Changing Sediment Sources in the Mio-Pliocene Etchegoin Formation on the Western Bakersfield Arch and Elk Hills, California

Janice Gillespie, Ariel Auffant, Luz Sophia Cobos, Adam Mahan, Robert Negrini, Daniel Steward, and Sean Taylor
Department of Geology, California State University Bakersfield, 9001 Stockdale Hwy., Bakersfield, CA 93311

The late-Miocene/Pliocene Etchegoin Formation on the western fringes of the Bakersfield Arch and at Elk Hills oilfield was deposited in an environment that represents a transition from deep marine to shallow tidal environments. The basal Etchegoin consists primarily of clays and silts that are rich in biosiliceous material. West of the Arch, these biosiliceous intervals are replaced by sands in the upper part of the basal Etchegoin—indicating the presence of a coarse clastic source of sediments to the west. However, fine-grained, biosiliceous-rich sediments persist throughout the basal Etchegoin further to the east on the western fringes of the Bakersfield Arch. The basal Etchegoin thins over top of the Elk Hills and Coles Levee anticlines, filling in seafloor topography created by earlier folding events. Above the basal Etchegoin are the sand-rich Calitroleum and lower Gusher zones. Net sand isochore maps of these zones indicate that sediment was supplied to the study area from both the east and west. A sand-poor, low energy depocenter occupied the eastern Elk Hills/ western Coles Levee area on the western fringe of the Arch.The eastern source area achieved dominance on the western fringe of the Arch during deposition of the upper Gusher zone. This is illustrated by the thick sequence of amalgamated sands that appear within the upper part of the Gusher zone in Coles Levee and eastern Elk Hills. These sands signal the progradation of the Kern River fan delta completely across the Arch. Three-dimensional seismic shows sea level fluctuating over the Arch during deposition of the upper Etchegoin and overlying San Joaquin formations—possibly due to tectonics or variations in sediment influx rates. Parasequences show onlapping transgressive sequences stepping eastward. The highstand intervals were subsequently filled by progradational sequences originating from the northeast. The upper Etchegoin contains brackish water fossils indicating a transition to a tidally influenced environment. Thus the sequences deposited as part of the upper Etchegoin appear be less affected by subsidence and more influenced by sediment influx into a nearly-filled basin.

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