Sequence and carbon isotopic stratigraphy of the Neoproterozoic Roan Group strata of the Zambian copperbelt

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Precambrian Research 190 (2011) 70– 89

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Precambrian Research

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equence and carbon isotopic stratigraphy of the Neoproterozoic Roan Grouptrata of the Zambian copperbelt

tuart Bull a,∗, David Selleya, David Broughtonb, Murray Hitzmanc, Jaques Cailteuxd,oss Largea, Peter McGoldricka

CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, 7001 AustraliaIvanhoe Nickel and Platinum Ltd., 2 Maude St., Sandton 2146, South AfricaDepartment of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, United StatesDepartment of Research and Development, Groupe G. Forrest International, Lubumbashi, Congo

r t i c l e i n f o

rticle history:eceived 11 October 2010eceived in revised form 27 July 2011ccepted 28 July 2011vailable online 24 August 2011

eywords:eoproterozoicryogenianatangan Supergroupoan Groupequence stratigraphyarbon isotope

a b s t r a c t

The Neoproterozoic Roan Group in northern Zambia is host to numerous world class stratiform sediment-hosted Cu ores that occur around the transition from basal continental clastics to an overlying shallowmarine succession. The latter can be considered in terms of six sedimentary facies deposited within amixed clastic and carbonate barred basin margin environment. Sequence stratigraphic analysis, basedon palaeo-bathymetric cycles defined by vertical facies changes in sections constructed from detailedlogging of diamond drill cores, allows the definition of seven sedimentary sequences. Accommodationfor the lowermost two sequences, which host the bulk of the copper ores, was generated by half-grabendevelopment during active extension. The subsequent two sequences record denudation of tectonicallygenerated relief and evolution to a laterally extensive, low-relief, basin margin carbonate platform duringa period of tectonic quiescence. The uppermost sequences that coincide with the Mwashia Subgroup,record the resumption of tectonically generated accommodation that may reflect the onset of the breakupof Rodinia.

13
Carbon isotopic profiles through the Roan succession in Zambia show two secular ı C excursions ofmore than 10‰ to values of <−5‰ that can be correlated to the global curve for the Cryogenian. Theupper excursion occurs below the Sturtian Grand Conglomérat and is therefore interpreted to record theIslay anomaly. The lower excursion is interpreted to record the Bitter Springs stage, which occurs priorto the Sturtian glacials in a number of other Cryogenian sections where, as is the case in the Roan Group
t asso
section in Zambia, it is no
. Introduction

The Central African Copperbelt is a part of the Neoprotero-oic Katangan basin succession characterised by the presence of ateast 14 giant and numerous smaller stratiform sediment-hostedu deposits. These extend from northern Zambia into the south-rn part of the Democratic Republic of Congo (Fig. 1). This paper isocused on the southern part of the system, the Zambian Copper-elt (ZCB), where the ores occur around the margin of a northwestrending basement inlier termed the Kafue Anticline (Fig. 2). The

ajority of copper deposits occur in the basal unit of the Katanganupergroup, the Roan Group (Fig. 3 ), in and around a carbona-eous shale unit (the Ore Shale or Copperbelt Orebody Member).

Abbreviations: LST, lowstand systems tract; TST, transgressive systems tract;ST, highstand systems tract.∗ Corresponding author. Tel.: +61 3 62267634; fax: +61 3 62267662.

E-mail address: [email protected] (S. Bull).

301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2011.07.021

ciated with glacial strata.© 2011 Elsevier B.V. All rights reserved.

The Roan Group has been the focus of numerous stratigraphic andsedimentological studies, commencing with the seminal work ofGray (1932). However, the chief tool employed for stratigraphicanalysis to date has been litho-stratigraphy, which used in isolation,cannot adequately deal with the lateral facies variation inherent tobasin margin settings. This has led to confusion in the definitionand interpretation of the sedimentary facies architecture of the ZCB(e.g. van Eden and Binda, 1972; Cailteux et al., 1994; Selley et al.,2005).

Recent work has inferred that the abrupt transition from thebasal continental Mindola Clastics Formation to the marine con-ditions recorded by the overlying Copperbelt Ore Member (Fig. 3),occurred in response to linkage of a subset of the pre-existing densenetwork of normal faults into through-going master structures(Selley et al., 2005). This paper applies the concepts of sequence
stratigraphy to the marine part of the Roan Group (i.e. up sequencefrom the level of the Copperbelt Ore Member; Fig. 3). The prin-ciple aim is to overcome the problems imposed by lateral facieschanges, by identifying the key surfaces that are used to define
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S. Bull et al. / Precambrian Research 190 (2011) 70– 89 71

Fig. 1. Map of the Lufilian Arc showing the major tectono-stratigraphic elements and the Zambian (ZCB) and Congolese (CCB) Copperbelts.M

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odified after Selley et al. (2005).

he elements of the sequence stratigraphic model (e.g. sequenceoundaries, transgressive and maximum flooding surfaces; Vailt al., 1977; Van Wagoner et al., 1988). These are used to subdividehe Roan Group into laterally correlative depositional sequences,hereby refining the basin architecture, allowing comparison withontemporary basin models, and advancing our understanding ofhe context of formation of the world class sediment hosted Cueposits.

The presence of sedimentary carbonates throughout most of thearine part of the Roan Group provides the opportunity to use C

sotope chemo-stratigraphy to supplement the sequence stratig-aphy. This provides additional control on correlating the Roanroup strata within the ZCB, and also allows comparison with theell developed global ı13C curve for the Cryogenian (e.g. Halversen

t al., 2005; Macdonald et al., 2010).

. Regional geology

.1. Katangan Supergroup

The Neoproterozoic Katangan Supergroup is a <5–10 km thicketa-sedimentary succession overlying older Proterozoic meta-orphic terranes (Fig. 1). It is subdivided into the basal Roan

and overlying Nguba and Kundelungu groups (e.g. Franc ois, 1995;Batumike et al., 2007). The Roan Group (Fig. 3) comprises clas-tic sedimentary rocks succeeded by carbonates that have beeninterpreted to record a rifted continental margin associated withthe breakup of Rodinia (e.g. Kampunzu et al., 1993, 2000; Temboet al., 1999). The overlying Nguba and Kundelungu groups are car-bonate and siliciclastic successions with basal diamictites termedthe Grand Conglomérat or Mwale Formation and Petit Con-glomérat or Kvandamu Formation respectively (Franc ois, 1973;Batumike et al., 2007). They are interpreted to record ongoing basindevelopment and widening (e.g. Buffard, 1978; Kampunzu et al.,1993).

Inversion of the Katangan Basin is recorded by the LufilianOrogeny, which produced an arcuate, north-verging fold and thrustbelt that spans the border between northwest Zambia and theDemocratic Republic of Congo (Fig. 1). The timing of this event isconstrained to be between the ca. 735 Ma age for the youngest vol-canic rocks in the basin (Key et al., 2001), and the ca. 525 Ma agefrom U mineralisation interpreted to occur within late (D2) faults
(Kampunzu and Cailteux, 1999). Peak metamorphic conditions forthe Roan Group strata in the ZCB range from sub-greenschist toupper greenschist metamorphic grade attained at ∼530 Ma (Johnet al., 2004).

72 S. Bull et al. / Precambrian Research 190 (2011) 70– 89

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Fig. 2. Map of the Zambian Copperbelt showing the dodified after Selley et al. (2005).

.2. Roan Group

The Roan Group in Zambia (Fig. 3) has traditionally been sub-ivided into a predominantly clastic Lower Roan Subgroup, aredominantly dolomitic Upper Roan Subgroup with local volcanicnits, and an overlying dolomitic and dolomitic siltstone successionith intercalated conglomerate, sandstone and carbonaceous shale

ermed the Mwashia Subgroup (e.g. Mendelsohn, 1961; Cailteuxt al., 2007). More detailed litho-stratigraphic subdivision of theower Roan Subgroup has been carried out at several mines androspects. This has resulted in a confused series of partly over-

apping deposit/area specific stratigraphic schemes, even whereitho-stratigraphic units are, in fact, clearly continuous (Binda and

ulgrew, 1974; Cailteux et al., 1994). The situation becomes moreonfused where lateral lithofacies variations do occur, in the form

able 1tratigraphic nomenclature of the Roan Sub Group on the western (Clemmey, 1976; Selle

Sub-Group Western Kafue Anticline

Formation Member

Mwashia

Upper RoanLower Roan Kitwe Antelope Clastics

Chambishsi DolomiteNchanga Qzit

Rokana evaporites

Copperbelt orebody

Mindola Clastics Kafue ArenitesBasal Quartzites

d drill holes and drill hole section used in this study.

of a general increase in grain size and proportion of the clasticcomponent and concomitant decrease in the proportion of car-bonate from west to east across the Kafue Anticline (e.g. Gray,1932; Mendelsohn, 1961; Binda, 1994). The various formation andmember level stratigraphic terminology that has been applied tothe Lower Roan Subgroup is summarized in Selley et al. (2005)(Appendix 1).

This study employs the stratigraphic scheme first proposedby Clemmey (1976) for the Katangan succession on the west-ern side of the Kafue Anticline and used by Selley et al. (2005)(Fig. 3; Table 1). The Lower Roan Subgroup is subdivided into a
basal formation dominated by arkosic sandstones and conglom-erates termed the Mindola Clastics, overlain by a more diversefacies association of coarse- and fine-grained clastics and minorcarbonates termed the Kitwe Formation. The latter is divided into
y et al., 2005) and eastern (Fleisher et al., 1976) side of the Kafue Anticline.

Eastern Kafue Anticline

Member Formation

Glassy Qzit Hangingwall formation Upper Argillaceous Qzit

Marker GritLower Argill. Qzit. Ore formationA OredodyInter A/BB OrebodyInter B/C

C orebody Footwall formation

S. Bull et al. / Precambrian Res
earch 190 (2011) 70– 89 73
5 members, the lowermost of which, the Copperbelt Ore Member(originally termed the Ore Shale e.g. Binda and Mulgrew, 1974), isa dolomitic siltstone with carbonate and carbonaceous shale cor-relates (Fig. 3).

