Abstract

Extensive, active-margin continental-slope deposits are not commonly preserved in the rock record. The middle and upper portions of the Holz Shale (Ladd Formation) in Black Star Canyon, northern Santa Aria Mountains, represent such a sequence where shale and mudstone strata are dissected by large, coarse-grained channel-fill deposits. Channels preserve evidence of filling primarily by conglomeratic debris flows, high-density turbidites, and classic Bouma low-density turbidites; slumping and traction-current mechanisms were less important. Associated with channels are pebbly mudstones, poorly developed levee facies, and submarine chute and gully deposits. Interbedded turbidites, contourites, and hemipelagic sediments dominate interchannel strata. Hemipelagic sediments exhibit sedimentologic fabrics that range from biologically dominated (homogeneous, bioturbated) to physically dominated (fine-scale, planar-laminated, anaerobic). This variation in fabric as well as associated digenetic information indicates that the anaerobic zone, although generally at some depth below the sediment-water interface, at times migrated up into the overlying water column. Foraminiferal assemblages within hemipelagic sediments are dominated by agglutinated forms which indicate deposition at bathyal depths. Macroinvertebrate evidence includes 1) the interchannel paleocommunity, dominated by the bivalve Inoceramus and the deposit-feeding trace fossil Chondrites and 2) the submarine-channel paleocommunity, comprised mainly of the trace fossils Thalassinoides and Ophiomorpha . Previous studies have demonstrated that these Late Cretaceous active-margin environments included a narrow continental shelf. Abundance of terrestrial plant material, paucity of displaced shelf faunas, well-rounded conglomerate clasts, and the coarse-grained texture of these deposits suggest that one or more of the Holz Shale submarine channels were receiving sediment directly from terrestrial environments. Paleogeographic reconstructions have postulated Cenozoic lateral and rotational displacement of associated Upper Cretaceous strata in the Transverse Ranges of southern California. Paleocurrent and other sedimentologic analyses of Holz Shale strata support these reconstructions and suggest that a submarine-fan complex, which was fed by Holz Shale submarine channels, is presently located to the north in the Simi Hills and Santa Monica Mountains.

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