Sedimentology, Ichnology, and Stratigraphy of the Cenomanian Buda Formation in west-central Texas, U.S.A.: Integrating multiple datasets in the analysis of Upper Cretaceous Lagoonal Chalks

The Upper Cretaceous Buda Formation is a highly bioturbated, naturally fractured chalk unit deposited on the Comanche Shelf of the Texas Gulf Coast Basin. This unit is a key stratigraphic datum, and its hydrocarbon (oil and gas) and water resources have been exploited since the early twentieth century. Despite its stratigraphic and economic importance, the depositional dynamics, and controls on the stratigraphic architecture of this formation remain poorly understood. Here, based on the integration of high-resolution stratigraphy, sedimentology, and ichnology, the Buda Formation is interpreted to have been deposited in a broad lagoonal paleoshelf (likely < 40 m deep, above SWB), with variable degree of restriction (semi-restricted to nearly open) with the open sea. This unit comprises two 4th-order (high-frequency) depositional sequences regionally recognized in the west and central Texas regions. The basal sequence is composed of the western Lechuguilla and Red Light members, which laterally correlate with the “lower Buda” member of the central Texas region. The upper sequence, on the other hand, comprises the western Love Station member coeval with the central Texas “upper Buda” member. Important thickness variations, lateral/vertical facies changes in the Buda Formation are a function of an irregular paleotopography, characterized by a broad shelf with inherited positive- and negative-relief paleostructures (e.g., San Marcos Arch, Terrel Arch, Maverick Basin, Stuart City/Sligo paleoreefal trend), and climate-driven sea-level fluctuations, under possible eustatic control. The ichnologic content of the Buda Formation in west-central Texas is characterized by abundant Thalassinoides isp. and Chondrites isp., among eighteen other ichnospecies that illustrate the Cruziana Ichnofacies. This suite may be partially overlapped by suites representative of the Glossifungites and Trypanites ichnofacies at several stratigraphic levels. The trace-fossil content of the lagoonal Buda Formation chalk is like those of (open) shallow-water shelf-sea chalks. However, it differs from them by higher ichnodiversity and ichnodisparity. This relatively richer endobenthic community in the Buda Formation chalk may be the result of greater environmental variability in the lagoonal realm and the effect of differential taphonomic processes in comparison to the open ocean counterparts. Likewise, the ichnologic character of the Buda Formation, particularly (1) the dominance of the Cruziana Ichnofacies, (2) the common occurrences of overlapping Glossifungites and Trypanites suites, (3) the absence of Zoophycos isp., and (4) the highly fluctuating bioturbation intensity and ichnodiversity suggest shallow-water carbonate deposition in lagoonal waters above SWB. Ichnologic information is, therefore, in agreement with interpretations from sedimentological data. Some of the burrows (e.g., Thalassinoides) occurring in the Buda Formation substantially increased the porosity and permeability of this formation, thus, potentially contributing to enhanced hydrocarbon storage and production. These biogenic structures might also have favored the development of natural fractures and stylolites in the Buda Formation