Cenozoic Rock Units of the Mojave Desert

Home , Crowder Formation, Punchbowl Formation, Soledad Pass

CENOZOIC ROCK UNITS OF THE MOJAVE DESERT

Thomas W. Dibblee, Jr_ 316 East Mission Street Santa Barbara, California 93101

ABSTRACT

The deeply eroded surface of the Pre-Cenozoic basement complex of plutonic and metamorphic rocks of the Mojave Desert is overlain in many parts by Cenozoic sedimentary and volcanic rocks. The oldest are elastic sedimentary rocks of Paleocene-Eocene age, known only along the southwestern and northwestern margins of the western Mojave Desert. These are marine deposits mostly southwest of the San Andreas fault zone, and fluviatile sediments northwest of the Garlock fault zone.

Within the Mojave Desert volcanic and sedimentary deposits of early middle Ter- tiary (Oligocene to middle Miocene) age are the oldest and most widespread Ter- tiary rocks. They are extensive in the western and central parts, and present locally in the northern and eastern parts. They are composed primarily of pyro- elastic rocks, and volcanic breccias and flows of rhyolitic to basaltic rocks erupted from many vents and fissures within these parts of the desert terrain. Intercalated with these eruptive rocks are clastic sedimentary rocks composed of agglomerates, fanglomerates and local minor amounts of finer sediments. The age of this assemblage may range from Oligocene to middle Miocene. At one place, a radioactive age (K-A) of 23.5 m.y.b.p. was obtained from rhyolite.

All these deposits acumulated on an eroded basement platform in parts of the Mojave Desert region that evolved into depositional basins. The most extensive are in the western and central parts which are designated as the western and central Mojave Desert basins, respectively. In the latter basin, more than 10,000 feet (3,100 m) of these deposits accumulated, in part on an irregular basement surface of as much as 10,000 feet (3,100 m) of relief. Elsewhere, thicknesses and basement relief are less than half that amount.

The basement terrain south of the western and central Mojave Desert basins is notably devoid of middle Tertiary volcanic and sedimentary rocks. This area was probably re-elevated and supplied coarse detritus to the depositional basins.

Minerals of economic value in the volcanic and sedimentary rocks are mainly ores of gold and silver in the Mojave, Calico and Ludlow districts, barite near Bar- stow, manganese near Ludlow, and perlite, pumice, and zeolite near Opal Mountain, and agate and jaspar in many places.

The lower middle Tertiary volcanic and sedimentary rocks are overlain unconformably by terrestrial sedimentary formations of middle Tertiary (Miocene) age. These formations are as thick as 5,000 feet (1,700 m) and consist of clastic sediments and lacustrine clays. These formations were deposited in valleys which contained lakes. In the western Mojave Desert basin, they accumulated probably in two valleys that became the West Antelope and Kramer basins, and in a valley

Geology and Mineral Wealth of the California Desert - South Coast Geological Society within the central Mojave Desert basin that became the Barstow basin. All these terrestrial basins were elongated east-west. In the Kramer and Barstow basins, lacuotrine clay contains valuable mineral deposits such as borates and strontium minerals.

The only other sedimentary formations of. Miocene age in the Mojave Desert occur in and north of Cajon Pass, where they overlie basement, and possibly, concealed under the alluviated area north of Palmdale.

Sedimentary formations of late Tertiary (latest Miocene(?) - Pliocene) age are present mostly in marginal parts of the Mojave Desert. None are known in the Kramer nor Barstow basins. The most extensive are in a deep basin or trough along the Garlock Fault near Randsburg and include the Ricardo Formation, as thick as 5,700 feet (1,900 m), exposed in the El Paso Mountains. In the mountains east of Randsburg, Pliocene sediments as thick as 5,000 feet (1,700 m) and volcanic rocks were deposited probably in this same trough. Near Baker, Pliocene sediments about that thick are exposed in the Avawatz and Soda Mountains.

Deposits of Quaternary age consist of alluvial deposits and scattered eruptions of basalt. Most of the basalts were erupted on right-slip fault zones trending northwest, diagonally across the Barstow basin.

INTRODUCTION

The deeply eroded surface of the crystalline basement complex that evolved during the preceding orogenies is overlain in much of the Mojave Desert region by Ceno- zoic sedimentary and volcanic deposits. The pre-Cenozoic metamorphic and plutonic rocks of the basement platform will be referred to collectively as basement rocks, in contrast to the overlying unmetamorphosed Cenozoic sedimentary and volcanic rocks described in this report.

The oldest Cenozoic deposits are sedimentary rocks of early Tertiary age exposed locally adjacent to and southwest of the San Andreas and northwest of the Garlock faults that bound the western part of the Mojave Desert. Volcanic and sedimentary rocks are exposed over large parts of the Mojave Desert. Quaternary alluvial deposits fill the desert valleys, and include local eruptions of basalt lava.

Geographic features of the western Mojave Desert are shown on Figure 1. Those of the eastern part are shown in the preceding article (Dibblee, 1980, this volume). Also shown in that article is a diagrammatic sketch of the Cenozoic as well as older rock units. Named Cenozoic Formations of the Mojave Desert are listed by DeCourten (1979, p.235-247).

CRETACEOUS EROSION

There are no sedimentary deposits of known Cretaceous age anywhere on the Mojave Desert region, but thick marine clastic deposits of that age are widespread in the Transverse and Southern Coast Ranges to the southwest and west. From this condition, it is evident that during Cretaceous time, the region now the Mojave Desert was a probable mountainous terrain of basement rocks undergoing possible continuous uplift and erosion during and following emplacement of batholithic rocks at depth that culminated in the Laramide orogeny. This highland terrain is designated as the Mojave uplift. The presumed enormous amount of detritus eroded

o h ;I: 1.1 50 Mlles 8i • • -•'. • ••••.)•.:i o / •N 64 r, ,- • .! • r %, •.":. • -• *- t A • i• 01 •tc•.4„ • . • .• • , r ?, 1.)1 • . • .; • ;"/ F Nyt. • • ' .‘. , o /3 (.15:1 I " • •..% • i ). - / • ; : •,1 • if9 l' • ,•;.4 •,:‘:)‘" <‘Z. , 1.. • 1. 1•'-' ...IT -••''Í' • -." 1- m. • ."- " ,'••• 1 •• I •v• „ Cr 1 • I • `J, YY •-•*/ • S,14 -- 1.` 1 IS:s , r) A s \ .*11 I • ' ‘ e ,• „ Aim 1-•;.11 )1.V 1v!,: \'‘‘•:) f•N c - 1 1. - 1/4 . \IC IA 5.; • • 1.,‘ ,N t" GEOGRAPHIC FEATURES . 3 1.1 'N I •'• ' •.\ •\ 1. Tehachapi Mountains ‘`, r 2. Sierra Nevada Fs. • • 3. El Paso Mountains :,• 4. Summit Diggings 5. Lava Mountains 6. Red Mountain S./ 7. Eagle Crag Mtns. 24. Bristol Mountains tt•- ' -•" • ...••••5 i'••1 8. Avawitz Mountains 25. Marble Mountains •"- •.. 9. Soda Mountains 26. Clipper Mountains ••• 10. Shadow Mountains 27. Lead Mountain 11. Antelope Valley 28. Bullion Mountains 12. Antelope Buttes 29. San Bernardino Mtns. 13. Rosamond Hills and Soledad Mountain TOWNS 14. Castle Butte T Tehachapi Ma Manix 15. Kramer borate district Ra Randsburg Ba Baker 16. E. Kramer borate dist. At Atolia Newberry 17. Dramer Hills Mojave Hector 18. Gravel Hills, Opal Mtn. Rosamond La Lavic and Black Mountain Lancaster Lu Ludlow 19. Mud Hills Palmdale A Amboy 20. Calico Mountains Pe Pearblossom Lv Lucerne Valley 21. Alvord Mountain Cp Cajon Pass Bb Big Bear Lake 22. Daggett Ridge and Barstow Newberry Mountains • Daggett 23. Cady Mountains • Yerrno

Figure i INDEX MAP OF THE MOJAVE DESERT Showing features and towns mentioned

43 within the highland was transported outward, mostly southwest and west to the sea, where it is accumulated as the thick marine deposits of Cretaceous age now exposed in the Transverse and Coast Ranges.

