Stratigraphic evolution of the Los Barriles basin, Baja California Sur, Mexico

Abstract

The Los Barriles basin, located north of the San José del Cabo basin, is part of the Gulf Extensional Province. It is a half graben bounded by the Los Barriles fault system on its western side. Our study confirms that this system is not part of the San José del Cabo fault but an independent structure. The basin fill includes three stratigraphic sequences: SE 1, SE 2, and SE 3. Los Barriles and Trinidad Formations constitute SE 1. The Los Barriles Formation is in contact with the marginal fault and on the eastern margin it rests unconformably upon the plutonic and meta- morphic basement. The unit is composed of conglomerate and sandstone formed in alluvial fans. The marine Trinidad Formation is composed of sandstone and mudstone. Both formations interfinger and constitute fan deltas without foresets, suggesting rapid subsidence along the Los Barriles fault. Calcareous nannofossils in SE 1 point toward an age in the nannofossil zone NN11 (Late Tortonian-Messinian). Thus, faulting and subsidence occurred since the late Miocene, indicating that the Los Barriles fault system is older than the San José del Cabo fault, which had been activated during the Pliocene. The Refugio Formation (SE 2) rests unconformably on the Trinidad Formation; it consists of fossiliferous sandstone and conglomerate originated in a shallow marine coastal set- ting. Laterally, it interfingers with younger portions of the Los Barriles Formation, forming a fan delta setting of probable Pliocene age. The El Chorro Formation unconformably covers all older units and, locally, even Los Barriles fault system. It comprises sandstone and conglomer- ate formed in an alluvial fan depositional environment. These deposits interfinger towards the present coastline with fossiliferous sandstone and conglomerate formed during the last interglacial period, representing SE 3.