lava Mountains area. For description, these remnants have been combined into three groups on the basis of the composition of the cap- ping material. This grouping may separate surfaces that originally were parts of the same pediment; it is unlikely, however, that any of the three groups include remnants of different pediments. The largest of these pediment remnants is exposed in the northwestern part of the mapped area and is illus- trated in figure 20. Except where dissected, it consists of a gently sloping surface covered with a thin layer of weathered granitic debris. Its north and east edges, however, are deeply dissected and the pediment de- stroyed where adjacent areas have been faulted down- ward or lowered by erosion. The pediment surface was originally all graded west-southwestward to the level of the valley west of the Lava Mountains to which it is connected by a narrow alluvial strip (in see. A32). FicurE 20.-Profile view of the dissected pediment (P) cut on Atolia Quartz Monzonite in the northwest corner of the mapped area. The trace of the thrust fault along the north side of the quartz mon- zonite area is indicated by the dashed and dotted line. The Garlock fault (G) is exposed along the near edge of the outcrops but is obscure when viewed from this direction. View looking south. The surface is now slightly warped and the eastern half forms an arch that plunges east-northeastward. In the north-central part of the mapped area, several pediment relicts dip 2° to 5° north or northeast (fig. 21). Near the east edge of their areal extent, however, in sections B26, B27, and B35, they dip north, and are about on the same grade as, and may have been con- tinuous with, the pediment described in the next para- graph on which the Christmas Canyon Formation was deposited. The capping material of the north-central group of relicts consists of older gravels which are, in every instance noted, composed of rocks that are on the uphill projection of the surface, indicating that no appreciable deformation has occurred since their for- mation. The underlying rocks are mostly those of the Bedrock Spring Formation, and beds dipping as much as 60° are truncated by the erosional surface. In the northeastern quarter of the mapped area are It was formed just remnants of a buried pediment. FIGURE 21.-Remnants of gravel-covered pediment surface cut on Bed- rock Spring Formation dominate the left-foreground (Qg) ; Quater- nary andesite flows deposited on the same surface are well exposed in the central part of the photograph (Qa). Dashed lines are the contacts between these flows and the Quaternary gravels or the underlying Bedrock Spring Formation. Dome Mountain is the highest point on the skyline. View looking southwest. ECONOMIC GEOLOGY 55 prior to deposition of the Christmas Canyon Forma- tion, which remains as a capping up to 200 feet thick. Most of this pediment was cut on the Bedrock Spring Formation, but the northeasternmost part was cut on metamorphic rocks and Atolia Quartz Monzonite. The part of this surface that was underlain by the Bedrock Spring Formation was nearly flat in Christmas Can- yon time, whereas the remainder had slight relief. Since its burial by the Christmas Canyon Formation, the east end of this pediment has been warped into a broad arch. Though all three pediments were constructed at about the same time, each has subsequently recorded differ- ent tectonic activity : the pediment cut on Atolia Quartz Monzonite has been slightly warped into a broad arch parallel to the Garlock fault ; the pediment cut on Bed- rock Spring Formation in the north-central part of the area has remained undeformed; the pediment cut on pre-Quaternary rocks in the northeast quarter of the map has been partly warped into a broad arch by the Dome Mountain anticline. AGE All three pediments in the area are probably of early or middle Pleistocene age. The pediment cut on Atolia Quartz Monzonite is an extension of a surface that is veneered by older gravels (in see. A83) ; these deposits rest on a surface that was almost undissected, indicating that the age of the pediment surface is only slightly older than the sediments that cover it. The pediment in the north-central part of the mapped area was also only slightly dissected at the time the Quaternary ande- site and older gravels were deposited on it, and the pedi- ment in the northeastern part of the map is only slightly older than the Pleistocene(?) Christmas Canyon For- mation which rests on it. As noted previously, rem- nants of these two surfaces nearly grade into each other, and this evidence indicates a similar age for both sur- faces. All three surfaces are thus dated as slightly older than the lower or middle Quaternary formations which rest on them, and appreciably younger than the deformed and truncated middle and upper Pliocene rocks on which they are cut. The pediment in the northeastern part of the area is also slightly warped where it crosses the axis of the Dome Mountain anti- 'cline, whereas the pediment surface in the north-central part of the area is not; this difference is probably due to an uneven intensity of folding along this axis in Quaternary time. SHORELINES Along the northeast edge of the area, an excellent shoreline was cut during the highest stand of Searles Lake at an elevation of about 2,250 feet, about 630 feet above the present lake surface (fig. 17). Through- out much of its length, the shoreline takes the form of benches 10 or 20 feet wide with wave-cut cliffs 5 or 10 feet high. Lacustrine tufa was deposited in this part of the lake at several levels between 2,200 and 2,250 feet. As noted previously (p. 44), these deposits are probably more than 50,000 years old, but are more recent than the last major displacement on the Garlock fault in this area. A very faint shoreline(?) is visible at two places on the north shore of Cuddeback Lake (in E32 and G4) at an elevation of 2,660 feet, approximately 110 feet above the present playa level. This shoreline(?) is defined by the upper limit on the slope of a slight concentration of fine sand. Although there is no evidence of wave erosion or of lake-shore tufa, the occurrence of similar deposits at the same elevation on the two hills would be a highly unlikely coincidence unless they were formed by a lake. A better defined shoreline, indicated by beach sands and sandbars, lies about 10 feet above the present playa surface. ECONOMIC GEOLOGY METALS In the Lava Mountains, gold mining has been at- tempted only in Christmas Canyon. Two adits, the longer about 300 feet, extend into Atolia Quartz Mon- zonite, and two shafts connect them with the surface. The mine has been inactive since about 1930. Some of the prospect pits in the Lava Mountains were probably made in the search for gold, for the claim notices asso- cited with them were dated in the early 1900's-5 to 10 years after gold was discovered at Randsburg. Most of the mines and prospect pits in the region east of City Well resulted from the search for silver-an outgrowth of the discovery in 1919 of the California Rand (Kelley) silver mine. One of these, the North Rand mine, in section D2T-h, was described by Hulin (1925, p. 141). It consists of a 400-foot shaft inclined to the south at about 60°; it is apparently all in the propylite of the Almond Mountain Volcanics. This mine reportedly (Joe Foisie Johannesburg, oral commun., 1955) had ore near the surface but none at depth. Material on the dump consists of altered tuff and sandstone containing small crystals of pyrite dis- seminated throughout the rock and concentrated along some of the fractures. Another mine, just south of the mapped area, about half a mile southwest of the North Rand mine, is reported to have a shaft about 500 feet deep which was too hot to be continued deeper (Joe Foisie, oral commun., 1955). 56 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY In section D24-q, a shaft estimated to be more than 100 feet deep is at the contact between the propylite and subpropylite rocks of the Almond Mountain Vol- canics. Samples from the dump are fine-grained gray- ish yellow-green volcanic rocks that contain dissemi- nated pyrite. A flurry of mercury prospecting occurred at approxi- mately the same time as the search for silver. There was no production from this area. The Steam Well, in section D25, was drilled as a mercury prospect. RADIOACTIVE DEPOSITS On the north side of the mapped area, along the Gar- lock fault, is a large prospect for radioactive minerals called the Alpha Beta Gamma mine. It is reported to have a radioactive count of 3X background (Walker and others, 1956, p. 13). No development work was underway when visited in 1960. WATER The water supply for the towns of Randsburg and Johannesburg comes from the wells in the southwest corner of the mapped area. T'wo of these wells, as re- ported by Thompson (1929, p. 221), were 380 and 400 feet deep with water levels at 375 and 390 feet, respec- tively. The water, which is of good quality, must be pumped about 6 miles with a net vertical lift of about 150 feet; the sale price in 1956 was about one cent per gallon. Bedrock Spring, in the northeastern part of the area, consists of an adit about 10 feet long that projects into volcanic rock and has a few inches of brackish water standing on the floor. It was reportedly (Joe Foisie, oral commun., 1955) dug for water about 1900 by the members of the Spangler family. No springs or other evidence of near-surface water were found elsewhere in the mapped area. Within the area shown on the map, there are no highly favorable areas for ground-water accumulation. The areas that drain west and south-potential water sources for Randsburg and Johannesburg-are not large. The area that drains northward is more favor- able because the collection area is larger and the Gar- lock fault might serve as a ground-water barrier to bring water nearer the surface, although there is no evidence of near-surface water along the fault. Water found in the northward-draining area might be brack- ish or saline from contact with the saline parts of the Christmas Canyon and Bedrock Spring Formations, and from disseminated salines deposited in the alluvium during the higher stands of Searles Lake. In section D25, a well was drilled about 1920 as a prospect for mercury. This 415.6-foot well, known locally as Steam Well, brings to the surface water vapor at a temperature of 96°C (W. R. Moyle, Jr., written commun., 1961). It lies along a fault parallel to the Brown's Ranch fault zone. ZEOLITE AND ALUNITE Immediately surrounding the Steam Well is an area of light-tan, yellow, or white rock that consists of an intimate mixture of alunite-KAl; (SO,), (OH),-and opal. The area of outcrop of this material is about 500 X 1,200 feet ; if it extends 100 feet downward, about 3 million tons of this material are available. A small area of subpropylite about 1,000 feet southwest of the Steam Well contains a large percent of the zeolite clinoptilolite; the extent of this material was not determined. GRAVEL Gravel has been quarried from the alluvium in Christmas Canyon since about 1948 for use as road metal on nearby blacktop roads. Apparently this is the only gravel in the area that has