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

27.1 (0.5) TURN RIGHT (north)
at the pole with reflectors (Post # 2
2404—Oak Grove Road).
27.2 (0.1) PARK at utility road.
STOP P-2—Johnston Corners
Pliocene GWD. The sedimentary
section at this locality starts at
the railroad tracks to the north. It
begins with coarse gravels that are
angularly overlain by a section of
fine-grained sand, very fine gravelly
sand, and calcareous sand and silt.
Some calcareous beds are strongly
cemented, similar to the upper
carbonate cemented caps of late
Pleistocene GWD (e.g., Forester
et al, 2003). The section continues
south of the road and is capped
on the skyline by a pediment
marked by a veneer of boulders. A
carbonate-cemented bed nearby
contains silica, which yielded an
approximate U-Pb age of 2.3 Ma
(K. Maher, written commun. to Miller, 2005). This date
suggests a correlation with the beds we saw at Stop P-1
in Barstow. Return to vehicles, Retrace south to National
Trails Highway.
27.7 (0.4) Stop at National Trails Highway. TURN
RIGHT and proceed west.
28.7 (1.0) Continue past Hinkley Road and Hodge.
33.6 (5.6) Continue past Holcomb Ranch Road.
34.5 (0.9) TURN RIGHT (north) on Indian Trails Road.
Proceed toward railroad.
35.2 (0.7) TURN LEFT (west) on the south railroad
frontage road.
36.3 (1.1) Pass through a dip and proceed 0.2 miles to an
outcrop on the left (south).
36.5 (0.2) STOP P-3—Helendale Ash (Reynolds and
Cox, 1999). Helendale Ash sampled here is a Mono “W”
type tephra, indicating an age range from 5.0 to 0.7 Ma.
It best correlates with a sample from the Ventura section
that is between 0.9 and 1.18 Ma. The Helendale Ash is
located at the base of the Mojave River gravels (2500 ft
elev.), and overlies the fine sediments of the Victorville
Basin. This sequence suggests that the river had finished
filling the Victorville Basin by ~1 Ma, and then began
carrying coarse sands farther east. The elevation of this
~1 Ma ash helps constrain the section that we will see at
r. e. reynolds and d. m. mller
Stops P-4 and P-5. RETRACE east along the south side of
the railroad for 0.2 miles.
36.7 (0.2) TURN RIGHT (south) through a break in the
fence and follow OHV tracks in wash to reach terrace top.
37.2 (0.5) Complex intersection. TURN NORTH, then
bear west along bluff road. Do not turn south toward
houses or pole line road.
37.5 (0.3) Proceed west, passing a left (south) turn. Note
lag gravels on the terrace top.
37.9 (0.4) Bluff Road drops off terrace top (elevation
2600 ft) into broad canyon.
38.1 (0.2) Stop P-4—Helendale Section (2480 ft elevation)
in wash bottom. Note red paleosols, and green pond
sediments that probably represent fine sediments of the
early Pleistocene Victorville basin. We will see Mojave
River sands at higher elevations as we follow the road
uphill to the southwest. The GWD capping the bluffs may
correlate with those at Johnston Corners and farther east
at Lenwood, suggesting a broad, marshy, east–west valley
bottom during the late Pliocene and earliest Pleistocene.
38.3 (0.2) Join a road that proceeds south to the pole line
road. PARK on hill crest where space is available.
38.6 (0.3) Stop P-5—Helendale GWD at 2600 ft on
ridge top supported by GWD carbonates. This unit is
similar to those above Stop P-4. About 10 km to the westnorthwest,
Dibblee mapped similar white beds as lake
20 2010 Desert Symposium