The C2 and C3 formations of the Schisto-Calcaire Subgroup (West Congo Supergroup) in the Democratic Republic of the Congo: An example of post-Marinoan sea-level fluctuations as a result of extensional tectonisms

Accepted Manuscript The C2 and C3 formations of the Schisto-Calcaire Subgroup (West Congo Supergroup) in the Democratic Republic of the Congo: An example of PostMarinoan sea-level fluctuations as a result of extensional tectonisms F. Delpomdor, L. Tack, J. Cailteux, A. Préat PII: DOI: Reference:

S1464-343X(15)00150-8 http://dx.doi.org/10.1016/j.jafrearsci.2015.06.005 AES 2296

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

28 September 2014 3 June 2015 5 June 2015

Please cite this article as: Delpomdor, F., Tack, L., Cailteux, J., Préat, A., The C2 and C3 formations of the SchistoCalcaire Subgroup (West Congo Supergroup) in the Democratic Republic of the Congo: An example of PostMarinoan sea-level fluctuations as a result of extensional tectonisms, African Earth Sciences (2015), doi: http:// dx.doi.org/10.1016/j.jafrearsci.2015.06.005

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The C2 and C3 formations of the Schisto-Calcaire Subgroup (West Congo Supergroup) in the Democratic Republic of the Congo: An example of Post-Marinoan sea-level fluctuations as a result of extensional tectonisms

Delpomdor, F.1, Tack, L.2, Cailteux, J.3, Préat, A1.

1

Université Libre de Bruxelles, Belgium, e-mail: [email protected],

2

Royal Museum for Central Africa, Belgium

3

Forrest International Group, Democratic Republic of the Congo

Abstract

In the Lower Congo region, the Ediacaran Schisto-Calcaire Subgroup consists of five carbonatedominated formations (C1 to C5). They record tectono-eustatic sea-level fluctuations controlled by several short-time extensional tectonic events occurred in the whole basin, followed by the development of the Araçuaï-West Congo Orogen between 630 Ma and 560 Ma. The uppermost units of the C2 Formation, i.e. C2d and C2e members, consist of open marine to peritidal/sabkha cycles of 1 m to 4 m in thickness formed during in a Highstand Systems Tract (HST). The unexposed transition between the C2 and C3 formations is interpreted as a ‘final’ HSTphase which initiated the burial of the carbonate ramp by prograding siliciclastics or an early Transgressive Systems Tract (TST) phase. The carbonates of the C3 Formation represent open marine shallowing-upward cycles of 3 m to 8 m in thickness, with deposition at the top of massive oolitic barrier shoals during a TST which flooded the entire the Neoproterozoic West Congo Basin. During the highstand, contributions of river water and land-derived material inputs occured, intermittently according to the semi-arid to arid conditions that prevailed in the restricted inner ramp and in the sabkha facies belts. In term of geochemistry, the disturbed δ13C trends of the post-Marinoan C2 and C3 carbonates rather reflect early diagenetic variations related to (i) the mixing of carbonate rocks with 13C and 18O depleted fluids including decarboxylation during early organic diagenesis and deep burial, or (ii) the meteoric alteration during carbonate stabilization, than temporal signals of the global ocean chemistry. This observation does not negate the stratigraphic utility of δ13C ratios for intrabasinal correlations.

1. Introduction

The Neoproterozoic Era heralded dramatic climatic changes, global carbon cycling and atmospheric oxygen budget (Knoll et al., 1986; Derry et al., 1992; Knoll, 1992; Des Marais, 1994; Strauss, 1997; Hoffman et al., 1998; Canfield, 1999) strongly interfering with global

tectonics (Kaufman et al., 1993, 1997; Halverson et al., 2007; Young, 2012). During this time, the low-latitude distribution of continents (Kirschvink, 1992; Hoffman et al., 1998; Hoffman and Schrag, 2002; Li et al., 2008) is suggested to have favoured a cool climate due to the nature of continental chemical weathering, atmospheric and oceanic circulation, and the magnitude of planetary albedo (Halverson et al., 2010). These effects led to several icehouse periods distributed at wide intervals throughout three or four global glaciations (poorly constrained c. 770-735 Ma Kaigas, well constrained c. 715-680 Ma Sturtian, well constrained c. 660–635 Marinoan, extremely well constrained c. 585-582 Ma Gaskiers; Frimmel et al., 1996; Dempster et al., 2002; Kendal et al., 2004; Lund et al., 2003; Fanning and Link, 2004; Hoffman et al., 1996, 2004; Zhou et al., 2004; Condon et al., 2005). These glaciations were followed by a sudden return to a greenhouse climate due in part to the increase of atmospheric carbon dioxide from volcanic degassing enhancing a warming of the surface of the Earth (Snowball Earth Hypothesis; Kirschvink, 1992; Hoffman, 1999; Hoffman et al., 1998; Kennedy et al., 2001; Hoffman and Schrag, 2002). The icehouse and greenhouse periods are evidenced by the presence of diamictites and cap carbonates throughout all continents (Young and Gostin, 1991; Young, 1995; Wang and Li, 2003; Deynoux et al., 2006; Allen, 2007; Li et al., 2008). However, other causative hypotheses have been proposed as alternative explanations for the Snowball Earth model (carbonate burial in Robert, 1971, 1976; high orbital obliquity in Williams, 1993, 2000, 2008; ocean stagnation in Kaufman et al., 1993; Grotzinger and Knoll, 1995; continental or zipper-rift models in Eyles, 1993; Young, 1995; Eyles and Januszczak, 2004). Although the Neoproterozoic has been widely studied using stable isotope geochemistry to establish regional or global inter-basinal correlations (Derry et al., 1989; Asmeron et al., 1991; Kaufman and Knoll, 1995; Bartley et al., 2001; Melezhik et al, 2001; Shields and Veizer, 2002; Halverson et al., 2005, 2010; Kaufman et al., 2006), the temporal variations of primary isotope signatures as worldwide proxies for seawater

composition are still debated (Bristow and Kennedy, 2008; Frimmel, 2009; Knauth and Kennedy, 2009; Derry, 2010). Literature on extreme greenhouse conditions of the Snowball Earth is abundant and focussed on geophysics, geochemistry and modeling (Kirschvink, 1992; Hoffman et al., 1998; Hoffman and Schrag, 2002; Donnadieu et al., 2004; Pierrehumbert, 2005; Goddéris et al., 2006; Le Hir et al., 2007). However, sedimentology and isotopic data concerning the interglacial environments remain are often largely ignored. The principal objective of our paper is to contribute to the debate on the Snowball Earth climatic events in Central Africa. We present detailed litho- and microfacies descriptions, sequence stratigraphic records and geochemical data of post-Marinoan deposits in the upperpart of the C2 and the C3 formations of the Schisto-Calcaire Subgroup of the Lower Congo region (Democratic Republic of the Congo). We show that in the Araçuaï-West Congo Orogen (AWCO), short-time extentional tecton-eustatic events occured between 630 Ma and 560 Ma. Finally, we discuss implications on the sedimentological processes and the

palaeoenvironmental interpretations that occurred immediately after the Marinoan global glaciation.

2. Geological setting

The West Congo Belt results from Gondwana amalgamation during the Pan-African orogeny (870 Ma to 566 Ma; Kröner and Stern, 2005). It stretches over 1300 km from southwest Gabon, across the Republic of the Congo (RC) and the Lower Congo region of the Democratic Republic of the Congo (DRC) up to northwest Angola (Fig. 1). The West Congo Belt is a fold-and-thrust belt with structural polarity to the east (Tack et al., 2001). The Paleoproterozoic Kimeza basement (2.1 Ga magmatic paragneisses and amphibolites) is

thrusted on top of progressively younger Neoproterozoic sequences with decreasing regional metamorphism from amphibolite facies to the west, up to unmetamorphosed rocks to the east (Frimmel et al., 2006). From oldest to youngest, the West Congo Supergroup (Fig. 2-A) is divided into the riftrelated Zadinian (siliciclastic sediments overlain by tholeiitic basalts; 1000 – 920 Ma) and Mayumbian (felsic volcano-plutonic complex; 920 – 910 Ma) groups. The tholeiitic basalts and overlying felsic rocks mark a typical bimodal (sub)surface assemblage, which is itself overlain by the sedimentary West Congolian Group (910 – 566 Ma; Frimmel et al., 2006; Tack et al., 2001, 2011). The latter is subdivided into four subgroups (Delhaye and Sluys, 1923; Lepersonne, 1951, 1974; Cahen, 1978). The Sansikwa Subgroup (1650 m) consists of continental siliciclastic sediments and is rift-related (Frimmel et al., 2006; Straathof, 2011). The Haut-Shiloango Subgroup (800–700 m) consists mainly of siliciclastic sediments capped by carbonate rocks (Frimmel et al., 2006; Poidevin, 2007; Delpomdor and Préat, 2013; Delpomdor et al., 2014). The Schisto-Calcaire Subgroup (1000m) consists of pelite and carbonate rocks. It is shelf-related (Frimmel et al., 2006; Poidevin, 2007; Delpomdor and Préat, 2013) and includes five formations (C1 to C5; Fig. 2-B): (i) cap carbonates (C1), (ii) shaly limestones and dolostones (C2), (iii) oolitic limestones (C3), (iv) stromatolitic limestones and dolostones (C4), and (v) organic-rich oolitic limestones (C5) (Delhaye and Sluys, 1923; Bertrand-Sarfati, 1972; Lepersonne, 1974; Delpomdor et al. submitted). A negative δ13C shift of ± 13‰ - consistent (in absolute values) with the negative swing of the Trezona anomaly (Frimmel et al., 2006; Delpomdor, 2007; Poidevin, 2007; Straathof, 2011; Delpomdor and Préat, 2013; Delpomdor et al., 2014) – marks the transition between the HautShiloango and Schisto-Calcaire subgroups. The Mpioka Subgroup (1000 m) consists of siliciclastic rocks considered as a late-orogenic molasse (Frimmel et al., 2006).

