Taphofacies of coastal lakes based on mollusk death assemblages: a case study of Mirim Lake

Abstract

Mirim Lake is the second-largest lacustrine body in Brazil, stretching for some 185 km and covering about 375 thousand hectares of water surface. This lake is part of the Patos-Mirim System, one of the world’s largest complexes of coastal lagoons, which is the result of the rise and fall of sea level over transgressive-regressive cycles triggered by glacio-eustasy during the Late Pleistocene and Holocene. Its characteristics and dynamics give it singular importance for studies on the genesis of shell accumulation in lagoons. This study aims to improve the knowledge of how the taphonomic signatures reflect the environmental characteristics and dynamics of Mirim Lake. Categorization of the arrangement and packing of death assemblages, as well as the orientation of shells, supported the biostratinomical approach. The taphonomic signatures included disarticulation, fragmentation, and corrosion of shells. Corrosion is the primary damage observed in the bioclasts, varying from a single loss of color to complete degradation. Differences in the characteristics of death assemblages observed in the dune fields and the lacustrine plain led to the recognition of two taphofacies. A more significant number of whole shells was observed in the dune field taphofacies, while sharp fragments and open-articulated bivalves in hydrodynamic unstable positions characterize the lacustrine plain taphofacies. The genesis of death assemblages and the taphonomic signatures of bioclasts were linked to three sedimentary dynamics attributed to the environment of Mirim Lake. Keywords: biostratinomy, taphonomic signatures, shell deposits, dune field, lacustrine plain.