Micromorphology of sepiolite occurrences in recent lacustrine deposits affected by soil development.
Sepiolite ([Mg.sub.4][Si.sub.6][O.sub.15][(OH).sub.2].6[H.sub.2]O) and palygorskite ([(Mg, A1)].sub.2] [Si.sub.4][O.sub.10](OH).4[H.sub.2]O) are common clay minerals in soils and surface sediments in semi-arid regions, where they occur in conditions that favour authigenesis of magnesium silicates and the preservation of inherited occurrences (Singer 2002). Although micromorphological characteristics of sepiolite and palygorskite are highly relevant for understanding the formation and evolution of these minerals in earth surface conditions, they are only poorly documented in the literature, mainly through studies that discuss only a single type of feature (Mees 2010).
The present study deals with sepiolite in pan deposits of the south-western Kalahari, as part of a wider investigation of distribution patterns of authigenic minerals in lake basins of the Aminuis region in eastern Namibia. Two basins of this group are known to contain abundant sepiolite (Mees 1999a, 2001), which is also the case for palustrine calcareous deposits in an area to the north of this region (Kautz and Porada 1976; Mees 1999b, 2002). In the course of a study of the deposits of a larger number of pan basins near Aminuis, thin-section observations demonstrated that sepiolite widely occurs in the form of various pedofeatures, varying in nature and abundance within and between profiles or basins. The aim of the present study is to discuss the significance of the micromorphological features of these sepiolite occurrences, as a contribution to understanding the development of sepiolite-rich surface formations.
The Aminuis region of the south-western Kalahari in eastern Namibia has a high concentration of dry lake basins (pans), whose development is related to disruption of a palaeodrainage system that is part of the Nossob River basin (Lancaster 1986) (Fig. 1). Surface deposits in the area consist of reddish Kalahari sands, which overlie silicified Cenozoic limestone in at least part of the region. These formations cover a pre-Cenozoic Karroo substrate consisting of shales with dolerite
sills (Geological Survey Namibia 1979), the latter appearing in outcrop around the Omongwa, Toasis, and Okeriko pans. The pans have previously been documented to contain unconsolidated lacustrine deposits in some basins (Mees 1999a, 2001). Radiocarbon dating of spring-related calcareous deposits in the Omongwa and Otjimaruru basins (Lancaster 1989) provides a minimum age of ~33 ka for pan-associated sedimentation in the study area. The present climate is semi-arid (300 mm average annual rainfall, 3000 mm potential evaporation; Thomas and Shaw 1991). Climatic conditions in the past varied in the course of successive arid and humid stages during the Late Pleistocene and Holocene (Thomas and Shaw 2002; Heine 2005), but no correlation is possible between these stages and those recorded by the deposits described in the present report.
Material and methods
Pan deposits were sampled in nine basins (Fig. 1, left pane), in shallow profile pits. Sampling sites were mainly along a N-S transect at Hugus and along an E--W transect at Toasis (Fig. 1, fight pane). At Okeriko (three sites), Otjimaruru (five sites), and Tugus (four sites), the sampling sites are numbered from north to south. For the other basins, one or more selected profiles were considered for this study: Okozondje, site 1 (centre); Gui-Ams, site 2 (north) and site 4 (south); Otjiwarongo, site 2 (centre); Wolfpan, site 1 (north) and site 2 (south).
[FIGURE 1 OMITTED]
The Hugus, Toasis, and Okozondje pans are the only studied basins where the deposits could be sampled up to a depth of ~1 m. In all other basins, the deposits are too coherent and sampling was generally limited to the upper 20 cm. Thin sections were prepared for 167 samples, after impregnation of undisturbed oriented blocks with a polyester resin. The sections were described using the concepts and terminology of Stoops (2003). The mineralogical composition of the deposits was determined by X-ray diffraction (XRD) analysis of bulk samples and of the clay fraction, the latter involving the use of five standard treatments (Mg saturation, Mg saturation and glycol treatment, K saturation, K saturation and heating to 350 and 550[degrees]C). Sand, silt, and clay contents were determined for the fraction remaining after decalcification with a 1 M sodium acetate buffer (wet sieving at 63 [micro]m, followed by repeated sedimentation for clay separation). Other parameters determined for all samples were carbonate content (calcimetry), pH (1:2.5 suspension in water), and the chemical composition of extracts (1 : 5, ion chromatography).
The deposits of the Hugus, Toasis, and Okozondje basins show the same sequence of two lithological units, marked by a difference in gram size, carbonate content, and mineralogical composition between the units, separated by a boundary representing a hiatus (Table 1; see also Mees and Van Ranst 2011). At these three sites, the upper part of the lower unit represents a buried surface horizon, containing euhedral sparitic calcite crystals in the groundmass and in pores (Mees and Van Ranst 2011). The upper part of the upper unit is generally marked by a granular field structure.
