One, two or no record of late neoproterozoic glaciation in South-East Cameroon?

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  • One, two or no record of late neoproterozoic glaciation in South-East Cameroon?

    V. Caron a,, G. Mahieux a, E. Ekoma FRE 3298 Gosystmes, Facult des Sciences, 33 Rue Stb Laboratoire de Gologie des Ensembles Sdimentaires,cCentre de Recherche Gologique et Minire, Garoua, CadCentro de Pesquisas Geocronolgicas, Instituto de Geoc

    a r t i c l e i n f o

    Article history:Received 7 May 2010Received in revised form 15 September2010Accepted 15 September 2010Available online 22 September 2010

    Keywords:

    1. Introduction

    Prolonged glaciations and short-living glacial pulses have oc-curred repeatedly at mid- to low latitudes between 750 and580 Ma, and left their marks on continents formerly within Rodinia(Kennedy et al., 1998; Evans, 2000). Lithostratigraphic and chemo-stratigraphic data from Neoproterozoic rocks provide evidence for

    two Cryogenian glaciations, during the Sturtian Epoch (ca. 750 Ma;Frimmel et al., 1996) and the Marinoan Epoch (ca. 640 Ma;Hoffmann et al., 2004; Williams et al., 2008), respectively, andfor one post-Marinoan glaciation during the Ediacaran referred toas the Gaskiers glaciation at ca. 580 Ma (Bowring et al., 2003; Knollet al., 2004; Alvarenga et al., 2007). The worldwide distribution ofglacial debris associated with these episodes of global refrigerationhas led to much controversial proposals of models for partly (Hydeet al., 2000) versus entirely frozen oceans (Hoffman et al., 1998;Hoffman and Schrag, 2002).

    Corresponding author.

    Journal of African Earth Sciences 59 (2011) 111124

    Contents lists availab

    an

    lseE-mail address: vincent.caron@u-picardie.fr (V. Caron).and N110), were identied in two different localities, in the west of the study area on siltstones of theKol Member, and in the east on limestones of the Atog Adjap Member, respectively. N60-oriented striaeindicate ice ow towards the WSW. Assigning an age to these features remains problematical becausethey were not found associated with glaciogenic deposits. Two hypotheses can equally be envisaged,i.e., either the striated surfaces are correlated: (1) to the Gaskiers (or Neoproterozoic post-Gaskiers) gla-ciation and represent the youngest Ediacaran glacial event documented in the southern Yaound belt; or(2) to the Late Ordovician Hirnantian (Saharan) glaciation, thereby providing new data about Hirnantianice ows in Central Africa.

    2010 Elsevier Ltd. All rights reserved.NeoproterozoicCameroonMarinoanGaskiersHirnantianDolostone1464-343X/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.jafrearsci.2010.09.004ane b, P. Moussango c, M. Babinski d

    Leu, 80000 Amiens, FranceUniversit de Yaound I, Cameroonmeroonincias, Universidade de So Paulo, Rua do Lago, 562, So Paulo, SP 05508-080, Brazil

    a b s t r a c t

    Severe climate changes culminating in at least three major glacial events have been recognized in theNeoproterozoic sedimentary record from many parts of the world. Supportive to the global nature ofthese climatic shifts, a considerable amount of data have been acquired from deposits exposed in Pan-African orogenic belts in southwestern and western Africa. By comparison, published data from thePan-African belts in Central Africa are scarce. We report here evidence of possibly two glacial eventsrecorded in the Mintom Formation that is located on the margin of the Pan-African orogenic Yaound beltin South-East Cameroon.In the absence of reliable radiometric data, only maximum and minimum age limits of 640 and 580 Ma,

    respectively, can at present be applied to the Mintom Formation. The formation consists of two litho-stratigraphic ensembles, each subdivided in two members (i.e., in ascending stratigraphic order theKol, Mtou, Momibol, and Atog Adjap Members). The basal ensemble exhibits a typical glacial topost-glacial succession. It includes diamictites comprising cobbles and boulders in a massive argillaceoussiltstone matrix, and laminated siltstones followed by, in sharp contact, a 2 m-thick massive dolostonethat yielded negative d13C values (

