Lower Cretaceous Alisitos Formation at Punta San Isidro: Coastal sedimentation and volcanism

Abstract

The Lower Cretaceous (Aptian-Albian) Alisitos Formation is well exposed at Punta San Isidro and adjacent sea cliffs on the Pacific shores of Baja California near Eréndira, Mexico. Continuous coastal outcrops define a local stratigraphic succession less than 100 m in total thickness, including repetitious tuff, sandstone and conglomerate units interbedded with discontinuous limestone beds rarely more than 2.5 m thick. The limestone beds are biostromal units that include scattered oysters and/or corals, as well as units dominated by the rudistid bivalve, Caprinuloidea perfecta. Two distinctive conglomerate units are composed of andesite cobbles colonized by encrusting oysters in a quasi rocky-shore setting. One sandstone unit includes abundant fossil wood with tree limbs up to 55 cm long and 5 cm in diameter. Proximal volcanic activity is indicated by a series of dikes that cut through pyroclastic beds and lead to a 10-m thick andesitic flow that caps the succession at Punta San Isidro. Compared with thicker intervals of the Alisitos Formation elsewhere in Baja California that are dominated by andesitic flows and offshore limestone, the Punta San Isidro sequence offers a window on a back-reef environment adjacent to a paleoshore that received pyroclastic lahars from a terrestrial origin or mass flows from shallow submarine explosions. Recovery of marine life and the renewal of a carbonate substrate followed successive episodes of volcanism and massive erosion along an active coastline. This scenario is very different from depositional processes of the Upper Cretaceous (Campanian-Maastrichtian) Rosario Formation that cannibalized and subsequently formed a regional unconformity against tilted Alisitos strata with substantial topographic relief in the Eréndira region.