Petrographic and geochemical features of sinkhole-filling deposits associated with an erosional unconformity on Grand Cayman

摘要

On Grand Cayman, exposures of dolostones belonging to the Cayman Formation (Miocene) represent an erosional unconformity that has been developing since the late Pliocene (～3.6 Ma). Sinkholes that developed during this time have remained open or become partly to fully filled with various combinations of rootcrete, breccias, loose limestone and dolostone lithoclasts, and white, red and orange limestones. These sinkhole-filling deposits have different geochemical attributes to the Neogene and Pleistocene marine carbonates that form the bedrock of the island. The deposits in the sinkholes formed in response to the variations of sea level, climate, exposure, and vegetation that developed during the period when the erosional unconformity was developing. The rootcrete, oncoids, red and orange limestones are terrestrial in origin, whereas the limestone and dolostone lithoclasts and white limestones are derived from marine deposits. On the erosional unconformity, intense root activity led to the formation of rootcrete and terrestrial oncoids but also selectively blackening reworked marine carbonates. The red and orange limestone matrices, which formed under more arid conditions, contrast with the other sinkhole-filling deposits that formed during periods when the climate was semi-arid to humid. The distinctive REE signatures of the sinkhole-filling deposits, which are different from those of the bedrock limestones and dolostones, can probably be attributed to trace amount of terra rossa and/or airborne Saharan-derived dust that are present in those deposits.