GENESIS OF RECTANGULAR COASTAL RE-ENTRANTS IN COMPOSITE CARBONATE ISLANDS

Islands that fall in the composite island category of the Carbonate Island Karst Model (CIKM) commonly contain rectangular coastal re-entrants known as bokas, initially documented from Curacao in the Caribbean, but known as well from islands as far away as Rodrigues Island in the western Indian Ocean. Similar features can be found in inland settings of these island types, such as the gullies of Barbados or the valleys of Cape Range, northwestern Australia. Originally considered to be the result of the collapse of epigene stream caves carrying allogenic water from non-carbonate inland regions through a coastal band of carbonates, we propose that these features instead result from the complex interplay of surface stream incision, glacioeustasy, and tectonic uplift. The bokas of Curacao and the gullies of Barbados contain cave fragments in their valley walls, as well as remnant massive speleothems. These cave speleogens and speleothems were the evidence used to propose the epigene cave hypothesis, but the width of these valleys would require such a stream cave to be of gigantic proportions. Our proposal hypothesizes that surface stream incision of the coastal band of carbonates during glacioeustatic sea-level lowstands would create a valley that subsequently becomes flooded by marine water during a glacioeustatic sea-level highstand, and flank margin caves are formed in the valley walls. Each subsequent sea-level lowstand drains the flank margin caves, and subaerial speleothems are deposited. The flank margin caves promote valley wall retreat as they are denuded, and further sea-level highstands create new flank margin caves and valley widening progresses. Tectonic uplift removes these valleys from further sea water invasion, and the process ends, leaving wide valleys with cave remnants and eroded speleothems.