THE IMPORTANCE OF OPHIOMORPHA ICHNOFABRIC IN PLEISTOCENE SHALLOW-MARINE CARBONATES: EXAMPLES FROM THE BAHAMAS AND FLORIDA

The trace fossil Ophiomorpha is ubiquitous in Pleistocene shallow subtidal carbonate grainstones of the Bahamas and Miami Limestone of south Florida. These burrows are robust, with thick walls distinctly pelleted on exterior surfaces and smooth on the interior; outside diameters typically are 2-5 cm. Burrow systems are highly complex, commonly forming irregular mazes of branching shafts and tunnels, without set patterns. Burrow tubes are filled or partly filled with sediment, or preserved completely open. Ophiomorpha ichnofabric indices frequently yield maximum measurements on outcrops. In Pleistocene carbonates of the Bahamas and south Florida, Ophiomorpha is attributed to the burrows of callianassid shrimp, by analogy with modern callianassids that dominate deep-tier faunas of bank-top, shallow-subtidal environments throughout the Bahamas, Florida Keys, and similar tropical carbonate settings. Callianassids are a powerful agent of bioturbation; their activities can totally transform original depositional fabrics to distinctive ichnofabrics, and their complex, lined burrows produce biologically engineered plumbing systems in sediments at shallow depth (up to 2 m or more) beneath the seafloor. A major implication of Ophiomorpha ichnofabric in tropical grainstones and grain-dominated packstones is potential for generation of substantial levels of ichnogenic macroporosity and permeability. Ichnogenic macroporosity includes intra- and interburrow porosity; the former resulting from dense Ophiomorpha occurrences that produce well-connected macroporous systems. Burrow tubes commonly remain open, or are later washed clean or cleared of fill via dissolution, generating intraburrow macroporosity. Interburrow porosity can be enhanced within the matrix surrounding well-cemented burrow walls of complex Ophiomorpha systems. This porosity enhancement can proceed via transport and/or dissolution of matrix, resulting in voids between more resistant burrows. Commonly, intra- and interburrow macroporosities develop together, creating highly permeable rock. Macroporous zones reported from Biscayne aquifer limestones in south Florida provide a prime example of ichnogenic macroporosity for comparison with other carbonate aquifers and reservoirs.