Distribution of Fault Controlled, Wave-Tide Dominated, Prograding Oolitic Shoals of the Miocene Carbonate-Evaporite Successions of the Ar-Rajmah Group, Al-Jabal Al-Khdar Uplift and Soluq Trough, Cyrenaica, NE Libya

摘要

This work focuses on the tectonics, depositional processes, and their relationship with the distribution of the oolitic facies along the basin strike in the Cyrenaican Miocene carbonate-evaporite ramp of NE Libya. Detailed regional facies relationships were determined from 29 field measured stratigraphic sections and 14 spectral gamma-ray profiles constructed using a hand-held gamma-ray scintillometer at 0.5 m intervals, as well as 101 readings of paleocurrent data measured from large scale planar and trough cross-bedding, herringbone cross-bedding, and the progradation direction of unidirectional oolitic clinoforms. The oolitic facies of the Cyrenaican Miocene Platform are mapped. Also, the directional paleocurrent data are plotted on the maps of the Early Miocene, Middle Miocene, and Upper Miocene rock intervals. The Ar-Rajmah Group Miocene carbonate rocks record two second-order supersequences that comprise six third-order sequences. The lower Miocene Benghazi Formation is 46 m maximum thickness, is dominated by red algal reefs and bioclastic packstones, and contains some oolitic grainstone. The Middle and Upper Miocene Wadi Al-Qattarah Formation is 26 m and 25 m maximum thicknesses, respectively, and dominated by continuous oolitic grainstones, microbialites that associated with evaporites and siliciclastics. The oolitic grainstone facies are contained within two curved faults that have roughly north-south orientation and run parallel two each other. The spacing between these two faults is 40 km in the south narrowing down to 20 km towards the north. The maps show progradation of the oolitic grainstone facies throughout the Miocene from the fault lines toward the basin center. Azimuths of the large scale planar and trough cross-bedding, herringbone crossbedding, and the unidirectional clinoforms of the oolitic grainstone facies that are between the two faults are parallel to the curved fault lines. However, the azimuths of the same sedimentary structures of the oolitic grainstone facies that are at or close to the western fault line (Lower Escarpment) are roughly perpendicular to the curved fault line. The geometrical distribution of the wave-tide dominated prograding oolitic shoals of the Cyrenaican Miocene is fault controlled. These continuous oolitic outcrops that extend along the basin strike for more than 135 km are good analogues to study for subsurface reservoirs of similar setting.