A glacigenic interpretation of a neoarchaean (≈2.78 Ga) volcanogenic sedimentary sequence in the Nnywane formation, Sikwane, Southeast Botswana

Journal of African Earth Sciences 35 (2002) 163–175 www.elsevier.com/locate/jafrearsci

A glacigenic interpretation of a neoarchaean ( 2:78 Ga) volcanogenic sedimentary sequence in the Nnywane formation, Sikwane, Southeast Botswana Benson N. Modie

*

Geological Survey Department, Private Bag 14, Lobatse, Botswana Accepted 26 July 2001

Abstract Neoarchaean volcano-sedimentary rocks of the Nnywane formation exposed at Sikwane, Botswana, exhibit palaeo-depositional features that allow the sequence to be interpreted with a glacigenic and volcanic-influenced depositional model. The sedimentary deposits are divided into two facies, a lower Grey facies of tuffitic sandstones, associated with diamictite and conglomerate units and shales, and an upper Red sandstone facies. A volcanic member is inter-layered with shales in the Grey facies, representing penecontemporaneous volcanism. Basal diamictite in the Grey facies contains striated and faceted clasts that reflect an origin as basal debris and which are typical of subglacial facies. Evidence supporting glaciation includes diamictite masses and isolated stones contained within shales, which are interpreted to indicate ice-rafting processes. The Grey facies is associated with a locally preserved smooth-textured tuffitic mudstone that is interpreted to represent glacial rockflour. Ice was probably formed in elevated areas during extensional tectonics associated with the major magmatic event of c.2.78 Ga in Southeastern Botswana, which is linked to the Limpopo Orogeny. Episodic sedimentation is evident in the Grey facies perhaps indicating the influence of glacial melt-waters and intermittent sedimentation associated with volcanic activities. The depositional environment consisted of a volcanic-determined and glacially influenced lake system. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Neoarchaean; Nnywane formation; Southeast Botswana

1. Introduction There are few known examples of ancient Precambrian glacial sequences described in the geological literature, perhaps owing to the unavailability and destruction of older rock exposures by erosion and tectonic deformation through time (Crowell, 1983; Eyles, 1993). Post-depositional tectonic deformation and associated metamorphism have often obscured diagnostic sedimentary depositional features in ancient Precambrian sequences (Eriksson et al., 1998). Recent studies on Pleistocene glaciated basins on the North American continent seem to indicate that the bulk of the Earth’s glacial record consists of glacigenic sediment that has been deposited and reworked in marine settings (Eyles, 1993). Other reasons given for the subdued approach to *

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ancient glacial sedimentology include the fact that most glacial sequences are non-economic, particularly in the field of petroleum prospects (Eyles and Miall, 1984). There are five major glacial events known in Earth history, namely early Proterozoic, late Proterozoic, early Palaeozoic, late Palaeozoic and late Cenozoic (Crowell, 1983; Edwards, 1986; Tucker, 1991; Eyles, 1993). The most widely documented glacial event in the Southern Hemisphere is the late Palaeozoic episode, which resulted in the accumulation of widespread glacial deposits and the development of large coal reserves during Permo-Carboniferous times before the break-up of Gondwana. In the Southern African region the late Palaeozoic episode is represented by the widely documented glacial deposits of the Dwyka group belonging to the lower Karoo Supergroup (e.g. Visser and Loock, 1982; Crowell, 1983; Von Brunn and Gravenor, 1983; Key et al., 1995; Visser, 1995). The oldest deposits of suspected glacial origin in the Southern African region were first described from

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diamictites in the Archaean Witwatersrand Supergroup of South Africa (e.g. Tankard et al., 1982; Crowell, 1983). Subsequent work on the Southern African subcontinent in Archaean deposits of the Pongola Supergroup (Mozaan group) of South Africa and Swaziland, involved a glacial interpretation for part of the sequence (Von Brunn and Gold, 1993). Recently, Young et al. (1998) described further evidence supporting glaciation in the Archaean Mozaan group which they ultimately termed the Earth’s oldest reported glaciation. As with other palaeoenvironmental interpretations the diagnosis of a glacial sedimentary setting is based on critical evaluation of all available lines of evidence. However, not all the necessary data for a complete evaluation are usually available, particularly with regard to ancient Precambrian rocks. Research into the identification of glacigenic deposits was set back by the introduction of the turbidity current theory in the 1950s (Edwards, 1986). The latter provided an alternative ex-

planation for the origin of diamictons and diamictites, which had hitherto been attributed exclusively to glacial origin, and hence regarded as tillites. This meant that a glacial origin for a rock could no longer be strongly based on its texture, but instead requires an in-depth facies analysis based on sedimentary features of significant depositional processes. It is also worth mentioning that the products of glacial-related processes include the deposits of many distinct environments such as rivers, lakes, continental shelves and margins (Eyles and Miall, 1984; Eyles, 1993). This makes the task of distinguishing true glacial deposits a difficult one. The deposits of recent glacial environments are generally comprised of massive subglacial tills associated with lenticular and stratified conglomerates and sandstones, and often include rhythmically laminated mudrocks with occasional scattered clasts deposited from rafted ice (Tucker, 1991). A perhaps more useful approach in environmental interpretations to distinguish true glacial tills and tillites

Fig. 1. Geological map of the Mochudi area, Southeast Botswana, showing the location of the study area at Sikwane.

