EARTH SURFACE PROCESSES A N D LANDFORMS, VOL. 19,299-317 (1994)
DURICRUST DEVELOPMENT A N D VALLEY EVOLUTION:
PROCESS-LANDFORM LINKS IN THE KALAHARI
DAVID J. NASH'
Department of Building, University of Brighton, Mithras House, Lewes Road, Brighton, BN2 4AT. U.K.
PAUL A. SHAW
School of Geology and Environmental Science, University of Luton. Park Square, Luton, LU1 3JU. U.K.
AND DAVID S. G . THOMAS
Department of Geography, University of Shefield, Shefield, SIO 2TN, U . K .
Received 30 March 1993
ABSTRACT
Duricrusts are an important landscape component of the Kalahari region of central southern Africa. Their exposures
within the dry valleys (rnekgacha) of the Kalahari provide some of the most widespread surface outcrops of the terrestrial
Jurassic to Holocene Kalahari Group sediments. Exposures have been extensively'used in the construction of lithostratigraphic sequences, on the assumption that valley systems have incised their courses through a pre-existing duricrust
sequence. Recent work, however, has identified the role of groundwater erosion processes in valley development, which
may have influenced duricrust formation.
Studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the
presence of a valley. Analyses of calcretes and silcretesin a series of profiles and thin sections from the Letlhakeng area of
Botswana also indicate extensive alteration and diagenesis in association with former higher water tables. Sedimentary
sequences within duricrust host materials can be identified but there is no evidence for correlation of duricrust cements
between exposures. Profile studies from the Auob Valley in Namibia, however, suggest that this valley has incised through
a sequence of duricrusts. Caution is advised in future attempts to correlate duricrust types on the basis of valley exposures,
with the recommendation that where such exposures are used in a lithostratigraphic context, only duricrust host material
characteristics and not cementing materials should be considered.
KEY WORDS
Kalahari Duricrusts Groundwater weathering and erosion Dry valleys
INTRODUCTION
Dry valley systems (termed mekgacha, singular mokgacha, in Botswana) are a major landform component of
the Kalahari region of southern Africa, with extensive valley networks throughout Botswana, Namibia and
South Africa (Figure 1). The mekgacha have been considered to have formed during periods of former wetter
climate in the Late Tertiary or Quaternary (Grove, 1969) and to have incised their courses into the relatively
subdued Kalahari landscape. As such, along with other palaeolandforms generally cited as indicators of climatic change (such as the lake basins of the Middle Kalahari; see Thomas and Shaw (1991) for a review),
mekgacha are often regarded as having palaeoenvironmental significance.
Mekgacha are also important in a geological context as they contain extensive exposures of the Jurassic to
Holocene Kalahari Group sediments (South African Committee for Stratigraphy (SACS), 1980) which
dominate the near-surface geology of much of the Kalahari region. These sediments, consisting of a
*Previously at Department of Geography, University of Sheffield, Sheffield, S10 2TN, U.K.
CCC 0197-9337/94/040299- 19
0 1994 by John Wiley & Sons, Ltd.
300
D. 3. NASH, P. A . SHAW AND D. S. G . THOMAS
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sequence of terrestrial conglomerates, marls, duricrusts and unconsolidated sands, cover an area of 2-5 million km2 and reach a maximum thickness of over 300 m (Thomas, 1984, 1988; Thomas and Shaw, 1991). The
most extensive surface outcrops of duricrusts are found within mekguchu, exposed as low cliffs and in valley
floors, in addition to forming entire valley flanks in more incised sections.
Attempts at establishing a regional stratigraphy for the Kalahari Group have been hampered by a dearth
of surface exposures in the generally flat terrain of the region, together with the extensive cover of Kalahari
Sand which blankets other lithologies. Correlative studies have been largely reliant upon information from
boreholes drilled as part of exploration for water and mineral resources (Thomas, 1988). Where surface
DURICRUST DEVELOPMENT
30 1
exposures of the Kalahari Group do occur, they are of potential importance in a stratigraphic context. As
such, the exposures of calcrete, silcrete and intermediate duricrust types within the Auob, Nossop and
Molopo valleys of the southwestern Kalahari have becn used in lithostratigraphic studies of the youngest
components of the Kalahari Group sediments (Malherbe, 1984; Thomas et al., 1988).
The basis for much of this lithostratigraphic work is the premise that mekgacha have developed by gradual
downcutting through a pre-existing Kalahari Group stratigraphic sequence, with present-day surface duricrust exposures genetically unrelated to the valleys in which they occur. Recent work (Shaw and De Vries,
1988; Nash, 1992; Nash et al., 1994), however, suggests that Kalahari valley development may not have proceeded simply by fluvial incision, but also by groundwater processes such as sapping and deep-weathering
(see Higgins (1984), Howard et al. (1988) and Baker (1990) for reviews of the operation of these processes). If sapping and deep-weathering were important in mekgacha formation, the relationship between
duricrusts and valleys might be more significant than the consensus view that the latter simply provide a
setting for exposures of the former. Lithostratigraphic studies based purely upon such exposures may be
inappropriate, for while duricrust host materials may contain identifiable sedimentary sequences that could
be regionally correlated, the duricrusts themselves might have been substantially altered during valley development due to groundwater movement.
