Comparison of Sequences Formed in Marine Sabkha (Subaerial) and Salina (Subaqueous) Settings--Modern and Ancient

Abstract

Marine evaporites occurring in modern subaqueous (salina) settings and subaerial (sabkha) settings are different. Subaqueous Holocene evaporites occur as shoaling-upward lacustrine sequences up to 10 m thick. They are evaporite dominated and are composed primarily of bottom-nucleated crystals that may be deposited as massive, laminated, or rippled units. Each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments. In plan view, they show a well-developed bull's-eye pattern with a sulfate center and a carbonate rim. In contrast, subaerial (sabkha) evaporites occur as part of a laterally prograding, shoaling-upward, peritidal sequence in which the supratidal unit is usually no more than 1 m thick. Sabkha sequences are matrix dominated, not ev porite dominated, with the bulk of the sulfate phase occurring as diagenetic nodules, enteroliths, or diapirlike structures. These sulfates were formed during syndepositional diagenesis by replacement and displacement processes. The various facies of the sequence tend to accumulate in belts parallel with the shoreline. Relative to the sea level or the brine level, sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows. Some of the characteristics that distinguish Holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies, even when gypsum has been converted to nodular anhydrite. The distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates.