Proterozoic microbial reef complexes and associated hydrothermal mineralizations in the Banfora Cliffs, Burkina Faso

Sedimentary Geology 263–264 (2012) 144–156

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Proterozoic microbial reef complexes and associated hydrothermal mineralizations in the Banfora Cliffs, Burkina Faso J. Javier Álvaro a,⁎, Daniel Vizcaïno b a b

Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain 7, Jean-Baptiste Chardin, Maquens, 11090 Carcasssonne, France

a r t i c l e

i n f o

Article history: Received 1 February 2011 Received in revised form 3 November 2011 Accepted 30 November 2011 Available online 8 December 2011 Keywords: Stromatolite Thrombolite Chalcedony Rift Taoudeni Basin West African Craton

a b s t r a c t The Proterozoic Guena-Souroukoundinga Formation of the Mopti arm (Gourma Aulacogen, southerm Taoudeni Basin) consists of a shale-dominated succession, up to 200 m thick, with scattered microbial reef complexes. Quarry exposures of the Tiara reef complex allow reconstruction of a transect across back-reef peritidal laminites, reef margin and peri-reef ooidal shoals, and fore-reef slope strata. Microbial carbonate productivity nucleated on isolated palaeohighs during transgression, whereas its end was controlled by two tectonically induced drowning pulses that led to the successive record of onlapping kerogenous limestones and pelagic shales. Reef carbonates are crosscut by fractures and fissures occluded by hydrothermal mineralizations, which are related to the rifting activity of the Gourma Aulacogen. The Tiara reef complex is similar to other Proterozoic reefs in being composed nearly entirely of stromatolites, although calcimicrobial (filamentous) and thromboid textures are locally abundant, which contrast with their scarcity or absence in coeval stable-platform microbial reefs of the northern Taoudeni Basin. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Precambrian stromatolite reefs possess all the properties of true ecological reefs. Their reefal framework results from a combination of the intertwining growth by calcified microbial organisms, penecontemporaneous and early-diagenetic cementation, and sediment infiltration. In this context, individual microbial mats and buildups can be considered the frame-building element of stromatolite and thrombolite reefs due to their trapping, binding, and in situ calcification mechanisms (Grotzinger and James, 2000; Narbonne et al., 2000). Thrombolites are a distinctive type of microbialites characterized by a non-laminated, macroscopically clotted (or thromboid) fabric, where mesoclots are surrounded by later void-filling cement or sediment. Because of the similarity in microstructure and interpreted origin, thrombolites are currently considered intergradational with stromatolites (or laminated fabrics), in many cases forming yet another taphonomic variant (Turner et al., 1997, 2000a,b). Thrombolites have been reported from strata as old as 1.9 Ga (Kah and Grotzinger, 1992) and are still forming today (Moore and Burne, 1994). Worldwide intergradation and intergrown relationships between stromatoid and thromboid fabrics occurred in Proterozoic reefs (e.g., Turner et al., 1993, 1997; Knoll and Semikhatov, 1998; Narbonne et al., 2000). In the West African Craton, stromatoid-dominated reefs occupied large areas of the northern Taoudeni Basin, developing isolated ⁎ Corresponding author. E-mail addresses: [email protected] (J.J. Álvaro), [email protected] (D. Vizcaïno). 0037-0738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2011.11.005

reefs and barrier and fringing reef complexes (Moussine-Pouchkine and Bertrand-Sarfati, 1978; Bertrand-Sarfati and Moussine-Pouchkine, 1985, 1988; Bertrand-Sarfati et al., 1991; Kah et al., 2009). However, comparatively little attention has been paid to the isolated reef complexes that developed in the southern Taoudeni Basin, where a rifting episode (leading to the onset of the so-called Gourma Aulacogen; Reichelt, 1971; Bertrand-Sarfati and Moussine-Pouchkine, 1983a,b) took place. The patchy exposures of Proterozoic carbonates fringing the Birimian craton represent nucleation and growth of stromatoid– thromboid consortia related to the southern propagation of the Gourma rift. The aim of this paper is: (i) to document the microbial carbonate productivity and related stromatoid vs thromboid fabrics exposed in the walls of an open quarry situated in the vicinity of the Banfora Cliffs, NW Burkina Faso; and (ii) to discuss the possible link of vein and pore hydrothermal mineralizations confined to fissure-dike swarms with rift-related fracturing. This study represents a new approach for elucidation of the geodynamic factors that controlled the hydrothermal ore mineralizations in the Gourma rift system. 2. Geological setting and stratigraphy In the West African Craton, the infill of the intracratonic Taoudeni Basin is less than 3 km thick in average and 1.2–0.4 Ga old. Some linear subsiding troughs, inferred by aeromagnetic data (Bayer and Lesquer, 1978; Toft et al., 1992) and remote sensing (Simon et al., 1982), are delineated in its southern edge. The thickest sediments of the Taoudeni Basin (~8 km) occur in one of these depocentres, named the Gourma Aulacogen (Moussine-Pouchkine and Bertrand-Sarfati, 1978; Black et

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al., 1979; Bronner et al., 1980; Lesquer and Moussine-Pouchkine, 1980; Clauer et al., 1982; Cahen and Snelling, 1984; Bertrand-Sarfati and Moussine-Pouchkine, 1988). This rift displayed a triple junction of failed arms, named Gourma, Mopti and Nara troughs (Fig. 1A). The southernmost prolongation of the Mopti trough, close to the Banfora Cliffs (Fig. 1B), is characterized by a regional positive gravimetric anomaly, which may be related to a thinner crust (Rechenmann, 1965; Bronner et al., 1980). The Gourma Aulacogen opened and deepened toward an embayment of the Hoggar–Iforas belt in the craton. It is interpreted as the failed arm of a rift that preceded the 800–700 Ma Pan-African oceanization elsewhere (Bayer and Lesquer, 1978; Black et al., 1979; Lesquer and Moussine-Pouchkine, 1980; Deynoux et al., 2006). After unsuccessful ocean opening, the rift was finally closed to the East about 600 Ma by a Pan-African age collisional suture (Villeneuve and Cornée, 1994; Villeneuve, 2005, 2008; Caby et al., 2008). The opening of the Gourma rift coincided in the northern Taoudeni Basin with the widespread development of carbonate platforms. In the Adrar of Mauritania, the Atar Group forms a ca. 700 m-thick succession composed of sandy, stromatolite-bearing carbonate/shale sequences (Bertrand-Sarfati et al., 1991; Kah et al., 2009) (Fig. 2). The age of the Atar Group has been poorly constrained: Rb–Sr geochronology performed on glauconite and illite in shale intervals provided ages from 998±32 Ma to >694 Ma (Clauer et al., 1982; Clauer and Deynoux, 1987). However, recent Re–Os organic-rich sediment geochronologic analysis from drill cores of the Atar Group has recently indicated that it is ~200 Ma older (1109±22 to 1105±37 Ma), so that at least part of the carbonate

