Journal of African Earth Sciences, Vol. 20, No. 1, pp. 7-15, 1995 Copyright Q 1995 Elsevier Science Ltd

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OWJ-5362(95)0004&2

Late Cretaceous alkaline saline lake complexes of the Kalahari Group in northern Botswana

P.I. DU PLESSIS and J.l? LE ROUX

Department of Geology, University of Stellenbosch, Private Bag 5018, Stellenbosch 7599, South Africa

(Received 14 September 1994 : accepted 18 November 1994)

Abstract - Block faulting in northern Botswana during the Late Cretaceous created grabens which were subsequently filled in by erosion of the nearby horsk. The resultant sedimentary succession forms the Mmashoro Formation at the base of the Kalahari Group. The graben sub-basins are partly interconnected, with younger sediments overstepping the horsts in response to expanding lacustrine and marginal lacustrine environments. The basal deposits consist of alluvial fan conglomerates eroded from the fault scarps, grading downslope and upward into ephemeral stream floodplain, sandflat and dry mudflat deposits. In some of the grabens sandflat deposits grade into perennial saline lake sandstones and finally layered cherk precipitated from hypersaline brines under increasingly arid conditions. The successions thus constitute classical alkaline saline lake complexes.

R&umC - La tectonique d’effondrement (“block faulting”) au Botswana septentrional au C&ad terminal est B l’origine de grabens par la suite remplis par les &dimenk r6sultant de l’&usion des horsts adjacenk. Cette succession s6dimentaire constitue la Formation de h4masharo qui est situ& a la base du Groupe du Kalahari. Les sous-bassins du graben sont partiellement connectt%, avec les s&liments les plus jeunes surmontant les horsts et indiquant un environnement lacustre ou lacustre marginal. Les d@6k de base sent constitu& de conglom&ak de c6nes alluviaux provenant de l’t%usion des escarpements faill6s passant aussi bien vers le haut que vers le bas & des d6pi3k gr&eux et argileux & structures laminaires planes ainsi qu’& des dep6k r&&ant de courants fork et eph&m&res. Dans certains grabens, les d6p6k gr&eux laminaires passent 1 des gIps de lacs salt% p&ennes et finalement B des cherk ruban& pr&ipit& de saumures hypersalines dans des conditions de plus en plus arides. Les successions constituent done des complexes typiques de lacs salCs alcalins.

INTRODUCTION

Apatite fission track dating in southern Africa indicates a significant cooling at 75-70 Ma, which is assumed to have coincided with a period of accelerated erosion (Brown 1990). This is supported by evidence for a major drop in sea-level around the southern African coast at this time (M. Bremner and J. Rogers pers. comm. 1992), which suggests uplift of the subcontinent. In northern Botswana, it appears that this tectonic event was accompanied by WNW-trending block faulting with displacement in the order of tens of metres, which was followed by erosion of the horsts and rapid in-filling of the grabens.

Although natural exposures of the basal Kak&ari Group are scarce due to the extensive sand cover of the Gordonia Formation, numerous borrow-pits were examined, combined with information from boreholes, geophysical surveys, SPOT, Landsat TM, and aerial photographs. Detailed field work was restricted to four study areas in northern Botswana, namely the Nata, Orapa-Letlhakane, South Sua Pan and Mmashoro districts, which fall partly within the perimeters of the Figure 1. Location of study areas and towns in northern Botswana.

7

8 P.1. DU PLESSIS and J.I? LE ROUX

younger Makgadikgadi Basin (Fig. 1). Only the Mmashoro Formation at the base of the Kalahari Group is described here.

SUBENVIRONMENTS

Hardie et al. (1978) distinguished ten subenvironments in modem saline lake depositional complexes: alluvial fans, ephemeral and perennial stream floodplains, sand- and mudflats, ephemeral and perennial saline lakes, dune fields, springs, and shoreline features. The deposits of six of these subenvironments can be recognized as facies of the Mmashoro Formation.

