Influence of Sediment Generation Rates on Piedmont Behavior!
This manuscript is in revision for Quaternary Research
Piedmont processes are sensitive to the sediment generation in source basins. At East Range Road in Ft. Irwin, the source basins have low sediment generation rates and thus affect the surface processes on the distal piedmont.
Aerial photograph of the East Range Road piedmont. Source basins are uplifted Tertiary fan deposits. The source basins merge with proximal piedmont at the large dashed line. The proximal piedmont is incised and merges with an active, unchannelized surface at the smaller dotted line. Source basins (ERV-UB) are outlined by thin black, dashed lines. Thick black lines spaced at 1 km intervals represented transect locations (3 km long; ERT designation) from which 21 samples spaced at 150 m apart were amalgamated. Black boxes represent soil pit locations (EP1 and EP2). The eroding proximal piedmont sample (ERV-P) is an amalgamation of sediment collected from channels draining only incised alluvial surface (sample location shown by 3 white dots). The sample that contains sediment sourced from both the source basins and the eroding piedmont (ERV-LB) was amalgamated from samples collected at sites marked by two gray dots. Faint white lines on piedmonts are roads used by tanks and wheeled vehicles.
Photographs of the East Range Road piedmont surface. A. Backhoe on incised proximal piedmont surface at EP1. Foreground of photograph shows active channel about 1 m deep. B. Photograph of distal wash surface on the East Range Road piedmont near ERT-4. Off road vehicle use (predominately tanks) has destroyed the piedmont drainage network and most vegetation.
SEDIMENT BUDGET FOR EAST RANGE ROAD
Schematic sediment budget for East Range Road piedmont. Uplands (ERV-UB) supply 75% of sediment to distal piedmont (white) while the eroding proximal piedmont (shaded gray; ERV-P) supplies 25%. The percentage of upland- and proximal piedmont-sourced sediment is calculated from the average nuclide activity of the mixed sediment entering the distal piedmont (EV-LB).
EAST RANGE ROAD PIEDMONT SURFACE HISTORIES
Cosmogenic 10Be data for pits EP1 and EP2. Data points represent mid-point of depth interval. Vertical error bars represent sample depth interval. Horizontal error bars represent 1sigma analytical uncertainty. Within panel EP1, the black line shows model fit of 76,000 yr of surface stability. Within panel EP2, black line shows model fit of three periods of deposition followed by surface stability. Dashed lines represent buried soil horizons. Number range represents period of stability in years before present for each buried soil.
Such modeling suggests the timing and duration of stability associated with each buried soil: 7,000 to 10,000 yr at 118 cm, 15,000 to 25,000 yr at 84 cm, and 20,000 yr at 43 cm. Sediment deposition rates at the site of pit EP2 range from 40 to 150 mm ky-1 between 118 and 84 cm, 80 to 100 mm ky-1 between 84 and 43 cm, and 250 mm ky-1 from 43 cm to the surface. The total time represented by these scenarios is between 57,000 and 75,000 yrs, sufficient to develop the observed soils. The top 43 cm of pit EP2 is interpreted as a latest-Holocene age soil (< 2,000 yr).
SEDIMENT TRANSPORT DOWN EAST RANGE ROAD
Nuclide activity increases down piedmont. Solid black squares represent transect data on wash surface (ERT 2-6). Open square represents amalgamated transect sample characterizing proximal piedmont surface sediment and ephemeral channel sediment (ERT-1). Open circle represents upland source basin sediment datum (ERV-UB). Gray circle represents eroding proximal piedmont datum (ERV-P). Black circle represents sediment mixture of ephemeral channel sediment and sediment eroded from piedmont (ERV-LB). Uncertainties at 1 sigma are smaller than symbols and thus not visible.
Best fit of nuclide mixing model (black line) to the 10Be data (squares). We used the model described in Nichols et al. (2002) RMS error of the model is 37,000 atoms. Model results suggest that sediment moves down the East Range Road piedmont, on average, at decimeters per year. These rates are similar to the rates modeled for the Iron Mountain piedmont and the distal wash surface of the Chemehuevi Mountain piedmont and suggest slow and regular sediment movement down wash surfaces.
LONG-TERM HISTORIES OF PIEDMONTS IN THE MOJAVE DESERT
Depositional histories for proximal (P) and distal (D) soil pits on Iron Mountain (IM), Chemehuevi Mountain (CM), and East Range Road (ER) piedmonts. The Iron and Chemehuevi Mountains have higher rates of sediment generation and more continuous sediment deposition. East Range Road with lower rates of sediment generation only records pulses of deposition on the distal piedmont.