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productivity recorded in the Atar Group is definitively Mesoproterozoic and not Neoproterozoic in age (Rooney et al., 2010). Broad correlations of this episode of carbonate production can be extended into the Gourma Aulacogen (Moussine-Pouchkine and Bertrand-Sarfati, 1978; Bertrand-Sarfati and Moussine-Pouchkine, 1983a,b) (Fig. 2). In the surroundings of the Bandiagara Plateau (Gourma trough, Mali; Fig. 1A), Keita (1984) distinguished a thick (>1000 m) stromatolite-dominated carbonate unit (Irma Group) overlain by two mixed (carbonate-bearing) units, the Sarnyéré and Massi formations, the three ones grading laterally into the Dimamou Formation (Reichelt, 1971; Simon, 1979; Bertrand-Sarfati and MoussinePouchkine, 1983b). This upper Mesoproterozoic carbonate-dominated succession pinches out westward (Bové and Madina-Kouta sub-basins and Hodh-Tambaoura region; Deynoux et al., 2006) and southward, along the Mopti trough. In the latter, some scattered carbonate exposures are known in the vicinity of the Banfora Cliffs (Lajoinie, 1960; Deynoux, 1971; Marcelin and Serre, 1971; Ouedraogo, 1981), on which this paper is focused. Due to their patchy distribution, their lithostratigraphic assignment has been traditionally ambiguous: e.g., these carbonates were recognized as part of the Bobo-Dioulasso and Toun formations (Deynoux, 1971), a nomenclature subsequently changed into the Guena-Sourokoundinga, Bonvalé and SomandéniKiébani formations by Ouedraogo (1981) (Fig. 2). These carbonatebearing strata are sandwiched between two weathering-resistant packages of sandstones and conglomerates: (i) the underlying Tin, Takalédougou, and Kawara-Sindou formations that form the Banfora Cliffs; and (ii) the overlying Bandiagara Group that form the

Fig. 1. Geological maps of the West African Craton. A. Mesoproterozoic and Neoproterozoic troughs and rifting arms that form the Taoudeni Basin. B. Geological sketch of the Banfora Cliffs, in the vicinity of Bobo-Dioulasso. Panel A is modified from Villeneuve (2005). Panel B is modified from Deynoux (1971) and Ouedraogo (1981).

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Fig. 2. Lithostratigraphic chart of the Mopti and Gourma troughs (or arms) of the Gourma Aulacogen; * marks the stratigraphic setting of the Tiara quarry. After Deynoux (1971), Ouedraogo (1981) and Deynoux et al. (2006).

Bandiagara Plateau (Mali) and is related to the onset of the PanAfrican I or Bassaride orogen (ca. 660–650 Ma). 3. Materials and methods The carbonate exposures of the Guena-Souroukoundinga (or Bobo-Dioulasso) Formation in the Tiara open quarry (geographical coordinates: N11° 05.834′, W04° 33.453′) were analysed using a combination of: (i) field observations of facies to determine the arrangement of reef frame, sediment fills, and solution discontinuities, thereby identifying the history of reef development; (ii) approximately 60 samples collected from all over the reef complex, studied in polished slabs and thin sections to identify microbial textures and cements, provided a database of information about reef growth conditions and diagenetic processes; and (iii) SEM microprobe analysis to determine the nature of critical components. The qualitative mineralogical composition of the samples was determined by the X-ray powder diffraction method in the Zaragoza and Lille I Universities. Vein and pore mineralizations hosted in the Tiara reef complex were studied in rocks neighbouring and confined to fissure-dike swarms. 4. Carbonate geometries and relative timing of events Despite the widespread distribution of homogeneous shales sandwiched between the weathering-resistant Tin and Bandiagara sandstones (Fig. 2), some scattered carbonate hills, up to 20 m thick and 60 m across, were reported and mapped by Deynoux (1971), Marcelin and Serre (1971) and Ouedraogo (1981) (Fig. 1B). During three decades, these have been the target of exhaustive quarry exploitation and some of them are still exposed in a series of active and disused quarries. A fresh exposure in the walls of an active quarry situated in

the vicinity of Tiara (Fig. 3) allowed direct observation of critical facies and made possible the reconstruction of the margin of a reef complex (the complete three-dimensional reconstruction of the reef complex is precluded by quarry exploitation and the reduced exposures of laterally equivalent carbonate hills; Ouedraogo, 1981). The Tiara reef complex stands in stark relief as the surrounding shales, which butt directly into the side of the reef facies, have been preferentially weathered and eroded. Carbonate exposures of the reef complex thin from a maximum of 20 m in the quarry (with a diameter of roundabout 60 m) to 1.2 m in the pillar relics and disused quarried walls found, over a distance of 60 m, to the West. This decametre-scale carbonate lens is capped by a discontinuous kerogenous limestone bed, up to 5 cm thick and bounded by two major discontinuity surfaces (D1 and D2 in Fig. 3). The lower discontinuity (D1), which marks the top of the reef complex, represents a solution unconformity surface associated with fracturing, fissuring and hydrothermal precipitation. The downward movement of the fractured and mineralized blocks shows a maximum throw of 2 m alongside listric fault planes (Fig. 3). The upper discontinuity (D2) represents a major facies rearrangement and marks the end of carbonate production. The relative timing of processes is assessed from examination of crosscutting relationships between fractures, hydrothermal mineralization and major discontinuities (D1 and D2): (i) the hydrothermal vein network associated with fracturing crosscuts (so postdates) the reef complex growth; (ii) the subsequent fractured and mineralized reef complex was sealed by the kerogenous limestone; and (iii) the first centimetres of the overlying shales contain recognizable clast lags derived from the underlying hydrothermal veins and kerogenous limestone. Therefore, discontinuities D1 and D2 sharply separate a succession of sedimentary events that will be described below by chronological order.

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Fig. 3. Panoramic view of the Tiara open quarry showing the main depositional geometries, facies associations and hydrothermal vein network described in the text.

5. Reef complex growth Where exposed, the basal strata of the preserved reef-complex relic consist of green shales overlain by thin to medium bedded, slightly argillaceous oncoidal, peloidal, and stromatoclastic floatstone and rudstone. We will follow Turner et al.'s (2000b) subdivision of a reef complex surrounded by slopes into a reef margin (or reef belt) separating a fore-reef slope (talus and pelagic strata) and a backreef setting (including reef flats and lagoons). 5.1. Back-reef peritidal laminites They consist of light grey, horizontal, thin-bedded laminites bounded by low-relief scouring surfaces (Fig. 4A). Both laminites and interbedded peloidal wackestone display common fenestrae and crinkled (“cryptalgal”) laminae. Small, generally less than 1 mm, irregular fenestrae, parallel to lamination and lined or filled with microspar and spar, are ubiquitous. Laminites commonly display alternation of organic-rich and fenestral laminae. Despite the lack of sedimentary structures reflecting subaerial exposure, the presence of fenestrae and crinkled lamination suggests a low-energy, peritidal environment. 5.2. Peri-reef ooidal shoal complexes Light grey, ooidal grainstones to packstones, 0.2 to 1.2 m thick, form small isolated lenses, up to 4 m wide. Sedimentary structures include small to medium low-angle, laminated- to cross-stratified beds and local horizontal-parallel laminae. They show a mixture of abundant ooids and subsidiary oncoids and coated and uncoated peloids, micritic clasts exhibiting fenestral fabrics, and aggregates. Radial-fibrous ooids

are moderately sorted, variably rounded, and range from 0.2 to 2 mm in diameter; nuclei are generally peloids (Fig. 4B). Primary interparticle pores are lined with a thin isopachous rim of fibrous crystals, up to 200 μm in length, overgrown by a bladed to equant calcite cement. Well-sorted ooids, rounded oncoids, and abundance of micritic intraclasts are all consistent with a shallow-water carbonate deposition that took place in high-energy settings above fair weather wave base. That, combined with the distribution into broad belts, in some cases interfingering with the reef facies association described below, suggests that most of these deposits formed as wave-swept, ooidal-dominated subtidal sandflats. 5.3. Reef margin facies Several reef geometries are distinguishable in the walls of the Tiara quarry. They consist of light to dark grey, stromatoid, intermixed stromatoid–thromboid and cement-dominated fabrics, and can be subdivided into the following inter-grading microbial and cement, fabric types: (i) Stromatoid bioherms. They are 2–5 m wide and up to 1.2 m thick and consist of crinkled to laterally linked domes. Laminae exhibit variable inheritance from underlying substrate and consist of uniform and graded laminae of micrite and microsparite. Carbonate grains, such as ooids, peloids, and intraclasts form a substantial portion of some stromatolites (up to 30% in volume) as either disseminated grains or distinct grainy laminae. Peloids with irregular shape range from well-rounded to angular; they are poorly sorted and show grain sizes between 20 and 100 μm. A vertical succession of stromatoid morphologies is common, ranging from lower crinkled to upper domal shapes. In the latter,