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from diamictons and diamictites of the many different depositional environments is to employ the method of coded lithofacies in vertical profile descriptions as introduced by Eyles et al. (1983). In this method the diamicton or diamictite, coded D, is used as the basic terminology with no genetic connotations, and is followed by a description of the internal texture represented by either c or m for clast-supported or matrix-supported textures, respectively. The basic code is extended to include structural symbols, i.e. m (massive), s (stratified), and g (graded); these are in turn succeeded by interpretive genetic symbols shown in parentheses, e.g. (r) resedimented, (c) current reworked, and (s) for sheared. Other facies associated with the diamicton or diamictite, which commonly consist of sand(stones) and mud (stones) are coded S and F, respectively, with the basic codes succeeded by sedimentary structural symbols such as r for rippled and h for horizontal lamination, etc. The systematic profiling of suspected glacial deposits through such lithofacies descriptions greatly enhances the rec-

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ognition of typical glacial diagnostic features that include substrate and basal debris deformation and erosion, and ice-rafted clasts. This paper discusses the interpretation of the depositional environment of a Neoarchaean volcanogenic sedimentary sequence that belongs to the Nnywane formation of the Lobatse group exposed in Southeast Botswana. Rocks assigned to the Nnywane formation are preserved in a WNW-ESE striking ridge exposed at Sikwane village in the Mochudi map area (Fig. 1), and also extend across the border into South Africa, in the Derdepoort area. Recent geological field investigations revealed the occurrence of depositional features that resemble glacial attributes in the volcanogenic sedimentary rocks of the Nnywane formation. In this regard, this paper presents an interpretation of the depositional processes that prevailed during accumulation of the Nnywane formation, by considering the data in a sedimentological and glacigenic perspective.

Fig. 2. Simplified geological map of the Sikwane area showing the distribution of the Nnywane formation and the location of measured sections.

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2. Geologic setting The geology of Southeast Botswana consists of an Archaean terrain that forms part of the greater cratonic province of Southern Africa known as the Kaapvaal Craton. The history of Archaean crustal evolution in this region is represented by a varied assemblage of rock units (Figs. 1 and 2) that commences with a basement complex of massive and foliated granitoids and gneisses (Jones, 1966, 1973a,b; Key, 1982, 1983; Nkala et al., 1994; Modie, 1999). The basement complex has not been dated but a similar association in north-central Kaapvaal Craton in South Africa is reported to have stabilised at approximately 3170–3060 Ma (Walraven et al., 1990). Crustal evolution progressed with the emplacement of a major gabbroic pluton known as the Modipe Gabbro. Preliminary dating of the Modipe Gabbro through Rb– Sr (whole rock) and K–Ar (pyroxene, plagioclase) gave imprecise ages ranging between approximately 2600 and 1900 Ma (McElhinny, 1966). However, the Modipe Gabbro has a minimum age of approximately 2.78 Ga that was recorded from the Gaborone granite (e.g. Walraven et al., 1994, 1996), which intrudes the gabbro. The volcanic and sedimentary units of the Lobatse group (Fig. 2) represent the occurrence of a volcanic event during the Archaean crustal evolution. These units consist of massive rhyolite porphyries of the Kanye formation, and tuffs, porphyries and volcaniclastics of the Nnywane formation (Modie, 1999). A major granitic pluton is represented by the Gaborone granite that consists of an inner porphyritic, rapakivi granite, a medial medium-grained equigranular granite, and a marginal phase comprised of microgranite and granophyre (Key and Wright, 1982; Key, 1983; Sibiya, 1988). The result of recent single zircon age determinations (Fig. 3) from both the Lobatse group and the Gaborone granite yielded a similar average age of 2.78 Ga (Grobler and Walraven, 1993; Walraven et al., 1994, 1996; Wingate, 1998). The new age determinations indicate that the

Lobatse group and the Gaborone granite belong to a single major magmatic event that took place at this time in Southeast Botswana. Field evidence does, however, indicate the relative order of emplacement of certain units (Fig. 3). Moore et al. (1993) and Chaoka et al. (1998) recently linked this major magmatic event to a possible continental collision during the Limpopo Orogeny, resulting in extensional tectonics in Southeast Botswana.

3. Facies descriptions The sedimentary sequence of the Nnywane formation is divided into two facies associations, namely a lower Grey tuffitic sandstone with shales, and basal diamictite and conglomerate, overlain by a Red sandstone facies (Fig. 4). The sedimentary facies form a thin wedge with a maximum thickness of about 150 m and an exposed strike length of approximately 4000 m. The two sedimentary facies are sandwiched between two major volcanic units that form the rest of the Nnywane formation (Fig. 4).

4. Grey facies This facies is dominantly Grey-coloured, consisting of basal diamictite, conglomerate, granule-rich to pebbly lithic-sandstone, and shales. Lithological associations within the Grey facies indicate the existence of three broad fining-upward cycles. These are represented by vertical facies variations from a basal rudaceous unit succeeded by arenaceous and argillaceous facies. A typical facies association commences with a diamictite (Fig. 5) consisting of angular clasts and boulders (Dmm), and grades into a conglomerate with relatively rounded clasts (Dms(c)) interbedded with sandstone (S) and shale lenses (F). Throughout the sequence sandstone beds commonly alternate with shales, but progressively

Fig. 3. Neoarchaean stratigraphy and radiometric age determinations from Southeast Botswana.

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Fig. 4. Measured sections of the Nnywane formation sedimentary sequence (see Fig. 2 for location). Grain-size scale depicts: clay–silt grade (c–s), sand grade (s) and gravel (g).

thin and diminish up-sequence and are eventually replaced by the shales. A volcanic micro-breccia is found intercalated with the shales in one place (Fig. 4, Section 1). The Grey facies unconformably lies above a substratum of flow-banded porphyritic lavas. 4.1. Basal diamictite The basal diamictite unit has a poorly sorted matrixsupported texture (Fig. 5) comprised of variable-size

(diameter) clasts ranging from large boulders (about 50 cm) near the base to pebble between 1 and 7 cm near the top. The dominant clasts in the basal diamictite consist of volcanic porphyry and tuff, with minor occurrences of chert and shales. Matrix in the basal diamictite locally varies between mudstone-rich and sandstone-rich, with the latter containing fragments of feldspar phenocrysts. The typically non-homogenous diamictite locally shows fewer clasts totally suspended in a tuffitic-mudstone matrix. Elsewhere along strike the basal diamictite is

