Abstract: The Sacramento Shale, a Potential Key Bed for Estimating Post-Miocene Crustal Shortening in the California Coast Ranges

Michelle Silk, D. L. Jones

Along the eastern front of the Central Coast Ranges, stratified rocks above the midcrustal brittle-to-ductile transition zone are contractionally deformed into subparallel folds and thrusts attendant to major splays of the San Andreas Fault System. Current work suggests regional deformational styles have historically alternated between compression and extension due to slight modifications in movement of the Pacific and North American Plates. The focus of our work is to determine if it is possible to decipher the complex structural/tectonic evolution of the region by attempting to (1) bracket the timing of deformational episodes and, (2) estimate the amount and distribution of crustal shortening. This probably cannot be done for older episodes owing to fault re-activation. It may be possible however to determine a generalized estimate of the most recent (post-Miocene) compressional episode by using selected, datable, Mesozoic and Tertiary units that are eastwardly displaced over Sierran basement along the currently active imbricate system of tectonic wedges as key beds in detailed cross-sections and palinspastic reconstructions. Our supposition is that the Sacramento Shale, an informal unit of the Great Valley Sequence, can be used as the Upper Cretaceous marker because of its (1) biostratigraphically datable age (Late Campanian based on its radiolarian fauna), (2) stratigraphic correlation with the type Pinehurst Shale in the Berkeley Hills, (3) geographic distribution: Northeast/Southeast of Mt. Diablo and in Pleasant Valley north of the Sacramento Delta and (4) inclusion in currently active compressional deformation (exposed at the surface via imbricated back thrusting along the Coast Range's eastern front).

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California