Isotopic compositional characterization of siderite nodules: Implications for the reconstruction of the depositional paleoenvironment of the Takoradi Shale Formation, around Essipon, Ghana

Abstract

The late Devonian to early Carboniferous Takoradi Shale Formation (TSF) of the Sekondian Group outcrops along the western coast of Ghana is typically composed of hard, compact, black or dark grey fissile shale or sandy shale, rich in organic matter. The upper part of the formation is characterized by inclusions of large discoidal nodules of compact, siderite or clay ironstone. The geological history of this formation, however, has not been robustly established. We carried out petrographic and stable isotopic studies on these siderites in order to constrain and understand the depositional environment of the TSF. The results obtained will add to the reconstruction of the geologic history of the formation. A total of 20 samples of the siderite nodules hosted in the shales of the TSF were analyzed for their carbon and oxygen isotope compositions using the laser spectroscopy technique. The δ18O (VPDB) values of the analyzed samples range from -7.04 to 3.72, whereas δ13C (VPDB) values typically range from -6.07 to -9.4 per mil. The observed δ18O values are typical of carbonates precipitated from 18O depleted marine pore waters, suggesting that precipitation of the siderite took place under reducing conditions. The C and O plots place the nodules in the boundary between marine and continental environments, with a larger percentage being marine. The textural fabric of the siderite is marked by separate episodes of calcite veining which reflect the different stages in the diagenetic history of the nodules and the TSF concurrently. The study therefore suggests that the siderite nodules formed during early stages of diagenesis in a marine to near shore environment followed by encroachment of meteoric water. Thus, marine siderite precipitation ceased and calcite became the more stable phase occurring as late forming veins in the siderite nodules, evidenced in the textural fabrics.