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Depositional

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environments of the Meso- to Neoproterozoic Ghanzi-Chobe belt, northwest Botswana

BENSON N. J. MODIE

Geology Department, University of Wales, Aberystwyth, Dyfed SY23 3DB, UK

(Received 10 February 1995: revised version received 31 January 1996)

Abstract-The Ghan&Chobe belt comprises a volcano-sedimentary basin sequence located in northwest Botswana. This sequence started to accumulate at about 1106 Ma ago and was deformed during the Pan-African event between approximately 750 and 500 Ma. The stratigraphy consists of an older Kgwebe Formation and a younger Ghanzi Group. The Kgwebe Formation represents the first phase of basin evolution that involved extensional tectonics and intracratonic rifting with subsequent bimodal volcanism and the development of lacustrine to fluvial systems. The Ghanzi Group represents the second phase of basin evolution marked by an extensive rift enlargement episode, resulting in a shallow marine basin with the deposition of siliciclastic sedimentary rocks and carbonate beds. The marine basin consisted of a shallow shelf environment dominated by storm and fairweather conditions. Stratabound copper sulphide mineralization was deposited at a redox interface between an alluvial facies and a marine tmnsgmssive sequence. The final development of the sedimentary environment involved a progradational shoreline with the deposition of a red arenite facies. Intermediate sub-environments consisted of mixed clasticcarbonate lagoons and possible progradational deltas. Rifting in the Ghanzi-Chobe belt was aborted before the development of an ocean resulting in a failed intracontinental rift basin.

Resume - La chaine de Ghanzi-Chobe au nord-ouest du Botswana comporte une sequence volcano-&iimentaire, qui a commence A se deposer vers 1106 Ma et qui a et& deformee pendant l’evenement pan-africain entre approximativement 750 et 500 Ma. La stratigraphie comporte B la base la Formation de Kgwebe surmont&e par le Groupe de Ghanzi. La Formation de Kgwebe correspond a la phase initiale de l’evolution du bassin, suite 21 une tectonique d’extension accompagnee de rifting intracratonique et se poursuivant par un volcanisme bimodal ainsi que par le developpement de systemes lacustres et fluviaux. Le Groupe de Ghanzi represente la seconde phase dans l’evolution du bassin. Elle est marquee par un episode d’agrandissement considerable du rift, conduisant a un bassin marin peu profond avec depot de roches ddimentaires silicoclastiques et de pas&es carbonat&s. L.e bassin marin &it caracteri& par un environnement de plateau continental peu profond ou alternaient tempOtes et conditions m&%ologiques plus calmes. Une min&ahsation stratdide en sulfures de cuivre s’est form&r a l’interface redox entre des facies alluviaux et une sequence marine transgressive. Le developpement final de l’environnement &dimentaire a evolue vers des conditions de rivage progradant avec depot de facies arenaces rouges. Des sous-environnements intermediaires consistaient en lagunes a sedimentation mixte clastique et carbonatee et en d’eventuels deltas progradants. L.e rifting dans la chaine de Ghanzi-Chobe a ce& sans le developpement dun ocean, conduisant ainsi a un bassin de rift mtracontinental avorte.

INTRODUCTION

Although geological investigations on the Ghanzi- Chobe belt have been undertaken by many workers (e.g. Passarge, 1904; Walker, 1973,1974; Wright, 1956,1958; Thomas, 1%9,1973; Reeves, 19781985; Ruxton, 1980, 1981; Litherland, 1982; Borg, 1988a, 1988b; Borg and Maiden, 1987,1989; Huch et al., 1992; Schwartz and Akanyang, 1994; Modie, 1994;), knowledge of the sedimentological evolution of the depositional sequences remains minimal. This is due to a lack of exposure caused by a thin veneer of Cainozoic superficial deposits, termed the Kalahari Group (Walker, 1973,1974; Thomas, 1969,1973) which overlie more than 90% of the Ghanzi-Chobe belt. The Ghanzi-

Chobe belt poses many other geological problems such as: structural complexity, caused by the Pan-African Damaran orogeny, which resulted in open to tight, upright and isoclinal folds and thrusts (Thomas, 1969, 1973; Walker, 1973; Anglo American Botswana, 1992; Huch et al., 1992; Schwartz and Akanyang, 1994); the absence of biostratigraphic data and the paucity of radiometric age determinations. Attempts to establish the history of evolution of the Ghanzi-Chobe belt have been largely from a regional perspective, based on correlation of regional structural trends, radiometric ages and facies similarities particularly with areas in Namibia (Watters, 1977; Reeves, 1978,1985; Key and Rundle, 1981; Hegenberger and Berger, 1985; Borg, 1988a, 1988b; Camey et al., 1994).