All other basins typically have a hard massive surface layer (1-2 cm thickness), followed by an interval with a fine granular field structure (up to ~15 cm depth), followed by more massive coherent deposits. Fine granular material is absent only at Otjimaruru and Otjiwarongo, characterised by a coarser fragmented field structure. The deposits at Tugus and Otjimaruru have a largely non-calcareous groundmass. At Gui-Ams and Okeriko, the deposits are calcareous and have a markedly higher micritic carbonate content in the basal part relative to higher parts.
The XRD analyses demonstrate that sepiolite occurs in the deposits of all studied pan basins, generally as the dominant clay mineral (Fig. 2, Table 1). It is absent only in the lower unit at Okozondje and in a white basal interval at site 8 in the Hugus basin. Muscovite is also present in all samples, as a subordinate phase. Smectite occurs in the lower unit at Hugus, Toasis, and Okozondje, and in some samples of the Otjiwarongo pan; the highest relative amounts are recorded for Toasis, where it is the dominant clay mineral in some parts (Fig. 2). Palygorskite is absent in all studied samples. In thin sections, sepiolite is recognised for the micromass and for various pedofeatures as a fine-grained constituent with low interference colours (first-order grey), which distinguishes this mineral from all others that are identified for the clay fraction by XRD analysis. The interference colours are only slightly masked for occurrences with the most pronounced brownish colour (see further).
Below, the nature and occurrence of various types of sepiolite occurrences observed in thin sections are described. General features are first reported, followed by results for specific pan basins, first for those of the Hugus--Toasis--Okozondje group, then for the others.
The pan deposits are typically calcareous, with micritic carbonates masking all other micromass components. The groundmass is non-calcareous at Tugus and Otjimaruru, but the deposits include peds with a calcareous core in the lower part of the sampled sequence at Tugus, whereas oolitic calcite aggregates are present in the upper part of the deposits in both basins (see Fig. 6d).
[FIGURE 2 OMITTED]
In intervals with low micritic carbonate content, as well as in carbonate depletion features (see further), sepiolite is recognised as pale yellowish fine material, typically with a stipple-speckled b-fabric. The groundmass has a unistrial b-fabric in several samples from the (smectite-bearing to smectite-dominated) lower unit at Toasis, in intervals with and without layering.
Sepiolite coatings are common in the pan deposits, showing strong variations in abundance and thickness between samples. The colour of the coatings varies between basins, ranging from very pale brownish (e.g. Tugus, Otjimaruru) to bright yellowish brown (e.g. Okozondje), generally corresponding roughly to the colour of the groundmass.
In intervals with a granular microstructure, the coatings occasionally cover the sides of channels that cross the fine-granular material (Fig. 3a). In intervals with sparitic calcite crystals in pores (Mees and Van Ranst 2011), sepiolite coatings, if present, cover the crystals (Fig. 3b).
Sepiolite-bearing coatings range in nature from pure sepiolite accumulations to coatings of calcareous fine material with recognisable sepiolite (Fig. 3c). Coatings of calcareous fine material that are not recognised to contain an admixture of sepiolite occur as well (Fig. 3d). The sepiolite-bearing coatings are commonly layered, with alternating sepiolite-dominated layers and layers of calcareous material that is generally similar to the fine material that is part of the groundmass. The latter occasionally include silt-sized quartz grains. In some layered thick coatings, truncation features are present, and one coating contains enclosed layered fragments (Fig. 3e). Layered coatings include occurrences along the sides of wide sediment infillings, which generally represent planar vertical structures in the field.
At Hugus, sepiolite coatings are relatively common and generally thick in the lower unit; their occurrence in this unit is confined to the upper part, as observed for the deepest profiles (sites 2, 3, 8). In the upper unit, coatings are quite rare and generally thin (Fig. 3f); in this unit, they are entirely absent in the centre of the basin (sites 2, 3, 8), and they are lacking in the upper part of the interval at all other sampling sites. Only one occurrence of a thick layered coating is recognised for the upper unit, in the lower part of the interval (site 4).
At Toasis, sepiolite coatings in the lower unit are confined to two profiles in the western part of the basin (sites l, 2), where they only occur in the lower part of the interval, and to three eastern sites (sites 6, 7, 11), where coatings are only present in the upper part of the unit (Fig. 4a). In the upper unit, coatings are much less abundant and they mainly occur in the upper part of the interval; in this unit, coatings similar to those in the lower unit occur in only two profiles (sites 7, 10), in the basal part of the interval.
At Okozondje, coatings occur only in the upper part of the upper unit, and locally in veins or pockets of upper unit material that continue into the lower unit. The highly calcareous basal interval at Okeriko includes layered coatings that are similar to those recognised for the lower unit at Hugus and Toasis (Fig. 4b).
[FIGURE 3 OMITTED]
In deposits of the Tugus and Otjimaruru basins, with a non-calcareous groundmass, coatings are recognised in a majority of samples, in all main units, including the interval with a granular field structure (Fig. 4c).
