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The Role of the Early Diagenetic Dolomitic Concretions in The C. R. Geoscience 348 (2016) 609–618 Contents lists available at ScienceDirect Comptes Rendus Geoscience ww w.sciencedirect.com Stratigraphy, Sedimentology (Palaeoenvironment) The role of the early diagenetic dolomitic concretions in the preservation of the 2.1-Ga paleoenvironmental signal: The Paleoproterozoic of the Franceville Basin, Gabon a a, b Nathaelle Onanga Mavotchy , Abderrazak El Albani *, Alain Trentesaux , a c c Claude Fontaine , Anne-Catherine Pierson-Wickmann , Philippe Boulvais , b d e Armelle Riboulleau , Lauriss Ngombi Pemba , Florent Pambo , f Franc¸ois Gauthier-Lafaye a Institut de chimie des milieux et mate´riaux de Poitiers (IC2MP), UMR 7285 CNRS–INSU–INC, universite´ de Poitiers, 86000 Poitiers, France b Laboratoire d’oce´anologie et de ge´osciences (LOG), UMR 8187 CNRS, universite´ de Lille, 59655 Villeneuve d’Ascq cedex, France c OSUR, laboratoire de ge´osciences, UMR 6118 CNRS, universite´ de Rennes-1, 35000 Rennes, France d Universite´ des sciences et techniques de Masuku, Franceville, Gabon e Socie´te´ COMILOG, groupe ERAMET, Moanda, Gabon f Laboratoire d’hydrologie et de ge´ochimie de Strasbourg, UMR 7517 CNRS, 67084 Strasbourg, France A R T I C L E I N F O A B S T R A C T Article history: Dolomite concretions from Paleoproterozoic organic-rich sediments of the Franceville Received 26 May 2016 Basin (Francevillian) in southeastern Gabon are studied. These concretions, belonging to Accepted after revision 16 August 2016 one of the rare well-preserved Proterozoic sediments in the world, are mainly observed in Available online 27 October 2016 the shaly levels of the Francevillian B1 Formation. The concretions often show a central pyrite-rich layer. The decrease in the carbonate content from the centre to the edges, and Handled by Sylvie Bourquin the carbon isotopic signal of carbonate within the concretions indicate a concentric growth of the concretions prior to compaction and precipitation of carbonate associated Keywords: with the degradation of organic matter during early diagenesis. From its geochemical Concretions signature and texture, dolomite was not recrystallized. Moreover, the interior of the Paleoproterozoic Francevillian concretions shows well-preserved accumulations of microbial mats. These concretions Palaeoenvironment thus appear to represent an exceptional record of the biogeochemical signature of the Gabon crucial period when the oxygen rose for the first time in Earth’s atmosphere. If their rare- earth element pattern does not seem to reflect that of Paleoproterozoic seawater, they appear as potential tools for estimating the original oceanic temperature in surface sediments. ß 2016 Acade´mie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/). 1. Introduction Authigenic carbonate s.l. (calcite, dolomite) is produced in sediments during early diagenetic reactions, primarily associated with oxidation of organic matter through sulphate and iron reduction (i.e. Schrag et al., 2013), or * Corresponding author. E-mail address: [email protected] (A. El Albani). by aerobic respiration (Coleman, 1985; Gautier and http://dx.doi.org/10.1016/j.crte.2016.08.002 1631-0713/ß 2016 Acade´mie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 610 N.O. Mavotchy et al. / C. R. Geoscience 348 (2016) 609–618 13 18 Claypool, 1984). Dolomite concretions are common in 2000). In particular, stable isotope data (d C and d O) are sedimentary rocks of all geological ages (Marshall and used to address microbial activity in sediment porewater Pirrie, 2013), and are particularly present in organic- and ambient temperature of formation (McArthur et al., matter-rich sediments (Coleman, 1985; Curtis, 1977). 1986; Ritger et al., 1987). Significant data on the Indeed, bacterial oxidation of organic matter locally biogeochemical cycle of carbon in the geological past have increases the alkalinity of sediment pore water, thus been provided by studies of carbon isotope variations in favouring the growth of concretions (e.g., Bryce and limestones and dolostones (e.g., Hotinski et al., 2004; Izon Knauth, 1992; Gautier and Claypool, 1984; Hoareau et al., 2015; Veizer and Hoefs, 1976). Nevertheless, similarly et al., 2009; Hudson, 1978). to oxygen, the carbon isotopic composition of carbonates is Carbonate concretions are developed in a variety of host likely to change in response to re-crystallization during rocks, including shales and sandstones. They display deep-burial diagenesis and low-grade metamorphism (e.g., different shapes and sizes, ranging from septarian cracks Bryce and Knauth, 1992). Thus, the preservation of the in mudstones (Boles et al., 1985, Thyne