Alluvial fan facies Although basal conglomerates of the Kalahari Group

reach thicknesses of up to 100 m near the western and southern margins of the Kalahari Basin (Mabbut 1955; Smit 19771, they form a relatively minor component of the succession in northern Botswana. Here they are normally confined to the proximity of faults, although gravels also fill shallow troughs eroded into the underlying bedrock at some distance from the fault scarps.

The conglomerate is typically light grey to light brown with a sandy clay-rich, calcite-cemented matrix. Clasts are poorly sorted, angular to subrounded and matrix-supported, comprising calcrete, siltstone, basalt, quartz, agate and carnelian derived from the underlying Karoo Supergroup. No sedimentary structures were observed, although this may be due to poor exposures.

The iron oxide in the matrix of the conglomerate

suggests subaerial deposition, whereas the poor sorting and roundness, clay-rich matrix and the presence of unstable minerals such as plagioclase and hypersthene indicate limited reworking by currents. Although debris flow may have played a role along the fault scarps, no conclusive evidence for these processes could be found.

The conglomerate is interpreted as alluvial fan wedges with midfan braided channels, which are typically less than 1 m deep (Hardie et al., 1978).

Ephemeral stream floodplain facies Conglomerates decrease in thickness away from the

fault scarps and grade laterally as well as vertically into sandstones, which dominate the basal Kalahari Group in the study area. Sandstones interpreted as ephemeral stream floodplain facies generally occur near the base of the Mmashoro Formation in the proximal to medial parts of the basins. They vary in colour from yellowish brown to dark reddish brown due to the presence of iron oxide (hematite and goethite). Petrographically, the sandstones are fine to coarse with poorly to moderately sorted, subangular to rounded grains of quartz (>80%), plagioclase, chert, hypersthene and augite, set in a clayey, carbonate-rich matrix. Calcite and chalcedony cements are ubiquitous. Thin section studies of fresh rock samples show continuous carbonate overgrowths on quartz grains, with the remaining voids being filled by silica.

In spite of local calcretization, silcretization or ferricretization, primary sedimentary structures have been preserved in several localities. These are dominated by trough cross-lamination, but also include low- and high-angle tabular and ripple cross-

Figure 2 Gypsum beds in sandstone of the ephemeral stream floodplain facies, Mmashoro Formation.

Late Cretaceous alkaline saline lake complexes of the Kalahari Group in northern Botswana 9

lamination, as well as upper and lower phase planar lamination. Mudstone rip-up clasts commonly occur near the base of the sandstones. In the Nata area northwest of Dukwe, wavy bedding is a prominent feature in markedly channellized sandstone.

Fining-upward mesocycles, of which up to four can be recognized in the Mmashoro area, generally start with upper phase planar lamination followed by large- scale troughs, smaller troughs, ripple lamination and in some cases massive, red to white sandy siltstone beds with abundant bioturbation. Saline minerals such as gypsum attest to a semi-arid environment (Fig. 2). The cycles are approximately 1 m thick and were probably deposited from waning currents following flash floods. The intermittent nature of the latter is indicated by the presence of rare, isolated vertical burrows of uniform diameter (Tigillites) and ,escape structures, suggesting a fast rate of deposition but enough time between flood events for some organisms to establish themselves in the upper part of the deposited sands (Howard 1975). The burrows are commonly truncated by flash flood deposits of the overlying strata.

As palaeocurrent directions in any particular area show little directional variation, a low-sinuosity, braided channel environment is envisaged. This is supported by the presence of multiple channels up to 1 metre deep, 4 metres wide and two metres apart at the base of the sequence in the Nata area.

Ephemeral stream floodplains typically consist of shallow channels and braid bars (Williams 1971; Twidale 1972; Frostick and Reid 1977). According to these authors, characteristic sedimentary structures include upper phase horizontal lamination, low-angle

planar lamination, megaripple and ripple cross- lamination, antidune wavy lamination, mud-chip pockets, mud-cracked clay drapes and steep-edged scour-and-fill, most of which have been observed in the study area. Fining-upward sequences reflecting a waning flow-regime are also typical. Near the source area highlands the deposits are mainly channel-fill conglomerates, whereas the middle of the floodplain is dominated by upper phase plane beds, trough cross- lamination and wavy lamination. Caliche, silcrete and gypsum crusts, as well as carbonate vadose cements and saline mineral intrasediment growths, indicate an essentially arid climate (Hardie et al., 1978).

