Late Pliocene abrasion platform from the Cantil Costero Formation of Baja California

An unusually extensive and well preserved marine abrasion platform of Late Pliocene age is present 84 m above sea leve1 north of El Rosario, in Baja California. The surface extends for 6 km along the coast and is up to 3 km wide. The abrasion platform is a major disconformity that separates siltstones of the Upper Cretaceous Rosario Formation from the basal conglomerates of the Upper Pliocene Cantil Costero Formation. During its creation under transgressive conditions, the platform was kept clear of nearshore sediments and colonies of the pholadid bivalve Penitella penita (Conrad, 1837) with densities of up to 200/mZ, bored into the underlying platform rocks. Tectonic uplift of the peninsula and the Sierra San Pedro Martir, starting about 2 m.y. ago, stimulated a massive progradation of land-derived clastics, which inundated the platform and locally exterminated the pholadid bivalves. This example clearly illustrates a model of transition from subtidal to intertidal conditions, in contrast to unconformities of strictly submarine origin.