DEFINITION AND INTERPRETATION OF EAGLE FORD (LATE CRETACEOUS, SOUTH TEXAS, USA) CHEMICAL FACIES BASED ON HIERARCHICAL CLUSTER ANALYSIS OF CORE-BASED CHEMOSTRATIGRAPHIC RESULTS

Major and trace element chemostratigraphies were generated at a three-inch sampling interval for three drill cores recovered from South Texas. Collectively, the drill cores preserve all or most of the Cenomanian-Turonian-age Eagle Ford Formation and portions of its bounding units (Buda Formation and Austin Chalk). In an effort to refine the interpretation of Eagle Ford depositional environments and to geochemically evaluate paleoceanographic changes across the early Late Cretaceous margin of South Texas, core chemostratigraphic results were evaluated using hierarchical cluster analysis (HCA), a technique that simplifies the variability in a complex data set and makes comparisons between chemostratigraphic data sets more holistic. It will be demonstrated that geochemical distinctions can be made between the various limestone-, marl-, and detrital-dominated intervals, and thus a suite of “chemical facies” can be defined. The HCA-defined chemical facies are largely identifiable from core to core, suggesting that they can be used to yield 1) greater insight into the subtleties of depositional variability at one location, and 2) a deeper appreciation of sedimentological and paleoceanographic conditions across a broad paleo-shelf environment.

Cores were scanned for major/trace element composition using a Bruker TRACER-III-SD ED-XRF. Raw chemostratigraphic data were calibrated to % (majors) and ppm (traces) concentrations using reference materials and updated methods originally outlined in Rowe et al. (2012). Quantitative chemostratigraphic results for each core were individually analyzed using HCA. Clusters identified using HCA were evaluated in greater detail by ranking the average concentration of each element in each cluster over the average concentration of each element in the entire core data set. The ranking scheme helped to assess which elements were most important in defining each cluster. For instance, it was determined that a cluster from core K-1 was (on average) 2.5 times higher in molybdenum concentration than the average of molybdenum for the entire core. Thus, the enrichment of molybdenum in that cluster is one of the features that best defines that cluster. It will be demonstrated how HCA and additional analysis of clustering will help link chemical facies to other types of core data.