Neogene Sedimentation of Continental Borderland Basins, Offshore Southern California: ABSTRACT

Abstract

Recent seismic stratigraphic analysis shows that the present borderland basins in offshore southern California did not begin to take shape until the late Miocene. The distribution of early Miocene and older strata is not affected by the basin configuration, and only the late Miocene and younger sediments are ponded in the basins. The late Miocene strata are characterized by the widespread Monterey-type diatomaceous sediments that form drape-like sequences covering all the topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying clastic sequences filling in the lows of the basins. Seismic facies analysis permits delineation of facies patterns of the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins such as Patton, Tanner, Santa Cruz, and San Nicolas, fine-grained mass-flow and hemipelagic sediments derived from the local insular sources dominate the basin-fill sequences. Catalina basin is a transitional basin filled with fine-grained turbiditic and hemipelagic sediments derived from both the continent and local insular sources. The nearshore basins, including Santa Barbara, Santa Monica, and San Pedro basins and San Diego trough, are filled with thick sequences of continent-derived sediments that mostly form prominent deep-sea fans. Approximate estimations of accumulation rates show that sediment accumulation in the offshore basins as well asmore » San Diego trough has remained fairly constant since the Pliocene, while the nearshore basins exhibit an increasing trend that might be related to the progradation of continent-derived sediments. Stratigraphic characteristics of borderland basin deposits indicate that the borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by mass-flow deposition in the Pliocene and Quaternary.« less