The Sedimentary Record of the Middle Permian Subduction Initiation in the El Paso Mountains, California

Jean L.Rains, Kathleen M.Marsaglia, and George C.Dunne
California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330-8266

In the El Paso Mountains of southern California an apparently continuous succession (947 m total) of Early Permian (~280 – 260 Ma) metasediments records the initiation of subduction along the western margin of the U. S. A stratigraphic section was described, measured, and sampled southwest of Mormon Flat; representative samples were thin sectioned and petrographically analyzed for texture and composition. Although metamorphosed up to greenschist level, sedimentary and mineralogical details are still discernible. Above a faulted(?) contact with Ordovician-Late Cambrian deep-marine passive-margin rocks, the section begins with a local basal conglomerate (~33 m), followed by metacarbonate turbidites locally silicified (~62 m), then mostly covered noncalcareous metashales and metasiltstones (~ 393 m). Upsection, the mudstones become more calcareous (~152 m); several beds of meta-arkose interrupt the upper ~64 m. An abrupt change to slightly tuffaceous calcarenite (~10 m) is followed by tuffaceous metasandstones with distinctly zoned plagioclase crystals that dominate the rest of the section (~297 m). Rare interbeds of quartz arenite occur ~15 m above the base of the tuffaceous metasandstones and bioclastic metalimestone near the top below the andesite flows. These results are consistent with first uplift (conglomerate), followed by subsidence and deeper water sedimentation (shale), with gradual shallowing (volcaniclastic and bioclastic sediments) into a volcanic edifice (andesitic lavas). This basin history is consistent with induced subduction along a transform plate margin.  

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California