MESSINIAN (LATE MIOCENE) KARST ON GRAND-CAYMAN, BRITISH-WEST-INDIES -AN EXAMPLE OF AN EROSIONAL SEQUENCE BOUNDARY

The Cayman Unconformity, which separates the Pedro Castle Formation (P liocene) from the underlying Cayman Formation (Miocene), is a sequence boundary that developed during the Messinian, when sea level was at a lowstand due to glaciation in the Southern Hemisphere. By the end of the Messinian, Grand Cayman was an atoll-like island that had an eleva ted peripheral rim that was up to 41 m above the central depression. T he Cayman Formation contains paleocaves and paleosinkholes that were l inked to the Cayman Unconformity. The topography on the Cayman Unconfo rmity is attributed to erosional processes, because (1) there is no ev idence of carbonates that formed by constructional processes (i.e., re efs, dunes) in the elevated peripheral rim, and (2) there is ample evi dence of dissolutional features in the Cayman Formation. The topograph y developed on the interior of Grand Cayman during the Messinian was u neven. A deep, basin-like depression, with its base as much as 50 m be low the peripheral rim, formed on the western part of the island. By c omparison, the floor of the depression on the eastern part of the isla nd was 20-30 m higher. The difference in the topography, which is a re flection of the amount of bedrock dissolution, suggests that the effec tive rainfall was highest over the western part of the island. The rel ief on the Cayman Unconformity and associated structures shows that ba se level during the Messinian karst development was at least 41 m belo w present-day sea level. This is also provides an estimate of the Mess inian lowstand position because the base level in oceanic karst settin gs is usually controlled by sea level.