Two diamictite formations, i.e. the 500 m-thick Lower Diamictite (overlying the Sansikwa Subgroup) and the 200 m-thick Upper Diamictite (overlying the Haut-Shiloango Subgroup) (Cahen and Lepersonne, 1981), have been ascribed respectively, a Sturtian (U-Pb age: 694 ± 4 Ma; Straathof, 2011) and Marinoan (Frimmel et al., 2006) age. Finally, the redbeds of the Inkisi Supergroup overlie unconformably the deformed rocks of the West Congolian Group. Their depositional age is suggested to be bracketed between 550 Ma (Pan African orogeny) and 320 ma (base of Karoo Supergroup) (Alvarez, 1995; Frimmel et al., 2006; Straathof, 2011; Kanda Nkula et al., 2011).

3. Methodology

The samples were collected in four drillcores (“Kwilu 1”, “Kwilu 2”, “Kwilu S” and “CICO”; Fig. 1) stored at the Royal Museum for Central Africa (Belgium). A dense and systematic sampling of 3 samples per meter has been realized and integrated in our lithological logs describing carbonate textures and main sedimentary structures. A total of 716 cores have been described and 189 samples were collected from these cores for classical petrography and selection of samples for stable isotopes (oxygen and carbon), major/trace and REE analyses in order to constrain paleoenvironments and diagenetic processes. The study of thin sections allowed definition of a standard sequence of seven microfacies (MF1 to MF7; Fig. 3), whose succession records a shallowing-upward evolution from upper subtidal (detrital flux and cyanobacterial mats) to evaporitic peritidal and sabhka environments. A brief classification of microfacies-type deposits is summarized in Table 1. Carbon and oxygen isotope compositions were analyzed in the most homogeneous facies (Table 2). Between 1 and 2 mg of powdered sample (whole rock) were reacted with 100% H3PO4 at 75°C to extract the CO2 from the dolomites. The amount of extracted CO2 at

−196°C and its d13CV-PDB and d 18OV-PDB were measured using a Thermo-Finnigan 252 mass spectrometer at Erlangen University (Germany). The reproducibility of the δ13CV-PDB and the δ18OV-PDB measurements is 0.06‰. Four samples were selected for Sr-analyses (whole-rock) on the basis of primary screening including thin sections: (i) the recrystallized phases were avoided, and (ii) the chemical degree of alteration based on Sr/Ca, Mn, Fe, Mn/Sr, and Rb/Sr allowed to avoid altered samples. Powdered samples were leached with 0.5 M acetic acid and centrifuged to separate soluble fraction from insoluble fraction. Strontium was eluted from solutions by ion exchange chromatography using a Sr-Spec resin. Sr isotopeswere measured on a VG54 multicollector mass spectrometer at Université Libre de Bruxelles (Belgium). 87Sr/86Sr values were normalized to 86Sr/88Sr = 0.1194. Eleven samples were investigated for their major, trace and REE+Y concentrations (Tables 2 and 3). They have been subjected to an aqua regia digestion, which consists of a 0.5 g digested sample at 90°C in a microprocessor controlling digestion block for 2 hours. The solution is diluted and analyzed by ICP/MS using a Perkin Elmer SCIEX ELAN ICP/MS at Activation Laboratories Ltd. (Canada). REE+Y concentrations were normalised with the Post-Archean Australian Shales (PAAS) composite (Taylor and McLennan, 1985). The detection measurement is limited to <0.1 for each REE concentrations. The high total REE concentrations were estimated as >25 ppm in our study. The REE concentrations less than 0.1 ppm necessitating sophisticated analytical techniques and instrumentation for measurement are not available because the detection measurement is limited to <0.1. PAAS-normalized element anomalies were calculated algebrically as follows: La/La* = La/(3Pr-2Nd), Ce/Ce* = Ce/(2Pr-Nd), Eu/Eu* = Eu/0.67Sm+0.33Tb), Pr/Pr* = Pr/(0.5Ce+0.5Nd), and Gd/Gd* = Gd/(2Tb-Dy). These anomalies were compared with anomalies calculated from a geometric average, and their differences on results obtained by these two methods are around 5%.

4. Drillcore descriptions

Three drillcores (Kwilu 1, Kwilu 2 and Kwilu S, along the Kwilu river) cut the horizontal C2d and C2e members of the ‘Limestones of Bulu’ (uppermost C2 Formation). They form a 200 m-thick composite carbonate succession from the C2d5 up to the C2e9 submembers of the C2 Formation and from the C3a to C3b members of the C3 Formation. The Kwilu 2 drillcore (Fig. 3-A) cuts the C2d and C2e members over 50 m of thickness. The C2d member has been collected from the C2d5 to the C2d7 submembers. The C2d5 to C2d6 submembers display 24 m-thick of grey planar to undulated laminated limestones (Figs. 4-A,B), and the C2d7 submember consists of grey evaporitic limestones with nodular and enterolitic structures (Fig. 4-C). The overlying C2e8 submember shows 26 m-thick of greenish limestones with nodular and enterolitic structures. The Kwilu 1 drillcore (Fig. 3-B), overlaps the upper part of the Kwilu 2 drillcore. It is composed of 25 m-thick of grey limestones (C2d7 submember) alternating with green/grey laminated limestones with evaporitic textures at the top (C2e8 submember). The Kwilu S drillcore overlaps the top of the Kwilu 2 drillcore. Over 25 m in thickness, it cuts alternations of green/grey laminated limestones (C2e8 submember; Fig. 4-D) and grey planar to undulated laminated limestones (C2e9 submember; Fig. 4-E). The C3 Formation is partly crossed over a total depth of 100 m by the CICO drillcore (Fig. 3D) drilled in the Lukala quarry. From the base to the top, it is composed by (i) 5 m-thick of alternations of green to grey and grey laminated limestones (Fig. 4-F), (ii) overlain by 53 mthick of grey laminated limestones (Fig. 4-G) including thin green laminae (C3a and the lowermost C3b members), and (iii) 37 m-thick of grey oolitic limestones (Fig. 4-H) with numerous levels of chert at the top (C3b member).

5. Depositional model

5.1. Microfacies description Microfacies descriptions are based on the recognition of grain and sediment types, as well as sedimentary structures and textures, from our 189 selected thin sections from the studied drillcores. Seven carbonate microfacies (MF1 to MF7; Table 1) constitute the standard sequence for the uppermost C2 and the C3 formations. Starting from MF1, they record a sedimentary evolution with a decrease of the paleobathymetry. Each microfacies is named after its depositional fabric according to the classification of carbonate rocks (Dunham, 1962; Embry and Klovan, 1972; Sibley and Gregg, 1987). It also takes account into the sedimentary structures and/or paleontological components. Each thin section has been etched with Alizarine red S and potassium ferricyanide for recognition of carbonate minerals.

5.1.1. Microfacies 1 (MF1): Detrital mudstones Description: Microfacies 1 consists of a laminar clayey detrital mudstones with discontinuous dark-coloured cyanobacterial laminae. The matrix is composed of inequigranular xeno/hypidiotopic microspar calcite crystals (5-10 µm in size) in non-planar mosaic fabrics (Sibley and Gregg, 1987). The laminae are submillimetric to millimetric in thickness, with detrital minerals, oriented following the plane of stratification, composed of subangular to angular quartz and feldspar silts, muscovite, chlorite and illite minerals (Delpomdor, 2007). Interpretation: MF1 is interpreted as lower subtidal lime muds deposited by suspension settling in low-energy open marine environments with sporadic land-derived siliciclastic inputs. The lack of storm- and wave-generated sedimentary structures suggests that these

muds were deposited above the storm wave bases on a low-energy subtidal shallow mid-ramp setting.

5.1.2. Microfacies 2 (MF2): Laminar mudstones Description: Microfacies 2 is characterized by submillimetric- to millimetric-scale finegrained laminar mudstones enriched in quartz and feldspar silts, chlorites, illites and muscovite minerals. The MF2 displays light-coloured massive mudstones and thin continuous planar parallel dark-coloured pyrite-rich cyanobacterial laminae (5-15 µm thick; Fig. 5-A), which show, under SEM, well-preserved colonies of thin segmented, unbranched, and slightly lobed filaments reported to Siphonophycus septatum species (Delpomdor, 2007; Préat et al., 2011). Light-coloured massive mudstones consist of xeno- to hypidiotopic microspar calcite crystals in non-planar mosaic fabrics, while dark-coloured cyanobacterial laminae composed of thin smooth continuous planar and weakly irregular levels of inequigranular xeno- to hypidiotopic microspar calcite crystals, ranging from 5 to 10 µm in size, embedded in nonplanar mosaic fabrics. Interpretation: The flat- to slighlty wavy laminated cyanobacterial mats developed on a lowenergy upper subtidal shallow ramp with strong input of land-derived siliciclastic particles. The occurrence of Oscillatorian Siphonophycus septatum mats (Delpomdor, 2007; Préat et al., 2011) are common in quiet shallow-marine subtidal to intertidal environments in arid to semiarid subtropical and tropical zones (Noffke et al., 2003).

