Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Updates To The Regional Geologic Map Of The Los Angeles 30′×60′ Quadrangle

Abstract

This year, the California Geological Survey will release an updated geologic map of the Los Angeles 30′×60′ quadrangle. Like previous maps in the CGS Regional Geologic Map series, the map will be on a 100,000-scale base map, but preserve as much detail as possible from original mapping at 1:24,000 or larger scale. The new map updates the one by Yerkes and Campbell published by the USGS in 2005. In updating this map, CGS has incorporated all new mapping of Quaternary units in valleys and basins throughout the region. This mapping, conducted for the CGS seismic hazard zoning program as part of the process of defining liquefaction hazard zones, provides new detail that helps to delineate late Quaternary depositional and tectonic processes in addition to seismic hazards. CGS has also completely updated the mapping of crystalline bedrock units in the San Gabriel Mountains, Verdugo Mountains, and San Rafael Hills. This update used the most detailed available source maps and attempted to reconcile maps with differing interpretations and nomenclature. This new compilation represents a complete and consistent interpretation where the detailed mapping and unit descriptions are from a single source, such as in the anorthosite complex mapped by Carter (1980), but in other areas, units described by one author may be extended based on contacts mapped by another. Additional mapping is needed to ensure that interpretations made for this compilation are correct and to fill in areas where the existing mapping is inadequate. In addition to the major updates noted above, CGS has updated numerous other areas based on map sources not used in the map by Yerkes and Campbell (2005). These more local updates include new mapping of parts of the active Hollywood, San Cayetano and Simi faults, updates to mapping and stratigraphy of the Plio-Pleistocene sedimentary rocks of the Ventura basin, updates to Tertiary units in the Soledad basin, and a complete revision of the way landslides are depicted on these regional maps. The density of landslides on some maps in this series made interpretation of the underlying geologic units and structures very difficult. To improve map clarity, only larger or more prominent landslides are shown; readers needing a more complete landslide map are referred to CGS 1:24,000-scale landslide maps. The map will be released for review and comments on the CGS web page: http://www.conservation.ca.gov/cgs/rghm/rgm/Pages/preliminary_geologic_maps.aspx.