A somewhat different stratigraphy is present on the easternside of the Kafue Anticline, where a well defined Copperbelt OreMember is absent. The most detailed stratigraphic scheme forthis area was developed at the Mufulira Mine (Fleischer et al.,1976). It divides the Lower Roan Subgroup into a basal Foot-wall Formation comprising well-sorted, non-mineralized arkosesand conglomerates generally correlated with the Mindola Clas-tics Formation (Table 1). This is overlain by the Ore Formation,comprising a mixture of well- and poorly sorted arkoses (locallytermed greywackes where they have bituminous cements), finer-grained clastics and minor carbonates that are correlative withthe lower part of the Kitwe Formation. An upper HangingwallFormation of well-sorted and poorly sorted arkoses underlies thecarbonate-dominated strata of the Upper Roan Subgroup, andtherefore correlates lithostratigraphically with the upper part ofthe Kitwe Formation (Table 1). The three ore horizons present atMufulira span the boundary between the two lower formations,with the lowest, C orebody, occurring at the top of the FootwallFormation and the A and B orebodies occurring within the OreFormation.

The Roan Group succession is affected to varying degrees bystratabound zones of brecciation (Fig. 3). Breccias are most com-mon in the Upper Roan Subgroup carbonate succession, some ofwhich are spatially associated with gabbroic intrusives. However,in the southern part of the Chambishi Basin, they step down sectionuntil they ultimately rest on the Mindola Clastics Formation (Binda,1994; Binda and Porada, 1995). Similar breccias occur in the Con-golese part of the copperbelt to the north, where they are morevoluminous and contain mega-fragments, the mineralized exam-ples of which constitute the Congolese orebodies (e.g. Cailteux andKampunzu, 1995).

2.3. Geochronology

Temporal constraints on the ZCB are sparse. The maximum agefor the onset of Roan Group sedimentation is based on a U–Pb zirconage of 883 ± 10 Ma for the Nchanga Red Granite (Armstrong et al.,2005; Fig. 3), which is unconformably overlain by the basal MindolaClastics Formation. The next age constraint is provided by volcanicswithin the Mwashia Subgroup which yield ages of ∼760 Ma fromtuffs in the Likasi area of the Democratic Republic of Congo northof the ZCB (Rainaud et al., 2002); 765–735 Ma from mafic lavasin northwestern Zambia (Key et al., 2001); and 752–741 Ma fromgeochemically similar gabbro sills and dikes intruding west of theZCB (Barron, 2003). Overall, this gives a period of >100 m.y., corre-sponding broadly with the lower half of the Cryogenian, in whichto deposit the Roan Group which has a total thickness in Zambia ofaround 2 km (e.g. Mendelsohn, 1961; Selley et al., 2005).

3. Methodology and limitations

3.1. Sequence stratigraphy

In proposing to apply the principles of sequence stratigraphic
analysis to the marine parts of the Roan Group, it is importantto acknowledge that there are limitations to the technique, someof which apply to Precambrian successions in general and oth-ers to the ZCB in particular. The principal element of sequence
Fig. 3. Schematic stratigraphic section of the Roan Group in Zambia.Modified after Selley et al. (2005).

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tratigraphy is the depositional sequence, defined as a relativelyonformable succession of genetically related strata bound bynconformities or their correlative conformities (e.g. Van Wagonert al., 1988). Recognition of a sequence, and its constituent ele-ents (systems tracts), relies on the identification of key bounding

urfaces, the sequence boundaries themselves and the trans-ressive and maximum flooding surfaces. In ‘classical’ sequencetratigraphy, involving clastic Mesozoic–Cenozoic passive marginuccessions, these bounding surfaces are usually identified using

combination of seismic data and down hole geophysical logs.ateral correlation of sequences, and sometimes their individuallements, is facilitated by biostratigraphic data. Precambrian suc-essions clearly lack these critical datasets, with the result thatorrelation of stratal elements is typically restricted to first orderepositional sequences and their constituent systems tracts (e.g.on der Borch et al., 1988; Krapez, 1996).

The lithofacies association of the marine influenced portion ofhe Roan Group in Zambia changes up section, from mixed clasticnd carbonate marginal marine (Kitwe Formation) to carbonate-ominated sediments (Upper Roan Subgroup). The thickness andxtent of the Upper Roan Subgroup indicates the establishment of

carbonate platform (Binda, 1994), and the intercalation of car-onates with clastic sediments indicates it is of the “attached”ype (i.e. it is connected to a continental hinterland; e.g. Schlager,005). The principles of sequence stratigraphy are the same inarbonate environments as they are in the clastic passive marginettings where they were developed. However, their applicationequires significant modification (e.g. Handford and Loucks, 1993;chlager, 2005), chiefly because whereas clastic sediments arextra-basinally derived, carbonate sediment is formed in situ.

The main constraints on undertaking a sequence stratigraphicnalysis of the Roan Group in Zambia are the lack of bothurface outcrop and seismic data. However this deficiency is some-hat counterbalanced by a comprehensive suite of diamond drill

ores from a large number of continuously cored holes fromhroughout the ZCB, augmented by limited exposures within openut and underground mines. As a result, the primary tools fordentifying the key bounding surfaces and stratal elements areacies/environmental changes that can be defined in vertical drillore intersections. The principle of using vertical facies changes tonterpret sea level history is based on Walther’s Law; if the contactsetween facies are conformable (i.e. they are not disconformities oraults), then their vertical sequence mirrors the original lateral dis-ribution of sedimentary environments. In terms of the key surfaceshat occur in the sequence stratigraphic model, sequence bound-ries are defined as “an abrupt basin-ward shift of facies” (e.g. Vanagoner et al., 1988), transgressive surfaces occur where basinal

acies abruptly overlie basin margin facies, and maximum floodingurfaces are marked by the deepest water facies present in a givenequence.

Another problem that has traditionally hampered stratigraphicnalysis of the Roan Group in Zambia is the presence of locallyoluminous breccias. These have been variously interpreted as dis-olution and/or migration of evaporites (e.g. Jackson et al., 2003;elley et al., 2005); or regional scale thrust faults facilitated by andocused on zones of evaporite dissolution (e.g. Cailteux et al., 1994;inda and Porada, 1995; Kampunzu and Cailteux, 1999); or syn-ectonic conglomerate complexes/olistostromes (e.g. Wendorff,005). Although favoring a tectonic origin for the breccias, Cailteuxt al. (1994) state that stratigraphic contacts within the Kitwe For-ation are generally transitional and non-tectonic. This is in accordith the outcomes of our study, which involved initial recon-
aissance examination of numerous drill cores, and subsequentetailed logging of key intersections. This indicated that althoughhe majority of cored intersections of the upper Kitwe Formationnd Upper Roan Subgroup are affected by some degree of breccia-
earch 190 (2011) 70– 89

tion, there is no evidence of stratigraphic or structural repetitionto support either of the syn-tectonic hypotheses. As a result, wefavor the evaporitic origin for the breccia units. The two key drillholes utilized in this study have relatively minor breccias, and thosethat are present are bounded by the same facies on either side,suggesting that any stratigraphic dislocation is minor.

Finally some comment needs to be made on the effects ofalteration and metamorphism of the ZCB strata. In general, the sed-imentary protolith for rocks in the Roan Group is clear. However,the entire succession has been subjected to protracted, multiplestages of diagenetic and hydrothermal alteration (see discussionin Selley et al., 2005) and ultimately greenschist facies metamor-phism. One result of this complicated post-depositional history hasbeen intense calcium-magnesium and potassic metasomatism ofargillaceous rocks (e.g. Darnley, 1960; Moine et al., 1986; Selleyet al., 2005). Subsequent green-schist facies metamorphism of themetasomatic assemblages has resulted in the extensive genera-tion of phlogopite. As a result, the proportion of phlogopite canbe used as a proxy for the original argillaceous component of thesedimentary protolith. Alteration and metamorphism have alsoresulted in the crystallization of most of the carbonate rocks in thesection. Thus, it is commonly not possible to determine whetherthey formed by biotic or abiotic processes. Exceptions are twothicker intervals, the Chambishi Dolomite and Upper Roan Sub-group, which locally preserve stromatolitic forms indicating theyare, at least in part, microbial in origin.

3.2. Chemostratigraphy

The chemostratigraphy of the Neoproterozoic is well studied,and efforts to establish a global ı13C curve have been ongoing (e.g.Halversen et al., 2005; Halverson et al., 2007; Macdonald et al.,2010). During the Cryogenian (i.e. during deposition of the Roanand Nguba groups in the ZCB), sedimentary ı13C and ı18O val-ues averaged around 5‰ and 25‰ respectively. The most recent Cand O isotopic studies of the ZCB have been aimed at determiningthe isotopic characteristics of the mineralized strata (Annels, 1989;Sweeney and Binda, 1989; Selley et al., 2005). The results show that,in accord with the global Neoproterozoic dataset (Halversen et al.,2005; Macdonald et al., 2010), unaltered sedimentary carbonatevalues were around 0–5‰ ı13C and 25‰ ı18O. However, carbon-ates associated with the mineralized Copperbelt Orebody Memberexhibited a coupled negative C and O isotopic shift towards val-ues of −25‰ ı13C and 10‰ ı18O. This was interpreted to indicatethat oxidation of organic C in argillaceous rocks was integral to themineralising process (Selley et al., 2005).

This study utilizes 137 new C and O isotopic analyses from sam-ples from two drill cored intersections that record representativesections of the Roan Group on either side of the Kafue Anticline. Iso-tope analyses were undertaken at the Central Science Laboratory atthe University of Tasmania using the technique of McCrea (1950).The results are expressed in terms of the ı (‰) notation relative toSMOW for oxygen and PDB for carbon and the precision of analysesfor both is ±0.2‰.

In general, Proterozoic carbonates have proven robust interms of preserving near primary C isotopic signatures throughpost depositional alteration (e.g. Veizer, 1992; Buick et al., 1995;Halversen et al., 2005) and metamorphism (e.g. Kaufman et al.,1991; Melezhik et al., 2005; Prave et al., 2009). Of greatest concernis the incorporation of ı13C depleted carbon produced by oxidationof organic matter during diagenesis (e.g. Halversen et al., 2005)or as demonstrated in the case of the Roan Group during miner-
alisation (Selley et al., 2005). Samples for this study were chosento minimize post depositional effects (i.e. they were free of veins,diagenetic growths and mineralization), and to further ensurethat the material analysed was representative of sedimentary

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arbonate it was sampled with a dental drill. However, to ensurehe best stratigraphic coverage, samples were taken from all threef the sedimentary facies that contain significant carbonate, theassive white dolomite, bedded grey dolomite and dolomitic

iltstone facies (Sections 4.2, 4.4 and 4.5 respectively). Since theatter two facies contain a proportion of organic carbon, three pairsf adjacent samples (i.e. <2 m apart) of massive white dolomite andolomitic siltstone facies were chosen from a diamond drill holehere the facies were repeatedly interbedded, to check for the

ffects of post depositional generation of ı13C depleted carbon (e.g.alversen et al., 2005). The variation in ı13C values was between

ample pairs was between 1 and 1.3‰ (Bull, unpublished data),n order of magnitude less than the >10‰ isotopic excursionsonsidered significant for correlation purposes.

