LATE QUATERNARY HISTORY OF THE VENTURA MAINLAND SHELF, CALIFORNIA

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. © 1990.