Landscape Evolution at an Active Plate Margin - Biological Science ...

Kushiro (1968) was the first to offer an explanation
for differences in composition of mantle-derived
basaltic magma. He suggested that depth of melting
was an important variable and that magmas from the
shallow mantle would tend to be quartz normative
tholeiites while magmas produced in the deep mantle
would be olivine normative and alkaline. Anderson
and Jessey (2005) suggested that one possible explanation
for the DS tholeiites, therefore, was that they
represented a shallower partial melt. This could be
related to their age and/or geographic position to the
west of other Mojave/Owens Valley basalt fields and
nearer shallow remnants of the east-dipping subduction
zone and East Pacific Rise (Yang, 2002). However,
Brown, Bruns and Jessey (2008) pointed out that
this does not account for the presence of iddingsite
which requires preexisting phenocrysts of olivine.
Therefore, initially, DS basaltic magmas may have
been as alkaline as those of other Owens Valley fields
and melting depth might not be an important variable
in compositional difference.
The Cudahy Camp and the Dove Spring formations
are biostratigraphically important because they contain
North American Land Mammal Age (NALMA)
transitions of middle and late Miocene faunas. The
late early Miocene Cudahy Camp Formation is nonfossiliferous,
while the Dove Spring faunas show the
transition from the Clarendonian through Hemphillian
land mammal age (13.4–7.3 Ma; Burbank and
Whistler, 1987; Loomis and Burbank, 1988; Whistler,
1991). Research in the local stratigraphic sequence
presents biostratigraphic, magnetostratigraphic and
tephrochronologic age parameters (Whistler and Burbank,
1992) that can be compared to other sedimentary
sections across the continent (Tedford and others,
2004). This stratigraphic section contains lithologic
clasts that relate it to nearby tectonic events. Previous
work suggested that clasts from the rising Sierra Nevada
Mountains were present in an upper portion of
the section that dates between 8.5–8.4 Ma (Whistler
and Burbank, 1992). Current studies (D. P. Whistler,
p. c. to Reynolds, 2008) suggest that “Sierran-looking
granitic clasts in place” have been found in a portion
of the section above Ash 15 along the Powerline Road
west of the campground that dates to 9.4 m.y. associated
with the Dove Spring Local Fauna. This suggests that
granites of the Sierra Nevada geologic province were
exposed by 9 Ma.
Return to vehicles and RETRACE EAST to CA 14.
D. R. Jessey and R. E. Reynolds
59.8 (1.0) Watch for cross traffic. TURN LEFT
(north) on CA 14. Note the brightly colored outcrops
of west-northwest dipping Ricardo Group lacustrine
and fluvial sedimentary layers as you drive north on
CA 14. The prominent ridge-capping black rocks are
basalt flows within the Ricardo Group.
63.3 (3.5) Continue past a left turn to Dove Spring
offroad vehicle area. Pleistocene terrace deposits crop
out along Dove Spring Wash. These sediments represent
deposition of a bajada between the Sierra Nevada
Range and the El Paso Mountains, and were dissected
when erosion breached the fault complex along the
south side of the El Paso Mountains. Sediments contain
small and large mammal species including Bison,
the Rancholabrean indicator mammal, and have
produced C-14 dates between 19–10 ka (Whistler,
1990, 1991).
64.4 (1.1) Continue past a right (southeast) turn to
Burro Schmidt’s Tunnel and northern access to the El
Paso Mountains. Continue northeast on CA 14.
65.5 (1.1) Divide (el. 3300 ft.) separates Fremont
Valley drainage system from Indian Wells Valley, the
latter draining east toward Searles Valley. The Scodie
Mountains of the southern Sierra Nevada are to the
west-northwest. Indian Wells Valley lies at an elevation
of 2300+ feet, only 300 feet above Fremont Valley.
Presumably the elevation difference results from
movement along the El Paso fault, but it could also
be related to transpression caused by a left step in the
Garlock fault (Smith, 1998).
73.4 (7.8) Pass Robbers Roost granitic spires at 10:00
(Reynolds, 2002).
77.3 (3.9) Continue past a left (west) turn on SR 178
to Walker Pass and Bakersfield. Walker Pass, at 5250
ft, is the obvious break in the Sierra. Joseph R Walker
“discovered” this pass in 1834 leading the Bonneville
expedition, the first wagon train to CA; it was later
used by Frémont in 1845. The pass became a major
route from gold fields at Keysville and Kernville to
Mojave and north to Reno (Pracchia, 1994).
77.7 (0.4) Continue past Bowman Road on the right
(El: 3700 ft.). Look uphill at 9:00 to see creosote rings.
These rings are clones of the original plant, and the
largest rings is 60 ft. in diameter and the plant is estimated
to be at least 9000 years old.
80.2 (2.5) Continue past a right (east) turn on SR 178
to Inyokern and Ridgecrest (El: 2310 ft.). Magnolia,
siding 16 (Inyokern) and Brown, siding 18, on the
14 2009 Desert Symposium