5.1.3. Microfacies 3 (MF3): Oolitic grainstones Description: Microfacies 3 consists of grainstones with lumps, oolites and rare oncolites (Fig. 5-D). Clastic components float in coarse-grained lighter-coloured drusy spar calcite cements. Oolites are predominantly spherical or ovoid with a dark equigranular microspar calcite fabric

(crystals less than 10 µm in size) in the cortex or consist of tangential and/or fibro-radial oolites, composite oolites and oolitic aggregates. Oolites exhibit fine-grained slightly pendant beard-like (Dunham, 1962) or lamellae microcrystalline cements (Moore and Druckman, 1981; Moore, 1989; Tucker and Wright, 1990). Millimetric-scale desiccation cracks and pseudomorphs of evaporitic crystals of gypsum and polyhalite, replaced by light equant or fine- to medium-grained spar calcite cements, are minor in this microfacies. Rare authigenic bipyramidal quartz silts (100-200 µm in size) are observed. Interpretation: MF3 is deposited in high-energy tidal-influenced shoal complexes separating the open marine facies belt from the restricted inner ramp and peritidal facies belts (see below). The fluctuations between high-energy tangential and low-energy fibro-radial oolites may result from rhythmic daily tides (Purser and Loreau, 1973) or oscillations of the relative sea-level. Purser and Loreau (1973) suggested that the tangential oolites occur during high tides, while the radial oolites and lumps are the result of low tides. Oscillating emersive periods of the oolitic barrier shoal facies belt are related to vadose phases, as indicated by the asymmetric meniscus calcite cements in the rim of oolites.

5.1.4. Microfacies 4 (MF4): Evaporitic mudstones Description: Microfacies 4 is characterized by light-coloured massive mudstones with abundant pseudomorphs of evaporitic crystals (replaced gypsum, anhydrite and polyhalite), consisting of light equant of fine- to medium-grained spar calcite cements. Pseudomorphs after evaporitic crystals, former crystals of gypsum, consisted of small-flattened lenticular crystals (50-300 µm in lenghth) with lozenge shapes (50 µm in size). Platy rectangular laths (50 µm in size) of replaced anhydrite by spar calcite cements, and small-sized crystals of polyhalite are common. The matrix consists of xeno- to hypidiotopic microspar calcite crystals in non-planar mosaic fabrics.

Interpretation: This microfacies represents backshoal carbonate muds deposited in lowenergy middle to upper intertidal environments under restricted and evaporative conditions.

5.1.5. Microfacies 5 (MF5): Evaporitic microbial bindstones Description: Microfacies 5 consists of thin submillimetric alternations of discontinuous darkand light-coloured planar and wavy pyrite-rich cyanobacterial bindstones with pseudomorphs of evaporite crystals (Fig. 5-B). The dark laminae consist of smooth to irregular and wavy layers of equigranular xeno- to hypiodiotopic microspar calcite crystals (5-10 µm in size) in non-planar mosaic fabrics, while the light laminae are composed of coarser hypidiotopic neomorphic microspar crystals (10-15 µm in size) in sutured mosaic planar-e fabrics. The microfacies texture displays often irregular and poorly sorted micritic peloids evolving in a muddy matrix (mudstone). The matrix contains (i) isolated randomly oriented swallowtail twins and platy rectangular laths of former gypsum and polyhalite, (ii) anhydritic nodules (200-400 µm in size) replaced by microspar calcite cements, and (iii) microenterolithic structures (millimetric to subcentimetric in scale), which deformed the bindstone facies during displacive growth of evaporitic minerals. Subvertical mud cracks and tepee structures filled by drusy calcite cements cross the microbial laminae. Interpretation: This microfacies represents the cyanobacterial tidal flat margin constituted of fragments of the mats in a non-marine subaerial evaporitic and hypersaline environment similar to a sabkha. Shearman (1963) has shown that the sedimentary profile of Trucial Coast in Abu Dhabi (United Arab Emirates) is divided into a shallow marine environment, i.e. the upper intertidal zone, and a coastal sabkha, i.e. the supratidal zone. The presence of microbial fragments or “chips”, evaporitic structures such as ‘nodular’ chicken-wire and enterolithic structures, are indicative of capillar evaporation and formation of evaporates by concentration

of brines in soft sediments. Tepee structures or mud cracks may interrupt the deposition and confirm episodes of emergence.

5.1.6. Microfacies 6 (MF6): Intraformational conglomerates Description: Microfacies 6 displays intraformational conglomerates with disrupted fragments of laminated mudstones (MF5) presenting a typical “puzzle” texture embedded in microspar calcite cements (Fig. 5-C). The microfacies is poorly sorted with inframillimetric to pluricentimetric angular and rounded pebbles or lumps of fine-grained microspar calcite matrix. Subvertical desiccation cracks (300 µm in length) filled with replaced gypsum, anhydrite and polyhalite are observed. Interpretation: MF6 records shallow-water upper intertidal to supratidal conditions in tidal flat environments with temporary periods of desiccation. Sheet-cracks, and step-like thin mud-cracks isolating lumpy patches forming intraformational conglomerates were reported in the inner margin of peritidal environments in the Bahamas (Hardie, 1977). Such sediments were sporadically subject to evaporation as recorded by the former precipitation of sulphates (Shearman, 1963; Kinsman, 1969).

5.1.7. Microfacies 7 (MF7): Microspartized and cherty mudstones Description: Microfacies 6 consists of pyrite-rich microsparitized (Fig. 5-E) and cherty mudstones (Fig. 5-F) with abundant pseudomorphs of evaporitic crystals. Interpretation: This microfacies is the result of hypersaline microsparitization and silicification processes in carbonate muds deposited in sabkha environments.

5.2. Interpretation

The assemblage of microfacies constitutes a standard sequence (MF1 to MF7) integrating the variations of sea-level, energy and salinity in a ramp setting. The sedimentary distribution of these microfacies in time and space is controlled by the tectonic evolution of the basin, the rate of carbonate production and the available accommodation space prevailing in the carbonate ramp system of the uppermost C2 and C3 formations from the Lower Congo region within the Neoproterozoic West Congo Basin (NWCB). The interpretation of the ramp geometry is based on the vertical changes of the shallow-water microfacies. A detailed depositional model for the uppermost C2 and C3 inner carbonate ramp is presented in Fig. 6. It displays two major settings, a mid ramp and an inner ramp, this latter comprising four main belts described here from distal to proximal settings as follows : (i) open marine environment with oolitic shoals, (ii) restricted facies, (iii) tidal flats and (iv) an episodic sabkha environment. Open marine shallow-water sediments consist of low-energy fine-grained carbonate mudstones enriched with land-derived siliciclastic inputs (MF1) and alternations of carbonate mudstones and bindstones with cyanobacterial mats (MF2). These microfacies represent a quiet inner ramp in the lower- to upper subtidal environments. These two facies are disconnected by high-energy tidal-influenced shoals (MF3). Restricted inner ramp assemblages, behind the MF3 shoals, consists of carbonate mudstones (MF4) with abundant casts of replaced sulphate salts developed in an extended ‘lagoonal’ area periodically submitted to a semi- to arid climate. Tidal flats are colonized by microbial communities (bindstones, MF5) and contain intraformational conglomerates with microbial chips, ‘nodular’ chicken-wire and enterolithic structures (MF6). Tepee structures and mud cracks highlight episodic interruption in the sedimentation with emergence of extended areas of the inner ramp (Riegl et al., 2010).

Tidal flat facies belts pass to the sabkha environments with hypersaline carbonate mudstones (MF7).

6. Stratigraphic architecture

The definition of the fundamental sequence stratigraphic concepts is independent of the type of depositional environments. However, the carbonate depositional systems are interpreted according to the sedimentary changes as a response of the interaction between authigenic (i.e. sedimentology resulting form the facies analyses), and allogenic processes, (i.e. sedimentation including climate, tectonics and sea-level changes) regardless of scale and age of the strata (Vail et al., 1977; Miall, 1997; Posamentier and Allen, 1999; Catuneanu, 2006). In our paper, the depositional systems are defined in relation to the geometry of the systems tracts and the sediment supply controlled by the sea-level changes, the types of stratigraphic surfaces or stratigraphic sequences (Vail, 1987; Sarg, 1988; Schlager, 2005). Due to the scarcity of continuous geological survey, only elementary parasequences or 5th order rhythms/cycles are defined. Each elementary parasequences are defined following a lithofacies hierarchy (Fig. 3) disposed from deepening (LF1) to shallowing-upward (LF3) intervals. This LF1/LF3 hierarchy records a ‘relatively conformable succession of genetically related beds or bedsets bounded by a flooding surface’ (Van Wagoner, 1995).

6.1. Lithofacies analysis

6.1.1. Lithofacies 1 (LF1): shales Description: this lithofacies consists into centimetre- to decimetre-thick well-bedded greenish, locally blue, massive shales, with rare detrital grains of quartz and feldspars. The

lower and upper contacts are sharp. In the Democratic Republic of the Congo, this lithofacies is absent (Delpomdor, 2007; Cailteux et al., submitted), while present in the Republic of the Congo, where it evolves laterally to open-marine limestones including stromatolites and oolites (Préat et al., submitted). Interpretation: LF1 indicate a low energy deposition formed in a quiet basin with sporadic sedimentary influx of detrital minerals deposited under deep conditions.

6.1.2. Lithofacies 2 (LF2): shaly limestones Description: lithofacies 2 exhibits centimetre- to decimetre-thick well to gently undulated bedded greenish shaly limestones with millimetre-scale planar to low-angle crossed laminations. The lower and upper contacts are sharp. Interpretation: LF2 includes the open-marine microfacies 1 (MF1 and represents low-energy deposition in open-marine lower environments.

6.1.3. Lithofacies 3 (LF3): limestones LF3 includes the open-marine and restricted lagoonal facies belts. Four sub-lithofacies (LF3a to LF3d) are recognized and described below:

6.1.3.1.. Sub-lithofacies 3a (LF3a): planar parallel and undulated laminated limestones Description: sub-lithofacies 3a consists of centimetre- to decimetre-thick beds of grey- to white laminae in a greyish to greenish limy matrix. Laminations are planar parallel, slightly undulated with occasional small-scale ripples and cross laminations. Bed bases and tops are sharp or slightly undulated.

Interpretation: LF3a comprises the open-marine (MF2) and lagoonal (MF4) microfacies and therefore represents a low-energy upper subtidal (MF2) to an upper intertidal restricted lagoonal (MF4) ramp setting.

