Coastal evolution of Late Cretaceous and Pleistocene rocky shores: Pacific rim of northern Baja California, Mexico

Cretaceous on Cretaceous and Pleistocene on Cretaceous unconformities are mapped throughout a 45 km(2) field area surrounding the village of Erendira on the Pacific coast of northern Baja California, Mexico. The unconformities represent a late Pleistocene rocky shore (120 000-130 000 ka) directly superimposed on a Campanian-early Maastrichtian rocky shore (77-70 Ma). Superb exposure and preservation of coastal features allow for precise paleogeographic restorations. Physical analysis includes a comparison of rocky-shore composition, configuration, and overall length. Paleoecological analysis permits the recognition and comparison of preferred wave directions. High-intensity, windward biotas during the Late Cretaceous included rock-encrusting rudists and coralline red algae, while low-intensity, leeward biotas were characterized by rock-encrusting oysters, scleractinian corals, and bryozoans. During the Pleistocene, the open coast was exploited by the byssate bivalve Mytilus and boring bivalves including Petricola and Penitella, while more protected settings were dominated by the gastropods Olivella and Saxidomus. Amino-acid analysis of Saxidomus nuttalli yielded an age estimate consistent with the last interglacial epoch (oxygen isotope stage 5e). Wave impact was oblique to the paleoshore during the late Pleistocene; onshore winds were generated from the west. In contrast, wave movement was generated from the east to southeast and parallel to shore during the Late Cretaceous. Field data are integrated with paleogeographic reconstructions and predictive models, including computer models for local and global oceanography. Where different paleogeographies and oceanographic models conflict, field mapping of this sort adds a much needed element of ground truth.