2010 Overboard in the Mojave - Biological Science - California State ...

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Stratigraphy and paleontology of the middle to late Pleistocene Manix Formation, and paleoenvironments of the central Mojave River, southern California __________ George T. Jefferson Colorado Desert District Stout Research Center, Anza-Borrego Desert State Park 200 Palm Canyon Drive, Borrego Springs, California 92004, gjefferson@parks.ca.gov Abstract The Manix Formation consists of lacustrine, fluvial and alluvial sediments that were deposited within and adjacent to Lake Manix. This middle to late Pleistocene pluvial lake was probably the largest along the Mojave River drainage system, which heads in the Transverse Ranges of southern California and runs northeast to Death Valley. During high lake stands, the lake covered ~236 km 2 of the central Mojave Desert. Lake Manix deposits record much of the early hydrological history of the upper Mojave River. Attaining an exposed thickness of ~40 m, the Manix Formation overlies playa lake deposits of the Pliocene-Pleistocene Mojave River Formation, and is locally overlain by latest Pleistocene and Holocene fluvial deposits. The Manix Formation is mappable as four laterally equivalent members. Generally from oldest to youngest these were deposited as: basin margin fanglomerates, alluvial and fluvial deposits, lacustrine and paralimnic deposits, and fluvial/deltaic deposits. The interfingering relationships of these sediments, especially between the lacustrine and fluvial systems, documents at least four major transgressive/regressive lacustrine events within the last 0.5 m.y. Transgressive lacustrine events are in phase with relatively cool/moist regional climatic conditions inferred to be present during even-numbered marine oxygen isotope stages. The chronology of these deposits is relatively well constrained by paleomagnetic data, U/Th series ages, tephra chronology, and 14 C ages. Molluscan, ostracode, and aquatic and terrestrial vertebrate assemblages have been recovered primarily from the fluvial, paralimnic and lacustrine deposits within the formation. Included are 55 taxa of fossil vertebrates that range between 350+ ka and ca. 20 ka, spanning possibly late Irvingtonian through late Rancholabrean North American Land Mammal Ages. The assemblage comprises extinct and extralocal extant taxa that reflect climatic and biogeographic conditions dramatically different from the present xeric environment. from Jefferson, G.T., 2003, Stratigraphy and paleontology of the middle to late Pleistocene Manix Formation, and paleoenvironments of the central Mojave River, southern California, in Enzel, Y., Wells, S.G., and Lancaster, N., eds., Paleoenvironments and paleohydrology of the Mojave and southern Great Basin Deserts: Boulder, Colorado, Geological Society of America Special Paper 368, p. 43-60. © 2003 Geological Society of America. 38 2010 Desert Symposium
stratigraphy and paleontology of the middle to late pleistocene manix formation Introduction Lake Manix was probably the largest in a chain of Pleistocene lakes that extended across the central Mojave Desert region of California along the Mojave River drainage (Fig. 1). The lake was located about midway between the headwaters of the Mojave River in the San Bernardino Mountains to the southwest, and Death Valley to the northeast, where the ancestral Mojave River episodically joined the Amargosa River and ended in Pleistocene Lake Manly. Lake Manly also was the terminus of the Owens River system, which drained the east side of the southern Sierra Nevada Mountains and basins within the northern Mojave Desert region (see Blackwelder, 1954; Blanc and Cleveland, 1961a, 1961b; Snyder et al., 1964). With a highstand of 543 m above mean sea level, Lake Manix occupied a cloverleaf-shaped area of ~236 km 2 (Meek, 1990) including the present Coyote Lake and Troy Lake playa basins and the Afton basin (Fig. 2). Well-developed wave-cut terraces and beach bars typify its eastern and southern shorelines (Meek, 2000). The ancestral Mojave River began to fill the Lake Manix basin ca. 0.5 Ma, and Lake Manix persisted with several, apparently short-lived low level stands until the late Wisconsin. Geologic and paleontologic investigations in Lake Manix basin began with Buwalda’s (1914) description of the Pleistocene “Manix Beds of the eastern Mojave Desert.” Buwalda described the stratigraphy, depositional history, and recognized the climatic implications of such deposits. A year later, in his summary of the extinct faunas of the Mojave Desert, Merriam (1915) confirmed Buwalda’s age assessment and compared the terrestrial vertebrate fossils from Manix to those from Rancho La Brea 2010 Desert Symposium Figure 1. Regional map of Pleistocene lakes along the Mojave River drainage. Pleistocene lakes (shaded) are shown at highest stands. The modern Mojave River course is indicated by a long-short dashed line. in Los Angeles, California. Since these early works, the geomorphology, stratigraphy, depositional history, and paleontology of the “Manix Beds” have been the focus of numerous studies. Manix basin and the long-lived lake it Figure 2. Generalized map of Lake Manix basin. The approximate maximum extent of the lake (shaded) is represented by a long dashed line at the 543 m topographic contour. Short dashed lines encompass mountains. The present Mojave River and Manix Wash drainages are represented by long-short dashed lines. Approximate trace of the Manix fault is shown as a solid line. A—Afton Canyon; C—Camp Cady; M—Manix railroad siding; MF—Manix fault; S —top of the reference stratigraphic section (see Figs. 6, 7) 39
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strontium prospects (Durrell, 1953)
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Mountains site contains 28% CaCO by
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Toomey Hills-Yermo Hills (Emerald B
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14.0 Ma (Pagnac, 2009). The Miocene
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Dokka, R.K. and C.J. Travis. 1990.
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Basalt is present east of the axis
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The first tungsten-mining boom at A
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Atolia in 1909. USGS Hess. Their sh
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here and now brand as a deliberate
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[Excerpts:] Thomas McCarthy, the Ra
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