. Sedimentary facies

Chambishi Southeast is a blind occurrence of Cu-Co min-ralization hosted by the Copperbelt Orebody Member on theorth-eastern margin of the Chambishi Basin, 5 km southeast ofhe Chambishi mine (Fleischer, 1984; Fig. 2). A 5 km long, east-est trending fence of holes from the area has been examined toetermine the sedimentary facies architecture of the “classical” (i.e.opperbelt Orebody Member present) ZCB stratigraphy (Fig. 4a).ecause the Roan Group strata dip away from the Kafue Anticlineasement inlier and the orebodies are focused around the Copper-elt Orebody Member at the base of the Kitwe Formation, mostrilling in the ZCB, including the Chambishi Southeast holes, areollared within the Kitwe Formation. As a result, with the excep-ion of RCB2 at the western end of the fence which is collared in the

washia Subgroup, drill core is only available for the basal half ofhe Kitwe Formation.

In broad terms, the Chambishi Southeast drill hole fenceFig. 4a) shows the typical facies architecture described by Selleyt al. (2005). The basal dominantly alluvial/fluvial Mindola Clas-ics Formation sandstones and conglomerates show abrupt andramatic lateral thickness and facies changes. In contrast, theopperbelt Orebody Member at the base of the Kitwe Formation,hich heralds the abrupt onset of sub-tidal marine deposition,

nd the predominantly inter-tidal strata above it, have much moreayer cake geometries. Using the well documented evolution of

odern rift basins as an analogue, Selley et al. (2005) proposed thathe abrupt change in environment and facies geometry recordedy the Copperbelt Orebody Member, occurred in response to

inkage of a subset of the numerous normal growth faults withhort strike lengths that had initially controlled deposition ofhe Mindola Clastics Formation. This produced a set of moreidely spaced master structures with longer strike lengths that

esulted in sedimentation of the Kitwe Formation in larger basinompartments. These faults could facilitate greater amounts ofubsidence, resulting in predominantly sub-wave base conditionsor deposition of the Copperbelt Orebody Member. In areas wherehe structural footwall remained elevated, such as the eastern endf the Chambishi Southeast section (e.g. DDH NN36; Fig. 4a), cleannter- and supra-tidal carbonate facies are lateral equivalents ofhe sub-tidal siltstone facies. These are locally termed barren gapsecause the carbonate is unmineralized, but have long been recog-ised to be stromatolitic bioherms developed on basement highse.g. Garlick and Fleischer, 1972; Clemmey, 1974; Fleischer, 1984).reas where the Mindola Clastics Formation pinches out againstioherm capped basement highs are the most favorable sites forineralization in both the footwall and the Copperbelt Orebody
ember, highlighting the influence of the basin architecture on
he location of ore deposits (Selley et al., 2005).Previous sedimentological studies of the Kitwe Formation have

nterpreted the mixed clastic and carbonate lithofacies to record

earch 190 (2011) 70– 89 75

sub- to supra-tidal sedimentation in a marginal marine envi-ronment (e.g. Clemmey, 1974; Binda, 1994). Sparse to abundantgypsum casts and anhydrite nodules (and carbonate and/or quartzpseudomorphs of same; Annels, 1974) appear in the upper part ofthe Copperbelt Orebody Member and occur sporadically through-out the overlying Roan Group strata. They have been recognised asbeing evaporitic in origin (e.g. Garlick and Fleischer, 1972; Annels,1974) and as a result arid conditions have been inferred duringdeposition of the Kitwe Formation. However, much of the anhy-drite present occurs as cements, vugs and veins. Annels (1974)interpreted this to reflect post depositional remobilisation intosecondary sites, and an original evaporitic origin for all anhydritephases is confirmed by the sulfur isotopic values, which are con-sistent with Neoproterozoic seawater sulfate (Selley et al., 2005).The base of the Upper Roan Subgroup is defined as the level atwhich carbonate becomes the predominant sediment (Fig. 3). Binda(1994) interpreted this to signify the development of an extensiveshallow water platform.

We concur with the general outcomes of previous sedimento-logical studies of the Kitwe Formation and Upper Roan Subgroup,which conclude that that these strata represent a cyclic evaporiticmarginal marine succession; that the Copperbelt Orebody Memberat the base of the formation represents a major transgression; andthat succeeding regressive and transgressive cycles occur in theoverlying strata (e.g. Binda and Mulgrew, 1974; Clemmey, 1974;Cailteux et al., 1994). However, for the purpose of our study, whichis concerned with the application of the principles of sequencestratigraphy, we need to understand the accommodation historyrecorded by the stratigraphic succession in more detail. This isachieved by documenting the cyclic variations in sedimentary envi-ronment (and hence relative sea level) with time, and interpretingthe cycles using appropriate facies models. To this end the KitweFormation and Upper Roan Subgroup are divided into six lithofa-cies based on the Chambishi Southeast drill section (Fig. 4a). Thefacies are described and interpreted below, and the spatial relation-ships envisaged between them are depicted on a schematic faciesmodel typical for a mixed clastic and carbonate barred basin margin(Fig. 4b).

4.1. Well sorted arkose facies

The well sorted arkose facies comprises pale yellow to whitecoloured, fine- to very-coarse-grained arkosic sandstone (Fig. 5a).The dominant sedimentary structure is planar lamination, buttrough cross beds and ripples also occur. The facies dominatesthe Mindola Clastics Formation (Fig. 4a) and the thickest andbest developed example in the Kitwe Formation is the NchangaQuartzite (which is actually an arkosic sandstone; Figs. 4a and 5a).However, thinner well sorted arkose facies intervals occur sporadi-cally within the poorly sorted arkose facies association, where theytend to be gradational to more argillaceous arkosic sandstones (Sec-tion 4.3). The Nchanga Quartzite has been interpreted as a littoralsand sheet deposited in beach to shore face environments (e.g.Binda, 1994). We concur with this interpretation and consider itas the most landward facies belt recorded in the Kitwe Formationto Upper Roan Subgroup intersection (Fig. 4b).

4.2. Dolomitic siltstone facies

This facies is grey to buff coloured (Fig. 5b) due to a high pro-portion of pale green phlogopite and dolomite, which reflects theoriginal argillaceous and calcareous composition of the protolith
dolomitic siltstone (e.g. Mendelsohn, 1961; Annels, 1974; Bindaand Mulgrew, 1974; Clemmey, 1974; Selley et al., 2005). Dolomiticsiltstone facies intervals are generally metamorphically recrystal-lized, obscuring primary sedimentary structures with the exception


locatio

oap(Cattda(aq

Fig. 4. (a) E–W drillhole section from Chambishi SE prospect (see Fig. 2 for

f locally well-developed planar lamination. Anhydrite nodulesnd their pseudomorphs are often present, and distinctive inter-enetrating casts after gypsum crystals are also locally abundantFig. 6a). The thickest example of the dolomitic siltstone facies is theopperbelt Orebody Member (Fig. 5b), but thinner interbeds occurt various levels higher in the section, and there is a laterally persis-ent unit immediately above the Nchanga Quartzite, locally termedhe Upper Ore Shale (Fig. 4a). Both the quartzite and the overlyingolomitic siltstone facies at this level are commonly mineralized
nd attain ore grade in the form of the upper ore zone at NchangaBinda and Mulgrew, 1974). The inferred fine original grain sizend lack of tractional sedimentary structures in this facies suggestuiet sub-tidal sedimentation. We interpret it to represent the most
n). (b) Facies model for a mixed clastic and carbonate barred basin margin.

basinward facies belt recorded in the Roan Group (Fig. 4b). The pres-ence of anhydrite nodules and gypsum casts attest to evaporiticdeposition during periods of hyper-salinity (Garlick and Fleischer,1972; Annels, 1974). In the case of the Copperbelt Orebody Mem-ber, the general increase in evaporitic components up section hasbeen interpreted to record upward brining (Binda, 1994), implyingprogressive evaporation of a restricted body of seawater.

4.3. Poorly sorted arkose facies

The poorly sorted arkose that characterises this facies is gen-erally pale to dark grey/brown in colour (Fig. 6b), reflecting asignificant proportion of phlogopite after an original argillaceous

S. Bull et al. / Precambrian Research 190 (2011) 70– 89 77

Fig. 5. Sedimentological features of the Kitwe formation (Fig. 4) at the Chambishi SE prospect, arrow indicates younging direction in each case. (a) Diamond drill corefrom hole number NN15 (Fig. 2). Central pale unit is the Nchanga Quartzite which forms the LST of sequence 2. It is underlain (to the right) but the poorly sorted arkosefacies interval that comprises the HST of the sequence 1, and overlain (to the left) by interbedded dolomitic siltstones (dark) and dolomites (light) of the TST of sequence 2.(b) Diamond drill core from hole number NN15 (Fig. 2). Central dark coloured dolomitic siltstone facies interval is the Copperbelt Orebody Member that forms the TST ofsequence 1. It is underlain (to the right) by arkosic sandstones of the Mindola Clastics formation and overlain (to the left) by interbedded carbonates and sandstones (pale)and dolomitic siltstones and sub-tidal carbonates (dark) of the poorly sorted arkose facies interval that comprises the HST of the sequence 1.