Present address: Geological Survey Department, P/Bag 14, Lobatse, Botswana

255

256 B. N. J. MODIE

- - - FAULT

SIIIAYM HIUS 8 I NAMIBIA

BOTSWANA

I correlativrjs / I

0 too 100 KM I I IWO NY

CC-Congo Craton - ICC-W craton 1

Figure 1. General location and geotectonic framework of the Ghanzi-Chobe belt. Inset shows the position of the Ghanzi-Chobe belt (GCB) in relation to the Kalahari craton (KC) and the Congo craton (CC). The black rectangular box (marked Fig. 3) shows the position of the Mabeleapodi hills area (after Borg, 1988b).

This paper has two objectives; firstly, to describe and analyse the sedimentary sequence in order to determine the changing depositional environments and secondly, to review several lines of evidence in an attempt to place the depositional sequence in a plate tectonic setting. Despite the problems caused by poor exposure and the post-depositional tectonic deformation of the Pan- African orogeny, the Neoproterozoic rocks in the Ghanzi-Chobe belt still widely preserve sedimentary structures (Modie, 1994), which have formed the basis of this study. The need for a more precise understanding of the history of sedimentation requires the description and interpretation of the petrofacies (e.g. Dickinson and Suczek, 1979; Dimarco and Lowe, 1989; Johnsson, 1990) and of the sedimentary structures by comparison with modern analogues (Selley, 1985; Reading, 1986; Bridge, 1993; McCormick and Grotzinger, 1993). Moreover, these data allow the sequence to be better constrained in terms of the plate tectonic setting at the time of deposition.

GEOLOGICAL SETTING

The Ghanzi-Chobe belt is a Meso- to Neoproterozoic, elongated volcano-sedimentary basin situated in

western Botswana (Fig. 1). To the northwest, the Ghanzi-Chobe belt is bound by the complex Damaran erogenic belt, widely developed in Namibia (Tankard et al., 1982; Miller, 1983), and to the southeast by the Kalahari craton. The northeast-trending Ghanzi-Chobe belt is approximately 500 km in length (i.e. from Mamuno near the Namibia border to the Goha and Shinamba hills in central northern Botswana) and 100 km wide, as deduced from regional aeromagnetic and gravimetric surveys (Reeves, 1985). Regional interpretations and correlations based on geophysical trends, radiometric ages, lithological associations and sediment-hosted stratabound copper sulphide mineralization have allowed the sequence in the Ghanzi-Chobe belt to be correlated with similar sequences in Namibia, e.g. the Klein Aub and Witvlei basins, and possibly with some areas of southern Zambia, e.g. the Irumide belt (Watters, 1977; Reeves, 1978,1985; Key and Rundle, 1981; Borg and Maiden, 1987; Borg, 1988a, 1988b; Hanson et al., 1988). Of great significance, in a more regional perspective, is the identification and investigation of the stratabound copper sulphide mineralization by both private exploration companies (e.g. U.S. Steel, 1978; Anglo American Botswana, 1992, 1993) and individual

Depositional environments of the Meso- to Neoproterozoic Ghanzi-Chobe belt 257

KLEIN AU6 SACS, 1980 Borg 8 Maden,~Q86

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Figure 2 Regional lithostratigraphical correlation of the Ghanzi-Chobe belt and the Klein Aub, Dordabis and Witvlei basins of Namibia. See also the similarity in the stratabound copper sulphide mineralization (after Borg and Maiden, 1989).

research workers (e.g. Ruxton, 1980,1981; Borg, 1988a,

1988b; Borg and Maiden, 1989), who have shown that there are great similarities between the sequences in the Ghanzi-Chobe belt and its correlati\ies in Namibia, particularly in the Klein Aub, Dordabis and Witvlei basins (Fig. 2). Consequently, these volcano- sedimentary basins have been interpreted as having developed in a continental rift setting, along the northwest margin of the Kalahari craton (Borg, 1988a; Hoffmann, 1989; Aldiss and Carney, 1992), thus forming a basement to the rocks of the Damaran orogen. Detailed descriptions on the nature of the mineralization in the Ghanzi-Chobe belt around the Lake Ngami/Ngwako Pan area can be found in the works of U.S. Steel (1978), Anglo American Botswana (1990-1993) Ruxton (1980), Borg (1988b), Borg and Maiden (1987,1989), Huch et al. (1992) and Schwartz and Akanyang (1994). The mineralization is described as being largely hosted by grey-green (chemically-reduced) argillitic facies of the middle formation of the Ghanzi Group and is generally disseminated or related to cleavage, fractures and veins.

