Lower Palaeozoic strata on the Pacific Plate of North America

Abstract

The portion of California (USA) and Baja California (Mexico) lying west of the San Andreas–Gulf of California plate boundary is part of the Pacific Plate, and consists of at least seven distinct lithological terranes (Fig. 1). The only pre-Carboniferous rocks previously identified in any of these terranes are the Pre-cambrian gneiss and plutonic rocks of the San Gabriel Mountains in California (Fig. 1, terrane III). Previous reports of Palaeozoic strata west of the San Andreas Fault in southern California are either erroneous¹ or unconfirmed (R. V. Sharp, personal communication). Fossils near El Volcan, Baja California, originally reported as possibly Palaeozoic², are now recognized to be early Triassic³. Unidentified cup corals and crinoid-like specimens were reported in metamorphosed limestone of the Gabilan Range, central California (Fig. 1, terrane I)⁴. Unidentified brachiopods and crinoidal debris in the Sierra Pinta of Baja California were assigned to the Carboniferous (Fig. 1, terrane VII; ref. 5). Fossils of Pennsylvania age were found in a block of limestone and quartzite in a mélange on Isla Cedros.⁶ We describe here the first Lower Palaeozoic rocks from the Pacific Plate; this carbonate–quartzite sequence contains Lower Ordovician conodonts and is exposed at San Marcos, 50-km south of the International Border between Tecate and Ensenada (Fig. 1).