GRIND-Project: Geochemical shifts and tectonic evolution in the Nama Basin (Late Ediacaran to Early Cambrian, Nama Group, southern Namibia)

First phase of the GRIND project (Geological Research through Integrated Neoproterozoic Drilling) is focused on the Ediacaran-Cambrian Transition (GRIND-ECT) in southern Namibia. The drills are cored into Lower Kuibis and the Upper Schwarzrand subgroups. Both form part of the Nama Group. Within the mixed siliciclastic-carbonate succession several ash layers occur. According to U-Pb TIMS ages of ash layers such sequences were deposited in a timeframe of c. 555 to 538.35 Ma during the Late Ediacaran to the Early Cambrian. Six boreholes were drilled into the sedimentary sequence and cover most parts from the Kanies to the Spitskop members. Whole-rock geochemistry in selected sediments provide clues to show shifts in the plate tectonic setting, the palaeo-environment and the evolution of life. In this regard, major and trace element contents in both subgroups are generally higher than values suggested for the Upper Continental Crust, while varying with respect to the Post-Archaean Australian Shale. Ratios such as SiO₂/Al₂O₃, Al₂O₃/Na₂O or K₂O/Na₂O indicate slightly higher compositional maturity in the lower subgroup than in the upper one. Trace element ratios such as Th/Sc, La/Sc, Th/Sc, Cr/Th and La/Co indicate predominantly felsic sources. However, elements associated with mafic input such as Sc, Cr, Co are more abundant in the upper subgroup, while elements such as Zr and Hf, related to higher felsic elements from stable areas, are more frequent in the lower subgroup. Such key elements indicate a change of the tectonic evolution from a passive margin setting to a continental island arc scenario.