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Does the Distribution of Encrusting Foraminifera at San Salvador, Bahamas, Provide a Model, for Small, Isolated Carbonate Platforms

Ronald Lewis
Auburn University


Foraminifera cemented to hard substrates have received relatively little research attention in distribution studies when compared to the vast literature devoted to the distribution of modern-day free species. However, these attached (encrusting) taxa are proving to be useful paleoenvironmental indicators. Importantly, some living species flourish within a narrow range of environmental conditions, governed by light penetration and water energy among other variables. Individuals are large and easy to recognize, and there are only a few dominant taxa. Because they are cemented to stationary objects such as coral heads and to clasts on the seafloor, they are less likely to be transported and abraded than are free tests. Research done at San Salvador island, located on a small isolated carbonate platform in the eastern Bahamas, shows a marked zonation from nearshore to offshore, which may be applicable to the interpretation of shallow-water carbonates.

Cobbles were collected along two transects on the west (leeward) side of the island from shore to the wall at the platform margin, and at numerous spot samples on the northern and northeastern (windward) side, including a bank barrier reef and the eastern platform edge. Encrusting foraminifera found on the undersides of cobbles were quantified using light microscopy in 10cm² quadrats: taxa and morphotypes were identified and differentiated according to taphonomic grade. In addition, ImageJ was used to determine area covered in high-resolution photographs taken in the laboratory.

Nearshore assemblages are dominated by well-preserved Homotrema rubrum. Patch reefs, especially those found at a distance from shore, are characterized by a relatively diverse assemblage with prominent Planorbulina spp., whereas bank barrier reefs are dominated by Homotrema rubrum, but may also include Gypsina plana. Shelf margin assemblages are the most distinct because they are dominated by the large, sheet-like Gypsina plana, with other taxa being sparse and poorly preserved. These assemblages are distinct, as shown by cluster analysis and non-metric multidimensional scaling (MDS) plots, especially when foram size is taken into consideration. The particular importance of three easily recognizable taxa – Homotrema rubrum, Planorbulina spp, and Gypsina plana – prompted the creation of a ternary diagram and simple on-shore-to-offshore model. Whether this model is broadly applicable will depend on the outcome of ongoing and future research at nearby outer islands (Cat Island and Long Island) and those further west.

AAPG Search and Discovery Article #90205 © AAPG Geoscience Technology Workshop, Permian and Midland Basin New Technologies, September 4-5, 2014, Houston, Texas