Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon)

Abstract

The mineral paragenetic sequence of the 2.1-billion-year-old (Ga) Francevillian basin is important for understanding the diagenetic fluid history that allowed the preservation of the oldest ecosystem, including bacterial and more advanced forms of life in the FB 2 Member. However, a full characterization of the clay mineralogy of the FB 2 microbial mat-related structures (MRS) and associated host sediments (sandstones and black shales) is yet to be determined. Petrographic, microscopic, and mineralogical analyses reveal the concurrent presence of authigenic vermiform kaolinite and sudoite in the MRS and host sediments. Kaolinite formed along cleavages of altered muscovite and as pore-filling during early diagenesis, while sudoite likely precipitated at the expense of kaolinite that undergone secondary dissolution later in the diagenetic sequence. The formation of sudoite was promoted by fault-controlled acidic and oxidized brines that might have migrated during the Francevillian basin inversion. These results imply that the porosity and permeability of sedimentary rocks dominantly control the mineralogical assemblage of the FB 2 Member.

Taphonomy of early life (2.1 Ga) in the francevillian basin (Gabon): Role of organic mineral interactions

“…The Franceville Basin is filled by a series of unmetamorphosed sedimentary deposits of Palaeoproterozoic age resting unconformably on an Archean basement (Figs. 1 and 2) (Aubineau et al, 2021;Bankole et al, 2015;Bros et al, 1992;Gauthier-Lafaye, 1986;Ossa Ossa et al, 2013;Thiéblemont et al, 2014;Weber, 1968a). The Francevillian siliciclastic fluvial and marine sediments are subdivided into four major lithostratigraphic formations, FA to FD, from oldest to youngest.…”

“…2); (Gauthier-Lafaye and Weber, 1989;Mayaga-Mikolo, 1996;Thiéblemont et al, 2009;Weber, 1968b). Sea-level fall that began during the deposition of the FB1c unit and extended during the deposition of the overlying FB2 Formation caused contemporary-level to increasingly oxic conditions in the water column with transient oxygendeficient bottom waters (Aubineau et al, 2021;Canfield et al, 2013;Ossa Ossa et al, 2018). Specifically, the FB2 deposits aged 2092 ± 48 Ma (Ngwal'ghoubou Ikouanga, 2022) are composed of Poubara sandstones interbedded with thin black shale levels (FB2a unit) and 5-m thick black shales with cm-thick siltstone layers (FB2b unit) (Fig.…”

“…Lithofacies analyses indicated that the black shale deposits of FB2b were emplaced in a calm, low-energy marine environment after a large discharge of sands (Reynaud et al, 2018). The relatively oxygenated water column, together with the cyanobacterial mats, may have favoured the development and preservation of complex macroorganisms (Aubineau et al, 2021(Aubineau et al, , 2018Canfield et al, 2013;El Albani et al, 2019a;Reynaud et al, 2018).…”

“…Although the mineralogical study of Palaeoproterozoic Francevillian rocks has already been carried out in part (e.g., Aubineau et al, 2021), a systematic study such as the one presented here is unprecedented. Clay minerals, specifically illite, mixed-layer minerals (MLMs) and chlorite, have been documented within or near the fossils but not on a large scale with as many samples as in this study (more than 40 fossils + host rock samples).…”

Geodynamic seawater-sediment porewater evolution of the east central Atlantic Paleogene ocean margin revealed by U-Pb dating of sedimentary phosphates

“…The CO 3 2− content of CO 3 2− -rich fluorapatite may also serve as a good indicator of post-depositional alteration (McArthur, 1978;McClellan, 1980;McClellan and Van Kauwenbergh, 1991). Besides, the assemblage of clay minerals in sedimentary deposits, is mainly used as proxies for fluid-rock interactions, paleoclimatic, paleoenvironmental, and thermal diagenetic events in sedimentary rocks of Earth's surface (Meunier, 2005;Wilson, 2013;Aubineau et al, 2021). Therefore, the integrated study of phosphate and clay mineralogy should provide pertinent Frontiers in Earth Science frontiersin.org information on secondary weathering and diagenetic history of the Amizmiz rock sequence.…”