Sandflat facies In the Orapa-Letlhakane and South Sua Pan areas,

ephemeral stream floodplain sedimentary strata grade laterally and upward into unchannellized, trough and low-angle cross-laminated sandstones characterized by white to pale reddish brown colours and moderate bioturbation. These sandstones are fine- to medium- grained and poorly to moderately sorted, with angular to rounded grains of quartz (85%) and heavy minerals set in a matrix of carbonate-rich mud (11%). Calcite, chalcedony and iron oxides coat grains and fill the pore spaces.

Bed contacts are commonly erosional and cross-cut by an irregular network of tunnels with variable diameter (0.5-3 cm) and Y-shaped branches lacking surface ornamentation (Fig. 31, which are classified as Thalassimides on a purely morphological basis (Simpson 1975). This suggests that sand initially deposited in fairly high energy conditions around the lake shores

Figure 3~\Thalussinoides burrows in perennial saline lake fads, Mmashorr, Formation.

10 P.I. DU PLESSIS and J.P. LE ROUX

Figure 4. Large-scale desiccation cracks with saline growth in sandflat facies, Mmashoro Formation.

was subsequently subjected to bioturbation in ponded water, which is supported by two observations. Firstly, Thalussinoides are typical of low to moderate energy regimes (Frey 1975), which indicates calmer conditions succeeding the erosive flood events that deposited the sands. Secondly, the fact that iron oxides generally coating the sand grains are absent from the back-filled burrows, suggests that bioturbation may have taken place in deeper, oxygen-depleted water conditions, probably as a result of lake waters rising and covering the surrounding sandflats during floods.

Subsequent subaerial exposure of the sands is shown by the presence of large desiccation cracks with saline mineral growths, as for example north-east of Letlhakane (Fig. 4).

Sandflats are narrow, sandy plains adjacent to saline lakes where braided channels of the ephemeral stream floodplain lose their identity, so that floodwaters disperse as unconfined sheetfloods (Hardie et al., 1978). The dominant component of the sediments is sand characterized by upper phase plane beds and wavy laminated beds, which in the present case may have been destroyed largely by bioturbation. Evaporative pumping commonly results in the formation of calcite coatings, pore cements and caliche crusts (Glennie 1970; Bull 1972; Lattman 1973). In the distal part of the sandflat, gypsum becomes more common as vug-filling cement.

Mudflat facies Many ephemeral saline lakes are fringed by a

subaerially exposed plain of fine-grained sediments (Hardie et al., 19781, which in the study area may be represented by the red sandy silt- and mudstone units

capping fluvial sandstones in the Mmashoro and South Sua Pan areas. The fact that some of these mudrocks can be traced over distances of more than 5 kilometres, suggests that they are probably not abandoned anabranches of the ephemeral stream channels. The siltstones show moderate to intense bioturbation (Thalassinoides), which probably destroyed the delicate millimetre-scale lamination considered to be typical of this subenvironment (Hardie et al., 1978). Most of the burrows have been filled in by brown fine- to medium- grained sand, which was perhaps blown onto the flats or washed in by successive floods.

Dry mudflat surfaces commonly display polygonal mudcracks and thin saline crusts (Hardie et al., 1978). In the Mmashoro area, these could evidently not be preserved due to erosion by floods depositing a younger cycle of fluvial sandstones, as indicated by red mudstone rip-up clasts at the base of the latter.

Expansion of the saline lakes during storm flooding probably covered the mudflats, so that currents decelerated rapidly and deposited finer particles in the resulting ponds. Organisms active in the adjacent, more permanent saline lakes subsequently spread out into the surrounding mudflats under these favourable conditions.

