LATE ALBIAN OCEANIC ANOXIA IN THE CHIHUAHUA TROUGH, SOUTH-CENTRAL NEW MEXICO

Multiple global oceanic anoxic events (OAEs) occurred during the Cretaceous Period and while these events have been identified in various locations around the globe, few have been identified in the Gulf Coast of North America. The Mesilla Valley Formation is Upper Albian shale exposed on the northeast flank of Cerro de Cristo Rey in south-central New Mexico that contains geochemical evidence of OAE 1c. The Mesilla Valley was deposited in a near shore shelf environment along the NE edge of the Chihuahua Trough and has been correlated with Upper Albian Washita depositional cycle WA4 in central Texas (Scott et al. 2001; Lucas et al. 2010). Deposition of the Mesilla Valley began in a relatively deep shelf marine environment and shoaled upward into the basal nearshore sands of the overlying Anapra Formation. A multi-proxy geochemical approach was taken for the purpose of constraining local redox conditions at the time of deposition. Analyses performed include total organic carbon (TOC), δ¹³C of TOC, δ¹³C in bulk carbonate and sequential iron extractions. Geochemical data are integrated with biostratigraphic data to accurately date the event and correlate it with standard Upper Albian zones. Carbon and iron data generated from the Mesilla Valley indicate a shallowing-upward suboxic-anoxic ferruginous depositional environment. Increasing TOC concentrations in conjunction with measurable negative to positive excursions in both δ¹³C_TOC (+1.6 ‰) and δ¹³C_carb (+0.7 ‰) are observed in the Mesilla Valley. These are common characteristic of OAE 1c in deep sea cores and outcrops. While the observed excursions may be lower in magnitude than their deep-sea counterparts, the correlation of geochemical data with existing biostratigraphic data substantiates the correlation of the Mesilla Valley event with OAE 1c. These data further suggest that this geochemical event may be recorded elsewhere in the Gulf basin and is of regional importance.