Comparison of flank margin cave development on San Salvador island, Bahamas, and Isla de Mona, Puerto Rico

Abstract

San Salvador Island, Bahamas is a 161 Km2 tectonically stable late Quaternary carbonate island located 600 km east-southeast of Miami FL. San Salvador contains numerous flank margin caves (phreatic karst features) that developed primarily in late Pleistocene eolianites. These caves developed during a short time in versy small fresh-water lenses. Cave elevations and Uranium-series ages from stalagmites indicate that all currently subaerial flank margin caves developed during the last interglacial seal-level highstand that was 6 m above current mean seal level 125,000 years ago (oxygen isotope substage 5e), which lasted no more than 14,000 years. The caves were formed by dissolution in the mixing zone at the margin of a freshwater lens that was elevated by the substage 5e highstand, and which resided within the small emergent portions of eolianite ridges. The flank margin caves have chambers with volumes greater than 1000 m3 on San Salvador; on other Bahamian islands, chambers as large as 14,000 m3 are known. Isla de Mona is 55km2, uplifted Late Miiocene-Pleistocene carbonate platform located in the Mona Passage 60 km west of Puerto Rico. Most of the island has 40 to 80 m high vertical cliffs to the sea which contain numerous flank margin caves at various elevations. A few cave chambers have volumes in excess of 100,000 m3. Paleomagnetic reversals observed in cave deposits and speleothems indicate the caves are at lease 780,000 years old as pre-Pleistocene. This age indicates that the caves may have developed in a stable, freshwater lens prior to Pleistocene glacio-eustasy, and have been subsequently tectonically elevated to their current position. The great size of the flank margin caves indicate mixing-zone dissolution in a large freshwater lens over an extended period of time. Flank margin caves in Isla de Mona, and some in the Bahamas contain evidence of two cycles of dissolution, in that speleothems and the cave wall rock to which they are attached show evidence on contemporaneous phreatic dissolution. In the Bahamas, the cause is most likely a minor change in sea level during the last interglacial highstand which caused a brief period of emergence, during which the subaerial speleothems grew, followed by re-submergence and dissolution. On Isla de Monta either tectonic motion, or the onset of glacio-eustasy, created the conditions that caused renewed phreatic dissolution after a period of subaerial exposure and speleothem precipitation. Despite differences in rock age and geologic setting, both San Salvador and Isla do Mona show evidence of re-invasion of the flank margin caves by dissolutionally aggressive water following a vadose interval. The flank margin caves have very similar morphologies and characteristics, and the only major difference is attrutable to the larger lens size and the longer duration of stable lens position on Isla de Mona. The data indicate that dissolution occurs rapidly in these environments, and despite the development of large voids, the same geochemical environment can be re-established after an emergence episode.