UPPER ALBIAN CHEMOSTRATIGRAPHY OF COMANCHE SHELF INTERIOR WASHITA GROUP, SOUTHWEST TEXAS

The objectives of the research are to 1) test for petrographic, geochemical and paleontologic evidence of Oceanic Anoxic Events-OAE1c in the Burt Ranch Member of the Fort Lancaster Formation and OAE1d at the top of the Fort Lancaster Formation of the Upper Albian Washita Group, southwest Texas, 2) correlate new δ¹³C chemostratigraphic data in a biostratigraphic framework with global reference curves, 3) reconstruct the paleoenvironment of the interior shelf, and 4) identify the processes at the iron-stained Albian/Cenomanian sequence boundary at the top of the formation.

To achieve these objectives, two stratigraphic sections at Fort Lancaster and East Iraan were measured and 51 samples were collected for petrographic and isotopic geochemical analyses. The lithofacies at the lower part of East Iraan section and Fort Lancaster section upper part are characterized by mudstone, mudstone-wackestone with echinoids, mollusks, suggesting low-energy open marine with intermediate depth. By contrast, the stratigraphically higher samples are dominated by bivalve-echinoid wackestone-packstone, bioclastic peloid grainstone associated with shallow, normal marine with high energy. Mudstone with sparsely distributed bioclast, wackestone are common in Fort Lancaster section lower part. In addition, the fossil assemblages observed in thin sections consists of bivalves, foraminifers, echinoids, ostracods, gastropods and calcareous algae. One endemic benthic foraminiferal biozone- Paracoskinolina coogani was recognized at East Iraan, which can be correlated with Devils River Limestone along the Pecos River and is time-equivalent with the middle Upper Albian. These data will also be used to define the shoaling-up cycles bounded by hiatus or erosional surface. As to the isotopic analyses, δ¹³C_carb and δ¹⁸O_carb whole-rock data are indicators for the presence of OAEs. An obvious negative δ¹³C_carb shift in the uppermost part of the East Iraan section suggests the possibility of OAE1d. To further confirm the hypothesis, a positive shift of total organic carbon is necessary, because the production and preservation of organic matter is favorable in anoxic conditions. Also, cross-plots of δ¹³C and δ¹⁸O reflect the diagenetic change based on the stable isotope trends in response to the different diagenetic circumstances.