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Fig. 5. Basal diamictite showing large blocks of lava and tuff in a sheared matrix of tuffitic mudstone.

replaced by a very fine-grained, soft and smooth-textured, and strongly fractured tuffitic mudstone. The latter shows a characteristic conchoidal fracturing. The basal diamictite (Dmm) occasionally shows discrete angular volcanic porphyry clasts that display striated and faceted/smoothened planes (Fig. 6), and also locally exhibit a sheared matrix (Fig. 5). In addition to striations developed on intraclasts, some are developed on the substratum (Fig. 7). Striations are oriented in various directions and also show variable dips (Figs. 6 and 7). The basal diamictite locally shows an abruptly gradational top contact due to sudden replacement with finely laminated shale interbedded with shaly sandstone. In other places the basal diamictite is succeeded by conglomerate, however the contact is not exposed. 4.2. Conglomerate-sandstone-shale An association of conglomerate, sandstone, and shale facies stratigraphically succeeds the basal diamictite

Fig. 6. Part of basal diamictite showing development of striations and smoothened surfaces on clasts in various orientations (e.g. represented by four pens shown in the picture). Camera lens cap diameter is 5.5 cm.

Fig. 7. Volcanic substrate beneath diamictite in Fig. 6 showing slickenside striations. Striations indicate various orientations (e.g. steep to horizontally aligned pens), suggesting origin by basal debris erosion.

facies, and depicts a cyclic fining-upward facies relationship. 4.2.1. Conglomerate About three conglomerate units are recognised (Fig. 4, Section 1). The matrix-supported conglomerate facies consists of relatively rounded clasts (e.g. Dm-(c)) that are dominated by volcanic porphyry and minor sandstone. Clast-sizes range from pebbles to boulders, however, there is no particular trend in size distribution. There is a general upward increase in matrix in the succession from an almost clast-supported texture with fine-grained matrix to a matrix-supported texture dominated by medium-grained sandstone. Within the conglomerate facies are interbedded, thin (35 cm thick), very coarse-grained to pebbly sandstone beds, containing thinly laminated shale beds (e.g. Dms(c)). The interbedded sandstone beds appear to be relatively more persistent along strike and show variable thicknesses, whereas the shales are only intermittently developed as thin lenses about 10 cm thick. Conglomerate units also display variable thicknesses. 4.2.2. Sandstone Progressively fining-upward coarse- to medium- and fine-grained, locally pebbly sandstone facies succeed the conglomerate facies, and alternate with shale facies. Within the sandstone facies individual beds also show internal normal grading (Sg), and faint parallel lamination (Sh), and in places also contain thin shale layers. Normal graded bedding is defined by granule-rich and pebbly layers developed at the bases of beds. Overall individual bed thicknesses thin upwards relatively from 130 cm near the base to about 5 cm thick at the top. Contacts between sandstones and shales are sharp, with the sandstone bases often having load-casts. The sandstones are poorly sorted and are comprised of litharenites and lithic wackes composed of volcanic ash

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facies is composed of fine tuffitic material. At one locality the shale facies contains two large lenses of bedded tuff measuring 13 m  3.5 m and 16 m  3 m (Fig. 9). These bodies also locally contain discrete sandstone boulders. Contacts between the shale facies and the oversized clasts are sharp and discordant, and there are also some loading effects shown on the shales. There is minor development of mudstone-filled fractures within the oversized clasts.

5. Red facies Fig. 8. A finely laminated mudstone unit with isolated pebbles; this unit sharply succeeds the diamictite facies shown in Fig. 5. Pencil is 13 cm long.

fragments. The matrix is predominantly clay minerals, fine micas including chlorite, and fine opaques, with accessory quartz and muscovite. 4.2.3. Shale Shale facies alternate with and progressively replace sandstone facies up sequence. Locally, shale units indicate thickening up-sequence from about 2 cm near the base to about 140 cm thick at the top. Individual beds often show either thinning or thickening along strike. Shales display fine lamination (Fl) that defines finingupward sedimentation pulses. The fine lamination, which commonly ranges from a millimeter to a centimeter scale, can also be less than a millimeter in thickness. Locally, the shale member shows a very finely laminated unit with some isolated pebbles of volcanic material (Fig. 8). The isolated pebbles show large diameters of about 2–3 cm in comparison to the enclosing laminae, which are of an order of millimeters. The shale

Fig. 9. Large bedded tuff lens contained in finely laminated shale (shales are shown as dark exposures immediately beneath the geological hammer and to the extreme lower left of the photo).

This facies consists of a pebbly, moderately sorted, medium- to coarse-grained Red sandstone, with some conglomeratic layers, and is only shown in Section 3 of Fig. 4, described from the Southeastern side of the exposures at Mabalane and Sikwane (Fig. 2). Petrographic investigations indicate a sublitharenite dominated by quartz with minor fragments of chert, felsite, volcanic ash, mudstone, and some internally strained polycrystalline quartz. The Red facies overlies the Grey facies with a discontinuous basal conglomeratic layer. Clasts in the basal conglomerate layer (Gm) consist of veinquartz, banded chert and feldspar porphyry. Two conglomeratic layers are locally developed at different stratigraphic levels. Sedimentary structures consist of poorly discernible planar parallel lamination and crossbedding.

