Outer Flats Research Articles

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

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.

Surficial Physical Sedimentary Structures of Bahia la Choya

I observed and measured the orientations of common physical sedimentary structures on the beach and spit, estero, tidal channel, and inner and outer tidal flats of Bahia la Choya. The spit is characterized by wind ripples and the beach typically displays swash marks. Mudcracks typify the high marsh portions of the estero while the meandering estero channel contains ripples and megaripples. The inner and outer tidal flats are dominated by small, ebb-oriented, straight-crested to undulatory current ripples. The current ripples are often superimposed on flood-oriented ridges and runnels of the outer flat. Rill marks, tool marks, and current crescents occur sporadically. Ebb-oriented megaripples, small current ripples, and primary current lineations occur in the tidal channel.Ripple crests trend from N15E - N40E on the southern tidal flats to N10W - N20W on the northern flats. The crests roughly parallel the shoreline and are normal to tidal currents.