Abstract
Neogene calcium sulphate deposits at the northern Western Desert, Egypt, are interpreted as shallow photic subaqueous evaporites. Study of the evaporite sequences at Gebel El-Hagif and Dir El-Biraqat areas reveals two principal structures similar to those observed in the Messinian gypsum in and around the Mediterranean Sea. The structures, arranged from base to top are: (1) cryptalgal laminites and stromatolitic structures built up by cyanobacterial (blue-green algal) communities in gypsite and gypsarenite units; and (2) selenite (specchiolino or mirror like gypsum) represented by (a) disordered selenite, and (b) vertically arranged selenite. Cyclic increase and decrease in salinity are represented by interlamination of gypsum and cryptalgal carbonate laminae. General increase of salinity is represented by the general increase in crystals size of gypsum from gypsite to gypsarenite to twinned gypsum crystals more than 5 cm in length at the top of the sequence. A regressive evaporite basin model is suggested for the deposition of the studied evaporite sequence. Dehydration of gypsum into anhydrite through an intermediate bassanite phase, and the subsequent transformation of secondary anhydrite into secondary porphyrotopic gypsum, are the main diagenetic processes affecting the studied Neogene evaporites.
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