Pluvial Lake Manix, Mojave Desert, USA: Effects of Subbasin Integration on Sedimentary Record
Pluvial Lake Manix, in the Mojave Desert of south-central California, was the terminal basin of the Mojave River until the late Pleistocene when it drained east to form Lake Mojave. Previous studies that focused on outcrops in the central Manix subbasin interpreted multiple glacial-pluvial lacustrine episodes, of which all but the last two (OIS 2 and 4) were represented by deep-lake sediments. Based on sedimentary analysis of a 45-m core from the Manix subbasin and outcrops in the Afton area to the east, we interpret a major subbasin-integration event that incorporated a previously separate valley and significantly impacted sedimentation patterns throughout the greater Manix basin. In addition, at least one of the deep-lake intervals is now thought to represent fluctuating shallow lakes and mudflats.
In the Afton subbasin, playa and distal-fan deposits of a closed basin are locally overlain by dramatically deformed lacustrine muds that represent large, reworked but semi-intact blocks of Lake Manix sediment derived from the central Manix subbasin upstream. These muds are overlain by fluvial gravel and sand that represents the initial establishment of stream drainage from the central subbasin, followed by a rapidly upward-fining sequence of sand, silt, and mud that mark the flooding of the newly incorporated subbasin with Lake Manix (Mojave River) water. Stratigraphic correlations suggest that this subbasin-integration event occurred just before or shortly after deposition of a distinctive tephra layer within the Manix subbasin; the tephra has not yet been found in the Afton subbasin. The ~7 m of sediment above the tephra had been interpreted as perennial-lake deposits, but new mapping and core analysis suggest that these sediments were deposited in a fluctuating environment of shallow lakes and mudflats. Deposits in the Afton subbasin interpreted as correlative with the 7-m interval consist of perennial-lake muds intercalated with sands that thicken and coarsen shoreward. These relations indicate that lake depth and distance from fluvial inputs were greater in the newly incorporated Afton subbasin than in the correlative Manix subbasin. By this time the Manix area, having been the depocenter of Lake Manix for possibly hundreds of thousands of years, had lost much of its sediment storage capacity; thus, relatively small changes in runoff and lake level could easily cover or expose previously deposited sediment. Sediment patterns in the core suggest that millennial- to centennial-scale cycles of wet and dry periods may be recorded by alternating soils and fluvial, shallow-lake, and mudflat beds.