The Mintom Formation (new): Sedimentology and geochemistry of a Neoproterozoic, Paralic succession in south-east Cameroon

Journal of African Earth Sciences 57 (2010) 367–385

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Journal of African Earth Sciences

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The Mintom Formation (new): Sedimentology and geochemistryof a Neoproterozoic, Paralic succession in south-east Cameroon

V. Caron a,*, E. Ekomane b, G. Mahieux a, P. Moussango c, E. Ndjeng b

a UMR 8157 Géosystèmes, Faculté des Sciences, 33 rue St. Leu, 80000 Amiens, Franceb Laboratoire de Géologie des Ensembles Sédimentaires, Université de Yaoundé I, Cameroonc Centre de Recherche Géologique et Minière, Garoua, Cameroon

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 February 2009Received in revised form 4 November 2009Accepted 6 November 2009Available online 20 November 2009

Keywords:NeoproterozoicCameroonDiamictitePeliteDolostoneLacustrine carbonates

1464-343X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jafrearsci.2009.11.006

* Corresponding author.E-mail address: [email protected] (V. Ca

This paper presents a lithologic and stratigraphic description of the Neoproterozoic (ante- or syn- Pan-African orogeny) Mintom Formation (new) of southeastern Cameroon, and provides a new facies and geo-chemical analysis of the sedimentary succession, formerly referred to as the upper Dja series. The MintomFormation can be subdivided from base to top into four members that record a general increase in car-bonate content. The members (all new) from lower to upper are: Kol Member (diamictite and pelite),Metou Member (dolostone), Momibolé Member (calcareous pelite), and Atog Adjap Member (limestone).Although the lithostratigraphic architecture looks very similar to that of well-documented syn- andpost-glacial Neoproterozoic deposits, physical evidence of glacial influence is absent.

By contrast with other Central African Neoproterozoic carbonates deposited in ramp settings, the suc-cession does not contain open marine facies. Limestones consist of monotonous subhedral microspariticcalcite mosaics and display occasional microbial laminae. These observations force reevaluation of bothprevious paleoenvironmental interpretations of the deposits and their comparison with neighboring Edi-acaran carbonates.

We assume that the graded basal succession from diamictite to laminated pelitic facies is compatiblewith emplacement of mass flow deposits in toe-of-slope setting during regional uplift. Interpretation ofthe overlying Métou dolostone is uncertain though sedimentological and geochemical properties point toa likely quiet depositional setting. The upper part of the Formation, including the Momibolé and AtogAdjap Members, is conspicuously laminated, in places rhythmically and ripple-bedded, suggesting shal-low subaqueous and calm depositional conditions only interrupted by occasional slumps indicative of alocally steepened bottom topography. Evaporitic fabrics and fenestral pores further indicate shallowwater, possibly peritidal, environmental conditions. In spite of indications of shale and post-depositionalcontamination, rare earth elements (REE) plus yttrium (Y) patterns obtained from carbonate samplespoint to a non-marine origin for the Atog Adjap limestone, but instead deposition in lacustrine or lagoo-nal settings under freshwater influence.

This interpretation suggests that the Mintom Formation formed in a small-scale palaeodepression, iso-lated from the open marine environment, where confined lagoonal or lacustrine sedimentation devel-oped.

The final Neoproterozoic evolution of the Mintom Formation was dominated by erosional features,including striations and stair-cased groove structures reported for the first time here, and revealingthe passage of glaciers of likely Ediacaran age.

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1. Introduction

The present study is based on a geological reexamination of theupper Dja series, proposedly erected to Formation status as theMintom Formation to avoid confusion with the much older lowerDja series (Alvarez, 1995; Vicat et al., 1997). It includes new re-

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ron).

cently collected data and detailed sedimentological analyses ofthe main facies identified in the field. The Mintom Formation is asubhorizontal, supposedly Neoproterozoic unit that includespelitic and carbonate deposits covering an area of 300 km2 in thesoutheast of Cameroon. It is up to 100 m thick but may locally thindown to a few tens of meters according to drill core data (Van-houtte and Salley, 1986). The figuration of the upper Dja serieson geological maps is unclear. It is shown either: (1) resting uncon-formably on the Pan-African Yaoundé (North Equatorial Fold) Belt

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368 V. Caron et al. / Journal of African Earth Sciences 57 (2010) 367–385

to the north and thrust onto the Congo craton to the south (Vicatet al., 1997; Fig. 1); or (2) discordant upon the Congo craton andoverthrust by the Pan-African Yaoundé Belt (Vicat et al., 2001);or (3) unconformably overlying both these major tectonic units(Gazel and Guiraudie, 1955; Vanhoutte and Salley, 1986). It followsthat the Mintom Formation may either predate, accompany orpost-date the Pan-African collisional event that took place between620 and 600 Ma, and the subsequent nappe tectonics that contin-ued around 580 Ma (Oliveira et al., 2006; Toteu et al., 2006a). It istherefore not clear whether sediments were emplaced in a contextof extensional or compressional tectonic dynamics.

The aim of the present work is to define the Mintom Formationas a formal lithostratigraphical unit and to reconstruct its variousdepositional environments reflected by lithological variations. Ma-jor facies include carbonate and non-carbonate sediments, most ofwhich are azoic and laminated, in places rhythmically. Simplified,they comprise from base to top a diamictite, dark grey argillaceouspelites, dolostones, grey to purple calcareous laminites and creamy

Fig. 1. Simplified geological map of Cameroon showing major lithotectonic unit

banded limestones. A simplified description has been given by Ga-zel and Guiraudie (1955) and particularly Vanhoutte and Salley(1986) who identified upward lithological and geochemicalchanges, but did not decide to subdivide the succession in formalunits. The basal transition from diamictite to pelite to dolostonecompares with well-documented glaciogenic successions else-where. As such it poses the question to what extent the MintomFormation could reflect syn- and post-glacial sedimentation, andtherefore be related to any of the Neoproterozoic glacial eventswell documented from siliciclastic and carbonate successions inAfrica and other continents but so far not reported in Cameroon.

The study, based on geological mapping, facies interpretation,and geochemistry, including major and trace element determina-tions, allows us to revise and re-interpret many of the earlier find-ings. We show that the Mintom Formation differs markedly fromother Neoproterozoic carbonate rocks in the region to which ithas so far been associated, especially in the Central African Repub-lic (CAR) and the Republic of Congo (RC).

s and location of the study area (arrowed). Compiled from various sources.

V. Caron et al. / Journal of African Earth Sciences 57 (2010) 367–385 369

2. Methods

This study is based on field observations carried out over areconnaissance field season during 2007, and a two-week periodin the dry season of 2008 during which seven stratigraphic sectionswere measured and sampled. Rocks were examined in well-pre-served cliffs and stream sections in an otherwise poorly-exposedterrain covered by dense tropical forest. Approximately 50 thinsections were prepared from rocks collected in the field. All thinsections were examined and described by transmitted-light andcathodoluminescent petrographic microscopy. Cathodolumines-cence (CL) microscopy was carried out on all thin sections usinga Technosyn 8200 MK II instrument (cold cathode) at operatingconditions of 15 kV gun potential and 0.35 mA beam current.

Analyses of clay mineralogies are based on decalcified andrinsed samples by repeated centrifugation. Clay particles were sep-arated following Stokes’ procedure. X-ray diffraction (XRD) analy-sis of oriented powder mounts of bulk samples was performedusing a Bruckner D4endeavor X-ray generator at 40 kV and25 mA running conditions. Mounts were scanned from 2� to32�2h at 0.02�2h/s using Co radiation. XRD analyses were run onuntreated, glycolated, and heated samples for 2 h at 490 �C. Theprocedure allows identification of swelling minerals such as smec-tite, and distinction of chlorite from kaolinite and smectite (Holt-zapffel, 1985). Clay content and semi-quantitative percentages ofeach clay minerals were obtained from diffractograms analysedwith MacDiff 4.2.5 software. The Kübler index of illite ‘‘crystallin-ity” was also determined using XRD techniques in order to identifydiagenetic stages, especially to detect the transition from deep dia-genesis to very low-grade metamorphism, i.e. anchizone and itsimmediate limits (Kübler, 1964, 1967; Kisch, 1983). In addition,the Esquevin index was calculated because it also integrates valuesmeasured on the illite 5 Å reflection, which is less affected than theillite 10 Å reflection by overlap by other phases, such as smectiticinterstratified phases and intermediate K/Na-rich micas (Batagliaet al., 2004).

The major elements composition of 32 samples was investi-gated with inductively coupled plasma-atomic emission spectros-copy (ICP-AES). Samples were subjected to an alkaline fusion inthe LiBO2 and the remaining material was treated with HCl. About5 ml of filtered fusion solution was subsequently diluted with35 ml of HNO3, and these quantities were doubled or tripled forstandards and drift corrector. The recovered solutions were ana-lyzed on a Vista Pro (Varian) device.

Percentages of carbon, nitrogen, and sulfur contained in 48samples were determined using a CHNS-O Flash Elemental Ana-lyzer (EA) 1112 series. Bulk samples were finely ground. The pro-cedure consists in dropping 1–2 mg of powder with 3–5 mg V2O5

to facilitate combustion into a quartz tube at 1260 �C with constanthelium flow. A few seconds before the sample housed in a tin cupdrops into the combustion tube, the stream is enriched with a mea-sured amount of high purity oxygen to achieve a strong oxidizingenvironment which guarantees almost complete combustion/oxidation.

Rare earth elements (REE) and yttrium (Y) were analysed tounderstand whether the Mintom sediments display a marine orfresh water/lacustrine geochemical signature. Trace element con-centrations of 21 samples were measured by inductively coupledplasma mass spectrometry (ICPMS) using a Perkin–Elmer 5000ICPMS instrument. Fifty milligram of sample powder were dis-closed in a mixture of HCl and HClO4 acids at 120 �C in sealed Tef-lon containers for 1 week. The containers were rinsed with diluteHNO3 and the solutions boiled to dryness. The residue was dis-solved in HCl and HClO4 and evaporated to dryness a second time,before being redissolved in a mixture of HNO3, HCl and HF at

100 �C. The analysis was performed using the IM100 analyticalpackage, in which a weighted average of instrument responsesfor three certified reference materials prepared in the same man-ner as the unknown was compared with the instrument responseof the unknown solution for each element. The nominal zero con-centration was assumed to be equal to the measured responsefrom acid blank. Typical lower limits of detection are 0.09 ppm(La), 0.08 ppm (Nd, Y), 0.02 ppm (Er, Dy, Gd, Pr, Sm), <0.01 ppm(Eu, Hu, Ho, Lu, Tb, Tm, Yb). Full datasets can be made availableon request.

3. Geological setting

3.1. Regional context

The study area is located in the Central African Region in thesoutheastern Cameroon about 380 km of Yaoundé at the boundarybetween the Pan-African Belt and the Congo craton (Fig. 1). ThePan-African Belt is oriented NE–SW to ENE–WSW and is character-ized by the presence of NE–SW shear zones (e.g., Adamaoua andSanaga faults; Fig. 1) and by the southward thrusting of its south-ern limit onto the Congo craton. Rocks in the belt are metasedi-mentary and volcano-sedimentary (various schists and gneisses,migmatites, amphibolites and quartzites), and metaplutonic (gab-bros, garnet–pyroxene bearing diorites and granitoids). Metamor-phism under amphibolite to granulite facies occurred between640 and 600 Ma (Pin and Poidevin, 1987; Nzenti et al., 1988; Pen-aye et al., 1993; Toteu et al., 2001). The plutonic rocks in the beltgenerally were emplaced prior to, during and after the Pan-Africandeformation. In southern Cameroon, the Pan-African Belt is repre-sented by the Yaoundé Group (Fig. 1) that comprises schists,micaschists and gneisses. Overthrusting of the Yaoundé Grouponto the Congo craton to the south and the Dja Group in the eastis coincident with an orogenic evolution that lasted about 20 Mabetween ca. 620–600 Ma (Toteu et al., 2006a), and is associatedwith the opening and closure of volcano-sedimentary extensionalbasins in a back-arc setting (Toteu et al., 2006b).