  • There is a considerable amount of data documenting the occur-rence of Neoproterozoic glacial and glacially-inuenced eithermarine or terrestrial deposits in the Pan-African orogenic belts ofWest Africa (e.g., Ponsard et al., 1988; Proust and Deynoux,1994; Porter et al., 2004; Deynoux et al., 2006; Alvaro et al.,

    2007), and southwestern Africa (e.g., Flling et al., 2000; Fllingand Frimmel, 2002; Halverson et al., 2004). By comparison, pub-lished data from the Pan-African belts located north of the Congocraton in Central Africa are few (Frimmel et al., 2006; Wendorffand Key, 2006; Poidevin, 2007). Little is known about the

    Fig. 1. (A) Distribution of alleged Neoproterozoic sedimentary rocks in Africa. (B) Simplied geological map showing the outcropping location of Neoproterozoic stratasurrounding the Congo Craton, including the Mintom Basin in South-East Cameroon.

    ng(C

    112 V. Caron et al. / Journal of African Earth Sciences 59 (2011) 111124West Congo(DRC)

    Fouroumbala BasinBakouma

    (CAR)

    Ba

    Mpioka MolasseUpper Mixtite Fm.

    Cap carbonate

    HautShilohangoSubgroup

    Lower Tillite Fm. Lower Tillite Fm.

    Cap carbonate

    BakoumaDolomite

    Fm.

    Bili Limestone Fm.

    Dialinga Fm.

    BougboulouSubgroup

    Bonda Tillite Fm.

    Kembe NakandoSandstone

    Mbania pelite

    SansikwaSubgroup

    BaLimest

    Schisto-Calcaire

    SubgroupBimbo S

    F

    Conglo

    Pan-

    Afric

    an O

    roge

    nyan

    d na

    ppe

    tect

    onic

    s

    Fig. 2. Lithostratigraphy of Neoproterozoic strata in Central Africa, including the study aCAR: Central African Republic; DRC: Democratic Republic of Congo. The age of the Mintoshown by opposite black arrows.ui BasinAR)

    Southeast Cameroon(This study)

    Gaskiers

    GlacialEventsCryogenian Tonian

    Ediacaran

    Glaciation(580 Ma)

    MarinoanGlaciation(635 Ma)

    (660 Ma)Sturtian

    Glaciation(750 Ma)

    Lower Dja series(Age unknown)

    nguione Fm.

    andstonem.

    merates

    ?

    Min

    tom

    For

    mat

    ion

    Kol Diamictite ?

    MtouDolostone ?

    Momibolargillaceous and

    calcareous siltstone

    Atog AdjapLimestone

    rea in South-East Cameroun (modied after Frimmel et al. (2006), Poidevin (2007)).m Formation is not radiometrically constrained, hence its given arbitrary duration is

  • EarV. Caron et al. / Journal of AfricanNeoproterozoic deposits fringing theYaoundBelt inCameroon, anditwas not until recently that the lithostratigraphy ofNeoproterozoicstrata has been described in detail in South-East Cameroon (i.e., theMintom Formation; Figs. 2 and 3; Caron et al., 2010).

