FROM WAVE TO TIDAL DOMINATED COASTLINE IN THE PLIOCENE OF BAJA CALIFORNIA SUR, MÉXICO

During the main extensional period, within the Baja California central domain, several depocenters were formed including the El Boleo basin, followed to the south by the Bahía Concepción basin, and to the southeast, the San Nicolás basin. The San Nicolás basin, a half graben, is associated with the transfer zone coupled to the Bahía Concepción accommodation zone. The Miocene extensional process resulted in the uplift and exhumation of the regional Cretaceous granodiorite basement in the Concepción peninsula, Punta San Antonio, and San Juan Londó valley. The late Pliocene San Nicolás Formation is a lithostratigraphic unit with four members as follows: 1) the lowermost Tobas San Antonio member, 2) the alluvial fan Los Volcanes Member, 3) Lodolita Arroyo Amarillo Member, and 4) the marine La Ballena Member. Most of the marine sedimentary beds in the lower part of the San Nicolás Formation are flat lying, and reflect low rates of sedimentation characterized by shell beds, followed by periods of shallow water deposition with evidence of storm activity in the form of reworked marine conglomerates which are interpreted as lag deposits within a storm related sequence. In the upper part of the stratigraphic section, the units are characterized by thick cross-bedded layers, included large-scale sand wave deposits up to 18 m thick. These types of sedimentary structures have being interpreted as produced by strong unidirectional currents, related to extreme tides. Most of the sedimentary units in the San Nicolás Formation reflect a post-extensional episode that involved rapid initial subsidence, and then a reactivation associated with low rates of sedimentation, followed by periods of shallow water deposition dominated by storm activity, and the presence of large scale sand wave deposits which delineate extreme tidal conditions. In contrast, at Punta Chivato, and in the Bahía Concepción area, all the marine units are assigned to the lower Pliocene San Marcos Formation, and upper Pliocene Marquer Formation. Here, the marine strata reflect a post-extensional episode that involved low rates of subsidence,associated with moderate rates of sedimentation, followed by periods of shallow water deposition dominated by very low energy.