EARLY TERTIARY DEPOSITS AND EROSION

Within the Mojave Desert region there are no sedimentary nor volcanic deposits of known early Tertiary age. In the Transverse Ranges, southwest of the San Andreas fault, is exposed as much as 6,900 feet (2,100 m) of marine sandstone, shale and conglomerate in the San Francisquito-Bouquet Canyon area west of Palmdale, and also in the Devil's Punchbowl area southeast of Pearblossom (Fig. 4, third and fifth columns). In these exposures, it is mapped and described as the San Fran- cisquito Formation, Paleocene-Eocene (Dibblee, 1961), 1967a, p.44-46, 50-51). It overlies basement rocks and is overlain by Oligocene and Miocene terrestrial formations.

In Cajon Canyon south of Cajon Pass, and on the northeast side of the San Andreas fault, granitic basement is overlain by the basal part of the San Francisquito (?) Formation, which in turn is overlain unconformably by, or in fault contact with Miocene terrestrial deposits (Dibblee, 1967a, p.45-50; Woodhurne and Golz, 1971, p.8-9; Foster, 1980, this volume), (Fig. 4, sixth column).

,AV g éi •c) V IL Carbonate Borate Tuff Tuff Volcanic -60041Y-ft bed bed bredcia flow breccia

11111111111111100001 % 111111111111111111111111 rso = 1111111101111111111101 - 5000' Granitic landslide Basalt Andesite Dacite, rhyolite, or Pedite breccia quartz latite felsite

- 4000'

Fanglomerate Sandstone Siltstone Shale Siliceous and conglomerate bed - 3000' FOSSIL HORIZONS Unconformity 6 Plutonic and Plant Marine Vertebrate metamorphic rocks 2000' CorreWaonline mollusk Queried where doubtful

1000'

Figure 2. Explanation of symbols and vertical scale for VERTICAL figures 3, 4, 6 and 7. SCALE

Stratigraphic Figure PRE-TERTIA TERT RY (n) RY IA Q UATERNARY GE Th, Trc, Qoa, A and Pa leocene and Eocene Miocene Pleistocene Qof, older Horned (s) mernbers)—lower Bedrock Ricardo --• --Tw -\----- 11111 - 4 lin • —... ._ XI 6 % - • 0 _ . '" • 0 3. • • . . -•0 • =. Ta-ii+ • A + " • 4. . • • (f i " . Cache . — . ,. 0 IT - units % indicate k. - • ...... • . . '+' . + Monolith- i 4 — .01 Tk ' — gs. fanglomerate—Pleistocene " area .* - 0 • - ... — ' . .0. . + . 21 - .• - : - . alluvium,

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non _ I

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""': l . Mountain north \ • the Tap, -1 ,i - _ (n) ''.! /I l' l -

", - , i /

and pl. northeastward / — ....a_.1....1.. andesites, - + ...... • . • , or + . . • ...... • . ° . - • ' ' --• 0 O 00000.0 ...... _ ..._. .- vicinity "-- '1 . . . - .

+ • • — C 0 Last - Tu, . • (figs.63, - 000 0 . . . o . -• Qoa- south • + Trc 4. + 0 ...... 6 • 0 - . Redrock-

. - Canyon 2 Tg .8 0 1 0 0 _ ar 0. 4. Basalt, a . . • — ... . + 0 . 0 unnamed 0 . 0 - . ea • .. Chance 0 . • + - 0 00.0 . . • o .. 0 0 .. . 0 0 ' + of . .-"- • into . (n) , .. 0 .

CI 64) . 00 0 Tbs, the and of 8 00 °. ' 0 - • a Garlock • „, ...... 0 . 1 1, .. • • • a .Trc . Goler 0 , . , 0 ,, the ' . 00 45 +2 Mountain- 0 C. • 51 Tg to 1 (fig. area _ 0 Black • P Qb .• .... 0 + 0 0 ° .. o 0 0 0 CO 0 • . • Gulch 0 0 fault 65) Garlock the 0 ' 0 0 (n)

0 .

. • Red l iill Tg, Tr, Tw, Ta, Tk, Tbo, o - .., .. 1/1 °Q8a.: ,,, ... --,,- , • 0 0 • • Toad 10 Th area ".--i (fig. .-' Horned . .. Fault Mountain - \ Witnet andesites—upper Goler 0 rhyolitic Kinnick Bopesta - rocks—Pliocene ,,,. ,. 1V,‘11.„ . - i Tr ' -- CIA, o '../‹..3.i. • +' 1 f 62) Hills 1 ‘7 . (s) - • i ., i , ;‘ i • ‘...-, , 1 /, •;- .1 1 Formation—Eocene ‘ - .:.

1 r., ‘ i Formation—lower ' ..

Formation—middle from felsites—middle(?) '. Formation—upper ,1„--j.1":, f VIEMIBIIIINIIIIPH ...' / • • .. • • '‘,. . + + MIIIIPM.W...IIII 4 ._ - , r,‘ . e • " • • - ,,, 4 +,,,, 4 - e 0 • • o area. .- + + 4 o

Qoa 1 /-,-.1 " • Tbs. Tap „ ...... Diggings ... Summit ,- o -' area (fig. Tu 4 4 + , , . . , • ..... 1 0 0 0 the + . + 4 - • .•.. , 0 -i, I 0 .... o + 4 + • ' - o' e and ° 68) 4 + + • • - 4 • — ,, (s) --- e o . • ' • . + + 4

- -. Monolith- % --• / , r , 1 From midcUe Tertiary and "• ••° '• ' • •••L:. """ ; ::: ip k, ,,,, o... o g g go o 'g Miocene Miocene Miocene n g o Mountain area : ; ; I . (fig. Miocene ' : 1 - • 0 % 7 Paleocene • • • - 2 1 0 ° o o Red . o ; ! : : • % -0% , : 0- • -• 0 ' °"'" 67) • • • ! • • • • • • ° : 0 0 0 c• (s) •• •. •° A : 5B : : : ; : . o ° ° o ° ' ° ' • ° 4 1 • • • • k,,,r1 n o . .c( o w TERTIARY PRE- TERTIARY QUATERNARY

Hungry Valley Cajon u.i area (s )1 Palmdale San Francisquito Soledad Pass Devils Punch- Pass tu (.9 area (n)l Canyon (s) area (s) bowl area (s) area (n) LI < (T . 814 , R . 19 W). < (fig. 34) (fig. 26) (fig. 27) (fig. 29) (fig. 31)

I I

RY RY ne

RNA ,---_-•,____, tocene toce ...... TE is is • - oa • • 0 °Qoa * 0 ATERNA UA U

• • • • .. Ple Ple Q _____ . •

. . . . • . TQ • .000000 • 000 0 0 00 0 00 Qoa e a

• — — —_ — 00000000 ...... 00000000 000.. ••••--••—• oo c000aa CD :::Qos: C ••••Thv • • • • - • ..... • • • • • • • • ... - •• .0 .• 0•••• 0 0.0 00000. := — • 00•••00 0.• • • ...... -0ed Tpb • •• Tc • 0 - - -• • e * • 0 ee • • 0 • 0 "0..“00 • ••••• ...... • ••• • 0 en • 0000400 00 0 — ? . . . • • • - ...... 0

a., _• ...... c . -

• liocene a.) _ —• • • • o .. P IARY TIARY o • • • • . . d RT EC. oaO000. • ••• •••• .00 0 0 00 an TER

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1 (n) and (s) indicate exposures are north or south of San Andreas fault

Stratigraphic units (nonmarine unless otherwise Tc, Crowder Formation—Pliocene noted): Tpb, Punchbowl Formation—lower Pliocene and Qoa, older alluvium (gravel) and Qos, older alluvium upper Miocene (sand)—Pleistocene Tvq, Vaqueros(?) Formation (marine-brackish)----lower Thy, Hungry Valley Formation of ' Crowell (1950)— Miocene and Oligocene(?) upper Pliocene Tv, Vasquez Formation—lower Miocene(?) and Tpv, F'eace Valley Beds of Crowell (1950)—middle(?) Oligocene Pliocene Tsf, San Francisquito Formation (marine)—Eocene(?) Tay, Anaverde Formation—Pliocene and Paleocene Figure 4. Exposures in the vicinity of the San Andreas Fault from the Hungry Valley area southeastward to Cajon Pass. From Dibblee, 1967a, pl. 4.

