Abstract

Subsurface structure maps of granitic basement rocks and the Campanian Dobbins Shale indicate a difference in strike on the east side of the Sacramento Valley and opposite dips on the west side, where basement rocks continue to dip west, yet overlying strata are uplifted and now dip east. This dip divergence defines an east-tapering wedge observed on several seismic lines. Correlation of outcrops to subsurface units with seismic data acquired across the Rumsey Hills indicates that this wedge of rocks, probably Franciscan, has been driven between the Great Valley sequence and underlying crystalline basement. Detailed structure maps from 600 wells and several hundred miles of seismic lines indicate that considerable post-Eocene deformation has occurred, although outcrops and reconstructions of the Princeton Gorge suggests a western high was present prior to this. The Rumsey Hills and other shallow structures on the west were formed by eastdipping thrust faults that are part of the roof thrust of the wedge. Also, the structures overlying these thrust faults are more tightly folded to the south, yet become broad and open to the north. These structures, including the Rumsey Hills, appear to be formed by northward translation of the deep structural wedge. On the east side of the basin, where basement and overlying strata have different strike orientations, relationships are still enigmatic because it has not been discerned if the different strike orientations are due to loading of individual thrust sheets, or a separate regional deformation prior to the Campanian.