Miocene–Pleistocene tectono-sedimentary evolution of Bahı́a Concepción region, Baja California Sur (México)

Abstract

Bahı́a Concepción is one of the largest fault-bound bays in the Gulf of California. It is one of the area's best examples of an extensional basin, and an accommodation zone related to the Late Miocene extension in the Gulf of California region. Extensional tectonics in the Gulf region initiated from middle to early Late Miocene, in a simple east–west manner. The units within the Comondú Group are tilted between 14 and 45° in opposite directions (E/W), as a direct result of a single main extensional episode. This event produced subsidence that created the depocenters for nearshore marine basins. The oldest sedimentary units present in the region are assigned to the Late Miocene–early Pliocene Tirabuzón Formation. Up-faulted granodiorite basement occurs on the Peninsula Concepción, and at Punta San Antonio as a direct result of the Late Miocene extensional episode. Extension on the Bahı́a Concepción zone was responsible for development of a divided half-graben structure first flooded in Late Pliocene time. The tectono-sedimentary evolution of Bahı́a Concepción is recorded by three stratigraphic stages: (1) pre-rift strata represented by the Comondú Group, (2) a syn-rift unconformity, and (3) a post-rift strata represented by the Late Miocene to Pliocene flat laying sedimentary units. This triad of stratigraphic stages clearly varies from other basins along the margin of the Gulf of California. Upper Pleistocene terrace deposits generally conform to the 6–12 m elevation regionally associated with substage 5e. Their uniformity in elevation inside and outside Bahı́a Concepción indicates that tectonic uplift was locked in step throughout the immediate region at least since that time.