Abstract: The Provenance of Middle Tertiary Sandstones of the San Emigdio Range and Tehachapi Range, Southern California

Mariana L. Rhoades, Peter G. Decelles

Eocene to Lower Miocene sandstones and shales in the San Emigdio Range of southern California were deposited in fan deltas and braid deltas along the tectonically active southeastern margin of the San Joaquin basin. The sandstones are arkoses and lithic arkoses. Framework grains include quartz, K-feldspar, plagioclase, and volcanic/metavolcanic, metamorphic, and sedimentary/metasedimentary lithic fragments. Modal point-count data plot within the uplifted basement and dissected magmatic arc fields on provenance discrimination diagrams, suggesting dissected Sierran and Mojave block sources. ^egr_Nd(t) values of -7.85 and -6.68 from two sandstone samples in the Tejon Formation (Eocene) also suggest a Mojave block source.

Modal averages show distinct differences between sandstones deposited in various subenvironments along the basin margin. In general, eolian (QFL=80,16,4) and beach (QFL=75,13,12) sandstones are more mature than fluvial (QFL=40,47,13), nearshore (QFL=51,41,8), Gilbert delta (QFL=49,39,12), and shelf sandstones (QFL=41,46,13). The relative immaturity of the shelf sandstones probably is a result of sand bypassing from nearshore and deltaic settings onto the shelf during storms.

Climate does not appear to have exerted a major control on sandstone composition. Quartz-rich sandstone in the Uvas Conglomerate (Lower Eocene) and the upper San Emigdio Formation (Upper Eocene) may have resulted in part from hot, humid climate. Feldspar-rich sandstone in the Pleito Formation (Lower Oligocene) may reflect climatic deterioration at the Eocene-Oligocene boundary.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California