The Mintom Formation was formerly referred to as the upperDja series of Neoproterozoic III (Ediacaran) age (Alvarez, 1995), acomponent of the Neoproterozoic Dja Series or Dja Group (Fig. 1)that also includes the lower Dja series found further east in CAR(Alvarez, 1995; Fig. 1). By comparison with the Schisto-Calcairesubgroup in RC (Alvarez, 1995; Frimmel et al., 2006), its deposi-tional setting has been similarly envisaged as a carbonate rampassociated with an overall regressive tectono-eustatic period thatpreceded the paroxysmal phase of the Pan-African orogeny (Alva-rez, 1995). Sediment accumulation was thought to occur undermarine influence in graben-like basins trending N–S parallel tothe western Great Lakes Rift (Vicat et al., 2001; Poidevin, 2007).However, correlation of the upper Dja series to other Neoprotero-zoic carbonate units in central Africa remains largely conjecturaland uncertain because the succession has never been studied indetail until now. It could well be considered a time-equivalent ofeither Ediacaran (i.e., Schisto-Calcaire subgroup, Haut-Sanghalimestone) or Cryogenian (i.e., Haut Shiloango subgroup) carbon-ates in RC, or Ediacaran Bangui limestones in CAR (e.g., Poidevin,1976; Dianzenza-Ndefi, 1983; Alvarez, 1995; Frimmel et al.,2006; Poidevin, 2007; Frimmel, 2009).

3.2. Age of the Mintom Formation

The age of the Mintom deposits is constrained by neither geo-chronological nor fossil data. The Formation unconformably over-lies the Archean Congo craton dated at 2900–3000 Ma (Toteuet al., 2001). All measured fold axes in the Mintom Formation


follow a similar trend that is subparallel to the Yaoundé Belt. Thevergence of folds is therefore parallel to the south–southeastthrusting movement of the Yaoundé nappes onto the Congo craton,thereby indicating that the Mintom Formation has been deformedduring the Pan-African orogeny. Moreover, the succession wasmetamorphosed in the epizone (see below), and as such under-went post-depositional processes that also affected Ediacaran car-bonates in RC, which were similarly folded and metamorphosedunder greenschist facies during the Pan-African collision (Dianzen-za-Ndefi, 1983; Vicat and Vellutini, 1983). These observations arein agreement with the low-grade metamorphism that character-izes the second episode of the Pan-African deformation, and theonset of southward-directed nappe tectonics recorded in thesouthern end of the Yaoundé Belt, known as the Mbalmayo Groupdomain (Nédelec et al., 1986; Oliveira et al., 2006).

It follows that sediments of the Mintom Formation are mostlikely older than 580 Ma when the Pan-African nappe tectonicsended (Toteu et al., 2001, 2006a; Ngako et al., 2003). As such, theywere probably accommodated in a basin that formed in responseto crustal rifting associated with either the geodynamics of thePan-African belt converging towards the Congo craton (Toteuet al., 2001, 2004) or the breakup of Rodinia between ca.1000 Ma and 700 Ma. The Mintom Formation is therefore consid-

Fig. 2. (A) Distribution of the sedimentary units comprising the Mintom Formation in(complemented and modified after Gazel and Guiraudie (1955), Vanhoutte and Salley (1River, (2) Ebom M’selek, (3) Dakar sur le Dja, (4) Marché Mondial. d1–d4: drill holes.

ered to have been deposited in a time-interval comprised betweenyounger Cryogenian or older Ediacaran (i.e. during the MarinoanEpoch, Williams et al., 2008; Fig. 3).

4. Stratigraphical nomenclature

The name ‘‘Dja Series”, derived from the Dja River in the Min-tom basin (Fig. 2), was first used by Gazel and Guiraudie (1955).The Dja series was divided into two sub-units referred to as theupper Dja series of supposedly Ediacaran age, and the lower Djaseries of Tonian age (Alvarez, 1995; Vicat et al., 1997; Poidevin,2007; Fig. 3).

A mining research project financed by the United Nation Orga-nization started in 1978 to evaluate the economic value of the car-bonate rocks in the Mintom basin. Results of the geological surveywere published in 1986 and concluded that the upper limestonescould be of interest for the mining industry (Vanhoutte and Salley,1986). Although Vanhoutte and Salley (1986) provided detailedsedimentologic descriptions of five drill cores complemented bymajor element analyses, they neither described nor interpretedthe various depositional units that we formerly define below. Coresare unfortunately lost and therefore could not be used in the pres-ent work, unlike the geochemical data.

the Mintom region in south-east Cameroon showing study sites and type sections986)). (B) Transect modified after Vanhoutte and Salley (1986). (1) Mouth of Métou

Fig. 3. Simplified lithostratigraphy of the Neoproterozoic strata in the Central African Region, including the lower and upper Dja series in southeastern Cameroon andnorthwestern Republic of Congo (RC). Adapted from Alvarez (1995, 1998) and Tack et al. (2001). Note that the Mintom Formation is included in the Marinoan Epoch thatspans the late Cryogenian and Ediacaran (Williams et al., 2008).


The upper Dja series is here erected to Formation status underthe name Mintom Formation to avoid confusion with the lowerDja series. The Formation is named after the locality of Mintom.It is limited below by an unconformable contact with the ArcheanCongo craton and above by recent soft clayey cover and hard later-itic crusts (Fig. 2). The Formation is subdivided into four constitu-ent members (Fig. 4), namely in ascending stratigraphic order Kol,

Fig. 4. Lithostratigraphy of the Mintom Formation formally defined in the present paperdrill hole data for the Momibolé and Atog Adjap Members are displayed in brackets. Targillite cover or lateritic crust.

Métou, Momibolé, and Atog Adjap on the basis of field, sedimento-logical, and geochemical evidence.

Given the scarcity and limited thickness of most sections crop-ping out along the Dja riverbanks for the Mintom Formation(Fig. 2), it becomes problematic to define a reference cross-sectionproviding a valid stratotype for the entire formation (Fig. 4). Con-sequently, we consider the type sections for the constitutive mem-

. Thicknesses are given according to field observations. Estimated thicknesses fromhe upper deposits of the Atog Adjap limestone are covered with either recent soft


bers of the Mintom Formation as being ‘‘provisional” until betteroutcrops can be accessed.

5. Kol Member

5.1. Contact relations

Deposition of the Mintom Formation is assumed to commencewith the accumulation of massive to crudely stratified boulder-richdiamictites found to rest unconformably upon the Congo craton inthe westernmost part of the study area (Fig. 2). Diamictites arehere informally named the lower Kol Member. At the type section,the outcrop displays up to 5 m of diamictite.

Although the contact with the overlying upper pelitic Kol Mem-ber has not been directly observed, such a proposed subdivision isbased on the transition towards the top of the lower Kol Memberto boulder-poor diamictic argillite, indicated by a general decreaseof the clast/matrix ratio.

Fig. 5. Examples of some typical lithologies and outcrop features of the upper Dja Formatboulders within the diamictite (white arrows) and the unconformable contact with the uboulder and rounded clasts (white arrow) in argillaceous matrix of the diamictite facies.the upper Kol Member. White arrows point to fractures crosscut by furrows. (D) StronglyMember at the type section (Fig. 2) in the Métou riverbed. (E) Planar-bedded facies of M(dark grey) and carbonate-rich (lighter grey) parallel beds, possibly related to low-denMember. (F) Symmetrical ripples on bed surface of the Momibolé Member with well-de

The upper contact of the pelite unit with the overlying dolomi-crosparite Métou Member has not been observed.

5.2. Lithofacies

The basal diamictite consists of poorly sorted admixtures ofclasts in an argillaceous matrix (Fig. 5A). The pebble- to boulder-sized clasts are mostly subrounded (Fig. 5B). No preferential orien-tation of elongate clasts has been observed. Clast lithologies consistexclusively of quartzite derived from local basement rocks.

Outcrops of the upper Kol Member facies occur sporadicallydown river from the previous site, and are typically poorly ex-posed. The best section is located at the mouth of the Métou River(Fig. 2) 1 m of the Kol Member pelite is exposed.

The upper pelitic Kol Member is brown to dark grey (Fig. 5C),finely laminated with laminae parallel planar, and thinly beddedfrom cm to dm thick in places. Beds are locally folded with foldsverging to the south/southeast parallel to the south-trendingthrusting of the Yaoundé belt onto the Congo craton, and fold

ion. (A) Lower Kol Member at the base of the succession. Note the rounded quartzitenderlying Congo craton (black arrows). (B) Close-up view of heterometric quartzite

(C) Thin, shallow, and straight furrows oriented N110 of possible glacial origin uponweathered outcrop of massive, faintly laminated, deformed dolostone of the Métouomibolé Member at the type section (Fig. 2) displaying alternating carbonate-poor

sity turbidity currents. White arrow points to the overlying Atog Adjap limestonefined undulating parallel laminae in section.

Table 1Major element composition (% and wt.%) of bulk samples from the Mintom Formation; n, number of analyses; SD, standard deviation, Min, minimum value; Max, maximum value.

Kol Member (pelitic sub-unit) Métou Member Momibolé Member Atog Adjap Member

n Mean SD Min Max n Mean SD Min Max n Mean SD Min Max n Mean SD Min Max

Element (%)CaCO3 8 19 15.9 1 41 2 95 1.4 94 96 27 45.6 17.2 19 78 18 79.3 6.9 62 89

Element (wt.%)CaO 2 17.2 11.7 8.9 25.5 3 28.4 1.3 27 29.4 4 29.8 2.5 26.5 32.5 10 40.4 8.7 24.9 47.9MgO 2 5.4 2.8 3.4 7.4 3 19.3 0.2 19.1 19.4 4 4 1.8 2.2 6.2 11 3.9 2.7 1 9.9SiO2 2 37.6 8.1 31.8 43.3 3 6.4 1.5 5.4 8.2 4 26.1 4 21.7 30.8 11 15.4 7.9 8.8 30.8Al2O3 2 10.1 3 8 12.3 3 1.5 1.55 1.5 1.6 4 6.8 1.3 5.7 8.5 10 3 2.2 1.5 8Fe2O3 2 4.8 1.5 3.8 5.9 3 0.8 0.1 0.7 0.9 4 3.3 95 1.5 4.1 10 1.6 0.9 0.9 3.6


axis oriented N160. Composition of the pelitic facies consistsmostly of illite, subsidiary CaCO3 (Table 1), and rare detrital quartz.Petrographic features include alternating infra-millimetric micro-crystalline translucent laminae and dark clayey cryptocrystallinecentimetric planar layers (Fig. 6A). Occasional dolomite rhombs(ca 50 lm) occur together with rare manganese dendrites. Organiccarbon is present in variable amounts (range 0–3.9%).

The uppermost surface of the section is striated and includestwo sets of straight shallow furrows with constant orientationsN60 and N110, respectively (Fig. 5C). These features were also rec-ognized in another locality upstream where directions of thin shal-low grooves are identical. Reported for the first time here, thestriated a surface is interpreted to reveal the passage of glacierswith westward ice-flow from stair-cased groove structures foundat the surface. Subglacial abrasion likely developed upon a cohe-sive substrate suggesting that the underlying sediments had beenpreviously lithified or frozen. Present on flattened surfaces locatedat similar altitudes, the striations also occur upon the Atog AdjapMember, suggesting that these features of possible glacial originpost-date the Mintom Formation.

