Tectonic and environmental perturbations at the Permian-Triassic boundary: insights from the Blue Nile River Basin in central Ethiopia

The Blue Nile River Basin contains a thick fluvio-lacustrine sediment succession of Permian to the Jurassic age. Its evolution is linked to extensional tectonics during the break-up of Pangea in the aftermath of the Carboniferous-Permian glaciation. We collected sandstone samples from several sections in order to study the tectonic evolution and possible impacts of environmental perturbation around the Permian-Triassic boundary. Based on thin-section petrography, bulk-rock geochemistry, heavy mineral spectra, and detrital zircon U-Pb ages we are able to establish a provenance model for the Permian-Triassic basin-fill evolution. The results reveal distinct differences between Lower Permian and Upper Permian to Upper Triassic sediments. The Lower Permian sandstones are rich in feldspar, carbonate cement, and relatively unstable heavy minerals like apatite and garnet. The chemical index of alteration and trace elements suggest little chemical weathering and proximity to the source area. Upper Permian to Upper Triassic sandstones, however, contain a large amount of ultra-stable heavy minerals, and geochemical data point to intense chemical weathering, reworking, and/or recycling. In the Lower Permian, detrital zircon U-Pb age spectra are dominated by Pan-African and Tonian ages, whereas Upper Permian and Upper Triassic samples show a higher proportion of old zircons and young zircons (c. 1 Ga and c. <541 Ma) probably from intraplate magmatic rocks. The results show that during the Upper Permian and Triassic, uplift and unroofing were happening accompanied by climate change.