Mexican archives for the major Cretaceous Oceanic Anoxic Events

Resumen

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 Peña Formation. However, recent research suggests
that the base of the La Peña 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 México 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.