6. Métou Member

6.1. Contact relationships

The Métou Member is patchily distributed along the Métou Riv-er (Fig. 2), and is typically highly weathered and poorly preserved(Fig. 5D). Although, the stratigraphic contact with the overlyingMomibolé Member has not been recognized, the latter crops outin the riverbed tens of meters upstream with its characteristic darkgrey finely laminated facies. This suggests that the contact be-tween both units is likely sharp, though possibly rapidlytransitional.

6.2. Lithofacies

The thickness of the Métou Member at the type section isapproximately 2 m. Rocks are pale-yellow to pinkish white, faintlylaminated in places, but appear more commonly massive due toweathering. Two samples have yielded CaCO3 contents above94%. Whole-rock CaO and MgO contents (Table 1) of three othersamples indicate that the Métou Member consists of dolomite orCa-dolomite. The Métou Member is texturally a dolomicrite and lo-cally a dolomicrosparite that is characteristically brecciated andexhibits a red luminescence under cathodoluminescent light(Figs. 6B and C). Fractures between breccia are filled with red-luminescent dolomicrospar and dolospar, and a later stage yel-low-luminescent calcitic spar cement (Fig. 6B). Peloids are occa-sional in the dolomicritic facies (Fig. 6C). Organic carbon contentis typically low (range 0–0.85%).

7. Momibolé Member


Although the basal contact of the Momibolé Member with theKol Member is unclear, being either rapidly transitional or sharp(see above), the relationship with the overlying Atog Adjap Mem-ber is underlain by an angular unconformity.

7.2. Lithofacies

The Momibolé calcareous pelite crops out repeatedly along theDja river from Momibolé, taken as the type section where the unitis 2.5 m thick, to the vicinity of Atog Adjap about 50 km down-stream (Fig. 2). The unit is affected by south–southeast vergingfolds with axes oriented N150-N160. Available drill hole data (Van-houtte and Salley, 1986) indicate that the Momibolé Member maybe at least 70 m thick (Fig. 7). It is typically a planar-bedded faciesthat consists of alternating centimeter-scale parallel beds withinternal mm thick laminae giving a rhythmic aspect to the facies(Fig. 5E). Alternating cream and brown to purple colour-bandingis associated with compositional contrasts between beds, namelycalcareous vs. argillaceous respectively. Creamy pale-brown bandsare enriched in CaCO3 (average 63.8%, range 45–78%) when com-pared to their purple counterparts (average CaCO3 = 33%, range19–41%). Bedding surfaces are typically sharp though erosionalscours have been observed, commonly at the base of carbonatebeds. Symmetrical ripples are locally well developed (Fig. 5F). Atthe type section, carbonate-rich layers become progressively morefrequent and thicker from base to top. Such upward changes ofgeochemical properties are also evident from major element anal-yses with the lower part of the Momibolé Member being relativelyenriched in SiO2, Al2O3, and Fe2O3, and depleted in CaO (Fig. 7)compared to the upper part (Figs. 7 and 8). The MgO concentra-tions for these samples are comprised between 2–6 wt.%, pointingto their likely dolomitized nature, the latter assumption being sup-ported by the presence of dolomitic rhombs in silica- and calcite-filled fractures. The clay fraction was separated and showed to bedominated by illite (90–100%), and contains minor diageneticsmectite (0–9%) as well as kaolinite (<1%) and chlorite (<1%)(Fig. 8). Calculation of organic carbon content from CHNS–O anal-ysis yields amounts ranging from 0% to 4.2% (Fig. 8).

Microscopically, fabrics are variable depending on the nature ofbeds and their stratigraphic position. Purple layers towards thebase of the Momibolé Member display a micro- to cryptocrystal-line texture of silt and clay with faint dark parallel laminae tensof lm thick (Fig. 6D). Intercalated thin clear laminae consist ofmicrospar crystals of possible detrital origin. Minute opaque min-erals (? hematite) are present throughout together with pyriteframboids and occasional very fine-grained angular detrital quartz.

Up section, microfabrics evocative of evaporitic structures canbe recognized within some brown cm-thick beds. Translucent mac-

Fig. 6. Microphotographs of the Mintom Formation rocks. (A) Faintly laminated argillaceous pelite from the upper Kol Member. (B) Red luminescent brecciated dolomicritefrom dolostone of the Métou Member. First generation of dolomicrospar cement in fracture exhibits a red luminescence whereas the following stage of calcite cementation isyellow luminescent. (C) Same as B under transmitted light with peloids (arrowed). (D) Alternating planar argillaceous and microspar laminae from the Momibolé Member atbase of the type section. (E) Alternating undulose broadly parallel bands displaying light brown cryptocristalline, translucent crystalline and dark brown argillaceous layersinterpreted as evaporitic features. Elongate crystals (arrowed) in dark brown matrix could represent pseudomorphs after evaporitic crystals, possibly gypsum. Top of MomiboléMember at the type section. (F) Calcite veins in Momibolé pelite are mostly parallel to bedding. They are interpreted to result from post-depositional metamorphism. Note theelongate and slightly curved calcite crystal laths displaying comb texture (arrow). (G) Micro- and macrospar layers separated by thin mud films, convoluted as a resultmetamorphic deformation. Initiation of metamorphic schistose foliation cutting through normal bedding planes (arrowed) resembles crenulation cleavages (Vernon, 2004).Atog Adjap limestone Member. (H) Fenestrae in recrystallized microspar after micrite from Atog Adjap limestone. (I) Same as H under cathodoluminescent light showing redluminescent euhedral to subhedral dolomite precipitates in fenestrae pores. (J) Concentrated tiny pyrite crystals produce enigmatic features of possibly biological origin (?faecal pellet). (K) Alternating micro-/macrospar layers and micritic (?) biofilms (arrowed) from the Atog Adjap Member at the type section. (L) Syntaxial and possibly compositeveins of calcite characteristic of low-grade metamorphism (Vernon, 2004). Crystals display comb texture. Curvature of the crystals towards the median line of the vein recordslater stage deformation. Momibolé Member. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


Fig. 7. Lithologies and vertical distribution of major elements reconstructed from core geochemical data and descriptions in Vanhoutte and Salley (1986). Note that bothlithofacies and geochemistry help distinguish the Momibolé and Atog Adjap units of the Mintom Formation. The Atog Adjap limestone is characterized by overall higher CaO,lower Al2O3, Fe2O3, MgO, Na2O, and K2O than the underlying variably calcareous Momibolé laminite.


Fig. 8. Vertical distribution of sulfur, nitrogen, total carbon, organic carbon, and CaCO3 contents, plus clay mineralogies and abundance of the various clay minerals in theMomibolé Member and overlying Atog Adjap Member at the Momibolé type section. CaCO3 content is highly variable in the Momibolé unit and reflects alternations ofcarbonate-rich and carbonate-poor beds likely related to climate-driven changes in sediment input (see text). Note that the unconformity between the two units is underlinedby prominent spikes of high organic carbon and sulfur concentrations possibly coincident with sediment condensation.


roscrystalline, pale-brown microcrystalline, and homogenous darkcryptocrystalline layers produce alternating up to 2-mm thick

bands parallel to bedding (Fig. 6E). Macrocrystalline layers arefilled with either mosaics of calcite and quartz crystals or elongate


calcitic laths arranged in comb texture. Isolated calcitic fibers lo-cated preferentially in the dark interlayers resemble pseud-omorphs after evaporitic crystals, possibly gypsum (Fig. 6E).

The banded microfacies is markedly different from the horizon-tal microstructures produced by veins parallel to bedding (Fig. 6F).The latter result from the sealing of cracks and fractures by elon-gate crystals that typically curve across the veins as a result of con-tinuous growth during low-grade metamorphic rock deformation(Wiltschko and Morse, 2001).

Pale brown creamy beds exhibit micro- to macrosparitic fabricsarranged in alternating laminae that compare well with the planarlaminated facies of the overlying limestone unit.

8. Atog Adjap limestone Member


The Atog Adjap Member overlies the Momibolé Member withangular unconformity. It is bounded by a thick recent lateritichorizon.

8.2. Lithofacies

Limestone beds exhibit folds and deformed structures identicalto the ones recognized in the underlying units. The Member is abanded to finely stratified pale yellow limestone that displays alter-nating cm to dm thick carbonate (micro-) crystalline beds and mmto cm thick grayish calcareous pelitic beds (Figs. 9A and B). Sedi-mentary structures include planar cross-beddings and slumps to-gether with asymmetrical ripples on bed surfaces. Neitherbioturbations nor any other macroscopic biogenic feature has beenobserved. CaCO3 content approximates 80% (Table 1). From drillhole data, the Atog Adjap limestone may be up to 30 m thick.

Fig. 9. (A and B) Field photographs of the banded (A) and locally laminated (B) AtogAdjap limestone.

On the microscopic scale, limestone beds frequently exhibit atexture of interlocking calcite crystals. They display either homo-geneous or alternating microspar and spar textures with thin inter-layered, locally convoluted, dark muddy laminae (Fig. 6G and K).Graded layers from macro- to cryptocrystalline calcispar and con-taining sub-angular quartz grains are present. Fenestrae fabrics oc-cur, particularly in homogenous microsparitic microfacies(Fig. 6H). Subhedral and euhedral rhomb-shaped crystals in fenes-trae are red luminescent dolomite (Fig. 5I). MgO concentrations ofthe Atog Adjap samples (Table 1) indicate that their mineralogyconsists of calcite but also confirms the presence of dolomite. Pyr-ite framboids are common to abundant, and tiny pyrite crystals arelocally concentrated producing enigmatic features (Fig. 6J) of likelybiological origin. Organic carbon is present though typically inminor amounts (range 0–1.1%), with the exception of sample08MOM24 located immediately above the basal unconformity(Fig. 8) that yielded high organic carbon and sulfur values of 8.9%and 0.65%, respectively.

9. Geochemical data

Geochemical data are graphically displayed in Figs. 7, 8 and 10.REE + Y data are listed in Table 2, and presented in shale-normal-ized diagrams using Post-Archaean Australian Shale for normaliza-tion (PAAS; Taylor and McLennan, 1985). Shale-normalizedelemental anomalies for La, Ce, Eu, and Gd have been calculated,and equations used (see Bau and Dulski, 1996; Bolhar et al., 2004)are as follows: La/La* = La/(3Pr � 2Nd), Ce/Ce* = Ce/(2Pr � Nd),Eu/Eu* = Eu/(0.67Sm + 0.33Tb), and Gd/Gd* = Gd/(2Tb � Dy).

Some significant differences in both major and trace elementconcentrations exist between the four members of the MintomFormation, except for a relatively low Mn content in all samples(mean Mn = 5 ± 3.8 ppm). Overall, an increase in Sr (ppm) occurswith progressively younger age of the depositional units.

9.1. Kol Member

Only one sample of the lower Kol Member diamictite was ana-lyzed and yielded the highest SiO2 (>95 wt.%) content of all Min-tom Formation samples. The sample has slightly enriched lightREE over heavy REE (Nd/Yb and Pr/Yb >1), shows a weak positiveEu anomaly (Eu/Eu* = 1.2), and is further characterized by signifi-cant negative Ce (Ce/Ce* = 0.5), Gd (Gd/Gd* = 0.63), and La (La/La* = 0.84) anomalies. The Y/Ho ratio of 27.8 is close to the PAASvalue of 27.3.

