The Use of Plant Trace Fossils to Differentiate Transgressive-Phase from Regressive-Phase Quaternary Eolian Calcarenites, San Salvador Island, Bahamas

Throughout the Quaternary, sea level was at least 10 meters below its present elevation ~85% of the time, with little to no carbonate sediment production associated with vertical-walled carbonate platforms such as the Bahama Banks. Only during interglacial conditions when sea-level rise has flooded the top of the platforms is sediment production significant. At those times carbonate sediment is rapidly produced in large volumes, and eolian calcarenites develop on the remaining dry ground adjacent to their source beaches. These early dunes are referred to as transgressive-phase eolianites. Further dune production is modest until the end of the interglacial, when sea level begins to fall and surf zone processes pass through the platform lagoons, where stored carbonate sediments are remobilized into beaches and a second episode of dune production occurs. The resulting dunes are regressive-phase eolianites.

These two eolianite packages bracket the leading and trailing portions of individual sea-level highstands. The transgressive-phase dunes are invaded by the fresh-water lens as sea level continues to its acme, but regressive-phase dunes are not invaded by a fresh-water lens during the sea-level highstand in which they were deposited, resulting in a dissolutional porosity and permeability inequality between the two dune packages. Criteria have been developed to identify transgressive-phase and regressive-phase eolianites; however, the one with the most potential is based on plant trace fossils, variously called rhizomorphs, rhizocretions, or vegemorphs. Extensive field work has demonstrated that vegemorphs are found preferentially in regressive-phase eolianites, and that the presence of vegemorphs disrupts the fine-scale eolian bedding. Transgressive-phase eolianites have notably fewer vegemorphs, and as a consequence, exhibit undisturbed fine-scale laminations. Vegemorph presence or absence is readily observable on vertical faces, and so paleodunes exposed at sea cliffs, in quarries or road cuts, or in caves can be easily categorized as transgressive-phase or regressive-phase deposits.