Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia

Pergamon Journal of African Earth Sciences, Vol. 27. NO. 1, pp. 1 l-25, 1998

o 1998 Elsevier Science Lfd

Pll:SO899-5382(98)00043-8 All rights reserved. Printed in Great Br!tam

0699.5362/98 519.00 + 0.00

Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of

Botswana and Namibia

DAVID J. NASH and PAUL A. SHAW Earth and Environmental Science Research Unit, University of Brighton, Mithras House,

Lewes Road, Brighton, BN2 4AT, UK Department of Environmental Science, University of Luton, Park Square, Luton,

LUI 3JU, UK

Abstract-Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO,) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade duricrust types from the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses. @ 1998 Elsevier Science Limited.

Resume-Les encroQtements de silcrete-calcrete, form& en surface ou pres de la surface par precipitation chimique d’un ciment comprenant un melange de silice et de CaCO,, ont et6 largement reconnus dans la litterature geologique, geomorphologique et pedologique mais n’ont pas et6 a ce jour decrits ou classes de facon systematique. Le papier presente une revue des definitions precedentes des poles du spectre silcrete-calcrete et identifie ensuite les relations majeures entre silice et carbonates dans des types d’encrofitements du Kalahari de Botswana et de Namibie. Trois types majeurs sont identifies a partir des associations silice-carbonates: encroutements air la silice secondaires apparait dans une matrice calcaire, varietes 00 le carbonate secondaire apparait dans une matrice siliceuse et materiaux 00 les ciments de silice et de carbonates dans la matrice semblent avoir precipite en m&me temps ou tres rapidement I’un apres I’autre. Dans chacun de ces trois groupes, des sous-types sont identifies selon que les materiaux secondaires ont remplace ou ont et6 remplaces dans un encrofitement pre-existant. Enfin, une procedure pratique de definition simple des encroatements de silcrete-calcrete est suggeree a partir de la combinaison des analyses chimiques et de l’examen des lames minces. o 1998 Elsevier Science Limited.

(Received 14 July 1997: revised version received 15 December 1997)

INTRODUCTION The geological, geomorphological and pedological duricrusts when indurated. The most commonly literature contains a plethora of definitions of occurring varieties of duricrust are calcrete the various types of surface and near-surface (where the host material is cemented by CaCO,), chemically precipitated crust, collectively termed silcrete (silica cement), ferricrete (iron oxide

Journal of African Earth Sciences 17

D. J. NASH and P A. SHAW

I

ANGOLA ___/-M--- Victoria Falls 1

w_ ‘, ZIMBABWE

- perennial channels

ephemeral channels - and dry valleys

Tshaboig

0 kms 200 1 I

SOUTH AFRICA 9. s 20-E, 24’ E ,

a alluvial sediments

approximate limit of J-I-L Kalahari Group

sediments

m sample sites

?? major towns

- - - international boundary

Figure I. Locations of sampling sites in Botswana and Namibia, together with the extent of the Kalahari Group sediments.

cement) and gypcrete (gypsum cement) (Goudie, to treat each of the different types of crust as 1973). Of these types, calcrete is the most discrete geochemical entities, partly as an aid geographically widespread, covering -13% of to communication. However, almost all detailed the Earth’s land surface (Yaalon, 1981). studies recognise that duricrusts contain a range

A common feature of the many general of constituents, with, for example, a typical accounts of duricrusts (e.g. Goudie, 1973, 1983; calcrete containing various clay and Fe rich Summerfield, 1983a; Dixon, 1994) is a tendency minerals and secondary silica in addition to the

12 Journal of African Earth Sciences

Silica and carbonate relationships in silcrete-calcrete in tergrade duricrusts from the Kalahari

calcium carbonate cementing agent. In many cases it may be difficult to identify a dominant cement type, and hence a specific duricrust type, particularly where a crust contains almost equal proportions of different cementing agents. This may arise due to a number of factors including co-deposition of minerals, constructional and degradational processes occurring within a pedogenic environment, and post-formational near-surface diagenetic alteration of a pre- existing crust, all of which can lead to the generation of extremely complex intergrade duricrust forms.

The most widely reported suite of intergrade duricrusts are those varieties in which the cementing agent is a mixture of silica and CaCO,, termed ‘silcrete-calcrete intergrade duricrusts’ in this study. Silcrete-calcrete intergrade duricrusts have been mentioned in studies of silcretes and calcretes from various parts of the world, including North America (Price, 1933; Sidwell, 1943; Brown, 1956; Swineford and Franks, 1959; Aristarain, 1970; Reeves, 1970, 1976; Hay and Wiggins, 1980; Vaniman et al., 19941, South America (King, 19671, southern Africa (Wright, 1978; Watts, 1980; Summerfield, 1982, 1983a; Shaw and de Vries, 1988; Nash et a/., 1994a, b), North Africa (Smith and Whalley, 1982; Thiry and Ben Brahim, 1990) and Australia (Arakel, 1986; Arakel et a/., 1989). The wide variety of silcrete-calcrete intergrade duricrusts can frequently lead to considerable difficulty in distinguishing between silcretes, cal- silcretes, sil-calcretes and calcretes, particularly when mapping duricrusts in the field. Shaw and de Vries (1988) and Nash et a/. (1994a, b), for example, record not only calcretes and silcretes exposed in the flanks of valleys around Letlhakeng village in southeastern Botswana, but also partially calcified silcretes and partially silicified calcretes, with combinations of these intergrade duricrusts types often occurring within the same profile.

Mazor et a/. (1977, p.208) highlight this problem in a study of duricrusts from northern Botswana:

“As both end-member types [silcrete and calcrete] have the same whitish colour and mode of occurrence in the field, and as mixed complexes are common, we suggest the use of the term ‘Crete’ for the whole group and the terms calcrete and silcrete to be reserved for chemically analysed clear-cut cases”.

Clearly, defining all silcrete-calcrete intergrade duricrusts as ‘cretes’ is a far from ideal solution, as without an understanding of the nature of a

particular duricrust it is difficult, for example, to ascertain the conditions leading to its formation and hence the environmental significance of the material. As a first step towards assessing the variety of intergrade duricrust types, this study considers the characteristics of intergrade calcretes and silcretes from the Kalahari Group sediments of central southern Africa. Samples are selected from a variety of locations across the Kalahari Region previously studied by the authors (Fig. I), where duricrust outcrops occur in the floor or flanks of pans and dry valleys (Shaw and de Vries, 1988; Thomas and Shaw, 1991; Shaw et al., 1991, 1992; Nash et al., 1994a, b; Shaw and Nash, in press). Chemical analyses and inspection of petrographic thin- sections of indurated horizons are used in order to identify the major silica-carbonate relationships within duricrust cements. These relationships are subsequently used to provide a framework for the classification of different varieties of silcrete-calcrete intergrade duricrusts.

PREVIOUS DEFINITIONS OF ‘CALCRETE’ AND ‘SILCRETE’

Before attempting to identify different intergrade duricrust types from the Kalahari it is necessary to outline both the general and geochemical definitions which have been put forward for the end members of the silcrete-calcrete spectrum. The term ‘calcrete’ was first coined by G. W. Lamplugh in (19021, to describe CaCO, cemented gravels in the Bay of Dublin, with ‘silcrete’ also introduced to provide a basic terminology for silica rich crusts (Lamplugh, 1902, 1907). ‘Duricrust’ was put forward by Woolnough (1927, 1930) as an all-embracing term to describe the full range of calcrete, silcrete, ferricrete and alcrete crusts recognised at the time. Magnesicretes, gypcretes, phoscretes and dolocretes were subsequently identified (Goudie, 1973).

Calcrete General descriptions of calcretes are provided by, amongst others, Netterberg (19691, Goudie (1973, 19831, Watts (19801, Milnes and Hutton (19831, Machette (19851, Wright and Tucker (19911, Dixon (1994) and Watson and Nash (1997). The most useful definition of a calcrete is that provided by Wright and Tucker (1991, p.l), based upon definitions given in Goudie (1973) and Watts (19801, as it stresses the formational processes and hydrological setting of carbonate accumulation:



“Calcrete is a near surface, terrestrial, accumulation of predominantly calcium carbonate, which occurs in a variety of forms from powdery to nodular to highly indurated. It results from the cementation and displacive and replacive introduction of calcium carbonate into soil profiles, bedrock and sediments, in areas where vadose and shallow phreatic groundwaters become saturated with respect to calcium carbonate”

Despite the large number of published analyses of calcareous duricrusts there have been few attempts to chemically define calcretes; the majority of studies simply provide details of bulk chemical composition and descriptions of mineralogy with little attempt at classification. One of the earliest attempts to define a global chemical composition for calcrete was by Goudie (1972, 1973) who found the mean composition of world calcretes from analyses of over 300 samples to be 79.28% CaCO, (42.62% CaO), 12.30% SiO,, 3.05% MgO, 2.12% AI,O, and 2.03% Fe,O,. These mean figures do, however, mask the considerable spatial and within-profile variability in calcrete composition (Dixon, 1994). For example, Goudie (1973) notes that levels of carbonate in Indian calcretes are lower than the global average whilst Australian samples are frequently much higher. Aristarain (1970) and Dixon (1994) suggest that North American calcretes also differ from the global mean with on average 34.93% CaO, 30.68% SiO,, 1.81% MgO, 1.88% AI,O, and 1.00% Fe,O,. Such variability can be attributed to three main factors. Firstly, the relative proportions of carbonate and silica will vary between individual samples dependent upon the percentage of skeletal quartz grains and other host materials within the calcrete. Secondly, variability can occur within and between individual profiles dependent upon the types of calcrete present and the degree of induration (Watts, 1980; Milnes and Hutton, 1983; Netterberg, 1980); powder calcretes generally contain the lowest proportion of CaCO, whilst hardpan varieties have a much higher CaCO, content. Thirdly, the percentage of dolomite [CaMg(CO,),l present within a calcrete will influence its bulk chemistry, with Netterberg (1980) suggesting that only duricrusts containing less than 5% dolomite by mass of total carbonates be termed calcrete. There is also a considerable lack of consistency in defining the chemical boundary distinguishing calcretes from other duricrust types. Dixon (1994, Table 5.5) includes chemical analyses of Australian calcretes with CaO contents as


low as 9.68%; the same sample contains 63.12% SiO,, placing it nearer to a silcrete in terms of bulk chemistry.

Silcrete There have been fewer accounts of siliceous duricrusts compared to those of calcretes, mainly owing to the less widespread occurrence of silcrete on a global scale. The most useful overviews include those of Frankel and Kent (1937), Goudie (19731, Langford-Smith (19781, Summerfield (1983a), Milnes and Thiry (1992) and Watson and Nash (1997). However, despite their relatively limited distribution compared to calcrete, silcretes have been more precisely defined.

Siliceous duricrusts are termed silcrete sensu strict0 when they contain at least 85% silica (Summerfield, 1983a) with particularly pure silcretes containing in excess of 95% silica. They can be distinguished from orthoquartzites in that they generally exhibit a porphyroclastic as opposed to a more even-grained texture (Hutton et a/., 1978). Silcrete is regarded as a product of the cementation or replacement of surficial materials such as rocks, sediments, saprolite or soils by various forms of secondary silica, including opal, cryptocrystalline quartz or well- crystallised quartz (Milnes and Thiry, 1992). This silicification is not associated with high- temperature volcanism or metamorphism but is a low temperature physico-chemical process (Summerfield, 1979, 1981, 1983a, b; Milnes and Thiry, 1992).

A number of chemical analyses of silcretes have been published which demonstrate the essentially limited variability of silcrete bulk chemistry (e.g. Callender, 1978; van Dijk and Beckmann, 1978; Wopfner, 1978; Summerfield, 1982; 1983d; Nash et a/., 1994b3. The most ‘pure’ silcrete reported to date is from the northern part of the Ruhr Deposit at Nerriga, New South Wales, Australia, containing 99.49 SiO,, 0.07% TiO,, 0.14% AI,O,, 0.07% Fe,O,, 0.01% Na,O and 0.02% K,O (Callender, 19781, but silica contents as low as 82.00% have been reported for a ferruginous palaeosurface ‘silcrete’ on the Dalhousie Anticline, Australia (Wopfner, 1978). The other major geochemical variation apart from silica content is due to variable amounts of TiO, within silcretes associated with colloform structures and geopetal drapes within the duricrust fabric (e.g. Summerfield, 1983a, b, c, d; Milnes and Thiry, 1992). These features are usually enriched in anatase (TiO,) (Smale, 1978; Watts, 1978) and often ZrSiO,, and may

Silica and carbonate relationships in silcrete-calcrete in tergrade duricrusts from the Kalahari

Figure 2. Siliceous void fill within sil-calcrete from the eastern flank of the Auob Valley at Kalkheuval Farm, near Gochas, Namibia. The void fill shows a sequence of disordered chalcedony ICj and quartz (01 at the centre of the void. Note that the calcite matrix in the vicinity of the void is partially replaced by silica, but that replacement is limited to approximately 1 mm from the void wall (sample DN AUOB 115: crossed-polarised light).

contain l-3% or more anatase, whilst silcretes without colloform structures commonly contain less than 0.2% anatase (Hutton et a/., 1972; Summerfield, 1979, 1983a, b, c).

VARIETIES OF SILCRETE-CALCRETE INTERGRADE DURICRUST IN THE KALAHARI

The major recent reviews of duricrusts (e.g. Goudie, 1983; Summerfield, 1983a; Milnes and Thiry, 1992; Dixon, 1994; Watson and Nash, 1997) recognise secondary silica within calcrete as the most important, if not only, silica- carbonate association commonly occurring within duricrusts. However, thin-section and geochemical analyses of Kalahari duricrusts included within this study, together with a review of the results of previous studies of duricrust micromorphology and geochemistry (e.g. Watts, 1980; Summerfield, 1982; Shaw and de Vries, 1988; Shaw et al., 1991; Arakel et al., 1989; Thomas and Shaw, 1991; Nash et a/., 1994a, b), suggest that there are three main silcrete- calcrete intergrade duricrust types. These can be grouped into duricrusts where secondary silica occurs within a primary calcareous matrix,

varieties where secondary carbonate occurs within a dominantly siliceous matrix, and crusts where the precipitation of carbonate and silica appears to have been contemporaneous. Within each of these types there are a variety of silica- carbonate associations which will now be discussed in turn with reference to examples from the Kalahari.

