Sequence stratigraphy, depositional history, and middle to late Quaternary sea levels of the Ventura shelf, California

Abstract

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.

Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California

“…2 and 4C). Pre-LGM and post-LGM stratigraphic units are characterized by relatively similar seismic facies because deltaic deposition associated with the Santa Clara and Ventura Rivers was apparently continuous from the LGM lowstand through the post-LGM period of sea-level rise (Dahlen et al, 1990;Dahlen, 1992). In this area, the contact between Q and overlying H was mapped using a combination of factors: recognizing subtle differences in seismic facies characteristics (e.g., an upward decrease in reflection amplitude) between lowstand deltaic deposits and transgressive marine deposits; tracing stratigraphic horizons southeastward from angular unconformities mapped to the northwest; and mapping and tracing horizons from the base of buried channels inferred to have formed during the LGM lowstand (following Dahlen, 1992).…”

“…4) reveal shallow (within 5-10 m of the seafloor) asymmetric folding in post-LGM strata with the fault tip locally imaged (Fig. 4A) at a depth of <200 m (also noted by Dahlen, 1992). On the Figure 4 profiles, the width of the shallow (above the transgressive surface of erosion) fold limb ranges from ~140 to 220 m, forelimb dips range from 2.5° to 7° and steepen with depth, and the amount of relief on folded reflections decreases landward from ~10.5 to 6.7 m. West of the shelf beneath the slope, inferred late Pleistocene strata are involved in broad folding of the forelimb above the projected Oak Ridge fault tip (Fig.…”

“…Sediment thickness maps developed for parts of this area have previously been presented at scales ranging from ~1:235,000 to 1:1,000,000 (e.g., Dahlen et al, 1990;Dahlen, 1992;Slater et al, 2002;Draut et al, 2009;Sommerfield et al, 2009). This earlier work does not incorporate the CSMP (2016) high-resolution bathymetry and most of the comprehensive seismic reflection data used in this study (Fig.…”

California State Waters Map Series: Offshore of Santa Barbara, California

“…As with sediment discharge from rivers, the largest wave events and the highest sediment transport rates on the shelf are typically associated with ENSO events. The shelf is underlain by variable amounts (0 to 14 m) of upper Quaternary shelf, estuarine, and fluvial sediments deposited as sea level fluctuated in the late Pleistocene (see sheet 9 of this report; see also, Dahlen, 1992;Slater and others, 2002;Draut and others, 2009).…”

Comments on potential geologic and seismic hazards affecting coastal Ventura County, California

“…The offshore portion of the Oak Ridge Fault runs very close to Platform Grace (Greene and others, 1978;Dahlen, 1992;Shaw and Suppe, 1994;Kamerling and Nicholson, 1996;Kamerling and others, 1998;Sorlien and others, 2000). The onshore portion of the Oak Ridge Fault appears to be the westward continuation of the thrust fault that caused the 1994 M 6.7 Northridge earthquake .…”