The Proterozoic sequence (Fig. 3) in the Ghanzi- Chobe belt consists of two main lithostratigraphical sub- divisions, namely an older basal Kgwebe Formation (approximately 2500 m thick), and a younger Ghanzi Group (approximately 5000 m thick). The Kgwebe

Formation is dominated by a bimodal volcanic suite composed of porphyritic rhyolitedacite flows, tuffs, minor ignimbrites and basalt with minor intercalated are&es (Thomas, 1969,1973; Huch et al., 1992; Schwartz and Akanyang, 1994; Modie, 1994). The overlying Ghanzi Group is volumetrically the largest lithostratigraphical unit and is divided into three formations (after Huch et al., 1992), namely the Lower D’kar Formation, Upper D’kar Formation and an uppermost Jakkalsputs Formation. The three formations consist of siliciclastic sedimentary rocks of variable compositions with subordinate carbonate beds. The two Proterozoic sequences described above are unconformably overlain by Phanerozoic sequences of the Karoo Supergroup and Kalahari Group (Fig. 3). The former is preserved in an 8 km wide and more than 20 km long, down-faulted, northeast-trending trough and consists of sandstone and basalt with local dolerite dykes and conglomerate (Thomas, 1969,1973; Reeves, 1978). The Kalahari Group is a complex Cainozoic lithological unit comprised of sands, calcretes, silcretes and river and pan sediments (Thomas, 1969, 1973; Walker, 1973,1974; Huch et al., 1992).

The igneous volcanic rocks of the Kgwebe Formation yielded Rb-Sr whole-rock radiometric ages ranging from 820 to 1020 Ma from the rhyolite-dacite unit

B. N. J. MODIE

Depositional environments of the Mesc- to Neoproterozoic Ghanzi-Chobe belt 259

INUSALSPUTS subordinate lImestone beds. cross-bedding and wave rlpplts.

parallel lamination+fluiug-up pulses. cl-00Utic limestone. rbythmWts-)cycltc tit&g-up pale siltstone to dark mudstont pulses.

red arenitt facits, cross-bedded. pebbly and granult-rich layers. red mudstone iutraclasts.

channellzed Rows

high-matrix grty sandstone, parallel and normal graded lamination. dark mud&one intraclasrs. DISTAL ALLUVIAL

FAN SYSTEM.

FLUVIAL TO BEACH

KGWEBE

vesicular basalt, interbedded volcautclasUcs

Figure 4. Lithostratigraphy and interpretation of the sedimentary depositional environments, based on the Mabeleapodi hills area (stratigraphical terminology after Huch et al., 1992).

(Boocock, 1968; Harding and Snelling, 1972; Key and Rundle, 1981). These whole-rock Rb-Sr ages are imprecisely defined as a result of metamorphic resetting and thus represent minimum ages. A new, more reliable U-Pb single zircon age of 1106*2 Ma was obtained from a rhyolite sample from the Mabeleapodi hills and has been interpreted as the age of crystallization of the rhyolite (Schwartz et al., in press) and hence the approximate age during which the sequence within the Ghan&Chobe belt began to accumulate.

The sequence in the Ghanzi-Chobe belt has suffered sonic degree of tectonic deformation and shows low- grade metamorphism dominated by chlorite, fine white micas, epidote and tremolite. The argillitic facies shows a growth of fine micas or muscovite and chlorite along cleavage planes, whereas the arenites and basalt have been deformed by brittle fracturing (Walker, 1973; Anglo American Botswana, 1992). The low-grade metamorphic assemblage described above is related to folding and cleavage, believed to be associated with the Pan-African Damaran deformation. The deformation has been dated at 650-700 Ma by Key and Rundle (1981),

based on the younger ages from Rb-Sr whole-rock analysis and K-Ar analysis on porphyries and deformed basalts of the Kgwebe Formation, respectively.

The following section describes the sedimentology of the Proterozoic sequence in the Ghanzi-Chobe belt. It should be emphasised, however, that in spite of the tectonic influence, sedimentological terms are preferred here for the purpose of clarity and to conform to the subject of this work. The Mabeleapodi hills area (Figs 1 and 3) was chosen as the study area for a detailed sedimentological analysis because it provides all the lithostratigraphical units found in the Ghanzi-Chobe belt, albeit with poor exposure, and photogeologically appears less deformed (Huch et aZ., 1992; Modie, 1994).

