Abstract

A small tract of heretofore-unrecognized Paleogene rocks lies about 30 km northeast of Santa Maria and 1 km southwest of the Sur-Nacimiento fault zone near upper Pine Creek. This poorly exposed assemblage of rocks is less than 50 m thick, lies unconformably on regionally distributed Upper Cretaceous submarine-fan deposits, and consists of three units: fossiliferous lower Eocene mudstone, Oligocene(?) conglomerate, and basaltic andesite that has a radiometric age of 26.6 ± 0.5 Ma. Both the sedimentary and igneous constituents in the Paleogene sequence are unlike those of known sequences on either side of the Sur-Nacimiento fault zone.

Conventional paleogeographic restorations need to be modified to accommodate the conditions of deposition and emplacement represented by the three rock units. The Paleogene sedimentary rocks near upper Pine Creek presumably are remnants of formerly widespread early Eocene bathyal deposits and locally distributed Oligocene(?) fluvial deposits southwest of the fault zone. The 26.6 Ma basaltic andesite, however, may not have extended much beyond its present outcrops. Although the history, nature, and amount of pre-Miocene slip along the Sur-Nacimiento fault zone are not resolved, an episode of Oligocene(?) displacement is required by the contrast in thicknesses; depositional patterns, and paleobathymetry of the juxtaposed rock sequences.

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