Page 257 - Sedimentology and Stratigraphy

244 Shallow Marine Carbonate and Evaporite Environments

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Fig. 15.20 (a) A barred basin, ‘bulls-eye’ pattern model of evaporite deposition; (b) a barred basin ‘teardrop’
pattern model of evaporite deposition.

chloride) at one extreme to carbonates deposited periods of lower salinity can accumulate within the
in normal salinities at the other. If equilibrium is basin deposits and be preserved when the salinity
reached between the inflow and the evaporative loss increases because hypersaline basins are anoxic.
then stable conditions will exist across the basin and There are no modern examples of very large,
tens to hundreds of metres of a single mineral can be barred evaporitic basins but evidence for seas preci-
deposited in one place. This produces a teardrop pitating evaporite minerals over hundreds of thou-
pattern of evaporite basin facies (Fig. 15.20). sands of square kilometres exist in the geological
Changes in the salinity and amount of seawater in record (e.g. Nurmi & Friedman 1977; Taylor 1990).
the basins result in variations in the types of evaporite These saline giants have over 1000 m thickness of
minerals deposited. For example, a global sea-level rise evaporite sediments in them and represent the
will reduce the salinity in the basin and may lead to products of the evaporation of vast quantities of
widespread carbonate deposition. Cycles in the deposits seawater. Evaporite deposits of latest Miocene
of barred basins may be related to global sea-level fluc- (Messinian) age in the Mediterranean Sea are evi-
tuations or possibly due to local tectonics affecting the dence of evaporative conditions produced by partial
width and depth of the seaway connection to the open closure of the connection to the Atlantic Ocean. This
ocean. Organic material brought into the basin during period of hypersaline conditions in the Mediterranean