SEDIMENTOLOGY OF THE KGWEBE FORMATION AND GHAN21 GROUP

Kgwebe Formation

The basal Kgwebe Formation represents the first phase of basin development in the Gharui-Chobe belt,

260 B. N. J. MODIE

which involved volcanism and sedimentation. Three

sedimentary members are recognized in the Kgwebe Formation, informally categorized into a lower, middle and upper member (Fig. 4). The three sedimentary facies represent stages of development of the sedimentary depositional systems and hence record the history of sedimentary basin evolution. The lower member is a fine-grained sandstone with an estimated thickness of approximately 1-5 m. This sandstone facies consists of a basal pebbly layer containing clasts of basalt, therefore indicating that sedimentation at this stage was locally derived from the volcanics. The associated sedimentary structures include wave ripple forms, mudstone intraclasts, desiccation marks and jointed sandstone- filled chert layers. These structures indicate sedimentation and reworking in shallow water to emergent conditions. As the sediments are not very thick, it is concluded that they were deposited in small- scale shallow lakes under semi-arid climatic conditions.

The depositional environment evolved from small lakes into a fluvial system depositing the middle member; this is a fine-grained, cross-bedded, red arenite facies (approximately 20 m thick) with numerous pebbly layers and a basal &&-supported conglomerate. The pebbly layers and conglomerate represent lag deposits due to fluctuating water stages in channelized flows. There are some isolated cobbles trapped in between well-sorted and well-laminated sandstone, which probably indicate material that slid into channels with minimal current energy, capable of transporting only fine-g-rained sand. The conglomerate, pebbly layers and isolated cobbles consist of material derived from the volcanic assemblages within the depositional basin, whereas the main sandstone facies source probably came from outside the depositional basin. Cross- bedding in the sandstone commonly show palaeocurrents to the northeast. The depositional environment seems to have consisted of a major axial- through fluvial system transporting fine-grained arkosic sediments flowing northeastward and interacting with local transverse streams eroding the volcanics. The latter deposited the pebbly and conglomeratic beds.

The next stage in the development of the depositional environment is represented by the upper member, which is a grey, medium- to coarse-grained, cross-bedded quartz arenite with rare pebbly layers and isolated clasts of porphyritic rhyolite. This facies indicates a change in source area and deposition in an environment of fluctuating high energy conditions, as evidenced by the increased grain-size, high mineralogical maturity and characteristically upper stage flow structures, e.g. planar parallel lamination, graded bedding and reactivation and erosional surfaces. There is some bi-directional stratification recognized in this facies, which probably indicates an environment influenced by current reversal processes such as a beach sub-environment; this would normally be characterized by high energy reworking processes of the surf, breaker

and swash zones. The depositional environment at the end of the Kgwebe Formation can be defined as having consisted of a fluvial system discharging into a beach sub-environment. In addition, the quartz arenite is succeeded by a non-exposed unit, approximately 200 m thick, indicated by a float (loose material) of finer grained facies, e.g. fine-grained sandstone, siltstone and mudstone, indicating low-energy, perhaps suspension deposits in a basin environment. The lack of exposure precludes a more precise statement regarding the exact nature of the depositional basin.

Ghanzi Group

The Ghanzi Group represents the second major phase of basin development in the Ghanzi-Chobe belt, which probably involved renewed rifting, resulting in an expansion of the depositional zones culminating in a marine incursion. The exact age of sedimentation of the Ghanzi Group is not known and therefore the precise temporal relationship between the first and second phases of basin development is also not known. The interpretation of deformation and primary sedimentary structures (e.g. folds, cleavage and cross- bedding) and petrographic analysis (e.g. the recognition of rock fragments) shows that the Ghanzi Group overlies the Kgwebe Formation. The exact nature of contact between the Kgwebe Formation and the Ghanzi Group is not known due to poor exposure, but it is likely that a major unconformity exists because of the fundamental change in the depositional environment across the boundary. The stages of basin evolution during the second phase are preserved by the depositional sequences of the Ghanzi Group, namely the Lower D’kar Formation, Upper D’kar Formation and the Jakkalsputs Formation (Huch et al., 1992; Fig. 4).