Samples 08METD02 and 08KOL06 of the upper Kol Member aredistinguished by their total REE + Y contents higher by two to threeorders of magnitude when compared to their counterparts fromoverlying and underlying units (mean REEtotal = 149 ppm vs.49 ± 25 ppm for the other Member samples). These samples arefurther characterized by relatively high Al (mean = 538 ppm), Fe(mean = 340 ppm) and Zr (mean = 110 ppm) contents, and a slightenrichment of Y in relation to neighboring REE (Fig. 10a). Othercharacteristics are negative Eu (Eu/Eu* = 0.95), Ce (Ce/Ce* = 0.91),and Gd (Gd/Gd* = 0.6) anomalies, the lack of La anomalies (La/La* = 1.02), weakly depleted light REE (mean (Nd/Yb)sn = 0.87),and slightly elevated Y/Ho ratios (mean = 32.8).

9.2. Métou Member

Dolostone samples of the Métou Member are characterized bythe lowest SiO2, Al2O3 and Fe2O3 contents, but not surprisinglyby high CaO and the highest MgO values when compared to theother samples of the Mintom Formation (Table 1). Métou Memberis also notable for its lowest REE + Y concentrations and its low Sr

Fig. 10. Shale-normalized REE + Y patterns of Mintom Formation rocks. (a) KolMember. (b) Métou (dolostone) Member. (c) Momibolé Member. (d) Atog Adjap(limestone) Member. See Fig. 2 for sampling localities.


(mean = 74 ppm), Zr (mean = 18 ppm), and Y (mean = 3.5 ppm)values (Table 1). Dolostone samples display relatively uniformshale-normalised REE + Y patterns (Fig. 10b) with a very slight lightREE depletion (mean (Nd/Yb)sn = 0.94; (Pr/Yb)sn = 0.95). Sampleslack Eu anomaly (mean Eu/Eu* = 1.01 ± 0.04), and are distinctlyCe (mean Ce/Ce* = 0.78 ± 0.15), Gd (mean Gd/Gd* = 0.6), and at alesser degree La (mean La/La * = 0.92 ± 0.08) depleted. A very weakY anomaly occurs (mean (Y/Ho)sn = 1.07) and Y/Ho ratios (27.7–30.5) compare with the PAAS value.

9.3. Momibolé Member

Momibolé samples contain moderate to high Al2O3, Fe2O3, SiO2,and CaO concentrations (Table 1). Momibolé Member samples are

enriched in Sr compared to the underlying units with values rangingfrom 282 to 747 ppm (mean = 483 ppm). Samples analysed byICPMS display remarkably consistent both REE + Y concentrations(Fig. 10c) and total REE contents (mean REEtotal = 80.8 ± 9.3), inde-pendent of sampling localities (Fig. 2). They show a slight lightREE depletion (mean (Nd/Yb)sn = 0.78; (Pr/Yb)sn = 0.77). No dis-tinct element anomalies are present yielding a flat REE + Y pattern.Weak negative La (La/La* = 0.97 ± 0.02) and Ce (Ce/Ce* = 0.92 ±0.02) anomalies are developed in all samples. Eu is depleted relativeto neighboring elements (Eu/Eu* = 0.9 ± 0.02) and Y is slightlyenriched relative to Ho with Y/Ho ratios ca. 29.

9.4. Atog Adjap Member

Atog Adjap limestone samples can be distinguished by their lowSiO2, Fe2O3, and Zr (mean = 34 ± 28 ppm) contents, but in particularby the highest CaO concentrations (Table 1). An outstanding geo-chemical characteristic of these carbonate rocks is their high Sr con-tent, which ranges from 118 to 2300 ppm (mean = 1433 ppm) andis coincident with low Al2O3 concentrations (Table 1).

The shale-normalized REE + Y patterns obtained for most of thelimestone samples are commonly flat and uniform (Fig. 10d) withvirtually no light REE anomaly ((Nd/Yb)sn = 0.99 ± 0.25). Sampleswith a slight light REE enrichment ranging from 1.05 to 1.4 displaythe highest Sr values consistently above 1500 ppm and up to2300 ppm. Element anomalies are weakly developed or absent(La/La* = 1 ± 0.08; Ce/Ce* = 0.89 ± 0.04), except for a significant Gddepletion, which is omnipresent in all samples (range Gd/Gd* = 0.53–0.65). There is no obvious Y positive anomaly (mean(Y/Ho)sn = 1.01 ± 0.1). Eu anomalies vary between positive andnegative (Eu/Eu* = 1.24 ± 0.6; Fig. 10d). However, one sample(07ATA4B) shows a significant Eu positive anomaly (Eu/Eu* = 3.1),together with the weakest Gd negative anomaly compared to theother samples (Gd/Gd* = 0.67), and high Sr content (ca. 2300 ppm).

10. Diagenetic and metamorphic alteration of the MintomFormation

The illite-cristallinity (IC) method (Kübler and Jaboyedoff, 2000)and application of the Kübler index (KI) (Kübler, 1969) and Esque-vin index (EI) (Esquevin, 1969) can satisfactorily be used to recon-struct structural and metamorphic histories in illite-bearing peliticrocks (Frey and Robinson, 1999). KI values for 50 samples are dis-played in Fig. 11. The majority of analyzed samples are character-ized by a low-grade metamorphism that evolved into the epizone.It follows that kaolinite and smectite are secondary minerals andpost-date epizonal heating of the rocks.

Epizonal metamorphic alteration is further evidenced at themicroscopic scale by the common occurrence of veins that repre-sent fractures filled with minerals, typically calcite and quartz.The internal structure of the veins displays elongate columnarcrystals displaying a comb texture (Fig. 6L). Curvature of the elon-gate crystals is common away from their basal contact with thewalls, providing evidence that deformation accompanied the veingrowth (Ramsey and Huber, 1983; Wiltschko and Morse, 2001;Fig. 6L). Such features are considered to be typical of low-grademetamorphic rocks (Rumble, 1989; Ague, 1991; Vernon, 2004).By contrast, high-grade metamorphic rocks commonly containmassive veins filled with granoblastic crystals resulting fromcoarsening and recrystallization of primary crystals (Vernon,2004.). There is no evidence of massive veins in the studied rocks.

Although metamorphic alteration is also evident outside theveins, where it has deformed and convoluted depositional laminae(Fig. 6G) and possibly biofilms, the preservation of laminar struc-tures (Fig. 6A and D), fenestrae (Fig. 6H), and peloids (6C) suggest

Table 2Trace element values (normalized) and geochemical data for the studied rocks (Mintom Formation).

ATOG ADJAP Member METOU Member

EBO04 AWA02 07ATA4A 07ATA4B 07ATD2 07ATA5 07MOM2 07MOM1 07MOM5 ATG6A 07MET1 07MET2 07MET3

REE total 31.569 46.357 34.1 33.414 69.252 63.141 90.533 41.391 20.498 31.157 18.034 24.766 24.623La 0.1338 0.1995 0.1636 0.1613 0.2626 0.2610 0.3827 0.1877 0.0914 0.1529 0.0919 0.1162 0.1120Ce 0.1256 0.1884 0.1407 0.1357 0.2626 0.2538 0.3756 0.1658 0.0842 0.1294 0.0628 0.1030 0.1005Pr 0.1438 0.2186 0.1687 0.1642 0.2990 0.2809 0.4111 0.1937 0.0997 0.1608 0.1019 0.1200 0.1155Nd 0.1463 0.2165 0.1808 0.1702 0.3280 0.2879 0.4230 0.2088 0.1018 0.1605 0.0979 0.1212 0.1142Sm 0.1874 0.2721 0.2108 0.2036 0.4252 0.3459 0.5189 0.2468 0.1189 0.1928 0.1099 0.1405 0.1405Eu 0.2019 0.3083 0.2657 0.6204 0.4056 0.3241 0.5157 0.2759 0.1269 0.2074 0.1037 0.1463 0.1463Gd 0.2082 0.2854 0.2382 0.2275 0.5000 0.3906 0.5622 0.2833 0.1266 0.2060 0.1073 0.1524 0.1524Tb 0.1964 0.2726 0.2093 0.1938 0.4703 0.3773 0.5271 0.2597 0.1189 0.1912 0.1021 0.1434 0.1447Dy 0.1923 0.2778 0.2137 0.1923 0.4701 0.3846 0.5128 0.2564 0.1068 0.1709 0.1068 0.1282 0.1496Y 0.2089 0.2937 0.1548 0.1581 0.4993 0.4256 0.5730 0.2315 0.1152 0.1437 0.1033 0.1344 0.1481Ho 0.1887 0.2704 0.1837 0.1746 0.4490 0.3865 0.5055 0.2402 0.1211 0.1715 0.0959 0.1322 0.1322Er 0.1789 0.2456 0.1614 0.1509 0.4386 0.3965 0.4772 0.2175 0.1123 0.1509 0.0947 0.1228 0.1298Tm 0.1877 0.2691 0.1580 0.1481 0.4938 0.4444 0.5358 0.2346 0.1259 0.1506 0.1012 0.1383 0.1432Yb 0.1649 0.2383 0.1291 0.1213 0.4617 0.4170 0.5004 0.1979 0.1227 0.1284 0.0940 0.1326 0.1326Lu 0.1640 0.2402 0.1178 0.1132 0.4457 0.4388 0.5058 0.1871 0.1201 0.1201 0.0970 0.1293 0.1386

Eu/Eu* 1.0604 1.1325 1.2635 3.0962 0.9215 0.9096 0.9887 1.0989 1.0669 1.0787 0.9663 1.0340 1.0309La/La* 0.9634 0.8958 1.1317 1.0593 1.0900 0.9783 0.9882 1.1496 0.9573 0.9466 0.8360 0.9879 0.9477Ce/Ce* 0.8888 0.8541 0.8981 0.8575 0.9727 0.9268 0.9410 0.9292 0.8628 0.8029 0.5931 0.8668 0.8599Gd/Gd* 0.5977 0.5894 0.6478 0.6710 0.5997 0.5820 0.6019 0.6173 0.6010 0.6088 0.5921 0.5992 0.5932Y/Ho 30.1604 29.5896 22.9670 24.6821 30.2921 30.0000 30.8782 26.2605 25.9167 22.8235 29.3684 27.7099 30.5344(Nd/Yb)sn 0.8873 0.9086 1.4009 1.4035 0.7105 0.6904 0.8454 1.0555 0.8295 1.2501 1.0422 0.9142 0.8608(Pr/Yb)sn 0.8722 0.9172 1.3073 1.3540 0.6476 0.6735 0.8216 0.9787 0.8123 1.2528 1.0846 0.9052 0.8710(Gd/Yb)sn 1.2624 1.1977 1.8454 1.8756 1.0829 0.9365 1.1237 1.4315 1.0319 1.6048 1.1418 1.1488 1.1488

MOMIBOLE Member KOL Member

MAR02 07ATA4C 07MOM3 07MOM4 METD02 KOL06 KOL02

REE total 79.473 73.033 94.208 76.515 187.999 109.745 31.656La 0.3429 0.3120 0.4160 0.3196 0.8152 0.4838 0.1280Ce 0.3317 0.3090 0.3945 0.3015 0.7161 0.4611 0.0766Pr 0.3613 0.3330 0.4428 0.3420 0.8063 0.5017 0.1744Nd 0.3678 0.3383 0.4448 0.3528 0.8204 0.5074 0.1861Sm 0.4468 0.4072 0.5441 0.4432 0.9928 0.6270 0.2757Eu 0.3981 0.3815 0.4843 0.4083 1.0176 0.5796 0.3417Gd 0.4893 0.4399 0.5622 0.4657 1.1545 0.6717 0.3476Tb 0.4574 0.4302 0.5401 0.4470 1.1189 0.6344 0.3127Dy 0.4487 0.4274 0.5342 0.4701 1.0897 0.5983 0.2778Y 0.4952 0.4370 0.5556 0.5152 1.4570 0.6681 0.2348Ho 0.4440 0.4268 0.5086 0.4773 1.1261 0.6014 0.2301Er 0.4316 0.4175 0.5018 0.4912 1.0456 0.5754 0.1965Tm 0.4716 0.4765 0.5407 0.5630 1.1111 0.6395 0.1926Yb 0.4418 0.4426 0.5195 0.5202 0.9482 0.5730 0.1706Lu 0.4434 0.4480 0.5289 0.5312 0.9561 0.5935 0.1570

Eu/Eu* 0.8841 0.9197 0.8922 0.9187 0.9838 0.9209 1.1868La/La* 0.9851 0.9686 0.9481 0.9975 1.0474 0.9866 0.8481Ce/Ce* 0.9351 0.9435 0.8949 0.9103 0.9038 0.9295 0.4711Gd/Gd* 0.5430 0.5740 0.5857 0.5861 0.5799 0.5969 0.6309Y/Ho 30.3864 27.8960 29.7619 29.4080 35.2509 30.2685 27.8070(Nd/Yb)sn 0.8325 0.7645 0.8563 0.6782 0.8651 0.8854 1.0913(Pr/Yb)sn 0.8176 0.7524 0.8524 0.6575 0.8504 0.8755 1.0225(Gd/Yb)sn 1.1073 0.9940 1.0822 0.8951 1.2175 1.1721 2.0381


that metamorphism altered but did not obliterate depositional fea-tures, thereby allowing sedimentological interpretation of the fa-cies recognized in the field.

