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Masters Thesis

Depositional environments of the Topanga Canyon Formation, central Santa Monica Mountains, California

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 two major cycles of marine retrogradation separated by delta progradation. The provenance for these strata is reworked lower Miocene and Oligocene formations from within the Santa Monica Mountains and more distant mafic and high-rank metamorphic source areas in the San Gabriel Mountains. Sedimentary environments represented in the Topanga Canyon Formation consist of wave-dominated, coastal deposits and river-dominated, coastal and fluvial deltaic deposits. At the basal portion of the sequence and within the Saddle Peak Member east of Malibu Canyon, wave-dominated coastal environments reflect an eastward-transgressing coastline, that was characterized by moderate- to high-energy waves. A black, fissile siltstone at the base of the sequence west of Malibu Canyon and comprising all of the Encinal Member within the study area, indicates restricted circulation within the offshore environment during marine transgression. An influx of coarse elastic sediment within the middle portion of the sequence signified the onset of river-dominated, deltaic sedimentation. The progradation of separate distributary systems caused a westward regression of the shoreline. Channel avulsion and lobe abandonment, followed by subsidence and a return to wave dominance, caused shoreface and delta-front deposits to interfinger locally within the middle undifferentiated Topanga Canyon Formation through Malibu Canyon. A northwest-southeast-trending shoreline is postulated during delta progradation. Fluvial deposits comprising the Fernwood Member east of Piuma Road form the upper delta plain within the prograding delta system. Both meandering and braided channel deposits were present at different times within the upper delta plain which reflects alternating conditions of stable flow and episodic or seasonal flood events. Strata east of Malibu Canyon, within the upper part of the undifferentiated Topanga Canyon Formation and Cold Creek Member, record an acceleration of basin subsidence and retrogradation of deeper marine environments. Although deltaic sedimentation continued, rapid basin subsidence caused an eastward trangression of marine environments and regression of the shoreline and delta-plain environments.

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