At Gui-Ams, coatings are abundant in the highly calcareous lower part of the deposits at one site (site 4), where some of the coatings show fine lamination. Some coatings occur along horizontal voids, whereby coatings covering the upper and lower sides are locally connected by bridging structures (Fig. 4d). At a second site in the same basin, groundmass aggregates are lined along all sides by sepiolite with parallel orientation, commonly with a gradual lower boundary (Fig. 4e, f).
In some samples, aligned elongated fragments of thin coatings, unrelated to pores, are present (Hugus, Okeriko) (Fig. 5a). A related feature appears to be clusters of fragments, without elongated aggregate shapes or preferential alignment, which is recognised for several basins (Fig. 5b).
Fragments of coatings
Much more common than fragmented coatings are isolated fragments of sepiolite coatings, both layered and massive, enclosed by the groundmass. They are easily recognised in samples with a calcareous groundmass. In intervals with low carbonate content, they are only detected as patches of oriented clay.
[FIGURE 4 OMITTED]
The fragments are partly larger than the thickness of coatings in the same interval, and they can also show a difference in the presence or type of layering with local undisturbed coatings. They are, in part, larger than the sand fraction of the interval in which they occur, and they are in some cases more similar in size to limestone fragments that are present. In contrast, some intervals contain sepiolitic clay aggregates that appear to be part of the sand fraction. This is recognised at Toasis for parts of the lower unit at sites 7 and 11, and also for the basal part of the upper unit at several sites.
At Hugus, Toasis, and Okozondje, fragments are absent in the lower unit. In the upper unit, they are generally more common in the upper part of the interval at Hugus and Toasis. In the Okozondje pan, fragments are abundant throughout the upper unit (Fig. 5c) and in vertical veins that penetrate the lower unit. The fragments at this site have a dark orange--brown colour, which also characterises the coatings and the groundmass in the same deposits.
At Tugus and Otjimaruru, fragments are very abundant in the interval with a strongly fragmented microstructure (Fig. 5d). In the overlying hard surface layer, characterised by a relatively massive microstructure, isolated fragments occur in a fine-grained groundmass with stipple-speckled b-fabric.
[FIGURE 5 OMITTED]
Highly calcareous deposits in the lower part of the deposits at Gui-Ams and Okeriko contain abundant sepiolite aggregates, grading to aggregates with a brownish colour. The aggregates are angular and often elongated at Gui-Ams, and they are rounded at Okeriko (see Fig. 4b).
One profile at Toasis includes a thin interval (1 cm) containing intercalations in the form of elongated, irregular patches of noncalcareous oriented clay, which are clearly different from possible carbonate depletion features that occur in the same band (site 2, 9-14 cm). Intercalations in another profile at Toasis are similar to coatings in the same sample and clearly represent deformed void-related features (site 7, 2-5 cm) (Fig. 6a).
Carbonate depletion hypocoatings
Sepiolite-dominated carbonate depletion hypocoatings are recognised in some samples from Hugus and Toasis as clearly orthic features with a stipple- to mosaic-speckled b-fabric and with enclosed sand grains (Fig. 6b).
At Tugus, some peds in the lower part of the profile have micritic carbonates in the centre, which also has a dark colour (site 1, 13+ cm). These peds should be considered as having thick depletion hypocoatings. The corresponding interval in other profiles at Tugus contains some peds with a non-calcareous dark core (Fig. 6c). At Otjimaruru, large fragments
with a unistrial b-fabric and a dark core appear to be the equivalent of these features (site 2, 7+ cm). At one of the Gui-Ams sites (site 2), coatings show some characteristics of depletion features, by having a gradual lower boundary and by covering the peds along all sides (see Fig. 4e, f).
Surface crusts and crust fragments
Surface crusts occur at most sites with coherent pan deposits. They are absent at Hugus and Toasis, where field characteristics of the surface interval include low consistency, high salt content, and the presence of a thin salt crust. A surface crust does appear at the third site with unconsolidated sediments (Okozondje), where surface occurrences of salts are absent.
The crusts are thin surface layers that generally show a weakly developed unistrial b-fabric, a layered fabric, and a massive microstructure (Fig. 6d). The unistrial b-fabric is well expressed only at Gui-Ams. At Tugus and Otjimaruru, the interval immediately below the crust commonly includes large vesicles, and in some samples the sand content is high along the base of the unit. The latter is also observed for the surface crust of the Okozondje profile.
In one sample, a buried surface crust occurs just below the crust along the present surface in part of the sample (Otjimaruru, site 4). A similar feature, at slightly greater depth and more disturbed, occurs at Wolfpan (site 2). At Gui-Ams, the nature of large fragments in the uppermost part of the deposits, showing a subhorizontal orientation, is similar to that of the present crust.