6.1.3.2. Sub-lithofacies 3b (LF3b): oolitic-pisolitic limestones Description: this sub-lithofacies consists into decimetre- to metre-thick stratiform or lenticular beds of whitish to light-grey massive oolitic limestones with dominant planar stratification, rare cross-bedding and intraformational conglomerates. Bed bases and tops are sharp. Locally, the bed top is marked by erosional surfaces. Interpretation: LF3b includes MF3 and represent high-energy oolitic shoals at the transition between the open marine mid/inner ramp and peritidal facies belts.

6.1.3.3.. Sub-lithofacies 3c (LF3c): planar parallel and undulated laminated limestones Description: LF3c exhibits centimetre- to decimetre-thick beds of grey- to white-coloured limestones with centimetre-thick greenish planar parallel to slightly undulated muddy laminations. Sedimentary structures as small-scale ripples and cross laminations are observed. Bed bases and tops are sharp. Interpretation: LF3c includes MF5 and represents subaerial evaporitic conditions.

6.1.3.4. Sub-lithofacies 3d (LF3d): nodular limestones Description: LF3d displays centimetre to decimetre-thick beds of grey to white nodular limestones within a greyish to greenish laminar limy matrix. Nodules are submillimetre to centimetre-sized, and show rounded, lenticular, ‘chicken-wire’ or enterolithic textures. The lower and upper contacts are sharp.

Interpretation: LF3d includes MF6 and MF7 and records upper intertidal to hypersaline supratidal conditions in tidal flats with temporary periods of desiccation.

6.2. Sequence stratigraphy Applying systematically the resetting of lithofacies sets for each flooding surface, 60 elementary parasequences are recognized in the uppermost C2 and C3 formations., Their general stacking allows to infer the relative sea-level fluctuations in the Lower Congo area, and is illustrated by a Fischer plot diagram (Fig. 7). The Fischer plot is a popular tool in cyclostratigraphy focusing the cycle-thickness variations of the elementary parasequences, their stacking patterns and is a key to characterize the third-order changes which depend upon the rate of the sea-level rise and the ability of carbonate production to pace the increase of accommodation space. Although Fischer plot diagrams have often been criticized as being too subjective and speculative, statistical analyses should be carried out or ideally have to be established with more than 30 or 50 cycles to be used with confidence (Saddler et al., 1993).

6.2.1. Sequence typology The uppermost C2 Formation succession can be divided into four parasequence sets (fourth order; 3-11, 12-20, 21-32 and 33-43; Fig. 7). Each elementary parasequence shows a gradual aggradational to progradational stacking patterns from open marine to tidal flat and/or sabkha environments (LF2 to LF3). The stacked patterns are missing between the end of the C2 Formation and the beginning of the C3 Formation due to the lack of well preserved outcrops or high density of vegetation in the field. The recognized elementary parasequences of the C3 Formation (Fig.7), correspond to shallowing-upward prograding lobes from open marine to near-shore settings with oolitic shoals that are identified in the parasequences n°46-53, 54-59 and 60.

6.2.2. Sequence interpretative The uppermost C2 Formation, i.e. C2d and C2e members, can be described as a low-energy open marine to tidal flat/sabkha depositional Highstand Systems Tract (HST) distributed all over the inner ramp areas (Fig. 7). During the HST, the carbonate production reached its maximum, reflecting the extended flooding within the NWCB. As consequence, these relative quiet conditions favoured the settling of widely extended carbonate muds (LF2) from the open marine to the tidal flat environments (LF3). The tidal flats were quickly enlarged when sediment accumulation rates exceeded the rates of increase in accommodation space leading progradational parasequences succeeding aggradational ones. A stratigraphic hiatus is revealed around thirty meters. It records either a ‘final’ HST initiating the burial of the carbonate ramp by prograding siliciclastics or an early Transgressive Systems Tract (TST) at the C2/C3 transition. However, the position of the Transgressive Surface (TS) at the top of the late HST or the base of the early TST cannot be located exactly due ro a lack of data (Fig. 7). The base of the C3 Formation is marked by several high amplitude TST patterns within the basin. However, no major changes in the distribution of lithofacies patterns are observed between the stacking patterns of the C2 and the C3 formations (Fig. 7). Nonetheless, the TST patterns show a much better differentiation in thickness than the HST occurring at in the end of the C2 Formation. The lithofacies patterns of the C3a member may reflect the development of tidal flat and sabkha environments during an early TST interval. The abrupt change of lithofacies patterns (LF2 to LF3) from low-energy shallow restricted inner ramp to highenergy open marine and massive oolitic shoals is visible at the base of the C3b2 member with the oolitic shoals covering wide areas of the inner ramp. As a consequence, the shallow restricted inner ramp, tidal flat and sabkha environments are flooded during a TST interval.

6.2.3. Sea-level fluctuation The sea-level variations may be illustrated using the Fischer plot diagram (Fig. 7), for which the amount of accommodation space available for sediments to fill is fundamentally controlled by rate of relative sea-level change. The environmental information (mainly sedimentology) and thicknesses provided by the elementary parasequences are taken into account here below, in order to determine the magnitude of relative sea-level change. The distribution of thickness carbonate cycles of the uppermost C2 and the C3 formations shows constant thinner elementary parasequences recording ‘thin’ peritidal cycles. The thickness cycle varies from 0.87 m (n = 9) to 4.03 m (n = 8), with an average value of 0.79 m (n = 41) in the uppermost C2 Formation and 3.65 m (n = 14) in the lower part of the C3 Formation. This leads to a thickness ranging between 1 m and 4 m for the peritidal cycles. The major transgression occurring at the base of the C3b2 is exhibited by a very thick ‘homogenous’ succession (42.7 m; n = 1) larger than the thickness of all the previous cycles. Although no elementary parasequences are clearly visible in this succession, the associated micro- and lithofacies sequence sets (26 m-thick only for the oolitic shoals) and the drillcore observations show that cycles within the oolitic shoals range between 3 m and 8 m in thickness (Cailteux et al., submitted).

7. Geochemistry

7.1. Stable isotopes The stratigraphic variations of C and O isotopes in the uppermost part of the C2 Formation and in the C3 Formation are illustrated in Figs. 8-A,B and C. Cailteux et al. (submitted) discuss in detail the geochemistry of the C3 Formation. All stable isotope data are available in

Table 2. The uppermost part of the C2 Formation displays, in the C2d member, δ13C and δ18O values varying from -4.8 ‰ to -4.5 ‰ (n = 8) and -12.4 ‰ to -9.4 ‰ (n = 8) respectively, while in the C2e member, δ13C and δ18O values are respectively between -4.2 ‰ to -3.8 ‰ (n = 7) and -12.3 ‰ to -10.6 ‰ (n = 7). No trends between isotopes and microfacies are observed. δ13C and δ18O values exhibit strongly positive correlations in the C2d (r = 0.70) and in the C2e members (r = 0.88). The C2 Formation yielded 87Sr/86Sr ratios displaying a significant variation from the C2d (0.70888; n = 1) to C2e (0.70788; n = 1) members. The C3 Formation exhibits, in the C3a member, δ13C and δ18O values varying respectively from -2.9 ‰ to -2.3 ‰ (n = 8), and -10.5 ‰ to -8.1 ‰ (n = 8), and in the C3b member, δ13C and δ18O values are between -2.5 ‰ to -1.5 ‰ (n = 11), and -9.7 ‰ to -9.1 ‰ (n = 11) respectively. No trends between isotopes and microfacies are observed. Nevertheless, weak negative δ13C values are measured in the oolitic limestones (MF5) of the C3b member (i.e. the C3b2 submember defined by Cailteux et al., submitted). The correlation between δ13C and δ18O values are strong with r = 0.51 in the C3a member, r = 0.83 in the C3b1 submember, and r = 0.72 in the C3b2 submember. The limestones of the C3 Formation gave no radiogenic 87

Sr/86Sr ratios of 0.70750 and 0.70766 (n = 2).

7.2. Major and trace elements (Table 2) The upper C2 Formation yielded CaCO3 contents (Fig. 8-E) between 28.5 % to 71.5 % (n = 3) in the C2d member, between 69.3 % and 85 % (n = 3) in the C2e member, while the C3 Formation ranges between 70.0 % and 81.8 % (n = 3) in the C3a member, and 96.8% to 99.5 % in the C3b2 submember (n = 2). The C2 and C3 formations display low MgCO3 contents with values ranging between 0.7 % and 10.6 % (n = 11). Whole-rock CaO (mean CaO = 42.2

%; n = 11) and MgO (mean MgO = 2.2 %; n = 11) contents confirm that the uppermost C2 and C3 formations are predominantly non-dolomitized limestones (Fig. 8-F). Al2O3 and Fe2O3 contents are commonly used as indicators of clay inputs. Our limestones do not contain clay materials as shown by moderate Al2O3 and Fe2O3 contents between 0.1 % and 5.2 % (n = 6), and 0.5 % and 5.7 % (n = 6) in the C2d and C2e members, respectively, and low Al2O3 and Fe2O3 contents ranging between 0.1 % and 2.4 % (n =5), and 0.5 % and 2.6 % (n = 5) in the C3a and C3b members, respectively (Figs. 8-G,H). However, the low Al2O3 concentrations coincide with the higher Sr concentrations (>1000 ppm; Fig. 8-D), which points to former sulphate casts replaced by calcite, as previously seen in our petrographical study All samples display very low K2O contents (between 0.0 ppm and 0.7 ppm; Fig. 8-I), while the MnO contents (Fig. 8-J) range between 1.3 ppm and 27.0 ppm (n = 3) in the C2d member, 7.5 ppm and 16.1 ppm (n = 3) in the C2e member, 2.5 ppm and 3.3 ppm (n = 3) in the C3a member, and 0.7 ppm and 0.9 ppm (n = 2) in the C3b2 submember. The PAAS-normalized trace element concentrations (Figs. 9-A,B) show significant variations according to the amount of clay content in the studied levels. All samples (n = 11) are strongly (i.e. the C3b2 submember) to moderately (i.e. in other stratigraphic levels) depleted in comparison to PAAS. The C2d samples (n = 3) display relative similar trends by comparison to the cap carbonates and clay-rich doloturbidites at the base of the C2a member (Delpomdor et al., in revision). The samples show relative depletions in Cr, Co, Cu, Zn, Rb, Zr, Hf and Th and strong enrichments in Sr, while the C2e samples (n = 3) are depleted in V, Rb, Zr and Th, and slightly enriched in Ni, Sr, Pb and U. The C3a and C3b samples (n = 5) show significant differentiations between high trace element concentrations in the C3a member by comparison to the C3b member. All samples exhibit similar depletions in V, Rb,

Hf and Th, and enrichments in Sr. All total trace element concentrations are lower than PAAS normalization.

