POLYGONAL FRACTURES IN OOID GRAINSTONES OF CAT ISLAND, BAHAMAS: A UNIQUE SEDIMENTARY STRUCTURE IN CARBONATE DEPOSITS

Polygonal fractures are a very common sedimentary structure in Holocene to Recent eolian and backshore carbonate grainstone deposits on Alligator Point, Cat Island. In general shape, these fractures resemble mudcracks, but they are produced in ooid-rich carbonate sand with little or no mud present. The polygons are 4- to 6-sided and up to 75 cm in diameter. The fractures are jagged and tightly fitting, and they can extend through multiple beds to maximum depths of ~80 cm. Occasionally the polygons are displaced and the resulting space is filled with sediment. Polygonally fractured deposits are composed of relatively well-sorted and well-rounded spherical to elliptical ooid grains of fine to medium sand size (mainly 150-350 µm diameter), with rare coarse sand grains (up to 600 µm diameter). Recent deposits are poorly cemented with thin rim or meniscus and pendant carbonate cements. Older Holocene deposits are more firmly lithified with fibrous, bladed, and equant cements.

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.