Nodular and enterolithic gypsum: the “sabkha-tization” of Salt Flat playa, west Texas

Abstract

The sediment in the continental sabkha of Salt Flat, west Texas, is typically millimeter-laminated and composed of evaporitic and diagenetic minerals including gypsum, dolomite, halite, calcite and aragonite. These laminated evaporites were originally deposited on the floor of a Late Pleistocene perennial brine lake and now constitute the matrix of the modern sabkha sediments in the playa. As climate changed from the more humid phase of the Pleistocene to the more arid conditions of the Holocene, the lake dried up and these laminated evaporites were deflated into gypsum dunes or overprinted by a sabkha-related hydrology. Near-surface gypsum-rich layers were transformed by capillary evaporation into coalescing nodular horizons. At the same time, in shallow vadose sediments (less than 1–2 m subsurface) the gypsum was dissolved and replaced either by halite or dolomite. Diagenetic gypsum was co-precipitated as gypsum overgrowths and a patchwork of cement from a highly saline interstitial brine (avg. 156,000 mg/l TDS). These processes of early-diagenetic enlargement and coalescence of gypsum crystals created Holocene gypsum nodules within the laminated host sediment. Diffuse growth of nodules in the lower vadose zone created chicken-wire textures. Gypsum growth in the upper vadose zone was often concentrated along particular layers and formed enterolithic bands. Pleistocene lacustrine evaporites were in effect “sabkhatized” by a fall in the lake water level.