[FIGURE 6 OMITTED]
The present study confirms the abundance of sepiolite, and absence of palygorskite, in pan deposits of the south-western Kalahari, in agreement with earlier results for the region (Mees 1999a, 2001). In other parts of the Kalahari, palygorskite is the dominant phase in calcretes, sepiolite appearing only as a late-stage precipitate (Watts 1980). In other studies of Kalahari calcretes, only palygorskite is detected (e.g. Netterberg 1982; Eitel 1994, 1995; Ringrose et al. 2002), whereby Tertiary and Quaternary calcretes differ only in palygorskite content (Eitel 2000). Near the study area, palygorskite occurs in Cenozoic limestone of the Gobabis--Okahandja area (Mees 2002), which may be the equivalent of Tertiary palygorskite-dominated calcareous formations in other parts of the Kalahari. Quaternary palustrine limestone does contain abundant sepiolite instead of palygorskite (Kautz and Porada 1976; Mees 2002), at least in the south-western Kalahari. As mentioned before, sepiolite occurs in recent pan deposits in this region (Mees 1999a, 2001), which is also observed for other parts of the Kalahari (Buch and Rose 1996; Atlhopheng and Ekosse 2007). For other pan basins, only palygorskite occurrences have been reported (Lake Ngami, Makgadikgadi Pan; Heine and Volkel 2010), as is the case for soils in depressions in the Steinhausen and Gollschau areas of central Namibia (Scholz 1968a, 1968b). In those studies, palygorskite and sepiolite are generally interpreted as authigenic precipitates (Kautz and Porada 1976; Watts 1980; Eitel 2000; Ringrose et al. 2002, 2005) (see also Singer et al. 1995), with the exception of assumed allogenic palygorskite in Quaternary calcretes (Eitel 1994, 2000) and occurrences derived from a Tertiary limestone substrate (Buch and Rose 1996).
Surface formations around the pan basins of the southwestern Kalahari are Kalahari sands, whose clay fraction consists of muscovite and dioctahedral smectite, at least in the Otjomongwa area (Mees 2001). Soils in the south-eastern part of Namibia are reported to contain palygorskite, and no sepiolite, in a region south of the study area whose northern boundary is not well fixed (Heine and Volkel 2010). This area with palygorskite-bearing soils in southern Namibia seems to extend into the eastern part of Namaqualand in South Africa, sepiolite occurrences being confined to the coastal area of that region (Singer et al. 1995). The lack of a source of sepiolite within the catchment of the studied pan basins implies that a clastic origin of its occurrence in the pan deposits can be excluded.
Unistrial b-fabrics are only observed for the smectite-bearing deposits of the lower unit at Toasis, including sandy-layered deposits. Therefore, the b-fabric only records settling of a possible precursor during clastic sedimentation. Thin section observations provide no indications about whether or not mineral transformation was involved in sepiolite formation.
Formation of sepiolite in earth-surface conditions, in a semiarid environment, requires high Si activity, at high pH, and high Mg concentrations. The formation of sepiolite rather than palygorskite is typical of saline-alkaline conditions, higher Mg levels, and low Al activities (Jones and Galan 1988; Singer 2002), whereby the last of these factors possibly explains palygorskite sepiolite sequences (e.g. Millot et al. 1961; Watts 1980). For both minerals, required high Mg activities can be related to Ca removal by associated calcium carbonate precipitation in calcareous soils (e.g. Yaalon and Wieder 1976; Watts 1980). In the Aminuis region, the presence of dolerite within the catchment might be a factor, as a rock type with a high content of weatherable Fe-Mg silicate minerals. A similar control has been implied for some palygorskite occurrences in soils (Anand and Paine 2002).
Formation of coatings
In principle, sepiolite coatings can be the result of in situ mineral formation or of clay-illuvial processes (e.g. Beattie 1970). In the studied deposits, the associated calcareous fine material, occurring as massive coatings (Fig. 3d) or as part of layered coatings (Fig. 3c), commonly clearly consists of translocated micromass material. In these coatings, the calcareous material is not an authigenic micritic carbonate precipitate, which, in authigenic clay coatings, typically occurs as dispersed micritic grains, often with gradual changes in concentration (Mees 2010). The illuvial nature of calcareous fine material in the coatings implies the same origin for associated pure sepiolite accumulations in pores. Layering thereby only records variations in the nature of translocated material reaching the site of deposition. Mechanical transport is also recorded by layered bands along the sides of intrusive features that penetrate the lower unit. Another indication for an illuvial nature is the common overall similarity in colour between the groundmass and the coatings in the same profile or interval. The occasional presence of truncation within layered thick coatings and the occurrence of coatings enclosing fragments of coatings could also be an indication (Fig. 3e), although truncation and mechanical reworking can also separate stages of authigenic mineral formation. The observation that palygorskite is easily disaggregated and dispersed (Neaman et al. 1999; Neaman and Singer 2004) should also apply to sepiolite. At the time of sampling, soil solutions were characterised by high salinity and high relative sodium content (Table 1), with contrasting effects on dispersion, but hydrochemical conditions may have been different during the period when the coatings were formed.
