ENIGMATIC TUBULAR FOSSILS FROM THE COCKBURN TOWN MEMBER (PLEISTOCENE, GROTTO BEACH FORMATION), SAN SALVADOR ISLAND, BAHAMAS: PRELIMINARY RESULTS FROM FIELD AND PETROGRAPHIC ANALYSIS

The Sangamonian reef to beach deposits of the Cockburn Town Member (Grotto Beach Formation, San Salvador, Bahamas) bear enigmatic mm-scale, tube-shaped fossils of undetermined origin. These fossils were initially noted by White et al. (1997) associated with an MIS 5e coral reef. Although suggestive of encrusting skeletal fossils in some exposures, our field and petrographic examination reveals that these tubular fossils are actually trace fossils in beach and inter-reef sand facies. The dominant trace fossils in this limited assemblage consist of horizontal to sub-horizontal, meandering, simple 1-2 mm diameter tubes (resembling spaghetti) that are preserved in peloid-ooid grainstone and display no discernable internal structure. The trace fossil assemblage forms a variety of interconnected patterns, largely due to false branching (overlapping and intersecting simple tubes as well as successive trace paths). Although identification of the component ichnotaxa is complicated by the interference of component trace fossils, potential tracemakers and trace behaviors are individually considered based on comparison of trace morphology and modern analogs. These trace fossils may represent a distinct portion of the Ophiomorpha-Skolithos nearshore carbonate sand ichnocoenosis or may belong to a different ecologic suite.

Although concentrated on the upper surface of lithified reefrock and beachrock with intraclasts, the traces are also preserved along seaward-dipping planar-stratified calcarenite beds that overlie trough cross-bedded calcarenite. The highest concentrations of these trace fossils (up to 68% of exposed surfaces) were in part the result of prior reef and beachrock cementation that created an impenetrable substrate below blanketing nearshore/shore sands. Apparent convex epirelief of these traces likely resulted from sand bioturbation, followed by selective early cementation of traces due to their high permeability. Subsequent erosion of less lithified surrounding sand exhumed the erosion-resistant trace fossils. However difficult to preserve, modern Bahamian beach sand surface (epichnial) traces with similar patterns may provide useful insights into the origin of the tubular, spaghetti-like trace fossils.