Core Chemostratigraphy and Elemental Geochemistry Along a Dip-Section, Pearsall Formation, Lower Cretaceous, Central to South Texas

Summary Elemental and mineralogical characterization of drill core provides critical information regarding the composition, diagenesis, and origin of sediments and sedimentary rocks. In successions characterized by subtle visual stratigraphic change, elemental and mineralogical constraints often yield quantitative and semi-quantitative signatures that can be used to show subtle or otherwise undocumented change. The elemental suite provided by energy-dispersive x-ray fluorescence (ED-XRF) is sizable (27 elements), and when used collectively, the elemental concentrations often define where samples should be taken for additional, more in-depth studies (e.g., microscopy, porosity/permeability, TOC/RockEval). Shallow drill cores of Early Cretaceous Pearsall Formation and bounding units recovered from Travis and Hays counties, Texas, were initially used to develop chemostratigraphic and mineralogical records of depositional change in a near-shore environment. Chemostratigraphic records for the drill cores were generated using a handheld Bruker ED-XRF spectrometer, and calibrated using geological reference materials. Hierarchical cluster analysis (HCA) of elemental results is utilized in each core to define chemical facies relationships within/between cores. Preliminary results from the Hammett Shale preserved in up-dip cores reveal two dominant detrital element clusters typified by variable enrichments in Al, K, and Si, that are differentiated from each other by their average concentrations of redox-sensitive trace elements (e.g., Mo, U, Cu). This finding suggests that the Hammett Shale was deposited under variably oxygenated conditions. Furthermore, HCA helps define the occurrence of two geochemically differentiable limestone clusters, and two different dolomite clusters. Samples partitioned into phosphate-enriched clusters are also defined in the Hensel Sand and the overlying lower Glen Rose Formation. These geochemically-defined sample clusters can be used to better define facies and facies relationships in a more quantitative fashion, and may be helpful for interpreting subtle variations inferred from well logs. Background Drill cores recovered from Central-South Texas, USA, preserve Aptian-aged records of mixed carbonate-siliciclastic strata that accumulated on the northern margin of the Gulf of Mexico during Cretaceous "Hot-House" climates. The cores include portions of the Sligo Formation, the Pearsall Formation and its various units, and the Glen Rose Formation. The cores provide paleoenvironmental constraints on the marginal marine conditions that existed at the time of deposition. Using multiple drill cores, a dip-section transect across the Aptian margin elucidates changes in paleoenvironmental conditions, facies relationships, and general rock characteristics (Figure 1). Specifically, stratigraphic changes in the chemical composition of the strata (chemostratigraphy) define chemical facies (chemofacies) that are related to more well-established lithofacies, determined through visual inspection and interpretation. Linking lithofacies and chemofacies relationships provides a stronger evaluation of paleoenvironment, and the changes in paleo-conditions along the dip-section core transect.