Perennial saline lake facies Two types of sandstone interpreted as perennial

saline lake facies occur in the study area. In the more distal parts of the grabens and towards the top of the sequence the Mmashoro sandstones are predominantly white to grey and unoxidized. They are massive due to extensive bioturbation, although lower phase plane beds are preserved in places. The cement is commonly

Late Cretaceous alkaline saline lake complexes of the Kalahari Group in northern Botswana 11

carbonate or silica. North and north-west of Letlhakane, massive, green,

intensely bioturbated sandstone occurs, which is moderately to well sorted with fine to coarse, angular to rounded quartz, plagioclase and basaltic grains in a glauconite-rich clay matrix. Calcium carbonate in the form of sparite forms continuous overgrowths on the grains. The remaining voids are filled by silica.

Trace fossils are abundant in both types of sandstone and consist of Thalassinoides as well as vertical cylindrical shafts with lateral, horizontal tunnels of the same diameter (about 2 cm). These may possibly represent Lenneu, as described by Hantzschel (1962). Surface openings and mounds of burrows are commonly undisturbed, and together with the absence of escape structures indicate slow, intermittent deposition without significant erosion (Howard 1975). In some instances the mounds have been washed out to form linear strips of grains slightly coarser than that of the surrounding sedimentary rocks, but the influx of water was apparently never strong enough to erode the burrow openings.

The burrows are commonly enhanced by diagenesis and weathering. Fresh samples display an increase in carbonate within the burrows, but leaching by acid rainwater produced hollow tubes. In some instances near the top of the sequence, the trace fossils contain more silica cement than the surrounding rocks and weather positively

Although ‘I’halassinoides is usually confined to the marine environment (Frey 19751, the probability that high, marine-like salinities were attained in the Early Kalahari lakes is supported by the occurrence of glauconite and small stromatolites in the upper parts of the succession. The absence of bioturbation in and just below the layered chert deposits at Letlhakane possibly indicates hypersaline conditions hostile to organisms, but Thdassinoides trace fossils become fairly common further north, perhaps suggesting less saline, deeper water.

The green, glauconitic sandstones probably reflect periods of shallow water with high salinity, following evaporative shrinking of the lakes during periods of low fresh water influx. This is supported by the association between glauconitic sandstones and rhythmites in some of the grabens. Glauconite is mainly restricted to marine continental shelf sediments in areas of moderate turbulence, some organic matter and low rates of sedimentation (Reineck and Singh 19801, suggesting similar conditions in the KaWtari lakes.

The pale, unoxidized sandstones are considered to represent deeper water and less saline conditions following catastrophic flood events.

Ephemeral hypersaline lake facies During the long dry periods between infrequent

floods, lakes slowly shrink and become saline by evaporative concentration, eventually reaching

saturation with respect to various solutes before drying up completely. Repeated floods and dry spells thus superimpose couplets of elastic and chemical sediments which alternate on a millimetre to centimetre scale, but can reach hundreds of metres in total thickness (Hunt and Mabey 1966; Baker 1958; Hardie et al., 1978).

In northern Botswana, rhythmic beds of chert and sandstone have a wide distribution, but are best preserved in the Mmashoro area where they cap perennial saline lake sandstones. The deposits, which locally exceed 5 metres in thickness, typically consist of couplets of layered black, grey or yellow chert and fine-grained green (glauconitic), horizontally laminated sandstone. Individual chert and sandstone layers are l-30 mm thick and can be traced laterally for tens of metres, indicating deposition under calm conditions.

Thin section studies show that the green sandstone layers have normal grading and moderate to good sorting. In thicker sandstone layers the amount of glauconite-rich clay increases upwards, so that the texture varies from grain-supported near the erosional basal contact to matrix-supported towards the top, implying waning energy regimes.

Rhythmites of the type described above are typical of hypersaline ephemeral lakes in semi-arid environments, and in fact occur in the present-day salt pans to the north of Orapa. They form by settling of sand, silt and clay from subsiding flood waters (elastic fraction), followed by the precipitation of silica (chemical fraction).

