POLYGONAL FRACTURES IN OOID GRAINSTONES OF CAT ISLAND, BAHAMAS: A UNIQUE SEDIMENTARY STRUCTURE IN CARBONATE DEPOSITS
To determine the possible causes of fracturing, we conducted laboratory experiments on ooid-rich beach sand from Alligator Point. The sand was placed in a clear glass container to dry at room temperature. Drying resulted in polygonal fracturing of the sediment surface similar to that observed in the field. These results were reproduced by moistening the sand with deionized water and allowing it to dry in layers 1, 3 and 5 cm thick. Our laboratory observations suggest that the polygonal fractures form by stresses generated by the reduction of interparticle porosity and re-packing of sand due to the loss of cohesion between grains during drying. These fractures are readily preserved by rapid lithification of carbonate sand. It is therefore somewhat surprising that such features have been rarely documented from other localities. In part this may be because such sedimentary structures could easily be mistaken for mudcracks. The paucity of documented examples of polygonal fractures in mud-free carbonate deposits suggests that the formation of this unique sedimentary structure may require sediment that consists of well-sorted, well-rounded spherical sand grains in areas subject to desiccation unlike the texturally and compositionally more heterogeneous skeletal and peloidal sediment common elsewhere in the Holocene of the Bahamas.