Fig. 6. Sedimentological features of the Kitwe formation and Upper Roan Subgroup in the Chambishi SE prospect (Fig. 4), arrow indicates younging direction in each case. (a)Original gypsum crystals now pseudomorphed by calcite, pyrite and Cu sulfides in the upper part of the TST of sequence 1 in DDH NN48 at 804 m. (b) Phogopitic sandstonesand siltstones (middle) of the poorly sorted arkose facies in DDH RCB2 at 1092 m. Note the coarse grained quartz crystals at top right (arrowed). (c) Thin carbonate interbed(top left) within the phogopitic clastics of the poorly sorted arkose facies in DDH NN48 at 763 m. (d) Stromatolitic textures within the massive white dolomite facies thatcomprises the Chambishi Dolomite in DDH NN48 at 692 m. (e) Typical section of the Chambishi Dolomite in DDH NN15 (Fig. 2). The dolomite is underlain (to the right) andoverlain (to the left) by interbedded phlogopitic clastics and carbonates of the poorly sorted arkose facies interval that comprise the upper part of the HST of the sequence 2and the lower part of the HST of the sequence 3 respectively. (f) Transitional contact between dolomitic siltstone and grey green pyritic dolomite of the bedded grey dolomitefacies (to the left) and massive white dolomite facies (to the right) in DDH RCB2 at 570 m. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of the article.)

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atrix. The unit is characterised by variable amounts of coarse-o very coarse-grained feldspar and polycrystalline quartz (Fig. 6b)hat occurs in diffuse massive beds, irregular patches and in someases crosscutting clastic dykes that are clearly de-watering struc-ures. The only other sedimentary structures discernible in corere bedding, locally defined by thin paler coloured sandstones andavy to planar lamination (Fig. 6b) and some intervals are massive

nd structureless (Fig. 6c). Anhydrite and carbonate are commonhroughout as patchy cements and locally irregular vughs andeins commonly elongate parallel to bedding. The poorly sortedrkose facies contains varying proportions of dolomitic siltstone,ell sorted arkose, massive white and bedded grey dolomite facies

nterbeds (Figs. 5a and b and 6c). These define cycles of sedimen-ation (parasequences) that are between 2 and 5 m in thickness. Ahick (>100 m) interval of the poorly sorted arkose facies compriseshe Antelope Clastics (or Shale With Grit) which is the upper-

ost unit of the Kitwe Formation (Fig. 4a) and can be correlatedlong the western side of the Kafue Anticline. The basal part ofhis unit is dominated by grey/green siltstones and fine sandstonesFig. 6b), with an increasing proportion of anhydrite and carbon-te up-section. Overall, the generally poorly sorted nature of theoarser-grained clastics and the presence of inter-bedded siltstonesnd carbonates in the poorly sorted arkose facies indicate relativelyow energy conditions. In combination with the widespread occur-ence of evaporitic minerals, we interpret these deposits to recordyclical inter-tidal sedimentation in a low-energy, arid, hyper salineetting, probably a salt flat or lagoon that occurs between thetrandline (well-sorted arkose facies) and the sub-tidal (dolomiticiltstone facies) belts (Fig. 4b).

.4. Massive white dolomite facies

The massive white dolomite facies comprises massive to thicklyedded recrystallized white dolomite. Anhydrite is generally abun-ant as cement patches, vughs and veins (Fig. 6d) and is locallyreserved in its original form as evaporitic nodules. Circular tovoid concretionary structures locally preserved in the thickerntervals of this facies are interpreted as sections through stromato-ites (Fig. 6d). The first thick example of the massive white dolomiteacies is the Chambishi Dolomite (Fig. 6e). This facies becomesrevalent in the Upper Roan Subgroup (Fig. 4a), however thinnereds occur sporadically in the lower part of the Kitwe Formationithin the poorly sorted arkose facies (Fig. 6c). The local preser-

ation of stromatolitic textures and association with evaporiticnhydrite nodules indicate this facies represents sub- to inter-idal stromatolitic reefs (Fig. 4b). Thinner examples, such as occurithin the poorly sorted arkose facies association, may represent

mall patch reefs, but as the carbonate is recrystallized, they maylso record clastic carbonate beds (e.g. ooid shoals). However, thewo thick occurrences, the Chambishi Dolomite (Fig. 6e) and espe-ially the Upper Roan Subgroup clearly reflect the development ofubstantial carbonate barrier reefs/platforms (Binda, 1994). Theseould have occurred on the shelf edge, and adjacent facies beltsould comprise the poorly sorted arkose facies association in the

ack-reef area towards the continental margin, and the dolomiticiltstone facies in the fore-reef area (Fig. 4b).

.5. Bedded grey dolomite facies

The bedded grey dolomite facies comprises dolomite that is paleo dark grey-green due to disseminated phlogopite and generallylso pyrite. In detail this facies encompasses a range of litholo-
ies grading between dolomite-rich and dolomitic siltstone-richnd members (Fig. 6f). The former comprises non-evaporitic greyolomite in which thin beds and laminae are defined by phlogopite-ich selvages after dolomitic siltstone. The latter comprises thinly
earch 190 (2011) 70– 89 79

interbedded grey dolomite and dolomitic siltstone. An interval ofthe bedded grey dolomite facies commonly occurs at the base of theCopperbelt Orebody Member and others occur sporadically withinthe overlying succession. Clemmey (1974) interpreted the intervalat the base of the Copperbelt Orebody Member to represent inter-tidal algal mats. We concur with this environmental interpretation,but consider that the quiet, reduced conditions indicated by thesignificant proportion of original argillite and pyrite respectively,suggest that the facies also spans sub-tidal environments. We inter-pret this facies to record inter- and sub-tidal aprons adjacent to thesupra-tidal reefs represented by the massive white dolomite facies(Fig. 5b). Once again, since the original sedimentary textures havebeen recrystallized during metamorphism, the precise mode of sed-imentation is unclear. In the dolomitic siltstone-rich intervals ofthis facies, the carbonate beds have sharp bases and appear to havebeen emplaced as gravity flows, presumably shed from the adja-cent reefs. The more massive to laminated intervals of this faciescould either represent more carbonate-rich and proximal examplesof this process and/or in situ microbial accumulations as proposedby Clemmey (1974).

5. Sequence stratigraphy

5.1. Western side of Kafue Anticline

DDH RCB2 from the Chambishi Basin (Fig. 2) is utilized as therepresentative section from the western side of the Kafue Anti-cline. This drill hole was collared in a major Roan Group brecciaunit that occupies the upper 400 m of the hole and is successivelyunderlain by; a 250 m thick mixed siltstone carbonate succession(Mwashia Subgroup); 150 m of typical Upper Roan Subgroup car-bonates; 450 m of Kitwe Formation with a 20 m thick CopperbeltOrebody Member at its base; and >100 m of Mindola Clastics inwhich the hole is terminated (Fig. 7). There are several trays ofcore missing, and a 10 m thick breccia underlain by a ∼100 m thickincipiently brecciated and altered zone occurs in the lower part ofthe Upper Roan Subgroup. The breccia zone is typical of those com-mon throughout the succession, and although some section may bemissing, overall stratal disruption appears limited, and the originalprotolith (inter-beds of the two carbonate facies) is clear.

The arrangement of facies in the DDH RCB2 intersec-tion has been interpreted in terms of a relative palaeo-waterdepth/accommodation curve to identify the key surfaces (Fig. 7).These are used to define the sedimentary sequences, and wherepossible, their constituent lowstand (LST), transgressive (TST) andhighstand (HST) systems tracts. Overall, the Roan Group strata inDDH RCB2 between the Mindola Clastics Formation and the brecciaat 400 m can be subdivided into six sedimentary sequences (Fig. 7).

5.1.1. Sequence 1The base of the first sequence is marked by a 4 m thick inter-

val of the bedded grey dolomite facies, which abruptly overliesthe Mindola Clastics Formation arkoses and is the oldest obvi-ously marine interval in the Roan Group. Carbonates are commonlypresent at the base of the Copperbelt Orebody Member andhave been interpreted to represent algal mats (Clemmey, 1974).In clastic-dominated successions carbonates often signify trans-gressive conditions (Handford and Loucks, 1993) and hence weinterpret the base of this interval as a transgressive surface. Itis overlain by 11 m of dolomitic siltstone facies, indicating thattransgression continued through the window suitable for carbon-
ate production to sub-wave base conditions. In detail, this intervalcan be subdivided into a basal 5 m thick non-evaporitic sequence,overlain by 6 m of siltstone with abundant evaporite pseudomorphsafter gypsum crystals (Fig. 6a) and locally anhydrite nodules. The


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ig. 7. Facies log of DDH RCB 2 (legend is the same as Fig. 3b) showing interpreteystems tract; MFS, maximum flooding surface; HST, highstand systems tract, SB,egative ı13C excursions discussed in the text). (For interpretation of the reference

on-evaporitic interval represents the deepest water conditionsttained during sequence 1 and so by definition contains theaximum flooding surface. The upper evaporitic siltstone inter-

al therefore represents the base of the overlying regressive HST,he upper part of which comprises 37 m of poorly sorted arkoseacies representing a continued regression to inter-tidal conditionsFig. 4b).

eobathymetry, sequence stratigraphy (TS, trasgressive surface; TST, transgressivence boundary) and C and O isotopic profile (red dashed lines highlight the threelor in this figure legend, the reader is referred to the web version of the article.)

5.1.2. Sequence 2The base of sequence 2 is marked by an abrupt regression from

the inter-tidal poorly sorted arkose facies deposits comprising the
previous HST, to 13 m of supra-tidal deposits of the well-sortedarkose facies locally termed the Upper or Nchanga Quartzite (e.g.Binda and Mulgrew, 1974; Clemmey, 1976 respectively). In mixedclastic and carbonate margins, widespread siliciclastic strata such

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s these often signify a significant drop in sea level and hence over-ie sequence boundaries (Handford and Loucks, 1993). We thereforenterpret the Nchanga Quartzite to represent a LST above the basalequence boundary. It is overlain by a transgressive surface anducceeding 20 m thick TST comprising interbedded carbonates andolomitic siltstones, one of the latter of which comprises the max-

mum flooding surface (Fig. 5a). The overlying HST is marked by regression to inter-tidal conditions represented by a 28 m thicknterval of the poorly sorted arkose facies.

