ARCHITECTURE AND SEQUENCE STRATIGRAPHY OF LATE TRIASSIC FLUVIAL SYSTEM, ABU SHAYBAH FORMATION, NW LIBYA

Abstract

The Abu Shaybah Formation (ASF) ranges in age from Carnian to Norian (Late Triassic), and is exposed from the foothill slopes of the Tarhuna-Gharyan scarp stretching westwards to Ar Rabitah and Al Khums to the eastward along the Jabal Nafusah. The lower boundary is sharp and unconformable with the Al Aziziyah Formation (marine deposits) and the upper is locally unconformable with the Abu Ghaylan Formation (marine deposits). Seven sections have been measured in the study area and the maximum thickness of Abu Shaybah Formation is about 254m, observed in the Wadi Ghan area. It consists of gravely sandstone, coarse sandstone, silty clay, and mainly mudstone facies. In the Abu Ghaylan road section, it is about 21m of mainly mudstone facies. Series of sandstone bodies, fining upward cycles, from 8 to 12m thick of cycles in fluvial meandering and 20 to 25m thickness of fluvial braided cycles. Four main facies associations can be characterized in ASF: Facies Association I: Fluvial braided, channel deposits (CH). It exhibits extensive sandstone coarse to very coarse grained, pebbly scattered and characterized by graded trough cross stratification, up to 15m thick of channels with no clay content. Facies Association 2: Fluvial meandering, its sandstone is medium to coarse grained, highly argillaceous, trough cross stratification point bar deposits overlain by 3 to 5m thick of shale (over bank and crevasse splay deposits), and isolated sand bodies lateral accretion. Facies Association 3: Flood plain and over bank sediments consist of red to green silty shale commonly mottled and content rootlet, up to 30m thick in Wadi Ghan section. Facies Association 4: It seems to be a marine transitional zone. These sediments of ASF were deposited under two strongly controlled forces (climate and tectonic), with large subaerial accommodation space rather than increases in subsidence rate in the study area. Sequence stratigraphic theory can be applied to these continental deposits, by defining base level and accommodation space, within the context of genetically related, coherent depositional sequences. Amalgamated multi-storey sand bodies are typical of low stand conditions, isolated ribbon sand bodies typify transgressive conditions (subdivided into early and late stages), and isolated multi-lateral sand bodies characterize high stand conditions.