7.3. REE+Y distributions (Table 3) The samples of the C2d and C2f members (Fig. 9-C) display relatively uniform shalenormalized REE+Y patterns (n = 7) with no distinct element anomalies. However, they show relative slight enrichments in light REE (mean (Nd/Dy)PAAS = 1.4; n = 7). The (Y/Ho) ratios are low, ranging between 27.3 and 39.3 (n = 4). The total REE contents are low with an average of 124.5 ppm (n = 3). REE+Y patterns of the C3a member (Fig. 9-D) are relatively flat with a relative slight depletion ((Nd/Dy)PAAS = 0.8; n = 1), no enrichment ((Nd/Dy)PAAS = 1.2 to 2.1; n = 2) in light REE and no depletion or enrichment in middle REE ((Y/Ho)PAAS = 1.1; n = 3). The samples show weak positive La, Pr, Gd, Ce, Eu and Ce anomalies (mean (La/La*)PAAS = 1.4; n = 5; mean (Pr/Pr*)PAAS = 1.1; n = 5; mean (Gd/Gd*)PAAS = 1.7; n = 5; mean (Ce/Ce*)PAAS = 1.2; n = 5; mean (Eu/Eu*)PAAS = 1.1; n = 3). The Y/Ho ratios are between 27.0 and 37.05 (n = 3) and are similar to the values of the underlying C2d and C2e members. The total REE contents vary between 57.3 ppm and 116.2 ppm (n = 2).

8. Primary or diagenetic signals?

The δ13C variations in the Neoproterozoic carbonates are often considered to reflect preserved primary marine signatures related to secular variations, with a residence time of carbon around 150 times the ocean mixing time (Kump and Arthur, 1999; Kaufman and Knoll, 1995; Hoffman et al., 1998; Fairchild et al., 2000; Hoffman and Schrag, 2002; Halverson et al., 2005). Due to the rapid mixing or exchange of carbon between the atmosphere and the ocean

surface reservoirs, the dissolved inorganic carbon δ13C composition in the pelagic ocean is disturbed as the δ13C composition of pelagic carbonates (Panchuk et al., 2005; Husinec and Bergström, 2014). Even so, the Neoproterozoic basinal correlations, based on carbon- and oxygen-isotopic variations, are commonly established from carbonate successions. However, large variations in δ13C values between modern shallow-marine carbonates and pelagic dissolved inorganic carbon are commonly reported (Patterson and Walter, 1994; James et al., 2001; Shen et al., 2005; Swart and Eberli, 2005; Giddings and Wallace, 2009; Swart et al., 2009), and imply that the correlations may be biased without robust sedimentological and diagenetic data. The causes of these carbon-isotopic variations are related to the fact that the δ13C values on shallow-marine carbonate sediments are influenced by a large number of effects, including biological processes (e.g. respiration, photosynthesis, bacterial sulphate reduction; Cummings and McCarty, 1982; Swart, 1983; Chafetz et al., 1999; Swart et al., 2009; Sarg et al., 2013), exchange between isotopically low carbon in meteoric waters and carbonates during lithification and early diagenesis (Melim et al., 2001; Knauth and Kennedy, 2009), carbonate mineralogy, and other diagenetic processes (e.g. burial, hydrothermal; Derry, 2010; Gammon et al., 2012). In our paper, no significant δ13C shift is reported in the uppermost C2 and C3 formations, implying that (i) either the factors influencing the δ13C composition of shallow-marine carbonates are the same, which appears to be excluded as the paleoenvironments are different (oolitic shoals, restricted facies, tidal flat, sabkha) or (ii) all samples were affected by the same diagenetic processes. Diagenetic processes (Fig. 10) can be identified if negative δ13C values are coupled with negative δ18O values (Gross and Tracey, 1966; Banner and Hanson, 1990; Kaufman et al., 1991). This is the case in all samples of the upper part of C2 and C3 formations as indicated by the good correlation between δ13C and δ18O. Their correlation points to (i) a possible

mixing of carbonate rocks with depleted 13C and 18O fluids including decarboxylation during deep burial (causing a decrease in δ13C and δ18O; Hoefs, 1987) or (ii) a meteoric alteration during carbonate stabilization including vadose conditions in the oolitic shoal facies (Fig. 11). Other tools such as Sr/Ca and Mn/Sr ratios and Mn concentrations are commonly used as diagenetic indicators (Brand and Veizer, 1980; Veizer, 1983; Jacobsen and Kaufman, 1999). The samples of the uppermost C2 Formation display Sr/Ca ratios ranging between 2.5 x 10-3 and 2.6 x 10-3 (n = 2) in the C2d member, and 6.6 x 10-3 for these two analyzed samples in the C2e member, while the sample of the C3 Formation displays Sr/Ca ratios smaller or equal (1.9 x 10 -3 to 8.9 x 10-3; Fig. 11-A) than modern seawater Sr/Ca ratio estimated at 8.6 x 10 -3. The cross plot of Sr/Ca ratios vs. Mn concentrations (Fig. 11-A) shows that the C2e and C3a carbonates are influenced by meteoric or mixed waters. However, the high Sr concentrations (>1000 ppm) in the C2 and C3 carbonates suggest strong evaporitic processes, with Sr incorporation in the host-rocks. Using the estimation for non-diagenetic limestones (i.e. Mn/Sr = 1.5 and Rb/Sr = 0.01; Bartley et al., 2001), the cross plot of Mn/Sr vs. Rb/Sr (Fig. 11-B) in the C2e and C3a members confirms a probable diagenetic effect rather than a primary origin signal. Otherwise, Delpomdor and Préat (2013) argued a possible regional thermal δ18O shift of ± 1.5‰ between the carbonate successions of the Schisto-Calcaire (Sub)Group from the DRC and Gabon, interpreted as the interaction, in ‘open’ or ‘closed’ systems, of West-Congo beltderived regional metamorphic fluids, from west to east, with the carbonate host-rocks. The effects of temperature on the δ18O values are well recognized as factors disrupting the Oisotopic signals, which incidentally, are often used as paleothermometer. Consequently, the oxygen-isotope values are not used in this paper to establish intra- and inter-basinal correlations.

We concluded that the disturbed δ13C trends of the carbonates of the post-Marinoan C2 and C3 formations reflect early diagenetic variations rather than temporal signals of global ocean chemistry, without rejecting the intrinsic stratigraphic utility of δ13C ratios for local or regional interbasinal correlations.

9. Geochemical interpretation: depositional conditions

9.1. Marine or non-marine depositional environment The previously recognized seven microfacies record a shallowing-upward standard sequence on a shallow open marine environment evolving to a peritidal margin. Although marine sedimentation is proposed in our sedimentary model for the upper part C2 and C3 formations in DRC, our geochemical data allow to discriminate between fully marine deposition or riverwater influenced depositional conditions. Since few years, new tools using REE+Y compositions as proxies for marine, lacustrine and hydrothermal waters were applied in carbonate rocks (Bau and Dulsky, 1996; Bolhar et al., 2004; Nothdurft et al., 2004; Bolhar and Van Kranendonk, 2007; Frimmel, 2009; Corkeron et al., 2012). Marine REE+Y patterns are typically characterized by uniform LREE depletion, enrichment in La, depletion in Ce, slight enrichment in Gd and Y positive anomaly (Zang and Nozaki, 1966), while REE+Y distributions of river waters are relatively flat with slight uniform LREE depletion and no distinct element anomalies (Goldstein and Jacobsen, 1988; Lawrence et al., 2006; Garcia et al., 2007). The REE+Y patterns of hydrothermal waters highlight typically a distinct positive Eu anomaly, LREE and MREE enrichments (Lawrence et al., 2006). All uppermost C2 and C3 samples exhibit uniform flat non-marine shale-normalized REE+Y distributions with typical modest LREE and MREE enrichments, absence of seawater-like Ce

and La anomalies (Figs. 9-C,D). This non-marine signature is again revealed by the Y/Ho ratios which are used as indicative proxies of marine and non-marine environments (Frimmel, 2009). All our samples display weak Y/Ho ratios ranging between 27.0 and 39.3 (n = 7; Table 3), in opposition to the ratios for open seawater ranging between 80 and 150 (Nozaki et al., 1997; Lawrence et al., 2006). River water signatures in nearshore environments display Y/Ho ratios below seawater values, i.e. below 60 (Lawrence et al., 2006). This suggests a freshwater influence during the HST and the beginning of the TST evolution in our studied series. However, the contribution of river water is expected to be intermittent, according to the semi-arid to arid conditions present in our restricted inner ramp and sabkha facies. The input of river waters is also revealed by the large amount of detrital materials in the C2d member, and locally in the C2e and C3b members, reflecting the development of nearshore discrete deltaic deposits during the episodes of high sea-level. The seawater elemental properties of the REE+Y distributions can be altered by various admixtures of terrestrial detrital contaminants in the pristine marine carbonates, producing river-water-like flattened elemental anomalies and decrease of Y/Ho ratios (Bolhar et al., 2004; Nothdurft et al., 2004; Frimmel, 2009). Th, Al and Zr concentrations are commonly used to assess shale contamination (Caron et al., 2010). All uppermost C2 and C3 samples show low Th (0.1 ppm to 6.1 ppm; n = 11), Al (0.04 % to 2.77 %; n = 11) and Zr (2.9 ppm to 17.6 ppm; n = 11) elemental concentrations reflecting a minimal continental-derived detrital input. In conclusion, the C2 and C3 carbonates of the Schisto-Calcaire Subgroup record a marine deposition, intermittently influenced by river water and land-derived materials in response of tectono-eustatic sea-level fluctuations occurred in the whole basin.

