Abstract
The Sidi as Sid Formation (Upper Cretaceous) outcropped in Jabal Nafusah in NW Libya is composed of very fine to very coarse-grained dolomite of the Ain Tobi Dolostone Member and marly limestone, dolomites, and calcareous shales interbeds with bedded gypsum of the Yifran Member. Five diagenetic dolomite types are recognized in the Ain Tobi Member. They are: Type-1, Fine and subhedral replaces mudstone/wackestone facies that show homogeneous luminescence pattern; Type-2, Very fine, anhedral to subhedral replaces packstone/grainstone facies and characterized by bright orange and yellow luminescence pattern; Type-3, Fine to medium, subhedral associated with quartz and has dirty orange luminescent cores and bright edges; Type-4, Medium to coarse, euhedral to subhedral crystalline fabric with orange luminescent cloudy cores and dully luminescent clean rims; Type-5 (baroque), Coarse to very coarse, subhedral with curved crystal faces and characterized by orange luminescent inclusion-rich cores and dully luminescent inclusion-free rims.
The environment of deposition of the Sidi as Sid Formation is part of an inner ramp which ranged from restricted or lagoonal ramp and open to semi-restricted ramp environments, which graded northwards to mid- and outer ramp environments. The Yifran Member represents a restricted ramp environment and the Ain Tobi Dolostone Member represents an open- to semi-restricted ramp environment. The ramp margin had been removed due to post-Cretaceous erosion and the outer ramp environment is represented by the Sidi As Sid Formation’s equivalent (Alalgah Formation) in the offshore Basin.