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Cenozoic Rock Units of the Mojave Desert

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Cenozoic Rock Units of the Mojave Desert 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 unconform- ably 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 41 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 con- dition, 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 42 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' 0' Figure 2. Explanation of symbols and vertical scale for VERTICAL figures 3, 4, 6 and 7. SCALE 44 Monolith- Lower • Redrock- Black Horned Summit Red Last Chance Mountain- w Cache Peak Jawbone Toad Hills Diggings Mountain o GE Canyon Goler Gulch area (n)I Canyon (n) area (n) area (n) area (s)1 area (s) area (s) .c( (f gs. 58,59) (fig. 62) (figs.63, 64) (fig. 65) (fig. 62) (fig. 68) (fig. 67) A _ .01 '''',, - tocene % IT 21., ,. , 6 is TERNARY •-•0 % .0. _—..., TERNARY UA Ple UA Q Q 6 - . - . Qoa- .. k..* • Qb • ° • .
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