“A Profile of an Alluvial Spit in a Drainage Channel to Silver Lake, Baker, California” : 1978

Posted on Saturday, January 16th, 2010

William T. Venner and Marcus Hamlin

This report is the analysis of a lake channel stratigraphic soil profile conducted by Baker High School students under the direction of William T. Venner. The soil profile was excavated approximately one half mile north of Baker, California, in a location where alluvial depositions from both the eastern and western drainages merge together at the southern terminus of Silver Lake.

ENVIRONMENT

The excavation was made in an alluvial spit in the lake channel, the drainage course connecting Soda Lake and Silver Lake. It lies along the axis of a pinch in a large structural depression between the Ivanpah Upland, Cima Dome and the Halloran complex to the east, and the Soda Mountains to the west.

Detritus is from three distinct sources: From the east, granitic arlcose (coarse feldspathic sand), gneiss and basalt clasts dominate. The area drained is much greater than the west-side watershed and reaches 4000-4500 feet.

To the west are the deeply eroded, but still structurally younger Soda Mountains. They contribute coarse detritus of mostly metamorphic rocks of great variety. The assemblages are distinctive and easily recognized. Maximum elevation at the watershed is about 3000-3400 feet.

The third source of sediment is Soda Lake itself; it is being dissected at its southern extremity by headwater erosion of the lake channel, contributing to the deposition of silt, clay, and evaporates.

SEDIMENTOLOGY

The excavation was made in an area where the different facies of a modern desert-basin sedimentary pile could be found in inter-tonguing relationship. In the Baker depression there are three main facies; they are described above. They are referenced to here as the Halloran, Soda Mountain, and playa facies, respectively.

The Halloran facies is fed directly into the channel by washes draining the great pediment ascending from Baker to the basalt flows east-northeastward. The washes run water on the average of four to six times a year and carry relatively well-sorted sand winnowed out of a granitic-detritus alluvial fan.

The Soda Mountain facies is fed into the channel along short and steep gradients; the washes flow only a few times annually, some years—not at all, under the impetus of severe local storms. They carry large quantities of coarse fragments and sand made up of quartz, feldspars, biotite, and various particles of fine grained metamorphic rocks.

The playa facies originates from the south from degradation of the lake channel itself as well as dissection of parts of Soda Lake. The evaporates originate in main from the lake, which is the terminal sump of the Mojave River.

ANALYSIS

The transport indicators point north, downstream towards Silver Lake. Material deposited in the channel is periodically scoured and redistributed by floods, accounting for the alternating layers of sands, clays, and gravels in sub-units 1-4 (su-1 to su-4). In all cases the material, regardless of facies, is moved to the north.

The color changes at the base of su-4 coincides with a change in lithology. The clasts increase radically in maximum size, beds thicken, and Anadonta fragments (indicators of fresh-water lake environments) appear.

Erosion was apparently more vigorous in the interval of deposition of su-5 and  su-6 than it is today. Su-6 has the texture and appearance of a mud flow. It is wholly unsorted and at least a foot thick. The alluvium overlying su-6 could have been only deposited by a strong stream or flood carrying abundant coarse rock. It is quite possibly that this elastic wedge represents deformation of the basin floor with respect to the Soda Mountains, since almost all of the larger fragments examined correlate with rock types presently exposed in those mountains, and that many of the stream valley profiles in the Sodas show evidence of entrenchment after a period of stability (Gneiss Hills, Tunnel Canyon area). Also, the modern playas themselves tend to be crowded against hills or steep escarpments on the west side of the basin, and grade more gradually into a vast, concave-profile pediment-alluvial fan eastwards. This arrangement suggests down-dropping in the not-too-distant past at the west edge of the depression by warping, faulting, or both.

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““A Profile of an Alluvial Spit in a Drainage Channel to Silver Lake, Baker, California” : 1978”

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