    Two distinct glaciogenic units have been recognised withinNeoproterozoic formations from the West and the North of theCongo Craton. The Sturtian event is represented by tillites, whichare not capped by carbonate beds, namely the lower Mixtite ofthe West-Congolian Group, the Akwokwo tillite (Lindian) of theeastern Democratic Republic of Congo (DRC), the GrandConglomrat in southeastern DRC, and the lower tillite of theFouroumbala Basin in Central African Republic (CAR; Key et al.,2001; Wendorff and Key, 2006; Poidevin, 2007). The Marinoanevent might be represented by the Upper Mixtite Formation ofthe West-Congolian Group (Frimmel et al., 2006) and the thin

    Fig. 3. (A) Simplied geological map of Cameroon showing major lithotectonic units and(Caron et al., 2010). Thicknesses are given according to eld observations. Estimated thicbrackets. (C) Geological map of the study area showing distribution of the sedimentary u(complemented and modied after Vanhoutte and Salley (1986), Caron et al. (2010)). Inth Sciences 59 (2011) 111124 113Banda tillite of the Fouroumbala Basin (Fig. 2). These are overlainby cap carbonates referred to as the Schisto-calcaire Subgroup inthe West-Congolian Group and the Bakouma dolomites in theFouroumbala Basin (Fig. 2; Frimmel et al., 2006; Poidevin, 2007).Direct evidence of younger glaciogenic deposits recording theGaskiers glaciation have not been documented from the latestNeoproterozoic Formations from eastern DRC to southern Camer-oon. It must be acknowledged, however, that this apparent lackof syn-Gaskiers deposits in Central Africa might be due to the ab-sence of radiometric dating for many glaciogenic sediments recog-nized in the region.

    The stratigraphy of the Neoproterozoic deposits in South-EastCameroun (Figs. 2 and 3), formerly referred to as the older lowerDja series and younger upper Dja series, is poorly documented,and their stratigraphic position is uncertain with respect to the

    location of the study area (arrowed). (B) Lithostratigraphy of the Mintom Formationknesses from drill cores for the Momibol and Atog Adjap Members are displayed innits and striated pavements within and above the Mintom Formation, respectivelysert shows the AB transect in C. Vertical scale exaggerated.

  • likely late Cryogenian to early Ediacaran age (Caron et al., 2010).

    standards and drift corrector. The recovered solutions were ana-

    Ealyzed on a Vista Pro (Varian) device.Isotopic analysis for Pb composition of the Momibol Member

    carbonates were carried out at the Center of Geochronological Re-search, University of So Paulo, and followed the proceduresdescribed by Babinski et al. (1999). Only the second leachate (L2)was analyzed. Samples were loaded with silica-gel and phosphoricacid onto Re laments and the isotopic compositions were deter-mined on a VG 354 multicollector mass spectrometer. Pb isotopicratios were corrected for a mass fractionation factor of 0.10%amu1 determined based on analyses of Common Pb StandardNBS 981 done during, and previously to, this work. Blanks were84 pg and had negligible effect on measured isotopic compositions.Isochron age was determined using the Isoplot software (Ludwig,1999); errors are reported at 95% condence level.

    Three samples were collected from weakly fractured portions ofthe Mtou dolostone and powdered for carbon and oxygen isotopeanalysis at the National Museum of Natural History. Carbonatesamples weighing 25 to 50 mg were reacted for 180 s with 100%phosphoric acid at 70 C in individual vessels in an automatedcryogenic distillation system (Kiel IV device) interfaced with aDelta V Advantage isotope ratio mass spectrometer. Analytical pre-cision is 0044% for d18O and 0013% for d13C. Delta (d) values are re-ported relative to the international PDB standard.

    3. Geological setting and stratigraphyDespite the limited extent of outcrops located on the banks ofthe Dja River, four lithostratigraphic units have been recognizedallowing reconstruction of the sedimentary architecture (Fig. 3)and interpretation of the deposits as being either lacustrine or la-goonal, conned in an isolated intracratonic basin (Caron et al.,2010).

    This work is focused on the analysis of the basal succession,which includes in ascending stratigraphic order diamictites, lami-nated siltstones and a thin dolostone bed (Fig. 3). Findings of recenteldwork are reported. In addition, previously unpublished andsome new stable isotopic data are presented. We particularly ad-dress the apparent discrepancy of having a typical glacial topost-glacial sedimentary succession, which in the light of pub-lished works on sedimentology and chemostratigraphy of Neopro-terozoic glacial deposits elsewhere could originate in the lateCryogenian (Marinoan Epoch) glaciation, and the absence of somediagnostic glaciogenic features in the deposits such as facetedclasts and dropstones.