The Lower D’kar Formation (approximately 2000 m thick) represents the initial stage of the second phase of basin development that involved renewed rifting. The sequence in this formation constitutes the distal facies of an alluvial depositional system characterized by initially high depositional rates, followed by reduced late stage rates and accompanied by winnowing flows. This is indicated by a general vertical and lateral facies variation from a poorly sorted, high mudstone matrix grey sandstone at the base to a better sorted red sandstone at the top. The latter shows development of pebbly layers and granule-rich layers, probably representing lag deposits of a channelized system. Sedimentary structures, which, include,-planar parallel lamination, sporadic cross-bedding, graded bedding, sharp erosive and non-erosive contacts and mudstone intraclasts, indicate periodic sedimentation with fluctuating depositional flow strengths.

The dominantly grey-green (chemically reduced) Upper D’kar Formation (approximately 1500 m thick) represents the development of a marine basin that probably resulted from thermal subsidence following

Depositional environments of the Meso- to Neoproterozoic Ghanzi-Chobe belt 261

Crossbeddlng

45 II=22

Ripple Crestunes

Figure 5. Palaeocurrent patterns for the Ghanzi-Chobe belt. Most data were ob- tained from the Mabeleapodi hills area and have been corrected for tectonic tilt.

the deposition of the Lower D’kar Formation. The marine basin deposited a finer-grained facies, which includes mudstone, siltstone and very fine- to fine- grained sandstone with subordinate (foolitic) limestone. The sedimentary structures are dominated by planar parallel lamination, which in the argillites constitutes fining-upward pulses, indicating suspension deposition below wave base. The characteristically periodic, thin and laterally extensive sandstone facies represents high energy activities such as storm events. A distinctively unique rhythmite facies consisting of repeated fining-upward pale siltstone to dark mudstone beds probably indicates the migration of sub- environments into the main depositional environment. Such rhythmites are usually a result of cyclic sedimentation between traction currents and suspended sediment load, such as in tidal influenced regimes producing tidalites. Suspended sediment may also be generated from delta slopes and from the influence of storm activities in marine shelf environments. The limestones were deposited in the shallow, warm waters of restricted lagoons and playa lakes (Huch et aZ., 1992) with the oolitic ones particularly in sub-tidal environments.

The final development of the depositional environment is represented by the dominantly red (chemically oxidized) Jakkalsputs Formation (approximately 1500 m thick), which is composed of well sorted, fine to medium-grained arkosic sandstone, interbedded with siltstone, mudstone and limestone. The. depositional environment is shown to have been characterized by current and wave reworking processes in the nearshore to shoreline environment; the associated sedimentary structures consist of planar parallel lamination, cross-bedding reactivation surfaces overlain by massive beds, ladder-back interference oscillatory ripples and straight-crested symmetrical ripple forms. The Jakkalspuk Formation overlies the Upper D’kar Formation with no obvious break and, therefore, both must have been deposited in the same broad environment; the two formations probably

represent an overall transition from a relatively deep, low-energy offshore-setting, to a shallower, high- energy, nearshoreonshore environment. The Jakkalspuk Formation marks the transition between continental depositional systems and the marine environment, as shown by the red bed facies, commonly associated with alluvial-fluvial systems, in contrast to wave-action structures of typically shallow-water basins.

PALAEOGEOGRAPI-IY

Due to inadequate exposure and the large distances between outcrops, palaeocurrent data are not statistically representative. However, it is expected that all the depositional sequences have had some influence from the palaeogeometry of the depositional basin and therefore their combined palaeocurrent patterns can be assumed to indicate the general dimension of the ancient basin of deposition (Fig. 5). Cross-bedding readings indicate a pronounced northeasterly directed current set. The majority of ripple crest-lines are orientated in a northeasterly to southwesterly direction with a secondary set striking northwest-southeast. The main northeast direction, shown by both cross-bedding and ripple crest-lines, parallels the present belt of exposure and is believed to indicate the trend of the ancient shoreline. A similar former shoreline trend was reported by Litherland (1982) working in the Mamuno area (Q.D.S 2220A and 22208) further southwest near the Namibian border. This interpretation is even more probable since the majority of the palaeocurrent measurements was obtained from the Jakkalspuk Formation (mostly from the Mabeleapodi hiIls area), which has since been interpreted as a shoreline facies. The majority of the palaeocurrent structure reflects the operation of shore-parallel currents and waves propagated normal to the shoreline.

TECTONIC SETTING

The basal Kgwebe Formation records the earliest tectonic history during the evolution of the Ghanzi-

262 B. N. J. MODIE

FOOTWALL-SOURCE ALLUVIAL FANS

Figure 6. Models showing typical sedimentary depositional systems associated with extensional tectonics. (a) Model depicting rift basin development from a half-graben with an interior lake basin. (b) Model depicting a half-graben with an axial-through fluvial system (for a detailed explanation see Leeder and Gawthorpe, 1487).