6. Facies interpretations 6.1. Grey facies 6.1.1. Basal diamictite The internal texture of a poorly sorted facies as shown by the basal diamictite (Fig. 5) can be interpreted to represent either a gravity flow deposit such as a debris flow or may indicate a glacial tillite deposit. By consideration of the inclusion of several striated and faceted clasts, which indicate origin as basal debris (Edwards, 1986), as well as the occurrence of striations on the substratum it seems likely that the diamictite unit represents a basal tillite. The basal diamictite also displays local shearing of the matrix, which might indicate subglacial deformation. Several studies have shown that striations are commonly developed in glacial environments, where they represent subglacial abrasion or erosion at the lower contact of a till (Edwards, 1986; Selley, 1988). Davison and Hambrey (1996) describe the occurrence of scouring, fracturing and brecciation in gneisses beneath a massive diamictite of the Proterozoic Stoer group in Northwest Scotland, and they attribute these to glaciotectonism. Similarly, Ronnert (1992) discusses

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subglacial deformation attributes developed below diamicton in Pleistocene glacial deposits of the Oak Creek formation of Southeast Wisconsin in the USA. The abundance of locally derived material in the basal diamictite at Sikwane indicates an intrabasinal source with proximal erosion and deposition possibly at the lower contact of a basal tillite. An abruptly gradational top contact locally displayed by the basal diamictite with overlying finely laminated shale and interbedded sandstone reflects contact with a subaqueous depositional basin. Such facies relationships can occur in subaqueous proglacial outwash environments, where the ice terminus is submerged (Edwards, 1986), and where sediment gravity flows transport sediment away from the ice margin with subsequent suspension deposition of finer facies above the coarse facies. The soft-textured and structureless tuffitic mudstone locally found in place of the basal diamictite is interpreted to represent consolidated rockflour. The latter is normally produced by the grinding action during glacial transport and is transported by glaciofluvial streams to the basin floor to form massive and featureless deposits (Crowell, 1983). Crowell (1983) also suggested that weathered consolidated rockflour may be recognised by the development of spheroidal structures. Such structures may be represented by the conchoidal fracturing pattern observed in the tuffitic mudstone.

6.1.2. Conglomerate The three conglomerate units (Fig. 4(m), Section 1) recognised reflect sedimentation cyclicity. Several geological factors may result in such cyclicity of sedimentation, e.g. tectonic uplift with rapid erosion and subsequent denudation of the source area or intermittent increase in both erosional and depositional energies related to climatic changes (Reading, 1986; Selley, 1988; Allen and Allen, 1990). A direct interpretation based on the internal facies evidence indicates high-energy current reworking processes progressively replaced by lowenergy processes up the sequence. This is shown by the transition from relatively rounded internal clasts with little matrix near the base to a dominant matrix near the top. The conglomerate units are thought to represent current reworked diamictite facies (Fig. 4). Interbedded thin sandstones containing finely laminated shale beds represent intervals of low water-stage. Overall, the conglomerate facies represent subaqueous deposition in an environment episodically fed with locally derived sediments, deposited by progressively waning current energies. The conglomerate unit that is shown to immediately overlie the basal diamictite (Fig. 4, Section 1) reflects origin through reworking and resedimentation of the basal diamictite by gravity flow mechanisms probably associated with glacial meltwaters. Consequently, the conglomerate facies is re-

garded to represent products of sub-aqueous outwash facies. 6.1.3. Sandstone The sandstones, which succeed the conglomerate units, are interpreted to form the intermediate facies in the sub-aqueous environment in which the conglomerate units were deposited. Fining-upward characteristics shown by the individual sandstone beds, such as the common graded bedding, together with the association of conglomerate with interbedded thin sandstone and shale, may have resulted from the replacement of largescale gravity flows by rapidly decelerating and waning turbulent processes. Such events may include shortlived catastrophic currents of turbidity flow types, including density underflows and overflows (Edwards, 1986; Reading, 1986). In addition to the preponderance of graded bedding the lack of traction current structures, such as cross-bedding and ripple forms, underlines the dominance of rapid and suspension deposition in these rocks. 6.1.4. Shale Shales also form an association with the conglomerate and sandstone facies, and are characterised by finingupward planar parallel lamination. The fining-upward lamination pulses represent material deposited from a suspended sediment load below wave base. The characteristic fining-upward lamination pulses are typical of turbidity and density current processes (Leeder, 1982; Johnson and Baldwin, 1986; Collinson and Thompson, 1989). Such processes may have been generated from fluvial in-flows or sub-aqueous gravity flows associated with slope failure. The latter seems more plausible if correlated with volcanism, which is indicated to have been active during deposition of the Grey facies, as shown by the occurrence of volcanic breccia interbedded with the shales (Fig. 4, Section 1). Volcanism is known to be associated with tremors that can cause slope instability and thus generate gravity flows. The large boulders of sandstone and lenses of tuff found in the shale facies (Fig. 9) are interpreted to represent dropped diamictite masses, which are diagnostic of ice-rafting processes. The diamictite masses were most probably carried basinward by floating ice as melt-out till. The ice raft ultimately melted and dumped the till during predominant deposition of suspended fine mud that formed the shales. Loading features shown in the shales indicate the weight differential effect of the possible diamictite pebble during deposition on fine mud. In addition, the mudstone-filled fractures represent possible injection structures developed during dumping of the heavy masses on fine mud at the time of deposition. Generally, both the sandstone facies and the shale facies exhibit isolated pebbles, which could be in-

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terpreted as dropstones, reflecting the actions of icerafting processes (Crowell, 1983; Edwards, 1986). The shale facies also locally show conglomeratic or pebbly layers that may indicate glacial origin. Some of the rhythmic laminations commonly displayed by the shale facies may represent varves. This, in addition to the cyclic characteristic shown by the whole of the Grey facies, may signify strong seasonal variations in meltwater run-off. 7. Red facies The dominance of quartz in this Red facies with few unstable components indicates periods of sediment reworking prior to deposition, in typically continental settings of low relief and slow sedimentation rates (Tucker, 1991). Traces of internally strained poly-crystalline quartz characteristic of metamorphic or granitoid basement rocks may reflect either the geology of the source area or the area traversed by sediments in transit. The inclusion of unstable grains such as volcanic ash and mudstone fragments reflects input from a proximal local provenance. This facies is regarded to have as its provenance area the basement rocks that form an extensive sub-outcrop of the area around Sikwane and to a lesser extent the Nnywane formation volcanics (Fig. 2). The sedimentary structures and lithological variation in this facies are inadequate to enable a complete interpretation of the ancient depositional processes. However, the occurrence of conglomeratic layers can be explained in terms of channelised flow systems with the development of high-energy lag deposits at high water stages (Reading, 1986). In this regard the poorly developed parallel lamination and cross-bedding, observed in this facies, may reflect the existence of upper or lower flow regime bed-forms produced by channelised streamflow deposits (Leeder, 1982; Collinson and Thompson, 1989). Fillmore (1993) attributed the rarity of crossstratification in beds dominated by crude horizontal bedding to processes of unconfined sheetflood-dominated flows in contrast to confined flows, which would favour development of bedforms. It is not clear however whether the paucity of bedforms in this case is a reflection of the depositional processes or just a result of the paucity of exposure. 8. Discussion 8.1. Depositional model The reconstruction of ancient depositional systems is usually determined by the recognition of sedimentary structures and rock types, and their description and interpretation (Tucker, 1988). However, although no two similar environments are ever exactly the same, the