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Fig. 4. Shoal-free cycles composed of basal peritidal laminites (pl) overlain by domal stromatolites, and subsequently interrupted by the onset of a proximal fore-reef slope breccia (frs); arrows mark scouring discontinuities; pen scale = 14 cm. B. Thin-section photomicrograph of ooidal grainstone showing peloidal nuclei; scale = 400 μm. C. Two individual columnar stromatolites separated by a laminated infill of peloidal packstone and micrite bearing scattered angular stromatoclasts; scale = 2 mm. D. Bundles of columnar stromatolites; scale = 5 cm. E. Thin-section photomicrograph of the previous columnar stromatolite exhibiting fenestral pores bounding organic-rich and organic-poor microbial laminae; scale = 500 μm.

inter-domal spaces are filled with micrite, peloids, ooids, aggregate grains, and stromatoclasts. As in the preceding facies association, primary interparticle and laminoid fenestral pores are lined by a thin isopachous rim of fibrous crystals, up to 200 μm in length, overgrown by a bladed to equant calcite cement. The vertical succession of stromatoid forms results in an increase in stromatolitic synoptic relief attributable to deepening of water depth due to relative rise in sea level (Petrov and Semikhatov, 2001; Kah et al., 2006). Laminoid fenestral pores appear to be of primary origin, probably resulting after microbial decomposition of organic matter. Well-laminated fenestral limestones and peloidal wackestone/packstone have abrupt vertical contacts with deeper domal stromatolites containing embedded allochems. (ii) Stromatoid–thromboid biostromes. Biostromes, less than 1.4 m thick and laterally persistent, are constructed almost entirely of closely spaced, columnar (and potentially branching) stromatolites, with a few domal stromatolites forming their basal parts. Their internal structure consists of a lower structureless part and an upper laminated part, composed of domal, columnar and/or branching stromatolites. Individual stromatolitic columns, 20–40 cm high and generally 2–12 cm in diameter, are slender and cylinder-like in shape, and show slight diverging patterns. Columns throughout the sheets are parallel with each other and can be oriented either vertically or inclined to the orientation of the biostrome. Cross-sections are circular, irregular or elliptical. Laminae are gently to steeply convex. The spaces

between individual columns vary from 1 to 8 cm and are filled with stromatoclast-rich floatstone and peloidal, oncoidal, and intraclastic wackestone and packstone (Fig. 4C–E). Textural intergradation of laminae with filamentous (calcimicrobial) and thromboid (non-filamentous or grumeux) laminar microstructures is distinct in thin section. Millimetre- to centimetre-thick, calcimicrobial laminae are composed of complexly anastomosing filaments embedded in microsparite. Microbial forms include micritic filaments and diffuse micritic streaks locally containing poorly preserved remnants of filaments (Fig. 5A). Filaments consist of non-septate threads up to 200 μm long, showing no evidence of segmentation or swellings. Bundles of micritic threads (Fig. 5B) are uniform in thickness (ca. 75 μm), branching and slightly curved, forming bushes. These upwardbranching fans, up to 500 μm, can be oriented vertically, inclined, or even parallel to the stratification plane, forming densely interwoven networks. This fabric shows lateral and vertical modifications in the density of filament packing (filament/interstitial cement ratio), the averaged orientation of bushes, and their taphonomic preservation. Grumeux or clotted micrite forms elongate to equant patches of micritic clots, with diffuse boundaries and interstitial blocky microspar (Fig. 5C–D). The clots tend to be spherical and relatively uniform in size, typically 50 to 100 μm across. They commonly form amalgamated clumps with little internal porosity. Silicified clumps are well preserved, whereas calcitic grumeux shows morphological intergradation of microbial textures, reflecting variable

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Fig. 5. Thin-section photomicrographs of stromatoid–thromboid textures. A. Radiating bundles of non-septate micritic filaments; scale = 150 μm. B. Bundles of micritic clots encrusting micritic pockets; scale = 200 μm. C. Bushes of micritic clots encrusting a flat intraclast (left) and a grumeux texture (right); scale = 200 μm. D. Grumeux texture composed of diffuse micritic clots, locally arranged in vertical and lateral bushes; scale = 300 μm. E–F. Isopachous crust surrounding a thrombolitic clast composed of radiating fans of crosscutting fibres; scale = 200 μm.

taphonomic states of microbial precursors (see, e.g. Turner et al., 2000b). Because gradation between clotted texture and distinct peloids is common, many peloids appear to be diagenetic. (iii) Encrusting cements. Some stromatoid–thromboid biostromes (and their reworked clasts) exhibit distinct acicular isopachous crusts. Flat to wavy crusts are less than 2 mm thick, and consist of single and upward stacked, subparallel to crosscutting crystal fans (Fig. 5E–F). Each fibre is straight, 2 to 5 μm across and 200 to 1500 μm long. Fibres and composite fans are straight, radiating upward and outward from stromatolite and thrombolite substrates. Their laminae are delineated by concentrations of finely dispersed organic matter, which outline accretionary surfaces perpendicular to the direction of crystal growth. Interference of fibres with other, laterally propagating fibre bundles is common as a result of competing crystal growth. Such laminated isopachous crusts are replacive rather than void-filling, contributed to reef stability, and mimic synsedimentary seafloor cements described in Archean–Proterozoic microbial reefs (e.g., Seong-Joo

and Golubic, 1999; Grotzinger and James, 2000). Their abiotic nature has been convincingly argued on the basis of observed crystal truncation, interpenetration of adjacent fibrous crystals, and their angular cross-sections. 5.4. Fore-reef slope facies associations Two facies associations occur interfingering between the reefmargin bodies described above and the homogeneous green shales that surround and onlap the reef complex: (i) Proximal fore-reef slope. The walls of the Tiara quarry exhibit bedded alternations of ooidal shoals and bioherms that are interrupted and pass laterally into reddish, irregularly bedded, intraclastic rudstone and grainstone beds. These strata, up to 1.6 m thick, have sharp scouring bases and chaotic intraclastic arrangements (Fig. 6A). Clasts are poorly sorted, angular to subrounded, coarse grained and contain a mixture of allochems

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dominated by peloids, stromatoclasts, aggregates, oncoids, ooids, and intraclasts, many of which are partly or wholly dolomitized (Fig. 6B). Oncoids are irregular or round in shape. On the weathered surface of limestones, laminated microbial coatings are light-coloured and their nucleus is generally dark-coloured. Nuclei include fragments of stromatolite reef limestones. The outlines of oncoids generally follow the shapes of the nuclei, and

reach up to 4 cm in diameter. Coatings are irregular, lumpy, and crinkly. Locally, the coatings bind grains into lumps. Close to the boundary between the enveloping stromatolites and intraclasts, the clasts show cracks lined with calcite cements. Microbial activity in the reef slope is well preserved and occurs as microstromatolitic (crinkled to domal) crusts in interparticle pores. Growth of cryptic microbial mats and biofilms in primary sedimentary pores

Fig. 6. Non-reef facies and microfacies. A. Fore-reef amalgamation of irregularly bedded rudstone, grainstone and breccia beds; arrows mark scouring discontinuities. B. Thin-section photomicrograph showing the irregular contact between an ooidal grainstone and a peloidal-intraclastic packstone; scale = 1 mm. C–D. Cryptic microbial coatings encrusting the walls of interparticle and intraparticle cavities and pores, finally occluded with bladed and equant calcite cements; scales = 500 μm and 200 μm, respectively. E. Detail of cracked intraclasts; scale = 500 μm. F. Field aspect of discontinuities D1 and D2 in the northern walls of the Tiara quarry; darker beds correspond to the kerogenous limestone; sh: overlying shales; scale = 1 m. G. Thin-section photomicrograph of the kerogenous limestone displaying interbedded shale slumped layers; scale = 500 μm.

