TIMING OF EXPRESSION OF PROGRADATIONAL COLLAPSE BLUE HOLES IN THE BAHAMAS: EXAMPLES FROM ELEUTHERA

Eleuthera, one of 29 islands located within the Bahamian Archipelago, is made of carbonates. These carbonates host different forms of karst, including blue holes - the focus of this research. Eleuthera has similar stratigraphy as other islands within The Bahamas, consisting of mostly eolianites. There are four major formations: Owl’s Hole, Grotto Beach, Whale Point, and Rice Bay. Owl’s Hole, Rice Bay, Whale Point, and the French Bay Member of the Grotto Beach are eolianites, while the Cockburn Town Member of the Grotto Beach Formation is subtidal, deposited during MIS-5e, ~120,000 years ago. Blue holes can form in multiple ways, this study analyzed blue holes formed through the progradational collapse of deep seated phreatic dissolution voids (conduits). The purpose of this study was to determine when blue holes were expressed at the surface through prograditional collapse.

Petrographic thin sections were made from samples of cap rock belonging to four progrational collapse blue holes on Eleuthera. These thin sections were analyzed to determine whether the depositional environment of the cap rock was subtidal or eolian, so that a maximum age for expression at the surface could be determined. The cap rock of all four blue holes’ were composed of subtidal facies (Cockburn Town Member), which suggests these blue holes were expressed at the surface post MIS-5e, <120,000 years ago. However, previous studies suggest the expression of blue holes occurred >300,000 years ago based on speleothem dating. In part due to this contradiction, more research is needed to better understand when the expression of progradational blue holes occurred.