ABSTRACT

The Pigeon Point Formation, of Upper Cretaceous age, is exposed along 18 km of the central California coast, about 55 km south of San Francisco. Facies units of the formation include varied types of mudstone, siltstone, sandstone, conglomerate, limestone, and coal, with numerous lithologies intermediate between these types. Coarsening upward throughout the formation is apparent although the stratigraphic order and thickness of the formation is only crudely determinable, owing to extreme faulting and lack of adequate exposure. Each facies is characterized by sedimentary structures whose origins are: (1) syndepositional; (2) post-depositional, chiefly by gravity and liquefaction; (3) erosional; and (4) biogenic. Fossils, which are rare in the formation, consist of forams and shallow-mar ne molluscs of Campanian and Maestrichtian age in addition to abundant plant fragments.

Interpretation of the petrology, facies associations, and sedimentary structures suggests that Pigeon Point deposition occurred along an embayed coastline. The shoreline lay immediately west or southwest of the elevated granitic/metamorphic complex of the Salinian block, which was the chief source of the sediments. Environmental conditions fluctuated greatly during deposition, as indicated by abrupt facies changes, including evidence of intervals of subaerial exposure (mudcracks) and possibly subaerial channel scouring. Abrupt lateral and vertical juxtaposition of extremely different facies units such as thin-bedded mudstone and boulder conglomerate suggests also that tectonic instability and (or) climatic-hydrologic fluctuations characterized the source land area during much of Pigeo Point time.