The coatings at Gui-Ams are an example of clay occurrences that seem to be authigenic rather than illuvial, as coatings that are marked by bridge-like connections in horizontal pores (Fig. 4d).
For the deposits of the Hugus, Toasis, and Okozondje pans, comprising two lithological units separated by a hiatus, sepiolite coatings in the lower unit can, in principle, have formed before or after deposition of the upper unit. Differences in the nature and abundance of sepiolite coatings between both units are an indication that the coatings predate deposition of the upper unit. However, some sepiolite occurrences in the basal part of the upper unit are similar to those in the lower unit. Also, the development of sepiolite coatings postdates the formation of sparitic carbonate infillings, which are covered by sepiolite if present (Fig. 3b). These carbonate occurrences formed before deposition of the upper unit, in a buried surface horizon that underwent little truncation after calcite formation, based in part on the recognition of similar vertical variations in distribution patterns of the carbonates between profiles (occurrence in the groundmass and in pores in the upper part, exclusively in pores in the lower part) (Mees and Van Ranst 2011). The absence of fragments of coatings in the groundmass of the lower unit at Hugus, Toasis, and Okozondje, recording a lack of disturbance after their formation, is another indication for coating development after deposition of the upper unit.
Sepiolite coatings occur in the upper part of the lower unit at Hugus and in the western part of the Toasis basin, and they occur in the lower part of the unit in the eastern part of Toasis, characterised by sandy layered deposits. The present distribution of sepiolite coatings seems to be related to variations in soil water behaviour with depth, whereby coatings are formed at levels where illuvial clay deposition is favoured. The absence or much lower abundance of coatings in the central part of both the Hugus and Toasis basins should similarly be related to factors such as bedrock depth and groundwater levels.
Coatings that cover the sides of macropores in intervals with a granular microstructure (Fig. 3a) indicate development of coatings at a recent stage. Recent formation, possibly related to flooding, is also suggested by the restriction of the occurrence of coatings to the upper part of the upper unit at Toasis. They are not necessarily part of the same generation as coatings in the lower unit in the same profile, below an intervening interval without coatings. The contrasting patterns for the upper unit at Hugus, where coatings are absent in the uppermost part of the unit in the (marginal) profiles in which they occur, indicate more complex controls on their formation or preservation.
The occurrence of sepiolite coatings in all studied basins suggests that sepiolite-bearing lake deposits throughout the region are affected. However, sepiolite coatings are notably absent in deposits of the Omongwa and Otjomongwa pans, characterised by a different sequence of lithological units and higher salinity (Mees 1999a, 2003). Clay coatings are entirely absent in the Omongwa pan deposits, and they only occur in the form of thin smectite coatings in channels in the lower part of the deposits of the Otjomongwa basin.
Fragmentation of coatings
Fragments are typically the result of the disturbance of coatings that were initially present in the horizon. Disturbance of coatings is clearly recorded by the presence of aligned elongated fragments, and other types of clusters most likely have a similar origin. Fragmentation is generally due to shrink-swell behaviour or bioturbation, and in some cases to salt crystallisation (Kuhn et al. 2010). Physical and biogenic pedoturbation has also led to a strongly fragmented microstructure in part of the deposits, which is comparable to structures reported for Tertiary sepiolite deposits (Hay et al. 1986; Bustillo and Alonso-Zarza 2007). Purely biogenic disturbance is incompatible with the presence of fragmented coatings consisting of aligned fragments (Fig. 5a). The greater abundance of fragments in higher parts of the upper unit at Hugus and Toasis also suggests surface-related physical processes. There are no indications that fragments in subsurface horizons are commonly derived from surface crusts, which is recognised for only a few profiles, restricted to the uppermost part of the deposits. In most basins, the surface crusts have a different type of b-fabric and layering than fragments in subsurface horizons.
In addition to local pedoturbation, fragments of coatings can also be the result of erosion and sedimentation involving lateral surface transport (Mees 1999b, 2010). The occasional occurrence of these processes is suggested by fragments whose size and nature are incompatible with being derived from coatings that are now present in the same interval. All fragments in the highly calcareous lower part of the deposits at Gui-Ams and Okeriko (Fig. 4b), with a strong difference in rounding between both basins, seem to be reworked aggregates of this type. These transported aggregates, as well as clay aggregates unrelated to coatings, are similar to sepiolite intraclasts described in other studies (Khoury et al. 1982; Hay et al. 1986).
