Sandstones, glaciers, burrows and transgressions: The Lower Palaeozoic of Jabel az-Zalmah, Al Kufrah Basin, Libya

Abstract

Field studies in the Jabel az-Zalmah region, at the northern flank of Al Kufrah Basin provide new insights into the stratigraphy and evolving depositional environments of Cambro–Ordovician through Devonian sediments. At the base of the succession, the Hawaz Formation is about 300m thick and is characterised by frequently Skolithos-burrowed, fine to medium grained sandstones with occasional silty intervals and rare conglomerates deposited in a generally high-energy tidal shelf setting. Bioturbation is pervasive, and includes spectacular Cruziana traces produced by Early to Mid Ordovician trilobites at the bottom and the top of the Hawaz Formation respectively. These strata were incised during the late Ordovician (Hirnantian) glaciation. Ice sheets flowing over Jabel az-Zalmah cut a palaeo-topography of up to 100m relief. This relief is overlain by a coarsening upward succession of clastic sedimentary rocks, recording re-advance of an ice sheet, which overrode the uppermost sandstones. Ice sheet retreat was accompanied by marine transgression and erosion, which left a thin transgressive lag-conglomerate, capped sharply by siltstones of the Tannezuft Formation. Progradation resumed after post-glacial maximum flooding, depositing tide, storm, and river-influenced parasequences of a major delta complex (200m thick Acacus Formation). Fluvial incision, and the development of a disconformity, preceded the deposition of the Tadrart Formation (50m thick), a succession of trough-cross bedded braided river deposits. Transgression, and the re-establishment of shallow or marginal marine systems, is represented by the extensively bioturbated Binem Formation in which Zoophycos burrows offer a tentative Middle Devonian stratigraphic fix. An understanding of the lithological character of the Lower Palaeozoic strata in Jabel az-Zalmah, together with the character of unconformities that punctuate the succession, will be vital for successful hydrocarbon exploration in this frontier basin which has, until now, been unsuccessful.