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Vryheid Formation and intrusive dolerite
Comprising predominantly of thick beds of yellowish to white cross-bedded sandstone and
grit alternating with beds of soft sandy shale. The sedimentary rocks are, however, so
extensively and widely intruded by dolerite sheets and dykes that the two lithologies are
considered to represent a single groundwater system. The distribution of these lithologies
is indicated in Figure 17 together with the positions of groundwater sample sources. Six
different modes of groundwater occurrences associated with these formations are listed.
These are (a) weathered and fractured sedimentary rocks not associated with dolerite
intrusions, (b) indurated and jointed sedimentary rocks alongside dykes, (c) narrow
weathered and fractured dolerite dykes, (d) basins of weathering in dolerite sills and
highly jointed sedimentary rocks enclosed by dolerite, (e) weathered and fractured upper
contact-zones of dolerite sills and (f) weathered and fractured lower contact-zones of
dolerite sills. Minor groundwater strikes are also often encountered.
The groundwater yield potential is classed as low since 83% of the boreholes on record
produce less than 2/l. The groundwater rest level is generally encountered between 5 and
25 m below surface. Numerous springs occur at lithological contacts such as where
sandstone overlies an impervious shale horizon, along fault zones and along impermeable
dolerite dykes. Groundwater seepage in lower lying areas contributes substantially to
sustaining the dry season flow in the stream systems that drain these landscapes. A
recharge of 4 to 5 % of mean annual rainfall is appropriate to this groundwater regime.
The general suitability of the groundwater for any use is indicated by the average EC value
of 57 mS/m and mean pH value of 7.5. The significant coefficients of variation for sodium,
chloride and sulphate possibly reveal contamination (perhaps associated with coal mining
activities) in some of the groundwater samples.
The significant coefficient of variation for nitrates indicates that a measure of caution is
required when considering this water for human consumption. The significant coefficient of
variation associated with the SAR data indicates that the sodium hazard must receive
greater attention than that of salinity when considering this water for irrigation use (Draft
Gauteng EMF, 2013).
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(Source: Draft Gauteng EMF, 2013)
Figure 17: Locality of the Vryheid’s formation
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Table 3: Groundwater chemistry of Vryheid’s formation
Element/Parameter
Statistics drawn from a population of 26 samples
(units)
pH
Minimum
Mean
Maximum
Standard
Coefficient
value
value
value
Deviation
Variation
(SD)
(%CV)
4.8
7.5
8.5
0.6
8%
3.7
57.0
344.0
55.0
96 %
33.0
400.0
1835.0
353.0
88 %
Calcium (mg/l)
1.0
38.0
184.0
32.0
84 %
Magnesium (mg/l)
1.0
24.0
174.0
26.0
108 %
Sodium (mg/l)
1.0
43.0
492.0
80.0
186 %
Potassium (mg/l)
0.3
3.6
38.0
4.5
125 %
Chloride (mg/l)
1.0
44.0
919.0
124.0
282 %
Sulphate (mg/l)
1.0
47.0
919.0
113.0
240 %
Total Alkalinity (mg/l)
12.0
162.0
539.0
106.0
65 %
Nitrate (mg/l)
0.1
3.9
80.0
9.8
251 %
Fluoride (mg/l)
0.1
0.4
2.6
0.4
100 %
Langelier
Saturation
-5.5
-0.8
1.2
1.1
Adsorption
0.1
1.8
31.0
3.9
Electrical
Conductivity
(EC) (mS/m)
Total
Dissolved
Solids
(mg/l)
Index (LSI)
Sodium
217 %
Ratio (SAR)
Source: Draft Gauteng EMF, 2013
2.11
Geology
The geology forms the foundation for the development of the landscape, soils and
vegetation cover. It is also the source of minerals that form an important part of the
economy of an area.
The ELM compromises of three dominant geology formations, which are the ECCA Group,
Kalahari Group, and the Transvaal Rooiberg Griqualand-West Group (Figure 18).