This paper aims to explore the process-landform relationships and potential genetic links between duricrust formation and Kalahari valley development, and hence assess the possible influence and implications
of groundwater weathering and erosion upon duricrusts. This is achieved by consideration of relationships
between valleys and duricrusts at a variety of scales (ranging between microscale to valley scale) from three
mekgacha systems within the Kalahari: the Rooibrak and Letlhakeng valleys in Botswana and the Auob
Valley in Namibia.
KALAHARI VALLEY DEVELOPMENT
The mekgacha networks of the semi-arid to arid Kalahari (Figure 1) can be subdivided into endoreic Middle
Kalahari and exoreic Southern Kalahari systems. Many valleys are of considerable antiquity, as indicated by
isolated Dwyka (Permo-Carboniferous age) tillite erratics occurring over a 50 km section of the headwaters
of the Black Nossop Valley (south of Gobabis, Namibia), the valley being incised into Precambrian Nama
Group quartzites, limestones and shales. The glacial erratics, dating to the Permo-Carboniferous glaciation
of southern Africa (Hegenberger and Seeger, 1980; Visser, 1983), were deposited within the Black Nossop
Valley and are not eroded remnants, suggesting the existence of this section of valley prior to the period
of glaciation. A great age for the Xaudum Valley is also suggested from borehole evidence (Union Carbide, 1980). Initial deposition of sediments in the valley occurred within a graben structure which predates
the onset of rifting in the Okavango Delta region (Rust, 1975), thus indicating a maximum Late Palaeozoic
age for the earliest deposits (Nash, 1992).
As noted above, two non-mutually exclusive explanations for the development of Kalahari mekgacha have
been proposed, namely that valleys evolved due to erosion by fluvial activity (due to perennial flow or lowfrequency high-magnitude flood events) or by a combination of deep-weathering and groundwater sapping
processes (Nash et al., 1994). Whilst there is clear evidence for the action of water within many valleys, such
as abandoned channels, lag deposits, buried shell material and terrace features (Nash, 1992), fluvial activity
does not explain many of the characteristics of mekgacha. Firstly, the morphology of many valley systems
exhibits three elements: a flat ‘dambo’ headwater section (often lobate in plan-form) which gives way
abruptly to a gorge-like section before reverting to a subdued form in lower valley reaches (Boocock and
Van Straten, 1962; Thomas and Shaw, 1991). This form could be attributed to the gradual recession of nickpoints, but the presence of an amphitheatre valley head containing relict spring lines in one valley (the
Gaotlhobogwe at Letlhakeng in Botswana) suggests the role of groundwater sapping processes in valley formation (Shaw and De Vries, 1988).
Valley location also appears to have been controlled by groundwater processes in many cases, particularly
deep-weathering by groundwater flowing along preferential subsurface flowpaths such as faults and
fractures (Nash et ul., 1994). Deep-weathering has been implicated as a factor in the development of African
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D. J. NASH, P. A. SHAW A N D D. S. G. THOMAS
dambos located upon outcropping or subcropping bedrock (McFarlane, 1989) but is of perhaps greater significance in the Kalahari where many valley systems are aligned parallel to geological structures buried at
depths of over 30 m beneath Kalahari Group sediments. There is considerable evidence for dissolution
and weathering of bedrock beneath vaileys (Nash, 1992) associated with subsurface fracture zones which
act as conduits for groundwater flow (Buckley and Zeil, 1984).
The last period of active recharge of Kalahari aquifers ceased at around 12 500 years BP (De Vries, 1984) at
the end of a widespread wetter period which had started around 18 000 years BP. Since that time, water tables
have declined, due to a combination of anthropogenic effects (Thomas and Shaw, 1991) as well as decreased
recharge. Present-day water tables are at considerable depths beneath valley floors, and as such groundwater
sapping processes could not operate today. There is, however, evidence that water tables have been significantly higher, even during the period of historical records. Livingstone (1858, p. 78) recorded ‘every salt pan
in the country. . . [having] a spring of water to one side’, whilst Hodson (1904) noted standing pools and nearsurface water tables common to valleys.
MODELS O F DURICRUST FORMATION
The duricrusts exposed in Kalahari mekgacha may have formed in a variety of ways. Prerequisites for the
formation of silcrete or calcrete, the most common Kalahari duricrusts, are a source of silica or calcium
carbonate, the availability of moisture to act as a transporting agent from the source to the site of precipitation (with the exception of airborne sources) and a mechanism to cause precipitation. When considering the
environmental conditions necessary for duricrust formation it is essential to note that two environments need
to be taken into account; one where silica or carbonate is made available in solution and another where it is
precipitated (Ollier, 1991).
Sources of precipitate, mechanisms of precipitation and the environmental conditions required for duricrust development are discussed by a variety of authors (e.g. Goudie, 1973, 1983; Summerfield, 1982; Chadwicket al., 1987; Wright and Tucker, 1991) and will not be further considered here. More important in terms
of the relationship between duricrusts and mekgacha is the mechanism by which silica or carbonate material
has been transported to form a particular duricrust. Two groups of mechanisms have been proposed, which
can be broadly categorized into those involving vertical transport of solutes and those where development
occus as a result of accumulation of material either due to biogenic activity or by lateral transfer of solutes
(Goudie, 1983). These mechanisms do not operate in isolation, since accumulation of silica and/or carbonate
can also occur through vertical transfer mechanisms following lateral transportation to a particular location.
