PIT CAVE DEVELOPMENT IN LATE QUATERNARY EOLIAN CALCARENITES

In carbonate islands, pit caves develop as vadose fast-flow routes to conduct meteoric recharge from the epikarst of the land surface to the fresh-water lens. On uplifted islands such as Guam, vadose fast-flow routes provide about 30% of the recharge to the fresh-water lens. Eolian calcarentites (or eolianites), such as those found in the Bahamas, represent a body of carbonate rock deposited under terrestrial conditions, the higher elevations of which have never been in phreatic conditions. The pit caves developed in these eolianites can occur in concentrations of over 100 per square kilometer. They rarely exceed 10 m in depth but may cross-connect to produce caves with over 50 m of horizontal extent. The pit caves commonly have a well-developed system of small feeder tubes developed within the epikarst that collect meteoric water. The great abundance of pit caves is misleading, especially in terms of the large water budget their numbers imply, as the evidence suggests that piracy of epikarst flow routes is common, and that pit caves become sequentially abandoned. Young eolian calcarenites are not necessarily prone to such piracy, as pit caves are equally abundant in the Mio-Pliocene carbonates of Isla de Mona. Pit caves have two main morphologies: simplistic vertical shafts that appear similar to those found in continental settings, and complex features that appear more like solution chimneys than vertical shafts. These latter features are interesting, in that preliminary data indicates that they develop on the flanks of the eolian ridges, and follow the foreset beds for short distances, commonly offset by direct vertical reaches. Pit caves nearer the top of ridges tend to behave more like direct vertical shafts in the area where the foreset beds level out at the ridge crest. From a speleogenetic point of view, the ability of the foreset beds to control vadose flow is surprising, given the high porosity (up to 40%) of these eolianites, and their well-sorted, homogeneous character. The ability of pit caves to widely collect water from the epikarst and deliver it quickly to the fresh-water lens indicates that they are an important factor in understanding both point and non-point pollution in carbonate island aquifers.