and Boles, 1989), pristine isotopic composition of Precambrian carbonates is circular or ovoid shape (i.e. cannonball concretions; often questioned (e.g., Veizer, 1989). McBride et al., 2003), or sheet-like bodies (i.e. elongate In the Francevillian basin (Fig. 1), dolomites are concretions) in sediment rocks related to the influence of observed in the Francevillian B Formation (FB1; Fig. S1). groundwater movement. Carbonate concretions are com- They are mainly found in more or less organic matter-rich posed of calcite, dolomite, ankerite, siderite and rarely dolomitic layers, in dolomitic black-shale horizons or as 13 ferroan magnesite of varied chemical composition. Concre- cement of sandstones deposits. The d C values observed in tions can be produced by simple cementation in sediment these dolomites show a positive signal corresponding to pores (e.g., Curtis et al., 1986; Raiswell, 1971), by the Lomagundi-Jatuli Event (Bekker et al., 2004; Melezhik cementation in pores plus replacement of detrital grains, et al., 2004), the origin of which is generally related with or by pushing the surrounding sediment (e.g., Chowns and oxidative weathering of organic carbon-rich deposits Elkins, 1974) or rock, during early or late diagenesis. under elevated levels of atmospheric oxygen (e.g., Bekker Carbonate concretions usually show chemical changes and Holland, 2012; Canfield et al., 2013; Holland, 2002). In from the core toward the edges (Curtis et al., 1986; Gautier, the Moanda area, known for its manganiferous dolomite 1982; Irwin, 1980). These changes are explained in terms deposits, dolomitic concretions can be observed within of progressive variations in pore-water composition, silty black shales. They correspond to the unique example allowing tracing the diagenetic evolution of the pore- of dolomitic concretions observed in the Francevillian water chemistry (e.g., Mozley, 1996; Raiswell and Fisher, formations. Fig. 1. Geological setting of Franceville Basin in Gabon (after Chevallier et al., 2002). N.O. Mavotchy et al. / C. R. Geoscience 348 (2016) 609–618 611 Here, we report a multi-approach study of selected (Fig. S1), which rest unconformably on the Archean dolomitic concretions from the Francevillian B1 Formation basement rocks (Weber, 1969; Pre´at et al., 2011). The FA in Moanda area. The aims of this study are the formation is essentially formed of fluvial and deltaic understanding of the mechanism of concretionary growth. sandstone deposits; the FB formation consists of marine Furthermore, considering that concretions formed during sediments and is further subdivided in several subunits; the early diagenesis stage, the geochemical data are tested the previously reported large colonial organisms (El Albani as proxies of environmental conditions during the first et al., 2010, 2014), were collected from the FB2b black period of increased oxic conditions in the atmosphere. shales; the FC formation is dominated by dolomites and stromatolitic cherts; the FD formation corresponds to 2. Geological setting black shales. More detailed geological information is supplied as supplementary material. 2.1. The Francevillian Basin 2.2. Dolomitic concretions The Francevillian Basin (Fig. 1) is a large foreland basin 2 containing 35,000 km of unmetamorphosed and unde- The sedimentary sequence visible in the Moanda formed sedimentary rocks. Strata were deposited in an section is about 36 m in thickness (Fig. 2). It corresponds epicontinental setting and crop out in the southeastern to the middle part of the FB1 Formation (about 2.1 Ga; Republic of Gabon. The series, 1000 to 2500 m thick, are Figs. 2, S1). It is characterised by an organic matter-rich subdivided into four lithostratigraphic units, FA to FD clay–silt facies where centimetre-scale silt-sandstone Fig. 2. Lithology of black shales sequence (FB1) at the Moanda area. Location of alignments of concretion samples. 612 N.O. Mavotchy et al. / C. R. Geoscience 348 (2016) 609–618 horizons and carbonate concretions are intercalated. The the carbonate concretions disappear (Fig. S2B), whereas uppermost part of the section is characterised by a siliciclastic materials become more abundant, showing microconglomeratic channel interval. some load cast, centimetric slumping, and small The sequence begins with black shales in which synsedimentary faults characterising a high sedimenta- centimetre-scale silt–sandstone levels are intercalated tion rate. At the top of the sequence, a 2.4-m-thick as well as alignments of carbonate concretions, set microconglomeratic bed is deposited above the silty parallel to the general stratification plane (Fig. 2). The black shales. The base of this conglomeratic interval
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