Stratigraphy and geochronology of Miocene and Pliocene volcanic rocks in the Sierra San Fermín and southern Sierra San Felipe, Baja California, Mexico

Abstract

Geological mapping and 40Arf39Ar geochronology in the Sierra San Fermin and southern Sierra San Felipe (between latitudes 30°30' and 30°44' N and longitudes 114°42' and 114°51' W), northeastern Baja California, reveal a history of Miocene to Pliocene volcanism from 21 Ma to at least 3 Ma. The area is located within part of the early to middle Miocene, subduction-related volcanic arc which reached the length of the Baja California peninsula. The study area also straddles two major, post-subduction vent areas 11 and - 6 Ma in age which have been dismembered by normal and sinistral strike-slip faulting within the Gulf of California Extensional Province. Prior to 11 Ma, basement-derived arkosic sandstone, basalt and pyroclastic flows, and volcaniclastic sedimentary rocks locally covered Cretaceous and older batholithic and prebatholithic rocks. Around 11 Ma, a 40-m thick regional pyroclastic flow, local hornblende andesite flows, and epiclastic volcanic units were deposited. At about 6 Ma, eruptions covered the region with an additional 220 m of pyroclastic flows, ash fall deposits, epiclastic volcanic units, and local rhyolite flows. Tuffs in this package thicken southwards across a major east-northeast-striking structure in the central Sierra San Fermin, which may be part of a caldera wall. The youngest of the tuffs in the 6 Ma sequence appear to have ponded against this structural barrier, causing the youngest units to be very thin or absent north of this boundary. Angular discordance between 11 and 6 Ma units indicates that major extensional -basins formed in this time interval. A hiatus in deposition throughout much of the study area occurred between 11 Ma and -6 Ma ago, although 8 Ma deposits crop out locally. This hiatus may not represent a lull in regional volcanic activity but merely the presence of local topography in the Sierra San Fermin with basins nearby, controlled by caldera collapse or normal faulting. Between 6 Ma and about 3 Ma, an additional 600 m of rhyolite and pyroxene andesite flows and breccias and pyroclastic flows were deposited in fault-controlled basins.