Inferring depositional environments from REE + Y patterns, interms of marine vs. lacustrine setting, should however be consid-ered carefully (see discussion section) because metamorphismmay have lead to the remobilization of some diagnostic elements(e.g., Eu).

11. Sedimentological interpretation

11.1. Kol Member

11.1.1. Lower Kol MemberRandomly scattered clasts in a clay–silt matrix as observed to-

wards the top of the lower Kol Member, together with the lack of

slumping features in the diamictite, has elsewhere been inter-preted as a result of deposition from floating ice (e.g., Williamset al., 2008). However, the typical features of glacially transportedmaterial (e.g., facetted or striated clasts; Wopfner and Kreuser,1986) have not been recognized. Further, the observation that clastlithologies are exclusively quartzite derived from local basementrocks suggests that the diamictite is not glaciogenic (Williamsand Schmidt, 1996). The occurrence of rounded boulders in aclayey matrix suggest instead that the lower Kol Member diamic-tites are subaqueous massive flow deposits emplaced in otherwisecalm depositional environments. No preferential orientation ofclasts and the fining-upward succession from basal diamictite tolaminated pelitic facies further support this interpretation. Massflows are indicative of tectonically active basins, and commonlytriggered during uplift associated with, for example, pulses ofextension in a developing rift system (Direen and Jago, 2008). As

Fig. 11. Esquevin diagram displaying Esquevin index (EI) vs. Kübler index (KI) datafor 50 samples from the constituent units of the Mintom Formation. Combined EIthat integrates values of the illite 5 and 10 Å reflection, and KI (illite crystallinityindex) provide a means of assessing the degree of diagenetic and metamorphicalteration of the rocks. The diagram shows that most samples have beenmetamorphosed in the epizone probably during the Pan-African orogeny.


a result, clasts are derived from the uplifted footwall of normalfault-bounded blocks, and their lithologies reflect local sources,as observed in the Kol Member, rather than extrabasinal.

11.1.2. Upper Kol MemberDark colour of this facies is likely related to the organic material

content and suggests deposition under reducing conditions. Themonotonous parallel planar lamination and bedding, the clayeynature of sediments, and the absence of current-related features,scours or tidal trough cross-beds, suggest deposition in low-energyconditions. Similar facies have been interpreted elsewhere as beingassociated either with lagoonal or deep-water environments. How-ever, evidence for subaerial exposure or evaporative conditions isabsent, thereby indicating that the observed sedimentologic andpetrographic features are more consistent with deposition by sus-pension either below wave influence or in environments devoid ofcurrent activity. The preceding set of evidence indicates that sedi-mentation occurred in the deepest-water settings when comparedto the other units.

11.2. Métou Member

The predominantly massive nature of the deposits due to poorexposure conditions makes interpretation in terms of depositionalenvironment difficult. However, faint planar laminations in places,the lack of graded beds and the dolomicritic texture of the sedi-ments are features consistent with suspension settling of micriteand low-energy conditions. However, the occasional occurrenceof fine-grained peloidal laminae could originate in episodic sedi-ment input by turbidity currents.

The origin of the brecciated microfacies is interpreted to relateto burial compaction rather than subaerial exposure for the follow-ing reasons: (1) peritidal fabrics such as solution vugs, fenestraeand pendant cements are absent; (2) breccia has only been ob-served on a microscopic scale; (3) tepee structures produced bysyndepositional deformation and resulting in brecciated facies ona macroscopic scale have not been observed; (4) cracks and frac-tures are randomly distributed and independent of bedding planes;(5) cracks are filled with a first generation of dolomite crystal

mosaics followed by a second generation of calcite cement thatcould be traced in the overlying beds, a common feature in ancientburied dolostones (Morrow, 1990a,b); and (6) the stratigraphic po-sition of the Métou dolomicrite overlying clayey sediments couldhave resulted in brecciation due to differential strain response toburial compaction.

11.3. Momibolé Member

Despite the lack of significant grading features, local scours atthe base of carbonate layers could result from low-density turbiditycurrents in a background of otherwise suspension settling condi-tions (Walker, 1984). Microfacies, particularly towards the base ofthe Momibolé unit at the type section, appear rhythmically lami-nated with alternating sub-millimetric clayey and microspar layers,which strongly resemble varved deposits in Pleistocene series. Awide range of depositional settings have been envisaged for lime-stone-argillite rhythmites, namely lacustrine, shallow marine tohemipelagic, and deeper water settings of epeiric seas (e.g., West-phal and Munnecke, 2003). Two main origins for the rhythmicitybetween carbonate-rich and carbonate-poor layers are commonlyproposed: (1) cyclic fluctuations in depositional conditions impart-ing repetitive changes in sediment input (e.g., Elrik and Hinnov,2007); and (2) diagenetic redistribution of calcareous material pro-duced by dissolution during burial (Westphal et al., 2000). The for-mer model is preferred here because of the preservation of thedelicate sub-millimetric laminae in the calcareous pelites and thelack of differential compactional features between carbonate-richand carbonate-poor layers that would be expected in the case ofpervasive dissolution and subsequent carbonate redistributionand cementation (Ricken, 1986; Westphal et al., 2000).

Rhythmical bedding associated with isolated dropstones hasbeen widely recognized in Proterozoic glacial rocks and is inter-preted to indicate seasonality in pro-glacial lakes (Tomazelli andSoliani Junior, 1997; Young and Nesbitt, 1999; Young, 2002). Suchlaminites arranged in a rhythmical manner are commonly inter-preted to be suspension deposits produced via seasonal freeze–thaw cycles. This interpretation is however inconsistent with theabsence of dropstones or lonestones in the Momibolé laminated fa-cies. An alternative interpretation would be that the rhythmically-bedded Momibolé Member represents shallow-water outer-shelfrhythmites as proposed by Alvarez (1998) for similar alternatingcarbonate and argillite beds of the Bobassa Formation in the Neo-proterozoic Bangui limestone in SW-CAR.

The upward transition from horizontally laminated claystoneswith occasional thin interlayered carbonate-rich laminae (Van-houtte and Salley, 1986) to alternating ripple cross-laminated cal-careous pelites and carbonate siltstones, together with thepresence of evaporitic features towards the top of the MomiboléMember, suggest a progressive shallowing, culminating in settingsperiodically subaerially exposed.

11.4. Atog Adjap Member

Fine-scale lamination and fine-crystal size of the limestone aresimilar to strata described in offshore to basinal depositional set-tings. However, the presence of planar cross-beddings and rareasymmetrical ripples provide evidence for current activity. Moder-ately developed slump structures and occasional erosional surfacesat the base of limestone beds suggest sediment accumulation upongently dipping palaeoslopes favoring low-density turbidity cur-rents. Although micro- and macrospar textures likely result fromrecrystallization of primary micrite, presence of graded layers frommicrospar crystals to cryptocrystalline matrix, and containing sub-angular silt size quartz grains suggest that carbonate grains may beof detrital origin. The occurrence of mud drapes, possibly microbial


films, intercalated between graded spar layers testifies to the epi-sodic, discontinuous character of sediment accumulation, withalternation of rapid depositional events and undisturbed phasesallowing mud settlement or biofilm development. Biogenicity ofthin micritic laminae in carbonate beds is assumed from theirexternal appearance. They are characterized by wavy and locallycomplex convoluted structures, which together with their densemicritic fabric, strongly suggest, though not unequivocally, amicrobial origin (Perry, 1999; Riding, 2000). Another evidence forthe temporal formation of microbial mats is fenestrae fabrics,which originate in organic matter decay and subsequent limitedoutflow of the resultant gas from the sediment. Fenestral poresin fine-grained carbonate facies have been abundantly reportedfrom rocks of various ages (e.g., Tucker and Wright, 1990), includ-ing Neoproterozoic, and are commonly attributed to peritidal envi-ronments (e.g., Grotzinger and James, 2000).

Euhedral shape of most dolomite crystals filling in fenestraepores suggests precipitation during early diagenesis of organic-richdeposits (Compton, 1988; Tucker and Wright, 1990; Moore et al.,2004), hence providing further indications of microbial activity inthe sediments.

At normal Earth surface conditions, precipitation of dolomite ispreferentially organically mediated (Warren, 2000). Suboxic toanaerobic conditions developing below the sediment–water inter-face favour degradation of organic material by sulfate-reducingbacteria, thereby increasing pore-fluid concentrations of CO2�

3 ,decreasing inhibitory effects of sulfate, and favoring dolomite pre-cipitation (Compton, 1988).

Although not necessarily organically-induced, abundance ofpyrite in the Atog Adjap Member indicates burial in redox condi-tions and the presence of free iron. Iron may either have come fromaeolian transport to surface waters, or be entrained in upwellingwaters (Johnson et al., 1999), or derive from fluvial sources (Haese,2005). However, siliciclastic input must have been limited consid-ering the low to moderate content of detrital material in the sedi-ments. The lutite size of quartz grains agrees well with an aeoliantransport.

Current-generated bedforms and evidence for peritidal sedi-mentation in this light-coloured facies indicate that sedimentsaccumulated in very shallow water settings.

12. Discussion

12.1. Marine vs. lacustrine depositional environment

The elaboration of refined depositional models for the MintomFormation is difficult due to the limited vertical extent and discon-tinuous nature of outcrops. Moreover, the lack of fossils, biogenicfeatures and diagnostic sedimentary structures makes it problem-atic to decide whether sedimentation took place: (1) in a terrestrialcontext either under or away from marine influence; or (2) in fullymarine environments.

Some sedimentologic features found in the Mintom formationare indicative of depositional environments coincident with eitherouter shelf marine settings, or lagoonal, or lacustrine environ-ments under low-energy conditions. These settings may be envis-aged for the Momibolé Member based on the presence ofrhythmical carbonate-rich and carbonate-poor beddings, the ubiq-uity of symmetrical, wave-generated ripples, and the lack of bi-directional sedimentary structures, but uncertainty arises whenconsidering the carbonate units, namely the Métou and Atog AdjapMembers.

