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

Stop M-3. Marker beds at the Columbus/Gem mine,
overturned stratigraphy, view east. SrB: strontium and
borate beds; BPL: brown platy limestone; MSL: massive
stromatolitic limestone.
STOP M-3—Columbus/Gem Mine. This overturned
section of Barstow Formation shows, in reverse stratigraphic
order, pink 18.7 Ma tuff (PST), strontium–borate
layer (SRB), three brown platy limestones (BPL), thick
granitic fanglomerate, and three strata of massive cylindrical
stromatolites (MSL), the stratigraphic lowest (now
the uppermost) of which is silicified. Farther south, across
a branch of the Camp Rock Fault, are aplitic granite
outcrops that, when uplifted in the Daggett Ridge event,
contributed to the granitic debris now found elsewhere in
the lower part of the stratigraphic section. Zircons from
the tuff at this location are ~18.7 Ma, indicating correlation
with the Peach Spring Tuff (Miller and others, this
volume). This suggests that the entire ~0.8 Ma duration
marker sequence at this locality is older than 18.7 Ma and
started being deposited around 19.5 Ma. Volcaniclastic
sediments between the brown platy limestones look like
their source was the Pickhandle Formation.
Stoddard Valley to the south has a similar,
but abbreviated, section of marker beds, with
large tubular stromatolites representing the
MSL, a single BPL layer and the PST (Wells
and Hillhouse, 1989; Miller and others, this
volume). The SrB horizon is apparently not
present in Stoddard Valley. RETRACE toward
powerline road.
94.8 (0.5) TURN LEFT (west) on dirt track.
95.3 (0.5) TURN LEFT (south) to avoid
gully.
96.2 (0.9) BEAR RIGHT (west) at Y turn.
97.7 (1.5) PARK.
STOP M-4—West Gem Exposures. Look
east at colorful badlands exposures which in-
r. e. reynolds and d. m. mller
clude a pink tuff similar to the tuff we saw at the last stop.
This pink marker tuff is useful in tracing this portion of
the stratigraphic section westward. The pink tuff sampled
at this locality has the same remnant magnetism as the
PST, suggesting a correlation (Hillhouse and others, this
volume). Here, unlike the section at Columbus/Gem, a
thin section of lacustrine beds lie above the pink tuff. The
marker sequence is not present here, apparently due to
the influx of granitic-sourced gravels that have thin lacustrine
beds within them. A Pleistocene analog can be seen
in Lake Manix alluvial gravel interbedded with lacustrine
silts (Miller and Reheis, this volume).
Central Daggett Ridge is visible to the west, past Ord
Mountain Road. That portion of the Miocene sedimentary
section is apparently separated from this portion by a
north–south fault, as the pink tuff is not present. The central
Daggett Ridge section contains the marker sequence
(MSL, BPL and SrB in drill records of US Borax).The
Daggett Ridge P/L locality (SBCM 1-109-2; Reynolds,
1991) has a fauna that contains small and large horses
suggesting the Hemingfordian/Barstovian transition at 16
Ma, similar to the faunal transition in the Toomey Hills
(next stop). Proceed northwest to power line road.
98.0 (0.3) TURN RIGHT (north) on the section line
road toward the power line road.
99.0 (1.0) Stop at the power line road. TURN RIGHT
(northeast) on power line road and retrace to Camp Rock
Road.
101.5 (2.5) Stop at Camp Rock Road. TURN LEFT
(north) and proceed to I-40.
103.5 (2.0) TURN RIGHT (north) on “A” Street and
cross I-40, entering the town of Daggett. The arrival of the
Stop M-4. West Gem locality. The pink tuff exposed here appears similar
to the friable, pink Peach Spring Tuff at the Columbus/Gem Mine (Stop
M-3). The Tuff is overlain by a thin sequence of lacustrine sediments.
12 2010 Desert Symposium