46 North of the Garlock Fault, about 6,500 feet (2,000 m) of stream-laid sediments, mapped and described as the Goler Formation (Paleocene), are exposed on the northwest slope of the El Paso Mountains, where they overlie basement rocks (Fig. 3, third and fourth columns), (Dibblee, 1952, p.19, 22-25; 1967a, p.98, 102-105). Similar and probable correlative deposits, but called the Witnet Formation, are exposed in the Monolith-Cache Peak area near Tehachapi (fig. 3, first and second columns), (Dibblee, 1967a, p.91, 94-95; Dibblee and Louke, 1970). In all these exposures, the cobbles in the conglomerates are smooth, rounded, and composed mostly of very hard rocks such as quartzite and porphyry, and were apparently transported long distances by large streams.

From the above data, it can be inferred that during early Tertiary time, the Mojave uplift persisted as a mountainous terrain of basement rocks undergoing erosion, from which the eroded detritus was carried outward southwest to sea to form marine deposits such as the San Francisquito Formation. The Goler and Witnet Formations are remnants of stream-laid sediments deposited in a river valley that may have drained southwestward to the sea between the Mojave and Sierra Nevada uplifts.

EARLY MID-TERTIARY HYPABYSSAL INTRUSIONS

In the Sunshine Peak area southwest of Lavic in the central Mojave Desert, a stock of dacite or quartz monzonite porphyry (quartz porphyry or monzonite porphyry, Jurassic (?) of Gardner, 1940, p.274-275) intrudes Cretaceous (?) quartz monzonite. Dikes extend northwestward from the stock into the quartz monzonite, and both are overlain by middle Tertiary volcanic rocks (Dibblee, 1966). Another stock of this porphyry with probably similar field relations is exposed southeast of Ludlow (Dibblee, 1967b). This rock, as well as dikeswarms of porphyritic rocks in other parts of the Mojave Desert, may have been emplaced in Oligocene or possibly early Tertiary time and probably record the earliest episode of very widespread volcan- ism that affected much of the Mojave Desert.

EARLY MID-TERTIARY VOLCANIC AND SEDIMENTARY ROCKS

Definition, Areal Extent, and Age

Within the Mojave Desert Region the oldest Cenozoic depositional series is an assemblage of predominantly volcanic rocks and some sedimentary rocks of probable Oligocene to middle Miocene age. This assemblage rests on the deeply eroded surface of the basement complex over much of the western and central parts of the Mojave Desert and in scattered areas in the northern and eastern parts. In some areas, it is locally known as the Neenach, Gem Hill, or Pick-handle Formations; in other areas it is unnamed. This assemblage of rocks is referred to collectively as the early mid-Tertiary volcanic and sedimentary rocks or assemblage.

The volcanic rocks of this assemblage were erupted from many volcanic vents and fissures in the western and central parts of the Mojave Desert, and locally in the northern and eastern parts and together with sedimentary detritus were deposited on the eroded surface of the basement complex. All these parts apparently subsided to become valleys. The largest of the valleys formed two large depositional basins, one in the western and one in the central Mojave Desert. These will be referred to as the western and central Mojave Desert basins.

47 This volcanic and sedimentary assemblage is notably absent from the southwestern part of the Mojave Desert within about 40 miles (64 km) of the San Andreas fault from the vicinity of Lancaster southeastward. This area, referred to as the South Mojave Desert uplift, apparently remained high or was arched upward in early middle Tertiary time when it supplied basement detritus to the sediments of the volcanic and sedimentary assemblage of the western and central Mojave Desert basins.

In the western Mojave Desert, the known and inferred areal extent of the early mid-Tertiary volcanic and sedimentary rocks is shown on Figure 5.

Within the Mojave Desert, this assemblage has yielded no diagnostic fossils. In most cases, its upper age limit is bracketed by fossils from the overlying Miocene (late Arikareean to Hemingfordian) sedimentary rocks. Its lower age limit is somewhat in doubt, but in the western Mojave Desert the age of the assemblage can be inferred from that of lithologically correlative formations of known age in the Transverse Ranges, such as the Vasquez, Sespe, Simmler and Tecuya nonmarine formations of Oligocene-early Miocene age. Radiometeric age dates of volcanic rocks in this assemblage would more closely determine its age as these become available. The volcanic and sedimentary rock assemblage is probably not the same age everywhere. From available data, most of it is probably of Oligocene age in the central Mojave Desert, but may be as young as middle Miocene in the Western Mojave Desert.

Exposures in the Western Mojave Desert

In the western Mojave Desert basin (Fig. 5), from Gorman eastward through Antelope Buttes, Rosamond-Mojave district, to the Kramer district, the volcanic and sedimentary assemblage is as thick as 2,000 feet (620 m), and it is composed mostly of light colored tuff-breccia. In west Antelope Valley it is called the Neenach of a basal tuff unit overlain by rhyolitic felsite and capped by a flow Volcanic Formation (Fig. 6, first column) (Dibblee, 1967a, p.58). It is composed of dark mafic andesite. The felsite has been radiometrically (K-A) dated as about 23.5 m.y.b.p. (Huffman, in Woodburne, 1975, p.66). Farther east, in the Antelope Buttes and Rosamond Hills (fig. 6, second and third columns), the assemblage is composed of tuff-breccia with local short rhyolitic-dacitic flow-breccias around nearby volcanic necks, of which Soledad Mountain is the most prominent. The tuff-breccia has been called the Gem Hill Formation (lower part of Tropico Group), and the rhyolitic rocks, the Bobtail Quartz Latite Member (Dibblee, 1958a, p.140- 141; 1963, p.164-186; 1967a, p.63-65).

Farther east, in the Kramer borate district (Fig. 6, sixth column), the volcanic and sedimentary rock assemblage is also predominantly tuff-breccia, but less than 1,000 feet (310 m) thick. It is the unnamed lower part of the Tropico Group (Dibblee, 1958a, p.139, I967a, p. 76-79), but presumably is correlative with the Gem Hill Formation. It is capped by the Saddleback Basalt. In the East Kramer borate district (Fig. 6, seventh column), this unit is granitic conglomerate. In the Kramer Hills (Fig. 6, eighth column), the lower part of the Tropico Group is as thick as 2,000 feet (620 m), and is composed of tuff-breccia, sandstone, shale, and dolomitic limestone. It is capped by the Red Buttes Quartz Basalt (Dibblee, 1958a, p.142-143; 1967a, p.79).

