DEVELOPMENT AND FUTURE FRONTIERS FOR CARBONATE COASTAL AND ISLAND KARST

Coastal and island karst as a subfield of general karst studies became established in the 1980s with the development of the flank margin model to explain caves in coastal and inland carbonate cliffs that had phreatic dissolutional surfaces, lacked epigenic forms, and were not wave action or tafoni in origin. Two decades of examination of localities world-wide generated two important additional concepts: 1) there is a significant difference between karst on islands, which is karst typical of that seen in interior continental settings, and island karst, which invokes the interaction of freshwater-seawater mixing and sea-level change to produce unique karst features; and 2) the Carbonate Island Karst Model (CIKM), which takes into account the diagenetic condition of the carbonate rock, and its distribution relative to insoluble rocks, if present. Continental carbonate coasts, such as Yucatan, became part of the model. The role of organic loading and decay, and discharge flow velocity, were characterized. A final significant development was that flank margin caves were seen to develop rapidly, within a few thousand years, but to persist under proper conditions, for millions of years. Coastal karren’s unique morphologies were seen to be an outcome of eogenetic carbonate rock in a coastal environment.

Many unanswered questions remain. The role of tidal forcing on flow dynamics and geochemistry in flank margin caves has not been fully explained. As the caves are mixing chambers within a diffuse flow system, cave flow is assumed to be laminar and phreatic morphologies are consistent with that view, but the actual nature of the flow, including vertical density convection, remains a topic of debate. An international meeting in 2014 failed to reach a consensus as to whether flank margin caves were hypogene or not. Two hydrological questions: the role of island size, and continental coastal discharge regimes, have had initial investigation, but more needs to be done. Initial study as to what actually happens at the wall-rock and fluid interface has been inconclusive. The nature of contaminant transport and residence time in coastal karst remains a frontier. Flank margin caves may be the most abundant dissolutional cave type in the world, warranting additional research.