ABSTRACT: Lithologic and foraminiferal analysis from the western part of the Valles–San Luis PotosÃ platform reflect a sea-level rise suggesting that the platform was drowned at the Cenomanian-Turonian boundary interval. Benthic and planktic foraminiferal content of the succession was identified to establish the transition from the shallow-water carbonate sedimentation with abundant larger benthic foraminifera, algae, rudists and other molluscs, corals, echinoderms, worms changing to hemipelagic-pelagic sediments containing calcispheres and opportunistic foraminifers (r-strategists), such as whitenellids, hedbergellids, heterohelicids and elongated chambered forms, as well as radiolarians. The El Abra Limestone is dated as the middle-late Cenomanian age (Pseudolituonella reicheliAssemblage Zone) and was deposited on a warm shallow-marine rimmed platform. Peloidal-bioclastic packstone-grainstone is the dominant texture of these rocks. Lithological variations determined by the high detrital influx were followed by a major transgression when wackestone and marly limestone of the lower part of the Soyatal Formation were deposited. The transition to deeper open marine environments is dated as latest Cenomanian/earliest Turonian represented by the Whiteinella archaeocretacea Partial Taxon Range Zone and coincides with a global sea-level rise and the Oceanic Anoxic Event 2 (OAE 2). Deeper overlying deposits of the Helvetoglobotruncana helvetica Total Range Zone (early/middle Turonian) are characterized in the study area by the occurrence of the keeled forms (k strategists). The Cenomanian/Turonian boundary is placed within the Whiteinella archaeocretacea Partial Range Zone in absence of ammonite markers. When theValles-San Luis PotosÃ platformwas flooded, the carbonate factorywas stressed due to the trophic resource modifications. Production of a larger amount of nutrients led to the extinction of the k-strategist benthic foraminifera; only small benthic foraminifera survived for a short period of time in the newly established mesotrophic conditions. High productivity continued and in the eutrophic episode the calcispheres, r-opportunists, intermediate taxa and the planktic foraminifera with elongate chambers proliferated. Later, complete drowning enabled the recovery of oligotrophic conditions with the occurrence of k-strategist keeled planktic taxa.
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