Taphofacies Patterns of Molluscan Faunas in the Taphonomically Active Zone (TAZ) of Corpus Christi and Nueces Bays, Texas

Alta S. Cate, Ian Evans

The earliest phases of the taphonomic process in a micro-tidal estuary have been described by examining the condition of shells and shell fragments accumulating in the upper sediment layers of the TAZ. Fragmented shells dominate all samples (85 wt %), fragmentation being achieved by predominantly biomechanical processes in this low energy environment. The most common forms of taphonomic alteration in the low energy coastal bays are shell dissolution and endolithic boring by sponges and algae/fungi. By plotting the ratio of biological alteration to alteration by dissolution, taphofacies patterns can be identified. Shell fragments show significantly higher percentages of alteration than whole valves, with the frequency of biological alteration being much higher than for who e valves. Small valves (less than 2mm long) showed less evidence of alteration than larger valves. Patterns yielded by whole valves differ from patterns derived from the total shell accumulation (fragments + whole valves), and crudely parallel existing, identified physical habitats. Whole shells from shallow bay margin locations and areas adjacent to inlets are characterized by high percentages of unaltered valves (57%) and altered valves are predominantly the products of biological alteration. These settings also correlate with high sediment shelliness. The interior, deeper parts of the bays are characterized low percentages of unaltered valves (25%) and altered valves are predominantly the products of shell dissolution, and these settings correlate with the lowest values of sediment sh lliness. Taphofacies that are identified with the large scale bay habitats exhibit considerable heterogeneity. When stations with similar taphonomic profiles are combined (using Chi square analysis), smaller scale taphofacies patterns can be seen to exist within the larger scale bay habitats.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995