Vertical transfer mechanism, which can be subdivided into per descensum and per ascensum models, are
reviewed by Summerfield (1983a) and Goudie (1983). The major feature of both models is that duricrust
formation may occur, usually within a soil profile, without the need for lateral movements of material in
solution. In contrast, duricrust development by the lateral transfer of material requires some form of depression to generate a hydrologic gradient.
A number of lateral transfer mechanisms can be distinguished, including lacustrine/pan and sheet-flood/
groundwater models. Under the former model, which is most applicable to arid and semi-arid environments,
the high potential evaporation rates and fluctuating pH conditions experienced within a lake or pan (containing standing water on a seasonal basis) may promote the weathering, leaching and potential neoformation of pan-floor clay minerals, thus providing a source of material for duricrust formation (see Summerfield
(1982, 1983a) and Jacobson et al. (1988) for reviews of this model).
Sheet-flood/groundwater models for duricrust formation have been discussed by a variety of authors, particularly in the context of Australian duricrusts (Stephens, 1971; Carlisle er d., 1978; Mann and Horwitz,
1979; Arakel, 1986; Arakel et al., 1989) but also for southern African examples (e.g. Netterberg, 1975,
1982). Duricrust accumulation by fluvial processes can involve deposition within channels or valleys, deposition from sheet floods, and/or lateral seepage of groundwater and throughflow water (Goudie, 1983). Valleys, like pans, are potentially important sites for the development of duricrusts since water tables are
generally closer to the surface in the vicinity of depressions. Mann and Horwitz (1979) distinguish
‘vadose’ from ‘phreatic’ (groundwater) calcretes, with vadose calcretes usually viewed as forming by
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DURICRUST DEVELOPMENT
A. Shallow groundwater system in a broad drainage channel.
Bedrock
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B. initial carbonate precidtation.
C. Growth of calcrete pods and domes.
D. Maturation of calcrete and surface reworking
Figure 2. The stages of development of a groundwater calcrete (after Mann and Horwitz, 1979)
processes such as evaporation at the capillary fringe (Goudie, 1983). Conversely, groundwater calcrete
usually develops within alluvially filled drainage lines with shallow water tables with precipitation occurring
in the phreatic zone (Mann and Horwitz, 1979, Figure 2).
Where renewed fluvial activity has occurred following a period of calcretization, pre-existing duricrusts
may be dissected and isolated from the valley base. For example, Mann and Horwitz (1979, p. 296) note
groundwater calcretes in Australia that formerly occupied a valley floor, now left as perched remnants after
a period of valley downcutting. A similar situation is encountered for dissected silcrete lenses within the Oligocene Fontainebleau Sand of the Paris Basin (Thiry et al., 1988; Figure 3). The silcrete lenses are superposed up the flanks of valleys, with the oldest lenses uppermost. The older lenses show greatest signs of
dissolution and weathering (Thiry and Millot, 1987), whilst those lower down, closer to the valley floor,
are comparatively fresh and unweathered. The lenses of silcrete ‘pinch out’ within a short distance of the
valley side, suggesting formation in association with the valley groundwater table (Thiry et al., 1988),
with successive lenses formed after continued down-cutting of the drainage network. Cemented valley deposits have also been used in a palaeohydrological context, such as the studies in Oman of indurated PlioPleistocene cemented gravel sequences now forming raised channels (Maizels, 1987, 1990).
THE RELATIONSHIP BETWEEN VALLEYS AND DURICRUSTS
The majority of studies of Kalahari duricrusts (with the exception of Summerfield (1982)) tend to overlook
the potential importance of process-landform links in duricrust formation, particularly in terms of the geomorphological setting and geochemical environment that certain landforms may provide. As has been mentioned above, the major assumption that has been made in many attempts to construct detailed
lithostratigraphic relationships for the Kalahari Group sediments (e.g. Malherbe, 1984; Thomas et al.,
1988), is that duricrust suites have developed independently of, and generally predate, the landforms with
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D. J . NASH. P. A. SHAW A N D D. S. G . THOMAS
.
A. Disseclion of the limestone cover and first silification in the
groundwater discharge zone.
Silcrete
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water table.
_ _ - _Groundwater table
C. The process is repealed, leading to a thicker leached profile above
Ihe water table and a deeper silcrete level. Note that uppermost
(oldest) silcrete lenses show evidence of partial dissolution.
Figure 3. Successive stages of the development of quartzite (silcrete) lenses in the Fontainebleau Sand of the Paris Basin (after T h i r y
et ul., 1988)
which they are now associated, i.e. that they have formed predominantly by pedogenic processes involving
vertical transfer of solutes within a soil or weathering profile. Kalahari duricrusts are, however, almost certainly polygenetic (Thomas and Shaw, 1991) and, in locations where formation occurred by non-pedogenic
processes, their development may be intrinsically linked to associated landforms. A major link between geomorphological setting and duricrust formation would potentially occur where groundwater erosion has
played a significant role in valley development; the presence of a valley would affect local hydrological conditions and allow for the accumulation of CaCO? and SO2 due to water table fluctuations and by movement
of lateral throughflow.
From the formational models presented in the preceding section, there are four ways in which a duricrust
exposed within a mokgacha at the present day may have developed.
1. Duricrust formation occurred due to pedogenic vertical transfer processes operating prior to valley incision, with duricrusts subsequently exposed by valley downcutting.
