Selenite facies in marine evaporites: a review
The term “selenite facies” describes evaporitic, primary precipitates of coarse-crystalline gypsum formed on a depositional floor. The crystals comprising these autochthonous facies commonly display competitive growth patterns. Modern marine selenite facies have been studied in coastal salt works (evaporative salinas) and in some natural coastal lakes where gypsum is precipitating or has been precipitating in recent times. In ancient marine formations, selenite crystals display two major facies groups: bedded to massive selenites; and domal selenites. Good examples of selenite facies occur in the Miocene of the Mediterranean region (Messinian) and in the Carpathian Foredeep (Badenian). In ancient examples, selenite facies have been preserved as pseudomorphic anhydrite at depth and as pseudomorphic secondary gypsum at outcrop. In such cases, their identification may be difficult, owing to the intense diagenetic transformations undergone by many evaporites. Ancient occurrences of selenite facies cover a wide range of settings: (1) flanking selenite shelves; (2) narrow flanking selenite shelves; (3) selenite platforms covering salt-filled basins; and (4) selenite basins. All occurrences are sensitive to changes in sedimentological conditions and require specific environmental configurations. In particular, they depend on the existence of an outflow of heavy brines to prevent oxygen depletion in the water column, and the accumulation of chlorides on the platforms and in the basins.
Keywords
Evaporite, gypsum, selenite, evaporitic platform, evaporitic basin, diagenesis.