    We further document the recent discovery of striated pave-ments on top of the Mintom Formation, and discuss the possiblecorrelation of this younger glacial event to either the Late Neopro-terozoic Gaskiers glaciation or a post-Neoproterozoic glaciation,namely the Ordovician Hirnantian glaciation.

    2. Analytical procedures

    The major element concentrations of samples were investigatedwith inductively coupled plasma-atomic emission spectroscopy(ICP-AES). Samples were subjected to an alkaline fusion in theLiBO2 and the remaining material was treated with HCl. About5 ml of ltered fusion solution was subsequently diluted with35 ml of HNO3, and these quantities were doubled or tripled forneighbouring Neoproterozoic strata in the DRC and CAR. The upperDja series has recently been redened as the Mintom Formation of

    114 V. Caron et al. / Journal of AfricanThe main geological features of Central Africa are intimatelyrelated to the late Neoproterozoic Pan-African orogeny, and consistin Cameroon of stacked south-verging thrust nappes, whichcollectively constitute the Yaound Fold Belt, also referred to asthe Oubanguide Fold Belt in CAR (Fig. 1). Rocks in the belts aremetasedimentary and volcano-sedimentary (various schists andgneisses, migmatites, amphibolite and quartzite), and metapluton-ic (gabbro, garnetpyroxene bearing diorite and granitoids). Insouthern Cameroon, the Yaound nappe was thrust directly ontothe Congo Craton, whereas in the Lomi region, the Yaound nappewas thrust over the Dja series composed of schists and dolerite, theage of which has not been precisely established (Fig. 1; Poidevinand Pin, 1986; Vicat et al., 1997). The Mintom Formation, whichforms the Neoproterozoic inll of the Mintom Basin located imme-diately north of the northernmost limit of the Congo Craton in thearea (Fig. 3A), is geologically part of the Yaound belt (Fig. 1), andas such underwent deformation and metamorphism during thePan-African orogeny (Vanhoutte and Salley, 1986; Caron et al.,2010). The southward vergence of folds together with evidencethat the succession has been locally either metamorphosed undergreenschist-facies conditions or is unmetamorphosed (Caronet al., 2010), are of particular importance as they provide some rel-ative age constraints in the absence of reliable radiometric dating(see below).

    Recent geochronological and structural studies have indeeddemonstrated that the Pan-African orogeny in Central Africa hasa multi-stage deformation history, characterized between 640and 580 Ma by two major phases of tectonic shortening referredto as D1D2 (ca. 640620 Ma) and D3 (ca. 620600 Ma), with dif-fering directions of broadly westeast and northsouth, respec-tively (Toteu et al., 2004). Ages obtained from granitoids andmetaplutonic rocks indicate that the onset of nappe tectonicswas likely diachronous westward and northward, starting at ca.660640 Ma in the Lomi region, ca. 620600 Ma in the Yaoundregion, and ca. 600580 Ma near Baa (Fig. 3A; Toteu et al., 2004,2006). In either sector of the belt, however, metamorphic assem-blages indicate that the nappe tectonics was initiated underhigh-grade conditions reaching the granulite facies in the deepernappes, and ended under medium to low-grade greenschist-faciesconditions in the shallowest nappes (Ndelec et al., 1986; Pin andPoidevin, 1987; Nzenti et al., 1988; Penaye et al., 1993; Toteu et al.,2001). Collectively, these data provide a minimum age for the Min-tom Formation of 580 Ma and a maximum age of 640 Ma (Caronet al., 2010). Unfortunately, this age interval of 120 Ma could notbe rened using the PbPb isochron method. The age of528 150 Ma obtained from six carbonate samples of the Momi-bol Member is not reliable. The isotopic ratios are extremelyhomogeno...

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