The Cenomanian-Turonian oceanic anoxic event (OAE-2) and continuous drowning up to the Santonian of the Western Valles-San Luis Potosi Platform, Central to Eastern Mexico: Biostratigraphy, chemostratigraphy and paleoenvironments

The Upper Cretaceous stratigraphic succession in the western part of the Valles-San Luis Potosi Platform (central to eastern Mexico) reflect the interaction between various paleo-oceanographic factors including sea-level change, tectonic factors, and the type and supply of sediments. The El Abra Formation is a shallow-water carbonate deposit that is overlain by the hemipelagic-pelagic Soyatal Formation, which represents the transition to a deeper, eutrophic, open marine environment in the latest Cenomanian-earliest Turonian (Whiteinella archaeocretacea Partial Range Zone). This unit is linked to a sea-level rise that occurred on a global scale and flooded the platform, coinciding with the Oceanic Anoxic Event 2 (OAE 2). This change is well–marked by faunal assemblages with a low diversity, and environmental stress–resistant species as well as values of delta 13C excursion, total organic carbon (TOC) and trace element signatures. These data suggest that in this time the ocean was oxygen deficient. During the early Turonian the platform was completely drowned, which led to re-establishment of the oligotrophic environment. These conditions favored for development of the planktonic foraminifera of the Helvetoglobotruncana helvetica Total Range Zone with the occurrence of keeled forms (k/strategists) even continuing as far as the late Santonian thus in the Soyatal interval two biostratigraphic zones are also recognized: the Dicarinella concavata Interval Zone (late Turonian-late Coniacian) and Dicarinella asymetrica Total Range Zone (early-late Santonian). This indicates that the studied part of the Valles-San Luis Potosi Platform remained flooded during this time, in contrast with Mediterranean Tethys regions where recovery of the shallow-water carbonate platforms occurred in the latest Turonian to mid-Coniacian. The planktonic foraminiferal assemblages of this interval are diverse, composed of large and complex keeled morphotypes, which indicate an open deep–sea environment.