Abstract
The Abu Ghaylan Formation is well exposed in north-central Jabal Nafusah, northwest Libya, between Wadi Ghan to the east and Ar-Rabitah to the west. Abu Ghaylan Formation is missing towards the east owing to erosional factors rather than depositional.
The Upper Triassic-Lower Jurassic stratigraphic section (Abu Ghaylan Formation) in the vicinity of Gharyan is easily distinguished at Abu Ghaylan and neighbouring areas where it lies between two red-coloured rock units, the Abu Shaybah Formation at the base and the continental clastics of the Kiklah Formation at the top.
The Abu Ghaylan unit further west grades into the Bir al Ghanam Formation near kaf Mantrus. The lithofacies of the Abu Ghaylan as described in literature is analogous to shallow shelf carbonates.
Two types of dolomite can be differentiated petrographically based on crystal size and precursor facies. The data gathered from XRD analysis suggest that the Abu Ghaylan dolomite types seem to be mainly non-stoichiometric and well-ordered.
Stable carbon and oxygen data are reported from dolomite and calcite from the Abu Ghaylan Formation. Surface dolomite samples have a mean δ^18O value of -1.66 per mil, and a mean δ^13C value of -0.28 per mil (all values are relative to PDB). Oxygen isotopic ratios become progressively lighter by depth, and reflect progressive lithification of the Abu Ghaylan sediments during burial.
The lack of significant isotopic signatures in both carbon and oxygen isotopes from estimated values for the Upper Triassic-Lower Jurassic carbonate precludes meteoric influence during Abu Ghaylan diagenesis. Abu Ghaylan dolomites generally have low trace elements (Sr, Mn, Na, and Fe) concentration.
Two mechanisms were involved in the formation of the Abu Ghaylan dolomite; they are hypersaline reflux and mixed water.