Stratigraphy, Lithofacies and Environmental Analysis of the Bahi Formation (Late Cretaceous) in the Northwestern Sirte Basin, Libya

The Bahi Formation forms the diachronous lower part of the Upper Cretaceous succession where it is known only from subsurface data in the western part of the Sirte Basin in north central Libya. It consist of interbedded sandstone, siltstone, conglomerate and shale. Core logs indicate that the formation overlies Palaeozoic sediments and occasionally Precambrian basement rocks with profound unconformity and is overlain by marine Upper Cretaceous sediments. It forms the basal unit of a late Cretaceous transgressive sequence which eventually affected virtually all of the country: the marine invasion was from the north or north-west. The thickness of the formation is variable and largely controlled by contemporaneous faulting. The lower part of the formation consists of fine red, silty sandstone with coarse unit at its base. The upper part is mostly with a coarse to very coarse-grained, cross-bedded sandstone and contains glauconite only in its uppermost part. Petrographically, the formation is classified as a subarkosic quartz arenite in composition with carbonate (dolomite and calcite), anhydrite and clay mineral (illite, kaolinite and chlorite) cements. Zircon, tourmaline and rutile are the most abundant heavy minerals throughout the formation. Geochemical studies of the major and trace elements indicate that K2O, Al2O3 and most of the trace elements are associated with feldspar and/or clay minerals. The relationship between the various trace elements shows that the Bahi sediment was derived largely from pre-existing sedimentary sources. The framework mineralogy and the discriminant function analyses of the major elements suggests that the Bahi Formation was derived from a craton interior and/or a passive continental margin. The mineralogy of the uppermost part indicates shallow marine deposition, probably in an oscillating shoreline situation whereas the sedimentological analyses strongly suggest that the bulk of the formation accumulated in fluvial systems which became established prior to the marine incursion