Taphonomic signature as a function of environmental process: Shells and shell beds in a hurricane-influenced inlet on the Texas coast

Abstract

Criteria for taphonomic processes, including dissolution, breakage, abrasion, size sorting and shell orientation, were statistically correlated against environmental parameters representing shell source, depositional environment and depth of burial, using samples obtained from San Luis Pass, a microtidal inlet on the Texas coast. Core intervals were differentiated by cluster analysis into shell-poor sands, slightly shelly sands, shell-rich sands and shell gravels based on their percent carbonate and percent shell gravel content. Boundaries between these sedimentary classes represent thresholds in physical processes forming the shell deposits. We propose that this classification scheme for shelly sediments may be useful for field descriptions in a variety of depositional and tectonic settings. The death assemblages were predominantly composed of whole and fragmental molluscan shells. Most shells show the effects of physical taphonomic processes such as shell breakage, edge rounding, surface abrasion and size sorting. A significant minority of the shells show evidence of minor chemical alteration (dissolution). Patterns of dissolution, edge rounding and surface abrasion differed significantly on shells in a mixed-faunal assemblage derived from different habitats (bay, inlet/beach, inner shelf and unrestricted). Thus, the habitat where a species lived, rather than its final depositional environment, controlled most aspects of its taphonomy. Only shell orientation and size-frequency distribution were significantly affected by final environment of deposition. The hypothesis that a discrete high energy (hurricane) “event” can imprint a unique taphonomic signature is, consequently, false. Although hurricanes physically winnow shell material, a single “event” merely concentrates shells with preexisting characteristics without further significant alteration. Depth of burial was poorly correlated with most biostratinomic effects, including dissolution. Significant dissolution must occur only near the sediment-water interface. Accordingly, quantified taphonomic characteristics might be useful in identifying the original habitats of species deposited in mixed accumulations; “taphofacies” reflect not only the final depositional process, but also taphonomic processes in the habitat of origin.