.1.3. Sequence 3The base of the third sequence is marked by a transgressive sur-

ace defined by the abrupt transition from the poorly sorted arkoseacies that comprises the HST of the underlying sequence 2, to3 m of massive white dolomite facies that comprise the Chambishiolomite (Fig. 6d and e). This is the first substantial carbonate-ominated, clastic-poor interval in the Roan Group and records thestablishment of a thick and extensive microbial reef. We inter-ret the Chambishi Dolomite to have developed on the outer partf a starved shelf. The thickness of the carbonate interval, and lackf inter-beds of either fore- or back-reef facies (with the excep-ion of one interval of dolomitic siltstone facies towards the topf the unit which we interpret as the maximum flooding surface;ig. 6e) suggests generally aggradational conditions. Once again theST is represented by a return to an interval of the poorly sortedrkose facies, in this case the unit locally referred to as the Shaleith Grit or Antelope Clastics Member (e.g. Binda and Mulgrew,

974; Clemmey, 1976 respectively). This is the thickest interval ofhis facies, and the basal 25 m differs slightly from the underly-ng occurrences. It is finer-grained and originally muddier overall,nterbeds of the well-sorted arkose facies are rare and carbonatesnd evaporites are absent. We interpret this to record a slightlyeeper and quieter back reef lagoonal environment (e.g. Schlager,005) reflecting the continued influence of the barrier reef systemrst recorded by the Chambishi Dolomite. The upper 75 m of the

nterval more closely resemble previous evaporitic intervals of theoorly sorted arkose facies, and is interpreted to record highstandrogradation of the inner shelf mixed clastic and carbonate inter-idal marginal deposits over the more basinal reef/lagoon system.

.1.4. Sequence 4The base of this sequence is marked by a transgressive sur-

ace comprising a return to carbonate-dominated sedimentation.equence 4 is the thickest sequence present, and the overlying TSTomprises up to 200 m of interbedded massive white and beddedrey dolomite facies. The maximum flooding surface is taken as thehickest interval of bedded grey dolomite facies, and the overlyingST as the >100 m thick aggradational interval of the massive whiteolomite facies between 650 and 790 m (Fig. 7).

.1.5. Sequences 5 and 6The thick HST that caps sequence 4 is overlain by a transgressive

urface at ∼650 m. This is marked by an abrupt return to sub-tidalonditions recorded by the bedded grey dolomite and subsequentolomitic siltstone facies intervals that represent the maximumooding surface (Fig. 7). The thin TST of sequence 5 is overlain by

thick regressive HST comprising sub-tidal bedded grey dolomiteacies overlain by supra-tidal massive white dolomite facies. Theatter facies is identical to that which comprises the bulk of thenderlying Upper Roan Subgroup (sequence 4), however we inter-ret this sequence as the basal element of the Mwashia Subgroup
Section 5.3.5). It is terminated by another transgressive surfacehat is overlain by a similar but thicker TST-HST cycle (sequence 6)hat is terminated by the breccia horizon that comprises the upperart of the DDH RCB2 intersection.
earch 190 (2011) 70– 89 81

5.2. Eastern side of Kafue Anticline

In order to document the Mufulira-type Roan Group succes-sion from the eastern side of the Kafue Anticline, where a welldefined Copperbelt Orebody Member is absent, drill cores fromMufulira itself were avoided because of the presence of substan-tial brecciation and stratal disruption above the Ore Formation(e.g. Cailteux et al., 1994; Binda and Porada, 1995). A diamondcore from the Itawa prospect 60 km to the southeast of Mufulirawas chosen instead (Fig. 2) because stratal disruption in this areais minimal. It was collared in the lower part of the Nguba Group(Fig. 8), and intersects ∼300 m of the Kakontwe Limestone Forma-tion, 40 m of dolomitic siltstones of the Kaponda Formation, and∼50 of Mwale Formation (Grand Conglomérat), the base of whichoccurs at 393 m. The underlying Roan Group intersection com-prises 140 m of interbedded siltstones and sub-tidal carbonates ofthe Mwashia Subgroup, 190 m of Upper Roan Subgroup carbonatesoverlying the Lower Roan Subgroup. Although thicknesses will begiven in metres, where unit boundaries are identified they are citedin feet in which the drill core and samples taken were marked up.

The Roan Group section in DDH Itawa 26 is continuous with theexception of a thin breccia and associated alteration zone towardsthe base of the Upper Roan Subgroup, however the carbonate facieson either side are identical so stratal disruption is interpreted to beminimal. Although the proportions are different, the facies in theDDH IT26 intersection are the same as those on the western side ofthe Kafue Anticline. The same facies model (Fig. 4b) has thereforebeen used to interpret the succession in terms of a palaeo-waterdepth/accommodation curve, and hence identify the key surfacesand define the sedimentary sequences (Fig. 8).

5.2.1. Sequence 1The base of first sequence is a transgressive surface marked by

the lowest interval of the poorly sorted arkose facies at 3080 ft.Sequence 1 is around 25 m thick and is dominated by clastic inter-tidal deposits of the poorly sorted arkose facies, with thin supra-tidal (well-sorted arkose facies) interbeds. A maximum floodingsurface marked by thin carbonate inter-beds representing sub-tidalpatch reefs separates a thin TST from a thicker HST.

5.2.2. Sequence 2The base of sequence 2 is marked by an abrupt return to the

well-sorted arkose facies at 2990 ft which is interpreted to repre-sent a sequence boundary. It is overlain successively by ∼10 m ofwell-sorted arkose facies interpreted as a LST (Fig. 8); a transgres-sive surface marked by an evaporitic carbonate unit; and ∼30 m ofinter-bedded sub-tidal carbonate, inter-tidal and supra-tidal clas-tics representing the TST and HST intervals.

5.2.3. Sequence 3The base of sequence 3 is once again marked by a sequence

boundary at the base of an interval of the well-sorted arkose faciesrecording a LST, which in this case is <10 m thick and is the unitreferred to locally as the Marker Grit (e.g. Binda and Mulgrew, 1974;Fig. 8). It is overlain by a transgressive surface marked by a return tointer-tidal (poorly sorted arkose facies) and then sub-tidal (massivewhite dolomite facies) deposits. Parasequences in the >30 m thickTST of sequence 3 are marked by the first occurrences of significantthicknesses of the dolomitic siltstone facies on this side of the KafueAnticline, one of which marks the maximum flooding surface.

5.2.4. Sequence 4
The base of sequence 4 is marked by a sequence boundary at the
base of a distinctive 20 m thick LST of the well-sorted arkose facieslocally referred to as the Glassy Quartzite (e.g. Binda and Mulgrew,1974; Fig. 8). It is overlain by a transgressive surface marked by


Fig. 8. Facies log of DDH Itawa 26 (legend is the same as Fig. 3b) showing interpreted palaeobathymetry, sequenced stratigraphy (abbreviations same as for Fig. 7) and C andO isotopic profile (red dashed lines highlight the two negative ı13C excursions discussed in the text). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of the article.)

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return to inter-tidal poorly sorted arkose facies deposits. Thesere succeeded by the carbonate dominated interval that compriseshe Upper Roan Subgroup in the published stratigraphic schemes.he carbonate-dominated sequence 4 is the thickest in the DDHT26 intersection, with a maximum flooding surface marked by annterval of dolomitic siltstone facies at ∼2370 ft subdividing an 80 mhick TST from a >150 HST.

.2.5. Sequences 5–7The thick HST of sequence 4, which is dominated by the mas-

ive white dolomite facies, is overlain by a transgressive surfacet ∼1700 ft which is marked by a return to a 15 m thick intervalf bedded grey dolomite facies. Together with an overlying inter-al of the dolomitic siltstone facies this defines a 20 m thick upwardeepening cycle (sequence 5; Fig. 8). This is succeeded by two moreimilar upward deepening cycles (sequences 6 and 7) of 40 and00 m in thickness beneath the contact with the overlying Grandonglomérat that forms the base of the Nguba Group.

. O and C isotopic chemostratigraphy

.1. Western side of Kafue Anticline

The down-hole C and O isotope data from the DDH RCB2 inter-ection are illustrated in Fig. 7. Overall the C and O curves broadlyirror each other, with most ı13C values between 0 and 5‰ andost �18 O values ∼25‰ which is typical for Cryogenian sedimen-

ary carbonate (e.g. Halversen et al., 2005). The notable exception ishe breccia zone at ∼780 m and its incipiently brecciated halo. Here18 O values fall abruptly to ∼5‰ from ∼900 m up to the level of thereccia, and then recover to “normal” values of >25‰ over the inter-al up to ∼730 m. ı13C values over the same interval are variablyepleted to −1 to 3‰ from ∼5‰ above and below. Oxygen isotopicalues are known to be affected at lower water to rock ratios thanarbon values (e.g. Sverjensky, 1981; Zheng and Hoefs, 1993) and as

result are often used as and indicator of post-depositional alter-tion (e.g. Brand and Veizer, 1980; Derry et al., 1992; Large et al.,001). This zone of anomalously light ı18O is therefore interpretedo have been affected by fluid flow and resultant alteration of thearbonate in and around the breccia zone.

There are three discrete negative excursions present in the DDHCB2 intersection (Fig. 7). The lowest is the large previously recog-ised excursion associated with sequence 1, in which ı13C valuesrend down to near −20‰. As noted in Section 3.2, the magnitudef this excursion is greater than can be explained by secular vari-tion and it is clearly associated with mineralization at this levelAnnels, 1989; Sweeney and Binda, 1989; Selley et al., 2005). It isherefore unlikely to be useful in terms of chemo-stratigraphy.

The second negative excursion spans the contact betweenequences 3 and 4 (Fig. 7). It begins in the HST of sequence 3, where13C values trend from +5 down to −7‰ over ∼100 m of section intohe TST of sequence 4, and then recover to >5‰ over the next 50 m.t is mirrored by an accompanying ı18O excursion from 25 to 15 ‰.s this excursion is not associated with mineralization or obviouslteration, and its magnitude is consistent with a secular variationn seawater chemistry (e.g. Halversen et al., 2005), it is a potentiallyseful chemo-stratigraphic marker.

The onset of a third negative ı13C trend is coincident with thease of sequence 5 that marks the base of the Mwashia SubgroupFig. 7). Values trend from +5 at the base towards −5‰ over ∼200 mf section below the major breccia unit at the top of the hole.
he ı13C values are once again mirrored by an accompanying ı18Oxcursion from >25 to <25‰. The magnitude of the excursion andbsence of associated alteration suggests it also represents a secularariation that may be useful for stratigraphic correlation.
earch 190 (2011) 70– 89 83

6.2. Eastern side of Kafue Anticline

The down-hole C and O isotope data from the DDH IT26 inter-section is illustrated in Fig. 8. Once again the C and O curves mirroreach other except in the region of the one breccia horizon presentat ∼2500 ft. Due to the paucity of carbonate in the Lower Roan Sub-group on this side of the Kafue Anticline, data from sequences 1,2, and 3 is sparse. It is therefore unclear whether the lowermostnegative ı13C excursion in DDH RCB2, which is associated with themineralization in sequence 1, is present.

