Sedimentology and Sequence Stratigraphy of the Middle to Late Miocene, Al-Jabal Al-Khdar Uplift and Soluq Trough, Cyrenaica NE Libya

Amrouni, Khaled S.; Pope, Micheal C.; El-Hawat, Ahmed S.

A sequence stratigraphic, chemostratigraphic and diagenetic study of the Middle to Late Miocene rocks in the Cyrenaica, northeast Libya, extends from the northwestern part of Al-Jabal Al-Khdar southwards to the Soluq trough, between 31°-33° N Latitude and 19° 30'-21° E Longitude. The sequence stratigraphic study involves determining detailed regional facies relationships from field and lab observations. The field work included measuring 25 stratigraphic sections bed-by-bed, 14 Spectral gamma-ray profiles constructed using a hand-held gamma-ray scintillometer at 0.5 m intervals, and annotating panoramic digital photomosaics. Special attention was given to identifying stratigraphic time surfaces. The lab work includes petrographic and diagenetic studies of 501 hand samples, thin sections and stable isotope (δ¹⁸O and δ¹³C) analyses. The sequence stratigraphic framework will be based on the sedimentological analysis, correlation of stratigraphic time surfaces and vertical stratigraphic sections, oxygen and carbon stable isotope profiles, and gamma-ray logs.

The Ar-Rajmah Group carbonate rocks record three parasequence sets separated by a sharp disconformity surface and maximum flooding zone respectively. The lower parasequence set is the Middle Miocene Benghazi Formation (40 m maximum thickness), the Middle and upper parasequence sets are the Late Miocene Wadi Al-Qattarah Formation (30 m and 31m maximum thicknesses respectively). The lower parasequence set is composed mainly of coral reefs, porites, red algae (rhodoliths), and open marine skeletal packstone containing large bivalves, gastropods, oysters, and echinoids. The Middle parasequence set has reworked red algae fragments at its base, shallowing upward into bioclastic grainstone, then cross-bedded oolitic grainstone. Microbialites (stromatolites, thrombolites, and laminites), and evaporites associated with pelletal mudstones, pelletal packstones, and siliciclastic fine to very fine quartz sandstone and green shale mark the upper part of this parasequence set. The siliciclastics appear only in the southern part of the field area. The upper sequence is dominated by continuous bodies of oolitic grainstone and microbialites associated with some bioclastic carbonates, red algae, and pellets. The shallowing upward parasequences range in thickness from 5 m to more than 15 m. This outcrop study of the Ar-Rajmah Group extends for more than 100 km along a dip profile and its excellent 3-D exposure makes it an analogue for ooid grainstone and microbial carbonate reservoirs in the subsurface within the Mediterranean region and globally.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013