Chobe belt as a depositional basin. The geological record shows that, in general, the association of sedimentary facies and volcanic rock assemblages, as exposed in the Ghanzi-Chobe belt, is a characteristic feature of extensional tectonic zones of the earth (Reading, 1986). The available data from the Ghanzi-Chobe belt does not allow the full understanding of the exact cause of the tensional stresses involved, i.e. whether the depositional basin evolved from an erogenic or anorogenic event. An anorogenic setting involving a thermally induced rift system seems favourable for the geotectonic evolution of the Ghanzi-Chobe belt. This is because of the lack of any line of evidence indicating an active continental margin, which is usually accompanied by a plethora of subduction-related attributes. Such attributes include a possible suture zone representing a former ocean, high-level intrusions accompanying subduction, fore-arc and back-arc settings and possible strike-slip basins resulting from oblique subduction (Hoal, pew. comm.). In southern Namibia, a similar (though occurring in three cycles) volcano-sedimentary assemblage of the Sinclair Sequence has been attributed to a subduction-related

geotectonic setting, a model well supported by both field and geochemical data (Hoal, 1993). In the Ghanzi- Chobe belt, the initial lacustrine and fluvial sedimentary depositional environments of the Kgwebe Formation indicate that the early stages of basin evolution involved the deposition of continental sedimentary sequences. In addition, the associated volcanic rocks, i.e. flow banded and folded rhyolite-dacite, tuffs, minor ignimbrites and vesicular basalt, are of typical sub-aerial continental character. Preliminary geochemical interpretations by Huch et al. (1992) have shown a continental affinity for the volcanic assemblages and therefore support a continental tectonic setting origin for the Ghanzi-Chobe belt.

Leeder and Gawthorpe (1987) have shown that extensional tectonic zones are generally characterized by half-graben or tilt-block systems whose sedimentary facies are controlled by uplift and subsidence, related to normal-fault displacements (Fig. 6). However, in the Ghanzi-Chobe belt the specific geomorphology of the depositional systems, at the time of deposition, cannot be deduced due to the present poor quality of exposure. The apparent lack of proximal coarse-grained facies,

Depositional environments of the Meso- to Neoproterozoic Ghanzi-Chobe belt 263

Figure 6. continued.

e.g. conglomerate and other debris flow deposits typical of fault margins, presumably indicates either distal facies or subdued rift faulting producing relatively gentle slopes.

Progressive basin development and sedimentation were tectonically controlled, as indicated by a clear increase in sediment supply and expansion of the depositional zones, upward through the stratigraphy. The first stage of basin development (Fig. 7a) was characterized by extensional tectonics which resulted in intracontinental rifting, followed by bimodal volcanism (ca 1106 Ma ago) and lake sedimentation of the lower member of the Kgwebe Formation sedimentary facies (Modie, 1994). The smaller volume of the syn-rift lower member facies probably indicates, as suggested earlier, a less active or moderate rifting episode resulting in subdued sedimentation rates. In a similar sequence in Namibia, Watters (1977) suggested an initial rift episode marked by emplacement of a basic magma into a relatively cool crust resulting in minor extrusives, presumably causing the subdued rift generation. Borg (1988b), however, suggested limited sedimentation due to the rapid extrusion of

large quantities of acid volcanics which filled up the developing basins. Following the volcanism, downsagging of the rift basin along the main bounding faults produced relatively high marginal areas leading to the development of fluvial depositional systems with sedimentation of the middle and upper members of the Kgwebe Formation sedimentary facies. The first phase of basin evolution, represented by the Kgwebe Formation sedimentary facies, can generally be explained in terms of two tectono-sedimentary models proposed by Leeder and Gawthorpe (1987); these models (Fig. 6) show an evolutionary trend from a continental basin with interior drainage (lake basin) to a continental basin with axial-through drainage (fluvial system).

During the second phase of basin development there appears to have been a renewed rifting stage, presumably accompanied by a change in the position of the major bounding faults which resulted in a more extensive basin, ultimately giving way to a marine incursion. This development is represented by the alluvial facies of the Lower D’kar Formation and the shallow marine facies of the Upper D’kar Formation

264 B. N. J. MODIE

0 A FIR3T PHASE OF BASIN DEVELOPMENT

II) deposition of the IQwebe Formation.

sxssssxxxssx

E

fine- to coarse-grained elastic sedimentary rocks.

tine-grained volcaniclastir sedimentary rocks. Fault acid and basic volcanics.

C Sediment

alkaltne igneous and supply direction metamorpMc basement.