The first negative excursion that is recorded in the section fromthe eastern side of the Kafue Anticline commences in the TST ofsequence 3 and terminates in the TST of sequence 4 (Fig. 8). Overall,ı13C falls over ∼80 m of section from around 7‰ down to −5 andthen recover rapidly to 3–7‰. The ı13C excursion is mirrored by aı18O excursion with values falling from >25‰ to <20‰.

A second ı13C excursion commences in the upper part of theTST of sequence 4, where values fall from ∼5‰ to close to 0‰over ∼30 m then rise to values of around 5‰ over the next 100 m.However, this is the interval over which the C and O curves aredecoupled and the negative ı13C excursion is broadly coincidentwith a positive ı18O excursion from around 25 to 30‰. Once again,we interpret this part of the isotopic curve with anomalous ı18O(and in this case also ı13C) values to reflect the effects of postdepositional fluid flow and alteration in and around the thin brec-cia horizon that occurs at around 2500 m. The associated negativeı13C excursion is therefore not considered significant for regionalcorrelation.

A third negative excursion commences at the base of sequence5. In this case ı13C values trend from >5 down to −6‰ over ∼200 mthrough the Mwashia Subgroup, with the most negative valuesoccurring towards the top of the unit, immediately below the GrandConglomérat that marks the base of the overlying Nguba Group.This negative ı13C excursion is mirrored by the ı18O profile inwhich values fall from ∼25‰ to ∼20‰. Above the Grand Con-glomérat, with the exception of two data points, ı13C and ı18Ovalues through 300 m of the Kaponda and Kakontwe Formationsdefine a flat trend with values around 2–3‰ and ∼28‰ respec-tively.

7. Discussion

7.1. Comparison of sequences and carbon isotope profiles acrossthe Kafue Anticline

Taken overall, there is a broadly similar upward evolutionin lithofacies association in the sections from either side of theKafue Anticline (Fig. 9), which has formed the basis for the for-mation level stratigraphic schemes previously proposed up to thelevel of the Upper Roan Subgroup (Table 1). The basal interval ineach case comprises coarse-grained clastic-dominated suabaeriallydeposited sediments (Mindola Clastics and Footwall Formations).These are succeeded by an interval of mixed coarse and finer-grained clastic and carbonate marginal marine sediments (Kitweand Ore and Hangingwall Formations) which record both the great-est facies diversity and the most marked lateral facies changesacross the Kafue Anticline. In the Upper Roan Subgroup, both sidesof the Kafue Anticline once more record a similar facies assemblage,in this case a carbonate-dominated platform succession. This is suc-ceeded by sub-wave base facies which characterise the MwashiaSubgroup on both sides of the Kafue Anticline.

The Ore and Hangingwall Formations on the eastern side of theKafue Anticline, and the Kitwe Formation on the western side, werecollectively referred to as the Mixed Formation by Binda (1994).The lateral facies and thickness changes at this level led to the


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roposal that the Mixed Formation represented a sandy shorelineedge that graded to deeper basinal deposits to the southwest into

he area of the Chambishi Basin, and that this was succeeded by aajor Carbonate platform in the form of the Upper Roan Subgroup

hat ultimately onlapped the entire Kafue Anticline. Overall ourequence stratigraphic analysis supports the Binda (1994) inter-retation. The fact that our analysis has identified four sequencesetween the Mindola Clastic Formation and carbonate platformhat comprises the Upper Roan Subgroup on either side of theafue Anticline (Fig. 9), validates the application of the technique

o this succession, and allows us to take the next step in refininghe stratigraphic architecture.

.1.1. Sequences 1 and 2
There is no clear basal sequence boundary to the initial sequence
n the western side of the Kafue Anticline (Fig. 7), as the underlyingrkosic strata of the Mindola Clastics Formation are subaerial fluvialeposits. The onset of sequence 1 is marked by an abrupt trans-

hic correlations between DDH RCB2 and DDH IT26, and potential correlation of theo color in this figure legend, the reader is referred to the web version of the article.)

gressive surface and the succeeding TST is thin, as the sub-tidalsiltstones that contain the maximum flooding surface occur only10–15 m above the base of the ∼70 m thick sequence. This necessi-tates the rapid generation of significant accommodation space, andgiven the clear half graben architecture of the immediately underly-ing Mindola Clastics Formation (Fig. 4), is consistent with the Selleyet al. (2005) model of fault linkage and rift climax at this time. Itis these strata that record the substantial negative ı13C excursiondown to values as low as −18‰, however, as discussed in Section6.1, this is interpreted to be a signature of the mineralization atthis level, and will therefore be unsuitable as an aid to correlationof this sequence. Above the maximum flooding surface the tectoni-cally generated accommodation space is progressively infilled, andthe clastic material that dominates the poorly sorted arkose facies
that comprises the bulk of the HST is interpreted to record erosionof elevated footwall blocks generated by the linkage of extensionalgrowth faults. As noted previously (Section 3.1), subdivision ofsequences to the parasequence level is generally impossible in Pre-

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ambrian successions (e.g. von der Borch et al., 1988; Krapez, 1996)nd is beyond the scope of this study. However, individual exam-les can be identified, particularly in the cyclic arrangement of theacies that comprise the poorly sorted arkose facies that dominateshe HST. Specifically, dolomitic siltstone and/or carbonate beds rep-esenting small patch reefs (deep inter-tidal to shallow sub-tidaleposits), overlain by poorly sorted arkose (inter-tidal deposits),verlain by well sorted arkose (supra-tidal deposits) record indi-idual upward shoaling parasequences.

The onset of the sequence 2 is marked by the well definedequence boundary at the base of the Nchanga Quartzite (Fig. 7).t is otherwise broadly similar to the underlying sequence, but isignificantly thinner (∼40 m), and the TST comprises a series ofhin dolomitic siltstones interbedded with clastic and carbonateasin marginal facies. Once again however, the maximum flood-

ng surface occurs within 10 m of the transgressive surface and isverlain by a relatively thick clastic-dominated HST. All of these fea-ures are consistent with the deposition of sequence 2 reflecting aecond, but less dramatic episode of rapid tectonically generatedccommodation.

On the eastern side of the Kafue Anticline, the absence of sub-ave base facies (i.e. dolomitic siltstone facies) in sequences 1 and

indicates less accommodation in this area at this time. As a resulthe facies belts represented are shifted to shallower environments,redominantly the supra- and inter-tidal clastics of the well-sortedrkose facies and poorly sorted arkose facies respectively (Fig. 8).ntervals of the former mark sequence boundaries and comprisehe LSTs, while the latter comprise the TST and HSTs. Inter-beddedarbonates recording sub-tidal patch reefs mark the deepest wateronditions (deep inter-tidal to shallow sub-tidal?) and hence theaximum flooding surfaces in this area. As is the case in sequences

and 2 on the western side of the Kafue Anticline, these occur lown both sequences, separating relatively thin TSTs from relativelyhick HSTs (Fig. 8). We consider this pattern to indicate that theectonic framework proposed by Selley et al. (2005) also controlledccommodation space on the eastern side of the Kafue Anticline.ue to the paucity of outcrops in the ZCB, the exact position of theaster structures that facilitated the accelerated accommodation

eneration that characterises sequences 1 and 2 has never beenetermined. However, with respect to the position of our two sec-ions within the regional structural framework, a comparison ofhe facies associations and accommodation characteristics (Fig. 9)learly indicate that the DDH RCB2 intersection was relatively prox-mal to a master growth fault and the DDH Itawa 26 intersectionelatively distal.

.1.2. Sequence 3On the western side of the Kafue Anticline, sequence 3 is dif-

erent in character to the underlying sequences. Its base is markedy a transgressive surface (Fig. 7), suggesting that transgressionesumed before relative sea level fell sufficiently to generate aarked basinward shift of facies on the top of the underly-

ng sequence. More importantly however, the TST represents therst accumulation of thick microbial carbonate, the Chambishiolomite, which could only occur if the supply of clastic sedimentad waned. This clearly indicates a marked change in the basinrchitecture that allowed considerable displacement of the shore-ine in response to relative sea level rise. While carbonate facies inhe preceding sequences record isolated patch reefs, we interprethe Chambishi Dolomite as the first substantial carbonate barriereef system developed on a starved outer shelf. Carbonate facies are
parse in the overlying HST, but the one sample analysed records a13C value ∼5‰ lighter than a sample from the TST (i.e. the Cham-ishi Dolomite) which marks the onset of a negative excursion athis level (Section 6.1).
earch 190 (2011) 70– 89 85

Sequence 3 on the eastern side of the Kafue Anticline is also dif-ferent to the underlying sequences, attesting to a change of basinarchitecture at this time (Fig. 8). However, in contrast to the west-ern side of the Kafue Anticline, carbonates are a minor componentof the TST, which instead is dominated by the poorly sorted arkosefacies indicating continued supply of clastic sediment to this region.The basal flooding surfaces that define individual parasequencesare marked in this case by dolomitic siltstone facies intervals thatprovide the first evidence of sub-wave base conditions in this area.Once again, sparse carbonates in the HST record the onset of anegative ı13C excursion (Section 6.2).

As with previous sequences, sequence 3 is thinner than itsequivalent on the western side of the Kafue Anticline (Fig. 9),indicating continued relative restriction in the generation ofaccommodation space in this area. However, the maintenance ofclastic sediment supply and the fact that subsidence was sufficientto generate sub-wave base conditions for the first time in thisregion, while the western side of the Kafue Anticline was starved ofclastic sediment, suggests a significant change in basin architectureduring deposition of sequence 3. Overall, we infer that sequence3 records the transition to a relatively low relief basin marginwith a different pattern of source regions for clastic sediment. Thisimplies that the elevated footwall blocks generated by the riftingthat controlled the previous two sequences were now denudedby erosion, and were not renewed by a further episode of activetectonism to generate the accommodation space for sequence3. If the accommodation space for sequence 3 was tectonicallygenerated, then it must have involved different growth structures,implying a different regional stress regime. Alternatively thesequence may represent a change from tectonic to eustatic controlon accommodation as tectonism waned.

