Late Quaternary variations in alluvial fan sedimentologic and geomorphic processes, Soda Lake basin, eastern Mojave Desert, California

Abstract

Alluvial fans at the front of the Soda Mountains at Zzyzx, in the Mojave Desert, California, have responded differently to late Pleistocene to Holocene climatic changes. The alluvial fans have been mapped in the field and the depositional facies interpreted as debris-flow and fluvial channel and sheetflood sediments. The relative age relationships of the fan segments have been determined primarily on the basis of soil development. The overall sequence has been established in relation to dated shorelines of late Pleistocene pluvial Lake Mojave, and the ages suggested by regional correlations. Six sets of alluvial fan deposits have been identified and labeled, oldest to youngest, Qf0–Qf5. Qf0 and Qf1 sediments predate pluvial Lake Mojave I (18.5–16.5 ka). Qf0 sediments are seen only in sections in fanhead trenches. Soil characteristics and regional correlations suggest an age for Qf0 much greater than for Qf1. Qf1 appears to date from the late Pleistocene, but prior to the Lake Mojave I highstand. Qf2 dates from the period following the Lake Mojave I and II highstands (18.4–16.6–13.7–11.4 ka) but prior to the youngest dated shoreline of pluvial Lake Mojave (10–9.3 ka). Fan depositional phases Qf3–Qf5 postdate the youngest lake shoreline, and are therefore Holocene in age. From the late Pleistocene to the Holocene there was a switch from deposition dominantly by debris-flow to fluvial channel and sheetflood processes, which was accompanied by changes in fan style from fan aggradation to progradation and dissection. However, during the mid Holocene (ca. 4.3–3.5 ka) the Qf4 sediments suggest a shortlived reversal of this trend with a local increase in sedimentation and a short-lived reversion to debris-flow deposition on some fans. Different fans along the mountain front responded differentially to climatic change over the period since the late Pleistocene, with the largest fans switching from debris-flow to fluvial processes first, and some of the smallest fans becoming inactive during the Holocene. The results indicate that the fan processes are controlled by water and sediment supply from the hillslopes, switching as these processes changed in response to climatic changes. There is no evidence for tectonically induced change over this period, and changes in fan geomorphology induced by base-level change are restricted to the toe areas of some fans. At the local level, topographic catchment thresholds control the response of individual fans to climatically induced changes in runoff and sediment supply. Harvey, A.M., and Wells, S.G., 2003, Late Quaternary variations in alluvial fan sedimentologic and geomorphic processes, Soda Lake basin, eastern Mojave Desert, California, in Enzel, Y., Wells, S.G., and Lancaster, N., eds., Paleoenvironments and paleohydrology of the Mojave and southern Great Basin Deserts: Boulder, Colorado, Geological Society of America Special Paper 368, p. 207–230. © 2003 Geological Society of America.