9.2. Redox conditions Some redox-sensitive trace elements such as Cd, Cu, Mo, Ni, V and U can be used to infer the redox conditions during the initial deposition. The V or Ni trace elements can be preferentially mobilized, under reducing conditions, to the sediments by organic matter and/or sulfide precipitation. These redox conditions may be confirmed by the vanadium-nickel fraction ((V/V + Ni) ratio; Lewan, 1984; Rimmer et al., 2004). Here, the (V/V + Ni) fractions (Fig. 8) vary from 0.0 to 0.4 in the C2d and C2e members (n = 6), and from 0.1 to 0.5 in the C3a and C3b members (n = 5). These values suggest clearly oxic conditions. Other evidence of oxidizing conditions is given by the indistinct to weak negative (Ce/Ce*) ratios (Fig. 8). Under oxidizing conditions, Ce3+ is oxidized to Ce4+ and the (Ce/Ce*) ratio shows a negative anomaly (Shields and Stille, 2001). Ce oxidation takes place preferentially at shallow water depths (Alibo and Nozaki, 1999). However, Ce anomaly may be negative during an increase of temperature, or absent in a low pH and/or hydrothermal precipitation. In addition, the PAAS-normalized Cr-Cu-Mo-U concentrations (Fig. 9) are lower than PAAS. These low values suggest oxidizing conditions, because, under reducing conditions, the CrCu-Mo concentrations are generally delivered to the sediment by adsorption on Fe and Mn oxy-hydroxides and/or organic matter scavenging, or sulphide precipitation (for Cu-Mo) (Algeo and Maynard, 2004). The absence of enrichment of these trace elements in the upper part C2 and C3 formations suggests that the redox conditions were oxidizing during sedimentation. Uranium is soluble under oxic environments, while, under reducing conditions, it is enriched by adsorption to organic matter and/or diffusion into sediment to form UO2 (Algeo and Maynard, 2004). In conclusion, the general depletion in trace elements in the limestones of the uppermost C2 and C3 formations provide compelling evidence that oxidized waters prevailed during the marine highstand and transgressive intervals.

10. Regional implication

10.1. Intrabasinal sedimentology According to Schermerhorn (1961), Alvarez and Maurin (1991) and Alvarez (1992), the carbonates of the Schisto-Calcaire Subgroup constitute a 2nd ordrer sequence formed during a sea-level rise that led to the drowning of extensive areas of the shelf. The latter recorded bimodal transgressive and regressive phases. The transgressive phase is linked to a glacioeustatic origin, reflecting the post-Marinoan sudden return to a greenhouse climate enhancing a global warming of the surface of the Earth, while the regressive phase is related to tectonoeustatic effects (Alvarez, 1995). Recent investigation (Delpomdor et al., submitted) suggests an extensional tectonic activity of the basin during the pre-, syn- and post-sedimentary deposition associated with the Marinoan glacial event. The resulting deposits mark two tectonically active deepening-upward subcycles associated to (i) the pre-Marinoan deposits reflecting a late stage of base-level rise or the early stage of sea-level fall, and (ii) the syn- and post-Marinoan sediments reflecting a late stage of base-level rise and forced regression, then followed by sea-level rise. The upper part of the C2 (or SCIb; Fig. 12) Formation in the DRC is interpreted as a HST stage (Fig. 12) favouring the development of carbonate systems along the shoreline and within the deep-water setting. In the RC, Alvarez and Maurin (1991) described sub-marine dunes between the Nianga-Niari and Comba aulacogens, reflecting a NE deepening of the basin. This trend is confirmed by shallower-water and hypersaline carbonate deposits in the DRC. The late HST is marked by the turn on of the siliciclastic sedimentation, with the increase of clay inputs at the C2 (or SCIb; Fig. 12) and C3 (or SCIc; Fig. 12) transition (Sikorski, 1958),

associated with a decrease of the carbonate productivity. The ‘final’ HST, initiating the burial of the carbonate ramp by prograding siliciclastic rocks, or an early TST are marked by the significant chemical change at the base of the C3 (or ScIc; Fig. 12) Formation (Cailteux et al., submitted). In the Comba aulacogen, the deposits are dominantly composed of low energy carbonates protected from the open sea by distal barrier reefs (Trompette and Boudzoumou, 1988; Bertrand-Sarfati and Milandou, 1989; Alvarez and Maurin, 1991; Alvarez, 1995). In Gabon and the DRC (Fig. 12), the deposits record inner ramp and tidal flat environments. The last transgression preceedes the second backstepping of (i) an open marine to lagoonal sedimentation in the entire basin with the oolite shoals, and (ii) an hypersaline lagoons at the end of the C3 (or SCIc; Fig. 12) Formation. Dadet (1969), Alvarez and Maurin (1991), and Cailteux et al. (submitted) indicated that the top of this formation has been exposed to erosion and/or karstification, suggesting a regionalscale turn off of the carbonate factory, during a probable short-time HST phase. In conclusion, the carbonate succession of the lowermost part of the Schisto-Calcaire Subgroup, i.e. the C1 to C3 formations, records short-time eustatic variations related to a periodic extensional tectonic events affecting the whole basin.

10.2. Tectonic significance During Neoproterozoic times, several distinct extensional tectonic episodes affected the São Francisco-Congo Craton in the area related to the development of the Araçuaí-West Congo orogen (AWCO) (Pedrosa-Soares and Alkmim, 2011). The São Francisco-Congo Craton episodically ‘unzipped’ since c. 1.70 Ga with development of a large ‘megarift’ system on the eastern margin of the São Francisco block and the western margin of the Congo block.

On the Brazilian side of the São Francisco-Congo Craton, the first rifting and anorogenic magmatic events started with, respectively, the deposition of the Espinhaço Supergroup and anorogenic plutons of the Borrachudos and Lagoa Real suites (E1 event; c. 1.70 Ga), the volcano-sedimentary Espinhaço Supergroup (E2 event; c. 1.57 Ga), and the syn-rift deposition of the Sopa-Brumadinho Formation from the Espinhaço Supergroup (E3 event; 1.18 Ga; Pedrosa-Soares and Alkmim, 2011). Zipper rifting progressed diachronously from south to north. On the African side, an local extensional episode (E4 event; c. 1000 Ma) is characterized by the peralkaline Noqui-type granite (DRC-Angola; Tack et al., 2001 ; Eeckout, 2014 ; Glorie, pers. comm.). The bimodal (sub)surface magmatism of the Zadinian and Mayumbian groups of the West Congo Belt (920-910 Ma) are coeval with the Tonian Salto da Divisa Suite (Brazil) and the Mayumbian Complex (Gabon). This magmatism traces the E5 event (c. 930870 Ma; Pedrosa-Soares and Alkmim, 2011; Thiéblemont et al., 2009). In Brazil, the E6 event is characterized by the deposition of the pre-glacial formations of the Macaúbas Basin and the development of Southern Bahia alkaline province (c. 735-675 Ma). In the West Congo Belt, the deposition of the Lower Diamictite Formation includes interlayered tholeiitic pillow lavas and feeder sills/dykes. They mark renewed extension is dated at c. 700 Ma (Thiéblemont et al., 2009; Straathof, 2011; Delpomdor et al., submitted). The rift-drift transition marked by oceanic crust production is documented on the Brazilian side of the AWCO at 660 ± 29 Ma (ophiolites of the Ribeirao da Folha Formation; Pedrosa-Soares et al., 2011; Babinski et al., 2012). After 660 Ma, a passive margin developed in the West Congo Belt with deposition of the upper part of the Haut-Shiloango Subgroup (c. 650 Ma; Frimmel et al., 2006), followed by respectively the Upper Diamictite Formation and the Schisto-Calcaire Subgroup (c. 630-575 Ma; Frimmel et al., 2006; Poidevin, 2007). These movements in the basin reflect extensional

post-E6 sub-events, marking tectono-eustatic fluctuations during the pre-, syn- and postsedimentary deposition associated with the Marinoan glacial event (Delpomdor et al., 2014; Delpomdor et al., submitted).

11. Conclusion

The uppermost part of the C2 and C3 formations represent a tectono-eustatic bimodal 3rd ordrer sequence controlled by the development of the Araçuaï-West Congo Orogen between 630 Ma and 560 Ma. The upper part of the C2 Formation, i.e. the C2d and C2e members, were deposited, under oxidizing conditions, in a low-energy open marine (MF1-MF2) to hypersaline tidal flat (MF5-MF6)/sabkha (MF7) environments during a HST phase. During this phase, the contributions of river water and land-derived materials occurred intermittently according to the semi-arid to arid conditions prevailing on the restricted inner ramp and the sabkha environments. Within the C3 Formation, the TST was characterized by the development, under oxidizing conditions, of tidal flats (MF5-MF6) and sabkha (MF7) environments, followed by a low-energy shallow restricted inner ramp (MF4) to low-energy open marine (MF1-MF2) and massive oolitic shoals (MF3) during a sea-level rise flooding the Neoproterozoic West Congo Basin. Locally, the oolitic shoals were exposed to fresh water conditions. The transition between the HST and TST phases is missing in our sequence model due to a stratigraphic hiatus. However, this transition could correspond to a ‘final’ HST initiating the burial of the carbonate ramp by prograding siliciclastics or an early TST phase. The peritidal sea-level cycles are estimated to 1 m and 4 m in thickness, while transgressive open marine cycles within the oolitic shoals vary between 3 and 8 m in thickness. Under passive margin conditions, the eustatic variations in the uppermost part of the C2 and C3 formations are rather the consequence of short-time extensional tectonic activities in the

whole basin including several sub-events in Central Africa than the consequence of the postMarinoan deglaciation related to the Snowball Earth climate event. In term of geochemistry, the disturbed δ13C trends of the carbonates of the post-Marinoan C2 and C3 formations rather reflect early diagenetic variations related to (i) the mixing of carbonate rocks with 13C and 18O depleted fluids including decarboxylation during deep burial of carbonates and also early organic diagenesis, or (ii) the meteoric alteration during carbonate stabilization, than temporal signals of global ocean chemistry. This observation does not negate the stratigraphic utility of δ13C ratios, that are usable for intrabasinal correlations.

