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

9.7 (3.8) Continue past the Basin
Road interchange. East Cronese
playa, visible to the north, is fed
by the Mojave River during floods.
It has an extensive record of past
lakes and human occupations
(Schneider, 1989: Warren and
Schneider, 2000). A recent paper
(Miller and others, this volume)
demonstrates late Holocene lakes
present during the Little Ice Age
(~ AD 1650) and Medieval Warm
Period (~AD 1290).
17.3 (7.6) Pass through faulted
Miocene sediments in road cut.
View west into Afton Basin which
was filled by Lake Manix in late Pleistocene time.
18.3 (1.0) Exit I-15 at Afton Road.
18.6 (0.3) Stop at Afton Road, TURN RIGHT (north).
18.8 (0.5) PARK north of frontage road, on north side of
off ramp.
STOP Q- 1—Afton Barrier Beach. Cuts along the
freeway and in drainage channels show that lagoonal deposits
capped by weak soils are overlain by beach gravels,
illustrating multiple lake occupations at this elevation.
Lake Manix reached highstands at or just below 543 masl
(1781 ft) at least three times during the late Pleistocene,
as suggested by Meek (1990, 2000). Highstands at this
elevation are marked by well-preserved constructional
beach barriers. Most such barriers are flat-topped with
sloping, gently rilled flanks. Retrace and PROCEED
WEST along freeway frontage road.
19.3 (0.5) STOP Q-2—Stromatolites. PARK. Cuts
along the road show breaks in lake deposits, one prominently
decorated by a row of stromatolite-capped pebbles,
indicating precipitation of calcium carbonate in clear,
shallow, sun-lit lake water. CONTINUE SOUTHWEST
on paved highway frontage road.
20.1 (0.8) Pavement turns sharply right.
20.2 (0.1) End of pavement.
20.3 (0.1) TURN LEFT on pipeline road and BEAR
RIGHT (north).
20.5 (0.2) PARK in quarry. Caution! Do not enter the
deep gully at the wash.
STOP Q-3—Dunn Wash Gravel Quarry. “Dunn” wash
exhibits lake expression in alluvial fan sediments (GPS
coordinates are 551615 E, 3880169 N, Reheis and others,
r. e. reynolds and d. m. mller
Stop Q-3. Medial distal fan deposits complexly interfinger with lacustrine sands and
muds. M. Reheis photo.
2007). The following is summarized from the report in
Reheis and Redwine (2008; Figs. 2 and 4), which contains
detailed description and measured sections.
At the Dunn Wash sites, at least four lake fluctuations
are recorded. Dunn Wash was an active drainage during
these fluctuations, and therefore lacustrine and alluvial
sediments are interbedded. These two depositional
environments are very difficult to distinguish in places
where alluvial gravel was only slightly reworked during a
subsequent lake-level rise.
The oldest lake unit (unit 1 in section M05-19, fig. 4 of
Reheis and Redwine, 2008) has basal lacustrine gravel
with thick tufa coats on clasts, overlain by green mud,
silt, and sand that coarsens and thins shoreward. The
tufa-coated gravel persists to an altitude of at least 539
masl. This unit is overlain by alluvial gravel of unit 2 and a
buried soil with a Btk horizon. The buried soil is overlain
by three packages of beach gravel and sand (units 3, 4, and
5) that rise and thin shoreward, typically with tufa-coated
clasts at the base of each package and separated by weak
soils or alluvial units. Lake units 4 and 5 can be traced
to an altitude of about 543 masl. Anodonta shells near
the base of unit 1 yielded a finite but minimum-limiting
age of 49,800±2,000 14C yr B.P. (table 1 of Reheis and
Redwine, 2008). A single preliminary U-series age on the
basal tufa is 71±25 ka ( J. Paces, U.S. Geological Survey,
written commun. to Miller, 2007). Lake unit 3 is only
locally preserved and may represent a minor fluctuation.
Anodonta shells within unit 4 below a weak buried soil,
yielded ages of 34,680 ± 260 and 31,900 ± 200 14C yr
B.P. At two other sites the uppermost unit 5 contained
Anodonta shells that yielded ages of 25,420 ± 120 and
26,030±100 14C yr B.P.
The gravels here are generally similar to alluvial fan
gravels, but the presence of Anodonta and lateral tracing
8 2010 Desert Symposium