Giant Desiccation Polygons of Great Basin Playas

摘要

Giant desiccation fissures observed on 39 playas (dry lakes) in the Great Basin are greater in size and causative rupture stress than are the typical, small, surface mud cracks. Giant open fissures may be a meter wide and more than 5 m deep, whereas the typical shallow mud cracks are a few centimeters wide and about 25 cm and more deep. The fissures form polygonal patterns that range from 15 to 100 m and more across, with intersections predominantly at approximately right angles. The fissures themselves follow sinuous, irregular lines. Thus, the patterns are for the most part 'irregular random orthogonal polygons'. The fissures form on hard, dry, compact playa crusts where desiccation has been rigorous and ground water is fairly deep. The contractive stress producing the fissures results from loss of moisture and appears to develop over a period of several years; however, the rupture probably takes place in minutes. The greatest release of stress frequently appears to occur below the surface. Tectonic forces and basin subsidence may be a factor in fissure formation. Fissures follow a cycle of growth and destruction: fresh, sediment filled, vegetated, and relict fissures comprise the major geomorphic forms. The patterns can be easily identified on airphotos, and can thus, through inference, offer reconnaissance information on the playa environment. (Author)