Upper Albian OAE 1d event in the Chihuahua Trough, New Mexico, USA

Oceanic anoxic events are clues to ocean processes and are correlation datums. In North America only OAE 1a and 2 are well documented. Based on a low-resolution sampling program, a multi-proxy geochemical approach constrained by a biostratigraphic framework was utilized to identify OAE Id in the upper part of the upper Albian Mesilla Valley Formation near El Paso, Texas. Chronostratigraphic and biostratigraphic evidence indicate that the OAE Id event in the Mesilla Valley section is located in the lower part of the upper Albian-Cenomanian Ovoidinium verrucosum zone, which correlates with the uppermost Albian Parathalmanninella appenninica and Stoliczkaia dispar zones. The chronostratigraphic age of the geochemical event in the Mesilla Valley Formation is uppermost Albian (97.39-9730 Ma). The classic geochemical signatures for OAEs are enriched total organic carbon (TOC) concentrations and coupled positive delta C-13 excursions. OAE Id at this location records TOC values ranging from 0.25 to 0.69 wt.% throughout the Mesilla Valley Formation, where TOC increases during the OAE (21.0-40.0 m) to more than 0.40 wt.%. Interestingly, the organic matter in the Mesilla Valley is dominantly type III, which indicates a pervasive terrigenous source. Although marine organic matter is abundant from the base into the middle of the proposed OAE interval, it is progressively replaced by terrestrial material above the OAE section during progradation. The delta C-13(organic) values record a positive delta C-13 shift of +1.6 parts per thousand from -26.41 to -24.80 parts per thousand. across the stratigraphic interval from 21.0 to 40.0 m, which correlates with OAE 1d. Mn and Fe geochemistry suggest the depositional conditions of the Mesilla Valley Formation were dominated by anoxic and possibly Fe-rich bottom waters, specifically during the time period associated with the OAE Id event. This interpretation is supported by the presence of Fe enrichment recorded by Fe-Total/Al and Fe-Highly (Reactive)/Fe-T with the lack of Fe-pyrite/Fe-Highly (Reactive) associated with Mn depletion. (C) 2013 Elsevier Ltd. All rights reserved.