Thermochronometric Investigation of Multiple Unconformities and Post-depositional Thermal History of a Fault Block in the Northern Western Desert, Egypt

Abstract

Detrital apatite and zircon (U-TH)/He analysis across a recently discovered unconformity in the Western Desert of Egypt provides new insight into the tectonic evolution of northeastern Africa. The unconformity juxtaposes Cretaceous and Cambrian-Ordovician strata, and poses a potential problem for the mapping of a Jurassic hydrocarbon source rock in and near the Faghur Basin. This study utilizes detrital apatite and zircon (U-Th)/He analysis of samples collected from two boreholes in the Western Desert to elucidate the timing and magnitude of this unconformity. Data from detrital minerals further provides insight into the tectonic and thermal evolution of source terrane (ie. timing of exhumation). Detrital zircon (U-Th)/He (DZHe) ages from Cretaceous to Tertiary formations above the unconformity represent the cooling ages of source terranes with respect to the ZHe closure temperature. These ages can indicate timing of geological processes such as tectonic uplift or erosional unroofing. Statistical analysis of the DZHe ages within formations support a series of models for the influx of sediment during distinct periods of tectonic activity. The ZHe data supports two main conclusions. First, DZHe ages from below the unconformity postdate Paleozoic strata, and support Hercynian-related uplift of a fault block during the Carboniferous in the northern Western Desert of Egypt. This paleo-structural-high shed sediments into onlapping Jurassic and Early Cretaceous units and remained subaerially exposed until the Late Cretaceous. Second, ZHe ages above the unconformity predate Mesozoic-Cenozoic strata, and the Safa Sandstone and Alam El Buib (AEB) Formation contain Triassic to Jurassic detrital components, which reveal the presence of a rapidly exhumed fault block associated with Triassic-Jurassic rifting. Detrital apatite (U-Th)/He (DAHe) data is limited by the current geothermal gradient to the near surface Moghra formation, but provides a record of sediment influx from Late Cretaceous to Late Eocene compressional events of the Neotethyan margin.