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and cavities is a well-known process in Proterozoic–Cambrian microbial and microbial-archaeocyathan reefs (Zhuravlev and Wood, 1995; Vennin et al., 2003; Álvaro and Clausen, 2006, 2010; Álvaro et al., 2006a,b). This suggests the lack of photosynthetic micro-organisms in these coelobiontic microbial associations. Where proximal fore-reef slope strata interfinger with reefmargin bodies, early-diagenetic cementation includes isopachous and equant sparry calcite cements. Isopachous rim cements grow radially from the clast surfaces (Fig. 6C–E). They consist of bladed crystals, about 150 μm in length. Sparry calcite cement commonly overlies the isopachous rim cement and occludes the remaining interparticular pores. It occurs as a drusy mosaic of equant, subhedral to anhedral crystals, 15–300 μm in size. Granular sparry calcite cement (anhedral to subhedral crystals, 50–100 μm in size) occurs in geopetal structures, which are randomly disposed. Radially from the quarry centre, cements progressively disappear and claystone content increases. These layers occur in both sheet and scouring forms, and are interpreted to represent proximal downslope deposits transported by gravity flows and reworked by bottom currents. Proximal forereef slope facies contain poorly-winnowed mixtures of peritidal and reef-margin sediment types, suggesting that sediments were really washed over the margin and deposited in similar highenergy conditions. The occurrence of centimetre- to decimetresized stromatoclasts testifies to the synsedimentary lithification of rigid reef frameworks in high-energy, erosive depositional environments. Grapestone formation implies that allochems were cemented in thin sheets and later reworked, a process that is limited to environments with intermittent bottom stability and low amounts of carbonate mud (Batten et al., 2004). The wellrounded shape of clasts and their cracked aspect reflect the semiconsolidated condition of the substrate during clast reworking. Cracks formed probably as a result of differential lithification patterns. The poorly consolidated clasts were also overgrown by early-hardening stromatolitic crusts. (ii) Distal fore-reef slope. Strata consist of decimetre-to-centimetre alternations of crudely bedded, matrix- to clast-supported, intraclastic rudstone/floatstone, grading laterally into poorly sorted wackestone and shale. Clasts, ranging from proximal pebble to distal granules (angular the former and rounded the latter), are chaotically oriented, poorly sorted, and locally arranged in slumped beds punctuated by scouring contacts. Distal granules are draped by bedded shales, which also occur in the interstices among clasts. This heterolithic slope-apron forms a belt (at least 20 m wide in the preserved pillar relics surrounding the Tiara quarry) of reefderived material. The apron tapers distally into wedges of reef detritus that interfinger with homogeneous shales. Downslope transport and reworking in the preserved slope-apron are evidenced by local breccia and mass wasting, soft sediment deformation, and superposed intraformational erosive surfaces. 5.5. Reef-margin cycle stacking patterns As stated above, the Tiara quarry does not preserve a complete reef complex, due to recent quarry activity. Although its complete 3D reconstruction and total size cannot be estimated, the quarry preserves a transect across back-reef peritidal laminites, reef margin and peri-reef ooidal shoals, and fore-reef slope strata forming a slope-apron belt, at least 20 m wide. Their facies associations are vertically stacked into two kinds of repetitive cycles, 0.4–2.8 m thick and bounded by scouring surfaces (Fig. 3). Cycles are distinguished based on the presence/absence of ooidal shoals: (i) shoal-free cycles consist of thin-bedded, peritidal laminites capped by crinkled stromatolites that grade vertically into domal and columnar morphologies (Fig. 4A); and (ii) shoal-bearing cycles

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show ooidal-dominant grainstones sandwiched between thin-bedded fenestral laminites and columnar stromatolites. The latter also display common interruptions of the idealized cycle by proximal fore-reef slope wedges bounded by scouring surfaces. Distal slope facies associations are non-cyclic intervals. Stromatolite boundstones are stacked in decimetre-thick packages that show increased synoptic relief of stromatolites and represent amalgamated, deepening-upward cycles ranging from high-energy shallow-water substrates (laterally flanked by ooidal shoals) into deeper, quiet-water settings. Stromatolite formation probably occurred through biomediated combination of chemical precipitation of mineral crusts and early burial cementation, as well as mechanical sedimentation with trapping, binding and/or rapid cementation of granular sediments at the sediment/water interface. Conspicuous marine cements and microbialite-derived intraclasts suggest that water movement affected reef growth at most stages. Accumulation of terrigenous sediments on microbial growth surfaces was insignificant in the reef microbialites. The vertical distribution of stromatolite morphologies within the reef facies shows that most depositional cycles resulted from autocyclic processes, although scouring discontinuities and reefgrowth interruption by sharp changes in accommodation space are evidenced by the episodic interbedding of fore-reef slope deposits. Neither the stromatolite boundstones nor the grainstone shoals formed continuous reef or shoal-rimmed margins in a palaeohigh, surrounded by the shales of the Guena-Souroukoundinga Formation, but a complex arrangement of small bioherms and biostromes interbedded with ooidal grainstone shoals. Cyclic repetition of these facies indicates that autocyclic processes coeval with sea-level fluctuations and tectonic instability contributed to periodic shedding of shallow-marine debris to the upper slope, followed by the rebuilding of shoals and reefs at the reef margin. 6. Unconformity surfaces and capping facies The reef-complex relic described above is capped by a discontinuous kerogenous limestone bed that is bounded by the two major discontinuity surfaces introduced above: D1 (a solution unconformity that seals a mineralized and fissured reef complex) and D2 (Fig. 3). 6.1. Fissure swarm and hydrothermal mineralization Underlying D1, there is a network of dikes and fissures crosscutting the reef complex that show no preferential orientations. Veins exhibit common variations in their strike and dip, thickness (b20 cm), and a tendency to split to offshoots. The richest ore mineralization occurs in offshoots branching from the main vertical faults or at fracture intersections (Figs. 3 and 7A–D). Veins show common crosscutting relationships and their stepwise fill shows a simple bilateral symmetry: (i) a first generation dominated by olive-green, impure chalcedony, which precipitated on the walls of the fissures and fractures; and (ii) a whitish to brownish crystallization of pure chalcedony and quartz. Electron microprobe investigations, confirmed by XRD analysis, were conducted to characterize the silica mineralogy. Analyses from veins and disseminations define a silica mineral controlled by impurities (MnO: 8.5–33.9 wt.%, Fe2O3: 1.4–2.6 wt.%, MgO: 0.6–1.9 wt.%, BaO: 0.4–2.0 wt.%, and CaOb 0.4 wt.%). Subsidiary minerals are talc (probably a byproduct of hydrothermal reaction of dolomite and silica), biotite and its alteration byproduct (vermiculite), and anatase. Mn and Fe oxides are partly corroded and replaced by silica, which also occluded the remaining porosity of solution cavities and fissures. Silica is characterized by drusy megaquartz and microquartz, and by isopachous and botryoid chalcedonic fans. The fans, up to ~100 μm in diameter, show a sweeping extinction and radiate inward meeting along planar to gently curved interfaces indicating competitive and inward directed growth. The presence of chalcedonic rims lining the fissure walls suggests that silica precipitated directly into open spaces. A distinct silicification

