SUBSTRATES, TRACES, AND ICHNOFABRICS OF A MODERN BAHAMIAN TIDAL FLAT

Carbonate-sediment tidal flats are common along the margins of slightly hypersaline lagoons or “creeks” throughout the Bahamas. At Pigeon Creek on San Salvador Island, extensive tidal flats exhibit a three-part zonation pattern from black mangroves on the inland side to red mangroves to open sand flats with mounded topography created by the burrowing activity of the callianassid shrimp Glypturus acanthochirus. These tidal flat zones were tested for substrate firmness using the modified Brinell apparatus method as developed by Gingras and Pemberton (1999). The mangrove zones can be densely populated and burrowed by several species of fiddler and mangrove crabs. The substrate is soft and qualifies as a barely competent softground.

On the open flats, the large cones formed by callianassid burrowing activity commonly coalesce with time to form composite mound surfaces that become stabilized with the development of microbial mats. These stabilized surfaces set the stage for colonization by shallow-tier burrowers, such as the upogebiid shrimp, Upogebia vasquezi, and several species of fiddler crabs. Substrate firmness increases markedly on the surfaces of the mounds, owing to microbial binding effects. Firmness reaches its maximum on scoured areas near the main channel of the lagoon, although this surface still qualifies as a softground, not a firmground. The upogebiid burrows are complex, distinctive, and have high fossilization potential. Where preserved, they likely would indicate a sea-level highstand. The deep-tier callianassids bioturbate lagoonal sediments thoroughly, creating a distinctive ichnofabric for this facies, which is widely represented in Quaternary stratigraphic sequences in the Bahamas.