KARST DENUDATION ON THE COASTLINES OF THE ABC ISLANDS

The coasts of Aruba, Bonaire and Curacao are characterized by a series of Pleistocene-age reef terraces at discrete elevations with dissolutional caves formed in terraces at all elevations, large-scale linear coastal reentrants (bokas) associated with caves, and coastline erosional features formed by a combination of cave collapse and wave erosion. Field mapping has documented bokas and numerous flank margin caves associated with terraces and bokas. Field and remote sensing data established the relationship of bokas to a broad system of fluvial valleys, formed on interior Cretaceous volcanic rocks, that eroded through the terraces into underlying basaltic bedrock. The bokas are rectilinear re-entrants in coastal cliffs, commonly 7-10 m deep and ranging 10-40 m wide, running inland up to several hundred meters and terminating in broad valleys. Flank margin caves exposed along their vertical walls have facilitated wall collapse. Caves located in the lowest terrace that are not associated with fluvial drainage are exposed by ceiling collapse and eventually breached by sea-cliff retreat. As wave-influenced coastal erosion proceeds, coastline caves are degraded to natural bridges paralleling the coastline, and eventually evolve to short coastal reentrants. Our data show that eustatic sea-level changes, interacting with tectonic uplift, played an important role in the development of flank margin caves associated with the reef terraces/bokas. In addition, cave vs. terrace elevation data indicate the degree of dissolutional denudation of the terraces since deposition and exposure. The assortment of dissolutional/erosional features signify polygenetic processes contributing to cave and boka formation and erosional degradation of the coastline.