SURFICIAL GEOLOGY AND STRATIGRAPHY OF PLEISTOCENE LAKE MANIX, SAN BERNARDINO COUNTY, CALIFORNIA

The drainage basin of pluvial Lake Manix, in the central Mojave Desert of California, has been a focus of paleoclimate, surficial processes, and neotectonic studies by the U.S. Geological Survey since about 2004. Until the late Pleistocene, Lake Manix was the terminus of the Mojave River, which drains northeasterly from the San Bernardino Mountains; the river currently terminates in the Soda and Silver Lake playas. Southwestern lakes are sensitive recorders of precipitation and temperature on millennial to decadal timescales, but there are few detailed studies of pluvial lakes in this region. In 2005, a 45-m-long core was obtained from the thickest preserved sequence of lacustrine deposits for detailed study. Initial conflicts between the stratigraphy and sedimentology expressed in the core and interpreted from outcrops in previous work led to a need to map the exposed sedimentary section in detail.

Lake Manix occupied several subbasins at its maximum extent. This map focuses on the extensive exposures created by incision of the Mojave River and its tributaries into the interbedded lacustrine and alluvial deposits within the central (Cady) and northeastern (Afton) subbasins, and extends from the head of Afton Canyon to Manix Wash. The active left-lateral Manix fault, with numerous strands, cuts through the Cady and Afton subbasins, displaces all lacustrine and older units, and coincides with the former eastern threshold of Lake Manix at the head of Afton Canyon. Sheared zones in late Cenozoic fanglomerate units along this fault played a significant role in the drainage integration history, which included at least one catastrophic flood documented on this map. Lake deposits and fanglomerates are dated by radiocarbon, tephrochronology, and uranium-series techniques and by correlation to the Manix core, which has been dated using an age model that includes paleomagnetic excursions.