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sedimentary structures usually show major overlaps across distinct environments. In this regard data of high quality are usually required to establish the occurrence of discrete environments. For ancient Precambrian depositional systems however the data are often insufficient, resulting in only general considerations and interpretations due to the poor preservation of rock sequences and their facies attributes. Evidence obtained from the sedimentary sequence and the associated volcanics of the Nnywane formation, when viewed in unison, gives insight into the palaeoenvironment of deposition. The preserved outcrop of the sedimentary facies of the Nnywane formation, that forms a thin wedge of approximately 150 m in thickness and 4000 m in strike length, may reflect the general extent of the palaeo-basin of deposition. A depositional basin of this magnitude when considered in terms of the internal facies association shown by the Grey facies (Fig. 4) could be interpreted to represent a lake system. Major lakes are usually caused by tectonic subsidence and faulting, but lakes may also form due to glacial erosion and damming by ice and lava (Selley, 1985; Allen and Collisnon, 1986; Fitzsimons, 1992; Fletcher and Siddle, 1998). The close relationship of the sedimentary facies, particularly the Grey facies, with the volcanic units forming the rest of the Nnywane formation indicates a depositional basin associated with volcanic activity. A possible basin-forming mechanism during the deposition of the Nnywane formation may have consisted of a caldera-type lake, created by crater explosion and collapse. The caldera lake provided a sediment trap with a localised high preservation potential. A glacial influence on the depositional environment of the Nnywane formation is indicated through the recognition and interpretation of such features as is commonly found in many glacial environments, namely striations and ice-rafted deposits (Crowell, 1983; Ronnert, 1992; Davison and Hambrey, 1996). In this lake system diamictons and conglomerates were locally deposited as sub-glacial basal tillites and subaqueous debris flows, respectively, generated along the lake’s steep margins, while the other sediments were received through melt-water conduits and subaqueous fans. Icerafting processes supplied additional material (Fig. 10). The depositional system was characterised by both high- and low-energy processes as reflected by the association of conglomerate with sandstones and shale facies. Episodic rapid sedimentation dominated, resulting from gravitational depositional processes triggered by volcanic activities, in addition to the seasonal supply of glacial melt-waters. Several studies in which similar episodic sedimentation is documented include Quaternary sedimentary sequences from ice-dammed localities such as the Linda Valley deposits of western Tasmania (e.g. Fitzsimons, 1992), and those of Llyn Teifi in west Wales described by Fletcher and Siddle (1998). At Llyn

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Fig. 10. A schematic diagram showing a general depositional environment envisaged for the Nnywane formation sedimentary rocks.

Teifi the cyclicity shown by rhythmically laminated facies is attributed to climatic variations of annual, shortterm and diurnal periods (Fletcher and Siddle, 1998). Much of the graded sandstone facies and the rhythmically laminated shales of the Nnywane formation were probably brought into the deeper parts of the lake system by processes of density underflows and overflows (Fig. 10). Such processes are regarded to play a significant role in sediment dispersion and sorting, particularly in lakes characterised by thermal stratification where sediment-laden inflows would be separated according to density differentials in the lake waters (Allen and Collisnon, 1986). Thermal stratification during deposition of the Grey facies is a likely scenario given the influence of both volcanism and glaciation. Continentally derived siliciclastic sediments that formed the Red sandstone facies were probably brought into the lake through glaciofluvial systems. The latter usually consist of braided streams that develop into prograding deltas at the lake margin and ultimately act as a sediment source to the lake floor via turbidity currents (Fletcher and Siddle, 1998). The depositional system at this stage was basically a hydrologically open lake receiving minimal clastic detritals from the metamorphic basement. 8.2. Evidence for glaciation As in other ancient Precambrian rocks around the earth, unequivocal evidence supporting continental glaciation in the Neoarchaean Nnywane formation is problematic. This phenomenon has been explained by the fact that large volumes of direct glacial deposits are

usually reworked and redistributed by non-glacial sedimentary processes and deposited in other environments such as the marine or lacustrine settings (Eyles, 1993). (Only an iota of evidence to decipher glacial origin is usually obtainable in such deposits, and may even be just allusive.) In the Nnywane formation a case for glacial influence is based on the occurrence of striated and faceted intraclasts in the basal diamictite, in addition to the recognition of large, oversized sedimentary clasts and isolated pebbles preserved in rhythmically finely laminated shale facies. The recognition of striated and faceted clasts, and dropstones in laminated shale has been considered throughout the geological literature as good criterian to infer glaciation (Crowell, 1983; Edwards, 1986; Eyles, 1993). These attributes also seem to form the most common and significant features that correlate ancient glacial deposits to modern glacial deposits. Other studies indicate that striations on clasts can also be produced in non-glacial settings by abrasion during flow or by metamorphically induced clast rotation (Eyles, 1993). However, according to Von Brunn and Gold (1993) glacial striations are usually found to be deeper and occur in several directions. Nevertheless, in ancient glacial-influenced deposits that may have undergone transportation and resedimentation, striations on clasts could be subdued, particularly in softer lithologies, posing difficulties in distinction (Crowell, 1983). The occurrence of oversized sedimentary clasts within rhythmically finely laminated shale facies in the Nnywane formation (Fig. 9) provides the best clue to the existence of an ancient glacial front. This is an indication of diamictite masses carried by ice-rafting processes to

