Mexican archives for the major Cretaceous Oceanic Anoxic Events

Abstract

Oceanic Anoxic Events (OAEs) are interpreted as brief episodes of oxygen-depleted conditions in the global ocean that resulted from profound perturbations in the carbon cycle. These events favored widespread deposition of organic carbon-rich sediments and the subsequent formation of hydrocarbon source rocks. The most important of these events for the Cretaceous period are the globally recognized OAE 1a (early Aptian, Selli event), the OAE 2 (Cenomanian/Turonian boundary, Bonarelli event), and the Atlantic-restricted OAE 3 (Coniacian/Santonian boundary). In Mexico, several sedimentary successions of these ages are proved hydrocarbon source rocks and potential targets for oil and gas shale exploration; however, in most cases, it is unknown how these global events influenced redox conditions under which they were deposited. In general, there is little research to document and characterize properly these events. This work exposes and analyzes the current state of the study of these events in Mexico, and proposes new stratigraphic units to prospect and methodologies for further studies. The OAE 1a has been isotopically constrained in the northeastern part of the country within sediments with high organic carbon content in the lowermost part of the La Pena Formation. However, recent research suggests that the base of the La Pena Formation seems isochronous and younger than the OAE 1a. Accordingly, this event must be recorded in the underlying sediments of the Cupido/Lower Tamaulipas formations. Because of its age and lithostratigraphic features, the Agua Salada Formation of the Lampazos Platform also seems to be linked to this event. The OAE 2 has been documented in northeastern Mexico in the Agua Nueva and Indidura formations, and in southern Mexico in the uppermost part of the Morelos Formation. Trace metal enrichment in these rocks indicates that the emplacement of the Caribbean plateau probably played an important role in the record of this anoxic event across Mexico. Poorly oxygenated conditions during the Cenomanian/Turonian in northeastern Mexico lasted until the early Coniacian. Other stratigraphic units that probably record this event are the Agua Nueva, Eagle Ford, Soyatal, and Maltrata formations. The record of the OAE 3 remains unknown. We hypothesize that Coniacian/Santonian Mexican paleogeography and sedimentary pattern could trigger at least intermittent anoxic/dysoxic conditions favorable for organic carbon burial, and suggest searching for these conditions in the San Felipe, Indidura or Austin formations.