Duricrusts containing secondary silica within a calcareous matrix Silicification of calcretes is widespread in the Kalahari and appears to be an integral part of the process of calcrete formation, as silica is released when calcite replaces silicate minerals (Walker, 1960; Watts, 1980). The inverse solubility of calcite and silica is well-documented (see Goudie, 1983, for a review); high pH values (>pH 9) favour the precipitation of calcite and the solution of silica, whilst the reverse occurs at low pH values ( <pH 9). Hence, environments where carbonate is being precipitated are also locations where silica solution is commonplace, thus generating silica rich porewaters.

In general, it appears that more strongly indurated pedogenic calcretes (in the sense of

Journal of Afrkan Earth Sciences 15


Table 1. Bulk chemistry of silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia

Concentration in Weight %

Sample Location SiO9 Ti02 AI,Os FesOs MnO MgO CaO NasO K20 p205 so3 LO1 Total

DN AUOS 115 Kalkheuval 31.26 0.14 2.30 1.04 0.02 2.06 35.63 co.15 0.50 0.04 <0.012 27.62 100.64

PSI6711 a Nata River 69.32 0.06 1.53 0.34 0.01 0.63 6.33 0.62 0.87 <0.0023 co.012 99.71

DN OKWA 117 Tswaane 95.29 0.09 0.39 0.20 0.01 2.96 0.14 0.47 0.23 ~0.0023 co.012 99.78

PSl94l9a Nbvetwe Parr 46.64 0.05 1.54 0.69 0.02 2.86 27.66 <0.15 1.00 0.02 <0.012 20.36 100.78

PS/94/10 Ntwetwe Pan 85.38 0.06 2.23 1.07 0.01 1.63 6.49 0.56 1.56 <0.0023 <0.012 98.99

PSi8716c Sowa Pit 91.66 0.12 1.38 0.36 0.01 0.55 3.80 1.09 0.74 co.0023 co.012 99.71

PSi94l18 Sore Junction 94.44 0.04 1.00 0.42 0.01 0.85 1.76 0.71 0.53 <0.0023 co.012 99.76

Analysis by Energy Dispersive Polarised X-ray Fluorescence, with loss on ignition (LOI) analysed at 1 OOO°C. See the text for descriptions of sample locations.

the classifications of Gile et al., 1966; Bachmann and Machette, 1977; Machette, 1985) have higher silica contents which may represent part of an evolutionary sequence of crust maturation (Milnes and Hutton, 1983). During progressive calcretisation it would be expected that porewaters would become increasingly saturated with silica unless they were able to drain away, thus increasing the likelihood of silicification. There is, however, some disagreement over where silicification most commonly occurs within the calcrete profile. Reeves (1970) and Watts (1980) identify silica precipitation in the lower parts of pedogenic calcretes in the USA and Kalahari, respectively, Nash et al. (I 994a) record a tendency for the upper parts of calcrete profiles exposed in the flanks of valleys in Botswana and Namibia to be more strongly silicified, and Arakel et a/. (1989) note silicification throughout calcrete profiles developed within Australian palaeodrainages. Perhaps the best overall conclusion is that of Summerfield (1982) who suggests that occurrences of silicified zones within Kalahari calcretes are largely unpredictable.

There are three main occurrences of authigenic silica within Kalahari calcretes. The most widely documented form of silicification is where chalcedonic and opaline silica and quartz occur as late stage void fills, but there may also be replacement of carbonate by silica along the margins of voids, as well as more pervasive replacement of the matrix in the absence of voids. Silica void and vein fills are common in Kalahari calcretes and exhibit well documented sequences of authigenic silica precipitation.

Voids are most commonly lined by a thin (<5 ,um) opaline silica layer, followed by a zone of chalcedonic silica (40-90pm) which may include both length-fast and length-slow species, with a palisade or euhedral quartz fill which may partially or completely fill the remaining void space (Watts, 1980). This is illustrated in Fig. 2, which depicts the micromorphology of a silicified pedogenic calcrete from the Auob Valley in eastern Namibia (sample DN AUOB 115). This sample contains 31.26% SiO, (Table 11, which can be mostly attributed to secondary silica as the calcrete contains a relatively low proportion (approximately 5%) of quartz grain host material. Similar sequences of silica have been recognised in vadose calcretes in Australia (Arakel et al., 1989) where silicification also proceeds in association with dissolution, brecciation and recrystallisation of calcretes.