been commercially exploited. Most of the material in the Bedrock Spring Formation is too fine for such uses, although some beds in the coarser facies of the 'Christmas Canyon Forma- tion are suitable. FAVORABLE AREAS FOR FUTURE PROSPECTING The Lava Mountains area has been prospected many times. It seems unlikely that large deposits of any of the materials sought (gold, silver, tungsten, mercury, uranium, and thorium) have been overlooked in out- crop. Several exploratory shafts and adits in the southwestern part of the area have also sampled the concealed rocks. Although the chances of success are small, the most likely area for future discoveries seems to be within or near the elongated zone of propylitiza- tion. The presence of propylite, zeolite, and alunite is reminiscent of several other mining districts (for ex- ample, the Comstock Lode), though a genetic connec- tion between these minerals and ore has never been firmly established. The surface trace of the contact between the propylite and subpropylite has been exten- sively prospected in the past, and additional explora- tion of this zone at depth, combined with detailed geochemical sampling, might detect ore that has been overlooked. Some of the areas of subpropylite that lie outside of the main zone have equal promise, and they appear to have been prospected less carefully in the past. Similarly, the zone of bleached tuffs along the northwest side of the Brown's Ranch fault zone has probably undergone secondary hydrothermal alteration of some type, and these tuff areas, too, have probably VOLCANIC PETROLOGY 57 been explored less intensively than the main area of hydrothermal alteration. VOLCANIC PETROLOGY SUMMARY Modal, chemical, normative, and spectrochemical analyses of 25 volcanic rocks have been presented in this report. Observations on thin sections represent- ing about 200 other samples of volcanic rocks show that the analyzed rocks are representative of the units they came from. The analyzed rocks include samples from the oldest and youngest, from the most mafic and most felsic, and from the most altered and least altered of the widespread volcanic rocks in the area. They also include samples from mapping units considered the same though separated by several miles, and from map- ping units considered different though superposed. But in spite of the number of ways these analyzed rocks can be regarded as different, their mineral and chemi- cal compositions are strikingly similar. The volcanic rocks are virtually all porphyritic ande- site. Most contain plagioclase, oxyhornblende, and biotite phenocrysts, and many contain minor quartz. Under a microscope, plagioclase microphenocrysts, orthopyroxene, and clinopyroxene also are visible. The groundmass is slightly crystalline to glassy. The chemical compositions and the normative values further indicate that there is little difference. The range of SiO,; percentages in the analyzed suite is about 12 percent, and all but three samples fall within a range of 6 percent. The ranges of other major and minor components are similarly small. The alkali-lime index is about 58, indicating the suite to be calc-alkalic. Seventeen of the analyzed rocks have C.LP.W. norma- tive compositions of yellowstonose (1.4.3.4.), six are tonalose (II.4.3.4.), and two are lassenose (I4.24.). Even within this small range of rock compositions, though, most major and minor elements show percent- age variations that are clearly interrelated. All late Cenozoic volcanic rocks in the Lava Moun- tains area are part of one suite. This is indicated primarily by the compositional similarity of the rocks, and is strongly supported by field evidence which shows that these deposits were all formed within a relatively short period of geologic time, and were areally restricted to one small part of the Mojave Desert area. The evidence that these rocks are part of the same suite means that they are also genetically related, and that the differences and similarities in the nature of successively erupted rocks reflect processes that gen- erated the magma and transported it to the surface. Petrographic and petrochemical evidence suggests that the magma was generated as follows: Sometime prior to the deposition of the middle Pliocene Bedrock Spring Formation, differentiation of mafic magma pro- duced a magma of more felsic composition. By the time of the first eruption of this new magma, though, differentiation had virtually ceased, and diffusion and mixing were causing it to become progressively more homogeneous. The small amount of compositional vari- ation in the erupted rocks is chiefly a result of this early differentiation. Between middle and late Pliocene time, a magma of similar composition was added to the magma chamber; this resulted in some resorption of the crystals already formed, but did not cause any de- tectable shift in the chemical composition of subse- quently erupted rocks. The process that transported the magma to the sur- face is indicated by three lithologic variables correla- tive with age: (a) Most of the earlier volcanic rocks are products of explosive volcanism, whereas the later volcanic rocks resulted from effusive activity; (b) the fragmental volcanic rocks become more homogeneous in successive deposits; and (c) interbedded epiclastic rocks form thinner units in progressively younger vol- canic formations, showing that less time elapsed be- tween successive eruptions. These progressive changes are interpreted to mean that between middle Pliocene and early Quaternary times, additional conduits were formed by faulting, the rocks surrounding the conduits became heated, and eruptive pressures within the magma chamber increased. These processes combined to allow the molten rock to reach the surface more and more frequently even though solidified and brecceiated in the process, and ultimately they permitted it to reach the surface still fluid. PETROGRAPHY ORIGIN AND SIGNIFICANCE OF YOLCANIC LITHOLOGIES Most of the layers of tuff, lapilli tuff, and tuff breccia in the Lava Mountains area were probably deposited as mud flows (lahars) shortly after their components were erupted, although some may represent volcanic debris that was deposited in place. The layers are well indurated and massive, and contain appreciable percentages of both fine- and coarse-grained volcanic debris that is unsorted and angular. This debris was probably formed by at least three mechanisms: (a) Explosive eruption of volcanic material which formed a mantle of talus or airborne debris on the terrain; (b) autobrecciation in the volcanic neck of nearly solid lava (Curtis, 1954, p. 467-471) that was subsequently remobilized and extruded; and (c) extreme brecciation 58 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY of lava flows as they reached the late stages of solidi- fication. The individual layers of tuff, lapilli tuff, tuff breccia, and rubble breccia were probably deposited within a very short period of time. The few bedding planes that do occur within the layers of tuff and lapilli tuff sug- gest only a short delay in deposition, for the material that forms the lower contact appears to have been un- consolidated when deposition was renewed. Most of the coarser breccias are texturally massive. In a few of these layers, though, contacts defined by abrupt changes in rock colors suggest that their deposition may have been interrupted although these contacts do not show signs of an intervening period of erosion. The large fragments in the rubble breccias appar- ently formed when lava that solidified in volcanic necks was subsequently brecciated and extruded. The com- plete absence of volcanic bombs, bread-crust structures, and glassy edges among the component fragments means that the fragments did not solidify individually. Most rubble breccia layers do not contain pockets of very fine material or massive lava as if formed by late- stage brecciation of large lava masses or flows (Curtis, 1954, p. 462-464). Furthermore, they form layers too tabular to be considered talus formed on the flanks of a volcanic vent, yet contain so little fine material that they cannot be considered mud flows. Their properties are best explained if the lavas are considered to have been first solidified in the volcanic necks, subesquently brecciated, and then forced out as rubble. Brecciation may have resulted from autobrecciation (Curtis, 1954, p. 467), or from crushing as a result of great pressure from below. Material brecciated in this way, and sub- sequently transported to the surface, would consist of angular fragments intimately mixed with hot gases, hot water, or lava. This mixture might well have a mobil- ity that would allow it to flow far down the adjacent slopes to form tabular bodies. Matrix-free rubble breccia would result when the lubricating material was water and gas; flow breccia would result when it was lava. The massive volcanic flows in the Lava Mountains area were mostly of small extent. They formed by the extravasation of fairly viscous lava from many vents and probably did not extend more than 2 miles from their source (see p. 34). The flow breccia and flow conglomerate layers were formed by the same processes as were the flows, but contained angular or well- rounded fragments that were mixed with the lava prior to or just after its extrusion. The volcanic rocks found in the Lava Mountains area are distributed throughout the stratigraphic section so as to demonstrate a progressive change from explosive to effusive volcanic activity. The volcanics in the mid- dle Pliocene Bedrock Spring Formation are estimated to be about half tuff and lapilli tuff, and half volcanic breccia. The volcanic rocks in the lower half of the Almond Mountain Volcanics are sandy tuff, tuff, lapilli tuff, and tuff breccia, which generally grade upward into massive rubble breccia and flow breccia without any sign of a long break in the depositional history. The overlying very late Pliocene Lava Mountains Andesite consists of andesite flows and intrusives plus a few flow breccias. The Quaternary andesite consists entirely of effusive flows, plugs, and sills. An increase in the homogeneity of the extrusive frag- mental volcanic rocks also follows this chronological order. - Where the fragments in the pyroclastic breccias of the Bedrock Spring Formation are coarse enough to be identified as to rock type, they are found to include a large variety. In the lower part of the Almond Moun- tain Volcanics, a comparable variety of volcanic rock types is noted, but in the upper part, the breccias are nearly monolithologic. The Lava Mountains Andesite consists mostly of flows, but the fragments in the small percentage of flow breccias are generally all the same and commonly of about the same composition as the matrix lava. The extruded Quaternary andesites are all flows. The positions of the other late Pliocene ( ?) volcanic rocks in the chronological sequence are not clear, so it is not known whether or not they conform to this pattern. The frequency of volcanic eruptions probably also increased with time, grading from an intermittent ac- tivity in Bedrock Spring time to a point when such activity accounts for all the deposits formed in Lava