Paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile Basin, Ethiopia

Abstract

Redox-sensitive elements (Mo, V, U, Re, Cd, Co, As, Sb, Tl, Ni, Cr, Mn, Fe, Pb, Cu, Zn, Se) and proxies for detrital clastic sedimentation (Al, Ti, Sc, Th) along with total organic carbon (TOC) and total sulfur (TS) were analysed to constrain the paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile (Abay) Basin, Ethiopia. These samples were collected from the deep marine Antalo Limestone (Oxfordian-Kimmeridgian), Gohatsion Formation (Bathonian) and glauconitic shale-mudstone unit (Aalenian-Toarcian). Significant variations in redox-sensitive elements concentration within and between these shales were evident for variable redox conditions. In general, the concentration of proxies for detrital clastic sedimentation increases from the Antalo Limestone to Gohatsion Formation and further to glauconitic shale-mudstone unit shales, respectively. The Antalo Limestone shales show slightly higher enrichment in redox-sensitive elements than the Gohatsion Formation and glauconitic shale-mudstone unit shales. The Antalo Limestones shales have higher TOC content than the glauconitic shale-mudstone unit shales and become very low in the Gohatsion Formation shales. The TS content in contrast, is relatively lower in the Antalo Limestone shales, intermediate in the Gohatsion Formation shales and becomes higher in the glauconitic shale-mudstone unit shales. The Antalo Limestone shales were deposited under anoxic-suboxic conditions whereas the Gohatsion Formation and glauconitic shale-mudstone unit shales were deposited under suboxic-oxic conditions. The TOC content and redox conditions of the Antalo Limestone shales implies favourable organic matter preservation and future source rock explorations in the basin should be targeted there.