Vertical variations in bassanite distribution patterns in near-surface sediments, southern Egypt

Nile valley sediments of the El Adaïma area in southern Egypt contain bassanite (CaSO4·0.5H2O), associated with gypsum (CaSO4·2H2O), in the upper ∼1 m of the deposits. The presence of bassanite at this site, formed by dehydration of gypsum, is marked by variations in mode of occurrence with depth. In the lowest bassanite-bearing interval, bassanite occurs mainly along the contact between gypsum crystals in pores, which is related to the protection of bassanite from rehydration in those parts of the gypsum aggregates. At a higher level, bassanite occurs along the sides of lenticular gypsum crystals in the sediment matrix. Near the top of the studied sequence, bassanite mainly appears as inclusions in gypsum crystals, which represents a texture that developed by repeated dehydration and rehydration. Elsewhere in the upper part of the deposits, bassanite occurs as aggregates of small crystals in pores, without associated gypsum, which was entirely dehydrated to bassanite in these parts. These vertical variations in bassanite distribution patterns record differences in the diagenetic history of the deposits between depth intervals.