Taphonomic implications of shell surface textures in Bahia la Choya, northern Gulf of California

Abstract

The author collected shell samples of the bivalve Chione from the extensive tidal flat system of Bahia la Choya, northern Gulf of California, and examined their surfaces under the binocular and scanning electron microscopes. Samples were collected from environments ranging in degree of tidal exposure, energy level, and substrate: inner tidal flats, sandy outer flats, rocky outer flats, tidal channel, estero channel, salt marsh, beach, spit, and subtidal. Surfaces were compared with those of shells experimentally subjected to bioerosion, dissolution, and abrasion. In all, more than 1100 shells were examined and described. Bioerosion by microborers is by far the dominant process attacking shell material in Bahia la Choya. Microbored textures dominated shell surfaces from all sedimentary environments, with the exception of the beach, where abrasion dominated. Chemical dissolution is important only in the fine-grained salt marsh and inner flat environments. Many shells show composite textures, which reflect the concurrent action of more than one taphonomic agent. Shells from the salt marsh, for example, frequently show the effects of both microboring and dissolution. Overprinted textures can document transport among environments.