Supratidal Diagenesis of Carbonate and Non-Carbonate Sediments in Arid Regions: ABSTRACT

A supratidal sediment surface is the common end-product of shallow-marine and intertidal sedimentation. Such surfaces normally increase in area as sedimentation proceeds, and may have a variety of geometric shapes. Furthermore, they may be either attached to a coastline or be unattached. The sediments deposited may range from almost entirely carbonate to non-carbonate and may be fine- or coarse-grained. The original mineralogy of the sediments represents a relatively stable assemblage for the temperature and solution conditions of the marine environment. When in the supratidal position, under a different physical and chemical regime, diagenetic changes may occur. Many of the climatic variables which affect the marine environment affect the supratidal environment in a more severe manner. Solution compositions normally show small fluctuations in the marine environment, but pore solutions may undergo substantial dilution or concentration in supratidal areas. Solution changes depend largely on the balance between rainfall and rates of evaporation and evapotranspiration. The addition of land-derived waters may occur in inner parts of attached supratidal areas. In areas of diluted pore waters, the probable diagenetic trend in carbonate sediments will be toward formation of low-magnesium calcite. Where pore-water concentration occurs, dolomitization of the original carbonate sediments takes place. The dolomitization of coarse-grained skeletal carbonate sediments is slower than that of finer-grained materials. Dolomitization normally is preceded by a fairly large calcium loss from the pore solution as a result of interstitial precipitation of aragonite or gypsum, the latter occurring under more extreme conditions of evaporation. Gypsum may be formed seasonally, being leached during the wet season, and is likely to be preserved only under conditions of net evaporation. Under high net-evaporation conditions, anhydrite, typically of the nodular t pe, is a possible development. Under an extreme net-evaporation regime, halite is formed, but higher salts are unlikely. In non-carbonate environments the evaporite mineral developments will be similar. In carbonate sequences larger amounts of calcium sulfate minerals are commonly present as a direct result of dolomitization. The chemical evolution of the pore fluids in carbonate and non-carbonate sediments under net-evaporation conditions will vary greatly. In the carbonate sequence, a large magnesium loss results from dolomitization; sulfate can be removed almost completely because of the excessive amounts of calcium available, and the final solution is essentially a calcium chloride-type brine. In non-carbonate sediments there is little or no magnesium loss, but almost complete loss of calcium, largely as carbonate and sulfate minerals; less than 40 per cent of the available sulfate is removed, and the final solution is essentially a magnesium sulfate-type brine. An important variable which may have a critical effect on brine evolution is the bacterial reduction of sulfate. Early diagenetic changes which may occur in supratidal environments are controlled largely by original sediment characteristics and climatic variables. The variation in possible diagenetic developments is large. Overprinting of different diagenetic facies commonly End_Page 620------------------------------ occurs. The end product, seen in ancient lithified analogues, is understandably difficult to interpret. End_of_Article - Last_Page 621------------