Exposures in the Tehachapi Area

In the Monolith-Cache Peak area northeast of Tehachapi, and in Jawbone Canyon farther northeast, northwest of the Garlock fault, a unit of tuff-breccia with

N ' 1 VO° • " ... . , 5. _, 0 v 40 • \., ••••:1' , . . -- , A s Os...... ? ...- • ...... - 4,, . ,,, 10 _ --T- \ ••••••• 1(.7.1V I Alp! 'Pr ./.... 6-' : ....\,. 7/1 + i ' ,.., / ...... ''''' . - 1 ''' /../...- / + -- if 1- _ — ._ 2 . ------• ' ....i 0 ..-____ _ . + /../ %., + + -I-- - — • ,- 1 041af//, t ..-"*--f?t-1.-. \ .., ,/ . .---. • + 4- (t3 + \ : 446 ---1- '' ..,,,,11- ,\U l'' I' s. ctair _ _ / ,...„,- ..F• .,-. + ," -,' _ _ . , --. 4 , .., z 13 -,,,.11‘,, q1' ,,- ..- + (Q,,. + + , - -.4„ . , + r 4,/ _-- - . + + 4- -+-- + t +67 , j-1- -I-V -I:\ '•,-,. Desert- f ' ----7.- 4- + + et. p, lit-- -.A.,. if.,2 \ 4,, + ---' IP) + + + + + ' -'F,' "1-', _`...4 1 ,',;\ ., . ' -. I' \ N1Dasin 3 / ° I '', I -,-- _ / . 16 . , , \ i ..,..... t 1+ NT: F ‘‘ \ ' A. II .,_ .1t ./....'" ,...r -/ + +4 +bb' + 4-cf-_....-At. - ---- % 1 i, '.• A e ....,/ ..,..4._ +. + 41-1; ?' —4- _;;‹,, + , • 1 A 4 ..•-• 7- . .. --' -.4- 4- 4- 4- -I- + / ‘ \,,NY. olii/.4 11111P + 1 .., ----- \,...... - 1 \,,+ 4 , .. I 3 k,,.. 111,W/row -,•,:_ + + „( a i. Western Mojave+._ / = ?WA,: ,111, 0 -,(4...,.. .kl. .-..... Desert basint-'••• '', , „,,AP.: -,,,,,:„, . c*' 1 ..- - '16,11ifi - ',..0,',..... 1P7 / ' ict,,,. + + + ..." . -''.• ' ..... • ...... + + .../ • e-

• • ••••,... . '...:,:.... • - *..4 ".";:-.. .::•• EXPLANATION . • . .... 751^31 p , ,.. ..?...... "ir, ,,,,, fri . .k\ //n1.,.. ••••,..•-•,, 7 Marine sedimentary • ••••••.', Pit - NW /A VM \\' ifiS • • ..2:-...„ ... .

- • Terrestrial sedimenta.ry \'‘t, 2.•-••

[ C ] Terrestrial'-pyroclastie ? mvoyif 0.5 flj

Extrusive volcanic

0 LC 20 H.LES

Intensive volcanic (1- + 1 L+ _ Lnwland area: deposition of terrestrial Lowland Area: deposition of sediments followed hy submergence and pyroclustic sediments deposition of marine sediments

— — — Fault:probably active . _ Probable highland area: erosion Lowland area at times flooded by lake or nondeposition _

0040 ,11 Lowland area: deposition of Inferred outline of probable mountainous terrestrial sediments area: severe erosion

Figure 5. Paleogeographic map of westerp Mojave Desert region - Oligocene (?) and early and early middle Miocene times. From Dibblee, 1967a, p. 118

West end of Kramer East Krarner Antelope Rosamond Bissell Castle Krarner ui Antelope borate borate tu 0 Buttes Hills Hills Butte Hills Valley district district < (fig. 33) (figs. 39,40) (fig. 43) (fig. 44) (fig. 50) (figs. 35, 36) (fig. 46) (fig. 49)

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Stratigraphic units (nonmarine unless otherwise Miocene(?) noted): Tgh, Gem Hill Formation (of Tropico Group)—middle Qoa,older alluvium and Qof, fanglornerate—Pleis- or lower Miocene(?) to Oligocene(?) tocene Tgb, Bobtail Quartz Latite Member of Gem Hill Tm, Meeke Mine Formation—Pliocene and (or) Formation and Tbl, Bissell Formation (of Tropico Pleistocer e(?) Group)—Miocene(?) To, Oso Canyon Formation and To, Quail Lake Ttu, upper part of Tropico Group—middle Miocene Formation (marine •and brackish)—upper Tsb, Saddleback Basalt and Trb, Red Buttes Quartz Miocene Basalt (both of Tropico Group)—lower Miocene(?) Tn, Neenach Volcanic Formation—middle or lower Ttl, lower part of Tropico Group—lower Miocene(?) Miocene and Oligocene(?) or Oligocene(?) Tf, Fiss Fanglomerate (of Tropico Group)—upper Figure 6. Exposures at the west end of Antelope valley eastward to the Kramer Hills. From Dibblee, 1967a, pl. 4. 50 basalt flows mapped as the Kinnick Formation is exposed (Fig. 3, first and second columns), (Dibblee, 1967a, p.91, 94-96; Dibblee and Louke, 1970). It is about 2,000 feet (620 m) thick and is lithologically similar to the Gem Hill Formation, but was deposited probably in a different basin. The Kinnick Formation yielded meager mammalian remains suggestive of middle Miocene (Hemingfordian) age.

Exposures in the Central Mojave Desert

In the central Mojave Desert basin, from the Gravel Hills-Opal Mountain area through the Barstow-Calico area to the Newberry, Cady, Bristol, Marble and Clipper Mountains, and Alvord Mountains, the volcanic and sedimentary assemblage is more heterogeneous and variable in lithology and thickness. In the Gravel Hills, Opal Mountain, Mud Hills and Calico Mountains near Barstow and Yermo, the assemblage is as thick as 2,800 feet (900 m) and is composed primarily of tuff-breccia mapped as the Pickhandle Formation (McCulloh, 1952; Dibblee, 1967a, p.84-89, 1968, p.19-26, 1970) (Fig. 7, first through fourth columns). In most of these areas it includes local rhyolitic to dacitic flow-breccias near volcanic necks. In the low hills northeast of Barstow, it includes several flows of basalt and thin limestone strata deposited in lakes (Dibblee, 1970).

In the Alvord Mountains north of Manix, the assemblage is about 1,000 feet (310 m) thick and composed of the Clews Fanglomerate at the base on an irregular basement erosion surface, overlain by the Alvord Peak Basalt, in turn overlain by the Spanish Canyon Formation of tuff-breccia with basalt flows (Byers, 1963, p.15-26).

In the Cady, Bristol, Marble and Clipper Mountains, the early mid-Tertiary volcanic and sedimentary assemblage is as thick as 7,000 feet (2,200 m), and is a heterogeneous series of tuff-breccias, andesitic to basaltic lava flows, coarse agglomerates, and alluvial fanglomerates of either volcanic or basement detritus. The volcanic rocks were erupted from local vents or fissures (Dibblee and Bassett, 1966b; Dibblee 1967c).

In the area south of Ludlow, the assemblage is a series of more than 10,000 feet (3,100 m) of coarse tuff-breccias and andesite lava flows erupted from vents now filled with large andesitic intrusions. This must have been an area of intense and violent volcanic eruptions. The volcanic series overlies an irregular basement surface (Dibblee, 1966, 1967b).

In the Newberry Mountains, the early mid-Tertiary volcanic and sedimentary assemblage is an enormously thick series, as much as 18,000 feet (5,800 m), of intercalated tuff-breccias, andesitic and basaltic lava flows, and coarse boulder fanglomerates, and in some places breccias of basement rocks. The lava flows were erupted from north-trending fissure vents, now filled with dikes. The fanglomerates and basement breccias were derived from basement rocks now exposed to the south. This whole series buttresses out southwestward against a fantastically irregular basement surface of more than 10,000 feet (3,100 m) of relief (Dibblee and Bassett, 1966a, Dibblee, 1970). This condition leads some investigators to interpret the contact as a great thrust fault, but the whole series dips uniformly into it, and there is no gouge nor shearing at this contact.

The enormous amount of volcanic material erupted in early middle Tertiary time, and deposited with coarse alluvial detritus, indicates severe volcanic and tectonic activity during that time when the Mojave Desert uplift of basement terrain

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-'' , I Stratigraphic units (all nonmarine): Qoa, older alluvium and Qb, Black Mountain Basalt— Pleistocene Tba, Barstow Formation—upper and middle Miocene Tp, Pickhand le Formation and Tom, Opal Mountain Volcanic Formation—middle Miocene to Oli- gocene(?) Tj, Jackhamrner Forrnation—middle(?) or lower Miocene and Oligocene(?)

Figure 7. Exposures in the Gravel Hills area southeastward to the Mud Hills. From Dibblee, 1976a, pl. 4.

52 was severly disrupted. This orogenic event is probably the local ettect of the orogeny that started to break up the basement terrain that is now the Basin and Range Province, when the part north of the Mojave Desert was broken up into north-trending basins and ranges as fault blocks.

The disruption of the Mojave Desert terrain is also contemporaneous with diastro- phism followed by deposition of red beds in the Transverse and Coast Range areas in Oligocene-early Miocene time. During and since that time much of the detritus eroded from the basement highland terrain, now the Great Basin and Mojave Desert Sections of the Basin and Range Province, that was previously carried outward to sea, became entrapped in the internal basins that formed within these subprov- inces.

Minerals

Minerals of economic value in the early mid-Tertiary volcanic and sedimentary rocks, are mainly ores of gold and silver in veins (some very rich) in or near volcanic plugs in the Mojave, Calico, and Ludlow mining districts; veins of barite near Barstow and Ludlow; manganese near Ludlow; and perlite, pumice, zeolite, and tuff of commercial grades near Opal Mountains (Wright and others, 1953; Troxel and Morton, 1962; in this volume: Polvina, 19803 Fife, 1980a and 1980b; and Clark, 1980).

In some areas, rhyolitic rocks of this assemblage contain semiprecious stones suitable for cutting and polishing. These include deposits of agate, jasper and geodes in the Tehachapi area, Mojave area, Eagle Crag Mountains, Opal Mountain and Bullion Mountains.

MIDDLE TERTIARY SEDIMENTARY ROCKS

Definition and Areal Extent

In• the western and central Mojave Desert, the early mid-Tertiary volcanic and sedimentary rocks are overlain, in some areas unconformably, by a terrestrial sedimentary series as thick as 7,000 feet (2,200 m) of known and probable Miocene (late Arikareean to Barstovian) age, which in some areas contains a few thin beds of tuff and lava flows of basalt. This series is locally known as the Barstow Formation in the central Mojave Desert; and upper part of the Tropico Group, Fiss Fanglomerate, Oso Canyon and Quail Lake Formations in the western Mojave Desert (Dibblee, 1967a). In the southern Mojave Desert, it is known as the Punchbowl Formation of Cajon Canyon. All these units will be referred to collectively as the middle Tertiary sedimentary rocks or assemblage.

This sedimentary assemblage accumulated in several large valleys that formed as depositional basins in Miocene time in the Mojave Desert region. The basins evolved after an interval of crustal movements in early Miocene time that affected possibly the entire Mojave Desert region. The designation and areal extent of these terrestrial basins in the western Mojave Desert is shown on Figs. 8 and 9.

Some of the basins filled with Miocene and/or younger sedimentary deposits may be in part fault-bounded, but from their configuration they seem to have subsided as downwarps with general east-west elongation. The East Antelope and Cajon basins

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EXPLANATION

EE L Marine sedimentary Area submerged under sea: Highland area: erosion deposition of marine nr nondeposition sediments , Terrestrial sedimentary --- Lowland- area: deposition inferred outline of probable mountainous area: severe of terrestrial sediment.s erosion I Volcanic Li • " Fault. active Lowland area at times active flooded by lake Fault, probably

Figure 8. Paleogeographic map of western Majave Desert region - late middle and late Miocene times. From Dibblee, 1967a, p 119.

IC ARE16'' , EXPLANATION ICA Slt.0 • .-•, At SCALE o5 •w,CE ""` Stna s[cIPoNS

Alluvial sediments and basalt

Sedimentary and volcanic rocks

rL Plutonic and metamorphic rocks

Inferred limit of depositional t= bamin

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Figure 9. Probable areal extent of depositional basins filled with sedimentary and volcanic rocks of Tertiary age in western Mojave Desert, and their inferred profiles and depth, as based on surface exposures, gravity data, and data from test wells shown. From Dibblee, 1967a, p 113.

(Fig. 9) are east-trending downwarps of the basement surface of the south Mojave Desert uplift near the San Andreas fault zone. During Miocene time the western Mojave Desert became divided into the West Antelope basin and the Kramer basins (Fig. 9), or valleys, also with easterly trends, in which subsidence continued. Subsidence resumed or continued in the central part of the central Mojave Desert basin to evolve into the Barstow basin (Fig. 9), which became a large valley. Most if not all of these valleys must have been closed basins which contained lakes in their lowest parts.

Exposures in the West Antelope Basin

At the west end of Antelope Valley, which is the west end of the West Antelope sedimentary basin, the middle Tertiary (Miocene) sedimentary series is well exposed in the foothills (Fig. 6, first column), (Dibblee, 1967a, p.58-62). There it overlies the Neenach Volcanic Formation, and is composed of nearly 7,000 feet (2,200 m) of sedimentary deposits of late Miocene age. It is divisible into two units. The lower unit, the Quail Lake Formation, is mostly tan sandstone deposited along or near shore. This formation grades westward into marine semi- siliceous shale ("Santa Margarita"), and eastward and upward into the upper unit, the Oso Canyon Formation, composed of stream-laid fanglomerate of granitic and rhyolitic detritus, and red sandstones and mudstones. The Oso Canyon Formation extends, concealed, eastward into the West Antelope basin under west Antelope Valley.

Farther east, in the Antelope Buttes and Rosamond Hills, is exposed the Fiss Fanglomerate of volcanic detritus, which unconformably overlies the Gem Hill Formation of tuff-breccia (Fig. 6, second and third columns). The Fiss Fanglomer- ate (Dibblee, 1958a, p.136-I41; 1963, p.187-191; 1967a, p.63-67) may correlate with the lower part of the Oso Canyon Formation.

In the Bissell Hills at the east end of the West Antelope basin, and about 11 miles (17 km) northeast of Rosamond, tuff-breccia of the Gem Hill Formation is overlain by the Bissell Formation (Miocene ?). It is about 760 feet (250 m) thick and in ascending order is composed of lacustrine siliceous shale and carbonate strata, clay shale with thin magnesite strata, and granitic pebbly sandstone (Fig. 6, fourth column), (Dibblee, 1958a, p. 141-142; 1963, p.191-198; 1967a, p.67-69).

Exposures in the Kramer Basin

In the Kramer basin (Fig. 9), the middle Tertiary sedimentary assemblage, there referred to as the upper most part of the Tropico Group (Dibblee, 1958a, p.136- 139; 1967a, p.79-82), is exposed in the Kramer Hills (Fig. 6, eighth column), but in the intervening alluviated areas it is known only from logs of test holes drilled for minerals, and mine workings at the Kramer borax mine. At the Kramer borax mine, this unit overlies the Saddleback basalt and is composed of about 500 feet (130 m) of lacustrine claystone of which the middle 250 feet contains the sodium borate deposit, and is overlain by sandstone and granitic conglomerate (Fig. 6, sixth column).

In the east Kramer borate area, the upper part of the Tropico Group is composed of lacustrine clay shale with thin layers of calcium borate (colemanite). This shale is about 700 feet (220 m) thick, overlies granitic conglomerate and is overlain by granitic sandstone and conglomerate (Fig. 6, seventh column).

In both the Kramer and east Kramer borate areas the borate-bearing lake beds were formerly inferred to be of Pliocene age (Gale, 1946, p.335, 339-349, pls. 51, 52; Dibblee; 1958a, p.138, 142; 1958b). However, mammalian fossils found in sandstone above the borate deposit in workings at the Kramer borate mine, are diagnostic of early middle Miocene (early Hemingfordian) age (R. H. Tedford, in Dibblee, 1967a, p.82). The lake beds of both the Kramer and east Kramer borate districts are overlain by fanglomerate of granitic detritus. These lake beds and overlying fanglomerate thereby represent the Miocene sedimentary rocks in the Kramer basin.

