EVIDENCE OF LATE JURASSIC-EARLY CRETACEOUS EXTENSIONAL TECTONICS IN CENTRAL MEXICO: FACIES ANALYSIS AND PROVENANCE OF THE MARINE SUCCESSIONS OF PEñA DE BERNAL QUERéTARO, MEXICO

A clastic marine succession, exposed at Peña de Bernal, recorded an event of basement uplift an erosion during the Early Cretaceous time. The area is located at the edge of the Oaxaquia block formed by Proterozoic grenvillean-basement, which is the backbone of eastern Mexico. The basement of the area is made up of Triassic-Jurassic? subduction complex, covered unconformably by volcaniclastic rocks of the San Juan de la Rosa Fm., intruded by coeval intermediate sills that show peperitic textures at contacts with limestone and volcanic sandstone. This formation changes transitionally upward to the Peña Azul Fm. In this work we proposed four sedimentary faceis and two facies associations for these two formations. The San Juan de la Rosa Formation is made up of thin-bedded limestone with sparse shale and volcanic sandstone (facies SJR1), and volcanic sandstone and tuff interbedded with minor calcareous shale (SJR2). These rocks were not dated, but they are Titonian-Berriasian in age in neighboring areas. This submarine volcanic event marks the initiation of the marine Mesozoic succession in the area. The San Juan de la Rosa Fm, changes transitionally upward to Peña Azul Fm. of Lower Cretaceous age. This is formed by calcareous debris-flows and turbidites (facies association AF1), fine-and grained sandstone (facies F2); followed by conglomeratic sandstone, fine-grained sandstone, sandy limestone with fossil ostrea, and tuff (AF4). They all recorded shallowing-upward marine sedimentation. Peña Azul Fm. changes transitionally upward to cherty limestone of the Tamaulipas Fm. (facies TS1). According to the modal analysis of sandstones, La Peña Azul Fm. recorded uplift and erosion of the basement. They show inverted clast stratigraphy, with limestone and chert clasts (derived from litified limestone units) and volcanic clast (derived from San Juan de la Rosa Fm.) in basal strata. Upper strata contain metamorphosed sandstone, sandstone clasts, derived from the basement. Thus, we proposed that the sedimentary basin was originated by extensional processes in submarine conditions, where local basement highs were the source of the clasts. This period of horst and graben tectonics was followed by higher and more regional subsidence recorded in the facies TS1 of the Tamaulipas Formation of Albian age.