7.1.3. Sequence 4Sequence 4 is similar in character on both sides of the Kafue

Anticline (Fig. 9). At >250 m it is by far the thickest sequence in theRoan Group, and in this case the maximum flooding occurs approx-imately half way through the sequence, i.e. the TST and HST are ofsimilar thickness. The negative ı13C excursion that commenced insequence 3 peaks with values of <−5‰ in the lower part of the TSTof sequence 4 in both intersections, above which values quicklyrecover to >0‰ (Section 6.1). This should provide a useful markerfor correlating sequences 3 and 4 with other Roan Group sections.The overlying HST is dominated by the massive white dolomitefacies, and once again the lack of significant inter-beds indicatesa long period of aggradational sedimentation on a starved outershelf. With the exception of the brecciated/altered zone where theO and C curves are decoupled in each intersection (Sections 6.1 and6.2), the ı13C profiles are fairly flat through this interval. We concurwith Binda (1994) that the Upper Roan Subgroup (i.e. sequence 4)records the establishment of a regionally extensive carbonate plat-form. In terms of controls on accommodation we interpret this tosignify further waning of regional relief and lack of active tectonism.

7.1.4. Post Upper Roan Subgroup sequencesOn both sides of the Kafue Anticline, the transgressive sur-

face at the top of the Upper Roan Subgroup that marks the baseof sequence 5 is abrupt and easily recognisable, and the strataabove it are cyclic in nature (Fig. 9). In RCB2, there are twosequences present between the top of the Upper Roan Subgroupand the breccia in which the hole is collared, both of whichhave identifiable TST and HSTs separated by maximum flood-ing surfaces. The HSTs in each case are marked by a return to
a massive white dolomite facies that comprises the bulk of theUpper Roan Subgroup. In contrast, in IT26, there are no intervalsof the massive white dolomite facies in the three upward deep-ening cycles present at this level below the Grand Conglomérat,

8 an Res

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hich together with the lack of identifiable HSTs in this area,uggests deeper water conditions reflecting greater amounts ofccommodation on the western side of the Kafue Anticline at thisime. In both cases however, there is a systematic decrease in13C values of ∼10‰ through these post Upper Roan Subgroupequences 5 to 7 (Sections 6.1 and 6.2), which in the more com-lete section in DDH IT26 extends right up to the base of the Grandonglomérat.

On the basis of their stratigraphic position between the Upperoan Subgroup and the Grand Conglomérat, and the fact that they

nclude sub-tidal facies, sequences 5 to 7 are interpreted to belongo the Mwashia Subgroup. Previous litho-stratigraphic schemes forhis unit in the ZCB have considered it to comprise dominantlyrgillaceous sediments (e.g. Mendelsohn, 1961; Cailteux et al.,994), although conglomerate and sandstone interbeds have beeneported locally. In contrast, in the Democratic Republic of Congo,he Mwashia Subgroup has been divided into a predominantlyolomitic lower and predominantly argillaceous upper intervale.g. Franc ois, 1974), the latter of which in turn has been furtherubdivided into two sub-units comprising dolomitic siltstones, andarbonaceous siltstones and sandstones respectively (e.g. Cailteuxt al., 1994). In the most recent litho-stratigraphic scheme proposedor the Democratic Republic of Congo (Cailteux et al., 2007), the pre-ominantly carbonate strata previously termed the lower Mwashiaave been included in the underlying Dipeta Subgroup (uppermostansuki Formation), and the strata previously termed the upperwashia between the carbonates and the Grand Conglomérat have

een subdivided, from bottom to top into the Kamoya (dolomiticiltstones with basal conglomerates), Kafubu (carbonaceous shalesnd siltstones) and Kanzadi (sandstones ± interbedded shales) For-ations.A detailed investigation of the sequence stratigraphy of the

washia Subgroup in the ZCB is not possible with our dataset,ecause the unit is truncated by the breccia in DDH RCB2 (Fig. 7).owever we can make some observations about the nature of the

trata at this level. As noted previously (Section 5.3.3), in the con-ext of the basin fill as a whole, we concur with Binda (1994) that thehickness and extent of the Upper Roan Subgroup (i.e. sequence 4)eflects the establishment of a major shallow to emergent carbon-te platform system, indicating that the basin was effectively filledt this time. Once established, a carbonate platform of this typend extent is resilient to sea level changes, because the carbonatefactory” can keep up with all but the most dramatic increases inccommodation space (e.g. Wilson, 1975).

In this context we concur with Cailteux et al. (2007), that thergillaceous rocks that mark the base of the Mwashia Subgroupn their stratigraphic scheme signify an abrupt change in environ-

ental conditions in the form of a major/rapid rise in relative seaevel. This resulted in initial transgression and subsequent flood-ng/drowning of the pre-existing carbonate platform. The mostikely explanation for a relative sea level rise of sufficient magnitudeuch as to cause the demise of the established carbonate platform,s a resumption of tectonic/structural control on the generation ofccommodation space. Syn-sedimentary tectonism at this time isupported by the lateral sedimentary facies variation in sequences–7, and occurrence of volcanics at this stratigraphic level in theongolese Copperbelt (Cailteux et al., 2007). The generation of moreccommodation space on the eastern side of the Kafue Anticlinet this time is the opposite of the situation that prevailed duringeposition of sequences 1 and 2. This suggests that the control-

ing structures and resultant basin architecture of the sedimentaryequences that were deposited during periods of active tectonism
efore and after the hiatus in tectonism recorded by sequences 3nd 4 were different.
The facies variation at the level of the Mwashia Subgroup haslearly caused some confusion in lithostratigraphic correlations

earch 190 (2011) 70– 89

between the Zambian and Congolese parts of the copperbelt in thepast (e.g. Cailteux et al., 1994). However, in terms of future regionalcorrelation of upper Roan Group strata, we contend that the baseof sequence 5, which separates a period of minimum accommo-dation from a major transgressive event, and is accompanied bythe onset of the 10‰ ı13C excursion identified in this study, pro-vides a distinctive datum at this level that should be identifiablebasin-wide.

7.2. Tectonic implications of the sequence stratigraphic model

In overview, the Katangan stratigraphic succession has beeninterpreted in terms of a rifted continental margin (the Roan Group)which developed into a Red Sea-type proto ocean during depositionof the Nguba and Kundelungu groups (e.g. Kampunzu et al., 1993;Tembo et al., 1999; Kampunzu et al., 2000). Our sequence strati-graphic correlation spanning the Kafue Anticline is consistent withthis interpretation and allows for refinement of the early part of thebasin history. It suggests that the Roan Group can be considered interms of three tectono-stratigraphic cycles which reflect differentstages of basin development (Fig. 9).

Cycle 1 comprises the Mindola Clastics Formation andsequences 1 and 2 that represent the initiation, climax and wan-ing phase of active extension respectively. Cycle 2 comprises thesequences 3 and 4 that span the Chambishi Dolomite and UpperRoan Subgroup which record a period of tectonic quiescence duringwith the basin margin had evolved to a shallow, low relief system.Cycle 3 comprises sequences 5 to 7 within the Mwashia Subgroupthat record a resumption of tectonic control on the generation ofaccommodation space. Facies patterns indicate that the controllingstructures and resultant basin architecture were different to thosethat prevailed during sequences 1 and 2. Overall the characteris-tics of sequences 5–7 accord with the proposal that the onset ofcontinental breakup occurred at this time (e.g. Kampunzu et al.,2000).

7.3. Economic implications of the sequence stratigraphic model

The origin of sediment-hosted stratiform copper deposits hasbeen studied for more than a century, and there is broad agree-ment on many elements relating to their formation (e.g. Kirkham,1989; Hitzman et al., 2005). For example, the majority of sediment-hosted Cu deposits occur in basins where a basal continental redbed succession is overlain by lacustrine or marine sediments incor-porating reduced lithologies and evaporites. The mineralization isconsidered to occur during diagenesis, with mafic minerals incor-porated into the basal clastics, or volcanic units if present, providingthe copper, which is transported in oxidized fluids and precipi-tated where these interact with reductants in the overlying basinalstrata. The stratiform sediment-hosted Cu deposits that character-ize the ZCB are world class examples of this style of mineralization.Although the ore deposits have not been the focus of this study,our sequence stratigraphic model has several broad implicationsfor the relationship between basin evolution and mineralization.

Firstly, it is clear that the classical ores, which comprise laterallycontinuous stratabound zones of economic sulfides, are restrictedto the two basal sequences that record significant architecture onthe basin margin. This style of mineralization is absent once a lowrelief carbonate dominated facies architecture is established bysequence 3. In detail, the Cu sulfides in sequences 1 and 2 occurin the well-sorted arkose and dolomitic siltstone facies depositedduring sea level lowstands and the initial phase of transgression
respectively. On face value, the latter deposit type could be inter-preted to reflect the widely recognised redox-based character ofstratiform sediment-hosted Cu systems, i.e. the metals can onlyprecipitate where a suitable reduced “trap” unit is present (e.g.

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irkham, 1989; Hitzman et al., 2005). However, reduced dolomiticiltstone facies occur throughout the poorly sorted arkose faciesntervals that comprise the HSTs, albeit as relatively thin interbeds,ut in this case Cu sulfides are generally minor to absent. These

ntervals therefore appear to have been isolated from the metalransporting fluids. In contrast, the widespread occurrence of sul-de accumulations in the underlying well-sorted arkosic facies i.e.he Mindola Clastics-hosted orebodies stratigraphically beneathhe Copperbelt Orebody Member (e.g. Selley et al., 2005), andhe extensive mineralization in the Nchanga Quartzite at NchangaBinda and Mulgrew, 1974), suggest that intervals of this faciesere the principal stratal aquifers during the formation of the ore-

odies. The reductant involved in this case has been interpretedo have been migrated hydrocarbons (Annels, 1979; Selley et al.,005). One possible explanation for the overall distribution of sul-des, is that only reductant accumulations within and/or in closeroximity to the stratal aquifers, as was the case for tectonicallyontrolled sequences 1 and 2, were able to interact with the metal-earing fluids.

A second possible control on the timing of mineralization ishat the process required cross stratal fluid pathways. These couldave been provided by the basin controlling normal growth faultshat were clearly active during the onset of sequences 1 and 2 asvidenced by the significant topography that existed at this time.

related issue, given the spatial association of pinch outs of therinciple statal aquifer systems (i.e. the well-sorted arkose facies)nd the best ore grades (Selley et al., 2005), is that these pinchouteometries were required to facilitate metal precipitation. Theevelopment of both permeable cross-stratal fluid pathways in theorm of syn-sedimentary normal faults and of pinchout geometries,ould have been facilitated during the early phases of the syn-

ift sequences 1 and 2, but would have become progressively lessarked as activity on these structures waned, topography was sub-

ued, and a more layer cake stratigraphic architecture developedhrough sequences 3 and 4. It is interesting to note that our inter-retation of the basin history is that similar tectono-stratigraphiconditions began to prevail again during deposition of the Mwashiaequences, which are the host to a series of recent discoveries ofconomic mineralization (e.g. the Frontier and Lonshi deposits;elley et al., 2005).

