Chemolithofacies of the Upper Cretaceous Buda Formation and Austin Chalk Group, south-central Texas : a product of integration of lithologic and chemical data
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The Upper Cretaceous Cenomanian Buda Formation and Coniacian-Campanian Austin Chalk Group of Texas and northern Mexico are calcareous mudrock (chalk) successions deposited on the drowned lower Cretaceous Comanche platform. Investigations covering depositional systems, lithofacies, and diagenesis have been completed over the years, but no up-to-date chemostratigraphic studies have been conducted on either the Buda or Austin Chalk units. This investigation has supplemented sedimentological analysis with high-resolution X-ray-fluorescence (XRF) geochemical data. Two methods were employed to define chemofacies from an XRF dataset to test whether geochemical data can enhance description of the lithofacies and add insight to the lithostratigraphic analysis. Buda and Austin units are composed of four lithofacies in the subsurface of south-central Texas: (1) calcisphere globigerinid wackestone, (2) calcisphere globigerinid mud-dominated packstone, (3) calcareous mudstone, and (4) argillaceous calcareous mudstone. Observations of elemental curves plotted versus depth identified the alternation of calcite-rich chemofacies with dolomite-, clay-, and pyrite-rich chemofacies. Multiple iterations of hierarchical cluster analysis applied to the XRF dataset revealed seven chemofacies: (1) Ca-enriched, (2) Mg-enriched, (3) Al-enriched, (4) S-enriched, (5) P-enriched, (6) Zn-enriched, and (7) Ni-enriched. Neither chemofacies method is capable of defining lithofacies, but each can be used to substantiate mineralogical variability at the sub-lithofacies scale. The XRF-curve-based chemofacies identified large-scale oscillations in mineralogical composition while the HCA chemofacies identified mineralogical variability at the resolution of XRF sampling. Chemolithofacies are defined as lithofacies that are additionally characterized by chemofacies data. Delineation of the chemolithofacies is achieved by applying geochemical (and in this case mineralogical) descriptors to the lithofacies units based on the geochemical data. Ca-enriched calcisphere globigerinid wackestones and packstones are the dominant chemolithofacies of the Buda Formation and Austin Chalk Group. Calcisphere globigerinid wackestones, calcisphere globigerinid mud-dominated packstones, calcareous mudstones, and argillaceous calcareous mudstones enriched in dolomite, clays, and pyrite make up the other chemolithofacies. Variability in the chemolithofacies is attributed to rock units of more mixed mineralogy as a result of intermittent, sustained periods of increased extrabasinal sediment delivery to the shelf.