2. The duricrust developed as a direct result of the presence of a valley by accumulation of minerals in the
water table zone beneath the valley flanks, in the way proposed by Thiry et al. (1988), with subsequent
incision separating the duricrust from the valley floor.
3. The duricrust developed due to groundwater-related processes (‘phreatic’ types of Mann and Horwilz
(1 979)) with precipitation at or beneath a shallow valley water table. Subsequent downcutting and dissection has produced perched remnants.
4. Duricrusts formed under pan conditions (the pans located in a proto-valley), with precipitation caused by
pH changes due to lateral movements of water into alkaline pan conditions. The presence of seasonally
standing water in pans located within valleys may have generated localized environmental conditions suitable for mineral dissolution and/or precipitation.
DURICRUST DEVELOPMENT
305
Given these four scenarios, a simple dichotomy can be proposed in order to aid the interpretation of
duricrust-valley relationships on the basis of exposures within and beneath valleys. If a valley were to be
incised through pre-existing duricrusts, a regional duricrust stratigraphy might be present. identifiable
from exposure to exposure. Alternatively, a lack of such consistent stratigraphy may suggest that the crusts
developed as a direct result of processes related to the presence of the valley and the effects this would have
on local hydrologic gradients.
In practice, it may not be possible to uphold such a simple distinction and it may be better to regard the
two sides of this dichotomy as representing the readily identifiable end-members of all the possible scenarios for duricrust and valley interactions. Given the timescales necessary for both duricrust and valley evolution (Radtke and Bruckner, 1991; Nash et al., 1994), it is likely that major spatial and temporal shifts in
environmental geochemistry have occurred during the stages of valley development, with resulting changes
in the properties of pre-existing and still-forming duricrusts. Such changes may render any duricrust stratigraphy almost unidentifiable, particularly at a microscale. It is possible that during the course of incising
its channel by fluvial erosion, the development of a valley influenced local hydrological conditions and
caused diagenetic alteration to pre-existing duricrusts (and also led to the generation of further duricrusts, possibly by pedogenic processes operating within valley slope materials). The duricrusts may also
have been modified by, or developed in association with, another conduit for water movement, such as
a geological lineament or fracture which has subsequently acted a a focus for valley location. It is, however, very difficult to assess the impact of a valley upon local hydrology during the early stages of its development; clearly the development of any linear depression would have influenced flowpaths of groundwater
and throughflow by altering hydrologic gradients. Furthermore, the present-day status of water tables at
levels well below valley floors (Thomas and Shaw, 1991) raises problems for the analysis of contemporary
water table-valley interactions, thus precluding the use of modern analogues in the reconstruction of former groundwater flowpaths.
METHODS O F INVESTIGATION
In order to investigate the relationship between duricrusts and mekgacha, two main approaches were used.
The first was to investigate subsurface variations in duricrusts associated with valleys, by considering information on duricrust thickness, composition and type, principally from lithological borehole logs drilled in
the vicinity of valleys.
The second method concentrated upon surface exposures of duricrusts in two ways. The field distribution
of duricrust types was examined by mapping profile variations in selected valleys with extensive duricrust
exposures. From this technique, any Stratigraphy within valley duricrusts at the selected locations could
be noted. Thin-section analyses of duricrust samples were also undertaken to assess microscopic and
mineralogical indicators of the environmental conditions during duricrust development.
SUBSURFACE VARIATIONS IN DURICRUSTS FROM BOREHOLE LOGS
The identification of compositional and thickness variations in duricrusts beneath valleys from borehole
records is only possible where a sufficient number of boreholes containing detailed lithological information
are drilled along or, more importantly, across a valley. In the case of boreholes in the vicinity of Kalahari
mekgacha, this degree of logging accuracy is comparatively limited; few boreholes drilled in conjunction
with mineral or groundwater exploration can be considered sufficiently reliable. The spatial distribution
of boreholes is also highly variable, with only three areas containing a sufficiently large number of borehole
logs for detailed analyses to be undertaken. These are in the Rooibrak and Xaudum valleys, where exploratory drilling was carried out by Union Carbide (1979, 1980) and in the valleys of the Mmone/Quoxo system
immediately south of Letlhakeng village, where boreholes were sunk for groundwater and calcrete exploration (Gwosdz and Modisi, 1983). Results from the Rooibrak Valley and Letlhakeng Valley 2 (nomenclature
after Shaw and De Vries, 1988) are now considered.
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D. 1. NASH, P. A. SHAW AND D. S. G. THOMAS
Figure 4. Calcrete rubble in the Rooibrak Valley, 63 km due east of Ghanzi, Botswana
Duricrusts beneuth the Rooibrak Valley
The Rooibrak Valley (Figure 1) forms the topographically indistinct western part of the RooibrakPassarge valley system and, whilst being clearly identifiable on aerial photography, has a total relative relief
of less than 5 m and a width of only 300-400 m (Nash, 1992). The valley is flanked by low banks of calcrete
rubble for much of its length (Figure 4), with calcrete developed extensively beneath the valley floor. Variations in the thickness of calcrete beneath the Rooibrak (from boreholes drilled by Union Carbide (1979)) are
shown in Figure 5. From two transects drilled perpendicular to the main valley, it can be clearly seen that the
calcrete is in a lenticular form, thinning rapidly away from beneath the centre of the valley, where it has a
maximum thickness of approximately 8 m. The valley appears to be situated directly above a much deeper
trough in the underlying Ghanzi Group quartzites and shales.