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be deposited at a distance in quiescent environments outside of the influence of strong reworking currents. Additional evidence for ice-rafting processes is in the form of dropstones that are totally enclosed by fine laminae, which are much thinner than the diameter of the stone (Fig. 8). Another line of evidence that supports glacial influence stems from the recognition, within the Nnywane formation, of very fine-grained, soft-textured and conchoidally fractured tuffitic mudstone that resembles rockflour. The latter is reported to form huge volumes ahead of ice-fronts today, and is deposited on the sea floor, forming massive and featureless deposits (Crowell, 1983). A study by Eyles (1993) has, as its conclusions, the fact that ancient glacial deposits were most commonly deposited and preserved within extensional settings. The Nnywane formation was developed within an extensional tectonic setting during a major magmatic event in Southeast Botswana at c.2.78 Ga, which has been linked to continental collision during the Limpopo Orogeny (Chaoka et al., 1998; Modie, 1999). Extensional tectonic settings provide elevated areas through uplift, thereby creating favourable conditions for glaciation. Palaeomagnetic studies from the relatively older Modipe Gabbro (Fig. 1) have yielded palaeolatitudes of about 33° (Evans and McElhinny, 1966), which although not very high, when considered together with tectonic uplift can be considered compatible with a glacial interpretation. 8.3. Nature of glaciation The documentation of glaciation, which includes the reconstruction of its spread, character, and timing, depends largely on the completeness of the stratigraphic record (Crowell, 1983). For ancient glacial events, however, only a fraction of such a stratigraphic record is obtainable, and is usually fragmental across regions and the globe. As a result the role of local controls, as distinguished from global controls, on the initiation of glaciation and the extent of the ice cover, is still a subject of debate (Eyles, 1993). Detailed discussions concerning the origin of glaciation with regard to the Precambrian atmosphere as considered in Eyles (1993) and Eriksson et al. (1998) are beyond the scope of this paper. The glacial-influenced sedimentary facies at Sikwane is broadly correlated lithologically with several other similar exposures developed within the Nnywane formation throughout Southeast Botswana (Mapeo, 1997). However, no direct evidence of glaciation has been recognised in any of the other exposures except at Sikwane. It can be argued though that since the regional tectonic setting during development of the Nnywane formation was common at the time throughout Southeast Botswana, the sedimentary basins were probably developed under the same sub-regional tectonic control.

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Hence the sedimentary sequences preserved in local isolated sub-basins in the region may have been influenced by similar glaciation, and then most were reworked by normal sedimentary processes such that the record of glaciation was completely obscured. The question regarding the character of ice, as to whether the ice was from continental ice sheets and mountains or was just shore or river ice, cannot be resolved yet. It appears though from field evidence in Southeast Botswana that whatever glaciation may have taken place it was only of a local extent. Regionally, in the Southern African sub-continent evidence supporting Archaean glaciation has been discussed in several recent publications (e.g. Crowell, 1983; Eyles, 1993; Von Brunn and Gold, 1993; Young et al., 1998). Amongst the earlier speculations on glaciation in the Southern African sub-continent is Tankard et al. (1982) in which diamictites in the Witwatersrand group of South Africa were considered to represent tilloids. This interpretation was however later considered inappropriate due to lack of direct evidence, with the diamictites instead attributed to sediment gravity-flow processes (Martin et al., 1989; Von Brunn and Gold, 1993). Further work in the region by Von Brunn and Gold (1993) established the occurrence of glacial-influenced diamictite in the Archaean Pongola Sequence of South Africa and Swaziland. In a follow-up study Young et al. (1998) further elaborated on evidence supporting glaciation in the Pongola Supergroup and ultimately declared the sequence as representing the earth’s oldest glaciation. An extensive review of new radiometric age dates by Buekes and Cairncross (1991) allowed the Witwatersrand Supergroup to be placed in the same age bracket as the Mozaan group of the Pongola Supergroup, between about 2870 and 2940 Ma. The two sequences are further correlated on the basis of lithostratigraphic and palaeoenvironmental similarities (Buekes and Cairncross, 1991). Correlation between the Pongola Supergroup and the Witwatersrand Supergroup clearly supports the need for further investigations for possible glaciation during this early part of geological time. Although the age of the Nnywane formation, at 2.78 Ga (Grobler and Walraven, 1993; Walraven et al., 1994, 1996; Wingate, 1998), lies outside of the zone of correlation between the Pongola Supergroup and the Witwatersrand Supergroup (e.g. 2870–2940 Ma), radiometric age studies by Armstrong et al. (1991) indicate the possible occurrence of closely related tectonic and sedimentary events during the 3.2–2.7 Ga period of development of the Kaapvaal Craton. Similarly, Buekes and Cairncross (1991) describe a period about 2700–2800 Ma during which major tectonic disruptions took place. Some of these major tectonic events have been linked to the Limpopo Orogeny in models explaining the tectonic history of the Witwatersrand basin and events involving

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the Nnywane formation in Southeast Botswana (Armstrong et al., 1991; Moore et al., 1993; Chaoka et al., 1998; Modie, 1999). Because there seems to be no direct link between the inferred glaciation in the Nnywane formation and that reported in the Pongola Supergroup it is probable that the two glaciation events only formed local occurrences spread out in space and time. These may have formed during the tectonic events of the period 3.2–2.7 Ga (Armstrong et al., 1991) in response to rapidly uplifted rift margins and a possible upset in atmospheric CO2 , which could have provided a favourable condition for the formation of local ice masses (e.g. Von Brunn and Gold, 1993; Young et al., 1998).

