SULFIDES, STABLE ISOTOPES AND OTHER DIAGENETIC FEATURES OF THE MESOZOIC CARBONATE-MARL-SHALE SUCCESSION IN NORTHERN ETHIOPIA

Abstract

This study investigates the carbonate-marl-shale succession (usually called Antalo Limestone and Agula Shale) in northern Ethiopia. The purpose of this study is to explain the diagenetic features, diagenetic environments and factors controlling them. The study is conducted by characterizing the whole rock carbon and oxygen isotopes, trace elements, mineral and textural features. Thirty samples were analyzed for carbon and oxygen isotopes, Sr and Mn; the latter two elements are chosen because of their sensitivity to diagenetic alterations. Moreover, 24 thin sections and 5 polished sections were investigated using petrographic and ore microscopes. The diagenetic features identified include sulfidation, micritization, silicification, cementation, neomorphism, pressure solution, vein development and hematitization. Oxidation rims around early diagenetic sulfides, development of clear spary calcite cement, and development dissolution features are common in the Agula Shale and lower part of Antalo Limestone. The δ13C of the Antalo Limestone ranges from -0.23‰ to 2.22‰ PDB and δ18O‰ is between -7.42‰ and -3.16‰ PDB. In the Agula Shale δ13C is between -3.25‰ and -0.12‰ and δ18O ranges from -17.07‰ to -6.48‰. The Mn/Sr ratio is higher for the Agula Shale (2.12) compared to that of the Antalo Limestone (0.35). The paleo-temperature estimation from oxygen isotope data suggested that the temperature experienced by rocks of the study area is between 56 and 26°c. These diagenetic and geochemical data suggest that the diagenetic environment varied from marine-phreatic to meteoric-phreatic environment. Furthermore, the diagenetic environment of Agula Shale is different from that of Antalo Limestone. The Agula Shale shows more influence of meteoric diagenetic fluid than Antalo Limestone does, which may be due to the early Cretaceous uplift in Eastern Africa. Neither textural (mainly abundance of framboidal pyrite and absence of open space filling sulfides) nor paleo-temperature data indicate any hydrothermal activity in the basin. Sulfides in the succession are thus the result of normal diagenetic and fossilization process in a reducing environment during sediment burial.