Chemostratigraphy of a Mixed Carbonate-Siliciclastic Succession From South Texas: A Dip Transect of the Lower Cretaceous Pearsall Formation

Abstract

Critical compositional, environmental, diagenetic, and provenance information can be gained through detailed core-based elemental and mineralogical analysis. In successions characterized by subtle visual stratigraphic change, elemental and mineralogical constraints often yield quantitative and semi-quantitative signatures that may otherwise go unnoticed. Energy-dispersive x-ray fluorescence (ED-XRF) provides a sizable quantitatively-determined suite of >20 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 (Aptian) Pearsall Formation and bounding units recovered from South Texas were initially used to develop chemostratigraphic and mineralogical records of depositional change in a near-shore environment. Chemostratigraphic records were generated using a handheld Bruker ED-XRF spectrometer, and calibrated using geological reference materials. Large shifts in major element concentrations define the Hammet Shale, Cow Creek, and Hensel Sand in the up-dip (near-shore) records. These results will be compared with three down-dip Pearsall successions from South Texas. 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.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015