The calcrete body has a similar form to the groundwater calcretes described by Mann and Horwitz
(1979) and shows ‘pinching out’ similar to that in the duricrust lenses studied by Thiry et ai. (1988),
suggesting an intrinsic genetic relationship with the valley. From the cross-sectional form of the calCrete, it appears to have developed after the deposition of the Kalahari Sand, with field investigations
of surface samples and spoil from boreholes indicating calcretes which incorporate sand in a carbonate
cement. The form of the calcrete lens indicates formation by water moving laterally away from or into
the valley, although whether this is a result of permanently flowing or periodically standing water is
difficult to assess. Drilling along the valley (not included in the figure) does not show significant calCrete thickness variations.
Also of interest is the presence of the bedrock trough which could be indicative of deep groundwater flow
along preferential flow paths beneath the valley (Nash, 1992). Such flow could be responsible for weathering
of the quartzite bedrock immediately below the valley course, although the borehole records in Union Carbide (1979) provide no information on the weathered status of the bedrock.
DURICRUST DEVELOPMENT
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Figure 5. Thickness of calcrete beneath the Rooibrak Valley (after Union Carbide, 1979); location as for Figure 4
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D. J. NASH, P. A. SHAW AND D. S . G. THOMAS
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Bedrock
DURICRUST DEVELOPMENT
309
Duricrusts beneath Letlhakeng Valley 2
Letlhakeng Valley 2 in southeast Botswana (24°08'30"S, 25"05'00"E)was the site of reconnaissance drilling
to assess the resource potential of calcrete exposures in the Lethakeng area. Unlike the relatively shallow
Rooibrak, Letlhakeng Valley 2 is deeply incised, reaching a maximum depth of 25 m beneath the surrounding terrain. The valley flanks are composed of a range of calcareous and siliceous duricrust types, which are
overlain in places by slope deposits and shallow soil.
A series of six boreholes drilled in the eastern valley flank (Gwosdz and Modisi, 1983) show variations in
duricrust form away from the valley axis (Figure 6). The general sequence of duricrust types beneath the
valley flank consists of alternating layers of hardpan and weakly consoiidated calcrete, giving way to uncemented sand at depth. The duricrusts and sands are part of the Kalahari Group sediments, which are
approximately 30-40m thick beneath this section of the valley. Boreholes further away from the valley
show decreased thicknesses of calcrete and a tendency for calcrete types to be less well indurated. This suggests a similar relationship between duricrusts and mekgacha as indicated for the Rooibrak, with calcrete
formation occurring in association with valley-related water movements. There does not, however, appear
to be any lateral continuity between hardpan calcrete layers in adjacent boreholes; this precludes duricrust
formation beneath the valley flanks in the way proposed by Thiry et al. (1988).
MACRO- AND MICRO-MORPHOLOGICAL VARIATIONS IN DURICRUSTS
Studies of duricrusts in profile were undertaken in the valleys immediately south of Letlhakeng village in
Botswana, and also in the Auob Valley in eastern Namibia (Figure 1). At Letlhakeng, duricrusts were
studied in 60 cross-valley transects spaced at either 0.5 or 1 km intervals, dependent upon exposure quality, with all major variations in lithology, morphology and appearance noted, together with estimated thicknesses for individual duricrust units. In the Auob, study was at a reconnaissance level, with exposures studied
mainly on the west side of the valley owing to difficulties of access. In addition to field profile descriptions,
subsequent studies of duricrust thin sections were also undertaken.
Duricrust profiles in the Letlhakeng area
Duricrusts (including calcretes, silcretes and intermediate types) are exposed almost continuously in the
valleys to the south of Letlhakeng village (24"06'S, 25"02'E). Taking outcrops on both valley flanks into consideration, the total length of exposure in the area exceeds 80 km.
Figure 7 demonstrates long-profile variations in duricrust type from 28 transects over an 8 km section of
Letlhakeng Valley 1. Typically, duricrusts are exposed in low cliffs at the top of debris slopes, although only
debris slopes of > 5 m vertically are included in the figure. Within Valley 1 , the majority of duricrusts have a
grain-supported (GS-) fabric (see Summerfield (1983b) for definitions of fabric types), although in places
massive floating (F-) fabric silcretes and conglomeratic (C-) fabric types occur. As noted above, the valley
has an abrupt amphitheatre head, where a profile of massive GS- to F-fabric silcrete up to 7 m thick is
exposed (Transect H on Figure 7). In the valley head area, pseudokarst features are apparent and the silcrete
has a honeycomb appearance with well-developed tubular cavities (Shaw and De Vries, 1988). The profile
500m to the west of this (Transect 1) contains silcrete only on its south side, with a silicified calcrete to
the north. This lack of spatial continuity between profiles and between opposing valley flanks is common
in all transects. The only generalization that can be made is that where silcrete and calcrete occur in the
same profile, the former duricrust type is generally uppermost, with a gradual downwards decrease in silica
content most common.