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Fig. 7. Dike and fissure hydrothermal mineralizations. A–B. Thin-section photomicrograph with cross Nichols showing a stepwise pattern of silicification, in which a silicified oncoidal packstone (on) was fractured twice: the first phase (1st) was occluded with microquartz (mQ) (walls are arrowed) and the second one (2nd) with isopachous crusts and botryoid chalcedonic fans; scales= 1 mm and 500 μm, respectively. C. BSE photomicrograph of a chalcedonic infill (ch; walls are arrowed) crosscutting a microquartz replacement (mQ) where the concentration of MnO impurities (ox) is constrained; scale= 100 μm. D. BSE photomicrograph of silicified intraclasts embedded in a microquartz (mQ) matrix with marked concentration of MnO impurities (ox) and growth of fibrous silica crystals (fs) in remaining intraparticle pores; scale= 50 μm. E. Northwestern wall of the Tiara quarry showing discontinuity D1 (marked) and onlapping hemipelagic shales (sh). F. Block marked by boxed area in previous figure and embedded in the hemipelagic shales, reworked from the underlying mineralized reef complex, exhibiting a greenish silicified dike (si) separating an altered carbonate (talc — ta) and a Fe-rich dolostone (Fe-do); scale = 10 cm.

front extends down along fissures and the top of the reef complex, and forms a discontinuous rim following the inherited palaeorelief rich in white and rusty-brown, saccharoidal chert, which marks the outline of D1. The orientation of both veins and fissures, roughly perpendicular to bedding, and the presence of matrix-supported breccia at their bases, suggest that the opening of these fissures was due to hydraulic fracturing as a tensile response to movements of the related faults. Veins formed where the fluids flowed through larger, open space fractures and precipitated mineralization along the walls of fissures, eventually occluding them completely. The distribution pattern can be attributed to the discrete and episodic formation and reactivation of fissures in different sectors of the fissure–dike swarm. Each stage of reactivation of

fissures, or the opening of new ones, was accompanied by the input of new portions of hydrothermal solutions. Fracture development controlled hydrothermal mineralization replacement formed from fluids that were delivered from flow through a vertical, or near vertical, fault-conduit system. Examination of crosscutting relationships between the chalcedony veins and the host rock shows that the onset of the hydrothermal vein network postdates the reef complex, so stromatolites were not contemporaneous with the hydrothermal exhalation of epithermal veins. The presence of low-temperature hydrothermal minerals, such as chalcedony, suggests a range of maximum temperatures from 300 to 150 °C (Guidi et al., 1990). Fournier (1985) pointed out that the precipitation of chalcedony in hydrothermal environments requires

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extreme supersaturation of silica (in our case study, after complete depletion of manganese) and rapid decompression of hydrothermal fluids. Manganese impurities would be derived from mafic volcanic source rocks that occur in the neighbouring Houndé Greenstone Belt (Birimian craton; Béziat et al., 2008), which must represent the basement of the Mesoproterozoic Mopti arm infill. 6.2. Solution unconformity surface (D1) The discontinuity is easily identified by a distinct contrast in both colour and composition between: (i) the lower light-cream, partly silicified reef-complex relic crosscut by a fissure swarm occluded by olive-green ore mineralizations; and (ii) the overlying dark-bluish limestones and homogeneous green shales described below (Fig. 6F). D1 shows evidence of solution features, due to the local occurrence of decimetrescale sinkholes and centimetre-sized solution cavities and vuggy pores. The preserved sinkholes are enlarged at the contact with faults and fissures (Fig. 3), and do not occur in the surrounding slope wedge. Vuggy porosity consists of millimetre- to centimetre-sized, sparse and/or interconnected irregular voids. They are locally filled with a microbreccia composed of angular clasts derived from the host limestone and the mineralized veins and green shale. Vuggy intervals are not associated with bedding planes or significant vertical facies change and, consequently, their origin is attributed to dissolution operating at discrete levels where dissolution was non-selective. Concentration of vuggy pores has developed “spongy” fabrics, which consist of a network of interconnected pores, generally less than 2 cm across, with typically sharp smooth to ragged walls. There, the amount of dissolution porosity ranges between 15 and 40% in volume. Despite the presence of karstic features, the local character of the dissolution discontinuities and cavities, and their association with fractures and veins (Fig. 3), do not support a subaerial exposure, although this process is not incompatible with other ones. Other possibilities can be invoked to explain undersaturation (the prerequisite of dissolution) on a carbonate seafloor, under subaqueous conditions and hydrothermal influence (Stefano di and Mindszenty, 2000; Álvaro and Subías, 2011): (i) contact (by mixing) with submarine discharge of waters of chemical composition different from that of the surrounding seawater (submarine cold seeps and discharge of fresh-water and brine); (ii) contact with deeper and cold, undersaturated waters; and (iii) contact with ascending hydrothermal solutions, which on cooling may become undersaturated. In our case study, infill of dissolution cavities was repeatedly interrupted by faulting and fissuring, and the onset of epithermal vein mineralization. Water undersaturation may have been the result of rapid fault-controlled subsidence of the seafloor to depths where temperature was sufficiently low to bring about dissolution. Tensional fractures and fissures are also generally supposed to serve as conducting channel-ways for the ascension of hydrothermal solutions that would affect the undersaturation of the surrounding waters. 6.3. The sandwiched kerogenous limestone Onlapping the solution unconformity surface that seals the fissure– vein swarm (D1), there is a dark, bluish grey, laminated, kerogenous limestone (total organic content or TOC up to 20%), up to 50 cm thick (Fig. 3). The limestone contains millimetre-thick, dark grey, calcareous claystone interbeds, is discontinuous, and directly onlaps the inherited solution palaeorelief in the northern walls of the Tiara quarry (Fig. 6F– G). It is characterized by internal scouring surfaces capped by graded, intraformational packstone to wackestone rich in kerogenous muddy and shale clasts. Internal laminae are locally contorted and show soft deformation: folded angular clasts, centimetre-scale slumps, and discrete intraformational scouring discontinuities locally disturb the lamination. The kerogenous limestone interbed, sandwiched between the solution unconformity surface (D1) and the onlapping hemipelagic