Replacement of the calcite matrix along the margins of voids or veins is commonplace (also seen in Fig. 2) due to dissolution along void boundaries. Similar silicification features have been noted in Tertiary lacustrine limestones in the Paris Basin (Ribet and Thiry, 1990) where matrix epigenesis is restricted to the walls of voids and progressively diminishes with distance from the void or vein. Ribet and Thiry (1990) note a tendency for replacement to be greatest along the margins of major voids or veins as they would act as conduits for the greatest flow of silica bearing water through the material. This is less obviously apparent in many Kalahari calcretes, which contain a higher percentage of silica compared to the relatively pure lacustrine limestones (due to the incorporation of silica rich


Silica and carbonate relationships in silcre te-calcre te in tergrade duricrus ts from the Kalahari

Figure 3. Almost completely silicified calcrete from the mouth of the Nata River in Botswana. Note the opaline and length-fast chalcedonic void linings andgrain coatings, together with evidence of replacement of the calcite matrix in the vicinity of the void. One void WI contains a sequence of opaline silica, length-fast chalcedony with calcite at its core (sample PS/87/la: crossed-polarised light).

host materials) and are therefore less reliant upon external silica sources.

The third main occurrence of secondary silica in Kalahari calcretes is where silica has replaced the carbonate matrix material in the absence of voids. For example, widespread matrix epigenesis occurs in pan silcretes from the Nata River in Botswana (sample PS/87/la; Fig. 3) which have, on the basis of micromorphological evidence such as inherited glaebular structures, developed by the almost complete replacement of a pre-existing calcrete. The extent to which replacement has taken place is illustrated by the bulk chemistry of this sample which comprises 89.32% SiO, and only 6.33% CaO (Table I). Figure 2 also depicts other evidence of widespread silicification including chalcedonic and opaline silica void linings and replacement of the calcite matrix in the vicinity of voids. Similar replacement features have been described within the phreatic zone of Australian palaeodrainage calcretes by Arakel et a/. (19891, where spherulitic diagenetic chalcedony has replaced microcrystalline calcite. Diagenetic silica is also noted as randomly distributed cryptocrystalline masses within the calcite matrix of these Australian calcretes. In the case of Kalahari calcretes, pervasive silicification almost invariably occurs in combination with the other forms of silica enrichment described above.

Duricrusts containing secondary carbonate within a siliceous matrix Secondary calcite within silcretes is much less widely documented on a global scale, although Nash et al. (I 994b3 describe Kalahari silcretes which contain numerous inclusions of carbonate, mostly within voids. As in the case of silicified calcretes there appear to be three main types of carbonate occurrence within Kalahari silcretes; carbonate as a late stage void or vein fill, carbonate replacement of the silcrete matrix in the vicinity of voids or veins, and epigenesis of the silcrete matrix away from voids or veins.

The most common occurrence of carbonate within Kalahari silcretes is in the form of void- fills or veins, frequently associated with brecciation of a pre-existing duricrust. This can be seen in Fig. 4 where calcite and dolomite have been precipitated as a late stage fill overlying opaline silica within a void in a silcrete from near Tswaane borehole in the Okwa Valley, Botswana (sample DN OKWA 117). The silcrete is relatively pure, containing 95.29% SiO, (Table 11, but with approximately 3% carbonate present. Figures 5 and 6 (samples PS/94/9a and PS/94/10 from Ntwetwe Pan, Botswana) show multiple generations of calcite veins within silcretes. In these samples, calcite emplacement has brecciated both the matrix and host grains, and in some places dissects previous calcite veins. Sample PS/94/9a is more extensively


D. J. NASH and i? A. SHAW

Figure 4. Grain-supported to floating fabric silcrete from east of the Tswaane Borehole in the Okwa Valley, Botswana. Note the void fill IV) near the centre of the micrograph with evidence of partial dissolution, alteration of the void walls and calcite at the void centre (sample DN OKWA 1176: crossed-polarised light).

Figure 5. Breccia ted Cal-silcrete from Ntwetwe Pan, Botswana. Both the silcrete matrix IM) and skeletal quartz grains (Q) have been brecciated during displacive calcite emplacement /Cl (sample PS/94/9a: crossed-polarised light).

calcified than PS/94/10, as reflected in the bulk processes accompanied by diffusion of the chemistry in Table 1. In all these cases, dissolved silica has allowed calcite to invade carbonate rich porewaters appear to have the silcrete matrix. In contrast to the examples circulated through a pre-existing, fully-indurated of the silicification of calcretes described silcrete. above, replacement of the silcrete matrix by

Replacement of the siliceous matrix by carbonate is much more extensive along the calcite in the vicinity of veins can be seen margins of major voids as opposed to minor near the centre of Fig. 7 (sample PS/94/9a cracks. This is presumably due to the fact that from Ntwetwe Pan) where dissolution all carbonate must have originated from


Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari

Figure 6. Brecciated cal-silcrete from Ntwetwe Pan, Botswana, showing multiple generations of brecciation and calcitic vein fills /Cl (sample PS/94/1 Oa: crossed- polarised light).

