EARLY CRETACEOUS ANOXIC EVENTS 1C AND 1D ON TEXAS COMANCHE SHELF

Oceanic Anoxic Events (OAEs) are widespread deposits of organic-rich, dark gray shale yielding mainly pelagic fossils. Positive δ¹³C shift of ~0.5 to 1.5‰ and shifts in trace metal concentrations are typical of OAEs. Late Albian Stage (103 – 100 Ma) OAE 1c and OAE 1d are recorded in intra-shelf basins on the Comanche Shelf.

OAE 1c is in dark gray shale and interbedded micritic limestone of the Boracho Formation in the West Texas Fort Stockton Basin. Planktic foraminifera, tintinnids and exogyrid and gryphaeid oysters correlate these two intervals with the Pseudothalmanninella subticinensis Zone. Carbon isotope fluctuates between positive and negative peaks coincident with total organic carbon. This cyclicity suggests that bottom waters fluctuated between anoxic and dysaerobic conditions on the ramp slope. Up slope the Burt Ranch Member hosts ammonites and a moderately diverse bivalve fauna where bottom waters were aerobic. In the North Texas Basin near Fort Worth OAE 1c is represented by a positive δ¹³C shift of about 0.7 ‰ in the lowermost upper Albian Kiamichi Formation, which is composed of dark gray shale and thin-bedded limestone.

OAE 1d is in dark gray shale of the Mesilla Valley Formation at Cerro Cristo Rey near El Paso in the Chihuahua Trough. Total organic carbon increases 0.4% from 0.29 to 0.69 weight % and δ¹³C TOC-VPDB increases 1.61‰ from -26.41 to -24.80. Bottom-water anoxia and ferruginous-euxinic conditions are inferred by the ratio of highly reactive iron (Fe_HR) to total iron (Fe_T). In North Texas OAE 1d is recorded in the uppermost Albian Main Street Formation with latest Albian ammonites. This unit is composed of interbedded bioclastic limestone and dark gray shale having a positive δ¹³C shift of about 1.9 ‰.

The latest Albian anoxic event 1d is not recorded in platform carbonates of the Fort Stockton Basin. The uppermost Albian interval is composed of echinoderm-rudist-ooid packstones with Mexicaprina rudists. Water depth was within normal wave-base. Down-slope in the Fort Stockton Basin depths were possibly up to 100 meters. Bottom waters fluctuated between anoxic to dysaerobic suggesting that the low-oxygen oceanic water mass was shallow enough to flood into Comanche Shelf basins.