Desiccation Features and the Reconstruction of Paleosalinities

Abstract

ABSTRACT Simple experiments illustrate a marked correlation between the morphology of desiccation cracks and interstitial salinity in carbonate sediments. In a study delineating microfacies and depositional environments in the Grayburg Formation (Permian) of west Texas, a correlation was also seen between the size and spacing of mudcracks and their position in the supratidal environment. In laboratory experiments, carbonate muds of uniform composition were wetted with sodium chloride solutions of varying concentrations and allowed to desiccate under identical circumstances. With high repeatability, the less saline samples were the first to crack and, when experiments were carried to completion, possessed the largest desiccation features. Under controlled laboratory conditions, desiccation is related to vapor tension, a function of the ionic strength of pore fluids, complexing, and the activity of water. Grayburg carbonate muds from an offshore hummocky tidal flat possess deep desiccation features, profuse burrows, and exhibit a moderate array of relic bioclasts. Farther landward, coastal supratidal deposits possess shallow desiccation cracks, exhibit no burrows, and appear to have hosted only a limited or restricted indigenous fauna. The coastal sabkha-like deposits are further distinguished from their offshore exposed-flats counterparts by containing an early anhydrite cement in well-developed birdseye and teepee structures. If these faunal, structural, and mineralogical zonations are indeed indicators of syndepositional paleosalinities, then a correlation exists between salinity and the development of desiccation features in Permian sediments. Less saline offshore flats appear to b more deeply and profusely cracked than hypersaline coastal flats. A distribution of mudcracks analogous to the Permian examples has also been observed by other workers in modern carbonate sediments along the Persian Gulf. In some areas the magnitude of polygon development parallels the declining salinity profile encountered in moving seaward from hypersaline sabkha pore waters to areas characterized by more normal marine interstitial salinities.