LARGE FLANK MARGIN CAVES OF THE BAHAMAS: MEGAPOROSITY FROM THE LAST INTERGLACIAL?

Flank margin caves are abundant in the Bahamas, where they are believed to have formed by mixing dissolution in a fresh-water lens during the last interglacial sea-level highstand (oxygen isotope substage or OIS 5e). This highstand reached ~6m elevation and lasted ~12 ka. Hatchet Bay Cave and Ten Bay Cave (Eleuthera Island), Hamilton’s Cave and Salt Pond Cave (Long Island) and Lighthouse Cave (San Salvador) are the among the largest known flank margin caves in the Bahamas. The geometric parameters of these caves were used to determine: 1) areal coverage of the cave (the ratio between cave area and the area of the smallest rectangle that contains the cave); 2) cave megaporosity (the ratio between the volume of the cave and the volume of the rectangular block that can contain the cave); and 3) dissolution rates of the host limestone. On average, the areal coverage of the caves is 30% and megaporosity is 15%, values which surpass the calculations of Worthington (1999), for continental epigenic caves in telogenetic rock, by one and two orders of magnitude, respectively. Based on a 12 ka lens residence time, the rate of dissolution was calculated at an average of 108 m3/km2/year, an order of magnitude higher than values reported by Ford & Williams (1989) in epigenic conduits. The differences are explained by the eogenetic nature of the Bahamian host rocks and the powerful mixing-dissolution mechanism in a hypogenic environment.

There are some concerns with the current model: 1) The voids are large; was the time span of OIS 5e (~12 ka) long enough for such dissolution? 2) Phreatic cave chambers and ceilings are found at 10 m elevation. 3) Occurrence of partially-dissolved vadose speleothems in some caves (which would require a second sea-level highstand). An alternative explanation for the formation of these large caves is that they formed earlier than OIS 5e, during a higher and longer sea-level highstand, such as OIS 11. This would address the problems, but there is no compelling Bahamian evidence of such a highstand. Another explanation for the etched speleothems is that they could have been dissolved post OIS 5e, during a brief OIS 3 highstand, for which controversial stalagmite evidence exists in the Bahamas. Storm loading of the lens and perching of the water table by paleosols could be explanations for the ~10 m phreatic-dissolutional ceilings.