Evolution of polygonal patterns in stratified mud during desiccation: The role of flaw distribution and layer boundaries

Detailed analysis of diagnostic surface morphologies of fractures in natura l mud indicates that mud cracks systematically nucleate at the bottom of th e mud and propagate vertically upward toward the free surface and laterally outward toward adjacent cracks. Earlier generations of mud cracks rupture the set of desiccated layers altogether, forming polygonal patterns that ar e similar throughout the mud sequence. Later generations of mud cracks subd ivide each layer separately, forming markedly different polygonal patterns within individual mud layers. A simple mechanical model draws an analog bet ween cooling of granular materials and drying of mud. It shows that subfact defects at grain boundaries near the base may become critical before defec ts associated with fine particles at the top, even though the stress profil e due to drying is usually more tensile at the top. Thin-section analysis a nd sieving method indicate that grain boundaries at the bottom are several times longer than at the top of the mud due to the natural sorting of grain s, illustrating the validity of the model. This study suggests that stress variations with depth are less important for pattern evolution of mud crack s than previously theorized. However, nonuniform flaw distribution and surf ace discontinuities play a fundamental role during mud-crack nucleation and growth. These results have significance for several subjects, including ex perimental studies of cracking induced by desiccation, the previously sugge sted analogy between natural mud fracturing and basalt fracturing, and the use of mud cracks in stratigraphic interpretation.