Formation of surface crusts
Surface crusts with unistrial b-fabrics are formed by settling of fine-grained particles, which were either washed into the basin or resuspended more-or-less in situ during flooding. An association with flooding is suggested by the common occurrence of vesicles below the crust, and a higher sand content below the crust indicates size sorting (Fig. 6d). The absence of surface crusts at Hugus and Toasis seems to be related to the high salinity in these basins, resulting in disturbance of surface features by periodic salt crystallisation. It is not clearly related to the nature of the deposits (e.g. with good permeability that limits the occurrence of surface brines), because the deposits are similar to those at Okozondje, where a surface crust does occur.
Dissolution of carbonates along macropores is recorded for several basins by the presence of carbonate depletion hypocoatings (see also Vanden Heuvel 1966; Mees 2010). They are generally easily recognised as orthic features without parallel alignment of clay particles. Note that a lack of parallel alignment has also been reported for palygorskite coatings, interpreted to be non-illuvial (Singer and Norrish 1974; Kapur et al. 1993). A distinction with coatings is difficult for some studied samples. Coatings of well-oriented clay with gradual lower boundaries seem to be depletion hypocoatings with superimposed porostriation (Fig. 4e, f).
At Tugus and Otjimaruru, the occurrence of peds with a calcareous core, or with an equally dark non-calcareous core (Fig. 6c), demonstrates that at least the lower part of the sampled deposits consists of decalcified sediments. This is not necessarily the case for higher parts of the deposits, comprising unaltered calcite oolites (Fig. 6d).
Decalcification may have an effect on the availability and nature of eluvial materials, but the distribution of pedofeatures in the studied profiles provides no indication for this. A link between groundmass decalcification and the development of illuvial sepiolite coatings was already implied by Vanden Heuvel (1966).
In the south-western Kalahari, sepiolite is the only magnesium silicate that is recognised for surface deposits, in contrast to palygorskite-dominated calcareous formations in other parts of southern Africa. The cause of this discrepancy requires further investigation to assess the possible role of local factors.
Although authigenic sepiolite occurs as part of the groundmass, sepiolite coatings in the studied deposits are commonly illuvial. This implies that the presence of pore-related sepiolite or palygorskite occurrences, observed in thin sections or scanning electron microscope images, is not a criterion for the recognition of in situ formation of these minerals. The possibility of illuvial sepiolite or palygorskite is only rarely considered (e.g. Beattie and Haldane 1958; Vanden Heuvel 1966; Khademi and Mermut 1998), although it should be common in soils containing these minerals as part of the groundmass. Only for soils without groundmass occurrences is a non-illuvial nature apparent (e.g. Singer and Norrish 1974; Soong 1992).
The presence of sepiolite in the groundmass and in pores is a feature that the studied deposits have in common with consolidated palustrine deposits along the margins of pans of the neighbouring Omaheke region (Mees 2002). If conditions of sepiolite formation were similar for those older deposits, the possibility of early sepiolite development exists, which has implications for the relationship between carbonate enrichment and magnesium silicate formation.
