Abstract:

Abaco Island, located on Little Bahama Bank, is the most northeastern island in the Bahamian archipelago. Abaco exhibits typical carbonate island karst features such as karren, blue holes, pit caves, banana holes and flank margin caves. Landforms that resemble tropical cone karst and pseudokarst tafoni caves are also present. The three cave types of Abaco—flank margin caves, pit caves, and tafoni caves—are abundant, but each forms by very different mechanisms. Flank margin caves are hypogenic in origin, forming due to mixing dissolution at the margin of the freshwater lens. Since the lens margin is concordant with sea level, flank margin caves record the position of sea level during their formation. Flank margin caves exhibit phreatic dissolutional features such as bell holes, dissolutional cusps and spongework. Pit caves form as vadose fast-flow routes to the freshwater lens and are common on the Pleistocene eolianite ridges of the Bahamas. Pit caves are characterized by their near-vertical or stair-step profiles. Because pit caves form in the vadose zone, their position is not tied to sea level. Tafoni caves are pseudokarst features that form when the soft interior of an eolianite ridge is exposed to subaerial erosion. Since tafoni caves form by mechanical processes, they do not exhibit phreatic dissolutional features. Tafoni caves may be mistaken as flank margin caves by the untrained observer, which may cause problems when using caves as sea-level indicators. Each of Abaco’s unique cave types may preserve depositional and erosional features that are useful to the researcher in creating general geologic timelines. While these timelines may not give exact dates, they are useful in the field for understanding depositional boundaries and determining sequences of geologic events.