Acknowledgements

Actlabs laboratories are thanked for assistance with ICP–MS analyses. The geochemical as well as the O and C isotope analyses were carried out into Erlangen University in Germany (Professor Joachimski). We also thank the Royal Museum for Central Africa (RMCA) for the access for drillcores available for this study.

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Table captions

Table 1: Detailed descriptions of carbonate microfacies.

Table 2: δ13C and δ18O values, major and trace element compositions of bulk samples of the upper part of the C2 and of the C3 formations (Cailteux et al., submitted) in the Lower Congo region.

Table 3: REE+Y geochemical data of bulk samples of the upper part of the C2 and the C3 formations in the Lower Congo region.

Figure captions

Figure 1: Geological sketched map of the combined geological sheets (1:200,000) of Luozi and Kai Mbaku (S5/14 – SB33.3 sheet, after Gérards, 1964), Kinshasa (S5/15 – SB33.4 sheet, after Ladmirant, 1964), Ngungu (S6/14 – SB 33.9 sheet, after Lepersonne, 1974), Inkisi (S6/15 – SB33.10 sheet, after Ladmirant, 1971), extended to geological maps of the Republic of Congo (after Dadet, 1969) and Angola (after Laboratório Nacional de Investigação Científica Tropical, 1981). Location of the studied Kwilu 1-2-S and Lukala drillcores in black stars.

Figure 2: (A) Lithostratigraphic framework of the West Congo Supergroup in the DRC (after Delhaye and Sluys, 1923; Lepersonne, 1951, 1974; Cahen, 1978; Tack et al., 2001). *: U-Pb dating from Tack et al. (2001); **: U-Pb dating from Straathof (2011); ***: Pb-Pb and/or ArAr dating from Frimmel et al. (2006); ****: 87Sr/86Sr data from Poidevin (2007). (B) Lithostratigraphy of the Schisto-Calcaire Subgroup (after Delhaye and Sluys, 1923; Lepersonne, 1974; Delpomdor et al., submitted).

Figure 3: Detailed lithostratigraphy and lithology coupled with microfacies analyses. (A) Kwilu 2 drillcore (RMCA serial number RG41401-41.423), (B) Kwilu 1 drillcore (RMCA serial number RG41394-41400), (C) Kwilu S drillcore (RMCA serial number RG4135041393), (D) CICO quarry drillcore (RMCA serial number RG41424-41440). The Kwilu drillcores are located on the left side of the Kiwlu river at approximatively 5 km NW of the junction between the N1 highway and the Kwilu bridge. The CICO drillcore is drilled in the Lukala quarry at Lukala (CICO has been renamed CILU).

Figure 4: Sedimentary features of the uppermost C2 and C3 formations open marine to peritidal/sabkha facies in drillcores of the Kwilu river and Lukala quarry, Lower Congo region (DRC). See Figs. 1 and 3 for locations and lithostratigraphic logs. (A) Grey homogeneous limestones embedded of green clay-rich microbial limestones. Kwilu river, drillcore Kwilu 2, core 2-203, RG.41423, depth 49.50 m. (B) Decimetric-scale green microbial limestones with grey limestone intercalations - Kwilu river, drillcore Kwilu 2, core 2-151, RG.41417, depth 41.50 m. (C) Grey dolomitized limestones with tepee structures Kwilu river, drillcore Kwilu 2, core 2-098, RG.41410, depth 28.00 m. (D) Evaporitic limestones composed of green clay-rich dolomitized limestones evolving to nodular, laminated and banded interbeds of grey replaced anhydrite limestones - Kwilu river, drillcore Kwilu S, core S-71, RG.41392, depth 20.80 m. (E) Laminated and banded interbeds of grey replaced anhydrite limestones within green lime mud matrix - Kwilu river, drillcore Kwilu S, core S-39, RG.41389, depth 13.00 m. (F) Evaporitic sequence composed, from base to top, of green clay limestones evolving to centimentric-scale laminated grey limetone layers - Lukala quarry drillcore CICO, core 314, RG.41440, depth 95.00 m. (G) Massive grey limestones with finely green-coloured microbial laminae - Lukala quarry drillcore CICO, core 295, RG.41440, depth 88.00 m. (H) Alternated loosely-packed oolitic limestone beds including oncolites and lumps separated by high amplitude stylolitic joints - Lukala quarry drillcore CICO, core 14, RG.41424, depth 8.00 m.

Figure 5: Microfacies analyses. (A) Cyanobacterial laminae (MF2) in blocky calcite crystal cements - Kwilu river, drillcore Kwilu 2, core 2-203, RG.41423, sample BC2, depth 49.50 m. (B) Vertical deformation of cyanobacterial bindstone (MF5) by the growth of pseudomorphs of totally calcitized sulfate. Note the presence of relics of anhydrite crytals - Lukala quarry drillcore CICO, core 193, RG.41436, sample BC134, depth 53.00 m. (C) Intraformational

conglomerate (MF6) in blocky calcite crystal cement. The elements of the conglomerate are microsparitized and present a planar stratification - Lukala quarry drillcore CICO, core 106, RG.41430, sample BC162, depth 30.65m. (D) Oolitic grainstone (MF3) in equant calcite crystal cement. The oolites are radial to tangential. Note the presence of bladed calcite crystal cements on their rims - Lukala quarry drillcore CICO, core 5, RG.41424, sample BC188, depth 0.80 m. (E) Acicular pseudomorphs of sulphate in microsparitized mudstones (MF7). The pseudomorphs of sulfate are filled by calcite crystal cements - Lukala quarry drillcore CICO, core 136, RG.41431, sample BC154, depth 38.50 m. (F) Silicified mudstone (MF7) containing coarse rhombohedral dolomite crystals in association with anhydrite, silica (megaquartz crystals) and pyrites - Lukala quarry drillcore CICO, core 139, RG.41432, sample BC153, depth 41.80 m.

Figure 6: Ediacaran carbonate ramp geometry and microfacies distribution of the C2 and C3 formations in the Lower Congo region. The microfacies can be distinguished within the following facies belts: open marine shallow ramp (MF1 to MF2), oolitic barrier shoals (MF3), protected inner ramp environments (MF4), peritidal environments (MF6 and MF7), semi-arid to arid sabkha environments (MF7) (for classification of MF, see Table 1). Notice stromatolotic bioherm reefs in the open marine ramp observed in the RC (Trompette and Boudzoumou, 1988; Bertrand-Sarfati and Milandou, 1989; Alvarez, 1995).

Figure 7: Composite lithostratigraphic log of the C2 and C3 formations coupled with sequence stratigraphy and microfacies patterns, and Fischer plot diagram. The uppermost C2 Formation forms open marine to peritidal/sabkha cycles of 1 m and 4 m in thickness deposited in a Highstand Systems Tract (HST). The unexposed transition between C2 and C3 formations are interpreted as a ‘final’ HST initiating the burial of the carbonate ramp by

prograding siliciclastics or an early Transgressive Systems Tract (TST) position. Overlying carbonates of the C3 Formation represent open marine shallowing-upward cycles of 3 m to 8 m in thickness with the deposition of massive oolitic barrier shoals in TST position. Abbreviations: M, open marine; R, protected inner ramp; T, peritidal; S, sabkha; HST, highstand systems tract; TST, transgressive systems tract.

Figure 8: Vertical distribution of stable isotopes (δ13C, δ18O, 87Sr/86Sr), major and trace elements of the lower part of the Schisto-Calcaire Subgroup, i.e. C1 to C3 formations, in the Lower Congo region. The PAAS-normalized element anomaly is calculated as follows: Ce/Ce* = Ce/(2Pr-Nd).

Figure 9: PAAS-normalized trace element and REE+Y distributions for the C2 Formation (A and C) and the C3 Formation (B and D). See text for discussion.

Figure 10: δ13C and δ18O data patterns combined with microfacies-type deposits of the upper part of the C2 and the C3 formations of the Lower Congo region. See text for discussion.

Figure 11: Diagenetic alteration. (A) Sr/Ca vs. Mn elemental concentrations; (B) Mn/Sr vs. Rb/Sr ratios. Non-diagenetic carbonate rocks have Mn/Sr < 1.5 and Rb/Sr < 0.01. See text for discussion.

Figure 12: Cartoon with reconstructed Schisto-Calcaire stratal geometry coupled with depositional models in the Neoproterozoic West Congo Basin with distinct areas (NyangaNiari, Comba, Lower Congo, north Angola) projected onto a longitudinal line of NW-SE orientation. See text for discussion.

