SANDCRACKS AND SANDCHIPS: EXPERIMENTALLY PRODUCED SEDIMENTARY FEATURES IN OOID SAND AND GLASS BEADS

Every builder of sand castles is aware of a major difference in physical properties between dry and wet sand. Addition of a small amount of moisture turns cohesionless sand into a cohesive material that can aggregate easily due to adhesive capillary forces caused by the presence of liquid bridges between grains. In laboratory experiments with ooid sand and glass beads we aim to evaluate the role of composition and texture in the formation of unusual sedimentary features in moist sand.

We experimented with mud-free beach sand from Cat Island, Bahamas that is fairly well sorted and composed mainly of well rounded, fine to medium sand size (100-400 µm), spherical to elliptical ooids. Layers made of cohesive, moist ooid sand had a substantial amount of interparticle vuggy porosity, similar to that common in beach deposits. When left to dry at room temperature, the porosity was reduced due to loss of cohesion and repacking of grains. During this process the sand surface cracked and the resulting polygonal sandcracks resembled desiccation mudcracks. We also produced sandchips, similar in origin and morphology to muddy intraclasts or clay chips, by disturbing the surface of moist or cracked sand. Similar results were obtained in experiments with glass beads of the same texture as ooid sand.

Although polygonal desiccation cracks and intraclasts are usually associated with muddy deposits, our experiments demonstrate that such features can be produced in homogenous, mud-free, relatively fine-grained, well-sorted, round, spherical to elliptical sand of various compositions. Uniform size and regular shape of such sand appear to provide maximum density and homogenous distribution of liquid bridges for optimal cohesion so that sand can contract and crack polygonally when continuous films of water formed by liquid bridges break into isolated capillary films during desiccation. While texture seems to control the formation of sandcracks and sandchips, the composition of sand and interstitial fluids influence their preservation by rapid lithification. The presence of salt and carbonate cements favors preservation in eolian and beach carbonate sand as supported by field examples on Cat Island. The apparent paucity of these features in the geological record suggests that generally they are not easily produced and/or preserved.