Despite the lack of correlation between duricrust cementing material in these exposures, there is evidence
of stratigraphy within the calcrete and silcrete host materials. Conglomeratic duricrusts containing wellrounded pebbles of Waterberg Group (SACS, 1980) sandstones, quartzite pebbles and jaspers are common
within the valley. These conglomerates are widely recognized from borehole records throughout the Kalahari as forming part of the basal Kalahari Group sediments (Thomas, 1988), and lie unconformably above
Karoo Ecca Group sandstones and shales in this region (Thomas and Shaw, 1991). The lower 10m of the
310
D. J. NASH, P. A. SHAW AND D. S.G . THOMAS
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DURICRUST DEVELOPMENT
31 1
southern side of Transect 8 (Figure 7) contains conglomeratic duricrusts exhibiting two fining-upwards
sequences, with pebbles fining from 30-40mm at the base of each cycle to <20mm at the top. A pale
pink, often patchy, colouring is common to a number of profiles, usually found in profiles where Waterberg
pebbles also occur. This is most likely due to staining by iron oxides, although whether the Waterberg pebbles have acted as a source of iron is unclear.
Profile studies in two other valleys in the Letlhakeng area confirm the considerable variation in duricrust
lithology identified from Valley 1, both within and between profiles, with no evidence for a common stratigraphic sequence in the Letlhakeng area, except within host materials. For example, Letlhakeng Valley 2
(discussed above with regard to borehole studies) shows little consistency of duricrust distribution, except
a tendency for siliceous duricrusts to be more common towards the valley headwater sections. This lack
of stratigraphy suggests that duricrusts were either formed after the incision of the valleys or have been considerably altered during valley development. The duricrusts studied were highly variable in terms of fabric,
appearance and chemistry, even within individual profiles (confirming the observations of Shaw and De
Vries, 1988). There is no evidence for duricrust displacement by faulting, which, if present, could explain
a lack of correlation. Indeed, the only place where a local fault-line occurs at the surface (from Landsat
imagery interpreted by Mallick et al. (1981)) is in a western tributary valley of Letlhakeng Valley 3, and
no displacement since duricrust formation is evident.
The identification of fining-upwards sequences in Letlhakeng Valley 1 suggests that duricrust host materials did exist prior to valley development, but during (or even after) incision the duricrusts on opposing valley
flanks developed independently as a result of lateral throughputs of groundwater. Studies of groundwater
conditions associated with Australian valley calcretes (Arakel, 1986) indicate that groundwater chemical
characteristics vary significantly within catchments and particularly away from water bodies. In central
and western Australia such variations are associated with diagenetic alteration of duricrusts, particularly
the progressive silicification of calcretes in association with fluctuating water tables (Arakel et af., 1989).
Furthermore, the models of calcrete development discussed above (Mann and Horwitz, 1979; Arakel,
1986) indicate that the development of a calcrete body along a valley axis causes local disruption of groundwater flows as pore space within valley alluvial deposits is replaced by cement. As valley incision at
Letlhakeng progressed, it is possible that different parts of the catchment (and potentially opposing valley
sides) developed individual groundwater environmental conditions. This may have occurred both at the
time of primary duricrust development but also later with further valley incision, leading to diagenetic alteration of existing duricrusts. Such variations in groundwater environment, together with disruption of groundwater flow as duricrusts formed and later diagenetic alteration, would explain the lack of anything but a
general similarity between most opposing valley flanks. It would not, however, preclude the possibility of
correlation between duricrust host materials in different profiles, unless duricrust development was associated with substantial dissolution of parent materials.
Duricrust profiles in the Auob Valley
In contrast to the valleys around Letlhakeng, duricrust outcrops in the Auob Valley (Figure 1) show a
definite stratigraphy over much of the length of the valley between Stampriet in Namibia (24'20'S,
18'25'E) and the confluence with the Nossop Valley in the Northern Cape Province of South Africa
(26'28'S, 20'38'E). In this section, the valley is incised at depths of up to 30 m through the 'Limestone' (calCrete) Plateau (Mabbutt, 1957), with duricrust exposures outcropping at the top of rectilinear debris slopes
that form the valley flanks.
The typical sequence of duricrusts is exposed at Kalkheuval Farm (24"45'45"S, 18"44'25"E),where 5.5 m of
indurated iron-stained calcareous duricrust overlie approximately 3.2 m of a much softer white calcrete
(Figure 8). This vertical cliff of duricrust occurs at the top of a debris slope, 18-20m high, which slopes
at an angle of between 24 and 27". The two duricrust types have differential resistances, indicated by a
marked break of slope at the contact of the two lithologies. The more indurated material forms a vertical
cliff above a 45" slope of softer calcrete partially covered by the debris slope.
Within this simplified sequence of harder and softer duricrust there are, however, other lithological variations. The uppermost 2 m of the exposure are partly silicified, this induration contributing to the resistance of
312
D. J. NASH, P. A. SHAW AND D. S. G. THOMAS
Figure 8. The Auob Valley at Kalkheuval Farm, eastern Namibia (2445'45"S. 18"44'25"E)
the duricrust. The top of the cliff has an almost horizontal surface which, unlike exposures seen at
Letlhakeng, does not exhibit pseudokarst features. The upper plateau surface does, however, contain occasional hollows visible on 1 : 50 000 aerial photography, which may indicate removal of material in solution.
Pebbles of jasper and quartzite (up to 5 mm diameter) are incorporated into the upper sections, with a 30cm
thick conglomerate band occurring at 2.1 m below the top of the profile. Variations in the iron content are
indicated by differences in colour between upper and lower sections. No obvious lineations or bedding structures were evident within the profile.