Ocean, lacustrine and hydrothermal waters and their precipi-tates are markedly different in their REE + Y composition, henceoffering a potential means of deciphering palaeoenvironments for

ancient sedimentary rocks (Elderfield and Greaves, 1982; Van Kra-nendonk et al., 2003; Nothdurft et al., 2004; Bolhar and Van Kra-nendonk, 2007; Frimmel, 2009) provided that detritalcontamination or diagenetic and metamorphic alteration havenot blurred their primary elemental signatures.

REE + Y patterns for the Métou dolostones and the Atog Adjaplimestone do not display pronounced elemental anomalies, exceptfor consistent negative Gd concentrations, and as such lack a strongseawater signature reported to be characterized by positive Eu, Gd,and Y, and negative Ce anomalies (Bolhar et al., 2004). Instead therelatively unfractionated REE + Y patterns of the carbonate unitsresemble those yielded by modern river waters (e.g., Lawrenceet al., 2006), particularly by lake Naivasha (Ojiambo et al., 2003)and at a lesser degree lake Tanganyika (Barrat et al., 2000), andby non-marine stromatolitic carbonates (Bolhar and Van Kranen-donk, 2007). The lack of a La anomaly in the analyzed samplesand Y/Ho ratios (22.3–30.9) well below seawater values (i.e., ca.60) but instead that compare with PAAS values are in agreementwith a freshwater depositional setting. Frimmel (2009) docu-mented similar trends from the Neoproterozoic carbonates of theRosh Pinah Formation in Namibia and South Africa and interpretedthem as being of meteoric origin.

However, various admixtures of terrestrial detrital contami-nants to pristine marine carbonates can alter their seawater ele-mental properties to such a degree that REE + Y patterns areflattened, elemental anomalies are obscured and Y/Ho ratios low-ered to PAAS values (Bolhar et al., 2004; Nothdurft et al., 2004;Frimmel, 2009). Concentrations of Zr, Th and Al are commonlyused as monitors to assess shale contamination because they arepresent in detrital material, including clay minerals. In addition,co-variations between Y/Ho vs. Zr, Zr vs. total REE, and Al vs. Thhave been shown to testify to shale contamination (Bolhar et al.,2004; Frimmel et al., 2006; Bolhar and Van Kranendonk, 2007;Frimmel, 2009). Contamination must be suspected for the AtogAdjap carbonate samples on the basis of: (1) relatively high con-centrations of Zr (4–85 ppm) and Th (0.09–6.3 ppm); (2) positivecorrelation between Zr and total REE (R2 = 0.83; Fig. 12a); (3) goodpositive correlation between Al and Th (R2 = 0.96; Fig. 12b); (4) co-variations between Zr and Y/Ho (R2 = 0.68; Fig. 12c). Surprisingly,plotted Y/Ho vs. Ce/Ce* ratios (Fig. 12d) display a well-defined po-sitive correlation (R2 = 0.89) for samples suggesting that theirREE + Y compositions are actually pristine (Bolhar et al., 2004).Although Zr (15–21 ppm) and Th (0.9–1.2 ppm) concentrationsare also high in the Métou dolostones samples, the other trendsindicating contamination are less obvious (Fig. 11b–d), exceptfor a good positive correlation between Zr and total REE (R2 =0.77).

In addition, metarmophism in the epizone of the Mintom For-mation had the potential to remobilize Eu (Bau and Dulski,1996). The lack of significant Eu anomalies in all the carbonatesamples but one lends support for this assumption.

To conclude, Mintom carbonates have likely been variably con-taminated by detrital material. In spite of great care during samplecollection and handling, subsequent in situ contamination is not tobe ruled out. Hence, the use of REE + Y patterns for palaeoenviron-mental reconstructions is compromised.

Nonetheless, the evidence of detrital input could also indicatethat deposition occurred in near-shore environments close to ter-restrial material sources rather than in distal deep-water environ-ments. Presence of detrital quartz, relatively moderate to high Zr,and Y (2.8–13.5 ppm) could reflect continental inputs by rivers.Moreover, carbonate formation under freshwater influence eitherin lacustrine or brackish lagoonal environments could satisfactorilyexplain the non-marine signature of the Mintom carbonates, par-ticularly the Métou dolostone, and the flat REE + Y patterns(Fig. 10).

Fig. 12. Relationships between (a) Zr vs. total REE, (b) Th vs. Al2O3, (c) Y/Ho vs. Zr, and (d) Y/Ho vs. (Ce/Ce*)PAAS in order to assess contamination of the Mintom Formationcarbonates by terrestrial detrital material. Positive correlation between Zr and total REE, good positive correlation between Al and Th, and co-variations between Zr and Y/Hopoint to contamination of Atog Adjap samples. Surprisingly, plotted Y/Ho vs. Ce/Ce* ratios display a well-defined positive correlation for samples suggesting that their REE + Ycompositions are actually pristine (see text for discussion). Full square: Atog Adjap samples; hollow circles: Métou samples; full triangle: Kol samples; hollow hexagons:Momibolé samples.


12.2. Comparison with neighboring Neoproterozoic successions

The Congo craton has been affected in upper Proterozoic timesby extensional processes that led to the formation of several majortectonic depressions, among which the NE–SW trending Sanghaaulacogen and the NW–SE trending fault basin known as theHaute-Sangha trough, bounding the Ntem craton to the southand east, respectively (Fig. 1; Poidevin, 1985; Vicat et al., 1989).The latter contains thick Neoproterozoic strata, including the lowerDja series of Tonian age and the Haute-Sangha limestone of Ediac-aran age (Alvarez, 1995). The location of the upper Dja series in thenorth-westernmost part of the Haute-Sangha depression has pro-vided previous authors a strong argument that the upper Dja seriesand Haute-Sangha limestone are correlated (Alvarez, 1995, 1998;Poidevin, 2007). As such, both carbonate systems would have beenemplaced during a major transgressive–regressive cycle followingthe melting of the Cryogenian ice cap, namely the Schisto-Calcairesubgroup cycle first recognized in RC, and its correlative Banguilimestone in SW-CAR. The marine Schisto-Calcaire subgroup ofEdiacaran age has been shown to rest unconformably upon con-densed dolostones, which in turn overlie post-glacial red marlstransgressively emplaced upon glacial diamictites (Alvarez andMaurin, 1991; Alvarez, 1995). This succession compares remark-ably with both the Mintom Formation succession and Neoprotero-zoic glaciogenic and post-glacial deposits reported from all aroundthe world (e.g., James et al., 2001; Alvarenga et al., 2004; Deynouxet al., 2006; Nédélec et al., 2007; Alvaro et al., 2007; Corsetti et al.,2007; Shields et al., 2007; Shen et al., 2008). However, the studiedsuccession has yielded no physical evidence of glaciogenicitypointing to deposition during an interglacial period, most likelywedged between Cryogenian and Ediacaran glaciations (seeabove).

Palaeoclimatic indicators are scarce in the Mintom Formation.Absence of palaeokarst solution in the shallowest carbonate depos-

its, but instead the occurrence of evaporitic features would suggestaridity or semi-arid conditions (Esteban and Klappa, 1983). Precip-itates such as aragonite, either pristine or neomorphosed, and non-skeletal carbonate sediments indicative of warm-water deposition,including ooids, aggregates, and stromatolitic bioherms are absent.These attributes would indicate cool-water settings for the Mintomcarbonates (e.g. Nelson, 1988; James, 1997), unless they weredeposited in non-fully marine environments.

In support of this assumption, and in addition to the lack ofshelf-rim facies, sedimentary structures that characterize openmarine settings, such as hummocky cross-stratification (HCS),low-angle planar cross-beds, and/or bi-directional current indica-tors have not been observed.

As such, the Mintom Formation contrasts markedly with otherNeoproterozoic carbonate sequences in Africa and elsewhere thatyielded diagnostic features allowing their palaeoenvironmentaland palaeoclimatic interpretation as open marine or lagoonal,and warm-water in character (e.g. Alvarez, 1995; Germs, 1995;Moussine-Pouchkine and Bertrand-Sarfati, 1997; Day et al., 2004;Deynoux et al., 2006).

12.3. Depositional model

Fig. 13 displays a schematic reconstruction of the depositionalconditions proposed for the Mintom Formation. Stratigraphic rela-tionships between the successive deposits together with the faciesassemblages described above are proposed to reflect a lacustrine tolagoonal setting with possible episodic marine intrusion on the ba-sis of: (1) the lack of sedimentary and geochemical open marinesignature in the deposits; (2) cyclic alternation of thin to very thinparallel laminated argillaceous and calcareous pelites of the upperKol and Momibolé Members that suggest deposition into a lakeand repetitive changes in sediment input of possible climatic ori-gin; (3) evaporitic features; and (4) early diagenetic dolomite crys-

Fig. 13. Schematic reconstruction of the depositional environment envisaged for the Mintom Formation. The succession is interpreted as a lacustrine or lagoonal system in agraben-like syn-rift basin, with limited sediment input and possible episodic marine invasion.


tallization in fenestrae fabrics of the Atog Adjap Member. Althoughdolomite forms in many diagenetic environments (Morrow, 1990b;Tucker and Wright, 1990; Machel, 2004), precipitation of the AtogAdjap dolomite crystals could have been bacterially induced as re-ported from hypersaline lagoons in anoxic conditions (Vasconcelosand McKenzie, 1997).

In this context, the basal diamictite (lower Kol Member) inter-preted as a mass and debris flow deposit in low-energy environ-ments could represent a short-lived alluvial fan system.

Scarcity of terrigenous-clastic material in the deposits suggeststhat the lake or lagoon was sheltered from continental inputs. Ab-sence of terrigenous-clastic material but in the deepest waterdeposits, where it occurs as rare fine-grained sediments in ma-trix-dominated beds, could also indicate that the depositional set-ting was bordered by a low-relief topography.

Occasional slumps and structures associated with turbidity cur-rents require locally steepened palaeoslopes in an otherwise gentlyslopping, ramp-like, system.

The proposed setting is different from the depositional model ofwell-developed open marine carbonate ramps as envisaged for theSchisto-Calcaire subgroup, the Bangui limestone, and the Haut-Sangha limestone (Alvarez, 1995; Poidevin, 2007). We propose thatsediments of the Mintom Formation were deposited in a small iso-lated basin adjacent to a major tectonic depocentre under marineinfluence such as the Haute-Sangha trough (Alvarez, 1995; Vicatet al., 1997, 2001).

13. Conclusions

The present study is a reexamination of the Neoproterozoicupper Dja series in southeastern Cameroun that we redefine asthe Mintom Formation (new) to avoid confusion with the mucholder lower Dja series. Geological mapping, facies and geochemicalanalyses enable reconstruction of the vertical facies distributionand their interpretation in terms of depositional environment.

– The Mintom Formation comprises, in ascending order, four unitscharacterized by specific lithofacies, namely:(1) The Kol Member: diamictites of subaqueous environ-

ments gradationally passing to carbonate-poor clayeylaminites.

(2) The Métou Member: massive dolostone.(3) The Momibolé Member: alternating carbonate-poor and

carbonate-rich laminites.(4) The Atog Adjap Member: banded limestone.

– The vertical stacking of facies, which closely resembles thelithostratigraphic architecture of Neoproterozoic syn- andpost-glacial deposits elsewhere, does not however display anydiagnostic glacial feature.

