High resolution facies analysis and sequence stratigraphy of the Siluro-Devonian succession of Al Kufrah basin (SE Libya)

Abstract

New fieldwork data collected from the flanks of Al Kufrah basin, SE Libya, provide insight into the controls on Siluro-Devonian sedimentation patterns along the eastern part of the North Gondwanan platform. A stratigraphic and sequential correlation framework from the northern and eastern flanks of the basin is presented for the Tanezzuft, Akakus, Tadrart and Binem formations. Ten facies associations are recognised, classified within three process-driven groups, facilitating high-resolution sequence analysis and highlighting key regional unconformities.The basal, shaly Tanezzuft formation is organised into a regional transgressive cycle that overlies upper Ordovician glacially-related deposits of the Mamuniyat formation. The shale grade upward into the heterogeneous Akakus formation that comprises complex deltaic thin sandy strata interbedded with silty shale and siltstones, widely influenced by storm to tide related flows. In Jabal az Zalmah (northern flank), four 3rd order transgressive–regressive (T/R) depositional sequences were identified, whilst only one in Jabal ‘Asba (eastern flank), indicating significant erosion of the Akakus due to the Caledonian deformation. Depositional cycles are made of sheltered tidal flat deposits which pass upward into lower to upper offshore heterolithic deposits. They are bounded by regional disconformities interpreted as forced regression surfaces. The Silurian Tanezzuft and Akakus formations form together an overall 2nd order regressive succession.The Caledonian unconformity is interpreted as a mega-regional sequence boundary resulting from uplift and erosion during the late Silurian into the early Devonian. The thickness variability of the Akakus formation across Al Kufrah basin is strongly indicative of differential erosion which generated a complex basin physiography. Above, the Tadrart formation of lower Devonian age forms an aggrading braided-style fluvial sandy sequence of approximately constant thickness across the basin.A return to marine conditions by the mid to late Devonian resulted in deposition of the Binem formation as a series of three stacked 3rd order T/R cycles. These deposits comprise bioturbated, tide-dominated successions which preserve some evidence for periodic open marine flooding. Lying conformably on the Tadrart formation, the Binem formation forms an overall 2nd-order transgressive trend where a maximum flooding surface occurs within the uppermost bioturbated deposits.Stratigraphic correlations made on Siluro-Devonian deposits reveal a complex sequential organisation and also account for the impact of the Caledonian erosion on pre-tectonic strata. Post-Caledonian Devonian deposits develop on a smoothed topography associated with a regional uplift and can be easily correlated over the Al Kufrah basin.