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shales that characterize the Guena-Souroukoundinga Formation, was deposited on a slope to toe-of-slope setting that recorded gravityrelated mass movements. The occurrence of the bed on the northern walls of the quarry and the high kerogenous content suggest a sharp depositional event derived from a northern provenance. The facies is similar to some of the facies associations recognized in the Gourma arm of the Gourma Aulacogen, where the Dimamou Formation has recorded a succession of tilting and slope-related deposition of kerogenous limestones (Bertrand-Sarfati and Moussine-Pouchkine, 1983b). Its presence in the Tiara quarry would represent the maximum palaeogeographic distribution of the Dimamou Formation facies in the Mopti trough. 6.4. Drowning unconformity (D2) and onlapping hemipelagic shales Unconformity D2 marks the end of carbonate production and accumulation and is overlain by the greenish shales of the GuenaSouroukoundinga Formation. Several centimetres below D2, the limestone rock is characterized by pervasive dolomitization and silicification (Fig. 3). The first centimetres of the overlying shales also contain recognizable clasts derived from the underlying hydrothermal veins and the kerogenous limestone (Fig. 7E–F). The shale is thin-bedded and poorly laminated and encloses and onlaps the aforementioned carbonate facies associations. It therefore represents the background sediment preceding and post-dating the interval of carbonate production. The shale lacks physical sedimentary structures and is interpreted as mainly hemipelagic clay, in which storm wave base rarely impinged on the seafloor. Development of this drowning surface was followed by a rapid rise in relative sea level that definitively outpaced carbonate productivity. The sea-level rise most likely was probably caused by a combination of increased rates of tectonic subsidence and eustatic sea-level rise. 7. Palaeogeographic context in a rifting scenario The sedimentary architecture and aerial distribution of framebuilding carbonates in the Tiara quarry, and the partial preservation of laterally equivalent fore-reef slope and basin strata, suggest nucleation and development of a reef complex fringing a palaeohigh (Fig. 8). Although stromatoclasts occur embedded in the slope-apron facies, large-scale platform-margin collapse events were not recorded. Therefore, a steep escarpment margin cannot be invoked because of the absence of decimetre-sized lithified reef blocks in the fore-reef deposits that form the slope-apron belt. Microbial reefs nucleated on the edge and top of topographic highs surrounded by homogeneous hemipelagic shales. Their growth started over seafloor bumps provided by oncoidal and rip-up shaly clasts. The abundance of oncoids suggests that microbial coating played a primary role in the first episodes of substrate stabilization. Subsequent development of stromatolites and thombolites was restricted to the horst margin, which graded downward into fore-reef slope and basinal facies, laterally into well-sorted, high-energy carbonate shoals, and backward into peritidal laminites. Pervasive early cementation played an important role in lithification of marginal reef framework, grainstones, and rudstones to form rigid waveresistant margins. The paucity of clays in the reef facies suggests deposition of microbial boundstone in relatively clear water. Only the reef margin and the proximal fore-margin slope of the reef complex is preserved at Tiara, which precludes the reconstruction of the real extension of the postulated isolated horst or uplifted half-graben shoulder. The end of reef growth and final demise of carbonate productivity were related to sharp extensional pulses. Discontinuities D1 and D2 represent two drowning pulses that modified the depositional profile of the palaeohigh host and its related reef complex. D1, also related to fracturing (with displacement of marker beds up to 2 m deep), fissuring and hydrothermal mineralization, led to a short-term record of

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Fig. 8. Tentative sketch of a possible scenario at the Tiara reef-complex relic described in the text, representing the margin of an isolated horst or a half-graben shoulder; mineralization vein network, not represented here, postdates this palaeoenvironmental reconstruction and affected all the sedimentary bodies.

distal slope-related sediments sourced from the North. The high organic matter content of the kerogenous limestone can be attributed to the input of nutrient rich, deep waters that stimulated high phytoplankton productivity and a direct connection with the Gourma arm, where similar facies occur in the Dimamou Formation (BertrandSarfati and Moussine-Pouchkine, 1983b). D2 caused a complete shutdown of carbonate production: drowning led to sudden and substantial deepening and tilting of the platform. The series of events described above can be tied to the palaeogeographic evolution of the Mopti arm in a rifting scenario. In the southern Mopti arm, carbonate production and associated nucleation and development of microbial reefs were primarily influenced by tectonic activity. They were controlled by localized tilted fault-controlled palaeohighs, in association with the margins of fault blocks formed during successive pulses of rift extensional perturbations accompanied by hydrothermal activity. The final demise of carbonate production appears to coincide with another episode of extensional tectonics. As a result, the hemipelagic shales that dominate the Guena-Souroukoundinga Formation deposited in grabens, while the carbonate reef complexes occurred on the shoulders and flanks of tilted fault blocks primarily controlled by the evolution of the Gourma rift.

presently preclude useful comparisons with the Irma Formation of the Gourma arm (Fig. 2). Microbial frameworks of the Gourma Aulacogen contrast with the virtual absence of thromboid fabrics in coeval stable-platform microbial reefs of the northern Taoudeni Basin. In the Atar of Mauritania, Bertrand-Sarfati (1972a) and Bertrand-Sarfati and Moussine-Pouchkine (1985, 1988, 1999) described common packages of laminated and structureless micrite, stromatolite reefs, and calcareous siltstones. The reported broad energy levels were low (due to the abundance of carbonate mud and absence of sandstones) with sporadic, higher energy levels in a flat intracratonic basin (Bertrand-Sarfati and Moussine-Pouchkine, 1988). Recently, Kah et al. (2009) re-interpreted some of these stromatolite carbonates suggesting that the reefs did not form regional hydrodynamic barriers to wave or current energy. Only in the northeasternmost exposures of the Atar Group (Hank Group in Algeria), near the tectonic edge of the West African Craton, reef margins dropped steeply at their edges. The resultant platform geometry would be represented by stromatolitedominated intra-platform ramps separated by rimmed depressions 15–20 m deeper than the remaining platform (Bertrand-Sarfati and Moussine-Pouchkine, 1992; Kah et al., 2009). Unfortunately, we have no information about the possible presence of thromboid fabrics throughout the northeastern edge of the Taoudeni Basin.

8. Comparison with other reef complexes from the Taoudeni Basin The Tiara reef complex shows a variety of characters that can be compared, in terms of components, facies succession and palaeogeographic setting, with microbial reefs and reef complexes known from the neighbouring northern Taoudeni Basin (Atar Group; Bertrand-Sarfati, 1972a,b, 1983; Bertrand-Sarfati and Trompette, 1976; Bertrand-Sarfati and Moussine-Pouchkine, 1985, 1988, 1992, 1999; Kah et al., 2009) and the Gourma arm of the homonymous rift (Reichelt, 1971; Simon, 1979; Bertrand-Sarfati and Moussine-Pouchkine, 1983a,b; Keita, 1984). The Tiara reef framework lacks significant quantities of lime mud and is composed nearly entirely of stromatolites, although calcimicrobial (filamentous) and thromboid textures are locally abundant. The reef complex also represents the margin of an active palaeohigh fringed by a belt of breccias. A similar tectonic activity, responsible for the onset of large-scale slumps and olistostromes, and deposition of kerogenous limestones, has been recorded in the Gourma arm (Dimamou Formation; Bertrand-Sarfati and Moussine-Pouchkine, 1983b) (Fig. 2). Peritidal carbonates of the Sarnyéré Formation in the Gourma arm display a wide diversity of thromboid fabrics, described by Bertrand-Sarfati and Moussine-Pouchkine (1983a) as “porostroma boundstones”, “orbicular crusts” and “micropopcorn micrite formed by arborescent encrustements of dark micrite”. Unfortunately, the extent of dolomitization and the subsurface setting

9. Conclusions During the Mesoproterozoic, the southern Taoudeni Basin recorded a rifting episode characterized by the onset of a triple-junction rift: the Gourma rift. Whereas the Adrar trough (Atar Group) of the northern Taoudeni Basin represents the establishment of a carbonate platform with development of thick and extensive, microbial reef complexes, the Mopti arm of the Gourma Aulacogen only recorded scattered carbonate reef complexes. These represent the onset of spotted centres of carbonate productivity on the top of palaeohighs in a rifting arm, otherwise dominated by transgressive hemipelagic shale sedimentation. Quarry exposures of the Tiara reef complex show a transect across back-reef peritidal laminites, reef margin and peri-reef ooidal shoals, and fore-reef slope strata. The Tiara reef complex displays a distinctive internal architecture that records the interaction between microbial growth (both stromatoid and thromboid fabrics are conspicuous), and sharp changes in accommodation space primarily controlled by extensional tectonic pulses. The reef complex is crosscut by fractures and fissures occluded by hydrothermal mineralizations dominated by Mn-rich chalcedony. The end of carbonate production was controlled by drowning pulses, which led to the successive record of onlapping kerogenous limestones and pelagic shales.

