Abstract
The taphonomy of early soft-bodied organisms in Palaeoproterozoic sediments is not yet clearly understood, even though some locations where these fossils are found present all the conditions for exceptional fossil preservation. Indeed, the degree of fossil preservation has received attention, but better knowledge of the environmental conditions and associated taphonomic processes is also essential. In the Gabonese Francevillian Basin, the discovery of macrofossils (2.1 Ga) of multicellular organisms in black shales is an outstanding example of this degree of preservation. Indeed, the biological diversity, as a wide variety of fossil morphologies are observed, is associated with two major taphonomic processes – moulding (lenticular-shaped forms) or early pyritization, while the fossil host rocks were not deeply buried and were affected only by weak to moderate diagenesis. However, usually, the mechanisms of this preservation remain difficult to assess, as the original taphonomic processes are impacted by diagenesis and still misunderstood. In this way, by closely observing fossil mineralization in four morphotypes of macrofossils and associated host rocks from mineralogical and textural points of view, this work aims to provide some keys to a taphonomic comprehension of soft-bodied organism preservation. After the deposition of dead organisms on the clayey sediment, an illitization process, which depends on the availability of dissolved K driven by bacterial activity, started from the first stages of preservation by moulding the lenticular-shaped forms and proceeded in the pores of the other macrofossils after their pyritization. At the fossil level, the intensity of illitization is controlled by the mode of preservation and the evolution of the associated permeability. In the nonpyritized lenticular-shaped specimens, illitization was not achieved, preserving I-S mixed-layer minerals, while in the pyritized forms, the illitization degree was more extensive. In comparison, I-S mixed-layer minerals are almost absent in pyritized abiotic concretions. A second process, which occurred later, consists of general chloritization from fluid circulation. Our detailed results show that each specimen behaved like a microsystem with its own physico-chemical and mineralogical evolution during preservation/diagenesis. This finding allows us to propose a conceptual model of the taphonomic history, describing the fossilization stages for each type of specimen.