Late Quaternary history of the Ventura mainland shelf, California

doi:10.1016/0025-3227(90)90062-O

Marine Geology

Volume 94, Issue 4, September 1990, Pages 317-340

https://doi.org/10.1016/0025-3227(90)90062-O Get rights and content

Abstract

The Ventura mainland shelf off California provides an interesting example of sedimentary response to tectonic activity and eustatic sea-level fluctuations during the late Quaternary. A network of 370 km of high-resolution seismic-reflection profiles provides the basis for the study. The high local rates of uplift, as well as local basin subsidence, have resulted in two distinct styles of sedimentation. A late Pleistocene angular unconformity separates folded Plio-Pleistocene strata from undeformed upper Pleistocene and Holocene strata on the northern shelf. In contrast, the southern shelf is characterized by a thick section of stacked, progradational deltaic sequences containing a well-preserved record of late Quaternary sedimentation.

Sediment overlying the unconformity north of the Pitas Point fault ranges from 10 to 40 m in thickness, whereas the equivalent sequence in the adjacent southern shelf is 80–85 m thick. The distribution of upper Pleistocene and Holocene deposits is controlled by structural features including the Pitas Point and Oak Ridge faults, and the Ventura Avenue anticline, as well as proximity to fluvial input from the Santa Clara and Ventura Rivers.

Within upper Pleistocene deposits, a pair of buried marine terraces occur at depths of 33 and 50 m below present sea level (b.p.s.l.). Terrace deposits consist of a basal, coarse-grained, transgressive deposit which is transitional into a finer grained marine facies. These deposits are associated with a period of aggradation during minor late Wisconsinan sea-level cycles.

Deltaic deposits beneath the southern shelf 4 km north of Hueneme submarine canyon suggest that the Santa Clara River debouched into the sea at this location during the late Pleistocene. The Santa Clara River shifted southward during the late Wisconsinan and cut a large channel, now buried, near Hueneme canyon. This channel indicates that relative sea level was at least 113 m b.p.s.l. during the late Wisconsinan. A smaller channel indicates a fall in sea level to 46 m b.p.s.l. during the Holocene.

The Oak Ridge fault displays a general increase in offset seaward across the shelf, with a maximum vertical displacement of 17 m on the Saugus/upper Pleistocene contact. Displacements on the Pitas Point fault increase shoreward, attaining as much as 40 m of vertical displacement of the late Pleistocene unconformity. Nearshore, the Pitas Point fault vertically displaces the seafloor by 4 m.

