Evolution of a late triassic fluvial system, NW Libya

Abstract

Fluvial systems, channel sandbodies and valley fills have been widely studied and documented in the literature, with particular attention paid to their internal structures. Although fluvial architecture and channel body geometry have been widely linked to baselevel change, tectonics and accommodation space, few studies have taken a holistic approach of additionally incorporating climatic signatures, diagenesis and width/thickness (W/T) of low net to gross fluvial systems. To remedy this situation, one of the first studies of a series of excellent fluvial exposures of the Late Triassic Abu Shaybah Formation (216.5 -199.6 Ma; c.15.9 Myrs), located along the Jabal Nafusah of northern Libya have been examined. The fluvial deposits of the Abu Shaybah Formation have been recognized to be part of an incised valley fill situated along a major strike slip fault zone, now demarked by the Jabal Nafusah escarpment. The associated architecture of the fluvial systems are identified by channel sand bodies with low W/T <10 for the valley fills, due to incision along tectonic linements and stacking along faults. In comparison passing up section away from the lateral confinement the sand bodies develop into mobile channel belts of braided and meandering stream systems with W/T >10-15. The architecture of the Abu Shaybah Formation changes up section as it becomes less influenced by tectonic activity, while climatic signatures become more marked by variables in stream discharges and sediment flux, recognized by channel aggradation and degradation of the fluvial systems. Such an occurrence has been used to help correlate the fluvial succession laterally and recognise how tectonism provides the space for and distribution of rivers while climate controls the sediment supply and facies evolution. Detailed petrographie analysis of the Abu Shaybah Formation has revealed the importance of new clay mineral growths for the identification of sequence boundaries. This research demonstrates the importance of taking a holistic approach when studying fluvial successions and has important implications for low net to gross fluvial reservoirs as a potential exploration target in the Al Kufrah and Sirte Basins.