A HIGH-RESOLUTION CHEMOSTRATIGRAPHIC RECORD OF CENOMANIAN-TURONIAN STRATA, SOUTH TEXAS, USA

The Eagle Ford Formation of South Texas is classified as a carbonate-rich, organic mudrock that was deposited on the South Texas Shelf during the earliest Late Cretaceous. Buda limestone stratigraphically underlies the Eagle Ford Shale. Their sharp contact represents a major transgression during the middle Cenomanian. The contact between the Eagle Ford and overlying Austin Chalk is highly gradational. Continuous core from Atascosa County was analyzed using a Bruker Tracer IV-SD handheld XRF (X- ray flourescence) unit. The core contains over 200 ash beds, ranging from 2mm - 130mm thick; it also displays numerous facies changes that generally occur on the scale of cm. Data were collected automatically at a 10-mm sampling interval along the entire 90-meter-long core using a MCS-1000, XRF Core Scanner (DeWitt Systems, Inc.). Major (Mg-Fe) and trace (V-U) element analysis was carried out with no protective window covering the X-ray source. A steady flow of helium was purged through the nose of the Bruker Tracer IV-SD during major element analysis to increase system sensitivity to low energy elements such as Mg and Na.

The Eagle Ford is mineralogically dominated by calcite and various clay minerals whose quantification is complicated by the presence of ash beds. A recent study suggested that some of the quartz content may possess a biological origin (radiolaria). Initial findings indicate that many high frequency facies variations resulted from periods of basin restriction during which water-column oxygen levels dropped. This generally anoxic episode is indicated by enrichment of key trace metals, including Mo, V and U. Molybdenum values in the lower Eagle Ford range from below detection level to 200 ppm, with average values around 40 ppm. Upper Eagle Ford facies have much lower Mo values (generally <10 ppm, approaching the level of detection). The goal of the present study is to integrate highly-resolved chemostratigraphic results and core description in order to better characterize the paleoceanographic conditions across the Cenomanian-Turonian boundary. Recent work suggests that anoxic conditions experienced along the South Texas shelf during this time preceded the well-documented Oceanic Anoxic Event 2, implying that local/regional paleoceanographic conditions played a greater role in the occurrence of anoxia.