THE EXPRESSION OF THE OCEANIC ANOXIC EVENT 1A IN THE CUPIDO PLATFORM (NE MEXICO)

The Barremian-Aptian Cupido platform of northeast Mexico is part of an extensive carbonate platform system that flourished in the western margin of the proto-North Atlantic. Unlike other platforms in the region and in northern Tethys, the Cupido platform recorded continuous carbonate deposition combined with distinctive paleoenvironmental perturbations during the Early Aptian Oceanic Anoxic Event (OAE) 1a. In order to understand the Cupido response to this anoxic event, two shallow-water sections are investigated by means of petrography (microfacies), geochemistry and detailed sedimentological analysis. Before the OAE 1a, during the Barremian-Aptian transition the replacement of photozoan rudist-coral by mesotrophic/eutrophic orbitolinid-miliolid communities occurred associated with increased runoff and nutrient input during a major second-order marine transgression. Thereafter, oligotrophic, photozoan dasycladacean communities repopulated the carbonate ramp. Microbial-dominated ecosystems with scarce skeletal components flourished during the harbinger and the lower/middle part of the OAE 1a, attesting highly-stressed conditions. Remarkably is the occurrence of irregular/amoeboid-shaped patches that resembles the patchy-cloudy Lithocodium–Bacinella morphotypes. The biotic perturbation experienced by the carbonate producers is also documented by the increase in peloidal carbonates with rare biota. Such deterioration of the platform coincides with episodic increases in the detrital influx and phosphorous, associated with high chemical weathering rates and subsequent eutrophication. During the final part of the OAE 1a, carbonate sedimentation resumed with photozoan, ooid-dominated facies that seems to represent a recovery phase of marine alkalinity. Considering the vast evaporitic interior of the Cupido platform and the reported temperatures, the protective “kettle effect” emerges as a credible mechanism to explain the ongoing carbonate production of the Cupido platform during the OAE 1a