SEDIMENTOLOGY, STRATIGRAPHY, AND PALEONTOLOGY OF THE WESTERN GULF OF MEXICO DURING THE EARLY TO MIDDLE CAMPANIAN, USA

The Texas Gulf Coastal region was the “gateway” to the Cretaceous Western Interior Seaway (WIS). However, the biostratigraphy, faunal dynamics, and environmental history of this region is poorly understood. To improve stratigraphic correlation, as well as examine the environmental and faunal changes that occurred in the Gulf of Mexico (GOM) during the Campanian, a high-resolution, molluscan-based bio/lithostratigraphic analysis was undertaken of the Campanian deposits along the North Sulphur River in Texas. To construct this bio/lithostratigraphic framework, macrofossil, micro/nannofossil, sedimentological, and chronometric data were analyzed from the lower Campanian Gober Chalk and lower to middle Ozan Formation. This new data revealed that changes in water depth over time controlled both faunal and environmental dynamics in the GOM during the Campanian. The mudstone facies in the Ozan Formation contains parallel/ripple laminae suggesting deposition below storm-wave base, and abundant soft-substrate benthic faunas (i.e., inoceramids, nuculids, pteriids), which indicate a well-oxygenated sediment-water interface. An intraformational unconformity capped by a 0.5 m reworked, coarse, fining-upward, glauconitic unit occurs ~46 m above the base of the Ozan Formation. The coarser, lower ~20 cm of this unit is iron-stained due to oxidation and contains numerous invertebrates, vertebrates, burrowed white limestone fragments, and black phosphatic stones. The evidence suggests slow accumulation of sediments on a sediment-starved shelf or the deposition of a transgressive lag. The upper, finer ~30 cm of this unit is composed of mudstone with numerous, vertically oriented circular structures with a central tube composed of black phosphatic pebbles. The Ozan Formation becomes more calcareous and ammonite-rich above the condensed zone, indicating an increase in relative sea-level. A diverse microinvertebrate fauna occurs throughout the study interval, indicating normal-marine salinity conditions throughout deposition. The faunal data collected allows for a detailed analysis of the correlation between the Gulf Coastal Plain (GCP) and WIS provinces, and the role the western GOM and Tethys Ocean played on the diversity, biogeography, endemism, and faunal turnover in the WIS.