ABSTRACT
San Salvador Island is 156 km2 of Pleistocene and Holocene limestone. About half of the Pleistocene limestone has been explored for caves, and seventy-seven have been found. The caves occur in three terrains that have different cave densities and cave types: 1) a Sangamon (last interglacial) marine terrace (1–7 m amsl), 77 km2 (49%); 2) lithified carbonate dunes, or eolianites (>7 m amsl), 33 km2 (21%); 3) lakes and restricted lagoons, 35 km2 (22%). The 11 km2 (8%) of island area that is beach and Holocene rock is non-cavernous. Within these terrains, five major types of caves are recognized: flank margin caves (20); banana holes (21); pit caves (21); lake drains (10), and blue holes (4). The flank margin caves and most pit caves occur in the dunes. Banana hole caves and blue holes occur in the terrace. Lake drain caves open into the lakes and lagoons, and extend beneath the other terrains. The three terrains with caves have the following minimum mean cave densities: terraces = 0.54 banana holes/km2, 0.10 blue holes/km2; dunes =1.1 flank margin caves/km2, 1.2 pit caves/km2; lakes and tidal creeks =1.1 lake drain caves/km2. The combined cave density is 0.67 caves/km for the terrace; 2.3 caves/km2 for the dunes; and 1.2 caves/km2 for the entire island, excluding beaches and Holocene rocks. The actual cave densities may be significantly higher because it is unlikely that all caves have been discovered in the explored areas, and many other areas have not been searched for caves. The median floor areas of the three main air-filled cave types are: banana holes = 28 m2; flank margin caves = 96 m2; and pit caves = 10 m2. The largest known caves in each category are: banana holes = 327 m2; flank margin caves = 1,700 m2 ; and pit caves = 134 m2. Banana holes are thought to pose the greatest potential collapse hazard because they occur beneath the relatively level terrace where most development is likely to take place and because they have thin roofs (typically 0.3–1.5 m thick) spanning oval-shaped chambers that are commonly more than 5 m wide. Cave densities and cave size distributions determined from San Salvador Island should be used throughout the Bahamas and similar carbonate islands to estimate the probability of cave occurrence beneath proposed man-made structures and the potential damage that could occur. Estimates of the potential financial loss from cave collapse will assist in justifying a prudent amount of funding to assess and mitigate this geologic hazard.