In addition to occurring along much of the length of the Auob Valley, the stratigraphic sequence of harder
sil-calcrete overlying softer calcrete is also identifiable in directions perpendicular to the main valley axis. In
particular, the sequence can be traced along the Elephants Valley northwards from its confluence with the
Auob at Twee Rivier Farm (25"27'30"S, 19"26'30"E) and for at least 500m away from the main valley in
gullies which dissect the Limestone Plateau. This suggests that the duricrusts forming the plateau predate the present course of the Auob in this region.
Thin-section studies of duricrusts ,from Letlhakeng
A total of 94 samples from eight transects in the Letlhakeng area were studied in thin section; detailed sampling locations are given in Nash (1992). Sampling profiles were in the form of low duricrust cliffs or downslope profiles. All samples consist of a variable percentage of skeletal quartz grains (with additional feldspars,
opaque and other heavy minerals) set in a cement of either calcium carbonate or silica (in the form cryptocrystalline silica, disordered or fibrous chalcedony or microcrystallinc quartz). In addition, a number of diagenetic features are present, particularly silica and carbonate void-fills and evidence of cement replacement.
The micromorphological characteristics of many samples, particularly from profiles in the amphitheatre
valley head of Letlhakeng Valley 1 (24"09'35"S, 25"12'00"E) and on the eastern flank of Letlhakeng Valley 2
(24"08'30"S, 25"05'00~'E),contain evidence that duricrust formation in the Letlhakeng area took place in
association with some form of valley or depression, confirming the implications of studies of dtiricrust
DURICRUST DEVELOPMENT
313
Figure 9. Thin section LET V1 A18 (scale bar is 1.00mm long). GS- to F-fabric silcrete, with quartz grains set in a cryptocrystalline
quartz and disordered chalcedony matrix. Void-fill consists of (inwards) opaline silica to length-fast chalcedony to opaline silica to
length-fast chalcedony to opaline silica to length-fast chalcedony to length-slow chalcedony (with extinction crosses) to microquartz
and megaquartz (crossed polars)
profiles. Firstly, the fabric characteristics of silcretes from the head of Valley 1 (Figure 9) suggest direct silicification of a quartz sand host material, as opposed to passive replacement of either pre-existing bedrock or
calcrete. The GS-fabric silcretes contain no evidence of remnant bedding structures or grain dissolution,
which might be expected if replacement of bedrock had occurred. Furthermore, if the silcrete had developed
by replacement of a calcrete, the formation of which is invariably associated with grain displacement and, to
a lesser extent, dissolution (Wright and Tucker, 1991), a GS-fabric would be unlikely. Replacement silcretes
have been shown to exhibit a number of features, such as inclusions, irregular crystal boundaries, flamboyant extinction and non-competitive growth fabrics (Summerfield, 1978), none of which is present in the silcretes from the head of Valley l .
These factors suggest that the silcretes formed by direct silica precipitation and cementation within a
quartz sand framework. What therefore requires explanation is why over 7 m thickness of silcrete occurs
in a limited area of the valley head, with calcretes occurring immediately up- and down-valley. Cementation
would require an incursion of silica-bearing porewaters focused on this particular area. This would imply a
local hydrologic gradient to allow groundwater movement towards the. area, which would suggest the presence of a basin or depression. It is also possible that forced upwelling of groundwater occurred in this area
(associated with groundwater erosion processes), which could be due to the reduced permeability of preKalahari Group bedrock in this area (Shaw and De Vries, 1988).
Another indication of the development of duricrusts in association with a valley is provided by the presence of freshwater bivalve shells (< 1 mm long) within calcretes in the flanks of Letlhakeng Valley 2
(Figure 10). The valves have been replaced by calcite, making identification of the exact species impossible, but the fact that most shells are intact suggests deposition under relatively still-water conditions.
Furthermore, the presence of intact shells in 18 regularly spaced samples from an 8.7 m profile is indicative
of the maintenance of such conditions for a lengthy period. The deposition of these sediments would imply
314
D. J. NASH, P. A. SHAW AND D. S. G. THOMAS
Figure 10 Thin section LET V2 B23 (scale bar is 1.00mm long). M-fabnc calqrete, with quartz grains and skeletal shell material in a
microcrystalline calcite matrix (crossed polars)
the existence of some form of depression or valley. The calcretes have an authigenic nodular to massive
micromorphology, consistent with that of groundwater calcretes described by Wright and Tucker (1991),
and these fabric characteristics combined with the presence of Shell material suggest formation by a similar
process to the model proposed by Mann and Horwitz (1979).
Void-fill and replacement features within duricrusts provide an indication of the role of fluctuating valley
water tables in duricrust diagenesis in this area. In silcretes, sequences of void-fill (e.g. Figure 9) are consistent with the opaline silica-chalcedony-microquartz-megaquartz sequence identified by Summerfield
(1983b), reflecting both the changing composition of porewaters and the declining rate of flow of porewater
as voids were progressively infilled. Some void-fills also contain microcrystalline carbonate at their centre,
implying a shift in porewater pH at the time of precipitation.
Siliceous void-fills in calcrete profiles are, however, analogous to sequences identified from Australian calcretes (Jacobson et al., 1988; Arakel et al., 1989; Arakel, 1986, 1991) which have undergone diagenesis in
association with a fluctuating valley water table. Arakel et al. (1989) identify silica species and structures
(specifically void-fill and replacement features) that have developed above, below and within the zone of
groundwater fluctuation. These void-fills include alternating bands of fibrous chalcedony and thin layers
of opaline silica, producing a layered cement, with spherulitic chalcedony often filling voids. Replacement
and transformation features occur in many calcrete samples from Letlhakeng (e.g. in the basal sections of
the north side of Transect 2 on Figure 7), with the microcrystalline calcite matrix containing patches of
sparry calcite up to 6 mni in diameter. Replacement of calcite matrix material by disordered chalcedony,
cryptocrystalline silica and microquartz is common in many profiles, also indicating dissolution and precipitation, most probably beneath the water table (Jacobson et a/., 1988).
