OPHIOMORPHA IN PLEISTOCENE CARBONATES OF THE WIDER CARIBBEAN: DISTRIBUTION, CHARACTERISTICS, AND EFFECTS

Ophiomorpha is a common trace fossil in Pleistocene (MIS 5e) shallow subtidal calcarenites of the Bahamas (including Exuma, Great Inagua, New Providence, Rum Cay, and San Salvador islands), the Miami Limestone of southeast Florida, and similar facies on other carbonate islands of the wider Caribbean, such as the Caymans. Specimens typically consist of segments of lined, branching shaft and tunnel systems that are distinctly pelleted on exterior and smooth on inside surfaces. These systems represent the complex dwelling burrows of callianassid (ghost) shrimp [Decapoda: Axiidea: Callianassidae]. In carbonates, Ophiomorpha is robust, with outside diameters normally 1.5-4 cm or more, wall thicknesses of 3-5 mm, and burrow shaft and tunnel segments reaching lengths of 10s of centimeters. Pelleted burrow walls consist of well-cemented, sand-sized skeletal grains, usually with variable amounts of peloid and ooid grains and micrite. In thin sections, wall material commonly is darker in color and better cemented than burrow fill and surrounding matrix material. As a result, Ophiomorpha burrows tend to weather out in relief on outcrop surfaces.

Distinctive shaft and tunnel terminus structures are commonly preserved in shallow marine calcarenites of the Grotto Beach Formation at Harry Cay on Little Exuma, Bahamas and typically consist of 3 or 4 short and robust, horizontally oriented branches with blunt, closed ends. These structures can have diameters of up to 20+ cm with individual branch diameters of 4-5 cm. Similar forms have been found elsewhere in the wider Caribbean, indicating a significant geographic range for the callianassid species that made these distinctive terminus structures.

Finally, callianassids are deep-burrowers and commonly the dominant bioturbators in modern tropical, intertidal to shallow subtidal, sandy carbonate substrates throughout the wider Caribbean. This manifests in the Pleistocene carbonate rock record where the presence of Ophiomorpha can create a distinctive, maximum intensity ichnofabric of high macrosporosity and permeability. In the subsurface, such facies have the potential to be significant aquifers in carbonate settings.