Abstract: Analysis of piston cores from lower slopes of central and outer basins of the California Continental Borderland shows the presence of structures ranging from starved silt ripples and lenses to erosion surfaces and truncated burrows to cross-bedded units that testify to reworking of the sediments by bottom currents. Adjacent basin-floor piston cores do not reveal these structures, but exhibit the usual bioturbated hemipelagic mud and turbidites. Slope sediments generally contain more silt than the adjacent basin-floor clay silts. The slope grain-size distributions are multimodal as a result of mixing of hemipelagic mud and thin (millimetre scale) silt layers as a consequence of the sampling intervals at centimetre scale. Carbonate and organic carbon contents tend to be low during the glacial periods but variations from this pattern occur that are probably related to the shift in the upwelling associated with the California Counter Current between interglacial and glacial conditions. The evidence for reworking is most abundant from late in Marine Isotope Stage 5 (MIS 5) to early Stage 4 (MIS 4), and it decreases into the Holocene. No structures are observed in the Holocene sections of the slope cores. The temporal distribution of the reworking structures can be explained either by changes in the degree of bioturbation (bottom-water oxygenation), which would work to erase the structures, or variation in the intensity of lower slope circulation through time. The observed pattern is in rough agreement with documented changes in bottom-water oxygenation conditions for these basins. However, the occurrence of reworking is also dominantly within glacial intervals, where lower sea levels produce stronger circulation within the basins as a result of exposure of banks and resulting restriction of cross-sections of the deeper flow pathways. These observations add to the increasing evidence that sediment transport by bottom currents is not restricted to the intensively studied western boundary current drift deposits. The action of bottom currents on eastern boundary slopes can introduce subtle effects on what are commonly assumed to be continuous records.

ABSTRACT The accumulation rates of distal marine sediments reflect the depositional processes in a basin. Key parameters that control the character of the strata and the sediment accumulation rates include basin and margin topography, nature of the sediment input, relative-sea-level variation, and position of the basin with respect to dominant sediment sources. Accumulation rates calculated for a variety of basin settings from the Western North American continental margins provide a means of evaluating the significance of some of these parameters. Borderland-type margin basins adjacent to sediment input sources from the mainland accumulate sediments at the highest rates. Sediments deposited in these inner-tier basins are transported by sediment gravity flows and a variety of pelagic and hemipelagic processes. These basins have steep topographic relief that tends to focus sediments into areally restricted depocenters. Borderland-type margin basins separated from the mainland sediment input sources by significant topography (basins, islands, and ridges) accumulate sediments three to eight times slower than the inner-tier basins. These outer-tier margin basins receive much of their sediment through pelagic and hemipelagic processes. The components of the sediment in these basins (detritus, carbonate, biogenic silica, and organic carbon) are closely linked to paleoceanographic conditions during deposition. Distal marine sediments deposited in continental interior basins have accumulation rates that range between those of the inner and outer borderland-type margin basins. Although the continental interior basins are directly linked to the mainland drainages, their gentle topography leads to a broad areal distribution of the sediments in distal depositional environments.

Abstract In terms of the distribution of pre-Neogene rocks, the California Continental Borderland and associated coastal areas can be divided into the inner Borderland and the outer Borderland, each of which is floored with juxtaposed Mesozoic to Paleogene sedimentary sequences and a coeval melange resembling the Great Valley Sequence-Franciscan Complex couple of central and northern California. The stratigraphy of the Borderland shows local differences, and only the contemporary basins preserve more complete stratigraphic records that can be compared with each other. Seismic sequence analyses provide a basis for inferring that the Patton, Tanner, Santa Catalina, Santa Monica, San Pedro basins and San Diego Trough are floored with Franciscan-type basement, covered by Miocene to Holocene sediments. The Santa Cruz, San Nicolas, and Santa Barbara basins are underlain by thick Mesozoic to Paleogene sequences draped by Miocene strata and filled with Paleocene to Holocene sediments. Generally speaking, Mesozoic to Paleogene sequences are composed of shallow- to deep-water siliciclastic deposits. The Miocene sequences are dominated by volcaniclastic and biogenic sediments, and the Pliocene to Holocene sequences are dominated by deep-water mass-flow deposits. Based on the available chrono-stratigraphic information, a preliminary interbasinal stratigraphic correlation can be established. The tectono-stratigraphic history of the Borderland can be accounted for by the interactions between the East Pacific Rise and the North American continent. During Mesozoic to early Tertiary times, the Borderland was a part of an Andean-type continental margin characterized by the coeval accumulation of fore-arc basin sequences and trench melange. After the East Pacific Rise impinged on the North American continent in the late Oligocene, transform tectonism gradually replaced subduction as the triple junctions passed along the continent edge. In the middle Miocene, wrench processes transposed a piece of continental margin northwestward to form the outer Borderland, and caused formation of Borderland basins and deposition of widespread Miocene volcaniclastics and San Onofre-type sediments. After the major transform activity jumped to the present San Andreas fault in the late Miocene, tectonism dwindled in the offshore area and the residual transform forces gradually forged the present basin-and-ridge configuration. Available stratigraphic information indicates that the Cretaceous and Eocene apparently were periods of high sea stand, as evidenced by extensive marine incursion in the coastal area, whereas the Paleocene and Oligocene are periods of relative low sea stand. The Miocene is a period of high sea stand marked by extensive biogenic pelagic sedimentation. The Miocene-Pliocene boundary is characterized by a transition from biogenic sedimentation to mass-flow deposition. A period of slow mass-flow sedimentation in the late Pliocene is attributed to reduced terrigenous sediment input during the sea-level rise.

Abstract Profiles of slope-normal gradients of continental slopes of the U.S. Pacific margin can be related to the large-scale structural framework of the margin, and to the secondary effects of sedimentation. Tabulations of average gradient, steepest gradient, slope width and the relation of steepest segment to midslope depth define three major slope provinces. The first two regions form the area extending from the Agua Bianca Fault Zone and its offshore extensions off northern Baja California to the Mendocino Fracture Zone. The boundary between region 1 and region 2 has been taken at Point Conception. The boundary separates 1) the outer steep-slope province off the California Borderland, the Patton Escarpment, and 2) a northern province with gentler declivities, usually because of modification by deposition. Both provinces show possible effects of the intersection of oceanic fracture zones with the margin. The northern province (region 3), from Cape Mendocino to Juan de Fuca Strait, can be subdivided into two subregions approximately at Heceta Bank, Oregon. The major provinces correspond on the first-order level to the two main modes of margin interaction with the adjacent ocean plates, underthrusting and transform motion. Second-order factors are the location of fracture zones, and deposition. Within the California Continental Borderland, the shallower base-of-slope depths and narrower shelves produce slopes that have lower relief and are generally steeper than the main continental slopes. These are secondarily modified by depositional processes and mass failure. There is a general increase in depth of slope base and in steepness to the south, illustrating the regional depression to the south, as well as the decrease in sediment supply in that same direction. It is evident that regional characteristics of the morphology of slopes is a function of the structural setting and of the local sediment supply.