Mountains Andesite and Quaternary andesite times. Thick beds of nonvoleanic epiclastic material separating beds of volcanic rocks presumably indicate long periods of quiescence between eruptions, whereas little or no epiclastic material, even around the outer edges of the area containing volcanic rocks, indicate very short pe- riods of quiescence. In the Bedrock Spring Formation, zones of epiclastic rocks separating the volcanic units are hundreds and even thousands of feet thick. In the overlying Almond Mountain Volcanics, zones of epi- clastic rocks are generally a few hundred feet thick, and most of them are in the lower half of the section. The overlying Lava Mountains Andesite and Quaternary andesite contain no interbedded epiclastic rocks. VOLCANIC ROCK TEXTURES The volcanic rocks of all formations contain both megascopic and microscopic phenocrysts. All contain megaphenocrysts of plagioclase and most include bio- tite, oxyhornblende (or hornblende), and opaque min- VOLCANIC PETROLOGY 59 erals; some quartz is visible in most rocks. The micro- phenocrysts are generally plagioclase, pyroxene, and opaque minerals. The groundmass consists of small needlelike crystals of plagioclase ( ?), ecryptocrystalline material, glass, and a dust of opaque minerals. The plagioclase megaphenocrysts commonly are as large as 6 mm in. the long dimension, biotite and oxyhornblende (or hornblende) 3 mm, quartz 2 mm, and pyroxene crystals 0.5 mm. - The average sizes are usually a quar- ter to a half of these lengths. Plagioclase micropheno- crysts have average lengths between 0.1 and 0.2 mm. In many rocks some minerals are alined. Plagio- clase microphenocrysts and hornblende phenocrysts are commonly parallel; plagioclase megaphenocrysts, py- roxene, and biotite are rarely so. Many of the rocks contain vesicles, most commonly between 0.1 mm and 0.5 mm in diameter, which rarely exceed 5 percent of the rock. A few of these vesicles show a tendency to be elongated, but most are nearly spherical. DESCRIPTIONS AND PROPERTIES OF ROCK CONSTITUENTS PLAGIOCLASE Composition. some estimates of plagio- clase compositions were made by means of refractive indices, most were made by measuring extinction angles in sections approximately normal to (010) and (001). These measurements provide an estimate only of the minimum anorthite percentage in the part of the crystal that was measured. The An percentages indicated throughout this paper were estimated from such data applied to the curves constructed by Tertsch (1942, p. 209) for high-temperature (disordered) plagioclase crystals oriented normal to (010) and (001). Plotting these angles on similar curves for the low-temperature (ordered) feldspars would increase the indicated anorthite by 8 to 12 percent. Even though the method used may not give the actual chemical composition of the phenocrysts very accurately, it gives at least a roughly quantitative measure of the range of plagio- clase composition in individual crystals, and also per- mits a semiquantitative comparison of the compositions of similarly oriented crystals. Most of the plagioclase phenocrysts in these rocks are apparently high-temperature (relatively disordered) crystals. F. C. Calkins carefully studied a dozen repre- sentative thin sections, and summarizes the results of his study of magaphenocrysts as follows : The only simple way that I know of to get evidence regarding thermal state solely from thin sections on the flat stage is to measure concurrent angles in Carlsbad-albite twins cut nearly normal to a. * * * Carlsbad twins are not very numerous in these thin sections, and some of the phenocrysts that look like 735-720 0O-64--5 Carlsbad twins may be only coalescing crystals. I have found a moderate number that I believe to be Carlsbad twins, but only a few of these are cut nearly normal to a. One of these is in section 29-6. This I feel sure is a Carlsbad twin, though in part distorted. The albite lamellae are fairly sharp, and distinct basal cleavage is visible in one of the two Carlsbad individuals, showing that the crystal is cut, in that individual, very nearly normal to @. - Most of the inner part has the concurrent angles 30° and 12°. In a low-temperature plagio- clase this combination gives, on my diagram for Carlsbad twins in low-temperature plagioclase, the composition and a value for lambda of 45°. The latter figure is in conflict with the evidence afforded by the thin section, but on Tertsch's (1942) diagram for high-temperature plagioclase these extinction angles give, approximately, An, and lambda 65°, and this value for lambda is just about what it should be in a crystal cut normal to a. I regard this as evidence that that particular phenocryst was formed at high temperature, as is inherently probable. I got similar evidence from three crystals in two other slides. From this evidence, as far as it goes, I think it likely that the plagioclase in these slides is mostly, and perhaps all, high temperature. The results of my extinction angle measurements on about a thousand megaphenocrysts show that about 5 percent of the crystals have apparent compositions in at least one zone that are as high as Any;;, so that their true compositions are probably a little higher. Refractive index measurements indicate compositions below Any; for about 5 percent of the measured grains. A com- positional spread of at least An,, is thus likely. The observed range in apparent 2V confirms this spread. For each volcanic unit, the average of several hun- dred extinction angle measurements is within 4° of the others. This probably means that the average anorthite percentages are separated by less than 5 percent. These indicated average compositions are close to Any,. According to Tertsch's high temperature plagioclase curves (1942, fig. 3), the errors introduced by measur- ing sections close to but not normal to (010) and (001) are fairly small, and it is improbable that the true An percentage is more than 10 percent higher. The plagioclase megaphenocrysts and micropheno- «erysts in most of the volcanic rocks of this area thus have an average composition of calcic andesine. In- dividual megaphenocrysts in specimens from this vol- canic suite may differ by more than 30 percent anorthite content, and those in a single thin section by more than 20 percent, but the average anorthite content of all the plagioclase in any two units probably does not differ by more than about 5 percent. It is nearly impossible, however, to determine the overall composition of even a single phenocryst accurately in a thin section, because most of the phenocrysts are corroded, strongly zoned, and mottled. This uncertainty would exist even if there were an undeviating correlation between composition and extinction angles, and we were certain of the ther- 60 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY mal state of the plagioclases in these rocks, neither of which is true. The compositions given throughout this paper must therefore be regarded as estimates. The composition of the microphenocrysts is even more difficult to determine because they are not commonly twinned. The few determinations made, some of which are supported by measurements of the refractive index, suggest that microphenocrysts with an average com- position have 2 to 5 percent more anorthite. Zones.-The plagioclase megaphenocrysts are con- spicuously zoned in almost all thin sections of these volcanic rocks. The most common type of zoning is oscillatory normal, although normal and reverse zones are found. Strong mottling is present in the inner parts of many megaphenocrysts. This mottling is evident in thin sec- tions observed with crossed nicols, but is not visible in plane-polarized light. Its origin is not known, but it evidently reflects inhomogeneities in the crystal com- positions. Though common in megaphenocrysts, it is not found in their calcic rims or in microphenocrysts. Many of the megaphenocrysts have calcic rims (fig. 22A). This feature seems unusual, but it has been observed by Larsen and others (1938a, p. 233 and fig. 17) in volcanic rocks of similar composition from Colo- rado. - In the rocks collected from the Lava Mountains, these rims occupy the outer 5 to 15 percent of the crys- tal radius. They are always clear, whereas the inner parts of the crystals are commonly corroded and re- sorbed. The rim usually shows a small amount of smooth normal zoning. - Extinction angles indicate that the rim contains 5 to 10 percent more anorthite than the feldspar just inside it, and that it has approximately the same composition as the inner core. This inference is confirmed by the high refractive indices of the rims. Calkins (written commun., 1962) says of them, "As you have already noted, many of the phenocrysts have rims in which the extinction angle is larger than that of the plagioclase adjoining them on the inner side. I find by means of inclined illumination that these rims all have distinctly higher indices of refraction than the plagioclase inside them, which indicates that they are actually more calcic, not merely of different thermal history." The megaphenocrysts in a given thin section usually differ considerably in character of zoning, and there may be many zones in one phenocryst and few in an- other one nearby. In most of the slides, one- to two- thirds of the plagioclase megaphenocrysts have calcic rims, but even among this selected sample, the inner crystals have no zonal characteristics in common. Most of the microphenocrysts show a very faint smooth normal zoning in which the An content has a range of less than 5 percent. T winning.-Albite twins are visible in all these plagioclase crystals (fig. 224, D,). Most of the twin lamellae are not notably even or uniformly developed. In some crystals, the zones interrupt these twin lamellae, and in others they do not. Most of the twins that ap- pear to follow the Carlsbad law have an undulating twin plane. Other twin types are noted ; of these, peri- cline twins are the most common. Many twins that appear to follow the Carlsbad law could not be verified, so the extinction angles of Carls- bad twins in conjunction with albite twins were not used routinely as a means of estimating plagioclase composition. This uncertainty stems from the rela- tions observed among the small plagioclase erystals, where once-independent crystals have in a few instances become attached on their side faces; if this happened during the early growth stages of the megaphenocrysts, and those crystals had continued to enlarge while at- tached in that position, the results would resemble a Carlsbad twin, but its components would of course lack the precise angular relation necessary to make the ex- tinction-angle curves valid. Size and habit.