EVOLUTION AND EXPANSION OF MODERN CARIBBEAN CORALS FROM A RARE PLIOCENE- PLEISTOCENE BOUNDARY SECTION, BOCAS DEL TORO, PANAMA

Isla Colón in the Bocas del Toro archipelago of Panama (Caribbean) provides one of the few records of late Pliocene to early Pleistocene carbonate platform and reef deposition. Incomplete records from this interval are due to sea level lowstand and non-deposition. This gap has made the evolutionary record of corals temporally and spatially incomplete. Isla Colón, however, accumulated shallow water sediments during this lowstand period and was subsequently uplifted in the middle Pleistocene.

As part of a more regional Caribbean study to investigate evolutionary turnover (Budd 2000), coral fauna from Isla Colón and the stratigraphy of three key formations (Old Bank, La Gruta, unnamed Pleistocene) were sampled and age dated (using Sr isotope ratios, paleomagnetics). Our results indicate that the Old Bank Fm ranges from ~5 to 2.5 Ma and the La Gruta Fm ranges between 2.4 to 1.6 Ma. The youngest formation, the unnamed Pleistocene is between 1.2 and 0.8 Ma. A possible hiatus from 1.5 to 1.2 Ma may mark the beginning of uplift. These dates allow us to correlate the reef units to coral deposits Caribbean-wide.

The La Gruta carbonate platform records the expansion of the coral Acropora palmata as a major contributor to Caribbean reef systems. This species' role was crucial to reef development during the rapid, high-amplitude sea level changes throughout the Pleistocene. New paleoenvironmental data as well as observations of the island's morphology will help determine the geomorphology and paleoecology of this ancient reef system and allow us to fill the void in coral evolutionary history.