– The Mintom Formation represents an overall shallowing-upward succession from facies dominated by clay lithologiesdeposited in quiet deep-water environments, to facies domi-nated by carbonate lithologies deposited in shallow-water envi-ronments under wave influence where microbial activity andevaporitic conditions occasionally developed. The successionlacks both sedimentological and geochemical evidence of openmarine conditions, as reported from Neoproterozoic carbonateselsewhere, deposited either in shelf-rim or ramp settings.Instead, the Mintom Formation is interpreted as a lacustrine orlagoonal system, although episodic marine influence is envis-aged. Deposition likely occurred in a graben-like basin charac-terized by a gently slopping, ramp-like, bottom topography,which was locally or distally steepened to account for the pres-ence of slumps in the deposits.

– The minimum age of the succession is 580 Ma because thedeposits were deformed and metamorphosed during the Pan-African orogeny. The Mintom Formation sediments were depos-ited during an interglacial period, possibly wedged between Cry-ogenian and Ediacaran glacial episodes. It is overlain by erosionalfeatures, including striations and stair-cased groove structures,that we interpret to record the passage of a Neoproterozoic gla-cier, whose age has still to be determined.

– The preliminary results of the present study document an unu-sual Neoproterozoic example of well developed non-marine car-bonates that contrast with other Neoproterozoic open marineramp carbonates in central Africa, including for example carbon-ates of the West Congolian Group in the Republic of Congo(Alvarez, 1995; Frimmel et al., 2006; Frimmel, 2009), or the Ban-gui limestones in the Central African Republic (Poidevin, 1976,2007). Hence, the succession provides an opportunity to assesshow geotectonic (e.g. inherited topography, subsidence) and


genetic factors (e.g. hydrodynamics, biological activity, siliciclas-tic supplies) have interacted to influence carbonate formation inProterozoic times.

Acknowledgements

Sylvie Régnier, Léa-Marie Bernard and Laurence Debeauvais arethanked for their expert help in providing high-quality thin sec-tions, and clay and geochemical data, respectively. Accommoda-tion at Mékoto was kindly provided by Philémon Zo’o Zame fromthe Bureau of Economical, Technical and Financial Affairs. Contri-bution of Marie-Laure Dufossé towards organization of the fieldtripto Cameroon is gratefully acknowledged. Sébastien Vasseur isthanked for editing the final English version of the manuscript.Helpful comments of JAES Edtitor Pat Eriksson, Robert Buchwaldtand an anonymous reviewer were greatly appreciated and signifi-cantly improved the manuscript.

References

Ague, J.J., 1991. Evidence of major mass transfer and volume strain during regionalmetamorphism of pelites. Geology 19, 855–858.

Alvarenga, C.J.S., de Santos, R.V., Dantas, E.L., 2004. C–O–Sr isotopic stratigraphy ofcap carbonates overlying Marinoan-age glacial diamictites in the Paraguay Belt,Brazil. Precambrian Research 131, 1–21.

Alvarez, P., 1995. Evidence for a Neoproterozoic carbonate ramp on the northernedge of the Central African craton: relations with Late Proterozoic intracratonictroughs. Geologische Rundschau 84, 636–648.

Alvarez, P., 1998. Le réseau des fosses intrcratoniques du Protérozoïque supérieurd’Afrique centrale et la progradation de la rampe Nord-Afrique centrale d’ägenéoprotérozoïque. In: Vicat, J.P., Bilong, P. (Eds.), Géosciences au Cameroun, coll.GEOCAM, 1/1998, Press, Univ. Yaoundé 1, pp. 287–303.

Alvarez, P., Maurin, J.-C., 1991. Evolution sédimentaire et tectonique du basinprotérozoicque supérieur de Comba (Congo): stratigraphie séquentielle duSupergroupe Ouest-Congolien et modèle d’amortissement sur décrochementdans le contexte de la tectogenèse panafricaine. Precambrian Research 50, 137–171.

Alvaro, J.J., Macouin, M., Bauluz, B., Clausen, S., Ader, M., 2007. The Ediacaransedimentary architecture and carbonate productivity in the Atar Cliffs, Adrar,Mauritania: palaeoenvironments, chemostratigraphy and diagenesis.Precambrian Research 153, 236–261.

Barrat, J.A., Boulegue, J., Tiercelin, J.J., Lesourd, M., 2000. Strontium isotopes andrare-earth element geochemistry of hydrothermal carbonate deposits from LakeTanganyika, East Africa. Geochimica Cosmochimica Acta 64, 287–298.

Bataglia, S., Leoni, L., Sartori, F., 2004. The Kübler index in late diagenetic to low-grade metamorphic pelites: a critical comparison of data from 10 Å and 5 Åpeaks. Clays and Clay Minerals 52 (1), 85–105.

Bau, M., Dulski, P., 1996. Distribution of yttrium and rare earth elements in thePenge and Kuruman iron-formations, Transvaal Supergroup, South Africa.Precambrian Research 79, 37–55.

Bolhar, R., Van Kranendonk, M.J., 2007. A non-marine depositional setting for thenorthern Fortescue Group, Pilbara Craton, inferred from trace elementgeochemistry of stromatolitic carbonates. Precambrian Research 155, 229–250.

Bolhar, R., Kamber, B.S., Moorbath, S., Fedo, C.M., Whitehouse, M.J., 2004.Characterisation of early Archaean chemical sediments by trace elementsignatures. Earth Planetary Science Letters 222, 43–60.

Compton, J.S., 1988. Degree of supersaturation and precipitation of organogenicdolomite. Geology 16, 318–321.

Corsetti, F.A., Link, P.K., and Lorentz, N.J., 2007, d13C chemostratigraphy of theNeoproterozoic succession near Pocatello, Idaho, USA: Implications for GlacialChronology and Regional Correlations, pp. 193-205. (SEPM Special Publication,No. 86).

Day, E.S., James, N.P., Narbonne, G.M., Dalrymple, R.W., 2004. A sedimentaryprelude to Marinoan glaciation, Cryogenian (Middle Neoproterozoic) KeeleFormation, Mackenzie Mountains, southwestern Canada. Precambrian Research133, 223–247.

Deynoux, M., Affaton, P., Trompette, R., Villeneuve, M., 2006. Pan-African tectonicevolution and glacial events registered in Neoproterozoic to Cambrian cratonicand foreland basins of West Africa. Journal of African Earth Sciences 46, 397–426.

Dianzenza-Ndefi, H., 1983. Les sédiments du Protérozoïque supérieur et leurstransformations au Nord-Ouest de la cuvette congolaise (Afrique centrale):apport des datations par les méthodes Rb-Sr et K-Ar. Unpublished PhD thesislodged at the French Geological Society (SGF), 147 p.

Direen, N.G., Jago, J.B., 2008. The Cottons Breccia (Ediacaran) and itstectonostratigraphic context within the Grassy Group, King Island, Australia:a rift-related gravity slump deposit. Precambrian Research 165, 1–14.

Elderfield, E., Greaves, M.J., 1982. The rare earth elements in sea water. Nature 296,214–219.

Elrik, M., Hinnov, L.A., 2007. Millennial-scale paleoclimate cycles recorded inwidespread Palaeozoic deeper water rhythmites of North America.Palaeogeography, Palaeoclimatology, Palaeoecology 243, 348–372.

Esquevin, J., 1969. Influence de la composition chimique des illites sur leurcristallinité. Bulletin du Centre de Recherche Pau-S.N.P.A. 3, 147–153.

Esteban, M., Klappa, C.F., 1983. Subaerial exposure environment. In: Scholle, P.A.,Bebout, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments.American Association of Petroleum Geologists, Tulsa, pp. 1–95.

Frey, M., Robinson, D., 1999. Low-grade Metamorphism. Blackwell Science, Oxford,UK. 313 p.

Frimmel, H.E., 2009. Trace element distribution in Neoproterozoic carbonates aspalaeoenvironmental indicator. Chemical Geology 258, 338–353.

Frimmel, H.E., Tack, L., Basei, M.S., Nutman, A.P., 2006. Provenance andchemostratigraphy of the Neoproterozoic West Congolian Group in theDemocratic Republic of Congo. Journal of African Earth Sciences 46, 221–239.

Gazel, J., Guiraudie, C., 1955. Mission de la descente du Dja – Missions Géologiques1 et 2: Rapports inédits. Direction des mines, Cameroun.

Germs, J.B., 1995. The Neoproterozoic of southwestern Africa, with emphasis onplatform stratigraphy and paleontology. Precambrian Research 73, 137–151.

Grotzinger, J.P., James, N.P., 2000. Precambrian carbonates: evolution ofUnderstanding. In: Grotzinger, J.P., James, N.P. (Eds.), CarbonateSedimentation and Diagenesis in the Evolving Precambrian World. Society forSedimentary Geology, Tulsa, pp. 1–20 (SEPM Special Publication, No. 67).

Haese, R.R., 2005. The biogeochemistry of iron. In: Schulz, H.D., Zabel, M. (Eds.),Marine Geochemistry. Second Revised, Updated, and Extended Edition.Springer, Berlin, pp. 241–270.

Holtzapffel, T., 1985. Les minéraux argileux: préparation, anamysediffractométrique et détermination. Socitété Géologique du Nord, 136 p(Publication No. 12).

James, N.P., 1997. The cool-water carbonate depositional realm. In: James, N.P.,Clarke, J.A.D. (Eds.), Cool-water Carbonates. Society for Sedimentary Geology,Tulsa, pp. 1–20 (SEPM Special Publication, No. 56).

James, N.P., Narbonne, G.M., Kyser, T.K., 2001. Late Neoproterozoic cap carbonates:Mackenzie Mountains, northwestern Canada: precipitation and global glacialmeltdown. Canadian Journal Earth Sciences 38, 1229–1262.

Johnson, K.S., Chavez, F.P., Friederich, G.E., 1999. Continental-shelf sediment as aprimary source of iron for coastal phytoplankton. Nature 398, 697–700.

Kisch, H.J., 1983. Mineralogy and petrology of burial diagenesis (burialmetamorphism) and incipient metamorphism in clastic rocks. In: Larsen, G.,Chilingar, G.V. (Eds.), Diagenesis in Sediments and Sedimentary Rocks,Developments in Sedimentology, vol. 2. Elsevier, Amsterdam, pp. 289–493.

Kübler, B., 1964. Les argiles, indicateurs de métamorphisme. Revue de l’InstitutFrançais du Pétrole 19, 1093–1112.

Kübler, B., 1967. Anchimétamorphisme et schistosité. Bulletin du Centre deRecherche Pau-S.N.P.A. 1, 259–278.

Kübler, B., 1969. Cristallinity of illite. Detection of metamorphism in some frontalpart of the Alps. Fortschritte der Mineralogie 47, 29–40.

Kübler, B., Jaboyedoff, M., 2000. Illite crystallinity. Comptes Rendus de l’Académiedes Sciences Paris 331, 75–90.

Lawrence, M.G., Greig, A., Collerson, K.D., Kamber, B.S., 2006. Rare earth elementand yttrium variability in South East Queensland waterways. AquaticGeochemistry 12, 39–72.

Machel, H.G., 2004. Concepts and models of dolomitization: a critical reappraisal.In: Braithwaite, C.J.R., Rizzi, G., Darke, G. (Eds.), The Geometry and Petrogenesisof Dolomite Hydrocarbon Reservoirs. Geological Society of London, pp. 7–63(Special Publication 235).

Moore, T.S., Murray, R.W., Kurtz, A.C., Shrag, D.P., 2004. Anaerobic methaneoxidation and the formation of dolomite. Earth and Planetary Science Letters229, 141–154.

Morrow, D.W., 1990a. Dolomite: part 1. The chemistry of dolomitization anddolomite precipitation. In: McIlreath, A., Morrow, D.W. (Eds.), Diagenesis.Geoscience Canada, pp. 113–124 (Reprint Series No. 4).