-The plagioclase crystals in most of the volcanic rocks from this area are divided into two groups, and the crystals of each group contribute about equally to the total volume percent of plagioclase in these rocks. One group includes the larger crystals, which are referred to throughout this report as mega- phenocrysts; besides being larger, they characteris- tically also have strongly developed zones, round cor- ners, and length-to-breadth ratios between 1 : 1 and 3 : 1. The other group includes the smaller crystals, which are referred to as microphenocrysts; these crystals charac- teristically have weakly developed zones, square corners, and length-to-breadth ratios between 4:1 and 10:1. The properties of these two groups are illustrated by figure 227. For convenience, individual crystals have been assigned to groups on the basis of whether their lengths are greater or less than 0.3 mm, but in almost all instances this criterion separates crystals with the other distinctions as well. Megaphenocrysts tend to form crystal clumps in which some of the member crystals are nearly perfect in shape and others have only partially developed faces. A typical cluster of such crystals is seen in figure 224. If a calcic rim is present, it invariably envelopes the en- tire group rather than any one of the member crystals, indicating that the clumping occurred prior to the last crystallization of megaphenoccryst feldspar. Micro- phenocrysts are generally separate. VOLCANIC PETROLOGY 61 FicurE 22.-Photomicrographs showing selected mineral relations. Four of the five are samples from the Lava Mountains Andesite; one (C) is from the Almond Mountain Volcanics. The scale at the base of each photograph is in millimeters. A, a cluster of zoned plagio- clase crystals (in sample 97-38-0) showing a calcic rim (CR) and an underlying zone of inclusions (I), both surrounding the entire group of crystals. Note also the variation in the zoning pattern of the component crystals. Crossed nicols. B, quartz crystals in a glassy to semicrystalline groundmass (in sample 27-14). The nearly straight edges are believed to indicate that these were euhedral crys- tals that grew in the melt. Crossed nicols. C, biotite and plagio- clase crystals (in sample 124-22J). The biotite has been altered to opaque minerals along the edge not protected by plagioclase. This indicates that the alteration occurred after the two minerals had become attached. Plane-polarized light. D, typical thin section of Lava Mountains Andesite. Note the two sizes of plagioclase crystals (megaphenocrysts and microphenocrysts) and the differences in their dimensions. Large irregular crystal in upper right is partly resorbed quartz. Crossed nicols. F, microperlitic fractures in a glassy sam- ple of a Lava Mountains Andesite flow (sample 128-5). Note that the crystals are sharp edged and unaltered. Plane-polarized light. 62 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY Inclusions.-Almost all crystals of plagioclase, espe- cially the megaphenocrysts, have formless or runic in- clusions of clear or yellowish glass. Small acicular or prismatic apatite crystals, generally less than 0.1 mm long, are also common. Inclusions of opaque minerals (probably magnetite), oxyhornblende, pyroxene, bio- tite, chalcedony (?), zircon (?), and rutile(?), are less commonly found. ; Alteration.-In most of the rocks studied, the feld- spar is not heavily altered. Evidence of slight altera- tion and partial resorption is commonly found within a crystal. Alteration that affected the outermost rims of crystals is rare. - Only in the propylite and subpropylite have the feldspar been extensively replaced. In these, the feldspar crystals have been replaced chiefly by cal- 'cite, usually in the form of stringers laced through the crystal ; but secondary albite, sericite, and fibrous aniso- tropic minerals are also noted. BIOTITE Composition.-The pleochroic formula and the re- fractive index of flat-lying cleavage flakes of relatively unaltered biotite crystals suggest that most of them have compositions that appoach annite, the high Fe, low Mg member of the group. The pleochroism is strong, and the refractive index (y') ranges from 1.67 to 1.73. Color and pleochroism.-Pleochroism is normally strong, the most common formula being as follows: X=grayish orange (10VR 7-8/4-6). Y=pale brown (5YR 4/2). 7Z=-grayish brown (5YR 3/2). In thin section, the biotite in these rocks is commonly darker along cleavage planes or fractures. The darker parts of the biotite flakes have a higher refractive index than the lighter parts showing that the color and index are related effects. Size and habit.-In most rocks, biotite forms hexa- gonal packets of thin plates, measuring 1 to 2 mm in width. Some rocks, such as the late Pliocene ( ?) felsite, contain biotite only as very thin wafers or shreds. Inclusions.-Inclusions of opaque material are com- mon (fig. 220). Many of these appear to be magnetite. Plagioclase, twinned and zoned, is also found, and some of the plagioclase inclusions themselves contain in- cluded magnetite. A few biotite crystals also contain oxyhornblende and pyroxene but it is possible that they are alteration products rather than original inclusions. Alteration.-Deuteric activity has partially to com- pletely altered the biotite crystals in the majority of rocks. The first phases of alteration have darkened the crystals and formed equant opaque minerals (magne- tite?) around the outer rim of the erystal or embedded in its edges. More intensive alteration commonly ob- literated the euhedral outline of the opaque minerals. This alteration was a process that took place after most of the lava was solid; where biotite and plagioclase are in contact, there has been no alteration of the protected biotite edges, indicating that the crystals were fixed in this position prior to deuteric action (fig. 220). Many mineralogical source books do not acknowledge the existence of altered biotite with a cleavage-flake (y') index of more than 1.70, although. both field and labora- tory studies have reported examples of it. For exam- ple, field studies by Larsen and others (1937, p. 901) report that biotite crystals from rocks such as latitic andesite have y indexes ranging from 1.70 to 1.73. Laboratory studies by Kozu and Yoshiki (1929, p. 181-182) proved that high-index biotite could be formed by heating normal crystals (y=1.655) to tem- peratures of 1,000°C and apparently at 1 atmosphere pressure. In their experiments, the biotite began to change index appreciably at about 400°C and continued at a fairly uniform rate to the highest temperature measured. They found that at 900°C, y=1.700, and at 1,000°C, y=1.703. They also found that naturally oc- curring high-index biotite showed no marked change in properties within this range of heating. Annite has been synthesized by Eugster (1956, 1957), Ostrovskii (1956), and Eugster and Wones (1962) at vapor pressures between 500 and 4,000 bars, and tem- peratures between at least 400°C and 840°C. The position of its stability field is slightly sensitive to changes in water pressure and total pressure, but is highly sensitive to changes in temperature and oxygen partial pressures. Iron oxides, the common alteration products in the volcanic rocks of the Lava Mountains, are produced by a decrease in temperature, or increases in the partial pressures of oxygen which depend partly on the quantity of water in the system and the extent to which it is dissociated. With falling temperatures, the oxygen supplied by dissociation of water would nor- mally decrease, and additional oxygen must be supplied. In lavas, this may be accomplished by the relative in- crease in the amount of water available for dissociation when the groundmass solidifies, or by the introduction of oxygen from air. The shallow intrusive volcanic rocks in the Lava Mountains were probably not formed in a zone reached by atmospheric oxygen; the biotite in some of those rocks is altered, although generally less altered than in extrusive rocks, and this suggests the oxygen is derived from both sources. The darkening of biotite probably also took place at the time of alteration. In hornblende, the change to oxyhornblende accompanies the oxidation of Fe and loss of OH ; as the optical changes during alteration of VOLCANIC PETROLOGY 63 hornblende and biotite are similar, the chemical changes are probably also similar. Although it seems para- doxical that OH ions should be lost during a period of increasing water percentages and decreasing tempera- tures, this is interpreted to be a secondary result of oxidation; as Fe* is oxidized, some of it is removed from the biotite crystal lattices to form dispersed ion oxides, and the OH chemically bonded to this Fe is probably lost at the same time. AMPHIBOLE Composition.-Hornblende, which for emphasis in this report is sometimes called green hornblende, is found chiefly in the dikes and sills of Quaternary ande- site. The mineral is found only locally in extrusive rocks (such as the perlitic layer of Lava Mountains Andesite described on p. 34 and illustrated in fig. 223). Most of the hornblende crystals have extinction angles of about 15°, and the refractive indices are near a=1.68 and y=1.70. In the volcanic rocks of the Lava Mountains, oxy- hornblende is the most abundant amphibole. These oxyhornblende crystals have extinction angles between 5° and 10°, deep coloration, and refractive indices higher than those of hornblende. Although there seems to be no quantitative correlation between specific optical-property variations and specific composition changes, compared to hornblende, oxyhornblende is generally depleted in (OH) and enriched in Fe® (at the expense of Fe). The distinction between oxyhornblende and horn- blende is somewhat arbitrary as there seems to be little agreement among mineralogical source books about where the line should be drawn. With regard to oxy- hornblende, Winchell and Winchell (1951, p. 437) cite theoretical compositions of four "end members," none of which contains any Mg; the extinction angle (Z Ac) ranges from 0° to 15°, and y is as high as 1.80. Rogers and Kerr (1942, p. 287) state that the extinction angle range is 0° to 12° and that y is as high as 1.76. Larsen and Berman (1934, p. 221) use the equivalent term "basaltic hornblende" for "those members containing appreciable Ti with less Mg+Fe than hastingsite and less (OH) than the normal amphiboles." The optical properties of minerals so defined vary; extinction angles range from 0° to 21° and y is as high as 1.718. The minerals included by their definition contain more Mg than Fe. In the Lava Mountains rocks, the most conspicuous change in the pleochroic formula occurs between crys- tals having extinction angles just above and just below 10°. Because color is one of the more obvious differ- ences between the two amphiboles, but is difficult to measure quantitatively, the associated extinction angle of 10° has been used here as the dividing line between the two. Color and pleochroism.-The hornblende found in these rocks is generally green or straw colored in thin section. The pleochroism is marked but not intense. The oxyhornblende is more deeply colored and in- tensely pleochroic; its average pleochroic formula is as follows: X=light moderate yellow (5Y 8/6). Y=dark yellowish orange (10 YR 6/6). Z=moderate brown (5YR 4/6). All intermediate variations are found. Size and habit.-Both hornblende and oxyhornblende are found as elongated prismatic erystals, which in cross section have a lozenge shape. They are rarely more than 3 mm long and most are between 0.5 mm and 1.5 mm long. Inclusions.