9. Conclusions Consideration of limited geological information from the Nnywane formation has given insight into the palaeoenvironment prevailing at the time of deposition of the sedimentary facies. Sedimentologic and stratigraphic studies of the Nnywane formation reveal a volcanogenically determined depositional lake system with an apparent glacigenic influence. Basal diamictite in the sedimentary facies contains striated and faceted clasts that are interpreted to reflect origin as basal debris and possibly indicate tillite. Internal clast composition indicates a local source. Supporting evidence for glaciation stems from the recognition of large discrete sedimentary bodies of unsorted materials contained in shales, in addition to isolated dropstones which indicate ice-rafting processes. Further evidence for glaciation is indicated by the occurrence of lithologies akin to rockflour. Glaciation was probably of local extent rather than global and was associated with extensional tectonics during the magmatic event of c.2.78 Ga in Eastern Botswana, which is linked to the Limpopo Orogeny. The facies association consisting of conglomerate, sandstone, and shales reveals a cyclic pattern that represents episodic sedimentation. Episodic sedimentation probably resulted from seasonal sediment influx associated with glacial melt-waters and intermittent sedimentation triggered by volcanic activities. Volcanism is shown to have continued during sedimentation as indicated by the intercalation of a volcanic member with the shale facies. The overwhelming local volcanogenic source that is indicated by the Grey tuffaceous sedimentary facies reflects the occurrence of a dominantly closed local depositional system. The closing stage of sedimentation was characterised by the development of subaerial continental glaciofluvial systems bringing sediments from outside the lake system. Provenance at this stage consisted of the granitoids and gneisses of the Archaean basement complex.

Acknowledgements This paper is published with the permission of the Director, Geological Survey Department, Botswana. The paper has benefited greatly from comments and suggestions by Prof. Pat Eriksson (University of Pretoria) and Dr. Roger Key (British Geological Survey). I would also like to thank Dr. R.B. Mapeo (University of Botswana) for his comments and assistance during compilation of data for the manuscript.

References Allen, P.A., Allen, J.R., 1990. In: Basin Analysis: Principles and Analysis. Blackwell Scientific, Oxford, p. 451. Allen, P.A., Collisnon, J.D., 1986. Lakes. In: Reading, H.G. (Ed.), Sedimentary Environments and Facies, second ed. Blackwell Scientific, Oxford, pp. 63–94. Armstrong, R.A., Compston, W., Retief, E.A., Williams, I.S., Welke, H.J., 1991. Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand triad. Precambrian Research 53, 243–266. Buekes, N.J., Cairncross, B., 1991. A lithostratigraphic-sedimentological reference profile for the Late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. South African Journal Geology 94, 44–69. Chaoka, T.R., Kampunzu, A.B., Bagai, Z., 1998. Neoarchaean withinplate magmatism marking plate collision in the Limpopo Belt, Northern margin of the Kaapvaal Craton geochemical evidence from the Gaborone Igneous Complex (Botswana). In: McMullan, S., Paya, B., Holmes, H. (Eds.), Geological Survey 50th Anniversary Conference Abstracts Volume, p. 41, Mogoditshane. Collinson, J.D., Thompson, D.B., 1989. Sedimentary Structures, second ed. George Allen and Unwin, London. Crowell, J.C., 1983. Ice ages recorded on Gondwanan continents: Alex. L. du Toit Memorial No. 18. Transactions Geological Society South Africa 86, 237–262. Davison, S., Hambrey, M.J., 1996. Indications of glaciation at the base of the Proterozoic Stoer Group (Torridonian), NW Scotland. Journal Geological Society London 153, 139–149. Edwards, M., 1986. Glacial environments. In: Reading, H.G. (Ed.), Sedimentary Environments and Facies, second ed. Blackwell Scientific, Oxford, pp. 445–470. Eriksson, P.G., Condie, K.C., Tirsgaard, H., Mueller, W.U., Altermann, W., Miall, A.D., Aspler, L.B., Catuneanu, O., Chiarenzelli, J.R., 1998. Precambrian clastic sedimentation systems. Sedimentary Geology 120, 5–53. Eyles, N., 1993. Earth’s glacial record and its tectonic setting. EarthScience Reviews 35, 1–248. Eyles, N., Eyles, C.H., Miall, A.D., 1983. Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences. Sedimentology 30, 393–410. Eyles, N., Miall, A.D., 1984. Glacial facies. In: Walker, R.G. (Ed.), Facies Models, second ed. Ainsworth Press Limited, Ontario, pp. 15–38. Evans, M.E., McElhinny, M.W., 1966. The palaeomagnetism of the Modipe gabbro. In: Annotated Bibliography and Index of the Geology of Botswana to 1966. Geological Survey Department, Lobatse, Botswana, p. 31 (Compiled by Laughton, C.A.). Fillmore, R.P., 1993. Sedimentation and extensional evolution in a Miocene metamorphic core complex setting, Alvord Mountain,

