Lithostratigraphy, facies interpretation and depositional environment of the lower miocene gypsified stromatolites and microbial laminates, rabigh and ubhur Areas, Red Sea coast, Saudi Arabia

Abstract EN

The Lower Miocene evaporites in Rabigh and Ubhur areas belong to the Gehfa Member, Dafin Formation and Ubhur Formation, respectively.

In Rabigh area, the evaporite sequence is composed of gypsified rhizocretion (formed in supratidal sabkha) and a thick sequence composed of gypsified stromatolites and microbial laminites, and skeletal gypsum (formed in upper subtidal and intertidal environments).

This sequence is conformably overlying the fluvial siliciclastics of Al-Hakkak Member, Dafin Formation.

In Ubhur area, the evaporite sequence is composed of gypsified microbial laminites, highly enriched in fine silt-sized quartz grains that may form nodular mosaic structure.

The Ubhur evaporites are formed dominantly in supratidal sabkha environment, that intermittently submerged to the intertidal environment.

Microscopic investigation of the Rabigh and Ubhur evaporites indicate the dominance of secondary gypsum rocks that mantled with20-30 cm thick powdery anhydrite crust.

Relics of primary prismatic, lenticular and rosette gypsum crystals are observed in areas highly enriched in microbial micrite.

The dominance of stromatolites and microbial laminites structures indicate their formation in low salinity brine (60-150 g / l), absence of grazer and burrowers, and sufficient oxygenation in supratidal and shallow depositional environment.

On the other hand, the gypsum laminae indicate a relatively higher salinity of the brine (>150 g / l) and their deposition in permanent shallow subaqueous environment.

Periodic exposure of the subaqueous evaporites is evidenced by the presence of gypsified rhizocretion and displacive nodular mosaic structure.