Evaporitic carbonate shoreline sediments are deposited in an arid or semiarid climate by tidal currents which transport the sediment from the marine environment to the shore. The sediments accumulate primarily as tidal-flat deposits which normally prograde seaward, producing a vertical sedimentary sequence from marine to supratidal. The supratidal sediments are the most easily recognized. They are characterized by irregular laminations, desiccation features, and a general lack of fossil material. The intertidal sediments are best recognized by their position immediately below the supratidal sediments. Intertidal sediments are commonly pelleted carbonate muds with numerous burrows, a restricted fossil assemblage, and may contain algal stromatolites.
The arid or semiarid climate allows sea water to evaporate and become saturated with respect to gypsum or anhydrite when the water circulation is sufficiently restricted from the open ocean. The low permeability of the tidal-flat sediment can produce this restriction to circulation. A channel connecting a lagoon to the open ocean can also produce this restriction to circulation, but the channel must be so small compared to the size of the lagoon as to be inconsequential. Gypsum (anhydrite?) crystallizes out of the interstitial hypersaline water within the sediment and out of standing bodies of hypersaline water. Nodules, replacement crystals, and pore-filling crystals are formed within the sediment, and laminated evaporite sediments are formed from standing bodies of water. The precipitation of gypsum (anhydrite?) produces a dolomitizing fluid which moves down through the underlying sediment. Extensive reflux dolomitization is characteristic of evaporitic shoreline carbonate deposits.
Evaporites are very susceptible to removal by shallow ground waters. Therefore, they are rarely well preserved in outcrops. Comparisons of subsurface and outcrop evaporitic carbonate sequences commonly show that the anhydrite or gypsum has been leached from the outcrop samples leaving molds and occasionally producing solution collapse breccias. Calcification of anhydrite or gypsum and dolomite is common. The recognition of evaporitic shoreline sedimentation from outcrop samples, then, often necessitates establishing whether or not dissolution of evaporites and calcification of dolomite and anhydrite or gypsum have occurred.
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Recognition of Ancient Sedimentary Environments
This volume contains a series of papers presented as part of a symposium held in Dallas, Texas, April 1969, at the annual national meeting of the Society. The problem of recognizing ancient sedimentary environments in the stratigraphic record is basic to essentially every aspect of research in sedimentary rocks. The publication will summarize much of what we currently know concerning environmental interpretation.