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

The fossil avians clearly suggest the presence of a
variety of mildly saline or freshwater lake and lake margin
environments. Judging from the food preferences, food
procurement methods and nesting habits of extant bird
species (Cogswell and Christman, 1977; Garrett and
Dunn, 1981), open water, sandy beach flats, and extensive
reedy marshlands were the dominant lacustrine habitats
in Lake Manix ( Jefferson, 1985b). The seasonally extralocal
pattern of the migratory forms in the assemblage suggests
an overall cooler or more equable climate.
Regional terrestrial vegetation patterns, reconstructed
in part from packrat midden data (Spaulding et al., 1984;
Spaulding, 1990), permit inferences about the local
paleoflora. The alluvial slopes and low hills surrounding
Lake Manix probably supported a juniper-sage brushland,
and the nearby mountains most likely were covered with
a pinyon-juniper woodland (Spaulding, 1980, 1990;
Jefferson, 1987, 1991a). Local valley bottoms probably
supported patchy semidesert grasslands and desert
scrub. These floristic associations are consistent with
the inferred browsing habits of the majority of the fossil
mammals (Table 4) ( Jefferson, 1987, 1991a).
Depositional history
Upper Mojave River drainage
Before the uplift of the Transverse Ranges, internal drainage
typified most basins on the southern and central part
of the Mojave Desert block. The southwestern margin of
the block apparently drained across the San Andreas fault
zone, west to the Pacific Ocean (Meisling and Weldon,
1989). About 3–2 m.y. ago, as the Transverse Ranges
adjacent to this margin of the block were elevated along
the San Andreas fault, drainage direction shifted to the
northeast.
In the headwaters area of the ancestral Mojave River,
~100 km to the southwest of Manix, the Victorville Fan
complex was shed off the rising Transverse Ranges northeast
into the Victorville basin (Weldon, 1985; Meisling
and Weldon, 1989; Kenny and Weldon, 1999) (Fig. 1).
Here, magnetostratigraphic data provide a date of 1.95
Ma for the base of the ancestral Mojave River deposits
(Cox and Tinsley, 1999; Cox et al., 2003). Cox et al.
(2003) suggest that the river advanced from the Victorville
basin northward 50 km to Lake Harper basin sometime
after 0.78 Ma and probably between 0.57 and 0.47
Ma. They (Cox and Tinsley, 1999; Cox et al., 2003) argue
that the ancestral Mojave River continued to advance
eastward, overflowing Lake Harper basin, and (based on
Jefferson, 1985a) reached the Manix basin, ~50 km to
the east of the Harper basin, no earlier than ca. 0.5 Ma
ago (Fig. 1). This places the appearance of the ancestral
george t. jefferson
Mojave River in Manix basin during the deposition of the
lower wedge of Member B.
Lake Manix basin
A thick section of late Miocene and Pliocene fanglomerates
that presumably reflects regional extensional tectonics,
documents the initial formation of Manix basin. The
top of the fanglomerates may be laterally equivalent with
the base of the Mojave River Formation, which has an age
of ca. 2.5 Ma. From prior to 2.5 Ma until ca. 1 Ma, Manix
basin was internally drained (Nagy and Murray, 1991).
Subaerial oxidization of silts and clays, bedded gypsum,
and limestones in the Mojave River Formation reflect
the presence of ephemeral saline lakes and/or playas.
Fluvial deposits in the top of the upward-coarsening
Mojave River Formation, estimated to be ca. 1 Ma (Nagy
and Murray, 1991), record a westward-flowing drainage
system. Given the drainage history of Victorville and Lake
Harper basins (Cox and Tinsley, 1999; Cox et al., 2003),
it is unlikely that these early fluvial deposits represent
flow from an ancestral Mojave River as Nagy and Murray
(1991) have suggested.
No lacustrine deposits ranging in age between ca. 1
and 0.5 Ma have been identified in Manix basin. During
this period, Hale (1985) suggested that a large lake,
Lake Blackwelder, filled Death Valley, Soda Lake basin
(Hooke, 1999) and Manix basin (Hale, 1985). It is then
argued that this lake overflowed the southern end of the
Troy Lake basin arm of Manix basin southeast into the
Colorado River drainage via Bristol and Danby Valleys
(Hale, 1985). The existence of this lake has not been
confirmed stratigraphically or by the presence of high
elevation shorelines in the Manix basin region (Rosen,
1989; Brown and Rosen, 1995; Enzel et al., 2003). However,
westward paleocurrents in the fluvial deposits of the
upper Mojave River Formation (Nagy and Murray, 1991)
are consistent with the flow direction of Hale’s proposed
drainage system.
A second pulse of alluvial fan development, Member A
of the Manix Formation, which was probably tectonically
induced (Nagy and Murray, 1991), separates the upper
fluvial deposits of the Mojave River Formation from the
base of Member B. Fluvial and lacustrine sediments that
comprise the lowermost deposits in the lower wedge of
Member B, estimated to be ca. 0.5 Ma ( Jefferson, 1985a,
1994, 1999), document the appearance of the Mojave
River system in Manix basin.
Fluvial/lacustrine deposition in Lake Manix records
at least four major transgressive/regressive events over
the past 500 ka ( Jefferson, 1985a, 1994, 1991b) (the last
three events appear on the reference stratigraphic section,
Figure 6). Major lacustrine phases (shaded bands Figure
50 2010 Desert Symposium