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Hydrothermal manganese ores occur as epithermal veins crosscutting the reef complex, and were controlled by fissuring and fracturing processes. The formation of parallel-layered chalcedony bands is related to the formation of and reactivation of fissures. These served as conduits for the ascending migration of silica solutions that mixed with colder seawater during the ascending shallow stages of their circulation and progressively depleted in Mn, Fe, Mg, and Ba. The source of the latter would be located in the underlying Houndé Greenstone Belt (Birimian craton).

Acknowledgements Financial support for this study was provided by project CGL 201019491 from Spanish MICINN and EU-FEDER. An early manuscript benefited from peer and constructive reviews by two anonymous referees. Blanca Bauluz (Zaragoza University) and Philippe Recourt and Sébastien Clausen (Lille University) are thanked by XRD analysis and interpretation.

References Álvaro, J.J., Subías, I., 2011. Interplay of phosphogenesis and hydrothermalism in the latest Ediacaran rift of the High Atlas. Morocco. J. Afr. Earth Sci. 59, 51–60. Álvaro, J.J., Clausen, S., 2006. Microbial crusts as indicators of stratigraphic diastems in the Cambrian Micmacca Breccia, Moroccan Atlas. Sediment. Geol. 185, 255–265. Álvaro, J.J., Clausen, S., 2010. Morphology and ultrastructure of epilithic versus cryptic, microbial growth in lower Cambrian phosphorites from the Montagne Noire, France. Geobiology 8, 89–100. Álvaro, J.J., Clausen, S., El Albani, A., Chellai, E.H., 2006a. Facies distribution of LowerCambrian cryptic microbial and epibenthic archaeocyathan-microbial communities in the western Anti-Atlas, Morocco. Sedimentology 53, 35–53. Álvaro, J.J., Ezzouhairi, H., Vennin, E., Ribeiro, M.L., Clausen, S., Charif, A., Ait-Ayad, N., Moreira, M.E., 2006b. The Early-Cambrian Boho volcano of the El Graara massif, Morocco: petrology, geodynamic setting and coeval sedimentation. J. Afr. Earth Sci. 44, 396–410. Batten, K.L., Narbonne, G.M., James, N.P., 2004. Paleoenvironments and growth of early Neoproterozoic calcimicrobial reefs: platformal Little Dal Group, Northwestern Canada. Precambr. Res. 133, 249–269. Bayer, R., Lesquer, A., 1978. Les anomalies gravimétriques de la bordure orientale du craton Ouest Africain: géométrie d'une suture panafricaine. Bull. Soc. géol. Fr. 20, 869–876 (6e sér.). Bertrand-Sarfati, J., 1972a. Stromatolites columnaires du Précambrien supérieur du Sahara nordoccidental. Centre Recherche Zones Arides. Sér. Géol. 14, 1–245. Bertrand-Sarfati, J., 1972b. Paléoécologie de certains stromatolites en ´recifs des formations du Précambrien supérieur du groupe d'Atar (Mauritanie, Sahara occidental): création d'espèces nouvelles de ces en récifs. Palaeogeogr., Palaeoclimat., Palaeoecol. 11, 33–63. Bertrand-Sarfati, J., 1983. Les stromatolites anciens, mécanismes de croissance, rôle des micro-organismes et de l'environnement. J. Rech. Ocean. 8, 71–89. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1983a. Pedogenetic and diagenetic fabrics in the Upper Proterozoic Sarnyéré Formation (Gourma, Mali). Precambr. Res. 20, 225–242. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1983b. Platform-to-basin facies evolution: the carbonates of Late Proterozoic (Vendian) Gourma (West Africa). J. Sediment. Petrol. 53, 275–293. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1985. Evolution and environmental conditions of the Conophyton associations in the Atar Dolomite (Upper Proterozoic, Mauritania). Precambr. Res. 29, 207–234. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1988. Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African Craton. Sediment. Geol. 58, 255–276. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1992. Formation et comblement d'une dépression intraplateforme engendrée par la croissance d'un biostrome stromatolitique, Protérozoïque supérieur. Sahara algérien. C. R. Acad. Sci. Montrouge 315, 837–843. Bertrand-Sarfati, J., Moussine-Pouchkine, A., 1999. Mauritanian microbial buildups: Meso-Neoproterozoic stromatolites and their environment; six days field trip on the Mauritanian Adrar. Assoc. Sediment. Fr. 31, 1–103. Bertrand-Sarfati, J., Trompette, R., 1976. Use of stromatolites for intrabasinal correlation: example from the late Proterozoic of the northwestern margin of the Tadoudeni Basin. In: Walter, M.R. (Ed.), Stromatolites. Elsevier, Amsterdam, pp. 517–522. Bertrand-Sarfati, J., Moussine-Pouchkine, A., Affaton, P., Trompette, R., Bellion, Y., 1991. Cover sequences of the West African Craton. In: Dallmeyer, R.D., Lécorché, J.P. (Eds.), The West African Orogens and Circum-Atlantic Correlatives. SpringerVerlag, Berlin, pp. 65–82. Béziat, D., Dubois, M., Debat, P., Nikiéma, S., Salvi, S., Tollon, F., 2008. Gold metallogeny in the Birimian craton of Burkina Faso (West Africa). J. Afr. Earth Sci. 50, 215–233.

