Island Cave and Karst Research
John Mylroie (Emeritus, Department of Geosciences, Mississippi State University, Mississippi State, Mississippi, USA)
Gerace Research Centre, San Salvador Island, Bahamas
23 June 2010
Caves in the Bahamas can be quite large. Their formation controls are different from what’s seen on continents.
Hypogenic caves form in a manner decoupled from surface hydrology - they can form in the freshwater lens of carbonate islands.
Caves show phreatic formation - no turbulent flow, but instead they form from laminar flow.
CIKM - carbonate island karst model
Caves form as a result of freshwater/seawater mixing and sea level change.
Get eogenetic karst on carbonate islands.
Carbonate islands vary:
1) simple carbonate islands - Bahamas
2) carbonate cover islands (lens is partitioned) - Bermuda
3) composite islands - Barbados
4) complex islands (complex lens configuration) - Saipan or New Zealand
The Bahamas have simple carbonate islands - there’s no non-carbonate rock influences on island hydrology.
The geology of an island is going to tell you how the freshwater lens is going to behave
Freshwater lenses float on seawater. Freshwater mounds up and flows to ocean (head). For every 1 unit of freshwater mound thickness above sea level, there’s 40 times as much thickness to the freshwater lens below sea level.
If the aquifer is porous and permeable, get a small lens. If the aquifer is inefficient (close to an aquitard or aquiclude), will get a large top mound before the freshwater flows to the ocean.
The top and base of the freshwater lens are mixing zones. The base is the halocline. Below the halocline is saline ground water - the marine phreatic zone.
Caves form by mixing dissolution at the halocline. But this especially happens at the lens margin - there, lens margin flow velocities are high (think your thumb over the end of a garden hose), so reactants and products flush through quickly. This is a zone of enhanced dissolution - get flank margin caves.
Freshwater/seawater mixing - these two are miscible liquids, but they will stay separated unless they’re agitated.
d34S from cave gypsum in some flank margin caves indicates biomediation of sulfur has occurred, likely from anoxic conditions.
Expect caves along the halocline at the margin of the freshwater lens - active halocline caves - flank margin caves.
Also expect caves along the upper margin of the freshwater lens - water table caves.
Freshwater/seawater mixing results in dissolution at a microscopic and macroscopic scale.
On Sal Salvador Island, an historic water well has been discovered on North Point Peninsula - that doesn’t make sense considering the small size of land there and the proximity to the ocean. However, the Holocene-aged North Point Member limestones there have little organized porosity/permeability, so they hold lots of freshwater, despite being close to the sea.
Mixing zone dissolution cuts across cave wall rock, speleothem, paleosols. Speleothem can be shaved off.
Caves on carbonate islands are controlled by sea level position - sea level has varied ~130 meters over the last 2 million years.
During most of the Quaternary, sea level has usually been lower than now. Some spikes in sea level occurred during marine isotope stages 5e, 9, 11, 15 - sea level was above modern sea level at these times. Very little of Quaternary time was spent above modern sea level.
Flank margin caves form at the distal margin of freshwater lens, at the flanks of an island.
As cave chambers develop in the lens, the mixing zone steps landward to the rear of the flank margin cave chambers - the cave grows large & complex.
Flank margin caves originally have no entrance. Entrances form later by hillside erosion breaching the flank margin caves. They have a low, wide aspect, the result of the freshwater lens/mixing zone geometry.
Flank margin caves are oval in plan view. The entrance reaches its maximum width when hillside erosion breaches the cave at the halfway point. Ex: Tinian, Marianas.
U/Th dates from flank margin speleothem in the Bahamas show that none of these vadose features are >100,000 years old.
Bahamas blue holes may lead to caves containing stalagmites (may see modern mixing zone corrosion in these flooded caves) from glacioeustatic sea level lowstands.
Bahamas blue hole speleothem - U/Th dates range from 39,000 to >350,000 years old.
Get tidal flow in some caves (Lighthouse Cave on San Salvador Island, Bahamas).
Pit caves form by roots passing water down through the vadose zone toward the water table - get a pit cave - water quickly gets to the water table. Pit caves form independent of sea level position (only forming in the vadose zone).
Banana holes - these are isolated phreatic voids that have collapsed. They are ~10 m across and 2 to 3 m high. Banana hole surface densities have been measured up to 3000/km2.
Before extension of the San Salvador Island airport in the Bahamas, ground-penetrating radar was used to identify voids. Voids were filled in before airport extension paving. They didn’t do that on Andros Island. The Andros airstrip has had 100 collapses.
Caves at paleo-lens positions occur high up on islands experiencing tectonic uplift. Such caves ignore structure. Ex: caves in tilted/jointed limestones in New Zealand.
Paleosols act as aquitards/aquicludes - can get freshwater lens distortion, resulting in unusual flank margin cave morphologies. Ex: Hatchet Bay Cave on Eleuthera Island.
San Salvador Island has a negative water budget. Freshwater lenses get separated by upconed saline groundwater - several saline upcones occur on San Salvador.
The margins of freshwater lakes on Bermuda enlarge, despite the similar size and geology between Bermuda and San Salvador. But Bermuda & San Salvador have different climates & different water budgets.
Guam has vadose cave passages in white limestone, perched atop black volcanics.
Saipan Cave - phreatic lift occurs here due to that island’s complex geology.
Eogenetic karst is a shortcut in karst processes.
Primary conditions → eogenetic karst →
Burial → uplift/fracturing → telogenetic karst → caverns
Isla de Mona is a steep-walled island, exactly what San Salvador Island was like during Quaternary lowstands.
The world’s largest flank margin caves have a pre-Pleistocene genesis & tectonic uplift.
Flank margin caves form by diffuse flow, not by turbulent flow as in classic conduit caves.
Caves have been found along Bahamas platform walls at -105 m and -125 m depths.
So, flank margin caves last (2 m.y. at Isla de Mona) & only if sea level stays still for a while.
Small islands have flank margin caves - diffuse flow model is an artifact of sea level highs, resulting in small islands.
Large islands have conduit caves because they have huge catchment area - they look typically continental.
Carbonate deposition results in 12 x 1013 g/year of carbon released as CO2. [contra intuition]
Carbonate dissolution results in 11 x 1012 g/year of carbon sequestered as HCO3-. [contra intuition]
Precipitation of CaCO3 releases CO2. [contra intuition]