ABSTRACT Paleobathymetric and paleoenvironmental analyses of upper Cretaceous and Cenozoic sediments of the Santa Ynez Mountains and adjacent Ventura Basin illustrate that Paleogene, Neogene, and Quaternary depositional cycles have produced the widespread stratigraphic patterns present in the northern portion of the continental borderland of southern California. Cretaceous through Eocene sedimentation was centered in an east-west structural trough termed the Santa Barbara Embayment. This feature was filled from east-to-west as rates of sedimentation exceeded rates of subsidence in Paleogene time. Abyssal basin plain deposits were sequentially buried by prograding fan, slope, and shelf deposits terminating in widespread non-marine deposition and erosion in Oligocene time as documented by the Sespe Formation. Upper Eocene and Oligocene alluvial deposits of this latter unit neatly partition Paleogene and Neogene depositional packages in this area. Rapid subsidence and marine transgression occurred over wide areas of the California margin in latest Oligocene and early Miocene time initiated by collision of the Pacific and North American plates with subsequent birth of the San Andreas fault zone, translational margin tectonics, and creation of a complex series of bathyal Neogene basins. Continued subsidence of these basins and eustatic drowning of the adjacent Miocene strandline created a series of mid Miocene silled basins starved of terrigenous debris allowing relatively undiluted deposition of diatomites during a period of climatic deterioration and consequent intensified diatom productivity in the California Current system. A well developed oxygen minimum layer allowed preservation of laminated diatomites and diatomaceous muds in these basins ultimately forming the characteristic lithologies of the mid and upper Miocene Monterey Formation. Further tectonic reorganization of the continental borderland occurred in Pliocene time with uplift of the Santa Ynez Mountains and other highs and simultaneous acceleration of subsidence in adjacent synclinal areas including the Ventura Basin. These events led to the rapid progradation of Plio-Pleistocene terrigenous debris into the Ventura Basin, capping underlying diatomaceous basin muds, and filling the basin just prior to a late Pleistocene tectonic event which uplifted portions of these deposits to their present position above sea level initiating the present phase of borderland deposition.

Standard planktonic foraminiferal zones developed for tropical faunas can be estimated in mid- to high-latitude faunal assemblages of the northeast Pacific on the basis of recognition of major paleoclimatic-paleoceanographic events, quantitative distribution of Neogloboquadrina pachyderma morphotypes, and coiling ratio of this species in addition to the presence of key taxa. The late Neogene foraminiferal assemblages of DSDP site 173, off northern California, have been quantitatively analyzed and zoned by correlation with the central North Pacific DSDP site 310; multiple correlations of other microfossil groups support this correlation. Site 173 can thus serve as a reference section for zonation of sediments exposed at Centerville Beach, northern California, and in general for mid- to high-latitude faunas of the northeast Pacific. Correlation of planktonic foraminiferal assemblages of the Centerville Beach section with assemblages of site 173 indicate that the Eel River and Rio Dell Formations represent lower Pliocene to lower Pleistocene deposits. The base of the Eel River Formation lies within the upper part of foraminiferal zone N19, the Pliocene-Pleistocene boundary lies within the upper part of the middle member of the Rio Dell Formation, and extrapolation suggests that the top of the Rio Dell Formation is no younger than 0.7 m.y. This correlation is supported by analysis of other planktonic microfossil groups and by recent magnetostratigraphic work by Dodd and others and Kodama.

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.