155

Black, R., Caby, R., Moussine-Pouchkine, A., Bayer, R., Bertrand, J.M., Boullier, A.M., Frabre, J., Lesquer, A., 1979. Evidence for late Precambrian plate tectonics in West Africa. Nature 278, 223–227. Bronner, G., Roussel, J., Trompette, R., Clauer, N., 1980. Genesis and geodynamic evolution of the Taoudeni cratonic basin (Upper Precambrian and Paleozoic), West Africa. In: Bally, A.W. (Ed.), Geodynamics of Plate Interiors: Am. Geophys. Union, Geodyn. Ser., 1, pp. 81–90. Caby, R., Buscail, F., Dembélé, D., Diakité, S., Sacko, S., Bal, M., 2008. Neoproterozoic garnet-glaucophanites and eclogites: new insights for subduction metamorphism of the Gourma fold and thrust belt (eastern Mali). In: Ennih, N., Liégéois, J.P. (Eds.), The Boundaries of the West African Craton: Geol. Soc., London, Spec. Publ, 297, pp. 203–216. Cahen, L., Snelling, N.J., 1984. The Geochronology and Evolution of Africa. Clarendon, Oxford, pp. 290–385. Clauer, N., Deynoux, M., 1987. New information on the probable isotopic age of the Late Proterozoic glaciation in West Africa. Precambr. Res. 37, 89–94. Clauer, N., Caby, R., Jeanette, D., Trompette, R., 1982. Geochronology of sedimentary and metasedimentary Precambrian rocks of the West African Craton. Precambr. Res. 18, 53–71. Deynoux, M., 1971. Essai de synthèse stratigraphique du bassin de Taoudéni (Précambrien supérieur et Paléozoïque d'Afrique occidentale). Trav. Lab. Sci. Terre St. Jérôme, Marseille (B) 3, 1–71. Deynoux, M., Affaton, P., Trompette, R., Villeneuve, M., 2006. Pan-African tectonic evolution and glacial events registered in Neoproterozoic to Cambrian cratonic and foreland basins of West Africa. J. Afr. Earth Sci. 46, 397–426. Fournier, R.O., 1985. The behaviour of silica in hydrothermal solutions. In: Berger, B.R., Bethke, P.M. (Eds.), Geology and Geochemistry of Epithermal Systems: Rev. Econ. Geol., 2, pp. 45–61. Grotzinger, J.P., James, N.P., 2000. Precambrian carbonates: evolution of understanding. In: Grotzinger, J.P., James, N.P. (Eds.), Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World: SEPM, Spec. Publ., 67, pp. 3–20. Guidi, M., Marini, L., Scandiffio, G., Cioni, R., 1990. Chemical geothermometry in hydrothermal aqueous solutions: the influence of ion complexing. Geothermics 19, 415–441. Kah, L.C., Grotzinger, J.P., 1992. Early Proterozoic (1.9 Ga) thrombolites of the Rocknest Formation, Northwest territories. Palaios 7, 305–315. Kah, L.C., Bartley, J.K., Frank, T.D., Lyons, T.W., 2006. Reconstructing sea-level change from the internal architecture of stromatolite reefs: an example from the Mesoproterozoic Sulky Formation, Dismal Lakes Group. Arctic Canada. Can. J. Earth Sci. 43, 653–669. Kah, L.C., Bartley, J.K., Stagner, A.F., 2009. Reinterpreting a Proterozoic enigma: Conophyton–Jacutophyton stromatolites of the Mesoproterozoic Atar Group, Mauritania. In: Swart, P., Eberli, G., McKenzie, J. (Eds.), Perspectives in Carbonate Geology: Int. Assoc. Sedimentol., Spec. Publ., 41, pp. 277–295. Keita, N.D., 1984. Etude géologique des formations sédimentaires de la partie sudorientale du bassin précambrien supérieur et paléozoïque de Taoudeni au Mali (région du plateau de Bandiagara). PhD, Univ. Aix-Marseille, 210 p. Knoll, A.H., Semikhatov, M.A., 1998. The genesis and time distribution of two distinctive Proterozoic stromatolite microstructures. Palaios 13, 408–422. Lajoinie, J.P., 1960. Observations sur le Primaire de la région de Bobodioulasso (HautVolta). Bull. Soc. géol. Fr. 2, 208–212 (7e sér.). Lesquer, A., Moussine-Pouchkine, A., 1980. Les anomalies gravimétriques de la boucle du Niger. Leur signification dans le cadre de l'orogenèse pan-africaine. Can. J. Earth Sci. 17, 1538–1545. Marcelin, J., Serre, J.C., 1971. Notice explicative de la carte géologique au 1/200 000, Banfora-Sindou-Mangodara. Dir. Géol. Mines Haut-Volta. BRGM ed., Orléans. Moore, L.S., Burne, R.V., 1994. The modern thrombolites of Lake Clifton, Western Australia. In: Bertrand-Sarfati, J., Monty, C. (Eds.), Phanerozoic Stromatolites II. Kluwer Acad. Press, Dordrecht, pp. 3–19. Moussine-Pouchkine, A., Bertrand-Sarfati, J., 1978. Le Gourma: un aulacogène du Précambrien supérieur. Bull. Soc. géol. Fr. 20, 851–857. Narbonne, G.M., James, N.P., Rainbird, R.H., Morin, J., 2000. Early Neoproterozoic (Tonian) patch reef complexes, Victoria Island, Arctic Canada. In: Grotzinger, J.P., James, N.P. (Eds.), Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World: SEPM, Spec. Publ., 67, pp. 163–178. Ouedraogo, C., 1981. Etude géologique des formations sédimentaires du bassin précambrien supérieur et paléozoïque de Taoudenni en Haute Volta. Trav. Lab. Sci., St Jérôme, Marseille 45, 1–51 (sér. 10). Petrov, P.Yu., Semikhatov, M.A., 2001. Sequence organization and growth patterns of late Mesoproterozoic stromatolite reefs: an example from the Burovaya Formation, Turukhansk Uplift. Siberia. Precambr. Res. 111, 257–281. Rechenmann, J., 1965. Mesures gravimétrique et magnétique en Côte d'Ivoire, HautVolta et Mali méridional. Cah. ORSTOM, sér. Géophys. 5, 1–43. Reichelt, R., 1971. Géologie du Gourma (Afrique occidentale). Un seuil et un bassin du Précambrien supérieur. Stratigraphie, tectonique, métamorphisme. Mém. BRGM 53, 1–213. Rooney, A.D., Selby, D., Houzay, J.P., Renne, P.R., 2010. Re–Os geochronology of a Mesoproterozoic sedimentary succession, Taoudeni basin, Mauritania: Implications for basin-wide correlations and Re–Os organic-rich sediment systematics. Earth Planet. Sci. Lett. 289, 486–496. Seong-Joo, L., Golubic, S., 1999. Microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation: Mesoproterozoic Gaoyuzhuang Formation. China. Precambr. Res. 96, 183–208. Simon, B., 1979. Essai de synthèse sur les formations sédimentaires de la partie occidentale du Mali. Trav. Lab. Sci. Terre, St. Jérôme, Marseille 1–133. Simon, B., Brisset, A., Roussel, J., Sougy, J., 1982. Confrontation de la télédétection (analyse numérique et analogique, téléinterprétation à petite échelle) avec la

156

J.J. Álvaro, D. Vizcaïno / Sedimentary Geology 263–264 (2012) 144–156

cartographie géologique classique et les données gravimétriques du Mali Sud occidental (Afrique de l'Ouest). Bull. Soc. géol. Fr. 24, 13–22 (7e sér.). Stefano di, P., Mindszenty, A., 2000. Fe–Mn-encrusted “Kamenitza” and associated features in the Jurassic of Monte Kumeta (Sicily); subaerial and/or submarine dissolution? Sediment. Geol. 132, 37–68. Toft, P.B., Taylor, P.T., Arkani-Hamed, J., Haggerty, S.E., 1992. Interpretation of satellite magnetic anomalies over the West African Craton. Tectonophysics 212, 21–32. Turner, E.C., James, N., Narbonne, G.M., 1993. Neoproterozoic reef microstructures from the Little Dal group, northwestern Canada. Geology 21, 259–262. Turner, E.C., James, N., Narbonne, G.M., 1997. Growth dynamics of Neoproterozoic calcimicrobial reefs, Mackenzie Mountains, Northwest Canada. J. Sediment. Res. 67, 437–450. Turner, E.C., James, N., Narbonne, G.M., 2000a. Taphonomic control on microstructure in Early Neoproterozoic reefal stromatolites and thrombolites. Palaios 15, 87–111. Turner, E.C., Narbonne, G.M., James, N., 2000b. Framework composition of early Neoproterozoic calcimicrobial reefs and associated microbialites, MacKenzie Mountains, N.W.T., Canada. In: Grotzinger, J.P., James, N.P. (Eds.), Carbonate

Sedimentation and Diagenesis in the Evolving Precambrian World: SEPM Spec. Publ., 67, pp. 179–205. Vennin, E., Álvaro, J.J., Moreno-Eiris, E., Perejón, A., 2003. Early Cambrian coelobiontic communities in tectonically unstable crevices developed in Neoproterozoic andesites, Ossa-Morena, southern Spain. Lethaia 36, 53–65. Villeneuve, M., 2005. Paleozoic basins in West Africa and the Mauritanide thrust belt. J. Afr. Earth Sci. 43, 166–195. Villeneuve, M., 2008. Review of the orogenic belts on the western side of the West African craton: the Bassarides, Rokelides and Mauritanides. In: Ennih, N., Liégeois, J.P. (Eds.), The Boundaries of the West African Craton: Geol. Soc., London, Spec. Publ., 297, pp. 169–202. Villeneuve, M., Cornée, J.J., 1994. Structure, evolution and palaeogeography of the West African Craton and bordering belts during the Neoproterozoic. Precambr. Res. 69, 307–326. Zhuravlev, A.Yu., Wood, R.A., 1995. Lower Cambrian reefal cryptic communities. Palaeontology 38, 443–470.