Surficial Geology and Stratigraphy of Pleistocene Lake Manix, San Bernardino County, California By Marith C. Reheis,1 Joanna R. Redwine,2 Elmira Wan,1 John P. McGeehin,1 and D. Paco VanSistine1 1 U.S. Geological Survey 2 U.S. Bureau of Reclamation 2014 Scientific Investigations Map 3312 ABSTRACT Pluvial Lake Manix and its surrounding drainage basin, in the central Mojave Desert of California, has been a focus of paleoclimate, surficial processes, and neotectonic studies by the U.S. Geological Survey (USGS) since about 2004. The U.S. Geological Survey initiated studies of Lake Manix deposits to improve understanding of the paleoclimatic record and the shifts in atmospheric circulation that controlled precipitation in the Mojave Desert. Until approximately 25,000 years ago, Lake Manix was the terminus of the Mojave River, which drains northeasterly from the San Bernardino Mountains; the river currently terminates in the Soda Lake and Silver Lake playas. Pleistocene 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 of Lake Manix, and extends from the head of Afton Canyon to Manix Wash. The map illuminates the geomorphic development and depositional history of the lake and alluvial fans within the active tectonic setting of the eastern California shear zone, especially interactions with the left-lateral Manix fault. Lake Manix left an extraordinarily detailed but complex record of numerous transgressive-regressive sequences separated by desiccation and deposition of fan, eolian, and fluvial deposits, and punctuated by tectonic movements and a catastrophic flood that reconfigured the lake basin. Through careful observation of the intercalated lacustrine and fan sequences and by determining the precise elevations of unit contacts, this record was decoded to understand the response of the lake and river system to the interplay of climatic, geomorphic, and tectonic forces. These deposits are exposed in steep badland topography. Mapping was carried out mostly at scales of 1:12,000, although the map is presented at 1:24,000 scale, and employs custom unit nomenclature, with multiple subdivided lacustrine and alluvial fan units. In addition, many important units are very thin and cannot be mapped separately, or are covered by thin eolian sand, so these are commonly portrayed as stacks of units or combined units. These details are more accurately portrayed in the measured sections that accompany the map. Altitudes of many contacts were obtained using differentially corrected Global Positioning System (GPS) or, in some cases, lidar (light detection and ranging) data. 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DATA FILES List of files: 00ReadMe.txt SIM3312_pamphlet.pdf SIM3312_sheet1.pdf SIM3312_sheet2.pdf SIM3312.met Appendix_Photographs.zip tables.zip Datafiles directory manix_geol.gdb.zip directory contour10m dem10m_shd --- shaded relief of map area only Geology manix_geol_label manix_geol_line manix_geol_pnt manix_geol_sample SIM3312_shapefiles.zip directory: manix_geol_label.shp (dbf/prj/sbn/sbx/shp.xml/shx) manix_geol_line.shp (dbf/prj/sbn/sbx/shp.xml/shx) manix_geol.pnt.shp (dbf/prj/sbn/sbx/shp.xml/shx) manix_geol_sample.shp (dbf/prj/sbn/sbx/shp.xml/shx) HOW TO OBTAIN THE DIGITAL FILES The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at http://pubs.usgs.gov/sim/3312/ or http://dx.doi.org/10.3133/sim3312 and follow the directions to download the files. 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