Exposures in the Tehachapi Basin

In the Cache Peak area near Tehachapi and north of the Garlock fault, the Miocene sedimentary rocks are mapped as the Bopesta Formation (Dibblee, 1967a, p.91-96; Dibblee and Louke, 1970). It is composed of as much as 2,800 feet (900 m) of stream-laid arkosic sandstone with minor siltstone and conglomerate. It includes several flows of basalt, intertongues northward into and is overlain by andesite, and overlies the Kinnick Formation (Fig. 3, first column). The Bopesta Formation yields a mammalian fauna of late Miocene (Barstovian) age (Dibblee, 1967a, p.93).

Exposures in the Northwestern Part of the Barstow Basin

In the Gravel Hills and Mud Hills in the northwestern part of the Barstow basin northwest of Barstow, the middle Tertiary sedimentary rocks are mapped as the Barstow Formation (Fig. 7), (Dibblee, 1967a, p.86-90; 1968, p. 19-32; Link, 1980, this volume;), with the Mud Hills as the type area. In these areas, the Barstow Formation is as thick as 2,500 feet (800 m) and consists of alluvial conglomerate, sandstone, and lacustrine clay or shale, deposited in a valley which contained a lake at its lowest part. In the Gravel Hills, the conglomerate consists of volcanic detritus derived from the north and it unconformably overlaps northward over the Pickhandle Formation onto granitic basement. In the Mud Hills, the conglomerate is mostly of granitic detritus. In this area, the Barstow Formation contains several thin strata of tuff and limestone. In this area and in Black Canyon of the Gravel Hills, the Barstow Formation yielded a large mammalian fauna of middle and late Hemingfordian and Barstovian age (G. E. Lewis, in Dibblee, 1968, p.33-35).

Exposures in the Central and Eastern Parts of the Barstow Basin

Southeastward from the Mud Hills, the Barstow Formation extends into the Calico Mountains (McCulloh, 1952; Dibblee, 1970), where it is similar. In the eastern part, it is in large part composed of thin-bedded lacustrine clay shale that contains a calcium borate (colemanite) deposit (Wright and others, 1953, p.223- 225). The Formation contains upper Miocene (Barstovian) age horse teeth. It overlies tuff-breccia and volcanics of the Pickhandle Formation, and in the eastern part of the range is intruded by and overlain by andesite porphyry.

In the Alvord Mountains north of Manix, the Barstow Formation is about 1,250 feet (400 m) thick. It overlies tuff-breccia of the Spanish Canyon Formation and is composed of sandstone and granitic conglomerate. It yielded late Miocene (Bar- stovian) age horse teeth (Byers, 1953, p.15-35).

On the north slope of Daggett Ridge south of Daggett, coarse fanglomerates, basement breccias and volcanic rocks, and the basement complex together are overlain with great angular discordance by the Barstow Formation or its

equivalent, where it is composed of about 1,500 feet (460 m) of sandstone, mudstone and granitic conglomerate (Dibblee, 1970). This formation yielded fossils of late Miocene (Barstovian) age (R. H. Tedford, in Dibblee, 1970).

On the south side of both Daggett Ridge and the Newberry Mountains to the east, the equivalent of the Barstow Formation is composed of about 2,000 feet (620 m) of coarse fanglomerate of basement detritus derived from the south, and some locally derived volcanic detritus, includes some sandstone and tuff-breccia. This formation is unconformable on both the volcanic and sedimentary assemblage and the basement complex (Dibblee, 1964a, I964b, 1970; Dibblee and Bassett, 1966a).

In the Cady Mountains, middle Tertiary sedimentary rocks are exposed on the northeast slope, roughly 18 miles (30 km) east of Manix, and in several places on its southwest flank near Hector, where this series is slightly unconformable on the early mid-Tertiary volcanic and sedimentary assemblage. On the northeast slope, the Miocene sedimentary deposits aggregate about 2,000 feet (620 m) in thickness. They consist of a lower unit of mostly sandstone that contains several thin basalt flows and an upper unit of coarse fanglomerates of volcanic and granitic detritus. The lower unit yielded mammalian fossils of middle Miocene, early Hemingfordian(?) age (Bassett and Kupfer, 1964, p.22; Dibblee and Bassett, 1966b).

In the southwestern Cady Mountains, the early mid-Tertiary volcanic and sedimentary rocks are unconformably overlain in several places by a sedimentary unit of fanglomerate of volcanic detritus. This unit was erroneously assigned to the upper Tertiary ()ibblee and Bassett, 1966b). Later it was named the Hector Formation, with the type section exposed about 4 miles (6 km) northeast of Hector where it is about 1,600 feet (500 m) thick (Woodburne and others, 1974). It contains thin strata of sandstone and tuff in its lowest part that yeilded mamalian faunas of early Miocene (late Arikareean and early Hemingfordian) age, and one of its tuff beds was radiometrically dated as about 21 m.y.b.p. (Woodburne and others, 1974, p. 6-16).

In the foothills about 4 miles west of Hector, is exposed about 700 feet (220 m) of fine-grained sandstone and siltstone which may be equivalent to or younger than the Hector Formation. In the flat south of Hector, this unit might have been penetrated in several test holes drilled in 1964 by Congdon and Carey Company. One of these holes, 4 miles (6 km) southeast of Hector, penetrated lacustrine claystone with thin strata of crystalline evaporites composud of anhydrite with colmanite, calcite and strontianite (Dibblee and Bassett, 1966b).

Exposures in the Cajon Basin

Terrestrial sedimentary deposits, superbly exposed south of Cajon Pass, northeast of the San Andreas fault, have been mapped as the Punchbowl Formation, Miocene age (Noble, 1954b, ch. IV, pl. 5; Dibblee, 1967a, p.49, 53-54; Woodburne and Golz, 1971; Woodburne, 1975, p.67-68; in Foster, 1980, this volume). In this area it is about 5,500 feet (1,800 m) thick (fig. 4, sixth column); possibly as thick as 8,000 feet (2,600 m) (Woodburne and Golz, 1976, p.16; Woodburne, 1975, p.68). It is composed of stream-laid arkosic conglomerate, sandstone, and minor siltstone, unconformable on granitic basement and lower Tertiary marine sandstone where present in Cajon Creek. In some places, the Punchbowl Formation is underlain by a basal marine sandstone unit as thick as 360 feet (100 m) that contains an early Miocene ("Vaqueros") molluscan fauna. The Punchbowl Formation of Cajon Valley yielded a mammalian fauna of late middle (?) and late Miocene (late Heming- fordian ? and Barstovian) age (Woodburne and Golz, 1971; Woodburne, 1975, p.68).

58 The Punchbowl and younger formations here dip northward toward, and presumably extend into, the Cajon Basin (Fig. 9).

Mineral Deposits

Minerals of economic value in the middle Tertiary sedimentary rocks have already been mentioned. Nearly all are in lacustrine argillaceous sediments, most are saline evaporites. One of these, the Kramer sodium borate (borax) deposit near Boron, the largest source of boron in North America, is a large crystalline body as thick as 200 feet (65 m) in lacustrine clay. The same unit contains moderate amounts of calcium borate (colemanite) at the east Kramer deposit near Kramer Junction (Dibblee, I967a, p.125-127). Other deposits of colemanite occur in clay of the Barstow Formation in the east Calico Mountains (Dibblee, 1970). Near Hector, clay of unknown Cenozoic age contains anhydrite with some colemanite (Dibblee and Bassett, 1966b). In the eastern Mud Hills, the Barstow Formation contains minor amounts of strontianite. Near Ludlow, shale of Miocene (?) age contains moderate amounts of celestite with cabonate strata (Dibblee and Bassett, 1966b),and lacustrine clay contains hectorite, a lithium-bearing clay mineral (Sweet, 1980, this volume). Near Rosamond, clay of the Bissell Formation contains thin layers of magnesite (Dibblee, 1963).

Lacustrine clays of the Miocene sedimentary assemblage may contain hidden econom- ically valuable mineral deposits in several of its alluvium-covered basins that could be found only by exploratory drilling.

