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Sedimentary Deposits

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Sedimentary Deposits

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Summarised from Hydrogeology of the Main Karoo Basin: Current Knowledge and Future Research Needs WRC Report Project K860 (2001).


The lithostratigraphic units of the Karoo Supergroup outcrop concentrically around the Main Karoo Basin and include the following:

1. Dywka Group: The Group consists mainly of diamictite (tillite) which is generally massive with little jointing, but it may be stratified in places. Subordinate rock types are conglomerate, sandstone, rhythmite and mudrock (both with and without dropstones).  In the southern part of the basin, under the influence of the Cape Fold Belt, the diamictite display a distinctive ‘tombstone’ morphology as a result of selective weathering along axial-plane cleavage.




2. Ecca Group: This Group consists of the following Formations:


The Prince Albert Formation is confined to the south-western half of the Karoo Basin.  The northern facies is characterised by the predominance of greyish to olive-green, micaceous shale and grey, silty shale, as well as a pronounced transition from the underlying glacial deposits.  Dark-grey to black carbonaceous shale and fine- to medium-grained feldspathic arenite and wacke are also present.  The southern facies is characterised by the predominance of dark-grey, pyrite-bearing, splintery shale, siltstone and the presence of dark-coloured chert and phosphatic nodules and lenses.


The mudrocks of the Whitehill Formation consist of shale that is very thinly laminated and contains up to 14% carbonaceous material. The Whitehill Formation loses its distinctive lithological character towards the northeast with its lower part containing siltstone and very fine-grained sandstone.


Outcrops of the Collingham Formation are confined to the southern and western margins of the Main Karoo Basin.  The formation is generally between 30 and 70m thick and comprises a rhythmic alternation of thin, continuous beds (average 5cm) of hard, dark grey, siliceous mudrock and very thin beds (average 2cm) of softer yellowish tuff (K-bentonite).   In the western part of the area minor sandstone and siltstone units occur in the upper half of the formation, while the distinctive Matjiesfontein Chert Bed (0.2-0.6m thick) is present in the lower half.


The predominantly argillaceous Vischkuil Formation overlies the Collingham Formation in the southwestern part of the basin.   The Vischkuil Formation consists essentially of dark shale, alternating with subordinate fine-grained sandstone, siltstone and minor yellowish tuff layers. The shale units are thinly laminated or structureless, and contain phosphatic and calcareous lenses, and ferruginous layers with liesegang structures. Sandstone (greywacke) beds vary from 0.3 to 1.5m in thickness and increase in abundance upwards.


The Laingsburg Formation usually comprises four sandstone-rich units separated by shale units and is approximately 400 m thick in its type area.  The thick massive sandstone units (up to 30 m thick, with individual beds up to 4m thick) are fine- to medium-grained with sharp upper and lower contacts. They grade upward into planar-laminated siltstone and shale, which commonly contain coalified plant fragments and lenticular calcareous concretions.


The Ripon Formation consists of poorly sorted, fine- to very fine-grained lithofeldspathic sandstone alternating with dark grey clastic rhythmite and mudrock.


The Fort Brown Formation consists of rhythmite and mudrock with minor sandstone intercalations and displays an overall coarsening-upward tendency.  At certain localities, one or more fairly prominent sandstone units occur some distance below the upper contact.  Individual sand/silt and silt/clay layers comprising rhythmite units of similar thickness, ranging from a few millimetres to a few centimetres, are laterally persistent.


The arenaceous Waterford Formation overlies the Fort Brown Formation.   The formation comprises alternating very fine-grained, lithofeldspathic sandstone and mudrock or clastic rhythmite units.


The bulk of the Tierberg Formation comprises well-laminated, dark grey to black shale.  Some yellowish tuffaceous beds up to 10cm thick occur in the lower part of the succession along the western and northern margins of the Basin.  Calcareous concretions are common towards the top of the formation.  Clastic rhythmites occur at various levels in the sequence.


The Skoorsteenberg Formation is a lenticular, arenaceous unit located between the Tierberg and Kookfontein Formations in the southwestern part of the basin.  The six sandstone-rich units represent arenaceous submarine fan deposits, which are separated by equally thick basin-plain shale units.


The Kookfontein Formation overlies the Skoorsteenberg Formation.  The lower part of the formation comprises horizontally laminated dark-grey shales alternating with clastic rhythmites, which form minor upward-thickening cycles.  The cycles become more prominent towards the top of the Formation where they are up to 15m thick and consist of alternating siltstone and thin sandstone beds, capped in places by a thick sandstone bed.


The Waterford Formation, consists of fine- to medium-grained sandstone, siltstone, shale and rhythmite.  The lower part of the Formation is characterized by upward-coarsening cycles of sediments, which are capped by extensive sheet-like sandstones and alternating chaotic, slump and slide deposits.  The upper portion of the Formation consists of sandstone (±8m thick), siltstone, ball-and-pillow layers and channel-fill deposits.