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A conceptual model for river water and sediment dispersal in the Santa Barbara Channel, California
2004, Continental Shelf Research
The ephemeral Santa Clara River delivers large amounts of freshwater and sediment to the eastern Santa Barbara Channel during brief, episodic discharge events. This discharge into the channel was characterized here with shipboard measurements during floods of 1997 and 1998. Within approximately 1-km of the river mouth, the river discharge quickly stratifies into a freshened, turbid surface plume and a bottom nephloid layer. Observations immediately off the Santa Clara River mouth on a peak day of river discharge revealed that sediment rapidly settled from the freshened surface waters, as suspended sediment in the freshened surface plume contained only $\sim$ 6% of the sediment mass expected if the sediment mixed conservatively. On the two subsequent days the reduction of sediment mass in the surface plume continued at $\sim$ 50% per day. These observations suggest that river sediment undergoes rapid initial settling within $\sim$ 1-km of the river mouth, followed by somewhat slower rates of settling. Although we did not measure sedimentation or bottom boundary layer processes, our mass balance results suggest that almost all of the river sediment either escapes along or deposits upon the inner shelf seabed.
Physiography of an active transpressive margin basin: High-resolution bathymetry of the Santa Barbara basin, Southern California continental borderland
2002, Marine Geology
High-resolution swath bathymetry and backscatter data from the Santa Barbara basin reveal a distinct assemblage of morphologic and sedimentologic features characteristic of the tectonically active setting of the basin. Such characteristics include the north–south asymmetry of the basin, the presence of an intra-basinal structural and morphologic high, extensive mass wasting, and fields of mounds and pockmarks. The north–south asymmetry of the basin and the increased abundance of slope failure along the northern basin slope likely reflect higher rates of tectonic shortening and sediment accumulation along the northern basin margin. High sedimentation rates lead to high pore water content of the sediment which, in combination with seismic shaking, may explain the abundance of mass wasting features even on shallowly inclined lower basin slopes. Mounds and pockmarks are the surface expression of locally extensive fluid seepage which we interpret to be driven by a combination of high sedimentation rates, a high organic content of sediment, and anoxic bottom water conditions during extended periods in Neogene and Quaternary times. During ROV dives we observed active fluid seepage to have occurred preferentially on the shelf and on a structural and topographic high, presumably due to structural focusing of migrating formation fluids. The preferred alignment of some pockmarks along faults is consistent with structurally controlled expulsion of formation fluids from underlying Neogene units. Despite the high tectonic activity of the basin, fault scarps are rare and only observed in areas of low sediment accumulation on the shelf and on topographic highs. Mass balance considerations suggest that sedimentation rates averaged over interseismic time scales are too high and scarp growth rates too low for scarps to be characteristic features of the basin floor. Long-term sediment accumulation rates that account for the effect of sediment compaction are low enough, however, to lead to long-wavelength surface expressions of the most active faults.
Post-glacial sediments of the Californian shelf from Cape San Martin to the US-Mexico border
2002, Quaternary International
Previous workers of the Californian shelf from Cape San Martin to the US–Mexico border have assembled compilations of high-resolution acoustic seismic profiling data from a number of primary sources. To examine these data more effectively the region has been divided into four areas for convenience in mapping, and partly in response to natural geographic divisions. These data, plus additional detailed profiling off San Onofre south of Dana Point, have made possible calculations of the total volume of unconsolidated post-glacial sediments on this narrow, active transform margin shelf.
The San Onofre survey shows that there are several well-defined buried terraces cut into the bedrock surface. These occur at about −20, −30 to −35, −40 to −50, and −60 to −80 m below present datum. Dating reported in the literature suggests that the bulk of the sediments are Holocene. As sea level shifts from glacial to interglacial, much of the sediment stored on the shelf at high stand is reworked to deeper water during the fall to the following low stand. Previous studies suggest that for the large deposits off the Santa Maria, Santa Ynez and Ventura–Santa Clara Rivers, some sediment may remain after a sea-level cycle.
The estimated total Holocene (past 10,000 years) river discharge for the region, based on records of the past century, is about 130 km³. Thus the total shelf deposit of ca. 60 km³ represents about one-half of the Holocene siliciclastic contribution. However, the 60 km³ includes basal sediments in the three large river lenses that are probably pre-Holocene. A more probable Holocene-contributed volume is about 30 km³. Previous studies show that biogenic and fluvial contributions are generally <5% and 25–30% of the shelf sediments, so that the dominantly sandy shelf sediments of probable Holocene age approximate the coarse fraction of the river. The bulk of the fluvial contribution passes via lateral longshore transport into the adjacent basins via submarine canyons.
Little sediment from the areas of relatively high input passes to the starved areas which remain as non-depositional, low-deposition rate, or erosional surfaces. The major portion of the shelf is sediment-starved since only a few small ephemeral streams enter most of the cliffed coastal areas, and rocky surfaces are exposed in the inner and outer shelf.
Contemporary channel-levee systems in active borderland basin plains, California Continental Borderland
1996, Sedimentary Geology
Long-range large-scale side-scan (GLORIA) information, other seismic reflection profiling studies, and data from cores in the California Continental Borderland, have defined active levee-channel systems extending basinward from the lower fan of Hueneme-Mugu Submarine Fan, Redondo Submarine Fan, and Santa Cruz Canyon Fan in Santa Monica, San Pedro and Santa Cruz Basins, respectively. The Holocene distributaries have been created by a series of turbidity current events. These distributaries range in length from 10–25 km, and are wide (2–5 km) and low-relief (1–10 m) in their distal parts. They are also active conduits for nepheloid flows. Distributions of sedimentological parameters typically mimic the pattern of levee-channel systems. Organic carbon and biogenic carbonate content roughly outline the systems.
Channels are incised in the upper to middle fan areas, and become constructional leveed channels in the lower fan and basin plain as the channel gradient adjusts to maintain a graded profile. Thus sediment gravity flows are generally confined to channels in the upper fan zones, but deposit both channelized and over-bank deposits on the lower fan and basin floor.
The deposits show that the canyon-fan activity has continued during a rising sea level phase. It is evident that canyon headward erosion rates have been equal to or greater than the transgression rate, and that the canyon-fan systems have remained linked with their sediment sources.
Frequency of events was probably higher, and volumes of the events were often larger, during the glacially lowered sea level episodes. However, turbidity currents of sufficient volume to reach the basin floors continue to occur at century or multi-century intervals. As one progresses headward in each system, the number of flows per length of core increases. Small flows that do not pass beyond the distributaries are much more frequent, and may be decadal in frequency, or even more frequent in the Santa Monica Basin system.
These California borderland basins are probably typical of narrow active margins where rate of lateral sea level transgression is less than or equal to the rate of canyon headward erosion. The canyons maintain connections with sediment sources during sea level rise, and the systems therefore are active during the entire sea level cycle. Thus sediment supply is not a simple function of eustacy. This contrasts with the simplified sequence model developed on passive margins where canyons turn off as sea level rises.
Sequence stratigraphy, depositional history, and middle to late Quaternary sea levels of the Ventura shelf, California
1992, Quaternary Research
High-resolution shallow seismic profiles from the Ventura shelf reveal two contrasting, structurally controlled styles of deposition. On the northern shelf, folded Plio-Pleistocene strata comprising the south limb of the Ventura Avenue anticline are unconformably overlain by undeformed late Quaternary strata. In contrast, a thicker section of undeformed, prograding deltaic sequences preserved on the subsiding southern shelf contains a detailed record of at least five middle and late Quaternary glacioeustatic sea-level oscillations. A transgressive surface at −140 m in upper Pleistocene Saugus deposits is overlain by transgressive and highstand systems tracts. A shelf-margin systems tract subsequently developed as the sea level lowered, marking the end of Saugus deposition. Thereafter, over 50 m of post-Saugus upper Pleistocene sediment accumulated during a period of smaller-scale sea-level oscillations, followed by the development of a pair of marine terraces, presently at −50 and −33 m, and a large fluvial channel, now buried at −113 m. Correlations of Saugus and terrace marker horizons with the middle to late Quaternary paleosea-level curve suggest development of the Saugus marker horizons during oxygen isotopic stages 7-5e and formation of the terraces during Wisconsinan high sea levels. Maximum rates of vertical separation on the Oak Ridge and Pitas Point faults are estimated to be 0.13 and 0.30 m/1000 yr, respectively.
Quaternary sequence stratigraphy of the Brisbane River delta, Moreton Bay, Australia
1992, Marine Geology
The Quaternary geological history of the Brisbane River delta front and prodelta was interpreted using seismic stratigraphic and sequence stratigraphic principles. During the last major glacial sea level episode, the floor of the bay was exposed and eroded by stream channels. In the subsequent postglacial marine transgression, the palaeochannels of the Brisbane and Pine Rivers were backfilled with mud. Sea level continued rising until about 6500 years ago and a highstand has persisted since then. During the Quarternary, sediment deposition in Moreton Bay was controlled by sea-level fluctuations, channel migration, and palaeotopography. There were at least four depositional episodes prior to the last glacial period, plus the presenet postglacial episode. Periods of low sea level occurred between highstands and the sea bed exposed and eroded. Eight major seismic reflectors were recognised and six seismic sequences defined. The seismic sequences represent five highstand systems tracts and one transgressive systems tract. The relationships between eustacy and system tract type were applied to glacio-eustatic sea-level curves to determine relative ages of sequences in Moreton Bay. The application of sequence stratigraphy to high frequency sea-level fluctuations was demonstrated. The stratigraphic system established within the context of sequence stratigraphy will provide a reference for future work within the region.

