Paper No. 11
Presentation Time: 11:10 AM
AN ICHNOLOGIC MODEL FOR QUATERNARY CARBONATE ROCKS OF THE BAHAMA ARCHIPELAGO: IMPLICATIONS AND APPLICATIONS
CURRAN, H. Allen, Department of Geosciences, Smith College, Northampton, MA 01063, acurran@smith.edu
Modern carbonate sediment-producing environments and Quaternary rocks of the shallow subtidal platforms and islands of the Bahama Archipelago provide a natural laboratory for study of the ichnology of shallow-marine to terrestrial carbonate systems. In many cases, activities of modern tracemakers can be matched with counterpart trace fossils, enabling extension of interpretation to more ancient carbonate terranes. Five ichnocoenoses (ecologically distinctive assemblages of traces derived from activities of a given endobenthic community) within the
Skolithos and
Psilonichnus ichnofacies characterize the modern and paleodepositional environments and limestones of the Bahama Archipelago. Shallow subtidal to intertidal ichnocoenoses of the
Skolithos Ichnofacies are dominated by burrowing activities of callianassid shrimp.
Ophiomorpha, the fossilized burrow of callianassids, is the dominant trace fossil of shallow-marine grainstones and can create a distinctive, commonly maximum ichnofabric that can result in substantially increased levels of macroporosity and permeability important to development of carbonate aquifers and reservoirs. The cone-shaped trace fossil
Conichnus conicus commonly is present with
Ophiomorpha and is indicative of grainstones deposited in actively shoaling paleoenvironments.
The Psilonichnus Ichnofacies includes beach upper foreshore, backshore, and coastal eolian deposits. Psilonichnus upsilon, a distinctive trace fossil produced by burrowing activity of the ghost crab Ocypode quadrata, is characteristic of upper foreshore to backshore beds and can be used as a stratigraphic indicator of past sea-level position. Carbonate eolianites exhibit a high-diversity ichnocoenosis of insect-generated trace fossils and rhizoliths resulting from plant roots and prostrate stems. Trace fossils formed by insects can be large and complex and may result in distinctive ichnofabrics within eolianites. This has implications for identification of eolianites in cores, as the presence of burrows alone should not be used as a criterion to reject eolian origin. Future research may result in expansion of the number of recognized ichnocoenoses within Bahamian carbonates and should include greater utilization of the Bahama ichnologic model in the study of carbonates in cores.