-Inclusions are common, the most fre- quent being euhedral opaque minerals, probably mag- netite. Biotite, apatite, clinopyroxene (containing magnetite?), and plagioclase (possibly of secondary origin) are also common. Alteration.-The most common deuteric alteration products are opaque materials. - Others are plagioclase, epidote ( ?), and fine-grained pyroxene (?). In one rock the green hornblende was partly altered to chlorite (penninite?) and calcite. It seems clear, as a result of investigations by several mineralogists, that in the vicinity of 750°C, common hornblende changes to oxyhornblende at atmospheric conditions (Belovsky, 1891, p. 291; Weinschenk, 1912, p. 292; Graham, 1926, p. 122-123; Kozu and Yoshiki, 1927: and Kozu, Yoshiki, and Kani, 1927). In the studies by Kozu and his associates, common hornblende was subjected to ever-increasing heat in an atmosphere of air. Between 700° and 800°C, the extinction angle diminished from 12° to 0.3° and y increased from 1.687 to 1.720. Above and below these temperatures the changes were slight. At about 50°C, a marked increased in the rate of weight loss (calibrated as per- cent of weight loss per degree centigrade) was also noticed. Oxyhornblende, also subjected to similar treatment, showed little change in optical properties when heated, although the rate of weight loss changed slightly at 750°C. Synthetic hornblende with the optical properties of oxyhornblende has not been made, but probably the experimental data for biotite noted above are qualita- tively applicable; in both minerals, the optical changes produced by deuteric alteration are similar, and ap- parently the OH ions bonded to the Fe? ions are re- leased when the Fee*? is oxidized. 64 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY Field evidence in the Lava Mountains shows that hornblende crystals commonly existed in volcanic mag- mas up to the time of their extrusion, but that shortly thereafter they were changed to oxyhornblende. For example, this is shown by the flow of Lava Mountains Andesite in section E9-a, which consists partly of horn- blende-bearing perlite and partly of oxyhornblende- bearing red-brown andesite (see p. 34) ; the hornblende- bearing perlite apparently represents a chilled basal layer and thus approximates the mineralogical com- position of the lava immediately after its extrusion. Judging from the experimental data cited above, the temperature of this flow must have still been above 750°C at the time of extrusion (or was raised above this temperature by oxidation shortly afterward) inas- much as most of the hornblende crystals in the flow were changed eventually to oxyhornblende, but the other conditions necessary to promote alteration were fulfilled only in the very late stages of solidification. ORTHOPYROXENE The orthopyroxene in these rocks is hypersthene. In all the rocks examined, the orthopyroxene is optically negative and the optic angle is medium to large. In most rocks, the crystals are slightly pleochroic, the formula being: X=very pale orange (10YR 8/2). Y=Z=-clear, to very slightly bluish tint. Most of the erystals are stubby prisms that have well- developed prism faces and very faintly developed terminations. Many contain inclusions of euhedral opaque minerals, probably magnetite. Some crystals are stained orange or light brown around the edges, and such colors commonly form a band around the inner edge of the opaque alteration products. Where deuterically altered, the mineral has changed to a fine-grained opaque material or a fibrous greenish material, probably bastite. Experimental data that outline conditions under which pyroxenes alter to iron oxides are not available, but presumably an in- crease in the partial pressure of oxygen is one change that would promote this reaction. As temperatures drop below 500°C, pure enstatite alters to serpentine (bastite) at pressures below 2,000 bars and in an excess of water vapor (Bowen and Tuttle, 1949, p. 453, fig. 2). CLINOPYROXENE The volcanic rocks in the Lava Mountains generally contain only a few tenths of a percent clinopyroxene. In some rocks, the optic angle and extinction angle suggest that the clinopyroxene is pigeonite, whereas in others, augite or diopside are indicated. Clinopyroxenes are found in three habits. The most common habit is the same as that of the orthopyroxenes, namely, small stubby prisms. The second, which is much less common, is clusters of small, well-terminated crystals that converge inward from the walls of a cavity. The third is a band of small anhedral crystals form- ing a reaction rim around partially altered crystals of quartz. Some erystals contain inclusions of euhedral mag- netite(?) crystals. Where deuterically altered, clino- pyroxene generally is changed to fine-grained opaque minerals or to bastite. QUARTZ A fraction of a percent of quartz is found in most volcanic rocks in this area. Its most common form is that of irregularly embayed fragments. In many thin sections these fragments have a uniform reaction rim of fine-grained clinopyroxene, whereas in others they are free of such rims. The development of this reaction rim is generally consistent within a single sample, but it may be thick, thin, or absent in successive samples from the same mapping unit. In a small percentage of specimens, nearly euhedral crystals of quartz are pre- served (fig. 2225), indicating that this mineral formed during early stages of crystallization; in most speci- mens, though, quartz crystals are largely resorbed, showing that quartz did not remain stable during later stages. Many quartz crystals contain round or oval-shaped inclusions of yellowish or clear glass, and very small needles or prisms of apatite. OPAQUE MATERIAL Four types of opaque material are present: (1) Eubedral equant crystals that appear as squares or polygons in thin section, probably magnetite, although ilmenite may be present in appreciable percentages; (2) irregular dustlike fragments that appear black in conoscopic light; (3) clumps of equidimensional fine- grained material that most commonly are semiopaque and appear a deep red in conoscopic light; and (4) fibrous and hairlike fragments that are deep red or brown in conoscopic light. Most of these are presumed to be iron oxides. OTHER MINERALS Minerals that were noted but not specifically de- scribed above include the following: chlorite, calcite, zeolite, apatite, opal, chalcedony, cristobalite, K-feld- spar, sericite, alunite, epidote, and bastite. Minerals that are tentatively identified are: tridymite, chloro- phaeite, serpentine, and olivine. VOLCANIC PETROLOGY 65 GROUNDMASS Most rocks in this area have a hemicrystalline ground- mass that consists predominantly of microcrystals, crystallites, cryptocrystalline material, and glass. The glass is most often clear, slightly yellowish, or greenish. The crystallites and microcrystals appear to be plagioclase, although no definitive optical properties could be measured. Opaque materials generally form a light to heavy "dust." In figure 23, the refractive indices of the glassy portions are plotted against the silica percentages of 1.54 x 6 2 . * § 20 o o w o > 1.52 o o 0 o - o [ 9 o s.°¢, s o 1.51 0 t a 0 o 1.50 4 t T T t T 1 60 62 64 66 68 70 72 WEIGHT PERCENT SiO, Ficurs 23.-Diagram showing the relation between the SiO; content of the whole rock and the refractive index of the glassy part of the groundmass. The analyzed samples of volcanic rocks older than the Bedrock Spring Formation, subpropylite, and the propylite contained no glass. the whole rock. All the indices are between 1.50 and 1.53. According to George (1924, p. 365), this would indicate a silica range of 57 to 72 percent. This range of inferred silica percentages happens to be about right, but there is little correlation between the refractive index of the glass and the silica percentage of any one rock. An appreciable percentage of silica minerals and K- feldspar are commonly found as submicroscopic min- erals in volcanic rocks from other areas. X-ray diffrac- tion of the analyzed volcanic rocks from the Lava Mountains area, though, shows that with one exception, cristobalite and clays are the only detected minerals not also present as larger crystals reported in the modal analyses. The exception is the analyzed sample of upper Pliocene( ?) felsite which also contains large per- centages of submicroscopic K-feldspar ; that rock thus might be classified as a dellenite (or quartz latite). Probably most of the cryptocrystalline material in rocks is cristobalite, although some of it may consist of highly imperfect molecule-like arrangements of other rock-forming components that appear slightly aniso- tropic in polarized light but do not cause distinctive < X-ray diffraction patterns. MODAL COMPOSITION OF THE SUITE Average modal compositions of the analyzed volcanic rocks in the three major volcanic formations are listed in table 11. The modal composition of one sample of volcanic rock from the Bedrock Spring Formation is added for comparison. In rocks from all four forma- tions the groundmass is the most abundant material and forms from 60 to 90 percent of the total volume, aver- aging about 70 percent. Plagioclase is invariably pres- ent; the total volume in individual volcanic rocks varies from 10 to 30 percent, but in all four volcanic rock- bearing formations, the average percentage is very close to 20. Of this percentage, megaphenocrysts generally constitute one-third to two-thirds of the total. Biotite locally constitutes over 2 percent of the rock but more generally about 0.5 percent. Oxyhornblende or horn- blende make up as much as 10 percent of fresh rocks, but the average is less than 1 percent; it is typically about twice as abundant as biotite, but either mineral may be absent. Orthopyroxene and clinopyroxene form less than 2.5¢ percent of every rock and may be locally absent. Quartz exceptionally forms over 3 per- cent but the average is less than 1 percent. Opaque materials are invariably present and constitute between 1 and 11 percent of the mode total ; these figures, how- ever, are probably too high because of the tendency for fine-grained opaque minerals embedded within the thin section to be projected onto the top surface, the sur- face on which the point count is theoretically being made. Other minerals make up as much as 6 percent of the rock but average only a few percent. MODAL CHANGES WITH AGE The purpose in comparing the modal compositions of successively deposited volcanic rocks is to find if any changes are sequential. Figure 244 shows in strati- graphic order the modal composition of individual specimens collected from the type section of the Almond Mountain Volcanics and the overlying cap of Lava Mountains Andesite. Figure 242 shows a comparable set of modal analyses of rocks from the same formations collected about 1 mile north of Dome Mountain. These stratigraphic sections are presumably typical, and data from them illustrate the amount of modal variation (a) between successive volcanic strata of the Almond Mountain Volcanics, (b) between the eastern and western facies of the Almond Mountain Volcanics, and (c) within a single flow of Lava Mountains Andesite. The percentages of amphibole, biotite, and pyroxene in the modal analyses are determined not only by the percentages in the original lava, but also by the inten- sity of deuteric alteration. Hornblende, oxyhorn- blende, and biotite in these rocks are more readily altered than orthopyroxene and clinopyroxene; but in some rocks, the modal percentages of all are greatly reduced and the percentages of opaque minerals corre- spondingly increased. Of the upper three samples plotted in figure 244 two are relatively unaltered, and 66 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY TaBus 11.