GEOPHYSICAL STUDIES OF CUYAMA BASIN, CENTRAL CALIFORNIA

We use gravity and aeromagnetic data to examine the structure of Cuyama Valley, which lies west of the Big Bend of the San Andreas Fault and at the transition between the southern Coast Ranges and western Transverse Ranges in central California. The valley is underlain by a Cenozoic basin filled by both marine and non-marine sediments, which formed originally as a transtensional basin in the Miocene. Today the basin is within a transpressional tectonic regime. A west-northwest-trending, oblong isostatic residual gravity low of 25 mGal is centered on the valley, highlighting the accumulation of lower-density Cenozoic (and possibly Cretaceous) sedimentary units in the basin. Gravity gradients at the northern and southern margins of the basin may be used to estimate the degree of overthrusting of the crystalline basement. Preliminary inversions of gravity anomalies yield estimates for the thickness of sediments and sedimentary rocks to be as great as 8-9 km, roughly consistent with previously published cross sections. Basin depths are constrained, in part, by a handful of boreholes in the Morales Canyon oil field that penetrate pre-Cenozoic rocks at depths generally between 1.5-2.5 km, but in one case 5.0 km. Northwest-trending magnetic anomalies project into the Cuyama Valley, where they are subdued presumably because of the depth of the basin. The most significant, short-wavelength anomalies coincide with outcrops of Miocene alkali olivine basalt in the Caliente Range to the north. Gravity analysis will aid in defining the shape of Cuyama Basin beyond the limited portions where borehole and seismic observations are available, a definition that should improve understanding its genesis and groundwater and hydrocarbon potential.