--> Geobiological Events in the Cyrenaican Miocene Carbonate-Evaporite Sequences of Ar-Rajmah Group, Al-Jabal Al-Khdar Uplift and Soluq Trough, NE Libya

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Geobiological Events in the Cyrenaican Miocene Carbonate-Evaporite Sequences of Ar-Rajmah Group, Al-Jabal Al-Khdar Uplift and Soluq Trough, NE Libya

Abstract

This work focuses on the drastic geobiological changes that occurred on the Cyrenaican Miocene carbonate-evaporite ramp, NE Libya. The detailed regional facies relationships determined from 29 measured stratigraphic sections, and 14 spectral gamma-ray profiles. In addition, at least three carbonate biological communities (coralline red algae, Porites coral reefs, and microbial reefs) were analyzed through time in detail. The Ar-Rajmah Group Miocene carbonate rocks record six 3rd order sequences. The Early Miocene Benghazi Formation (up to 46 m thick) is dominated by oncoidal coralline red algae facies which interfingers with the bioclastic wackestone/packstone facies in the open marine subtidal environment. The Porites coral reefs are associated with bioclastic mudstone/packstone facies in the lagoonal environment. The Middle Miocene Wadi Al-Qattarah Formation is up to 26 m thick. It is dominated by tidal microbialite reefs intercalated with continuous ramp crest oolitic grainstones that are associated with thick evaporites and an estuarine tongue of green shale and quartz sandstone. The Middle Miocene microbialite reefs have three forms: stromatolites, thrombolites, and cryptalgal laminites. During the Middle Miocene, the Porites coral reefs completely disappeared due to siliciclastic influx, and the coralline red algae became restricted to the deep basinal area due to the continuous uplifting. The Late Miocene Wadi Al-Qattarah Formation (up to 25 m in thickness) and dominated by tidal microbial reefs and continuous ramp crest oolitic grainstones facies that associated with some evaporites. The Late Miocene microbialite reefs contain two forms: thrombolites and cryptalgal laminites. The Porites coral reefs dominated the restricted lagoonal environment, and the coralline red algae dominated the open marine environment of the Early Miocene, whereas tidal microbial reefs dominated the Middle and Late Miocene. The Middle Miocene continuous uplift severely limited the extension of the coralline red algal facies to the deeper areas and the siliciclastic influx caused the demise of the Porites coral reefs. These Middle and Late Miocene geological conditions resulted in the expansion of the tidal microbialites at the expenses of the open marine subtidal oncoidal coralline red algae facies and the lagoonal Porites coral reefs.