Stage 1. Extensional tectonics, rifting, b&nodal volcanism and volcaniclastlc deposition.

Stage IL Post-voluutism thermal subsidence and downsagging of basin along bounding faults __) raised basin margins and fluvial deposition.

Figure 7. Schematic summa ry diagrams indicating rift basin development during deposition of (a) the Kgwebe Formation and (b) the Ghanzi Group.

(Fig. 7b). The absence of volcanic assemblages in the Ghanzi Group probably implies a distal setting for the second phase of rift basin development. The increase in sediment supply indicated by the Jakkalsputs Formation, in relation to the underlying sediments, implies a major period of uplift in the source area. This could be related to the initial stage of Damaran deformation.

From the evidence considered above, rifting in the Ghanzi-Chobe belt was aborted before the on-set of sea- floor spreading and hence resulted in a failed intracratonic rift system. Ultimately, the failed rift basin was affected by the Pan-African orogeny, which resulted in a fold-thrust belt. This rift can be likened to

younger, narrow, fault-bounded rift valleys recognized world-wide, such as the East African rift system and the Benue trough of western Africa.

REGIONAL CORRELATION OF THE GHANZI-CHOBE BELT

As noted earlier, regional interpretations based on geophysical trends, radiometric ages, lithological associations and sediment-hosted copper deposits studies have allowed the sequence in the Ghanzi-Chobe belt to be correlated with similar sequences in Namibia (Figs 1 and 2) and possibly some areas of Zambia. The Gharui-Chobe volcano-sedimentary basin is commonly

Depositional environments of the Meso- to Neoproterozoic Ghan%Chobe belt 265

ur BASIN UCVCLUYMCN I

deposition of the Ghanzi Group.

NW SE

SE

I::1 Jakkalqmts Formation 7 I

I GHANZI Upper D’kar Formation

GROUP

Lower D’kar FormatIon J Kgwcbe Formation

[El igaeoas and metamorphic basement

Figure 7. continued.

correlated with the Klein Aub, Witvlei and Dordabis basins in south central Namibia (Watters, 1977; Borg, 1988a, 1988b; Borg and Maiden, 1987, 1989). These Mesoproterozoic basins are aligned along the northwestern margin of the Kalahari craton and were developed by continental extensional tectonics, which resulted in rift basins filled by thick sequences of volcanic and sedimentary rocks.

Lithological correlations

Lithological similarities of the Ghanzi-Chobe belt and its correlatives in Namibia have been discussed by several workers that include Toens (1975), Watters (1977), Ruxton (1981), Borg and Maiden (1987, 1989),

Stage I.

stage II.

Post Kgwebe Formation rIftlag (age m&own), basin expansion, alluvW depodtrolr (Lower D’kar Formation), tkemml subsldencc folIowed by m&ne incarsion (Upper Dlrar Formation).

upuft of source arep, prograduona! shorelIne deposItton (Jakkalqmts Fonnatlon), and closare of basin (probably related to carIy Damaran defornrptlon).

and Borg (1988a, 1988b). It is apparent from the lithological associations, e.g. mafic to felsic lava with volcaniclastic and sedimentary units (Fig. 2), that the basin in question wenf through a similar evolutionary trend which involved crustal extensional tectonics resulting in fault-bounded rift-grabens.

Geophysical correlations

Geophysical magnetic anomalies identified over the known outcrop area in the Ghanzi-Chobe belt indicate a linear belt approximately 100 km wide, which can be traced from the Namibian border in the southwest, to the border with Zambia in the northeast (Reeves, 1985). The sequences in Namibia are associated with elongate

266 B. N. J, MODIE

gravity highs, probably caused by basalt, which display a distinct linear feature defining the general morphology of the basins (Borg, 1988a).

Age correlations

The U-Pb single zircon age of 1106f2 Ma obtained from the Kgwebe Formation porphyry (Schwartz ef al., in press) is within error of an U-Pb zircon age of 1094f18/20 Ma from quartz-porphyritic rhyolites of the Oorlogsende Porphyry Member (Hegenberger and Burger, 1985) in eastern Namibia. The Oorlogsende Porphyry Member lies within a narrow belt of isolated outcrops which connects the sequences of the Ghanzi- Chobe belt to their correlatives in Namibia (Fig. 1). Hegenberger and Burger correlated the Oorlogsende Porphyry Member with a similar volcanic assemblage of the Nuckopf Formation in the Rehoboth area, near Klein Aub. The latter yields U-Pb radiometric age ranges between 1010 and 1090 Ma (SACS, 1980, p. 397). The latest age from the Kgwebe Formation (11061t2 Ma) nullifies the suggested younging of the rift-system from the Klein Aub towards the Ghanzi-Chobe belt, discussed in Borg (1988a). Borg’s (1988a) suggestion of lateral younging to the northeast has been challenged by Hoal (1993), citing as his arguments discrepancies in age constraints and the limited area within which such an age range is evident, among others.

