SURFICIAL GEOLOGIC MAP OF THE MADELINE AND MCDONALD PEAK 7.5-MINUTE QUADRANGLES, LASSEN COUNTY, CALIFORNIA

In conjunction with the primary goals and funding set forth by the US Geological Survey's Educational Component of the National Cooperative Geologic Mapping Program (USGS EDMAP program), this study applies field mapping techniques, stereoscopic aerial photograph interpretation, stratigraphic analysis, and an analysis of soil development into pluvial lake shoreline deposits to investigate the relative timing of lake-level fluctuations and alluvial deposition to better understand the Quaternary history and geomorphic response to climatic change within the Madeline Plains region of Lassen County, California. The scope of this mapping is limited to the surficial deposits within the Madeline and McDonald Peak 7.5-minute topographic quadrangles. Objectives outlined in this project are to determine the spatial distribution of Quaternary deposits associated with the rise and fall of pluvial Lake Madeline, an internally drained lake basin located in the southeast portion of the Modoc Plateau, and to develop a digital compilation of these data using a geographical information system database at 1:24000 scale or larger.

Evidence suggests that pluvial Lake Madeline reached a maximum elevation of 1667m, connecting what is now Grasshopper Valley, Dry Valley, and the Madeline Plains, covering a surface area of ~777km2. The preservation of shoreline remnants and distinct differences among fundamental soil properties delineates a relative age break between shoreline surface altitudes of 1654m and 1651m, indicating that a suite of approximately eight shorelines represent a minimum of two different ages of lake cycles. The geomorphic expression and areal extent of lake deposits strongly suggest that potential overflow of pluvial Lake Madeline occurred prior to the last glacial maximum, creating a hydrologic connection with Secret Valley and perhaps the Honey Lake segment of Lake Lahonton to the south. Correlations and relative age assignments are enhanced by a numerical age on the Trego Hot Springs tephra (14C age of 23.4ka B.P.) observed in several locations within the field area. In addition, alluvial fan deposits originating from Tertiary volcanic units that surround the basin suggest that significant sediment contributions occurred following the last pluvial highstand.