The specific location of diagenetic alteration of calcrete with respect to water-table levels cannot, however,
be identified at Letlhakeng. The samples in this study are from valley flank exposures as opposed to the
Australian studies which were based upon cores drilled in valley floors. Calcretes exposed at Letlhakeng
DURICRUST DEVELOPMENT
315
will have evolved in association with progressive lowering of the water table as valley incision proceeded.
Any diagenetic alteration will have been overwritten as the valley deepened, with calcretes eventually abandoned above the water table on the valley flanks. Thus whilst features such as the replacement of a calcite
matrix by chalcedonic silica are identified by Arakel et al. (1989) as indicating transformation within the
phreatic zone, their presence in Kalahari samples may be the product of a succession of diagenetic
changes. Although similarities can be identified between samples from Letlhakeng and Australian groundwater calcretes, direct analogies between the zones of silicification should not be drawn.
CONCLUSIONS
Studies of duricrust profiles from the Auob Valley and Letlhakeng present differing results for the overall
relationship between duricrust formation and the development of mekgacha. The remarkable consistency
between duricrust profiles from the Auob suggests that the valley has incised its course through a series
of pre-existing duricrusts. The down-cutting of the Auob does not appear to have generated significant
changes within the duricrust sequences now exposed within its valley flanks, thus validating the use of exposures from this part of the south-western Kalahari in a stratigraphic context. However, studies at Letlhakeng
suggest no such stratigraphy, except within host materials, and indicate considerable formation and alteration of duricrusts in conjunction with a former higher valley water table. Thus, whilst it may be appropriate
to assess lithostratigraphic relationships from duricrusts exposed in valleys in the south-western Kalahari,
studies in other areas, particularly where groundwater has been important in valley development, should
proceed with considerable caution.
The lack of an overall duricrust stratigraphy in the Letlhakeng area may be interpreted in either of two
ways: it may be that duricrusts have formed as a consequence of the presence of a valley (as suggested by
the presence of freshwater shell material within duricrusts in Letlhakeng Valley 2), or that movements of
water towards the valley over long time periods have altered pre-existing duricrust suites to the extent
that any previous stratigraphy is rendered unidentifiable. Clearly, a lack of evidence for stratigraphy does
not necessarily preclude the existence of duricrusts prior to valley incision, but means that careful identification of the mode of origin of duricrusts is required. In either case there is a clear link between the presence of
a valley and the contemporary morphology of duricrust exposures at both profile and micromorphological
scales, thus highlighting the need for recognition of the links between duricrusts and their geomorphological
setting. This potential genetic link between duricrusts and valleys is especially significant in the case of landforms such as the Kalahari valley systems, which are of considerable antiquity (Nash et al., 1994).Their great
age, combined with the evidence for groundwater circulation beneath valley floors and for former higher
water tables, suggests that few duricrusts are likely to have remained unaffected by the action of groundwater.
A direct link between duricrust development and valley evolution through the action of groundwater may
also be of use in establishing absolute ages for Kalahari mekgacha, particularly if the electron spin resonance
technique for dating crushed silcrete samples described by Radtke and Bruckner (1991) could be developed
to allow analysis at a microscopic level. This would enable absolute dating of individual layers of silica
within silcretes exhibiting multiple stages of silicification (admittedly with problems of identifying primary
precipitation and re-solution) and could provide a major contribution to the understanding of the long-term
tectonic and geomorphological development of southern Africa.
In conclusion, it is perhaps unfortunate that the main developments in the understanding of the stratigraphy of the Kalahari Group sediments have taken place from sites at the periphery of the Kalahari depositional basin, with few studies being undertaken in more remote central zones. Whilst parts of the southwestern Kalahari provide the best continuous exposures of these sediments, with a clearly identifiable duricrust stratigraphy underwritten by detailed records from an extensive network of boreholes, it cannot be
assumed that all other exposures away from this region can be easily correlated with such a sequence. If,
as is indicated by the examples included within this study, there is a strong relationship between the distribution of duricrust cementing materials and geomorphological features such as valleys, then it would be advisable to restrict all future studies of Kalahari Group stratigraphy to the characteristics of duricrust host
materials and not duricrust exposures in toto.
316
D. J. NASH, P. A. SHAW AND D. S. G. THOMAS
ACKNOWLEDGEMENTS
The authors would like to express thanks to the following organieations who provided funding for fieldwork:
the University of Sheffield, Palmers College and the Exploratian Fund of the Explorers’ Club, New York
(D.J.N.), the Royal Society (D.J.N., D.S.G.T.) and the University of Botswana (P.A.S.). Sample collection
was also carried out in conjunction with the Sheffield University Botswana Expeditions of 1989 and 1990,
assisted by the Royal Geographical Society, Manchester Geographical Society, Gilchrist Educational Trust
and British Airways plc. Original figures were drawn by Graham Allsopp and Paul Coles of the Department
of Geography, University of Sheffield.
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