Mineralization correlation

The volcano-sedimentary sequences of the Ghanzi- Chobe belt and their correlatives in Namibia show similar occurrences of stratabound copper sulphide mineralization (Ruxton, 1980,1981; Maiden ef al., 1984; Borg, 1988b; Borg and Maiden, 1987, 1989). The mineralization is hosted by a dark, reduced, argillitic facies underlain by red, oxidized, coarse-grained elastics defining a redox interface. The source of the metals is believed to be the basal basalt units which occurs in all the basins (Ruxton, 1980,198l; Borg and Maiden, 1987, 1989; Schwartz and Akanyang, 1994).

The Zambia link

There have been some suggestions, mainly based on regional gravity and aeromagnetic surveys (Reeves, 1985; Hanson et aZ., 1988), that the northeast-southwest structural trends in the Ghanzi-Chobe belt and its correlatives in Namibia and the Choma-Kalomo block (as well as the Irumide belt) in southern Zambia are continuous. This correlation has also been suggested by several workers (e.g. de Villiers and Simpson, 1974; Martin and Porada, 1977; Key and Rundle, 1981), albeit with little confidence due to the poor quality of the exposure and data. The latest age from the Ghanzi- Chobe belt (e.g. 1106 Ma) re-establishes the earlier notion of correlating (at least the earliest volcano-

sedimentary rocks) the Kgwebe Formation with the Chomo-Kalomo. This correlation was rejected by Hanson et al. (1988) based on the previous younger ages obtained by Key and Rundle (1981), e.g. 821 and 981 Ma for the Kgwebe and Goha porphyry, respectively. The latter forms the northeastern extension of the Ghanzi-Chobe belt, which is approximately 100 km southwest of the Chomo-Kalomo block. The age of 1106 Ma also just falls within the documented Mesoproterozoic (1350-1100 Ma) (Hanson et al., 1988) tectono-thermal history for the Choma-Kalomo block.

CONCLUSIONS

The Ghanzi-Chobe belt has undergone a two-phase mode of sedimentary basin development represented by the Kgwebe Formation and the Ghanzi Group (Fig. 7). The Kgwebe Formation represents the first phase, which was initiated by extensional tectonics and rifting with subsequent bimodal volcanism and elastic sedimentation in small lake environments. The Ghanzi Group indicates a major basin development stage during which there was renewed extension resulting in an expansion of the depositional zone, culminating in a marine incursion. The marine basin involved a shallow shelf environment dominated by storm and fair-weather conditions. Intermediate sub- environments consisted of mixed elastic-carbonate lagoons and possible progradational deltas.

Copper sulphide mineralization was deposited within a dark, chemically reduced argillitic facies forming the base of a marine transgressive sequence and underlain by a red, oxidized alluvial facies. The Cu is believed to have been derived through the leaching of underlying basalt. The final development of the sedimentary environment was marked by increased sediment supply and the deposition of a progradational shoreline facies, implying a major period of uplift in the source area, presumably related to the initial stage of Damaran deformation. The evidence considered in this paper indicates that the Ghanzi- Chobe belt represents a failed intracontinental rift basin that developed as part of an extensive, but segmented, linear rift system extending from south central Namibia.

Acknowledgements

This paper has been extracted from an unpublished M. Phil. thesis held at the University of Wales, Aberystwyth, and supervised by Dr. Maxwell Dobson. The thesis was carried out with financial support from the O.D.A. through the British Council. The data was wholly supplied by the Geological Survey Department of Botswana through its numerous mapping projects. Of particular note is a bilateral technical co-operation mapping project with the ‘Bundesanstalt fur Geowissenschaften und Rohstoffe’ (BGR) of Hannover, Germany, from which a handful of the data was

Depositional environments of the Meso- to Neoproterozoic Ghanzi-Chobe belt 267

collected. The author would like to thank very much Professor Borg (Martin Luther University, Halle, W&e&erg) and Dr. Hoal (Geological Survey, Namibia) for critically reading the manuscript and providing creative suggestions. He also would like to thank Dr. Roger Key and Dr. Read Mapeo for reading the manuscript and giving valuable comments and advice.

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