TESTING THE HYPOTHESIS ABOUT THE INFLUENCE OF GRAIN TYPE AND TEXTURE ON FORMATION OF POLYGONALLY CRACKED CARBONATE GRAINSTONES IN THE BAHAMAS
To test this hypothesis, we examined polygonally cracked grainstone from three different locations in the Exumas to address the role that size and shape of sand particles play in the formation of polygons. The samples examined were comprised of fine to medium sand (mainly 150-350 µm in diameter), with rare coarse sand grains (up to 500 µm). Sand grains were mainly well-sorted and very well-rounded ooids with thin cortices around nuclei of peloids or skeletal fragments. The sand also contained some rounded skeletal fragments (mainly mollusks and foraminifera), which are uncoated or have a very thin surficial cortex. Grains may be heavily micritized by microboring, but there is no mud present in the matrix. Instead, the sand is loosely lithified by mainly clear, equant meteoric calcite cement with isopachous rims, and meniscus and pendant morphologies.
These observations generally support the hypothesis that homogeneous composition and texture are key factors for development of polygonal cracking in grainstones. However, there could be other controlling factors such as: 1) air and sand moisture regime (i.e., composition and amount of fluids); 2) temperature, duration and rate of wetting and drying; and 3) depositional processes that produce unique sedimentary structures and fabric, including grain packing and porosity within sediment. With future studies on comparing polygonally cracked versus uncracked grainstones as well as examining conditions under which polygonal cracking is currently occurring, it should be possible to evaluate these additional factors to better understand the formation of polygonal fractures in sand and to predict situations where it might be possible to find more modern and ancient examples.