An Akouemma hemisphaeria Organic Macrofossils Colony Hosting Biodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) dated at 2.2 Ga

Abstract

A colony of silico-carbonate Akouemma nodules found in Akou River sedimentary formations of the Palaeoproterozoic Okondja Basin consists of two groups, spheroidal (ovoid) nodules and elongated nodules. These nodules, which consist of two hemispheres separated by a median disc, are composed essentially of micro-quartz associated with calcite of extra-polymeric substance (EPS) type, clay minerals, organic carbon and oxides and sulphides of iron. They contain tubular microfossils, pluricellular clusters, microorganisms and vesicles, and have undergone considerable deformation by mutual lateral compression in tabular beds. They were interpreted as biogenic nodules hosting microorganisms. We provide the following additional supporting evidence: Akouemma nodules exhibit internal fibro-radial fabrics initially composed of fibres and carbon particles; the initially well-organized structures are decaying and are in particles and fragments that are dispersed in the undeformed siliceous mass. These internal fabrics are strongly highlighted by Al- K- (Ti)-rich clay minerals that are often in close association with fibres and carbon particles. They had likely a vegetative reproduction by duplication. We infer that the Akouemma nodules are macrofossils of sessile soft-plastic body organisms. These macrofossils, recently dated at 2.2 Ga, are designated “Akouemma hemisphaeria” and bring a new vision to the “large colonial organisms” found in the Franceville Basin.