-Averaged modal compositions of volcanic rocks from the Almond Mountain Volcanics, Lava Mountains Andesite, and Qua- ternary andesite. [All values are volume percent. 17, average or mean (Elév >; s, standard deviation< Modal composition of one sample of volcanic rock from the Bedrock Spring Formation added for comparison ®(X-2) PY. ally P mee A f XH ] >, z, standard error ((N—)1/2)’ where X=percentage of mineral in indi- vidual sample and N=number of samples analyzed] Bedrock Almond Mountain Volcanics Lava Mountains Andesite Quaternary andesite Spring (9 samples) (9 samples) (2 samples) Mineral Formation (1 sample) o>. 4 WP 3 hes s T 8 82 % 8 82 T 8 82 Plagioclase megaphenocrysts______ 15. 1 9. 8 5. 5 1:8 10. 2 2. 9 1. 0 10. 2 0. 71 0. 50 Plagioclase microphenocrysts . _ _ __ 5. 1 10. 7 3.1 1. 0 10. 7 4. 1 1. 4 10. 4 . 99 70 (Total plagioclase) ___________ (20. 2)) (20. 5) 4. 5 1. 5 (20. 9) 4. 2 1. 4 (20. 6) 1:7 1. 2 PIOMLE. 2 ee nen ee oc ana nee nan aes : 6 w . 69 23 : . 20 . O7 76 . 36 .. 25 6. 3 0s relo Trace -~9 1.8 . 90 Oxyhornblende.:.:..-_._-___.:.:. 0 . 6 . 36 A2 1. 6 2. 5 +B 24 10 O7 Orthopyroxene. 1. 2 1: 3 sid . 26 ay 4 . 82 $27. +2 & 14 . 10 0 hess . 24 . 08 . 5 . 66 22 va 10 . 07 Quarta: Ls 2a l.. Bbc loll 0 va £87, 42 5 1.50 8 . 4 22 .16 Opaque 1. 5 4. 1 3. 1 1. 0 5. 9 3. 4 1. 2 3. 0 1. 0 1. 4 Other minerals. >-.._. . iA .9 1. 5 . 5 e/ . 87 . 29 5. 3 99 - 70 }}, 70. 2 113 5 7 1.9 69. 0 4. 1 1: 4 68. 6 1. 5 1. 0 one is highly altered. The contrasts between their | distributions. The statistical data for the Almond mafic mineral percentages probably represent the max- imum variation in these rocks that can be attributed to alteration. Figure 244 suggests an upward decrease in the per- centages of microphenocryst feldspar, orthopyroxene, clinopyroxene, and groundmass, and an upward in- crease in the percentages of megaphenocryst feldspar, oxyhornblende, and opaque minerals. All but two of these trends must be regarded as chance or a product of local conditions, however, because the stratigraphic section plotted in figure 242 does not repeat them. The upward increase of the opaque minerals is repeated in both sections, but this must only reflect alteration dif- ferences because it is not correlative with increasing iron or titanium percentages in the same rocks. The upward decrease in orthopyroxene percentages, how- ever, is repeated in both sections. This trend is prob- ably real because it shows better than normal consist- ency and is partly supported by the averaged data described below. The averaged modal compositions listed in table 11 show the volcanic rocks in successive formations to be quite similar. The relatively large sizes of the stand- ard deviations and standard errors are chiefly results of the small number of samples analyzed, but they also indicate that appreciable modal variation does occur within these rock units, and that the small differences between these averages might not be confirmed by addi- tional analyses. Figure 25 compares graphically the average values of each component in the three major volcanic formations, and shows their 50-, 75-, and 90- percent confidence limits-the limits that will, with these probabilities, enclose the true average if the com- ponents of these formations have statistically normal Mountain Volcanics and Lava Mountains Andesite are based on nine samples from each; those for the Qua- ternary andesite are based on only two, so they have uncertain significance when compared with those from the other two rock units. However, they permit ap- proximate limits to be set on the probabilities that the three successive formations actually have the averaged modal relations indicated, and on the range over which these averages might migrate with additional sampling. The average percentages of plagioclase megapheno- erysts, plagioclase microphenocrysts, and total plagio- clase are almost identical 'in the three volcanic formations. The slight differences between them are not significant. The confidence limits plotted in figure 25 indicate a 75-percent chance that more extensive sampling would not separate the megaphenocryst or microphenocryst averages by as much as 4 percent, or the total plagioclase averages by as much as 5 percent, but the relation between such averages is not indicated. Some of the differences between average percentages of biotite, hornblende and oxyhornblende, clinopyrox- ene, quartz, opaque mineral, nonopaque mineral, and groundmass are much greater, and testing the differ- ences between them (Arkin and Colton, 1950, p. 127) indicates chances ranging from 50 to 99 percent of their being confirmed by additional sampling. However, some of the indicated differences reverse their trends with time, and others are appreciable only between two of the three formations. None are indicative of pro- gressive changes in the magma. The confidence limits show the ranges over which these averages might migrate with complete sampling. The orthopyroxene averages differ from the others because they show a trend. The chances of the indicated VOLCANIC PETROLOGY w C - y- 124-23 J{ E 0 Q c £ * a $ - 24-23 F- = 3 ' . X 3 124-23 B- \ | | | | | V4 \ t * t T T T 7 * I 124-22 G- ia E= | § 124-22 M{ 5 8 3 -£ s § 124-22 k- @, o E 8 al o > 124-22 | . E C. r- s microphenocrysts T T T T THT T|T TT TM T RST TT I T T (T) IIO 2|0 (I) 2 t!) 5 | 0 5 10} 0 5) 0 5| 0 5|0 5 10 0N 5] 60 70 80 - - ho- lino- on- Plagioclase Biotite b'i'e°n'é’e oééwé'en ggiog- EXIESX' Quartz Opaques - Jopaques Groundmass A Es -32 97-38 24 © CD /O f t o ¢ f t o P EA 3 iv \ 1 1 1 I i 1 1 a. & 97-38 L E $2 g ' 3 total c 2 b f w £ 9 3 2 - o7-ssk 8 fs C C ! 6 z= G 6 & & u $ a Q? 0 7= g> 97-38 I go , JQQ E t .$ < 97-38 F & T T T I I I I I I T [T JT 0 10 20 o: 's l0 's (o ~ 5 ado . S510 slo s 10 lo - 70 sol R ta Horn- Oxyhorn- | Ortho-| Clino- Non- Plagioclase Biotite blende blende pggox- gzgox- Quartz Opaques - ppaque Groundmass B FicurE 24.-Diagrams showing the variation in modes of rocks from the Almond Mountain Volcanics and Lava Mountains Andesite. The locations of samples are described in table 21. Data are from tables 5 and 8; values are in vol and from the west side of Almond Mountain. ume percent. 124-22k is from a Lava Mountains Andesite sill in the Almond Mountain. shifts being confirmed by additional sampling are 75 percent for the lower pair of formations, and 55 percent for the upper pair. This modal trend may indicate contemporaneous settling of these crystals in the magma, but may also result from the greater alteration of mafic minerals in younger rocks. The percentages of mineral involved are too small, however, to be detected by comparison of chemical analyses. PETROCHEMISTRY CHEMICAL COMPOSITION OF THE SUITE MAJOR ELEMENT AND NORMATIVE COMPOSITIONS The analyzed volcanic rocks in this suite contain between 60 and 72 percent SiO,. All but three of them contain between 62 and 68 percent SiO,. The narrow The radii of the circles represent the average analytical errors calculated in table 18. 4A, samples are from the type section of the Almond Mountain Volcanics The stratigraphic positions of the lower five samples are shown in fig. 11. Sample Mountain Volcanics. B, samples are from 1 mile north of Dome range of rock compositions is striking. Similarly re- stricted are the average Al,O, percentages, which lie mostly between 15 and 16 percent. The comparatively wide scatter of FeO, and FeQ percentages reflects deuteric alteration; a more useful estimate of the sim- ilarity between formations is furnished by the percent- ages of total Fe as Fe,0;,, which are mostly close to 4 percent. Individual MgO values range from 0.5 to 3.5 percent, but most are near the middle of this range. The CaO percentages mostly lie between 3.5 and 6 percent, the Na,0 percentages between 3 and 5 percent, and the K,0 percentages between 2 and 3 percent. In all but one rock, the percentages of CaO) and Na:0 both exceed the percentage of K0 ; in about half of these, the per- & Co GEOLOGY AND VOLCANIC PETROLOGY, Plagioclase megaphenocrysts 0 5 10 15 20 25 Plagioclase microphenocrysts Ta T T T T 0 5 15 20 25 .—|:.:&|— Qa @ 8 © o ._l—_EF:|_ TI 3 8 m 2 E _r‘_:3_:_“g Ta o. T T T T T 0 5 10 15 20 25 30 f—|—\ f } 254 gs S © ”£21“ TI am j fl— Ta T T T T T T 0 0.25 0.5 0.75 1.00 1.25 1.5 I $ errr frr 5 2 x 4 6.6 ¢ $ ‘ > es t_ ___ 5 € . | 1 p s 5 x ._lfl:_‘L4 Ta (oxyhornblende) £ 0 Orthopyroxene 3 Clinopyroxene 3 T T T T T 1 d 0.25 0.5 0.75 1.0 1.25 1.5 LAVA MOUNTAINS, SAN BERNARDINO COUNTY Quartz Opaque minerals Nonopaque minerals 3 Groundmass 3 t 55 60 65 70 75 VOLUME PERCENT Qa (hornblende) TI (oxyhornblende) EXPLANATION Calculated average=Z 50 percent T5 percent 90 percent Confidence limits The probabilities that the range indicated by the bar includes the true average, providing that the formations have a statistically normal distribution of their components Qa, Quaternary andesite Tl, Lava Mountains Andesite Ta, Almond Mountain Volcanics FicurE 25.-Diagram comparing calculated average percentages of components in the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite. Data from table 11. centage of CaO exceeds that of Na,O. The variation in TiO; and P;0,;, percentages is about 0.5; the variation in MnO is about 0.05. The percentages of CO; and total H;0 vary greatly because they depend chiefly on the degree of alteration. In figure 26, the percentages of SiO; in these volcanic rocks are plotted against the percentages of oxides of Limits calculated by the t-distribution method (Bennett and Franklin, 1954, p. 153-157). other major components. The points representing per- centages of Al;0;, total Fe as Fe;0;, Na:0, K,0, TiO;, P,0;, and MnO show the least scatter along a line; the points representing percentages of MgO and CaO show more. The points representing H,O and CO, percent- ages show no trends because they are dependent on al- teration. Although even the most systematic of these VOLCANIC PETROLOGY - ~ I E bees . f 5c“; Q. X C 3p *~ 00 < Z T AB T T "I T T T n 60 62 64 66 68 70 72 a 5 - o & a 44 S o h- £ 1 s 3 - 3 Z 5 i Zj 1 T T T T T T a 60 62 64 66 68 70 72 4— - a #7 o & ul & Z S 1A = 0 6 7 o C a *" © & > Z § .s: o 2 $% s §§ ' PJ 2: 3 - 2 T T T T T T 1 60 62 64 66 68 70 o7'2 £ 8:~ a ® bo & s $ o 2 $ B 4 @ a - o x & Z atk T T T T T T 1 60 62 64 66 68 70 72 SiO,, IN PERCENT 26.