Abstract

The rocks of the Vizcaino Peninsula, Baja California Sur, include the largest continuous exposure of pre-Cenozoic rocks in the Continental Borderland of Peninsular California.

The oldest dated rocks are the upper Triassic volcanic-volcaniclastic sequence at Puerto San Hipolito. Several tectonic belts expose mafic-ultramafic rocks, apparently from the oceanic crust and at least Early Jurassic in age. In mid-Jurassic time the peninsula was intruded by tonalite plutons. These had been uplifted and erosionally exposed by Late Jurassic time, as tonalite debris appears locally in the Late Jurassic Eugenia Formation.

In Early Cretaceous time the Eugenia Formation was extensively intruded by andesite porphyry dikes. The Eugenia, together with the dikes, is unconformably overlain by more than 10,000 metres of Valle Formation, largely sandstone and shale with major lenses of conglomerate in its middle portion. The Valle Formation varies in age from Upper Albian to very Late Cretaceous and is overlain comformably by earliest Paleocene.

Diatomaceous and volcanic Miocene, arenaceous Pliocene, and thin, largely continental Pleistocene formations complete the stratigraphic sequence. Much of the faulting and folding in the Peninsula is of post-Miocene age.