Thesis (PhD (Soil Science))--Univ ersity of Stellenbosch, 2008. / The (semi-)arid Namaqualand region on the west coast of South Africa is wellknown
for its spring flower displays. Due to the aridity of the region, soils research
has lagged behind that of the more agriculturally productive parts of South
Africa. However, rehabilitation efforts after the hundred or so years of mining,
coupled with the increasing ecology and biodiversity research, have prompted
a recent interest in Namaqualand soils as a substrate for plant growth. The
area is also notable for the abundance of heuweltjies. Much of the previous
heuweltjie-work focussed on biogenic aspects such as their spacing, origin and
age, but although heuweltjies are in fact a soil feature, there have been few
published studies on the soil forming processes within heuweltjies. However, the
depositional history of the sediments on the Namaqualand coastal plain is well
constrained, which is in stark contrast to the paucity of data on their subsequent
pedogenesis. Given that the regolith has been subaerially exposed in some parts
for much of the Neogene, the soil formation forms an important part of the
sediments’ history. The primary aim of this thesis, therefore, was to examine the
soil features of the Namaqualand coastal plain to further the understanding of
pedogenesis in the region.
The regolith of the northern Namaqualand coastal plain, often ten or more metres
deep, comprises successive late Tertiary marine packages, each deposited during
sea-level regression. The surface soil horizons formed from an aeolian parent
material. The relatively low CaCO3 in the aeolian sands dictated the pedogenic
pathway in these deposits. The non-calcareous pathway lead to clay-rich, redder
apedal horizons that show a stronger structure with depth, and generally rest
directly on marine sands via a subtle discontinuity that suggests pedogenesis continues
into the underlying marine facies. The calcareous pathway lead to similar
clay-rich, redder apedal B horizons, but which differ in that they are calcareous, and rest on a calcrete horizon often via a stoneline of rounded pebbles. Deeper in
the profile, there is generally a regular alteration of sedimentary units, with the
upper shoreface facies showing reddening, and the lower shoreface sands remaining
pale. This seems to be a function of the grain size, since the upper shoreface
materials are coarser, and the redder parts of the lower shoreface are also associated
with slightly coarser sands. In some strata the oxidation of glauconite-rich
sediments resulted in an orange colour. In an area with abundant heuweltjies, a
strongly-cemented calcretized nest was present about 2 m deep within a silica cemented,
locally calcareous dorbank profile. Vertical termite burrows are present
up to 12 m deep, and appear to have been conduits for preferential vertical flow.
Soil formation and termite activity is at least as old as the Last Interglacial. E
horizons may have formed in a wetter Last Interglacial paleoclimate, but they
are still active in the present day.
The Namaqualand coastal plain, with its extensive areas of calcrete development,
is almost a textbook setting for calcrete development by inorganic processes.
However, these calcretes also show microscale biogenic features. These include
M rods, MA rods, and fungal filaments. Abiotic alpha-fabric seems dominant
in mature calcrete horizons, and beta-fabric in calcareous nodules in a calcic
B horizon above calcrete. The apparent absence of Mg-calcite and dolomite,
and abundance of sepiolite in the calcretes of coastal Namaqualand suggests
that these Mg-rich clay minerals are the main Mg-bearing phase. Deformation
(pseudo-anticlines) in the calcrete appear to result primarily from the displacive
effect of calcite crystallization. Although evidence of shrink/swell behaviour
is present in the form of accommodating planes, it does not appear to be as
volumetrically significant as displacive calcite.
Indurated light-coloured horizons that resembled calcrete but are non- to mildly
calcareous, break with a conchoidal fracture, resist slaking in both acid and alkali,
turn methyl-orange purple, and show a bulk-soil sepiolite XRD peak are
similar to palygorskite-cemented material (‘palycrete’) from Spain and Portugal,
and so were tentatively named ‘sepiocrete’. Sepiolite and palygorskite are often
reported from arid region soils but there has been no recorded cementation of
soils by sepiolite. The degree of induration in some of these horizons suggest that
amorphous silica could play a role in cementation, and so this thesis compares
the two silica-cemented horizons encountered in Namaqualand (silcrete and dorbank
(petroduric)) to these ‘sepiocrete’ horizons. Both silica and sepiolite are present in the matrix, although the degree to which silica and sepiolite dominate
seems to vary even within same horizon. It seems most probable that both
contribute to the structural properties of the horizon. Sepiolitic horizons do not
form a diagnostic horizon in the World Reference Base, Soil Taxonomy, or the
South African system. To fit the existing soil classification schemes, the terms
‘sepiolitic’ and ‘petrosepiolitic’ (in the same sense as ‘calcic’ and ‘petrocalcic’)
would be appropriate. The term ‘sepiolitic’ should be used for horizons which:
contain sepiolite in amounts great enough for it to be detected by XRD in the
bulk soil, peds (a fractured surface and not just the cutan) cling strongly to the
wetted tongue, and methyl orange turns from orange to purple-pink over most
of a fragmented surface. The term can be easily be applied as a adjective to
other hardpans where sepiolite is significant but not necessarily cementing, such
as ‘sepiolitic’ petrocalcic/petroduric. If the horizon is in addition to the above
criteria cemented to such a degree that it will slake neither in acid (so cannot be
classified as petrocalcic) nor in alkali (and so cannot be classified as petroduric)
then the term ‘petrosepiolitic’ would be appropriate. The ‘sepiolitic’ criteria distinguish
the ‘petrosepiolitic’ horizon from a ‘silcrete’, a silica-cemented horizon
which does not fit the definition of petroduric.