This study was funded by project G.0103.05N of the Fund for Scientific Research (Flanders).
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Manuscript received 25 October 2010, accepted 13 June 2011
Florias Mees (A,C) and Eric Van Ranst (B)
(A) Department of Geology and Mineralogy, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium.
(B) Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium.
(C) Corresponding author. Email: email@example.com
Table 1. Main mineralogical, textural, and chemical characteristics of selected sites Main macroscopic characteristics in field: b, brown; db, dark brown; dgr, dark green; gb, greyish brown; ggr, greyish green; gr, green; grb, greenish brown; lb, light brown; lg, light grey; lgr, light green; lgrg, light greenish grey; w, white; cl, clayey; ga, granular; h, hard; lay, layered; ma, massive; ss, high salt content; v, vesicular. Mineralogical composition: Ca, calcite; Do, dolomite; Qz, quartz; Sp, sepiolite; Sm, smectite (relative abundance of minerals based on peak height). CaC[O.sub.3eq] expressed as percentage of the sample weight without soluble salts. Texture: Sa, sand content (after removal of soluble salts and carbonates); Cl/Si, clay/silt ratio. Chemical properties: electrical conductivity (EC) in dS/m, cation and anion concentrations in cmol(+)/kg. * Lower unit; (+), ++, +++, (x), x, xx denote absence and relative abundance. n.d., Not determined Basin, site, depth Field Mineralogical composition Ca Do Qz Sp Sm Hugus, site 2 1-24 cm (1-9 cm) lg v +++ - + xx - 1-24 cm (9-15 cm) lg ga +++ - + xx - 1-24 cm (16-24 cm) lg ma +++ - - xx - 24-29/36 cm * db +++ + - xx (x) 29/36-49 cm * dgr +++ +++ - xx x 49+ cm (49-75 cm) * lgr cl +++ + - xx (x) Toasis, site 5 0-1.5 cm h ss + - ++ xx - 1.5-3 cm SS + - ++ xx - 3-7 cm b ga ++ - +++ xx - 7-21 cm lgr +++ - + xx - 21-36 cm gr +++ - +++ xx - 36+ cm (41-49 cm) * db + - + x xx 36+ cm (55-63 cm) * db lay ++ + + x xx Toasis, site 9 0-1 cm h lb ++ - +++ xx - 1-6 cm b ga ++ - +++ xx - 6-19 cm lgrg +++ ++ +++ xx (x) 19-36 cm grb ++ - +++ xx - 36+ cm (36-46 cm, sandy) * db lay ++ - +++ xx - 36+ cm (47-53 cm, light) * db lay + + + xx x 36+ cm (53-61 cm, dark) * db lay +++ - ++ xx x Okozondje, site 1 0-0.5 cm h + - + x - 0.5-16 cm (0.5-7 cm) b ga +++ - ++ x - 0.5-16 cm (7-16 cm) b ga ++ - +++ x - 16+ cm (20-30cm) * b +++ - (+) x - 16+ cm (35-45 cm) * gr +++ + ++ - (x) Otjimaruru, site 4 0-2 cm lg h - - +++ xx - 2-11 cm (grey) b v - - +++ xx - 2-11 cm (brown) b v - - ++ xx - 11+ cm (12-20 cm) cl + - ++ xx - 11+ cm (20-25 cm) cl ggr ++ - ++ xx - Gui-Ams, site 2 1-5 cm b ga +++ - + xx - 5+ cm (5-16 cm) lb +++ - 4 xx - 5+ cm (16-25 cm) lb +++ - +++ xx - Okeriko, site 2 0-0.5 cm lb +++ - ++ xx - 0.5-11 cm gb gr +++ - - xx - 11+ cm w c - +++ - xx - Basin, site, depth CaC[O.sub.3] Texture egiv. Sa C/S Hugus, site 2 1-24 cm (1-9 cm) 33 6 1.28 1-24 cm (9-15 cm) 27 4 0.44 1-24 cm (16-24 cm) 32 2 0.59 24-29/36 cm * 36 13 4.94 29/36-49 cm * 54 2 1.22 49+ cm (49-75 cm) * 45 3 3.41 Toasis, site 5 0-1.5 cm 13 32 1.38 1.5-3 cm 14 24 0.89 3-7 cm 14 17 0.85 7-21 cm 21 22 0.77 21-36 cm 13 25 0.71 36+ cm (41-49 cm) * 34 13 0.22 36+ cm (55-63 cm) * 28 8 0.21 Toasis, site 9 0-1 cm 11 42 0.57 1-6 cm 15 21 0.76 6-19 cm 39 n.d. n.d. 19-36 cm 28 47 2.06 36+ cm (36-46 cm, sandy) * 21 36 1.91 36+ cm (47-53 cm, light) * 21 5 0.55 36+ cm (53-61 cm, dark) * 31 13 0.55 Okozondje, site 1 0-0.5 cm 16 19 0.60 0.5-16 cm (0.5-7 cm) 18 3 0.79 0.5-16 cm (7-16 cm) 16 1 0.67 16+ cm (20-30cm) * 17 1 1.84 16+ cm (35-45 cm) * 8 23 2.31 Otjimaruru, site 4 0-2 cm 3 24 0.90 2-11 cm (grey) 