LATE TERTIARY SEDIMENTARY AND VOLCANIC ROCKS

Definition and Areal Extent

Terrestrial sedimentary and local volcanic formations of known and inferred late Tertiary (Clarendonian and Hempillian) age are present in basins near or along the northwestern and southwestern margins of the Mojave Desert near the Garlock and San Andreas faults, respectively. With the possible exception of the west Ante- lope basin, those formations are not present in any of the basins of the western and central Mojave Desert filled with formations of middle Tertiary (Miocene) age. This condition, as pointed out by Hewett (1954, ch.II, p.16), indicates that subsidence of those basins ceased after Miocene time, when new basins formed along the margins of the Mojave Desert region.

The late Tertiary sedimentary deposits are locally known as the Horned Toad, Ricardo, Bedrock Spring and Avawatz Formations along or near the Garlock fault zone, and the Anaverde, Crowder and Santa Ana Formations near the San Andreas fault zone. All these valley formations and associated volcanic rocks of late Tertiary age are referred to collectively as late Tertiary sedimentary and volcanic rocks.

Late Tertiary Sedimentary Formations Along and Near the Garlock Fault Zone

In the Tehachapi Mountain foothills north of the west end of Antelope Valley, the Miocene Oso Canyon Formation is unconformably overlain by terrestrial sediments mapped as the Meeke Mine Formation of inferred Pliocene and Pleistocene (?) age

59 (Fig. 6, first column), (Dibblee, 1967a, p.59, 62). About 1,500 feet (490 m) of this unit is exposed with the top eroded. It is composed of weakly indurated conglomerate and sandstone derived from rocks of the Tehachapi Mountains, and lacustrine clay. It unconformably truncates the Oso Canyon Formation northward onto basement rocks, and is strongly folded. Presumably the Meeke Mine Formation overlies Miocene sediments to •the east in the deeper part of the West Antelope basin under Antelope Valley. If this unit is of Pliocene age, this is the only basin in the western Mojave Desert where sediments of both Miocene and Pliocene ages are present.

In the Horned Toad Hills area northwest of Mojave, granitic basement is overlain by about 1,050 feet (310 m) of arkosic sandstone, pebble conglomerate, clay, and marl beds together mapped as the Horned Toad Formation (Fig. 3, fifth column), (Dibblee, 1958a, p.143; 1967a, p.97-99). It yielded a mammalian fauna of middle Pliocene (late Hemphillian) age (Tedford and Savage, in Dibblee, 1967a, p.98). This formation dips southeastward under the alluviated valley area near Mojave, and if so, it was deposited in a local shallow basin, designated as the Mojave Basin (Fig. 9).

Koehn Valley (also known as Cantil Valley) which contains Koehn Dry Lake at its lowest part, is the site of a deeply subsided trough or basin of low density material (sedimentary deposits) between branches of the Garlock fault zone, as indicated by a gravity geophysical survey (Mabey, 1960; Dibblee, 1967a, p.112), and several deep test holes. This basin, designated the Koehn basin (Fig. 9), may contain more than 10,000 feet (3,100 m) of sedimentary deposits.

Sediments that fill this basin are probably in large part those of the Ricardo Formation, which is exposed in Redrock and upper Last Chance Canyon of the El Paso Mountains elevated to the northwest (Fig. 3, third column), (Dibblee, 1967a, p.98-104). The Ricardo Formation is composed of a basal conglomerate and a pyroclastic unit, overlain by lacustrine clay with strata of sandstone, limestone, chert, and volcanic flows that grades upward and westward into fluviatile conglomerates. This formation unconformably overlies the basement complex and the Goler Formation, and it thins rapidly northeastward from about 5,700 feet (1,900 m) at Redrock Canyon to less than 700 feet (220 m) near Black Mountain and upper Goler Gulch (Fig. 3, third and fourth columns). The Ricardo Formation yielded a mammalian fauna of early Pliocene or latest Miocene (Clarendonian) age (Merriam, 1919, in Dibblee, 1952, p.30; 1967a, p.104).

In the Sumwit Diggings area, Red Mountain and Lava Mountains northeast of Rands- burg, is exposed a series of arkosic sandstone, conglomerate and red clays, and intercalated volcanic rocks (formerly mapped as the Rosamond Series, Miocene (?), Hulin, 1925, p.58-60). However, in the Lava Mountains and Red Mountain, where it yielded fossils of middle Pliocene (Hemphillian) age, it was named the Bedrock Spring Formation (Smith, 1964, p.15-23), with the type section in the Lava Mountains. In all these areas it overlies the basement complex, and ranges from 4,000 feet (1,300 m) to 5,000 feet (1,700 m) in exposed thickness (Fig. 3, sixth and seventh columns).

The Bedrock Spring Formation may have been deposited in the eastern extension of the Koehn Basin, and if so, it may extend westward, concealed under Koehn Valley. It may also extend eastward an unknown distance under Pilot Knob Valley along and near the Garlock fault, possibly as far east as the Avawatz Mountains, but there are no exposures to ascertain this interpretation.

60 On the southwest slope of the Avawatz Mountains, the basement complex is overlain by a series of fluviatile conglomerates, sandstones and clays called the Avawatz Formation that yielded a mammalian fauna suggestive of early Pliocene age (Hen- shaw, 1939, p.5-9). These beds must have accumulated in a trough that extended southeastward toward the Soda Mountains.

In the Soda Mountains, the basement complex is overlain by as much as 5,300 feet (1,700 m) of alluvial conglomerate of basement detritus and minor amounts of sandstone, clay, and local lenses of monolithic breccia. These beds are unfossil- iferous, but were correlated with the Avawatz Formation of the Avawatz Mountains on the basis of similar lithology (Grose, 1959, p.1514-1537). In the Soda Moun- tains, the beds are divided into a lower member of coarse alluvial fanglomerate a- bout 3,200 feet (1,000 m) thick, a middle member of fangolmerate and tuffaceous sandstone about 1,000 feet (310 m) thick, and an upper member of pebbly conglomerate. Lenses of limestone breccia occur in the lower and middle members, as mapped by Grose (1959)

In the Shadow Mountains northeast of Baker, the granitic basement is overlain by 1,460 feet (290 m) or more of northeastward-dipping terrestrial sediments of Miocene or Pliocene age composed of alluvial gravel, sand, clay, and several lenses of dolomite breccia, as described by Hewett (1956, p.86-90). These beds are lithologically similar to the Avawatz Formation as described by Grose (1959, p.1514-1537).

Late Tertiary Volcanic Rocks

Red Mountain and the Lava Mountains east of Atolia are composed largely of volcanic rocks, mostly andesitic porphyry, of probable Pliocene age. This rock, (called the Red Mountain Andesite by Hulin, 1925, p.55-58, p1.1) is intrusive into the Bedrock Spring Formation and overlies it as thick extrusive masses (Fig. 3, seventh column), (Dibblee, 1967a, p.106-107; Smith, 1964, p.23-28), emplaced and erupted in several generations. In the Lava Mountains, it was mapped as the Lava Mountain Andesite and Almond Mountain Volcanics by Smith (1964). These mountains are indeed remnants of volcanoes during Pliocene time. Scattered local eruptions of this andesitic porphyry occur southeastward from the Lava Mountains on the basement complex as far as the Calico Mountains, where it was called the Lane Mountain Andesite, some of which occurs on previously deformed middle Tertiary formations (McCulloch, 1952; Dibblee, 1967a, p.91; 1968).

In the Randsburg area the basement complex is cut by several volcanic plugs and many dikes of rhyolitic felsite of unknown Tertiary age. They are older than the Bedrock Spring Formation, because they occur as fragments in it and do not intrude it. They are therefore no younger than middle Pliocene, or may be of Miocene age (Dibblee, 1967a, p.105).

