Integrated bio- and lithofacies analysis of coarse-grained, tide-dominated deltaic environments across the Cretaceous/Paleogene boundary in Patagonia, Argentina

Abstract

Four facies associations representing, from sea to land, namely (1) tidal bars in distal delta front, (2) proximal tidal bar complexes, (3) tidal channel and tidal flat deposits, and (4) multi-episodic, tide-influenced channels, resulted fromthe bio- and lithofacies analysis of the siliciclastic Cretaceous/Paleogene Lefipán Formation (Patagonia, Argentina). Lithofacies analysis indicates the existence of coarse-grained, tide-dominated deltas fed with sediments of braided rivers, a rare type of delta that was developed at the western margin of a large marine embayment during the Cretaceous/Paleogene (K/Pg) boundary interval. Differences in taxonomic composition and ecological structure of the biofacies revealed variations in environmental factors, with salinity exerting the principal influence on biofacies. Salinity varied from oligohaline and mesohaline in the deltaic, high-stress environments, as typified by monospecific bivalve assemblages, to diverse and trophically more complex associations, indicating more normal marine and stable conditions. Thus, the biofacies provided an independent tool to check the results of the lithofacies analysis in complex transitional environments. A comparison ofMaastrichtian and Danian faunas suggests that the K/Pg boundary event disrupted the stable Maastrichtian communities via the extinction of dominating species, leading to less diverse butmore varied Danian assemblages. A shift in trophic structure to increased percentages of deposit-feeders in the Danian was not a direct consequence of impact-related environmental perturbations at the boundary in the studied onshore environment. This contrasts with previous studies of siliciclastic offshore settings,where such a shift in trophic compositionwas interpreted as the consequence of a productivity crisis at the boundary.