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^∗: Now at: Universität Freiburg, Albertstr. 23b, 7800 Freiburg (F.R.G.).

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Research paperLate Quaternary history of the Ventura mainland shelf, California

Abstract

Seismic Stratigraphic Interpretation and Petroleum Exploration

Sea-water intrusion in California

Calif. Dep. Water Resour. Bull.

Seismic stratigraphy of the Ventura mainland shelf: Late Quaternary history of sedimentation and tectonics

Quaternary stratigraphy and sedimentation of the mainland shelf of San Diego County, California

Seismic facies of shelfedge deposits, U.S. Pacific

Geologic evolution and Quaternary geology of the Santa Barbara basin, southern California

Study of Quaternary shelf deposits (sand and gravel) of southern California

Rep. State Calif. Dep. Boating Waterw., Beach Erosion Control Proj., Sacramento

The marine geology of the eastern Santa Barbara Channel with particular emphasis on the ground water basins offshore from the Oxnard Plain, southern California

U.S. Geol. Surv. Open-File Rep.

Sound velocity gradients in marine sediments

J. Acoust. Soc. Am.

Coastal tectonics

Quaternary chronology and rates of crustal deformation in the Ventura area, California

The depositional sequence as a basin unit for stratigraphic analysis

Holocene sea-level curves for Santa Monica shelf, California

Science

Potential sand and gravel resources in Santa Monica and San Pedro Bays, southern California

Research paper
Late Quaternary history of the Ventura mainland shelf, California