The Pietermaritzburg Formation comprises dark, upward coarsening, silty mudrock, which is heavily bioturbated.  Pene-contemporaneously deformed sandy and silty beds appear near the top of the formation.


The Vryheid Formation comprises mudrock, rhythmite, siltstone and fine- to coarse-grained sandstone (pebbly in places).  The Formation contains up to five (mineable) coal seams.  The different lithofacies are mainly arranged in upward-coarsening deltaic cycles (up to 80m thick in the southeast). Fining-upward fluvial cycles, of which up to six are present in the east, are typically sheet-like in geometry, although some form valley-fill deposits. They comprise coarse-grained to pebbly, immature sandstones - with an abrupt upward transition into fine-grained sediments and coal seams.


The Volksrust Formation consists of grey to black, silty shale with thin, usually bioturbated, siltstone or sandstone lenses and beds, particularly towards its upper and lower boundaries.  Thin phosphate and carbonate beds and concretions are relatively common.





3. Beaufort Group consists of the following subgroups:



In the southern and central parts of the Basin the Adelaide Subgroup consists of alternating bluish-grey, greenish-grey or greyish-red mudrock and grey, very fine to medium-grained, lithofeldspathic sandstone.


b) The early Triassic Tarkastad Subgroup is characterized by a greater abundance of both sandstone and red mudstone when compared with the Adelaide      Subgroup.


c) The Katberg Formation sandstones are light brownish grey to greenish grey, fine- to medium-grained and contain scattered pebbles up to 15 cm in          diameter within the coastal outcrops. Oval to spherical calcareous concretions, 3–10 cm in diameter, are also common.  The Burgersdorp Formation        sandstones are greenish grey to light brownish grey and are fine-grained.  The average thickness of sandstone units in the central portion of the             Formation in the main outcrop area is 2m.





4. Molteno, Elliot and Clarens Formations


The late Triassic Molteno Formation comprises alternating medium- to coarse-grained, ‘glittering’ sandstones and grey mudrocks, with well-preserved plant fossils and sporadic coal seams.



The late Triassic to early Jurassic Elliot Formation comprises an alternating sequence of mudrock and subordinate fine- to medium-grained sandstone.



The early to middle Jurassic Clarens Formation  represents the final phase of the Karoo sedimentation and consists mainly of wind-blown, fine-grained sandstone and siltstone.  Channel-filled wadi sandstones and horizontally-laminated sheet-flood sandstone are also present.  Minor interbedded sandstone, siltstone and mudstone represent localised playa lake deposits.






Dwyka Group

The Dwyka diamictite and shale have very low hydraulic conductivities [~ 10 -11 to 10 -12 m.s -1 and virtually no primary voids. The Dwyka Group constitutes a very low-yielding fractured aquifer and water is confined within narrow discontinuities like jointing and fracturing. They therefore tend to form aquitards rather than aquifers.  The few sandstone bodies deposited in the glacial valleys of the northern facies are very limited in extent, and sealed off by the diamictite or mudrock.  Since the Dwyka sediments were deposited mainly under marine conditions, the water in these aquifers tends to be saline. Exploitable aquifers thus only exist at few localities in the Basin, where sand and gravel were deposited on beaches or where the Dwyka Group was fractured significantly.  In general, the Dwyka Group is thus not an ideal unit for the large-scale development of groundwater.

Ecca Group

The Ecca Group consists mainly of shales that are very dense, they are often overlooked as significant sources of groundwater.  One should thus not neglect the Ecca rocks as possible sources for groundwater, especially the deltaic sandstone facies.  The permeabilities of these sandstones are usually very low.  The main reason for this is that the sandstones are usually poorly sorted.

Beaufort Group

The geometry of these aquifers is complicated by the lateral migration of meandering streams over a floodplain.  Aquifers in the Beaufort Group will thus not only be multi-layered, but also multi-porous with variable thicknesses.  The contact plane between two different sedimentary layers will cause a discontinuity in the hydraulic properties of the composite aquifer.  The life-span of a high-yielding borehole in the Beaufort Group may therefore be limited, if the aquifer is not recharged frequently.

Molteno, Elliot and Clarens Formations


The characteristics and depositional history of the Molteno Formation indicate that the Formation should form an “ideal” aquifer. This applies not only to the pebble conglomerates and coarse-grained sandstones at the base of the Formation, but also to the other sedimentary bodies.


The largest part of the Elliot Formation consists of red mudstone.  The Formation thus represents more of an aquitard than an aquifer.


The Clarens Formation consists almost entirely of well-sorted, medium- to fine-grained sandstones, deposited as thick consistent layers.  It is poorly fractured and has a very low permeability.  The Formation may therefore be able to store large volumes of water, but is unable to release it quickly.