Figure 7. Cal-silcrete from Ntwetwe Pan, Botswana, showing brecciation by late stage calcitic vein fills (C) together with matrix epigenesis (MI near veins in the centre of the micrograph (sample PS/94/9a: crossed-polarised light).

outside the silcrete body. The degree of replacement appears therefore to be largely governed by the ability of carbonate rich porewaters to interact with the void walls and the ability of any dissolved silica to be removed, both of which are controlled by the rate of movement of porewater through the silcrete.

The fact that the rate of porewater movement may control the extent of replacement explains

the general absence of matrix epigenesis away from voids or veins in well-cemented Kalahari silcretes. Figure 8 (sample PS/94/10 from Ntwetwe Pan) shows a rare example of a silcrete in which the siliceous matrix has been extensively replaced by carbonate; as in the case of silicified calcretes, epigenesis most commonly occurs in conjunction with the other diagenetic features previously described.



Figure 8. Cal-silcrete from Ntwe twe Pan, Botswana, showing widespread matrix epigenesis MIMI to calcite away from voids or veins ICI Isample PS/94/1 Obr crossed- polarised light).

Figure 9. Complex silcre te-calcre te in tergrade duricrus t from So wa Pit at the northern end of Sowa Pan, Botswana. The sample includes a fine quartz host material 101 cemented by in terla yered silica lSI and calcite /Cl (sample PS/87/6c: crossed-polarised ligh tl.

Duricrusts with contemporaneous silica and carbonate precipitation Duricrusts in which both silica and CaCO, appear to have been precipitated either contemporaneously or in close succession are much less common but have been reported from the Kalahari by Shaw et a/. (1991) and Harrison and Shaw (1995). Intergrade laminar duricrusts comprising detrital quartz grains within a matrix containing,

in places, almost equal proportions of carbonate and silica occur in the vicinity of Sowa Pit, a hyper-saline brine exploration trench in the floor of Sowa Pan in Botswana. These crusts occur in association with silcretes (previously described by Shaw et a/., 7 991; Harrison and Shaw, 1995) which are thought to be forming at the present day due to silica fixation by cyanobacteria. Intergrade duricrusts are mostly found on the

20 Journalof African Earth Sciences

Silica and carbonate relationships in silcre te-calcre te in tergrade duricrus ts from the Kalahari

Figure 10. Pisolithic silcrete from a core drilled through the floor of the Thamalakane River Valley in Botswana at its junc tion with the Boro River. The pisolith core comprises disordered chalcedony ID) and cryptocrystalline silica surrounded b y alterna ring layers of silica and rhombohedral calcite crystals ICI (sample PS/94/18: crossed-polarised ligh tl.

pan surface with in situ material restricted to the upper 10-l 5 cm of the pan sediments. Typical samples viewed in thin-section consist of wavy laminae of microcrystalline calcite interlayered with cryptocrystalline silica (sample PS/87/6c; Fig. 9). The bulk chemistry of a typical sample (PS/87/6c in Table 1) indicates that the material contains 91.66% SiO, and only 3.80% CaO, but it should be noted that the sample contains in excess of 35% quartz grains which will influence the overall silica content.

These crusts, which are morphologically similar to the siliceous duricrusts described by Shaw et al. (19911, also appear to be deposited in association with cyanobacteria. However, the most probable explanation for their formation is through lithification processes operating at and above the water table as a result of capillary rise within the vadose zone. The alternating siliceous and calcareous matrix materials appear to have been precipitated via a combination of evaporation and changes in pH and salinity levels in the pan surface sediments. Monitoring of the water chemistry of the brine in Sowa Pit indicates that the water pH typically fluctuates around pH 9.0 (Shaw et a/., 19911, the critical level of alkalinity governing whether silica or carbonate is precipitated (Watts, 1980; Goudie, 1983; Summerfield, 1983a). Influxes of fresh water to the pan surface could temporarily lower pH levels and allow precipitation of silica. It is also

likely that such fluctuations in pH, combined with a rise in water table levels, could lead to the dissolution of pre-existing crusts. Silica rich solutions are also known to be stable in the presence of carbonates, but highly unstable (i.e. liable to precipitate silica) in high salinity environments (Watts, 1980). Thus, any influx of water onto the pan surface would also encourage dissolution of salts within sediments and promote the precipitation of silica. As such, the localised occurrence of this particular type of duricrust may be due to the specific hydrological and hydrochemical setting.