Morrow, D.W., 1990b. Dolomite: part 2. Dolomitization models and ancientdolostonesand. In: McIlreath, A., Morrow, D.W. (Eds.), Diagenesis. GeoscienceCanada, pp. 125–140 (Reprint Series No. 4).

Moussine-Pouchkine, A., Bertrand-Sarfati, J., 1997. Tectonosedimentarysubdivisions in the Neoproterozoic to early Cambrian cover of theTaoudenni Basin (Algeria, Mauritania, Mali). Journal African Earth Sciences24, 425–443.

Nédelec, A., Macaudière, J., Nzenti, J.P., Barbey, P., 1986. Evolution métamorphiqueet structurale des schistes de Mbalmayo (Cameroun). Implications pour lastructure de la zone mobile panafricaine d’Afrique centrale, au contact ducraton du Congo. Comptes Rendus de l’Académie des Sciences Paris 303, 75–80.

Nédélec, A., Affaton, P., France-Lanord, C., Charrière, A., Alvaro, J., 2007.Sedimentology and chemostratigraphy of the Bwipe Neoproterozoic capdolostones (Ghana, Volta Basin): a record of microbial activity in a peritidalenvironment. C.R. Geoscience 339, 223–239.

Nelson, C.S., 1988. An introductory perspective on non-tropical shelf carbonates.Sedimentary Geology 60, 3–12.

Ngako, V., Affaton, P., Nnange, J.M., Njanko, T., 2003. Pan-African tectonic evolutionin central and southern Cameroon: transpression and transtension duringsinistral shear movements. Journal of African Earth Sciences 36, 207–214.

Nothdurft, L.D., Webb, G.E., Kamber, B.S., 2004. Rare earth element geochemistry ofLate Devonian reefal carbonates, Canning basin, Western Australia:


confirmation of a seawater REE proxy in ancient limestones. GeochimicaCosmochimica Acta 68, 263–283.

Nzenti, J.P., Barbey, P., Macaudière, J., Soba, D., 1988. Origin and evolution of the latePrecambrian high-grade Yaoundé gneisses (Cameroon). Precambrian Research38, 91–109.

Ojiambo, S.B., Lyons, W.B., Welsh, K.A., Poreda, R.J., Johannesson, K.H., 2003.Strontium isotopes and rare earth elements as tracers of groundwater–lakewater interactions, Lake Naivasha, Kenya. Applied Geochemistry 18, 1789–1805.

Oliveira, E.P., Toteu, S.F., Araújoc, M.N.C., Carvalho, M.J., Nascimento, R.S., Bueno, J.F.,McNaughton, N., Basilici, G., 2006. Geologic correlation between theNeoproterozoic Sergipano belt (NE Brazil) and the Yaoundé belt (Cameroon,Africa). Journal of African Earth Sciences 44, 470–478.

Penaye, J., Toteu, S.F., Van Schmus, W.R., Nzenti, J.P., 1993. U–Pb and Sm–Ndpreliminary geochronologic data on the Yaoundé series, Cameroon: re-interpretation of the granulitic rocks as the suture of a collision in the‘‘Centrafrican belt”. Comptes Rendus de l’Académie des Sciences Paris 317, 789–794.

Perry, C.T., 1999. Biofilm-related calcification, sediment trapping and constructivemicrite envelopes: a criterion for the recognition of ancient grass-bedenvironments? Sedimentology 46, 33–45.

Pin, C., Poidevin, J.L., 1987. U–Pb zircon evidence for a Pan-African granulite faciesmetamorphism in the Central African Republic. A new interpretation of high-grade series of the northern border of the Congo craton. Precambrian Research36, 303–312.

Poidevin, J.-L., 1976. Les formations du Précambrien supérieur de la region deBangui (République centrafricaine). Bulletin de la Société Géologique de France18, 999–1003.

Poidevin, J.-L., 1985. Le Protérozoïque supérieur de la République centrafricaine.Annales du Muséum royal d’Afrique centrale, Tervuren, Belgique, Série in 8ème,Sciences géologiques, 91, 75 p.

Poidevin, J.-L., 2007. Stratigraphie isotopique du strontium et datation desformations carbonates et glaciogéniques néoprotérozoïques du nord et del’ouest du craton du Congo. C.R. Geoscience 339, 259–273.

Ramsey, J.G., Huber, M.I., 1983. The Techniques of Modern Structural Geology.Strain Analysis, vol. 1. Academic Press, London.

Ricken, W., 1986. Diagenetic Bedding: Model for Marl–Limestone Alternations.Lecture Notes in Earth Sciences, vol. 6. Springer, Berlin. 210 p.

Riding, R., 2000. Microbial carbonates: the geological record of calcified bacterial–algal mats and biofilms. Sedimentology 47, 179–214.

Rumble, D., 1989. Evidence of fluid flow during regional metamorphism. EuropeanJournal of Mineralogy 1, 731–737.

Shen, B., Xiao, S., Kaufman, A.J., Bao, H., Zhou, C., Wang, H., 2008. Stratification andmixing of a post-glacial Neoproterozoic ocean: evidence from carbon and sulfurisotopes in a cap dolostone from northwest China. Earth and Planetary ScienceLetters 265, 209–228.

Shields, G.A., Deynoux, M., Strauss, H., Paquet, H., Nahon, D., 2007. Barite-bearingcap dolostones of the Taoudéni Basin, northwest Africa: sedimentary andisotopic evidence for methane seepage after a Neoproterozoic glaciation.Precambrian Research 153, 209–235.

Tack, L., Wingate, M.T.D., Liegeois, J.-P., Fernandez-Alonso, M., Deblond, A., 2001.Early Neoproterozoic magmatism (1000–910 Ma) of the Zadinian andMayumbian Groups (Bas-Congo): onset of Rodinia rifting at the western edgeof the Congo craton. Precambrian Research 110, 277–306.

Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition andEvolution. Blackwell, Cambridge. p. 312.

Tomazelli, L.J., Soliani Junior, E., 1997. Sedimentary facies anddepositional environments Gondwana glaciation in Batovi andSuspiro regions, Do Sul, Brazil. Journal of South-American Earth Sciences 10,295–303.

Toteu, S.F., Van Schmus, W.R., Penaye, J., Michard, A., 2001. New U–Pb and Sm–Nddata from north-central Cameroon and its bearing on the pre-Pan Africanhistory of central Africa. Precambrian Research 108, 45–73.

Toteu, S.F., Penaye, J., Poudjom Djomani, Y.H., 2004. Geodynamic evolution of thePan-African belt in Central Africa with special reference to Cameroon. CanadianJournal of Earth Sciences 41, 73–85.

Toteu, S.F., Yongue Fouateu, R., Penaye, J., Tchakounte, J., Seme Mouangue, A.C., VanSchmus, W.R., Deloule, E., Stendal, H., 2006a. U–Pb dating of plutonic rocksinvolved in the nappe tectonic in southern Cameroon: consequence for the Pan-African orogenic evolution of the Central African fold belt. Journal of AfricanEarth Sciences 44, 479–493.

Toteu, S.F., Penaye, J., Deloule, E., Van Schmus, W.R., Tchameni, R., 2006b.Diachronous evolution of volcano-sedimentary basins north of the Congocraton: insights from U–Pb ion microprobe dating of zircons from the Poli, Lomand Yaoundé Series (Cameroon). Journal of African Earth Sciences 44.

Tucker, M.E., Wright, V.P., 1990. Carbonate Sedimentology. Blackwell Publishing,Oxford UK. 482 p.

Van Kranendonk, M.J., Webb, G.E., Kamber, B.S., 2003. New geological and traceelement evidence from 3.45 Ga stromatolitic carbonates in the Pilbara Craton:support of a marine, biogenic origin and for a reducing Archaean ocean.Geobiology 1, 91–108.

Vanhoutte, M., Salley, P., 1986. Reconnaissance des calcaires de Minto – Projet derecherches minières, sud-est Cameroun. United Nations Development Program,Unpublished Report 91, 59 p.

Vasconcelos, C., McKenzie, J.A., 1997. Microbial mediation of modern dolomitesprecipitation and diagenesis under anoxic conditions (Lagoa Vermelha, Rio deJaneiro, Brazil). Journal of Sedimentary Research 67, 378–390.

Vernon, R.H., 2004. A Practical Guide to Rock Microstructure. Cambridge UniversityPress, UK. 593 p.

Vicat, J.-P., Vellutini, P.J., 1983. Sur la nature et la signification des dolérites dubassin précambrien de Sembe-Ouesso (Rép. Pop. Congo). Precambrian Research37, 57–69.

Vicat, J.-P., Gioan, P., Albouy, Y., Cornacchia, M., Giorgi, L., Blondin, P., 1989. Mise enévidence, sur la bordure ouest du craton du Congo, de fossés d’effondrementd’âge protérozoïque supérieur, masqués par les formations phanérozoïques dela cuvette du Zaïre. Comptes Académie Sciences Paris 309, 1207–1213.

Vicat, J.-P., Pouclet, A., Nkoumbou, C., Semé Mouangué, A., 1997. Le volcanismefissural des séries du Dja inférieur, de Yokadouma (Cameroun) et de Nola(RCA)–Signification géotectonique. Comptes Rendus Académie Sciences Paris325, 671–677.

Vicat, J.-P., Moloto-A-Kenguemba, G., Pouclet, A., 2001. Les granitoïdes de lacouverture protérozoïque de la bordure nord du craton du Congo (Sud-Est duCameroun et Sud-Ouest de la République centrafricaine), témoins d’une activitémagmatique post-kibarienne à pré-panafricaine. Comptes Rendus AcadémieSciences Paris 332, 235–242.

Walker, R.G., 1984. Turbidites and associated coarse clastic deposits. In: Walker,R.G. (Ed.), Facies Models, vol. 1. Geosciences Canada Reprint Series, pp. 171–188.

Warren, J., 2000. Dolomite: occurrence, evolution and economically importantassociations. Earth Science Review 52, 1–81.

Westphal, H., Munnecke, A., 2003. Limestone–marl alternations: a warm-waterphenomenon? Geology 31, 263–266.

Westphal, H., Head, M.J., Munnecke, A., 2000. Differential diagenesis of rhythmiclimestone alternations supported by palynological evidence. Journal ofSedimentary Research 70, 715–725.

Williams, G.E., Schmidt, 1996. Origin and palaeomagnetism of the MesoproterozoicGangau Tilloid (basal Vindhyan Supergroup), central India. PrecambrianResearch 79, 307–325.

Williams, G.E., Gostin, V.A., McKirdy, D.M., Preiss, W.V., 2008. The Elatina glaciation,late Cryogenian (Marinoan Epoch), South Australia: sedimentary facies andpalaeoenvironments. Precambrian Research 163, 307–331.

Wiltschko, D.V., Morse, J.W., 2001. Crystallization pressure versus ‘‘crack seal” asthe mechanism for banded veins. Geology 29, 79–82.

Wopfner, H., Kreuser, T., 1986. Evidence for Late Palaeozoic glaciation in southernTanzania. Palaeogeography, Palaeoclimatology, Palaeoecology 56, 259–275.

Young, G.M., 2002. Stratigraphic and tectonic settings of Proterozoic glaciogenicrocks and banded iron-formations: relevance to the snowball Earth debate.Journal of African Earth Sciences 35, 451–466.

Young, G.M., Nesbitt, H.W., 1999. Paleoclimatology and provenance of theglaciogenic Gowganda Formation, Paleoproterozoic, Ontario, Canada: achemostratigraphic approach. Geological Society of America Bulletin 111,264–274.

Documents

The Mintom Formation (new): Sedimentology and geochemistry of a Neoproterozoic, Paralic succession in south-east Cameroon