Surface Processes and Histories on Complex Piedmonts: An Example from the Chemehuevi Mountain Piedmont!
A detailed summary is explained in a manuscript published the American Journal of Science, 2005, v. 305, p. 345-368.
The Chemehuevi Mountain piedmont has several surfaces including incised bedrock (A), incised alluvium (B), a zone where the incised alluvium merges with an active surface (C), and an active wash surface (D). We used soils data and cosmogenic nuclides to constrain surface histories as determined from a 2 m deep soil pit dug into the incised alluvium (located ~6 km from the Chemehuevi Mountains) and from a 2 m deep soil pit dug into the active wash (located ~12 km from the Chemehuevi Mountains).
Cosmogenic nuclide depth profiles in alluvium can vary according to the surface processes. Stable surfaces have nuclide activities that decrease with depth (A). Recently deposited sediment or well-mixed sediment has uniform nuclide activities at depth (B). Surfaces that are slowly aggrading have nuclide activities that increase with depth (C). On some piedmonts, more than one of these processes may be represented in the soil profile.
Above are nuclide data for two soil pits dug into the Chemehuevi Mountain piedmont. Pit CP1 was dug into the incised alluvium ~6 km from the Chemehuevi Mountains while CP2 was dug into the wash surface ~12 km from the mountains. Data points represent mid-point of depth interval. Black lines show model fit. Dashed lines represent buried soil horizons. Error bars represent 1 sigma analytical uncertainty. Within panel CP1 are the following identifications: A. Modeled surface age. B. Modeled duration of depositional hiatus. C. Duration of deposition at 18 mm ky-1. Below C. nuclide data represent rapid deposition followed by burial. D. Total time represented in the soil pit. Within panel CP2 are the following identifications: the top 52 cm are well-mixed, A. Duration of depositional hiatus. B. and C. Duration of deposition of 37 mm ky-1 and 20 mm ky-1, respectively. D. The total time represented in the soil pit.
The soil pit data shows that piedmont surface processes have changed over time. The model ages and process rates at CP1 (the proximal site) and CP2 (the distal site) are geomorphically and temporally consistent. Deposition rates at CP1 from ~64 to 32 ky were 18 mm ky-1. Deposition rates at CP2 from ~70 to 34 ky were 20 mm ky-1. The similarity in both the timing and rate of deposition at these widely separated sites suggests that the piedmont as a whole was aggrading during the last interstade. At 34 ky, the deposition rate at the distal CP2 site increased to 37 mm ky-1 coincident with an increase in nuclide activity of the incoming sediment. This change in deposition rate and in nuclide activity of the sediment arriving at the CP2 site is both coincident and consistent with the changes occurring up piedmont at site CP1. At CP1, we infer a change from deposition to erosion at ~32 ky. Incision of the proximal alluvial surface (CP1) would both contribute more highly-dosed sediment to the down-piedmont sediment flux and increase the volume of sediment moving down piedmont.