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ABSTRACT

Analyses of foraminiferal biofacies and lithofacies within thick Cenozoic sequences exposed in the Santa Ynez Mountains, Ventura Basin, Santa Rosa and San Miguel Islands, Palos Verdes Hills, and the Santa Ana Mountains-San Joaquin Hills area of southern California clearly delineate major Paleogene and Neogene depositional cycles characterizing the tectonically complex portion of the Pacific rim assigned to the Southern California Continental Borderland, Emphasis is placed on estimated upper depth limits of selected species of benthonic foraminifera in reconstructing paleobathymetry of each of six sequences analyzed, with integrated assessment of modern microfaunal and sedimentologic patterns allowing identification of up to 40 lower bathyal through littoral paleoenvironmental boundaries. Each sedimentary column and associated paleobathymetric-paleoenvironmental curve is plotted both in relation to maximum unit thickness and to an updated Cenozoic time scale utilizing available planktonic zonal criteria, radiometric ages, and magnetostratigraphy. The lithostratigraphic-chronostratigraphic plots yield estimated rates of uplift, subsidence, and sediment accumulation for each paleoenvironmental segment identified (basin plain, slope, shelf, etc.), reflecting interplay between tectonic and depositional events during Paleogene and Neogene basin-filling episodes in the northern and eastern portions of the borderland. Paleogene deep marine sedimentation in this region was focused in an east-west trough (Santa Barbara Embayment) in the area of the present Santa Ynez Mountains and northern Channel Islands. Filling of this depocenter was slow during Late Cretaceous through early Eocene time when lower bathyal (≥2,000 m) basin plain and distal fan deposits accumulated at rates of 20–50 m/m.y. balanced by equally slow subsidence. Subsequently, great wedges of outer, middle, and supra fan sediments filled the trough from the north and east at rates of 200–300 m/m.y. during the middle Eocene. This accelerated to 500 m/m.y. during the late Eocene as subsidence waned to less than 100 m/m.y. and slope, shelf, and littoral facies transgressed westward over proximal fan and base-of-slope deposits. A tectonic pause accompanied by widespread nonmarine deposition and erosion occurred over the southern California margin during the Oligocene aided by a global eustatic event. Nonmarine deposition was terminated by abrupt and widespread subsidence (150–500 m/m.y.) during the latest Qligocene-early Miocene coincident with initiation of equally dramatic tectonic events elsewhere around the Pacific rim. Rapid subsidence produced a series of effectively silled middle bathyal Miocene basins momentarily deficient in terrigenous debris allowing relatively undiluted deposition of prolific numbers of diatom frustules from highly productive surface waters. Most middle to late Miocene basin sills hovered at depths within the oxygen minimum zone creating oxygen deficient (0.1–0.5 ml/l.) subsill water, effectively excluding well-developed megainvertebrate faunas capable of destroying bedding and thus facilitating accumulation and preservation of diatomaceous muds and laminated diatomites (Monterey Formation). Tectonic reorganization of the Miocene borderland basins began in late Miocene-early Pliocene time with further subsidence to lower bathyal depths in some synclinal areas. In addition, an increasing influx of terrigenous material in the form of local fan lobes and fine-grained detritus diluted diatom frustules and capped underlying diatomites with early Pliocene mudstones and distal sands. Major flexing of the borderland occurred about 3 m.y. ago (middle Pliocene) marked by rapid uplift (400–1,000 m/m.y.) of anticlinal interbasin ridges and borderland margins as illustrated in the Palos Verdes Hills and Santa Ana Mountains-San Joaquin Hills sequences. This same event was accompanied by dramatic increases in rates of sedimentation (>2,000 m/m.y.) and subsidence (> 1,000 m/m.y.) in synclinal nearshore depocenters such as the Los Angeles and Ventura basins. Both of these basins were filled to capacity by the late Pleistocene as signaled by rapid reductions in rates of sediment accumulation and subsidence (<1,000 m/m.y.). A major late Pleistocene tectonic episode then deformed borderland margins, basin sills, and interbasin ridges to their present configuration initiating modern depositional patterns.

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