Duricrusts deposited at the distal end of the Okavango Delta provide a second, similar, example of where silica and carbonate appear to have been precipitated contemporaneously within an individual duricrust (Shaw and Nash, in press). Duricrusts from cores drilled in the floor of the Thamalakane River, Botswana, at its junction with the Boro River, include pisoliths containing concentric laminae produced by alternating phases of silica and CaCO, deposition (sample PS/94/18; Fig. 10). The core of each pisolith comprises disordered length-fast chalcedony surrounded by cryptocrystalline silica. Alternating with silica rich zones are concentric bands of rhombohedral calcite crystals. The pisoliths are cemented by further cryptocrystalline silica to form a densely indurated silica-rich crust. The pisoliths contain

Journal of African Earth Sciences 2 1

D. J. NASH and /? A. SHAW

94.44% SiO, and only 1.76% CaO, suggesting that carbonate forms a minor part of the silcrete. The micromorphology of the pisoliths within the silcrete beneath the Thamalakane River suggests that, like the laminated crusts from Sowa Pan, the duricrust formed as a result of fluctuating pH conditions. However, in contrast to the surface samples described from Sowa Pit, the alternating hydrochemical conditions beneath the floor of the Thamalakane occur as a result of changes in the level of the water table within the channel in response to periodic flooding of the valley. A number of levels of pisolithic hybrid crust can be identified within the cores, reflecting the changing position of the vadose/phreatic boundary through time.

DEFINING SILCRETE-CALCRETE INTERGRADE DURICRUST TYPES

The above descriptions of the three main silica- carbonate associations found within the Kalahari intergrade duricrusts illustrate the great variety of materials within the silcrete-calcrete spectrum. The range of samples included within this study also highlight the difficulties of attempting to define any duricrust in terms of its bulk chemical composition. Definition of a material as a silcrete is relatively straightforward if it has a porphyroclastic texture and contains more than 85% silica (Hutton et a/., 1978; Summerfield, 1983a). However, no such chemical or petrographic definition exists for calcrete; materials are recognised as calcretes simply if they appear to be surface or near-surface crusts where a host material is cemented by or enriched in calcium carbonate. Only where a material is relatively pure CaCO, could a geochemical cut- off point be identified.

Clearly, before a definition of silcrete-calcrete intergrade duricrusts can be identified it is necessary to have a simple and consistent classification of the end members of the silcrete- calcrete spectrum. Ideally, this classification should not be based upon bulk chemistry, as has frequently been the case in the past, but should centre upon the chemistry of the matrix cementing agent. The underlying problem of the classification of duricrusts on the basis of bulk chemistry is that analysis masks the contribution of the volume and type of host material to the total duricrust composition. Many siliceous duricrusts may have a silica content of over 95% but in the case of a silcrete with a grain- supported fabric this may be made up of a high percentage of skeletal quartz. The problem is

2.2 Journal of African Earrh Sciences

even more pronounced in calcretes, where variations in the amount of silica may reflect both the skeletal grain content and also the presence of detrital and authigenic clay minerals, in addition to masking any post-formational silicification. However, whilst the analysis of matrix composition alone is an ideal situation, it may not be possible to separate the matrix from the host material in well-indurated duricrust types. In such cases, assessing matrix chemistry may only be possible through the use of elemental mapping of thin- or polished-sections using instruments such as EDAX or via the time- consuming process of point-count analysis of thin-sections. For practical purposes it may be simpler to continue the identification of a silcrete as a crust containing more than 85% silica on the basis of bulk chemical analysis (after Summerfield, 1983a) and introduce an arbitrary boundary of 50% CaCO, above which a duricrust is considered a calcrete.

Having established a simple definition of the end members of the silcrete-calcrete spectrum it should now be possible to define intermediate types. The samples described in this and other studies include situations where both silica and carbonate have replaced and/or have been emplaced within pre-existing duricrusts. In such cases, chemical analyses alone would only provide an indication of the percentage of carbonate and silica and could not be used to identify the direction of evolution if the material has been diagenetically altered. In practise, when chemical analysis demonstrates that a sample falls between a ‘silcrete’ or ‘calcrete’ it would appear better to examine the micromorphological characteristics of the duricrust and identify any evidence of replacement or emplacement in order to ascertain its exact type. The three major groupings outlined above provide a readily applicable basis for subsequent definition, which can then be combined with quantitative geochemical data. For example, where a calcrete is partially silicified, with silica emplacement in void spaces or replacement of a carbonate matrix, then it should be referred to as a sil- calcrete. Likewise, a material which was originally a pure silcrete but which has been enriched or partially replaced by carbonate should be referred to as a cal-silcrete.

The use of this classification scheme can be illustrated with reference to two of the duricrusts described in this study. For example, duricrust sample DN AU06 115 from the Auob Valley in Namibia comprises 31.26% silica and in excess of 60% CaCO, (Table I), which suggests that it


should be classified as a calcrete. Analysis of the duricrust in thin-section indicates that it consists of a calcareous matrix which contains numerous siliceous void fills, suggesting that it should be more correctly termed a sil-calcrete in order to reflect the secondary silicification of a pre-existing calcrete. In contrast, sample PSI 94/9a from Ntwetwe Pan in Botswana contains 46.64% silica and less than 50% CaCO,, which makes it difficult to define as either a silcrete or calcrete. Thin-section analysis of the sample, however, reveals that it is brecciated by veins of calcite and contains evidence of extensive calcite replacement of a pre-existing siliceous matrix. As such, it should be described as a cal- silcrete.

