Abstract

Stratigraphic and structural details in the juxtaposed Franciscan and Great Valley Sequences of the northeastern Diablo Range, California, allow restriction of the age of regional thrusting or gravity sliding. The Cretaceous section in the area consists of three sandstone-shale units: the Albian-Turonian Adobe Flat Shale, the Coniacian-Campanian Panoche Formation, and the Campanian-Maestrichtian Moreno Formation. Both the upper and lower parts of this section are absent locally, having been removed by erosion or faulting, or both. The middle unit, the Panoche Formation, varies in thickness. About 18,500 ft of Coniacian-Campanian Panoche section are exposed in Del Puerto Creek on the south, and a similar thickness of equivalent rocks is present in the Byron-Tesla area on the north. Between these areas, near Hospital and Ingram Creeks, the Panoche section, deposited over the same time range (Coniacian to late Campanian), has a maximum thickness of only 9000 to 10,000 ft. Although some of the section has been removed by faulting, much thinning has apparently resulted from differential deposition over a topographic high. This topographic high corresponds geographically with the location of the present Mount Oso Anticlinorium located in Franciscan rocks to the southwest. These relations imply that the Mount Oso Anticlinorium occupied the same relative geographic position during deposition of the Panoche beds. If so, no large-scale tectonic transport between Great Valley and Franciscan rocks, by regional overthrusting, underthrusting, or gravity sliding, has occurred since Coniacian time in the northeastern Diablo Range.

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