Paleoshoreline and Prograding Clinoforms of Oolitic Grainstones of the Miocene Carbonate-Evaporitic Sequences of the Ar-Rajmah Group, Al-Jabal Al-Khdar Uplift and Soluq Trough, Cyrenaica, NE Libya

摘要

This work focuses on the relationship between the unidirectional large scale clinoforms of oolitic grainstones and the paleoshoreline along 135 km in the Cyrenaican Miocene carbonate-evaporite ramp, NE Libya. Detailed regional facies relationships were determined from 29 measured stratigraphic sections, and 14 spectral gamma-ray profiles. Seven measurements of the progradation direction of oolitic clinoforms were taken at five different locations. The ramp oolitic grainstone facies was mapped and the azimuth of the unidirectional clinoforms measured data was plotted on the maps. The Ar-Rajmah Group Miocene carbonate rocks record six 3rd order sequences. The Lower Miocene Benghazi Formation is up to 46 m thick, dominated by red algal reefs, bioclastic packstones, and contains some oolitic grainstone. The Middle and Upper Miocene Wadi Al-Qattarah Formation is up to 26 m and 25 m thick, respectively, dominated by continuous oolitic grainstones and microbialites associated with evaporites and siliciclastics. The oolitic grainstone facies which is the focus of this study is spatially restricted between two parallel curved faults that run roughly north-south. These curved faults form the lower escarpment in the west (LE-fault) and upper escarpment in the east (UE-fault) and the spacing between them is 40 km in the south and 20 km in the north. The azimuths of the large scale unidirectional clinoforms of the oolitic grainstone facies between these two faults are in opposing directions to each other, but still parallel to the curved fault lines. In contrast, the azimuths of those clinoforms at or close to the western Lower Escarpment fault line (palaeoshoreline) are roughly perpendicular to the curved fault line and prograde to the east-southeast. The clinoforms are always within a depositional sequence that starts with subtidal bioclastic wackestone/grainstone sharply overlain by ramp crest oolitic grainstone, which may or may not be capped by microbial facies. The sedimentary structures of the oolitic grainstone facies are large clinoforms that change upward into either largescale trough cross-bedding or herringbone cross-bedding. The oolitic clinoforms are of 2-4.5 m thick and increase in thickness towards the north. The types of sedimentary structures and inconsistent direction of progradation indicate that the prograding oolitic grainstone clinoforms of the Cyrenaican Miocene were controlled by wave-tide depositional processes and tectonic fault lines running parallel to the paleoshoreline.