A third possible avenue by which basin development may con-rol the mineralization process in the ZCB relates to the salineature of the ore fluids (e.g. Annels, 1974; Unrug, 1988). Evaporiteseudomorphs are most common from the level of the upper part ofhe TST of sequence 1 through to sequence 4, and are in general besteveloped in the HSTs. Brine reflux, where dense residual brinesescend through the partially consolidated substrate is known toccur during formation of evaporitic strata (e.g. Warren, 2000). It isherefore possible, that the ore fluids responsible for the mineral-zation within the sequences 1 and 2 were brines generated duringccumulation of the overlying abundantly evaporitic sequences 3nd 4.

.4. Comparison of the C isotope profile with the globaleoproterozoic curve

The Neoproterozoic successions that occur globally and includeistinctive glacial strata at several levels have been correlatedsing secular ı13C trends, in combination with radiometric dat-

ng where suitable material is present (e.g. Halversen et al., 2005;rave et al., 2009; Halverson et al., 2010; Macdonald et al., 2010).he most recent compilation (Macdonald et al., 2010) includes four
iscrete negative ı13C excursions in strata deposited between 500nd 850 Ma, which are termed, from oldest to youngest, the Bit-ers Springs stage, the Islay anomaly, the Trezona anomaly and thehuram-Wonoka anomaly (Fig. 9). The upper three of the negative
earch 190 (2011) 70– 89 87

ı13C anomalies are associated with strata that record glacial eventstermed the Sturtian, Marinoan and Gaskiers events (Halversonet al., 2010; Macdonald et al., 2010).

When combined, the carbon isotopic profiles from DDH RCB2and IT26 provide complete coverage of the carbonates present fromthe upper part of the Lower Roan Subgroup to the middle levels ofthe Nguba Group (Fig. 9), the base of which is defined by the GrandConglomérat. This interval includes two negative ı13C excursionsthat occur in both intersections and are interpreted to be secularin origin. The upper ı13C excursion is the easiest to characterisebecause of its relationship the Grand Conglomérat in the completesection in DDH IT26.

The Grand Conglomérat has been interpreted to represent theoldest Sturtian Neoproterozoic glacial event (Key et al., 2001;Bodiselitsch et al., 2005; Batumike et al., 2007; Wendorff and Key,2009). Until recently, the age of the Sturtian glaciation has beenpoorly constrained (e.g. Halversen et al., 2005; Kendall et al., 2006).However ID-TIMS dates have now been obtained from volcaniclas-tic units interbedded with the glacial strata in Oman (711.5 ± 0.2;Bowring et al., 2007) and northwestern Canada (716.5 ± 0.2 Ma;Macdonald et al., 2010). The maximum age of the Grand Con-glomérat in Zambia, constrained by U–Pb zircon date of 765 ± 5 Mafrom volcanic units in the underlying Mwashia Subgroup in thewestern foreland region of northwestern Zambia (Key et al., 2001),is therefore permissive of a Sturtian depositional age. Key et al.(2001) also proposed a minimum age for the Grand Conglomératof 735 ± 5 Ma based on dating of volcanic breccias in tectonic con-tact with it in the western part of the Lufilian Arc in northwesternZambia. However Halversen et al. (2005) consider the latter date tolack stratigraphic control, because it is from the external fold andthrust belt of the Lufilian Arc, where Key et al. (2001) acknowledgethat strata are tectonically interleaved.

Strata beneath the Sturtian glaciation globally record a negativeı13C excursion termed the Islay anomaly (e.g. Prave et al., 2009;Halverson et al., 2010), in which values begin to fall from >5‰ wellbelow the glacial strata to <−5‰, and then recover to less negativevalues immediately below the glacial deposits (Fig. 9). The onset ofthe upper ı13C excursion documented in the Roan Group in our datacoincides approximately with the transgressive surface that marksthe onset of sequence 5 at the base of the Mwashia Subgroup. Itinvolves a total fall in ı13C from >5 to <−5‰ over sequences 5–7that span >150 m of section to the base of the Grand Conglomérat.On the basis of the age constraints on the Grand Conglomérat, andthe distinctive magnitude and structure of this negative ı13C excur-sion, it is interpreted to represent the Islay anomaly that can berecognised in Cryogenic sections globally (Fig. 9).

The lower negative ı13C excursion that is present in both of theRoan Group intersections spans the bulk of sequence 3 and peaksin the lower part of sequence 4 (i.e. the base of the Upper RoanSubgroup). In both intersections ı13C values fall >10‰ to values of<−5‰. On the basis that the only secular negative ı13C excursionto values of <0‰ in strata deposited prior to the Sturtian glaciationin the global record is the Bitter Springs stage, which is not asso-ciated with glacial strata (e.g. Halverson et al., 2010; Macdonaldet al., 2010), the lower negative ı13C excursion in the Roan Groupintersection is tentatively correlated with this event (Fig. 9). Ifthis interpretation is correct, then the ∼811.5 Ma age of the BitterSprings stage based on a U–Pb zircon date from a tuff unit in the cen-tral Ogilvie Mountains of northwestern Canada (Macdonald et al.,2010) provides the first age constraint on Roan Group strata belowthe Mwashia Subgroup. This correlation supports the contention,based on the sequence stratigraphic analysis, that at least some
of the Roan Group strata were deposited under conditions of tec-tonic quiescence, because the ∼100 My interval between the BitterSprings stage and the Sturtian glaciation is recorded by the ∼500 mof section comprising the Upper Roan and Mwashia Subgroups.

8 an Res

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Cailteux, J., Binda, P., Katekesha, W.M., Kampunzu, A.B., Intiomale, M.M., Kapenda, D.,


. Conclusions

This study employs sequence stratigraphic techniques basedn sedimentary facies analysis to provide a contemporary basinodel for the economically important Roan Group in the ZCB.

esults indicate that the marine part of the Roan Group (the Kitweormation and Upper Roan and Mwashia Subgroups) can be con-idered in terms of six sedimentary facies/facies associations andnterpreted using a facies model for a mixed clastic and carbon-te barred basin margin. Vertical facies variations define relativealeo-bathymetric cycles, in particular a series of discrete trans-ressive events that allow subdivision of the succession into sevenedimentary sequences. The sequence model seems robust, inhat it allows correlation of the sequences across the Kafue Anti-line which incorporates considerable lateral facies variation fromore basinal Chambishi Basin-type intersections relatively rich in

iltstones and fine-grained clastics, to more marginal Mufulira-ype intersections that have a higher proportion of coarse-grainedlastics.

Conditions of regional extension and half-graben formation arenferred to have prevailed during deposition of sequences 1 and 2,ogether with the underlying Mindola Clastics Formation. The latteromprises subaerial fluvial and alluvial deposits that record the ini-ial stages of rifting and sequences 1 and 2 record the rift climax andhe waning stages of rifting respectively. The sequences are thickern the western side of the Kafue Anticline and the constituent faciesore basinal in character, indicating that this region was more

roximal to the main accommodation controlling syn-sedimentarytructures than the eastern side.

The subsequent sequences 3 and 4 record the initiation andominance of stromatolitic reefal facies respectively. In basinevelopment terms, these strata record the transition to a region-lly extensive, low relief, carbonate-dominated basin marginlatform developed under conditions of tectonic quiescence. A neg-tive ı13C excursion that commences at the base of sequence 3nd peaks in the base of sequence 4 is interpreted to correlate tohe Bitter Springs Stage recognised in Cryogenic sections globally.ts recognition in the Zambian section may be a useful adjunct foregional correlation of the middle levels of the Roan Group through-ut central Africa.

The transgressive event that forms the base of sequence 5, whiche assign to the Mwashia Subgroup, is marked by sub-tidal facies

verstepping the preceding carbonate platform. It must, therefore,epresent a large and/or rapid rise in relative sea level and we inter-ret this to represent a resumption of the tectonically controlledeneration of accommodation space. Syn-sedimentary structuralontrol at this time is supported by the lateral facies changesnd volcanic units that occur at this stratigraphic level regionally.equences 5–7 incorporate a second >10‰ negative ı13C excursionown to values <−5‰ through the Mwashia Subgroup beneath therand Conglomérat. On the basis of the nature and magnitude of thexcursion, and the available chrono-stratigraphic constraints thatuggest a Sturtian age for the Grand Conglomérat, it is interpretedo represent the Islay anomaly recognised in Cryogenic sectionslobally.

The “classical” stratabound copper orebodies are restricted toequences 1 and 2 defined in this study that were deposited underonditions of active rifting. We consider this to be largely dueo the facies architecture of these sequences, in which oxidisedtratal aquifer units occur in hydrological communication withxtensive reduced shales. Footwall pinch outs generated duringyn-sedimentary faulting that controlled sequences 1 and 2, and
rines generated during evaporite formation in the HSTs of theseequences and in sequences 3 and 4, may also have been importantactors in ore genesis.
earch 190 (2011) 70– 89

Acknowledgements

This research reported in this paper was undertaken as partof AMIRA-ARC project P544, funded through industry sponsor-ship and the Australian Research Council Linkage scheme, andconducted jointly by CODES at the University of Tasmania andthe Department of Geology and Geological Engineering at Col-orado School of Mines. Contributors to this team effort includeCODES Ph.D. students Nicole Pollington and Mawson Croaker. Wealso wish to thank the corporate sponsors and managers of theAMIRA P544 project, and particularly the companies based in Zam-bia, including Anglo American-Zamanglo, Anglovaal Mining, FirstQuantum-Mopani and ZCCM. Undertaking the project would havebeen impossible without regular input and assistance from a num-ber of mine and exploration geologists, especially Dave Armstrong,Hugh Carruthers, Nick Franey, Peter Mann, James McMasters, JamesMwali, Claus Schlegal, Mike Stewart and Jon Woodhead. Reviewsby Richard Hanson and Galen Halverson have greatly improved thismanuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.precamres.2011.07.021.

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Documents

Sequence and carbon isotopic stratigraphy of the Neoproterozoic Roan Group strata of the Zambian copperbelt