-Variation diagram showing the relations between the oxides of the major elements and silica. do not fall along a smooth line, they define statistical trends as well as most suites of analyzed rocks. This further corroborates the evidence that the volcanic rocks of this area are all members of a single suite. Excluding the point representing the upper Plio- cene(?) felsite, the data for all rock-forming oxides except one define curves that slope in one direction or the other. The exception is the curve for K,0. This is a significant exception, and its implications are dis- cussed in the section on "Volcanic petrogenesis." Figure 27 shows the interrelations between percent- ages of Na,0+K,0, CaO, MgO, and FeO in selected volcanic rocks of the area. All percentages, £ gé 1 C35 x = a. £) 9 T T T T T wig. | ay 60 62 64 66 68 70 72 ts ~ ©o o & T ~* 0—1 SB ~ 380 M- cy- = T T T" T T "3 | = 60 62 64 66 68 70 72 = <8 a & =& "'% * may T T T ~ T T 1 = 60 62 64 66 68 70 72 ® C "o s 9 a g pal *+ 4¥* x % a ® e - (@) z & ® Q. 0 * 3 T 1 G o s = & * * 0 T T T T T T 1 60 62 64 66 68 70 72 l a a & ul a. 2 1-4 0) 5 he (@ 8 o- o - se oefgb$x "' ~ 's o T T T T T T > 60 62 64 66 68 70 72 SiO,, IN PERCENT EXPLANATION $ Quaternary andesite 09 Almond Mountain Volcanics 3 (subpropylite) Upper Pliocene(?) felsite ® Almond Mountain Volcanics (propylite) 0 o Lava Mountains Andesite * Bedrock Spring Formation G Volcanic rocks older than the Bedrock Spring Formation Almond Mountain Volcanics (eastern facies) ® Almond Mountain Volcanics (western facies) Data from tables 3-10. except the upper Pliocene(?) felsite (pf) lie in a distinct group. A faint elongation is detectable in all three diagrams, and this probably reflects differentia- tion. The distribution of the points, however, is not correlative with the age of the rocks they represent, and the differentiation trends were apparently created before eruptions began. By extrapolation, the alkali-lime index (Peacock, 1931, p. 57)-that percentage of SiO at which Na,0+K,0=Ca0O-is about 58. According to Pea- cock's classification, these rocks would be calc-alkalic. Table 12 lists the alkali-lime index values for 14 other volcanic areas in western North America selected to GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY 70 'OT-4 'g 'p Sorge; wou; Suudg yooipog ou} Wo}; yo01 aq quod 'ojtsto; (;)ouooofq doddn squasordoat {d jujoq 'pojiit0 ort pozp1[4doid oSW vore sufejUNOJ¢ BA¥T o} WLIOJ sYooI oru®IIOA ut QS pute "O'od+0%4T °OUD UoomMjog suomt[oilojut pojootos Supmoys ste13¥;p UopJELIEA-'2Z _ osw - ox +0%eN .* "9 foaq +094 peg peo VOLCANIC PETROLOGY T1 represent the range of volcanic suites of this area. Com- pared to most of these, the Lava Mountains suite is relatively deficient in calcium. The C.L.P.W. norms of the analyzed rocks are listed with the chemical analyses in the section on "Stratig- raphy." Seventeen of the rocks are yellowstonose with a symbol 1.4.3.4.; six (four from the western facies of the Almond Mountain Volcanics, and two from the Lava Mountains Andesite) are tonalose with a symbol II4.3.4.; two (the Pliocene(?) felsite, and the sub- propylite from the Almond Mountain Volcanics) are lassenose with a symbol 1.4.2.4. Tasur 12. -List of alkali index values (Peacock, 1981) of 14 volcanic areas in western North America Alkali Area index Source of data Lava Mountains, Calif_____- _ 58 This report, p. 69. Newberry volcano, Oreg__-- 58 Williams (1935, p. 297). Steen's Mountain, -- 58 Fuller (1931); index com- puted by Williams (1935, p. 297). Markleeville, Calif________. 59.8 Curtis.! Neovolcanic zone of Mexico. 60 Williams (1950, p. 265). Medicine Lake Highland, 60.5 Anderson (1941, p. 401). Calif. Paricutin region, Mexico.... 61.5 Wilcox (1954, p. 315). Paricutin volcano, Mexico... 62 Do. Crater Lake, Oreg...__.__._. 62 Williams (1942, p. 153). Clear Lake area, Calif______ 62 Anderson (1936, p. 661). Mount St. Helens, Wash... 62. 3 Verhoogen (1937, p. 292). Mount Shasta, Calif____--_- 63.7 Quoted by Williams (1942, p. 153). Mount Lassen, Calif_______- 63.9 Do. Glendora area, Calif_______._ 65 Shelton (1955, p. 71). 65.2 Eaton (1957, p. 311. See footnote on page 74). Los Angeles basin, Calif ___. ' Curtis, G. H., 1951, Geology of the Topaz Lake quadrangle and eastern half of the Ebbetts Pass quadrangle: Berkeley, California Univ., Ph. D. thesis. In figure 28, the normative compositions of selected volcanic rocks are plotted on the four sides of the tetra- hedron representing the phase system Q-or-ab-an at 5,000 bars water vapor pressure. All compositions but that of the upper Pliocene(?) felsite (pf) fall in a cluster. This cluster is nearly equidimensional, indi- cating that these phases have not participated signifi- cantly in differentiation. The lack of variety in the rocks of the Lava Moun- tains suite is emphasized by the small number of C.IL.P.W. normative rock names needed to describe them. The difference between yellowstonose and tonalose is solely in the class number (I and II), which depends on the ratio of normative quartz plus feldspars to dark minerals; the higher the Roman numeral, the higher the percentage of normative dark minerals. The similarities between these rocks (excepting lassenose) are more impressive: the same order, rang, and sub- rang result from very similar ratios of normative quartz to feldspar, alkalies to lime, and potash to soda. The lassenose is anomalous because of the higher ratio of alkalies to lime than in the rest of the rocks. Washington's normative tables (1917) can be used as a guide to other areas containing fine-grained rocks with similar normative (and thus chemical) compositions, and as an approximate measure of rock abundances at least in the areas that attracted the attention of vol- canic petrologists up to the time of his compilation. Of the three types found in the Lava Mountains, tonalose is the most common volcanic rock listed in Washington's tables, lassenose is next, and yellowstonose the least common. In the Lava Mountains suite, yellowstonose forms the greatest percentage rather than the smallest implying that the Lava Mountains rocks are deficient in normative dark minerals compared with most areas. When Washington's lists (1917) of yellowstonose, tonalose, and lassenose localities are tabulated, it is found that the fine-grained examples of these three rock types are concentrated in Cenozoic orogenic regions, namely the west edge of North and Central America, the western Pacific region, the West Indies, and the area surrounding the Mediterranean Sea. The yellow- stonose samples were chiefly derived from Central America, northern California, the Yellowstone National Park area, and the East Indies, Philippines, and Japan areas; the tonalose samples from northern California, southern Oregon, and the West Indies and Mediter- ranean areas; and the lassenose samples predominantly from northern California, southern Oregon, Arizona, New Mexico, and some from the Mediterranean area. The Lava Mountains rocks are thus representative of common although not prevalent volcanic rock types in orogenic regions. - Perhaps the major distinction of the suite in the Lava Mountains is the limited range of com- positions; most areas contain several types of volcanic rocks, even though only brief periods of time are repre- sented. The Lava Mountains are composed chiefly of one. MINOR ELEMENT COMPOSITION A total of 36 elements were sought in the spectrochem- ical analyses. Three of these, Fe, Mn, and Ti, were also included in the major element analyses. Those elements detected in virtually all the analyzed rocks are : B, Ba, Co, Cr, Cu, Fe, Ga, Li, Mn, Ni, Pb, Rb, Se, Str, Ti, V, Y, Yb, and Zr. - Those elements that were sought but not detected in any rocks are: Ag, As, Bi, Cs, In, La, Pt, Sb, Sn, U, and Zn. Elements that were ques- tionably detected in some rocks are: Ga (in 128-34, 97- 38K, and 97-381), Mo (in 41-34, 124-22K, 97-381, and 181-29), and Nb (in most samples). The lower limits of detection for the elements not found are listed in the following table. 72 GEOLOGY AND an 2 (10659) t VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY (12359) / fs3 s) & a Plagioclase & 3 2 4 1000° Beta quartz Plagioclase Beta quartz | or-ab feldspar (748°) 700° 1100° Plagioclase or (876°) or-ab feldspar an (1235°) Fisurs 28.-Phase diagram of the system Q-or-ab-an at 5,000 bars of water vapor pressure showing normative composition of selected volcanic rocks. others (1963, p. 34). are omitted. Data from tables 4, 5, 8-10. Lower limits of detection of elements sought but not consistently found in the volcanic and plutonic rocks Approxi- Approxi- Approxi- mate lower mate lower mate lower Element limit of Element limit of Element limit of sensitivity sensitivity sensitivity (ppm) (ppm) (ppm) 0.5 1 50 200 20 200 .8 10 5 10 100 500 20 2 200 50 20 100 In figure 29, the minor element contents are plotted against the SiO, content of the host rock. Sloping re- lations are indicated by the points representing Ga, Mn, Point pf represents composition of the upper Pliocene(?) felsite. Diagram adapted from compilation by Bateman and Normative compositions of selected volcanic rocks from the Lava Mountains area are shown ; propylitized rocks Se, Sr, Ti, V, and Y. Three elements-Cr, Ni, and Cu-show a steplike curve, the break coming between 64 and 65 percent SiO,. No slope is defined by the points representing B, Ba, Co, Cs, Li, Pb, Rb, Yb, and Zr. - The lack of sloping relations between SiO; and Ba, Cs, Pb, and Rb results from the same factors that cause the lack of slope in the K,O0-SiO, diagram, and they are discussed in the section on "Volcanic petrogenesis." The lack of slope on the curves for B, Co, Li, and Zr remains unexplained. Spectrochemical analyses of five volcanic suites from the western United States have been published by Nockolds and Allen (1953, tables 2, 3, 4, 7 ; 1954, table 16). A similar set of data has been assembled by VOLCANIC PETROLOGY 783 60 - * * so 44 ad -| *s .y * A § y* > B lo | ® & % + ® -x Li : *> s § $o 4" * id * : * o e Pas. s ig o * Mig @ 0 t ** T t t 1 0 y -* t t t 1 60 62 64 66 68 70 72 60 62 64 66 68 70 72 0 2500 - a 100 /* G + o o ® Ag * Rb e @ - ca a .* x 2000 - o e ** U ® @ e e e @ x- % Ba ® a: * y ® « 8.4- ‘t s * ® 1500 P e x 0 t t t t y 1 |6- x 60 62 64 66 68 70 72 1000 T T T T 1 60 62 64 66 68 70 72 23 4 8 $ 8e 10 4 ® °o*'fipx‘ @e - x 80 - * = ee © g 4 .* f a j * f- cs. I § 60 62 64 66 68 70 72 so - @ & e * & e 3000 - Co a0 - e a x ® s. . n e 2000 -| _ ® ® x 0 @ 20 44 + + x U & % '+ e Ke - © 3 § T T T T t 1 1000 /. "iate ® 60 62 64 be 68 70 72 x 0 T T T t t 1 100. - ® 60 62 64 66 68 70 72 g * ® 3000 80 - @ 4 5 ® e * % # e € e *** ‘u: ® B 60 ~ ° . © 2000 - *s stik " " (n, uf 4 a Cr ao - ¢"-¢v. a.. * Ti a x e 1000 - 20 _¢ x 0 & £ * 0 & T T T T T \ < 0 T t T T T maw, 60 62 64 66 68 70 72 a. 60 62 64 66 68 70 72 100 - Z ms hd e £ 30 e r **. 4.6% a (¢ e o ®s e ¥ £089 a $* he n: s re 6 x * @e .X zZ 20 - 0,0 o x @ & ~* + # 5 Cu # 0 t T T y t n 4 10 - 0 x @e > 60 62 64 66 68 70 72 r + i 20 - a o o 0 T y t T t m-, @ o F 60 62 66 68 70 rey" p44 ® ® > 3 20 - & e * *; ~ *s a **, ** *% ee x s e je (80° xia & 0 t t T t t == Ga - 10 (* a's 40% *- ¢ o 60 62 6 66 68 70 72 $- 0 t t t T T 1 4 e o ony &C 62 6 66 68 70 Fn. mealg *~ @ § o s # s a fon 400 + ® 0 e0 U e ® _ xx im ~ Line "mage le . f "a .~ Mn # 60 62 64 66 68 70 72 200 - s ® 150 - e o +2 ® e xe ® e e @e ® 0 t T T T t 1 Zr 100 {¢ % 80 x. x # 0 60 62 64 66 68 70 ® 50 £ ® I T I T T ~a "4 .* s ***%s'. 60 62 6 C 68 70 72 L 40 - ++ i0,, IN PERCENT Ni .