3 52 0.27 2-11 cm (brown) 3 30 0.53 11+ cm (12-20 cm) 6 12 1.39 11+ cm (20-25 cm) 10 6 1.09 Gui-Ams, site 2 1-5 cm 21 7 0.87 5+ cm (5-16 cm) 17 2 0.89 5+ cm (16-25 cm) 13 6 1.04 Okeriko, site 2 0-0.5 cm 28 22 0.71 0.5-11 cm 29 9 1.11 11+ cm 67 52 0.17 Basin, site, depth Chemical properities pH EC Ca Mg K Hugus, site 2 1-24 cm (1-9 cm) 10.17 27.9 1.05 0.65 12.32 1-24 cm (9-15 cm) 10.17 23.3 0.30 <0.02 8.89 1-24 cm (16-24 cm) 10.16 23.5 0.40 <0.02 10.47 24-29/36 cm * 10.14 33.7 0.73 <0.02 15.55 29/36-49 cm * 10.14 30.5 0.53 <0.02 13.67 49+ cm (49-75 cm) * 10.10 25.7 0.69 <0.02 11.58 Toasis, site 5 0-1.5 cm 9.40 16.8 0.58 <0.02 7.00 1.5-3 cm 9.37 38.7 3.43 <0.02 20.70 3-7 cm 9.60 12.3 0.40 <0.02 4.77 7-21 cm 9.53 18.7 0.46 <0.02 8.62 21-36 cm 9.44 18.5 0.53 <0.02 7.87 36+ cm (41-49 cm) * 9.35 40.4 0.66 <0.02 22.60 36+ cm (55-63 cm) * 9.33 40.5 0.59 <0.02 20.37 Toasis, site 9 0-1 cm 9.36 39.5 0.04 <0.02 18.21 1-6 cm 9.75 11.4 0.04 <0.02 5.05 6-19 cm 9.53 14.7 0.05 <0.02 5.72 19-36 cm 9.40 15.9 0.05 0.03 6.36 36+ cm (36-46 cm, sandy) * 9.40 22.6 0.05 0.03 9.05 36+ cm (47-53 cm, light) * 9.37 46.7 0.04 <0.02 21.28 36+ cm (53-61 cm, dark) * n.d. n.d. n.d. n.d. n.d. Okozondje, site 1 0-0.5 cm 9.04 5.9 0.11 0.12 1.19 0.5-16 cm (0.5-7 cm) 9.70 11.6 n.d n.d. n.d. 0.5-16 cm (7-16 cm) 9.77 11.0 1.02 <0.02 2.06 16+ cm (20-30cm) * 9.81 18.2 0.62 <0.02 3.88 16+ cm (35-45 cm) * 9.71 18.5 0.53 <0.02 3.98 Otjimaruru, site 4 0-2 cm 10.19 14.4 <0.02 <0.02 5.82 2-11 cm (grey) n.d n.d n.d n.d. n.d. 2-11 cm (brown) 10.13 13.1 <0.02 <0.02 7.44 11+ cm (12-20 cm) 10.28 16.8 <0.02 <0.02 8.32 11+ cm (20-25 cm) 10.21 13.1 <0.02 <0.02 7.38 Gui-Ams, site 2 1-5 cm 10.07 15.1 <0.02 <0.02 9.48 5+ cm (5-16 cm) 10.10 15.2 <0.02 <0.02 9.55 5+ cm (16-25 cm) 10.06 14.5 <0.02 <0.02 8.59 Okeriko, site 2 0-0.5 cm 9.38 2.2 0.06 0.48 1.05 0.5-11 cm 10.31 7.4 <0.02 0.4 4.08 11+ cm n.d. n.d. n.d. n.d n.d. Basin, site, depth Chemical properities Na Cl S[O.sub.4] Alk Hugus, site 2 1-24 cm (1-9 cm) 162.07 138.50 17.11 26.43 1-24 cm (9-15 cm) 134.66 115.89 13.29 21.13 1-24 cm (16-24 cm) 137.62 116.49 13.80 21.99 24-29/36 cm * 200.85 182.88 21.05 29.49 29/36-49 cm * 177.14 162.73 18.10 22.35 49+ cm (49-75 cm) * 145.66 135.27 13.99 17.78 Toasis, site 5 0-1.5 cm 76.95 91.12 7.12 1.86 1.5-3 cm 210.73 241.38 23.85 2.36 3-7 cm 55.77 62.37 7.61 2.41 7-21 cm 89.91 102.28 11.45 2.38 21-36 cm 87.20 100.75 10.32 2.10 36+ cm (41-49 cm) * 225.23 268.30 23.45 2.37 36+ cm (55-63 cm) * 218.32 250.42 22.10 2.48 Toasis, site 9 0-1 cm 246.20 259.83 11.00 2.40 1-6 cm 54.42 54.45 3.65 3.02 6-19 cm 74.36 75.97 4.35 2.47 19-36 cm 80.71 83.69 4.16 1.84 36+ cm (36-46 cm, sandy) * 120.84 126.65 5.71 2.03 36+ cm (47-53 cm, light) * 264.38 284.85 11.92 2.92 36+ cm (53-61 cm, dark) * n.d. n.d. n.d. n.d. Okozondje, site 1 0-0.5 cm 26.42 25.34 3.17 1.50 0.5-16 cm (0.5-7 cm) n.d. n.d. n.d. n.d. 0.5-16 cm (7-16 cm) 49.67 46.96 10.50 4.47 16+ cm (20-30cm) * 88.78 85.00 19.65 4.95 16+ cm (35-45 cm) * 92.63 90.99 20.28 3.55 Otjimaruru, site 4 0-2 cm 70.75 47.96 4.72 16.01 2-11 cm (grey) n.d. n.d. n.d. n.d. 2-11 cm (brown) 85.81 56.23 6.70 16.23 11+ cm (12-20 cm) 92.17 56.30 6.11 22.12 11+ cm (20-25 cm) 93.66 61.22 5.90 23.56 Gui-Ams, site 2 1-5 cm 71.76 68.99 12.15 9.20 5+ cm (5-16 cm) 76.42 69.37 13.25 9.63 5+ cm (16-25 cm) 69.67 63.91 11.88 9.37 Okeriko, site 2 0-0.5 cm 6.58 6.62 0.23 2.49 0.5-11 cm 36.18 24.33 2.71 12.80 11+ cm n.d. n.d. n.d. n.d.
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|Author:||Mees, Florias; Van Ranst, Eric|
|Date:||Sep 1, 2011|
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