The origin of chert layers in hypersaline lakes is somewhat controversial, and more than one process may be involved in any particular area. One of the most important factors controlling these processes is the alkahnity of the lake water. When the pH value exceeds 9, the solubility of silica increases dramatically (Bemer 1971; Krauskopf 19791, so that silica provided by the weathering of clay minerals and quartz grains is partially dissolved. In hypersaline lakes the pH can rise very high, often through the photosynthetic activities of phytoplankton (Peterson and Von der Borch 1965) or sulfate-reducing bacteria (Abd-el-Malek and Rizk 1963). Supersaturation of the lake water with amorphous silica as a result of evaporative concentration can eventually lead to a drop in pH, which would cause the silica to precipitate as a gel (Peterson and Van der Borch 1965).

In the Magadi Lake of the Rift Valley in Kenya, sodium silicates such as magadiite (NaSi,O,,(OH),.3~0) appear to form at the interface between dilute, stratified inflow and the underlying dense brine (Eugster 1969,198O). Near the sediment surface, the magadiite changes into flaggy chert (l-5 cm thick), which is related to leaching of sodium by more dilute runoff according to the reaction:

NaSjO,,(OH),.3H,O + H+ = 7Si0, + Na+ + 5H,O (Eugster 1969)

12 P.I. DU PLESSIS and J.P. LE ROUX

Spontaneous crystallization of quartz from magadiite can also occur, with the expulsion of Na+ taking place even in the presence of concentrated brines (Hay 1968, 1970).

Present-day groundwater brines in Sua Pan are of the Na-CO,-SO,-Cl type, whereas the pH in surface brine pools varies between 8.5 and 10.2 (Shaw ef al., 1990). It is thus conceivable that both of the processes outlined above could have led to the formation of chert layers. A third possibility is that they developed in a way similar to that of laminar silcretes presently forming in Sua Pan. These are restricted to the top 20 cm of the pan sediments and form largely by capillary rise in the porewater zone. Silica is either precipitated directly, or through the intermediate formation of sodium carbonate, which is then replaced by microcrystalline and chalcedonic silica (Shaw et al., 1990).

Desiccation cracks of several orders, some filled by red oxidized terrestrial sand, and small-scale tepee structures characterize the layered cherts in the study area. The tepees commonly appear as more or less equally spaced compressional buckles in cross-section, distorting one or more layers of the rock (Fig. 5). In plan view, they form an irregular polygonal pattern. Sediment within the fractures and related breccias is made up of allochemical silica and terrigenous sand grains derived from the adjacent environments.

Tepees develop as a result of repeated cycles of desiccation and thermal contraction, followed by cementation, hydration and thermal expansion. They are normally indicative of sub-aerial exposure, a semi- arid climate and periods of reduced sedimentation or

non-deposition (Riccardo and Christopher 197’3, which confimxs the ephemeral nature of the Kalahari saline lakes during deposition of these facies. The complexity of tepees usually increases with the length of subaerial exposure (Riccardo and Christopher 1977), which in this case appears to have been relatively short as the structures are simple. Their smaIl size can be attributed mainly to the limited thickness of the chert laminae.

Capping the rhythmites in the Orapa-Lethlakane area, is a matrix-supported conglomeratic breccia with a maximum thickness of 30 cm. This bed consists of angular fragments of previously layered chert cemented by silica. It may represent the final (senile) phase of tepee formation, or alternatively, a semi-consolidated chert bed broken up by subsequent flood waters.

In the Mmashoro area dome-shaped stromatolites are associated with the rhythmites. They are 4 cm high and up to 10 cm wide, showing irregular layering of alternating light and darker micrite with scattered sediment grains. In some areas the stromatolites display a vaguely pelleted structure or laminoid fenestrae, the latter probably resulting from the decay of organic matter (Adams et aI., 1991).

Shaw et al. (1990) described similar, floating colonies of cyanobacteria in present-day brine pools in Sua Pan. These form rounded, jelly-like masses a few to 60 cm in diameter, consisting of laminated couplets (up to 1 mm thick) interpreted as diurnal growth rings. Scattered quartz grains and colloidal aggregates of calcium carbonate represent trapped wind-blown material. The algal colonies thrive in pH conditions of about 9, decreasing in number and size with an increase or decrease in salinity.

