CONFIGURATION AND LITHOLOGY OF MESOZOIC BASEMENT BENEATH THE SANTA MARIA BASIN, CA FROM ANALYSIS OF BOREHOLE DATA

Stratigraphic information from oil and gas wells from the onshore Santa Maria basin suggests that the basin is floored by Mesozoic rocks of the Franciscan Complex, with lesser amounts of Coast Range ophiolite and overlying marine sandstones generally equivalent to Great Valley Sequence rocks. Understanding the spatial distribution of basement lithology helps unravel the basin structural evolution and bears on basement density assumptions and resultant gravity-based depth-to-basement models that delineate basin depth and shape. Previous investigations of these rocks interpreted various basement lithologic types and basement structure contour maps differed markedly. Two previous sandstone petrofacies studies separated Franciscan rocks from Great Valley-like rocks, but differed in the interpreted depositional setting of the Cretaceous sandstones.

We present a revised borehole stratigraphic compilation that attempts to reconcile these disparate lithologic interpretations. The compilation emphasizes the domainal nature of basement types; Franciscan rocks dominate, but Cretaceous rocks with Great Valley affinities are intercepted by boreholes in the northwest and southwest parts of the basin. The continuity and consistency of elevation of these strata suggest that they are more likely forearc Great Valley-type strata, rather than localized trench-slope deposits. Electric logs may be of use in distinguishing relatively undeformed Great-Valley like strata from sandstones surrounded by Franciscan mélange. Differing sandstone petrofacies and abrupt transitions from marine sandstone to Franciscan Complex rocks suggest the presence of faults within the basin, but provide no piercing points. In contrast to Paleogene marine sandstones found to the south of the basin, no Paleogene strata occur north of the Lompoc-Solvang fault. Cretaceous sandstones are locally present as far north as the Lions Head fault and may suggest that this fault juxtaposes Cretaceous rocks of differing age and lithology. Serpentinites exist as linear bodies that are aligned along fault traces and probably represent mobilized forearc rocks within transpressional faults. The serpentinites, both exposed and penetrated locally by drillholes, coincide with magnetic highs that may help map these rocks in the subsurface.