The Permian-Triassic transition in the Blue Nile Basin: insights from petrography and geochemistry of sandstones

Abstract. The Permian-Triassic transition was a time of climatic, tectonic as well as ecologic reorganizations at a global scale. Clastic sedimentary successions are a key archive to study the response of earth surface processes to such extensive disruptions on land. Here, we focus on the so-called Fincha Sandstone, a Karoo-equivalent fluvio- to lacustrine succession of Permian to Triassic age, deposited in the Blue Nile Basin of central Ethiopia. We use thin-section petrography, bulk-rock geochemistry, and heavy mineral spectra in order to decipher source rocks, as well as the possible contribution of chemical weathering and recycling. The results show distinct difference between Early Permian and Late Permian to Late Triassic sandstones.

Petrographically, the Early Permian sandstones are rich in feldspar, and unstable heavy minerals like apatite and garnet. The average chemical index of alteration, trace, and rare earth element concentrations suggest little chemical weathering and relative proximity to the source area. This was controlled by a high topography, alluvial and braided systems and a semiarid climate.

In contrast, the Late Permian and Late Triassic sandstones are quartzose, have a lower content of feldspar, and show ultra-stable heavy mineral assemblages rich in zircon, rutile, and tourmaline. The chemical index of alteration is around 80 to 90 %. This can be explained by a combination of recycling of mature platform sediments together with a humid climate reflected in deltaic-lacustrine deposits. Most probably, a corrosive environment around the Permian-Triassic Boundary has further fostered high sediment maturity.

In the Middle to Late Triassic, sandstones become gradually immature again, marked by a significant increase in lithoclasts and metamorphic heavy minerals such as garnet. This trend is less visible in geochemical data because lithoclasts are fine-grained quartzose and derive from low metamorphic terrains typical for the basement of the Arabian Nubian Shield. This makes the onset of axial, NW-SE directed sediment transport through the Blue Nile Rift Basin most probable.