Figure 5. Tepee struchxes in layered cherts of the ephemeral saline lake facies, Mmashoro Formation.

Late Cretaceous alkaline saline lake complexes of the Kalahari Group in northern Botswana 13

Alluvial fan Sandflat

I Ephemeral I Ephem,eral lake

Figure 6. Generalized distribution of s&environments in saline lake complexes of the Mmashoro Formation.

. / I 50, / . Thickness of Kalahari Group (m) . / /

/ I _ Measured transport dlrectlon

0 m 500 ?? Borehole

Figure 7. Fault system and borehole distribution in grabens of the South Sua Pan alpa.

Oncoids (larger than 2 mm) showing asymmetrical growth with wavy laminae and scattered fine sediment particles, are associated with the early Kalahari stromatolites and may be quite abundant in places. They are presumed to have originated from blue-green algae coating grain surfaces and binding loose sediment grains, as inorganic oncoids usually display regular concentric laminae and contain only one grain at their centre (Adams ef al., 1991). Peloids (grains composed

of micrite and lacking any recognizable internal structure) were also observed in thin sections, probably having formed by the micritization of algae.

DISCUSSION AND CONCLUSIONS

The general distribution of depositional subenvironments in the Mmashoro Formation is shown in Fig. 6. A more detailed illustration of graben in-fill

14 RI. DU PLESSIS and J.I? LE ROUX

architecture, based on detailed drilling, ground magnetic and gravity surveys in the South Sua Pan area, is presented in Figs. 7 and 8. This WNW-trending graben has two dolerite dykes on its southern edge, one of which is coincident with a major fault. The floor of the main graben has been broken up by secondary, NE- trending faults into minor horsts and grabens, which affected the earliest transport patterns. Initially, alluvial fan and braided stream deposits derived from the fault scarps dominated sedimentation in the basin, but as in-filling of the latter proceeded, sediments were supplied from further afield and transport became both perpendicular and parallel to the main fault along the southern edge of the graben.

The coarse-grained alluvial fan conglomerates and light brown, coarse-grained ephemeral stream sandstones at the base of the succession grade downstream and upward into red siltstones, which have been produced partly by weathering and erosion of underlying basalts and mudstones of the Karoo Supergroup. The wide lateral extent of these beds suggests a dry mudflat environment. The siltstones are in turn overlain by white to light brown, fine-grained sandstones with lenses of coarse- to very coarse-grained sandstone and moderate bioturbation, which possibly represent a transition between ephemeral stream floodplain and sandflat environments. These deposits grade upward into reddish brown, fine- to coarse-

grained, texturally immature sandstones with moderate bioturbation, which probably represent a sandflat environment. The absence of rhythmites from the South Sua Pan graben may be related to the extensive period of denudation that accompanied deposition of the overlying Letlhakane Stoneline Formation (cf Du Plessis 1993).

Acknowledgements We are indebted to the Anglo American Corporation,

De Beers Prospecting Botswana and the Debswana Diamond Company for their sponsorship and permission to publish the results of this study. Drs. B. Cairncross and R. Smith are thanked for their constructive reviews of the manuscript.

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[Letlhakane Stoneline Formation D

*-,,... ---.”

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1 0

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0

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??;: Facies 2 Facies 5 cl

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Figure 8. Vertical profiles of grabens in the South Sua Pan area, showing facies architecture of sedimentary in-fill. Facies I-Calcareous conglomerate (alluvial fan subenvironment); Facies 2-Light brown, coarse to very coarse-grained sandstone with rare bioturbation (ephemeral stream floodplain subenvironment); Facies 3-Reddish brown, fine- to coarse-grained sandstone with trough and low-angle cross-lamination and moderate bioturbation (ephemeral stream/sandflat s&environment); Facies 4-White to light brown, fine-grained, bioturbated sandstone (sandflat subenvironment); Facies 5-Red siltstone with moderate to intensive bioturbation (mudflat subenvironment).

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