Depositional Environments of the Topanga Canyon Formation, Central Santa Monica Mountains, Southern California

FLACK, MICHAEL E., Emcon Associates, Camarillo, CA, and A. EUGENE FRITSCHE, California State University, Northridge, CA

The Topanga Canyon Formation within the central Santa Monica Mountains of southern California is a sequence of lower through middle Miocene conglomerate, sandstone, and siltstone deposits that record a lower, wave-dominated, marine, retrogradational cycle; a middle, river-dominated, marine and fluvial, progradational cycle; and an upper, river-dominated, marine, retrogradational cycle.

The lower retrogradation consists of black siltstone (Encinal Member) below-wave-base shelf deposits west of Malibu Canyon, where circulation was restricted, and fossiliferous sandstone (Saddle Peak Member) shoreface deposits east of Malibu Canyon, where wave energy was moderate to high.

The middle progradation is marked by an onset of river-dominated deltaic sedimentation. Nonmarine sandstone and conglomerate (Fernwood Member) upper delta-plain (channel, point-bar, crevasse-splay, marsh, and floodplain) deposits east of Malibu Canyon prograde southwestward into marine sandstone and siltstone (undifferentiated Topanga Canyon Formation) delta-front, interdistributary bay, and prodelta deposits. Both meandering and braided channels existed on the upper delta plain. Channel avulsion and lobe abandonment, followed by subsidence and a return to wave dominance, caused wave-dominated shoreface and river-dominated delta-front deposits to interfinger locally.

The upper retrogradation consists of marine sandstone and siltstone (Cold Creek Member) distributary-mouth-bar, delta-front, and prodelta deposits that overlie the earlier fluvial deposits east of Malibu Canyon. Rocks of this age do not occur west of Malibu Canyon.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)