Response of the western proto-North Atlantic margin to the early Aptian oceanic anoxic event (OAE) 1a: an example from the Cupido platform margin-Gulf of Mexico, NE Mexico

Abstract

Integrated microfacies and geochemical analyses conducted on five stratigraphic sections in northeastern Mexico (ancentral western margin of the proto-North Atlantic) reveal major paleoenvironmental changes in shallow water and pelagic environments in the prelude and run-up of the early Aptian oceanic anoxic event (OAE) 1a. During the Barremian–Aptian transition, the replacement of photozoan rudist-coral by mesotrophic/eutrophic orbitolinid-miliolid communities in the Cupido platform occurred in association with increased nutrient input. Thereafter, oligotrophic, photozoan, green algal communities repopulated briefly the carbonate ramp. Microbial-dominated ecosystems and peloidal carbonates with scarce fossils flourished before and during the onset of the OAE 1a, attesting carbonate crisis and highly stressed conditions. Organic-rich sediments deposited under weak reducing conditions in some parts of the platform. Carbonate sedimentation at the end of OAE 1a was characterized by photozoan ooid-dominated facies documenting a recovery phase of marine alkalinity. The studied carbonate platform shows a response mode to OAE 1a marked by continuous carbonate production, presumably protected by the “kettle effect”, but accompanied by significant ecological perturbations and incipient drowning. The documented change shares attributes of both central/southern and northern Tethyan platforms response modes. The OAE 1a deposits in the pelagic setting are characterized by organic-rich, radiolarian-dominated carbonates accumulated under oxygen-restricted bottom water conditions and during enhanced nutrient input linked to intense chemical weathering. The intensity of the trace metal anomalies suggests that volcanic-hydrothermal activity associated with the construction phases of the Ontong Java LIP also influenced the elemental abundance in the proto-Atlantic water mass.