B.N. Modie / Journal of African Earth Sciences 35 (2002) 163–175 central Mojave Desert, California, USA. Sedimentology 40, 721– 742. Fitzsimons, S.J., 1992. Sedimentology and depositional model for glaciolacustrine deposits in an ice-dammed tributary valley, western Tasmania, Australia. Sedimentology 39, 393–410. Fletcher, C.J.N., Siddle, H.J., 1998. Development of glacial Llyn Teifi, west Wales: evidence for lake-level fluctuations at the margins of the Irish Sea ice sheet. Journal Geological Society London 155, 389–399. Grobler, D.F., Walraven, F., 1993. Geochronology of Gaborone Granite Complex extensions in the area north of Mafikeng, South Africa. Chemical Geology 105, 319–337 (Isotope Geoscience Section). Johnson, H.D., Baldwin, C.T., 1986. Shallow Siliciclastic Seas. In: Reading, H.G. (Ed.), Sedimentary Environments and Facies, second ed. Blackwell Scientific, Oxford, pp. 229–282. Jones, M.T. (1966). Geology of the Mochudi and Marico River area. Internal Report MTJ/37/66. Geological Survey Department, Lobatse, Botswana. Key, R.M., 1983. The geology of the area around Gaborone and Lobatse, Kweneng, Kgatleng, Southern and South East districts. In: District Memoir, vol. 5. Geological Survey Department, Lobatse, Botswana, 230 pp. Key, R.M., Wright, E.P., 1982. The genesis of the Gaborone rapakivi granite complex in Southern Africa. Journal Geological Society London 139, 109–126. Key, R.M., McGeorge, I., Aitken, G., Cadman, A., Tidi, J., Anscombe, J., 1995. The Karoo supergroup of south-west Botswana: new detailed information on the Dwyka and Ecca Groups. In: Barton Jr., J.M., Copperthwaite, Y.E. (Eds.), Extended Abstracts Vol. II Centennial Geocongress Geological Society South Africa, Johannsburg, pp. 781–784. Leeder, M.R., 1982. In: Sedimentology: process and products. George Allen Unwin, London, p. 344. McElhinny, M.W., 1966. Rb–Sr and K–Ar age measurements on the Modipe gabbro of Bechuanaland and South Africa. Earth and Planetary Sciences Letters 1, 439–442. Modie, B.N., 1999. The geology of the area around Mochudi. In: Bulletin, 48. Geological Survey Department, Lobatse, Botswana, 122 pp. Moore, M., Davis, D.W., Rob, L.J., Jackson, M.C., Grobler, D.F., 1993. Archean rapakivi granite-anorthosite-rhyolite complex in the Witwatersrand basin hinterland, Southern Africa. Geology 21, 1031–1034. Nkala, G., Key, R.M., Koosimile, I.D., 1994. The geology of the Marico River area. In: Bulletin, 39. Geological Survey Department, Lobatse, Botswana, 39 pp. Reading, H.G., 1986. In: Sedimentary Environments and Facies, second ed. Blackwell Scientific, Oxford, p. 615. Ronnert, L., 1992. Genesis of a diamicton in the Oak Creek Formation of south-east Wisconsin, USA. Sedimentology 39, 177–192. Selley, R.C., 1985. In: Ancient Sedimentary Environments, third ed. Chapman and Hall, London, p. 317. Selley, R.C., 1988. In: Applied Sedimentology. Academic Press, London, p. 446. Sibiya, V.B., 1988. In: The Gaborone granite complex Botswana, Southern Africa: An Atypical Rapakivi Granite-massif Anorthosite Association. Free University Press, Amsterdam, 446 pp.

175

Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, D.K., Minter, W.E.L., 1982. In: Crustal Evolution of Southern Africa: 3.8 Billion Years of Earth History. Springer, New York, p. 523. Tucker, M., 1988. In: Techniques in Sedimentology. Blackwell Science Ltd., p. 394. Tucker, M.E., 1991. In: Sedimentary Petrology, second ed. Blackwell Scientific, Oxford, p. 260. Visser, J.N.J., 1995. Permian geography and climate of Southern and central Africa: implications for the reconstruction of climates for supercontinents. In: Barton Jr., J.M., Copperthwaite, Y.E. (Eds.), Extended Abstracts Vol. II. Centennial Geocongress Geological Society South Africa, Johannesburg, pp. 808–811. Visser, J.N.J., Loock, J.C., 1982. An investigation of the basal Dwyka tillite in the Southern part of the Karoo basin, South Africa. Transactions Geological Society South Africa 85, 179– 187. Von Brunn, V., Gold, J.C., 1993. Diamictite in the Archaean Pongola sequence of Southern Africa. Journal African Earth Sciences 16, 367–374. Von Brunn, V., Gravenor, C.P., 1983. A model for late Dwyka glaciomarine sedimentation in the eastern Karoo basin. Transactions Geological Society South Africa 86, 199–209. Walraven, F., Armstrong, R.A., Kruger, F.J., 1990. A chronostratigraphic framework for the north-central Kaapvaal craton, the Bushveld complex and the Vredefort structure. Tectonophysics 171, 23–48. Walraven, F., Retief, E.A., Moen, H.G.F., 1994. Single-zircon Pbevaporation evidence for 2.77 Ga magnetism in Northwestern Transvaal, South Africa. South African Journal Geology 97, 107– 113. Walraven, F., Grobler, D.F., Key, R.M., 1996. Age equivalence of the plantation Porphyry and the Kanye volcanic formation, Southeastern Botswana. South African Journal Geology 99, 28–31. Wingate, M.T.D., 1998. Paleomagnetic test of the VaalBara hypothesis at 2.78 Ga. In: McMullan, S., Paya, B., Holmes, H. (Eds.), Geological Survey 50th Anniversary Conference Abstracts Volume, pp. 144–147. Young, M.G., vonn Brunn, V., Gold, D.J.C., Minter, W.E.L., 1998. Earth’s oldest reported glaciation: physical and chemical evidence from the Archean Mozaan group ( 2:9 Ga) of South Africa. Journal Geology 106, 523–538.

Geological map Geological map with brief explanation of the Mochudi area, QDS 2426A and part of 2426C; 1:125 000, 1973a. Compiled by Jones, M.T Geological Survey Department, Lobatse, Botswana. Geological map with brief explanation of the Marico River area, QDS 2426B; 1:125 000, 1973b. Compiled by Jones, M.T. Geological Survey Department, Lobatse, Botswana. Geological map with brief explanation of the Gaborone area, QDS 2425D; 1:125 000, 1982. Compiled by Key, R.M. Geological Survey Department, Lobatse, Botswana. Geological map with brief explanation of the Mathethe area, QDS 2525A; 1:125 000, 1997. Compiled by Mapeo, R.B.M. Geological Survey Department, Lobatse, Botswana.