CAN GRAVITY ANOMALIES BE USED TO MAP PRE-PLIOCENE SEDIMENTARY BASINS IN THE CENTRAL COAST RANGES, CALIFORNIA?

In relatively simple tectonic settings such as the Basin and Range, gravity anomalies correlate with Neogene basin-filling deposits, and the gravity anomaly can be modeled as a Cenozoic basin-fill component superimposed on a regionally varying pre-Cenozoic basement component. However, many sedimentary basins in the California Coast Ranges have had multi-stage basin-filling histories such that Pliocene and younger sediment accumulations are often poorly correlated with older Cenozoic sedimentary thickness, resulting in a complex gravity anomaly pattern. In these basins, Neogene tectonic deformation associated with the San Andreas Fault system has disrupted marine and non-marine depositional basins formed in an earlier tectonic environment.

We are modeling gravity anomalies in the multi-stage Cuyama and Carrizo Plain basins by separating the gravity effects of Pliocene and younger deposits from those of Miocene through upper Cretaceous deposits, with the goal of estimating how pre-Pliocene sediment accumulations have been disrupted since deposition. We utilize down-hole logs from oil and gas wells to constrain basin thickness estimates derived from depth-inversion of gravity anomalies. Our analysis shows that in the Carrizo basin, Pliocene and younger sediments are as thick as 1.5 km near the center of Carrizo Plain, and pre-Pliocene deposits have a smaller gravity signature than overlying deposits. In the Cuyama basin, the Pliocene and younger sediments are slightly less than 2 km in maximum thickness, and older sediments are 3-5 km thick. In western Cuyama basin, a narrow region of thick, interpreted-pre-Pliocene sediments extends to the northwest of the present-day valley. Restoration of ~30 km of right-lateral offset on the buried Russell Fault places this narrow region of sediments adjacent to similarly thick pre-Pliocene accumulations observed east of the Russell Fault in central and eastern Cuyama Valley. By analyzing the disruption of Miocene and older basins west of the San Andreas Fault in conjunction with evidence for piercing relations from bedrock outcrop and magnetic anomalies, we hope to define better the pattern and timing of deformation along the faults that segment this portion of the California Coast Ranges.