The playa mud flats of five closed basins in Nevada and California were examined to establish the characteristics of their ubiquitous mud cracks. The mud-crack patterns seen in plan view reflect local conditions of deposition, ground-water level, and exposure time. The purpose of this study was to determine how they appear in vertical cross section (as seen in rock outcrops or drill cores). Trenches and polished slabs of plastic impregnated surface samples reveal a variety of mud-crack cross sections, including: (1) sinuous, V-shaped fillings with lobate margins; (2) marked changes in width of a single filling; (3) branching fillings; (4) horizontal and obliquely dipping cracks; (5) breccia patterns created by mud-filled vertical and horizontal cracks; and (6) multiple cr ck fillings. These features are similar to characteristics used for identifying subaqueous shrinkage cracks in the rock record. Although subaqueous shrinkage is possible for the playa mud cracks, more important mechanisms for their formation are: (1) superimposed mud-crack patterns in areas of low sedimentation; (2) soil-like processes (internal drying, eluviation, clay expansion, and shrinkage); (3) partial erosion and mechanical slumping of mud-crack margins; and (4) plastic sediment flowage during

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rewetting. These observations imply: (1) the criteria for recognizing subaqueous shrinkage cracks need reevaluation and (2) a careful study of mud-crack characteristics and their vertical and lateral variations in stratigraphic sections may provide detailed data on basin morphology, hydrology, and sedimentation rates.

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