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Figure 18: ELM geology
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2.11.1 ECCA Group
As Gondwana moved north towards the equator, thick clay and silt beds were laid down in
a large sea that occupied the Karoo basin. These sediments now form shales and
sandstones, and are easily weathered and often present slope stability problems. The
ECCA group is only located in small portions in the South and Eastern part of ELM.
2.11.2 Transvaal, Rooiberg, Griqualand-west
The Transvaal Supergroup is an end Archaean/earliest Proterozoic platform succession
developed on the Kaapvaal Craton. It contains three unconformity-bounded sequences
that are preserved and exposed in two geographically separate areas– the Transvaal
basin, where it circumscribes the Bushveld Complex, and the Griqualand West basin at the
western Kaapvaal margin, that extends into southern Botswana beneath Kalahari cover as
the Kanye basin. The two basins are separated by a broad basement high, referred to as
the Vryburg arch.
The Transvaal Rooiberg Griqualand-West Group makes out a large portion of the ELM
geological formation.
2.11.3
Kalahari Group
As with the Transvaal Rooiberg Griqualand-West Group the Kalahari group makes out a
large portion of the ELM geological formation.
2.11.4
Sedimentary Rocks
Sedimentary rocks are deposited on the Earth’s surface from waste material, consisting
mostly of mineral grains and rock fragments derived from the weathering and erosion of
pre-existing rocks. This material is carried away from its source under the influence of
gravity. This is often by running water, but it may also be by wind or glaciers. It is
eventually deposited as flat-lying layers, usually but not always on the sea floor.
As this deposition continues, this sediment is then buried under an ever-increasing load of
overlaying material. This causes compaction of the loose sediment, as water is gradually
expelled from the pore spaces between the sediment grains. Mineral matter being
deposited in the pore-spaces between the sediment grains induces cementation. The
action of these two processes converts the sedimentary material into a solid rock. The
dominant sedimentary rocks in the ELM area are as follows:
•
Dolomite
Most dolomites were probably formed from limestone after its deposition as magnesia-rich
brines converted the original calcite into dolomite. If this occurred soon after deposition,
the original features of the limestone are often preserved, but otherwise dolomite typically
occurs as a massive and rather granular rock with sugary or saccharoidal texture.
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•
Mudstone and Shale
Shales and mudstones are extremely fine-grained sedimentary rocks, mostly consisting of
clay particles less than 1/256 mm in diameter. Produced originally by chemical
weathering, especially of feldspar, clay particles eventually settle out in still water as mud,
often on the sea floor. Such muddy sediment first forms soft and sticky clay before all its
water was driven out by ever-increasing loads of sedimentary rock, converting it into
mudstone or shale. Shales are more fissile rocks than mudstones, splitting easily along
the bedding into thin sheets, and forming paper shales if particularly fissle.
•
Sandstone
Sandstones are formed by sand grains of varying diameter. They can be between 1/16mm
and 2mm. These grains are most usually quartz, occurring as fresh grains without any
cleavage, and often greyish in colour with a slightly frosted surface. Other minerals may
be present together with fragments of fine-grained rocks. Feldspar can best be
distinguished from quartz by its rather turbid appearance and the presence of cleavage.
Sandstones typically have a granular texture. They often weather to form a slightly rough
surface.
2.11.5 Metamorphic rocks
•
Quartzite
Quartzite is a metamorphic rock composed almost entirely of quartz, formed by the
recrystallization of quartz-rich sandstone, and consisting of an interlocking mosaic of
quartz grains, all tightly welded together. Although a metamorphic rock, it resembles a
quartz-cemented sandstone (or orthoquartzite). Its detrital nature is often only revealed
by the presence of feldspar grains (or other distinctive grains such as blue quartz).
Quartzite often fractures along a multitude of very smooth joined-planes, displaying a
polished appearance which cut across the individual quartz grains in the rock.