Late Tertiary Sedimentary Formations near and Along the San Andreas Fault

Sedimentary deposits mapped as the Anaverde Formation (Wallace, 1949, p.790, pl. 1; Dibblee, 1960, 1961, 1967a, p.56-57) are exposed along a narrow sliver of the San Andreas fault zone, bounded by the San Andreas fault on the southwest and faults parallel to it on the northeast for about 40 miles (64 km), from Elizabeth Lake to Valyermo. Along this sliver the Anaverde Formation is upended and in part

61 sharply compressed into a syncline, with about 1,600 feet (500 m) of strata exposed. The deposits overlie granitic basement and consist of fluviatile arkosic sandstone with granitic pebbles and lacustrine claystone. Northeast of the San Andreas fault zone near Palmdale granitic basement is overlain by arkosic pebbly sandstone, also mapped as the Anaverde Formation (Fig. 4, second column), (Noble, 1953; Dibblee, 1960, 1967a, p.57) that dips north into and presumably extends into the East Antelope Basin.

No mammalian fossils have been found in the Anaverde Formation, but plant remains are suggestive of Pliocene (Hemphillian) age (Axelrod, in Wallace, 1949, p.79I). This formation may be correlative with the Peace Valley Formation, Pliocene (Hemphillian) of the Hungry Valley area of the Ridge basin southwest of the San Andreas fault (Dibblee, 1967a, p.56; Woodburne, 1975, p.22), (Fig. 9 and Fig. 4, first column), and possibly with part of the Punchbowl Formation (early Pliocene) in the Devils Punchbowl area (Fig. 4, fifth column), (Noble, 1954a).

In the Cajon valley the Crowder Fromation unconformably (?) overlies the Punchbowl Formation, which there is of late Miocene (Barstovian) age, and the Crowder is composed of about 2,000 feet (620 m) of stream-laid arkosic pebbly sandstone (Fig. 4, sixth column), (Dibblee, 1967a, p.49, 52-56). Eastward from Cajon Valley, it rests on the basement complex. It is overlain by Pleistocene alluvial sand (Harold Forma- tion) and gravel (Shoemaker Gravel) that forms the rim of Cajon Pass. No diagnostic fossils have been found in the Crowder Formation, but its age is inferred to be Pliocene on the basis of its stratigraphic position. It dips northward into the Cajon basin in which it forms part of the sedimentary fill.

Remnants of the Crowder Formation or its equivalent extend far eastward, high in the San Bernardino Mountains. South of Big Bear Lake it was mapped as the Santa Ana Sandstone (Dibblee, 1964c) where it is of similar lithology. In the north- eastern foothills of this range between Lucerne Valley and Yucca Valley is exposed about 300 feet (100 m) of arkosic sandstone mapped as the Old Woman Sandstone, Pliocene (?), overlain by a basalt flow (Dibblee, 1964d; 1967d; 1967e; 1967f).

Minerals

The late Tertiary sedimentary and volcanic rocks contain only a few mineral deposits. In Last Chance Canyon in the western El Paso Mountains, the Ricardo Formation contains strata of pure volcanic ash, of commercial quality, as thick as 20 feet (6.5 m) which has been mined for many years. Perlite from the top of an andesite flow-breccia in the lower part of the Ricardo Formation has also been quarried for light weight aggregate, also from this canyon (Dibblee and Gay, 1952, p.50-52). Near Palmdale the clay member in the upper part of the Anaverde Forma- tion contains many thin beds of gypsum.

QUATERNARY SEDIMENTARY AND VOLCANIC DEPOSITS

Pleistocene Alluvial Deposits

Elevated and dissected deposits of presumed Pleistocene age, unconformable on older formations, including those of Pliocene age, are extensively exposed in the upper slopes of the valley areas of the Mojave Desert region. They fringe the mountain areas where they consist of coarse gravels derived from the adjacent mountains, and they grade down-slope through finer gravels into pebbly sands.

The deposits dip under Holocene alluvial deposits in the lower parts of the valleys where the buried deposits probably consist of sands and clays. The alluvial deposits range up to about 1,000 feet (310 m) in exposed thickness. In some areas, such as in the Cajon Pass and Tehachapi foothill areas, the coarse gravel, as thick as 700 feet (210 m), is underlain by a lower unit, as thick as 400 feet (130 m) of pebbly sand and siltstone. At Cajon Pass this unit, assigned to the Harold Formation, Pleistocene (of Noble, 1953) by Woodburne and Golz (1971, p.67), overlies the Crowder Formation (Fig. 4, sixth column). The Manix Lake beds exposed on the Mojave River north bank near Manix (Dibblee and Bassett, 1966a) consist of several hundred feet of gravel, sand, and lacustrine clay that yielded a large late Pleistocene verterbrate fauna (Buwalda, 1914).

Quaternary Volcanic Eruptions

The only volcanic activity in the Mojave Desert region in Quaternary time was scattered eruptions of basalt lavas. One of these is at Black Mountain on the north slope of the El Paso Range north of the Garlock fault. Several others are scattered in the central Mojave Desert. Most of these are of presumed Pleistocene age. They are elevated and dissected; some are deformed along faults. Each is composed of thin sheets of basalt, made up of one or several flows aggregating as much as 150 feet (45 m) in thickness where each was erupted.

Black Mountain on the north slope of the El Paso Range is composed of the Black Mountain Basalt which is unconformable on the Ricardo Formation (Fig. 3, fourth column), (Dibblee, 1952, p.30; 1967a, p.102-104 and 109). The basalt is much dissected. Its source of eruption is not visible. Several other eruptions of basalt lava occur farther north near the Sierra Nevada foothills.

In the central Mojave Desert local eruptions of basalt lava are present on both flanks of the Barstow basin. Two are on the north flank about 20 miles (32 km) northwest of Barstow, at another Black Mountain, and a similar unnamed mountain to the northeast, separated by Opal Mountain. The basalt sheets that cover each of these mountains were mapped also as the Black Mountain Basalt, Pleistocene (Dibb- lee, 1967a, p.85, 109; 1968, p.38-40), (Fig. 7, second and third columns). At each of these sheets the basalt is as thick as 150 feet (50 m) and is unconformable on the Barstow Formation (Fig. 7, second column). Each of these lava sheets was erupted, probably from fissures, along major northwest-trending fault zones, where each sheet is thickest. Later, right-slip movements on these fault zones buckled, disrupted, and elevated each lava sheet, east of which each lava sheet is tilted eastward and partly dissected.

On the south flank of the Barstow basin, basalt was erupted at three places in Pleistocene time. One was erupted from Malpais Crater in the Rodman Mountains and flowed northward to Kane Canyon, south of Newberry (Dibblee, 1964b). This eruption is on basement about midway between the Camp Rock and Calico northwest-trending faults.

At Sunshine Peak, 14 miles (22 km) southwest of Ludlow, basalt was erupted from Sunshine Crater and two smaller craters northwest, on the Pisgah fault. The basalt was later buckled and faulted by subsequent movements on this fault (Dibblee, 1966).

Near Lead Mountain, 23 miles (36 km) southeast of Ludlow and southwest of the Ludlow fault zone, basalt was erupted from a crater. The lava flowed southeastward and northwestward.

63 All of these local basalt flows erupted from or near major northwest-trending fault zones in Pleistocene time when the faults were active. The positions of these eruptions are roughly aligned, but somewhat diagonal, with a more westerly trend than that of the faults.

Besides these Pleistocene eruptions, there are two undissected Holocene eruptions of basalt lava in the central valley area. One of these was erupted from Pisgah Crater, about 12 miles (17 km) west of Ludlow and east of the Pisgah fault. The other was erupted from Amboy Crater near Amboy, on Bristol dry lake. Both of these lava flows rest on Holocene alluvium and are unweathered.

The only basaltic eruptions in the eastern Mojave Desert are a cluster of craters and flows on granitic basement east of Baker, west of Teutonia Peak. The basalt in this area is composed of a sheet that erupted from many craters in late Pleis- tocene time, and another to the south that erupted from many craters, now cinder cones, probably in Holocene time, as described by Hewett (1956, p.103-105, 109; Berea, 1966; see also Katz and Boettcher, 1980, this volume). There is no active faulting in this area.

ACKNOWLEDGEMENTS

This paper was kindly reviewed by John F. Curran and Arthur R. Brown.

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