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Welcome to the Education Corner: Evaporite Formation and Accumulation
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Future Links: Deep Hypersaline Seas • Gypsum-Anhydrite Transformations: Blades • Rosettes • Subaqueous Selenite • Clastic Gypsum
Coastal salinas form in subsea-level depressions proximal to the margins of seas where evaporitic drawdown concentrates their mixed fill of marine and meteoric waters into hypersaline brines. Most of these coastal depressions represent shallow barred basins for which marine water is the primary but not exclusive fluid source. Barriers formed by calcareous coastal dunes characteristically control the influx of ocean water into the basin limiting it to minor spillover and groundwater resurgence (Warren 1989). Recharge by marine-derived groundwater occurs after evaporation lowers the salina brine surface to levels below that of the adjacent sea. With evaporation, salina brine temperature rises and solutes become more concentrated until super saturation leads to evaporite precipitation. Which evaporite minerals form and accumulate depends not only on the ionic composition and concentration of the brine but also on climate (Schreiber and Tabakh 2000). For example, halite may crystallize but fail to accumulate in settings where the relative humidity exceeds 65% (Kinsman 1976). Precipitation of bittern salts requires even lower relative humidity than 35% (Schreiber and Tabakh 2000) and brine temperatures above 40 degrees C ( d'Ans 1947; Casas et al. 1992). Precipitation of carbonate and gypsum has relatively few comparable climatic constraints, but this is not to say their accumulation is independent of climate. Warren (1989) obseves that the prominent development of boxwork limestone occurs only in salinas associated with semi-arid settings. Inorganic carbonate precipitation takes place at brine concentrations above 120 o/oo and gypsum forms in solar ponds whose salinity exceeds 150 o/oo (Logan 1987; Schreiber and Tabakh 2000).
The salinity structure of modern salinas is highly variable. As with hypersaline waters elsewhere (Logan and Brown 1986, Figure 24, p. 38; Morris and Dickey 1957), salinas may develop lateral salinity gradients with near vertical contacts. Hence, the evaporite sediment fill of salinas commonly displays a bull’s-eye pattern in which the sedimentary facies patterns mirror salinity gradients (Warren 1989). Carbonate accumulation at the inflow zone passes laterally into gypsum that in turn may pass into halite. Such a salina fill pattern characterizes Lake Macleoud in Western Australia.
Lake Macleod is a large (120 km x 40 km) coastal salina in Western Australia. A stranded Pleistocene carbonate sand dune ridge bars the Indian Ocean from free spillover into the MacLeod basin. This coastal ridge extends as an unbroken barrier that is 3-15 km in width and stands 20-90 feet above sea level (Logan 1981). Indian Ocean water presently feed Lake MacLeoud via marine springs located at the northern end of the basin and by seepage along the length of the barrier (Logan 1986). But the facies fill pattern suggests the existing hydrology evolved from a more open system through the development of an aquitard across much of the lake basin (Logan 1986)..
Seawater flows southward away from the springs and floods to varying extent portions of the present-day mudflat. In the absence of meteoric influx, the southward flow creates an extensive shallow brine lake in which the rate of evaporation determines its southern edge. Concentrations of hemipyramidal gypsum crystals and clastic gypsum precipitates occur along the brine lake margins..
Displacive and subaqueous vertically aligned gypsum precipitates form locally on the floor of the hypersaline lake. Logan (1986, 1987) also reports the formation of tiny gypsum prisms within the water column of the brine lake.
Today's evaporite deposition in Lake MacLeod fails to reflect the entire spectrum of evaporite deposition as shown by its sedimentary record. Beneath the present sedimentary surface lies a succession of subaqueous carbonate, gypsum and halite that measures over 50 feet (15m) in thickness (Logan 1981, 1986, 1987). Much of the halite accumulation consists of columnar halite, thus suggesting not only elevated concentrations, but also significantly greater lake depths relative to the present.
Not all coastal salinas exhibit depositional histories inclusive of halite deposition. Warren (1982a, 1982b, 1989) describes several unique Holocene lakes from Southern Australia where subaqueous gypsum dominates the fill history. Seawater and to a lesser extent meteoric seepage from adjacent dunes feed these lakes. Their sediment fill pattern typically has carbonate facies rimming the lake margin and gypsum facies occupying the central lake area (Warren 1989). Warren (1989) goes on to note that an abrupt and nearly vertical boundary typifies the carbonate to gypsum facies transition, thereby reflecting the horizontal salinity zonation of the lakes.
Spectacular growths of bottom nucleated gypsum crystals dominate much of the sedimentary fill of these coastal salinas. Subaqueous gypsum crystallization is a discontinuous process as evidenced by multiple dissolution truncations. Gypsum crystal growth interruptions stem from seasonal freshening of lakes and episodic exposure horizons. Biotic carbonate laminae mark many seasonal truncation surfaces. These consist mainly of fecal pellets produced by ostracod and brine shrimp blooms and lesser amounts of micritic sheaths formed around blue-green algal filaments (Warren 1989)..