* *> 20 - |6 Ro x@® gx ® ig 8 EXPLANATION T T T T T 1 60 62 64 66 68 70 72 + po 0 "A4 F3 Quaternary andesite Almond Mountain Volcanics subpropylite Pb 10~¢0 % o (subpropylite) Upper Pliocene(?) felsite @ 0 T T T 6's 7:0 ‘7‘2 ho Almond Mountain Volcanics 60 62 64 66 ropylite e Lava Mountains Andesite (p ) SiO,, IN PERCENT £ x Bedrock Spring Formation Almond Mountain Volcanics (eastern facies) 4 Voleanie rocks older than the \ Bedrock Spring Formation Almond Mountain Volcanics (western facies) 29.-Variation diagram showing the relations between the minor elements and silica. Data from tables 3-6, 8-10. 74 Eaton * for rocks of the Los Angeles basin. All six suites show general similarities with the suite from the Lava Mountains, although distinct differences occur in the quantities of Ga and Pb in the rocks from the Los Angeles basin area. The remaining five suites were not analyzed in the same laboratory as were the Los Angeles and Lava Mountains rocks, and the quantitative differ- ences between the two groups of results are not large enough to be necessarily significant. RELATIONS BETWEEN CHEMICAL AND MODAL CcoMPOSITIONS The volcanic rocks of the Lava Mountains show two significant correlations between the chemical and modal compositions; the SiO, content is directly proportional to the percentage of groundmass, and it is inversely proportional to the percentage of total plagioclase. Both relations undoubtedly reflect in part the higher viscosity and slower crystallization rate correlative with the S10, percentage of a melt. The correlation between S10, and plagioclase partly reflects compositional changes in the magma, however, as shown by a similar variation pattern between SiO; and normative plagio- clase (fig. 30). Of course all other elements that vary systematically with SiO); can also be shown to be re- lated to these two modal percentages, so the significance of these correlations is unknown. There is no detected correlation between the percentage of minerals con- 30 7 20 - "aa * 10 - PERCENT MODAL PLAGIOCLASE ® 60 62 64 66 68 70 72 PERCENT Si02 60 - h 50 -I * * 40 - PERCENT NORMATIVE PLAGIOCLASE 30 T T T T 60 62 64 66 68 70 72 PERCENT SiO, FicurE 30.-Diagrams showing variation between SiO; and modal and normative plagioclase in selected volcanic rocks. Propylitized rocks omitted. Data from tables 4, 5, 8-10. + Eaton, G., 1957, Miocene volcanic activity in the Los Angeles basin and vicinity: Pasadena, California Inst. Technology, Ph.D. thesis, table 11. GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY taining mafic constitutents and the percentages of Fe, Mg, Mn, Co, V, Ni, or Cr. A correlation probably existed prior to deuteric alteration, but the mafic min- eralogy has been changed so greatly that all such rela- tions have been obscured. CHEMICAL CHANGES WITH AGE The chemical data indicate few progressive changes in the composition of the melt. Figure 31 shows the percentages of major element oxides in successive layers of volcanic rocks that form the eastern and western facies of the Almond Mountain Volcanics, and in the Lava Mountains Andesite flows that immediately over- lie them. The upper three samples in figure 314 are from a single flow of Lava Mountains Andesite, and the diagram shows the chemical variation that exists between samples differing chiefly in the degree of deu- teric alteration. Each of the remaining samples repre- sents a different flow or breccia layer. The data in figure 31 show that any progressive change in the chemical composition of the magma was smaller than the sporadic and local changes, and that any such change is not reliably indicated by comparison of individual analyses that represent only one part of the volcanic province. The upper part of figure 31 shows faint trends toward lower percentages of SiO; and H0 in younger rocks, and toward higher percentages of Al;O;, total Fe as Fe;,0;, MgO, CaO, Na:0, TiO;, P;0;, and MnO in younger rocks. The lower part of the figure shows trends toward lower percentages of total Fe as Fe:O;, MgO, CaO, and MnO, and toward higher percentages of S10», Na,0, and TiO; in younger rocks. The small trends in Na.0 and TiO; percentages are the same in both sections, but the percentage differences between successive samples, however, commonly exceed the total range of the inferred trend, and the indicated trends are thus not very reliable. Any trends indicated by these data can be recognized only by a study of averages representing larger numbers of samples collected from more widespread areas. Table 13 lists the average percentages of major ele- ment oxides in the three most widespread volcanic formations. For comparison, the single analysis of volcanic rock in the Bedrock Spring Formation, and the average of two representative rocks from the Atolia Quartz Monzonite are added. The standard deviations and standard errors are listed ; they measure the scat- ter of the data that produced these averages, and the probable proximity of these calculated averages to the true ones. Chemical data on rocks of the Almond Mountain Volcanics and Lava Mountains Andesite are based on nine analyses of each formation, so the statisti- VOLCANIC PETROLOGY 75 124-23 J - a - w £ fol € £ gg 124-23 F- 7 =E o < > a A 124-23 B- s L F L r L i 1 & U 7 U \ * | V | X 124-22 q - ® S4 & fol : E , 124-22 M { © ~1 5 5 2C 3 @ |124-22 K - © Ol @ C ® ~ p 2 c 0 o > [124-22 | - O = £ < 124-22 F - ® - t t t mi- ct t te: am: t =+ T/T T m- Tt t t r t 64 65 66 67| 16 17) 3 4 $4 2 3| 3 4 513 512 3] 0 1} 0 110 0.1 1 2 3 Total Fe SiO Al>O3 |(as Fea O3) MgO CaO Na,0 K,0 TiO P, 0, | MnO Total H2O A a = - S €4 3 # % A §{}|7=> Cl D] . s C $109 |_ | _ a o & 1 | \ 7 / E "I \ % > 3 97-38 L - * 97-38 K - * 97-38 14 - £ fol a 5 w 3 0 s 5 97-38 F- - 2 0 6 > E < T T TT CH T|I T T TT fr TT TT TT Tt Ti- T T 61 63 65516 17] 3 5 1 2 3 | 4 5|3 4 5] 2 3] 0 1|0 110 0.1] 1 2 3 SiO Al,O, | Fe (as FepOq) Mgo CaO Na ,0 K0 TiOp P,O& | MnO Total H,O FisurE 31.-Diagrams showing the variation in the percentage of major element oxides in superposed rocks of the Almond Mountain Volcanics and Lava Mountains Andesite. Almond Mountain Volcanics. cal parameters are moderately accurate measures of the same parameters that would be calculated from a much larger number of samples. However, data on the Quaternary andesite are based on only two samples, and their statistical parameters are only approxima- tions. The similarity between most of the average values supports the conclusion that any magmatic trend correl- ative with the time at which the rocks were extruded 785-720 0-64-46 The location of samples is described in table 21. average absolute deviation as listed by Shapiro and Brannock (1952, p. 16). A, samples are from the type section of the Almond Mountain Volcanics and from the west side of Almond Mountain. graphic positions of the lower five samples are shown in figure 11. B, samples are from 1 mile north of Dome Mountain. The radii of the circles represent the Data from tables 5 and 8; all values in weight percent. The strati- Sample 124-22K is from a Lava Mountains Andesite sill in the is small. The average FeO, percentage increases and the FeO percentage decreases, but this only reflects the extent of deuteric alteration 'of rocks of similar chemi- cal composition, as shown by the nearly constant per- centages of Fe as total Fe,0;,. Table 14 expresses the same information in the form of norms. The second- ary Fe,0,-FeQ relation described above, here causes the progressive increase in hm and the decrease in mt with age. 76 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY TABLE 13.-Averaged major oxide compositions, by chemical analysis, of volcanic rocks from the Almond Mountain Volcanics, Lava Mountains Andesite, and Quaternary andesite. The major oxide composition of one sample from the Bedrock Spring Formation and the average of two representative samples (26-2 and 41-34) of Atolia Quartz Monzonite are added for comparison [Data from tables 2, 4, 5, 3, and 10. All values are in weight percent. 7, average or mean 2&3: ; s, standard deviation ( where X-percentage of oxide in individual sample and N-number of samples analyzed] 311155152] $5); sz, standard error (W); Represent- Bedrock ative Spring samples of Formation Almond Mountain Volcanics Lava Mountains Andesite Quaternary andesite Atolia Oxide (1 analysis) (9 analyses) (9 analyses) (2 analyses) Quartz Monzonite (2 analyses) I T 8 83 T 8 82 T 8 # T P _i ille. 64. 8 64. 8 2. 36 0. 79 64. 0 1. 52 0. 51 64. 2 0. 36 0. 25 67. 6 16. 6 16. 5 . 29 . 10 16. 6 . 26 . 09 16. 4 22 . 16 15. 8 1. 4 1. 8 . 15 . 25 2. 9 1. 01 . 84 3. 4 28 . 20 1. 7 cache cans 2. 2 1. 8 A41 . 24 1.2 1. 18 . 38 . 6 28 . 20 1. 2 (Total Fe as FeqOs) -| (3. 6) (8. 8) (. 56) (. 19) (4. 2) (. 57) (. 19) (4.1) { *C. 07) | *(. 02) (3. 0) eus 2. 0 1.0 . 90 . 80 2. 1 72 . 24 1. 2 36 . 25 . 58 4. 6 4. 4 . 58 . 19 4. 5 57 £19 4. 4 L. 07 L 02 2. 4 4. 5 3. 9 . 24 . 08 4. 2 24 . 08 4. 2 +.. 07 L 02 3. 9 (O eae. ds ean 2. 0 2. 6 . 26 . 09 2. 5 20 . 07 2. 6 '. 07 ' 02 3. 9 58 . 59 . 084 . 028 . 65 072 . 024 . 63 . 014 . 010 . 48 sree cea idan ios a wh a . 20 21 . 042 014 . 26 037 012 21 . 014 . 010 . 16 . 08 05 . 013 . 004 . 05 . 016 . 005 04 '. 007 & 002 107. M0 cul 1. 2 2. 0 . 88 29 1.1 . 55 18 2. 2 40 . 07 3.1 COJ ans . 27 05 |.: - MBP 1 10 Cid 1. 0 1 Calculation assumes maximum possible difference between analyses before rounding last significant figure. TABLE 14.-Averaged normative compositions of volcanic rocks from the Bedrock Spring Formation, Almond Mountain Vol- camics, Lava Mountains Andesite, and Quaternary andesite. The average of two representative samples (26-2 and 41-34) of Atolia Quartz Monzonite are added for comparison [All values are weight percents] Represen- Bedrock Almond Lava Quater- tative Norm Spring Mountain | Mountains nary samples of (C.LP.W.) Formation | Volcanics | Andesite andesite Atolia (1 sample) | (@samples) | (9 samples)|(2 samples)] _ Quartz Monzonite (2 samples) 18.2 20. 0 18. 2 19.1 24.5 0 L <. 0 1.4 11.6 15.6 14. 4 15. 6 28.0 38. 2 82.7 35. 4 35.6 33.0 19.2 19.7 19.1 18. 2 9.9 2.9 1.2 1.8 2.7 0 5.1 5.8 5.1 1.9 2.1 2.1 2.8 1.5 .2 1.7 1.2 1.2 .9 1.0 1.0 0 A 1.8 3. 4 .6 0 0 .4 .2 0 0 0 m 46. to _£E|_ irs T T T T T 1 T T T T T 1 15 16 17 18 19 20 0.4 0.5 0.6 0.7 0.8 0.9 43}; Qa fi_ Qa Total Fe JEEJH (as Fe;O3) W Fz?» Jflj—L TI ._£EI_‘ Ta ___$|_ Ta T T T T T 1 T T T I I I 1 2 3 4 5 6 0.1 0.2 0.3 0.4 0.5 0.6 r_1:j:1__ll os E‘s—I oa Mgo _n£$&1__4 TI Mno i TI ass C-- - 6 4 6 | 4 6 b © b 3 'o & * | 4. b 1 b 1 Loss-c6>a a55 ® tte ”We #$ - V az 4A A [ A n as as dd ay ) IN um ep no 19 09 eg a ost oslz oloe oloot o| ogoe _ ooptJoooz __ Q}91 ojo qJgot olos 9foot qloos oloz gjos 9joot qlgot qloose _ oos|9ot § > C H4 ze-vet 3 0 ls - Fi ce-rerg A 8 Z t= 0 o 0 & o +- LMzz-tet 3. g o C - -wee-vet "® 3 ©, 3 L -dze-vet ; % 1 1 } P \ I M 1 } h F I I 1 1 * I < £ P 1 4 I . U I I I I - ® p. y 5 2 < a o - -a sz-ver ®. § ms ct ® ® 3 [ -ree-ver @ 80 GEOLOGY AND VOLCANIC PETROLOGY, LAVA MOUNTAINS, SAN BERNARDINO COUNTY 4:§:1_—‘ os ,__r—_'EF'_—1— Qa ._F_EEL4 T1 4:33 Ta t \ t t ; 50 60 0 5 10 15 20 25 30 Pb t t t t 1000 1500 2000 2500 3000 3500 0 5 10 15 20 25 1 Sr 1 . _ aBL m Co jes -_." ics. Te t> U T T T 1 ( 10 20 30 40 50 60 ( 500 1000 1500 2000 2500 3000 "ls gs -fs . oa ; $2 fi TI Rb i TI {_ poses. Ta ~-