The California Continental Borderland began to form during late Miocene time. Only the latest Miocene and younger sediments are ponded in the contemporary basins. Upper Miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying ponded clastic sequences that fill the contemporary basins. A seismic facies analysis has defined facies patterns in the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins (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. Santa Catalina Basin is a transitional basin filled with fine-grained turbidites and hemipelagic sediments derived from both continental and local insular sources. The nearshore basins (Santa Barbara, Santa Monica, and San Pedro Basins and San Diego Trough) are filled by thick sequences of continental sediments that form prominent deep-sea fans.
The borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by detrital-sediment-gravity-flow deposition in the Pliocene and Quaternary. Accumulation rates estimated from Pliocene-Quaternary sediment thickness for each basin show that in the offshore basins, as well as in San Diego Trough, rates have remained fairly constant since the Pliocene. In contrast, the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent-derived sediments. Outer- and intermediate-basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence. Intermediate basins show an increase in mass-wasting deposits (slope facies) with time, which is the result of increasing fine-suspension slope deposition and eventual failure.