Animal-substrate interactions preserved in ancient lagoonal chalk.

Trace-fossil assemblages reflect the response of the benthos to sets of paleoenvironmental conditions during and immediately after sedimentation. Trace fossils have been widely studied in pelagic shelf and deep-sea chalk deposits from around the globe but never documented from ancient lagoonal chalk successions. Here we report the first detailed ichnologic analysis of a lagoonal chalk unit, using as an example the Upper Cretaceous Buda Formation from the Texas Gulf Coast Basin. In this unit, variable interconnection with the open ocean, accompanied by marked fluctuations in physicochemical parameters inherent to lagoonal circulation (e.g., salinity, hydrodynamic energy, bottom-water oxygenation), highly influenced the resultant trace-fossil content of the chalk. These lagoonal chalk deposits contain twenty ichnotaxa, displaying a clear dominance of Thalassinoides isp. and Chondrites isp., which are present in most of the bioturbated strata. The dominance of Thalassinoides isp., both in softgrounds as an element of the Cruziana Ichnofacies and in firmgrounds as a component of the Glossifungites Ichnofacies, highlights similarities with trace-fossil assemblages from shallow-water shelf-sea chalks. In contrast to both (open) shallow-water shelf-sea chalks and deep-sea chalks, the Buda Formation chalk exhibits more diverse assemblages and sharp fluctuations in ichnodiversity and ichnodisparity during relatively short periods of time. The increased ichnodiversity and ichnodisparity in this lagoonal chalk (in comparison with its open ocean counterparts) may reflect a complex interplay of taphonomic (i.e., incomplete bioturbation allowing preservation of shallow-tier trace fossils and ecologic (i.e., increased spatial environmental heterogeneity in the carbonate lagoonal setting) factors.