Abstract

The Rumsey Hills are the topographic expression of an anticline formed by fault-bend folding during Tertiary time. The east-dipping thrust underlying this fold is rooted at depth to the Coast Range thrust, here defined as a regional fault system along which the western basin edge has been uplifted. Two seismic reflection lines cross the Rumsey Hills and are presented to illustrate the relationship of the uplift and shortening of the western valley edge to the eastward (or northward) propagation of an underlying wedge of Franciscan rocks which has been tectonically inserted between basement rocks and the overlying Great Valley strata.

Geologic mapping in the Rumsey Hills area has provided many details about the complexity of these thrust faults. Stratigraphic geometries observed on seismic data indicate that uplift of the Rumsey Hills is post-Cretaceous. Downcutting of the Eocene Princeton Gorge, observed in outcrop and on seismic data in the Capay Valley, also indicate uplift started during or just prior to Eocene time. Pliocene and Pleistocene rocks are steeply tilted around the anticline also, and indicate that active underthrusting of the Franciscan wedge is occurring.

Several exploration wells have been drilled on this structure. None have been commercially successful, though there have been numerous small showings of gas and some very high flow rates of saltwater in pre-Campanian sandstones. Both the hanging wall and footwall blocks have been tested. Considerable exploration risk exists for trap and charge-timing due to the young age of this structure and the large amounts of uplift.

The data presented here support the model presented by Unruh and others (1991) for development of the California fore-arc region by tectonic wedging and underthrusting.