Difficulties with applying such a classification scheme occur in cases where almost complete replacement of a pre-existing duricrust has occurred and, in particular, where a duricrust has a complex evolutionary history. Examples of silcretes developed by the almost total replacement of a pre-existing calcrete have been previously reported, including silcrete lenses developed within calcretes in the Molopo Valley on the Botswana/South Africa border (Summerfield, 1982). In this case the pre-existing duricrust has been replaced to the extent that the original cementing agent no longer comprises more than half of the duricrust matrix. Such a sample would be termed a silcrete following the classification of Summerfield (1983a) as it comprises 90.70% SiO, and only 0.14% CaCO,. However, were the silica content less than 85% it would be more appropriate to describe the material as a sil-calcrete following the classification procedure suggested in this paper, and as illustrated for sample PS/94/9a above. This would more fully reflect the direction of diagenesis from a calcareous material towards a silcrete.

Nash et al. (1994b) describe siliceous duricrusts from the Okwa Valley in Botswana which have even more complex formational histories, having apparently formed in four stages. A pre-existing calcrete was silicified by the replacement of the carbonate cement. This siliceous duricrust was then partially calcretised with displacive brecciation occurring as a result of carbonate replacing the silica cement. Finally, chalcedonic and quartzitic voids fills were deposited with microcrystalline calcite deposited at the centre of some void spaces. The resultant duricrust profile includes materials which contain between 93.17 and 87.05% SiO, and 3.78 to 0.02% CaCO,, all of which would again be

defined as silcretes by Summerfield (1983a). In this case it is probably simpler to recognise the material as a silcrete whilst acknowledging its complexity. The greatest difficulty occurs in materials such as the duricrusts from Sowa Pit (Fig. 9) where silica and carbonate cements occur in relatively equal proportions and appear to have been deposited contemporaneously. In such cases, where there does not appear to have been any replacement or emplacement of cementing material, it may be more appropriate to define the material simply on the basis of the percentage of silica and CaCO, present within the matrix.

CONCLUSIONS

This study of silcrete-calcrete intergrade duricrusts has identified three major intergrade types on the basis of silica-carbonate associations within the matrix of duricrusts from the Kalahari. Silcrete-calcrete intergrade duricrusts can be subdivided into those where secondary silica is present within a calcareous duricrust matrix, varieties where secondary carbonate is present within a siliceous matrix, and those where silica and carbonate appear to have precipitated contemporaneously to form a duricrust cement.

On the basis of these associations, the following procedure is suggested for the practical and relatively simple identification of duricrust varieties within the silcrete-calcrete spectrum:

il Bulk chemical analysis of the duricrust, using, for example, X-ray fluorescence techniques, should be undertaken in order to ascertain whether a crust is readily classifiable as a silcrete or calcrete. If the duricrust contains more than 85% silica it should be defined as a silcrete, whereas if it contains more than 50% CaCO, it is a calcrete.

ii) If chemical analysis demonstrates that the sample falls into an intermediate category, then the duricrust should be carefully analysed in thin- section in order to identify the relationship between silica and carbonate within the matrix.

fii) The sample should be placed into one of the three categories identified within this paper, i.e. whether it is a calcrete containing secondary silica, a silcrete containing secondary calcite, or a duricrust in which silicic and calcitic cements appear to have been contemporaneously deposited. If the material falls into the former two categories it can be defined as either a cal- silcrete or a sil-calcrete, respectively. If it falls into the latter category then the material can be

0. J. NASH and l? A. SHAW

classified according to whether silica or calcium carbonate are dominant within the matrix.

iv) If petrographic analysis indicates that the material has a complex developmental history and it falls into more than one of the above categories, then it is more appropriate to classify it on the basis of the chemistry of the matrix cement.

ACKNOWLEDGEMENTS

The samples included within this study were collected during the period 1987 to 1995, during the course of individual fieldwork by DN and PS and as part of the 1989 and 1990 Sheffield University Botswana Expeditions and the 1994 Gilchrist Expedition. Funding and support was generously provided by a variety of bodies, including the following organisations: the Gilchrist Educational Trust, the Royal Geographical Society, the Manchester Geographical Society, the Royal Society, the Explorers’ Club (New York), the British Geomorphological Research Group, Barclays Bank plc, Tate and Lyle plc, British Airways plc, Palmer’s Sixth Form College Trust and the Universities of Brighton, Luton, Sheffield and Botswana. All are thanked for their support. Editorial Handling - G. W. McNeil.

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Documents

Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia