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Prispevek obravnava podobnosti v razvoju krasa kvartarne karbonatne platforme na otoku San Salvador in devonske karbonatne platforme na platoju Krasna na Moravskem. Za obe obmocji so znacilne jame, katerih nastanek lahko razložimo s "flank margin" modelom in so nastale v obmocju sladkovodnih lec med obdobji relativno visoke morske gladine, v casu relativno stabilne halokline, kar potrjujejo razlicne studije jamskih zapolnitev. V obeh primerih so jamske sedimentne zapolnitve genetsko primerljive - obalni in eolski sedimenti ter brece. The comparative study of a Quaternary carbonate platform (San Salvador Island, the Bahamas) and a Devonian Carbonate Platform (Krasna‡ Elevation, Moravia) indicates a great similarity in karst evolution. Caves on both sites are interpreted as flank margin caves associated with a freshwater lens and halocline stabilised during sea-level highstands. The sedimentary fill of both caves is genetically comparable - beach and aeolian sediments with bodies of breccias.

Sealevel curves are best developed on tectonically stable coastlines, like San Salvador, where eolianites preserve transgressive and regressive phases associated with Quaternary high seastands, while reef facies mark the highstands. At 11 locations around San Salvador, terrestrial molluscs ( Cerion ) from the eolianites, lagoonal bivalves ( Codakia ), and corals from the highstand deposits were dated by ESR. Volumetrically averaged sedimentary dose rates were calculated from sedimentary geochemistry and time-averaged cosmic dose rates from each sample’s current and past geologic contexts. Rice Bay Formation corals dated at 3.9 ± 0.3 to 7.1 ± 0.4 ka (OIS 1). Minimum ages for the Cockburn Town Member’s regressive phase ranged from 49 ± 6 to 75 ± 8 ka, correlating with OIS 3–4. Codakia dates showed that an OIS 5a sealevel approached modern levels at 91–78 ka. In situ corals from the Cockburn Town Reef averaged from 127 ± 6 to 138 ± 10 ka, correlating well with OIS 5e. Ages from the Reef’s rubble zones hint that some coral reefs grew as early as OIS 7, but were likely reworked during OIS 5. San Salvador preserves deposits from three mid to late Quaternary highstands above, and as many as three that closely approach, modern sealevel.

Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which they are named, is the result of their great depth, and they may lead to cave systems below sea level. Blue holes are polygenetic in origin, having formed: by drowning of dissolutional sinkholes and shafts developed in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and dissolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.

Banana holes are circular to oval voids with diameters ranging from 2 meters to more than 10 meters, and with depths up to 5 meters, which are found throughout the Bahamas. They are named for a specialty crop sometimes grown in the thick moist soils that accumulate in them. They commonly have vertical or overhung walls, and exhibit phreatic dissolutional morphology. Occasionally, banana holes are found with complete or nearly complete roofs.

Throughout the islands of the Bahamian archipelago fossil coral reefs are found from current sea level up to a maximum elevation of +4 m. 234 U 230 Th radiometric dates obtained from in situ corals from these reefs, by both alpha-count and mass-spectrometric techniques, indicates that they were all formed during Oxygen Isotope Substage 5e (ca. 125,000 years ago). Those data are consistent with a maximum sea-level highstand of +6 m during Substage 5e, and either no vertical motion of the Bahamas, or possible isostatic subsidence of up to 2 m during the past 120,000 years. No older in situ fossil corals, or other subtidal deposits, have been found subaerially exposed anywhere in the Bahamas. That finding suggests that late Quaternary (at least the past 300,000 years) isostatic subsidence has occurred at a rate of 1–2 m per hundred thousand years, and/or no pre-5e highstands were above modern sea level. An independent corroboration of the conclusions drawn about sea level amplitude and tectonic stability of the Bahamas from the coral reef data is available from examination of abundant flank margin caves (horizontal, phreatic dissolution caves) found above modern sea level throughout the Bahamas. These horizontally extensive air-filled caves have dissolutional ceilings with elevations that are restricted to +1 to +7 m, which is consistent with formation at the margin of a thin freshwater lens elevated by a past +6 m sea-level highstand. The restricted cave elevations, and the lack of stalagmites in these caves that are older than 100,000 years, are also consistent with cave formation during Substage 5e, and possible subsequent isostatic subsidence of a few metres. The subsurface geology of the southeastern Bahamas contains a long-term record (millions of years) that has been attributed to past tectonic activity along the North American/Caribbean plate boundary. While that record suggests differential subsidence across the Bahamas in the Tertiary Period, the data from fossil coral reefs (and subtidal deposits) and flank margin caves indicate that all Bahamian banks on which there are islands have been tectonically stable, and behaving similarly, for at least the past several hundred thousand years.

In Quaternary carbonate units composed mostly of eolianites, paleosols are important stratigraphic markers that help differentiate episodes of carbonate deposition tied to glacio-eustatic sea level fluctuations. Paleosols used in this manner can be misinterpreted, and thus lead to errors in interpretation of the geologic record. Some possible pitfalls include: failure to differentiate between terra-rossa paleosols and calcarenite protosols; failure to recognize that separate paleosols may merge laterally into composite paleosols; failure to recognize that single paleosols may bifurcate in highly weathered bedrock; and failure to recognize soil-derived material that infills karst features. The Quaternary carbonates of the Bahamas are used to illustrate these pitfalls, which may occur in carbonates of any age.

The Bahama Islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level. The development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens. Lens position is controlled by sea level, which in stable carbonate platforms like the Bahamas is a function of glacioeustatic sea level still stands. Caves in the Bahamas that are currently subaerial must have developed during past higher sea levels. During the Late Quaternary, sea levels higher than present have been relatively short-lived, and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation. The Bahama Islands are low-lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level. Past high sea level events greatly reduced the exposed land area of the Bahama Islands, thus also limiting both the catchment for and size of freshwater lenses. Caves must be younger than the rock in which they are developed; most subaerial Bahamian caves are found in limestones that are less than 150000 years old. Development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism. The mixing of discharging freshwater with tide-pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary. Bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes. Their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands.

This chapter provides information on the geology of Bahamas, and mentions that the geology of the Turks and Caicos is also similar. The Bahamian archipelago covers 300,000 km 2 , of which 136,000 km 2 is shallow bank, and 11,400 km 2 is subaerial land. The Commonwealth of the Bahamas comprises the majority of an extensive archipelago of carbonate islands and shallow banks in the western North Atlantic Ocean. The southeastern portion of the same archipelago consists of the Turks and Caicos Islands (British West Indies), and the submerged Mouchoir, Silver, and Navidad banks. Climatically, the Bahamas' temperature ranges from subtropical in the north to semiarid in the south. Today, the northern islands are largely covered by pine barrens with palmetto, but there are regions of limited broadleaf coppice. The Bahamas lie within the zone of the northeast trade winds, and that has resulted in the preferential occurrence of islands on the eastern (windward) side of most banks. As a result of the glacioeustatic control of limestone deposition in the Bahamas, the lithostratigraphic units of Bahamian islands are also allostratigraphic units that are usually bounded by terra rossa paleosols.