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

held, derives its name from the Manix Union
Pacific railroad siding (Fig. 2), 32 km east of
the city of Barstow, San Bernardino County,
California.
Latest Pleistocene and Holocene incision
by the Mojave River and local tributaries such
as Manix Wash, have exposed a 120+ m-thick
section of alluvial fan, playa, lacustrine and
fluvial sediments that records much of the
Pleistocene history of the central Mojave
River. Exposures in this area serve as the type
localities for the Pliocene-Pleistocene Mojave
River Formation (Nagy and Murray, 1991),
the middle to late Pleistocene Manix Formation
( Jefferson, 1985a, 1994, 1999), and the
Camp Cady faunal assemblage (Winters,
1954; Howard, 1955; Jefferson, 1985b, 1987,
1991a; Seiple, 1994).
The upward-coarsening Pliocene-Pleistocene
Mojave River Formation consists
primarily of fine-grained sediments deposited in an internally
drained playa basin (Nagy and Murray, 1991). These
deposits rest on top of the late Miocene and Pliocene
fanglomerates that initially filled Manix basin. Within
the Manix Formation, which overlies the Mojave River
Formation, locally derived alluvial and fluvial sediments,
and lacustrine deposits largely transported by the ancestral
Mojave River, appear to transgress/regress in consort
with climate changes over the past 0.5 m.y. ( Jefferson,
1985a, 1994, 1999).
The assemblage of invertebrate and vertebrate fossils
recovered from eroded river bluffs and badlands of the
Manix Formation dates from ca. 350 ka to 20 ka, and
includes extinct and extralocal extant taxa that reflect a
more equable climate and diverse biogeographic setting
than at present (Buwalda, 1914; Merriam, 1915; Winters,
1954; Howard, 1995; Jefferson, 1985a, 1985b, 1987,
1991a; Jefferson and Steinmetz, 1986; Steinmetz, 1987,
1988; Seiple, 1994).
The following discussion is an amended synthesis of
previous studies. It provides a summary of the stratigraphy
and depositional history of the Manix Formation,
and a brief discussion of deposits and events relevant to
the history of Lake Manix and the paleohydrology of the
Mojave River. Also included is an analysis of the fossil
assemblage recovered from Lake Manix and its paleoenvironmental
significance ( Jefferson, 1987).
Pliocene-Pleistocene Stratigraphy
Basement rocks exposed around the margins of Manix
basin are primarily pre-Cenozoic metamorphic rocks
george t. jefferson
Figure 3. Mojave River Formation. The view is to the northwest from
approximately 1 km east of the confluence of Manix Wash and the
Mojave River. Gypsum-cemented pale gray-green siltstones and
claystones form the more resistant horizons (also see exposures in midimage
of Fig. 5). Note geologist for scale.
and Tertiary volcanic and volcaniclastic deposits (Beyers,
1960; Bassett and Kupfer, 1964). The oldest fill within the
basin consists of the late Miocene and Pliocene fanglomerates
(older fanglomerate unit of Nagy and Murray,
1991; granitic fanglomerate of Meek and Battles, 1991).
These fanglomerates are overlain by Pliocene-Pleistocene
playa and fluvial sediments of the Mojave River Formation
(Nagy and Murray, 1990, 1991). These units, in turn
are usually overlain unconformably by fanglomerates,
alluvial, fluvial, and lacustrine sediments of the middle to
late Pleistocene Manix Formation (Buwalda, 1914; Ellsworth,
1933; Blackwelder and Ellsworth, 1936; Winters,
1954; Jefferson, 1985a, 1994, 1999; Budinger, 1992).
Late Wisconsin and Holocene fluvial sediments (Hagar,
1966; Groat, 1967; Meek, 1990) both cap and are inset
into the older sedimentary sequence.
Mojave River Formation, ca. 2.5–1 Ma
The Mojave River Formation consists primarily of
gypsiferous, reddish-tan and light gray-green claystones
and siltstones that total >80 m in thickness (Nagy and
Murray, 1991) (Fig. 3). The base of this upward-coarsening
sequence is not exposed in the type area, near the
confluence of Manix Wash and the Mojave River (Fig.
2). However, 4 km to the east, these deposits overlie a
30+ m-thick section of fanglomerates (older fanglomerate
unit of Nagy and Murray, 1991) that are exposed on
the upthrown, north side of the Manix fault (Keaton and
Keaton, 1977; McGill et al., 1988) (Fig. 4). To the south,
toward the Cady Mountains, the deposits grade up section
into fine to coarse-grained lithic arenites and granule
to cobble conglomerates.
In the most northern exposures, these deposits are
40 2010 Desert Symposium