2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 8:00 AM-12:00 PM


DUDASH, Stephanie L., Mountain GIS and Geological Services, 2849 Foxtail Drive, Montrose, CO 81401 and MILLER, D.M., US Geological Survey, 345 Middlefield Road, MS-973, Menlo Park, CA 94025, sldudash@hotmail.com

New geochronology and field investigations in the Coyote Lake arm of Pleistocene Lake Manix provide supplemental data that reinforce conclusions made by Meek about the termination of Lake Manix and later inundations fed by the Mojave River. Radiocarbon dates on Anodonta shells and their stratigraphic context in regressive lake deposits near the southeast barrier beach at Coyote Lake indicate that by 19.6 14C ka Coyote Lake had dropped to an elevation of ~530 m, more than 10 m lower than the highstand of intact Lake Manix at 21.5 ka in the Afton basin (Meek, 1999). Lacustrine mud is capped by the lower regressive lake deposits that consist of ripple-laminated sand and mud which grade to a tan sandy unit containing the 19.6 ka Anodonta. Above the Anodonta bearing regressive sand lies 125 cm of ripple-laminated sand and mud, part of which may be ~15 ka based on nearby dated sediments. From our observations it is reasonable to infer the lake shallowed by ~10 m by 19.6 ka and was separated from Lake Manix.

Several radiocarbon dates on Anodonta near Coyote Wash yield ages between 15.9–13.1 ka. The Mojave River apparently fed the basin at about 15 ka; its fluvial deposits are truncated and redistributed as shallow lake sand and mud of that age. Beaches associated with this stage of Coyote Lake are lower than the 543 m Manix highstand by just a few meters. Younger Mojave River inverted channels grade to a northern beach barrier (elevation 537 m) and lie stratigraphically above sandy muds that contain ~13 ka Anodonta, suggesting another lake stand at that time.

Understanding the paleohydrology of the Mojave River is essential for interpreting the history of Lake Manix and associated lakes. Previous researchers have suggested that: 1) the Mojave River intermittently flowed into and maintained lacustrine conditions in Troy and Coyote Lakes between ~18-11 ka (Meek, 1994; Enzel et al., 2003), and 2) based on desiccation cracks and eolian input in the Soda Lake basin between 16.6-13.7 ka, the Mojave River experienced a decrease in discharge due to climate shifts (Wells et al., 2003). Our data indicate that the Mojave River discharged into Coyote basin some of the time between ~16-13 ka. These data reinforce the need to consider upstream basin diversions in interpreting climate history of Mojave River flow into downstream basins.