Using Applied Micropaleontology in Subsidence Analyses to Constrain Plio-Pleistocene Fault Movement in the Ventura Basin, California

Nestor D. Phillips, Martin B. Lagoe

The Oakridge Fault is a major tectonic feature in the Ventura Basin of southern California, and its Late Cenozoic movement history and evolution are not well constrained or understood. Insight into the Late Cenozoic history of movement of the fault, and the general tectonic and depositional history can be derived from the Plio-Pleistocene deposits of the Ventura Basin. Subsidence analyses, utilizing detailed stratigraphic information, can provide better constraints for the movement of the Oakridge Fault and a clearer insight to the tectonic development of the Ventura Basin.

Backstripping is a subsidence analysis integrating decompaction of sediments, geologic age, and paleobathymetry to unravel basin history. Much higher resolution of structural movement can be obtained from increased chronostratigraphic resolution and refined paleobathymetric modeling of the marine record. A chronostratigraphic framework for this area is based on temperate Pacific planktic foraminiferal datums and climatically-controlled, coiling shifts in Neogloboquadrina pachyderma, both tied to paleo-magnetic stratigraphy. Refinement of the paleobathymetry was attained from re-analysis of modern benthic foraminiferal data.

Four sections within the basin are analyzed by backstripping: the Wheeler Canyon and Santa Paula Creek sections, north of the Oakridge Fault, and the Balcom Canyon and Texas Butler sections, to the south. Tectonic subsidence, sediment-loading subsidence, and total subsidence histories of the four sections provide insight into the tectonic development of the basin, while differential subsidence between sections north and south of the Oakridge Fault allow for an estimate of the fault's Pleistocene separation history.

Highlights of this tectonic analysis include early subsidence dominated by tectonic processes, a mid-Pliocene uplift event and later, basin subsidence due primarily to sediment loading. The total (net) movement of each section is the result of the interaction between tectonic movement and subsidence due to sediment loading. Isolation of the tectonic movement from the subsidence due to sediment loading was responsible for the delineation of three previously unrecognized tectonic uplift events. These uplift events were masked by the subsidence due to the tremendous sediment load deposited during the Late Cenozoic. The Oakridge fault appears to have been a normal fault during much of its Plio-Pleistocene history, only later being reactivated as a reverse fault.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995