MODELING BLUE HOLE DEVELOPMENT IN THE BAHAMAS

Blue holes are found across the Bahamian archipelago, and are formed through three mechanisms: 1) vadose dissolution during a previous sea-level lowstand; 2) bank margin failure; and 3) progradational collapse into a void at depth. Blue holes formed from progradational collapse are the most common type of blue holes found in the Bahamian platform interiors. These blue holes are formed in a similar fashion to the tiankengs of China, and cenotes of Yucatan.

Progradtional collapse requires a void at depth with enough accommodation space for collapse to reach the surface; or, there must be a removal mechanism (e.g. dissolution) that removes the collapse material as it forms, maintaining accommodation space. Both options can result from lateral conduit flow in dissolutional caves, as has been demonstrated in both China and Yucatan. Abandoned conduit caves from the MIS 5e sea-level highstand are unknown in the Bahamas, however blue holes intersect such caves at a variety of depths. Some of these conduit systems contain kilometers of passage, but while exhibiting tidal flow, are filled with static water (fresh water, sea water, or both).

The control of conduit development in these platforms appears to be island size. For small islands, as exist today (and even smaller during MIS 5e), diffuse flow discharges the fresh-water lens to the sea. A ~10 m drop in sea level would expose the entire Bahamian platform, increasing the meteoric catchment area by the square while the perimeter increases linearly. It has been previously hypothesized that this transition results in conduit development, as the diffuse flow regime becomes inefficient at higher area to perimeter ratios. During the Quaternary, sea level has been at many positions below the bank top; these positions are observed as the conduit depths seen in blue holes.

Blue holes have stalagmite ages >350ka, and depths up to 200+ m. Their progradational collapse was likely enhanced by buoyancy loading and unloading as Quaternary sea levels migrated. Offshore blue holes on the present-day flooded bank are infilled with Holocene sediments; tidal flow from these infilled blue holes may cause whitings. Deep blue holes on current islands, at elevations below 6 m, should have been infilled with MIS 5e sediments; therefore their expression by surface collapse as seen today occurred after MIS 5e.