Influence of karst denudation on the northwest coast of Curaçao
The northwest coast of Curaçao is characterized by a series of Pleistocene-age reef terraces at four discrete elevations with dissolutional caves formed in the terraces at specific elevations (highest to lowest terraces, in meters above sea level: 90-175 m, 50-85 m, 10-45 m, and 5-10 m). Large scale rectilinear coastal reentrants called bokas occur in the lowest terrace, and are hundreds of meters long perpendicular to the coast, tens of meters wide, and have steep, vertical walls of up to more than ten meters height. Prominent coastline erosional features formed by a combination of cave collapse and wave erosion are also present in the lowest terrace. Reconnaissance field mapping in March of 2011 and 2012 documented 17 bokas and identified and surveyed numerous flank margin caves related to the reef terraces and the bokas. Quaternary uplift is evident by the position of the four elevated reef terraces adjacent to the coast. Eustatic sea-level changes, interacting with tectonic uplift, played an important role in the development of flank margin caves associated with the reef terraces. The flank margin caves in the inland cliffs fronting the terraces have been exposed by cliff retreat. As the caves form at sea level, and the coral terrace was at wave base when alive, the difference in elevation between the caves today and the terrace today (commonly 2 to 6 m) is an indication of the degree of dissolutional denudation of the terraces since terrace deposition and exposure. A widespread system of fluvial valleys, formed on interior Cretaceous volcanic rocks, has eroded through the limestone terraces into the underlying basaltic bedrock. Large bokas are developed where these inland streams have incised through the lowest limestone terrace. Waves penetrate into the lower portions of the bokas. Their inland termini open to broad valleys on the volcanics. The bokas contain flank margin caves exposed along their vertical walls, including within the broad inland termini, which have facilitated boka wall collapse. Caves located in the lowest reef terrace that are not associated with ephemeral fluvial drainage are exposed by ceiling collapse and are eventually breached by sea-cliff retreat. As wave-influenced coastal erosion proceeds, these flank margin caves are degraded to natural bridges that parallel the coastline and eventually evolve to short coastal reentrants. The assortment of karst, marine, and fluvial features signify polygenetic processes contributing to boka formation and the erosional degradation of the coastline.