A more detailed geological representation is provided in Figure 19 and Figure 20.
Geology was also found to be a major gap in information.
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(Source: SDF, 2013/2014)
Figure 19: Geological profile
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Figure 20: Geological formation is ELM
2.12
Soils and Agricultural Potential
Mineral soils are largely derived from rocks or sediments that are constantly modified by
climate, plants, living organisms and animals so that, over a period of time, soil formation
occurs. The major factors that control the formation of soils are as follows:
•
Parent materials (geological or organic);
•
Climate (precipitation and temperature);
•
Biota (living organisms, vegetation, microbes, soil fauna and human beings);
•
Topography; and
•
Time.
The parent geological forms in the ELM area are described in Section 2.11 Geology. The
different parent types are responsible for certain soil properties depending on the
sediment build up and mineral content in the parent material and the above mentioned
factors.
The soil types occurring within the ELM boundary is outline in Figure 21.
Shale, quartzite, siltstone, sandstone, mudstone conglomerates
•
Soils are usually sandy-clay loams with a coarse sand grade in the topsoil;
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•
Soils are highly variable in depth. Average soil depth is however the shallowest of
all soils in the Gauteng province;
•
Soils are therefore wet in summer but dry in winter due to their shallow depth to
bedrock;
•
The westerly dip of the underlying bedrock ensures good lateral drainage except
where there is localized impedance due to the ‘damming’ effect of rocky-outcrops,
resulting in the accumulation of stagnant water;
•
Soils can be very stony, and stone lines are common;
•
The bulk density of both the top- and sub-soils is low;
•
Top-soils have low colour values and chromas due to a higher than average organic
matter content; and
•
These soils contain a higher percentage of organic carbon in the topsoil than that of
any other geological substrate.
Soils derived from shale parent material are slightly more fertile than those from quartzite
(Gauteng EMF 2013).
Dolomite, limestone, chert group
•
Dolomite derived soils are deep;
•
Only on ridge tops and steep slopes does the un-weathered rock come close to the
surface;
•
Soils are well-drained and appear to remain so even in bottomland- and river
terrace areas; and
•
The Griffin form is generally the wettest soil to develop on dolomite.
Solution and removal of dolomite by ground and surface waters result in the formation of
many caverns and underground channels, which often collapse. The normally horizontal
overlying strata are often crumpled into irregular folds as a result. These folds may be
seen quite close to the surface. They contain thin bands of chert, chalcedony and greyish
blue shales, forming a C-horizon in many soils.
Surface soils are classified as clays with a coarse sand grade. Fine sand is high, but
medium sand is low. Sub-soils are classified as silty clays. Soils on dolomite have on
average very high clay content. The stone content is generally low, but stone layers
containing either manganese concretions or chert fragments do occur.
Dolomite-derived soils have a high pH and low exchange acidity, most likely due to their
limestone origin. Although available P levels are low, the exchangeable bases Potassium
(K), Calcium (Ca) and Magnesium (Mg) are high, while exchangeable Aluminium (Al) is
low. The high levels of exchangeable bases on dolomite-derived soils can most probably
be ascribed to the parent material and high pH of these soils.
Diabase, gabbro, basalt, norite, diorite, dolerite group
Soils derived from these parent materials are not easily recognizable as they are
invariably buried below colluvial topsoil from the adjacent rock, from which there are
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usually separated by a stone layer. Diabase soils therefore mostly exist only as sub-soils
and their position as an important sub-division of soils relating to agricultural potential in
the area of the ELM. When occurring as colluvial material, these soils are recognizable by
their very much deeper weathering than the adjacent formations. The spheroidal
weathering of diabase rock is characteristic and is often a reliable indication of its
presence even when the rock has completely weathered. Diabase soils are high in silt, but
low in coarse fragments. Structure is invariably present and neocutanic horizons are
common.
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Figure 21: Soil within ELM
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