Table 1

Microfacies

Thickness

Sedimentary structures

Components and microfacies

Environment

MF1: detrital mudstones

Centimetric to decimetric interbedded layers of limestones; top gradual to MF2

Green-coloured clean structureless or laminated mudstones; millimetric-thick discontinuous discrete laminae; rare quartz grains

Fine-grained microsparite matrix with chloriteillite and quartz/feldspar minerals; inequigranular xeno/hypidiotopic texture (5-10 µm); smooth to crinckly dark-coloured cyanobacterial laminae

Basal unit of the open marine inner ramp; lower subtidal zone

MF2: laminar mudstones

± 5-20 cm; base gradual with MF1, top sharp to MF3

Green-coloured millimetric to centimetric smooth to crinckly laminated mudstone with interbeds of MF1; rare erosional surface

Millimetric-scale couplets with (i) dark organicrich laminar mudstones and (ii) light massive mudstones; (iii) smooth to crinckly laminae of equigranular xenotopic to hypidiptopic texture (510 µm); well preserved Siphonophycus septatum mats (Delpomdor, 2007; Préat et al., 2011)

Upper subtidal to lower intertidal zone

MF3: oolitic grainstones

Decimetric bedsets often associated with MF2-3; Beds separated by high amplitude anastomosed stylolite sets

Massive; planar crosslamination, low angle through cross-lamination; graded;rare erosional surface

Sand-sized oolites and lumps with micritic rim; tangential or radial cortex; rare bioclasts; pendant beard-like or lamellae microcrystalline calcite cements; evaporite crystals and cements; desiccation cracks

Tidal-influenced lower intertidal zone with oolitic shoals

MF4: evaporitic mudstones

Centimetric to decimetric bedsets often associated with MF5-6-7; gradual contact to MF5

Fine crystalline calcite matrix (5-10 µm); equigranular xenotopic to hypidiotopic mosaic texture; replacive evaporites (gypsum, anhydrite, polyhalite)

Basal unit of the protected inner ramp; mid- to upper intertidal zone

MF5: evaporitic microbial bindstones

Centimetric-scale layers often associated with MF6

Clean structureless mudstone; replacive evaporite crystals; rare laminations; dessication cracks Similar to MF2; millimetricscale laminations, even planar or wavy, often crinckly; replacive evaporites; gradual contacts; dessication cracks

Alternation of submillimetric-thick (i) dark finegrained crystalline organic-rich bindstones (5-10 µm) and (ii) light medium-grained organic-poor mudstones (10-15 µm); equigranular xenotopic to hypidiotopic texture; peloidal texture; replacive evaporites; bedded, banded, nodular and enterolite structures; mudcracks and tepees

Base of the peritidal inner ramp; upper intertidal to supratidal zone

MF6: intraformational conglomerates

± 10-30 cm; to gradual with MF5 and MF6

In-situ breccia; dessication cracks; recrystallized matrix

Puzzle-like texture; mudcracks; pseudomorphs of sulphates (gypsum, anhydrite, polyhalite)

Periodically emersive upper intertidal to supratidal

Table 2

Samples RG number Submember Lithology Drillcore

BC2 41423 C2d5 lst. Kwilu 2

BC7 41421 C2d5 lst. Kwilu 2

BC8 41421 C2d5 lst. Kwilu 2

BC11 41419 C2d5 lst. Kwilu 2

BC17 41417 C2d6 lst. Kwilu 2

BC20 41412 C2d7 lst. Kwilu 2

BC24 41410 C2d7 lst. Kwilu 2

BC26 41408 C2d7 lst. Kwilu 2

BC28 41406 C2e8 lst. Kwilu 2

BC30 41406 Ced8 lst. Kwilu 2

BC313 41397 C2e8 lst. Kwilu 1

BC33 41405 C2e8 lst. Kwilu 2

BC200 41393 C2e8 lst. Kwilu S

BC207 41392 C2e8 lst. Kwilu S

Stratigraphic level (m)

385.5

387.8

388.7

391.5

395.0

402.5

406.8

409.4

412.7

413.6

416.2

417.9

418.6

424.5

-4.6

-4.8

-4.8

-4.7

-4.8

-4.8

-4.5

-4.1

-4.0

-3.8

-3.9

-3.9

-4.2

-9.4 -

-10.5 -

-12.3 -

-11.2 28.5 10.6 16.1 5.1 5.2 5.7 0.7 12.4 8 64 56 11.9 147 65.8 80.6 31.8 285 17.4 < 0.1 77.3 0.3 2.57 6.1 1.3 0.30 2.5E-03 3.34 0.11

-12.4 -

-12.4 -

-12.3 64.8 6.7 36.5 3.2 2.5 2.8 0.4 27.0 4.6 34 25 2.5 88.4 334 50.8 19 680 12 < 0.1 155 0.2 3.94 2.9 1.2 0.28 2.6E-03 3.06 0.03

-11.8 85.0 0.7 47.9 0.3 0.1 0.5 0.0 7.5 0.2 1 1 2.5 78.3 9.51 5.4 1 > 5000 2.9 < 0.1 1290 < 0.1 8.02 0.4 0.9 0.01 -

-11.9 -

-11.3 -

-11.5 0.70789 84.3 2.3 47.5 1.1 0.7 1.1 0.2 16.1 2 8 13.3 5.7 20.3 15.7 8 7.2 > 1000 13 0.3 272 < 0.1 7.49 1 0.9 0.28 -

-10.6 -

-12.3 69.3 4.3 39.0 2.1 2.1 2.5 0.4 8.1 4.4 28 23 7 36.9 24.6 37.8 20.8 1830 16.2 < 0.1 71.3 0.4 7.5 3.9 1.4 0.43 6.6E-03 0.34 0.01

d13C (‰, V-4.5 PDB) d18O (‰, V-9.4 PDB) 87 86 0.70888 Sr/ Sr 71.5 CaCO3 (wt %) 5.6 MgCO3 (wt %) 40.3 CaO (wt %) 2.7 MgO (wt %) 0.3 Al2O3 (wt %) 0.8 Fe2O3 (wt %) 0.1 K2O (wt %) 1.3 MnO (wt %) 0.6 Sc (ppm) 14 V (ppm) 5.8 Cr (ppm) 1.7 Co (ppm) 31.6 Ni (ppm) 8.95 Cu (ppm) 4.5 Zn (ppm) 3.2 Rb (ppm) Sr (ppm) > 1000 8.3 Zr (ppm) 0.4 Nb (ppm) 28.6 Ba (ppm) 0.1 Hf (ppm) 6.67 Pb (ppm) 0.5 Th (ppm) 0.8 U (ppm) 0.31 V/(V+Ni) Sr/Ca Mn/Sr Rb/Sr

Table 3

BC2

BC17

BC26

BC28

BC33

BC207

BC101

BC123

BC128

BC165

BC189

RG number

Samples

41423

41417

41408

41406

41405

41392

41440

41438

41438

41429

41424

Submember

C2d5

C2d6

C2d7

C2e8

C2e8

C2e8

C3a2

C3a2

C3a2

C3b2

C3b2

lst.

lst.

lst.

lst.

lst.

lst.

lst.

lst.

lst.

lst.

lst.

Kwilu 2

Kwilu 2

Kwilu 2

Kwilu 2

Kwilu 2

Kwilu S

CICO

CICO

CICO

CICO

CICO

385.5

395.0

409.4

412.7

417.9

424.5

589.4

621.7

626.5

662.3

689.3

5.5 8.92 1 3.54 0.6 0.1 0.6 < 0.1 0.38 2.5 < 0.1 0.2 < 0.1 0.2 < 0.1

27 54.4 6.8 25.1 5 0.9 4.5 0.6 3.2 16.4 0.6 1.7 0.2 1.4 0.2

17 35.4 4.4 16.2 3.2 0.7 3.2 0.5 2.6 15.3 0.5 1.3 0.2 1 0.2

4.2 7.5 0.8 3.13 0.6 < 0.1 0.6 < 0.1 0.5 3.16 < 0.1 0.2 < 0.1 0.2 < 0.1

10.2 22.5 2.7 10.5 2.1 0.4 2.2 0.3 1.78 11.8 0.3 0.8 0.1 0.6 < 0.1

20.4 42.1 5 18.8 4 0.8 4.1 0.6 3.4 20 0.7 1.9 0.3 1.6 0.2

9 17.8 2.2 8.58 1.9 0.4 2 0.3 1.79 10.8 0.4 1 0.1 0.9 0.1

7.6 15.4 1.8 6.87 1.4 0.3 1.5 0.2 1.3 7.41 0.2 0.7 < 0.1 0.5 0.2

23 46.9 5.4 19 3.1 0.6 2.7 0.3 1.9 10.8 0.4 1.1 0.1 0.8 0.1

1.6 3.2 0.4 1.37 0.3 < 0.1 0.3 < 0.1 0.238 1.64 < 0.1 0.1 < 0.1 < 0.1 < 0.1

1 2.06 0.2 0.77 0.1 < 0.1 0.2 < 0.1 0.14 0.89 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1

1.27 1.25 0.98 1.48 1.01

148 0.96 0.98 0.91 1.15 1.04 27.33 1.01 1.57

101.7 1.07 0.95 0.92 0.96 1.04 30.60 1.13 1.42

1.49 1.14

0.96 1.06 1.01 1.25 1.00 39.33 1.46 1.53

123.9 0.99 1.05 0.98 1.11 1.01 2.57 1.06 1.03

57.27 1.02 1.15 0.98 1.14 1.01 27.00 1.00 0.83

1.08 1.13 1.01 1.42 0.99 37.05 1.37 1.20

116.2 1.09 0.96 0.95 1.65 1.03 27.00 1.00 2.08

0.86 0.85 1.18 1.09

1.36 1.23 1.34 0.90

-

-

Lithology Drillcore Stratigraphic level (m) La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Y (ppm) Ho (ppm) Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) REE total (ppm) Eu/Eu* La/La* Ce/Ce* Gd/Gd* Pr/Pr* Y/Ho (Y/Ho)PAAS (Nd/Yb)PAAS

1.55

0.94 1.37

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Highlights 1. The Schisto-Calcaire Subgroup consists of five carbonate-dominated formations. 2. Lithofacies show a peritidal to open marine inner-ramp deposits. 3. Deposition record tectono-eustatic sea-level fluctuations in the whole basin. 4. Geochemistry points a meteoric and /or thermal influence.