Sepiolite is more prominent than palygorskite in the XRD traces. The <0.08 μm
fraction is the only size fraction where palygorskite could be detected before
acetate treatment. It is unlikely that these fibrous clay minerals are inherited
from either the marine or aeolian parent materials, they appear to be pedogenic
in origin. Sepiolite and palygorskite are associated with the presence of calcite
in the soil profile. Trends in MgO, Al2O3 and SiO2 show that the soil clay
fractions lie on a mixing line between sepiolite and mica end-members, with a
contribution from smectite, and is consistent with the XRD and TEM results.
There is a good correlation between Fe2O3 and TiO2, which can be attributed
to the ubiquitously presence of mica. There was no TEM evidence of fibrous
mineral degradation to sheet silicates, nor for the evolution of mica laterally to
a fibrous mineral. SEM analyses show that much of the sepiolite/palygorskite
occurs as fringed sheets, but higher magnification often revealed these sheets
to be composed of fibres. These are found coating (rather than evolving from)
mica/illite particles, as free-standing mats, and are common on the grain-side of
cutans. Some of these textures suggest illuviation of the fibrous clay minerals,
but another explanation may be that sites such as that immediately adjacent to silicate grains have the highest concentration of silica for their formation.
There was no conclusive evidence for or against the presence of kerolite in the
clay fraction, although it does not appear to be a dominant phase in the <2 μm
fraction.
The hypothesis was that the permeable upper horizons in Namaqualand soils
constitute a shallow ephemeral aquifer, which can be considered the pedogenic
analogue of the saline lake environments in which sepiolite typically forms. The
chemical evolution of the soil solution and clay mineral genesis could therefore be
considered in the same terms as the geochemical evolution of closed-basin brines.
The Namaqualand coastal plain, like other maritime areas, shows a trend of decreasing
pH, increasing Ca and increasing Mg with increasing evaporation. This
can be explained by their seawater-influenced initial ratios, and is consistent with
the ‘chemical divides’ of the Hardie-Eugster model of brine evolution. Halite remains
undersaturated at all concentrations in the saturated paste extracts. At
higher concentrations, gypsum reaches saturation, and sulfate is removed from
solution. H4SiO4 activity remains unchanged for all levels of evaporation and
pH. Calcite remains close to saturation, and is only dependent on the HCO−3
activity and pH for the range of Cl− activity encountered. Most of the soils for
which there is a positive sepiolite identification show a positive sepiolite saturation
index. The sepiolite saturation index is independent of Mg2+ and H4SiO4
and only increases with increasing pH. Evidence of the pH control on sepiolite
saturation is that sepiolite is commonly associated with calcareous horizons.
Sepiolite precipitation is therefore more likely to be triggered when a solution
encounters a pH barrier than by the concentration of ions by evaporation. The
effect of a pH change on the sepiolite saturation index is much greater than that
of the effect on calcite. The marine-influenced high Mg coupled with the Hardie-
Eugster model of brine evolution offers an explanation for sepiolite-dominance at
the coast, and palygorskite-dominance inland. Coastal areas, unlike continental
areas, have Mg>HCO−3 initially, which results in an increasing Mg trend with
evaporation during the precipitation of sepiolite according to the Hardie-Eugster
scheme. The result is that after sepiolite precipitation is initiated by a geochemical
pH-barrier, Mg levels will rise causing the increasing (Mg+Si)/Al ratio to
continue to favour sepiolite precipitation. This suggests that once sepiolite has
begun to precipitate, the subsequent salinity with its accompanying Mg increase
makes substantial palygorskite formation unlikely to follow. The hardpan horizons in heuweltjies commonly grade from a ‘sepiolitic’ petrocalcic
in the centre through ‘sepiolitic’/‘petrosepiolitic’ to the petroduric horizon
on the edges. Noteworthy sepiolite-related pedofeatures in the calcrete include
‘ooids’ with successive sepiolite (hydrophilic and therefore a precipitational substrate)
and micrite/acicular calcite layers in the coatings; and limpid yellow
nodules with pseudo-negative uniaxial interference figures. They superficially
resemble the spherulites in the fresh termite frass. Their fibrous nature and low
birefringence, together with the low Ca, high Mg, Si composition, and molar
Mg/Si ratios consistent with sepiolite. The pedogenesis of the hardpans in the
heuweltjie is proposed to be as follows: enrichment of cations such as Ca and
Mg in the heuweltjie centre caused by termite foraging results in calcite and clay
authigenesis in the centre of the heuweltjie, leaving the precipitation of pure silica
to occur on the periphery. The decaying organic matter concentrated in the
centre of the mound by the termites is sufficient to supply the components for
calcite precipitation in the centre of the heuweltjie. Following calcite precipitation,
the pH is suitable for sepiolite precipitation. The movement of the Mg-Si
enriched water downslope, coupled with the decrease in HCO−3 and increase in
Mg2+ due to sepiolite precipitation, allows for the precipitation of the ‘sepiolitic’
zone on the outer side of the calcrete, and extend beyond the calcrete in some
heuweltjies.
The Namaqualand coastal plain is well positioned for further work on its regolith,
particularly because of the mining excavations which provide excellent exposures
of well-defined layers of the regolith down to bedrock. Soil formation and termite
activity is at least as old as the Last Interglacial, and so more detailed work would
further the understanding of the subaerial alteration history in southern Africa,
as well as providing better-constrained information on the Namaqualand soils
that can be used by land-use management and biosphere studies.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1182 |
| Date | 03 1900 |
| Creators | Francis, Michele Louise |
| Contributors | Ellis, F., Fey, M. V., University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Rights | University of Stellenbosch |
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