Ellisras Basin
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The Ellisras Basin is a geological basin that spans the border between South Africa and Botswana, extending west from the town of
Lephalale Lephalale (formerly Ellisras) is a coal mining town in the Limpopo province of South Africa immediately east of the Waterberg Coalfield. The town was established as Ellisras in 1960 and named after Patrick Ellis and Piet Erasmus who settled on a ...
(formerly Ellisras) in Limpopo province. Basin fill consists of sedimentary rocks of the
Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a perio ...
, with maximum thickness of 1,500 metres (4,900 ft). The principal economic resources in the basin are the Waterberg Coalfield in South African and the
Mmamabula Mmamabula is a planned coal mine and coal-fired power station to the east of the main road and rail corridor in Botswana between Gaborone and Francistown and south of the Serorome River. The power station would be near to the village of Mmaphash ...
coalfields in Botswana.


Location

The Ellisras Basin is about north of the main
Karoo Basin The Karoo Supergroup is the most widespread stratigraphy, stratigraphic unit in Africa south of the Kalahari Desert. The group (stratigraphy), supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Pennsylvan ...
. It extends east from the Kalahari Basin that underlies much of Botswana. The South African portion of the Ellisras basin is about from east to west and from north to south. The portion in Botswana extends much further to the north and south.


Structure

The basin formed at the same time as the
Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a perio ...
, and is around 310 to 180 million years old. It has the asymmetrical cross-section typical of a
half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...
, bounded on the north side by a fault zone that slopes steeply down to the axis of the basin, while the southern side slopes down more gently. On the north it is bounded by the Melinda Fault Zone (MFZ), a zone of
block fault Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in Earth's crust. Large areas of bedrock are broken up into blocks by faults. Blocks are characterized by relat ...
ing in the highly deformed and metamorphosed Archean-age rocks of the Limpopo Belt. The Eenzaamheid Fault Zone defines the southern boundary of the basin, where basin sediments abut older coarse-grained sandstones and minor
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s in the
Waterberg Group Waterberg is the name of two different mountains in Southern Africa, one in the Limpopo Province of South Africa, and one in the Otjozondjupa Region of Namibia. It may further refer to: * Waterberg Biosphere in South Africa * Waterberg Coalfield, ...
. There is an unconformity where the Karoo rocks of the basin overlie much older rocks. The southern flank of the basin is underlain by Waterberg Group rocks, which form between two-thirds and three-fourths of the basin floor. Mafic rocks of the Limpopo Belt underlie the basin floor in the northeast. The
Bushveld Complex The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It i ...
underlies the basin in the southeast.


Stratigraphy

There are few bedrock exposures in the Ellisras Basin; in most of the area, the relatively flat-lying sedimentary rock is covered by sands and soils. Accordingly, basin stratigraphy has been investigated primarily by drilling. Basin deposits reach their maximum thickness in the eastern portion of the basin, where the
Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a perio ...
is as much as thick; thickness decreases to the west. In the Ellisras Basin, the
Dwyka Group The Dwyka Group is one of four geological groups that compose the Karoo Supergroup. It is the lowermost geological group and heralds the commencement of sedimentation of the Karoo Supergroup. Based on stratigraphic position, lithostratigraphic co ...
( late Carboniferous to Permian) at the base of the Karoo System is represented by the Waterkloof Formation,
diamictite Diamictite (; from Ancient Greek ''δια'' (dia-): ''through'' and ''µεικτός'' (meiktós): ''mixed'') is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that ra ...
and other rocks of
colluvial Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combinati ...
and
glaciofluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvio ...
origin, associated with meltwater or floating ice from the
Karoo glaciation The late Paleozoic icehouse, also known as the Late Paleozoic Ice Age (LPIA) and formerly known as the Karoo ice age, was an ice age that began in the Late Devonian and ended in the Late Permian, occurring from 360 to 255 million years ago (Mya), ...
that produced the Dwyka Group tillite in the main Karoo Basin. Some sources also identify the overlying Wellington Formation, predominantly a mudstone, as a Dwyka Group equivalent. The Ecca Group, the second of the four groups in the Karoo Supergroup, is represented by the Wellington (alternatively assigned to the Dwyka Group), Swartrant (Vryheid Formation equivalent), and Grooteguluk Formations. The Swartrant Formation is predominantly
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
derived from a mafic source. It includes some coal. A 2-m-thick
tonstein Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning rock) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite. The clays often are ce ...
layer forms the base of the overlying Grootegeluk Formation, a 70- to 80-m thick unit that consists primarily of alternating layers of mudstone and coal. Grootegeluk Formation mudstone is primarily of a granodiorite composition, indicating that it was derived from different sediment source than the Swartrant Formation mudstone. Above the Grootegeluk Formation is the Eendragtpan Formation. This is a mudstone unit similar in composition to the Grootegeluk Formation mudstones, but it does not contain coal or other organic matter and is assigned to the
Beaufort Group The Beaufort Group is the third of the main subdivisions of the Karoo Supergroup in South Africa. It is composed of a lower Adelaide Subgroup and an upper Tarkastad Subgroup. It follows conformably after the Ecca Group and unconformably underlie ...
. The
Stormberg Group The Stormberg Group is one of the four geological groups that comprises the Karoo Supergroup in South Africa. It is the uppermost geological group representing the final phase of preserved sedimentation of the Karoo Basin. The Stormberg Group roc ...
is represented in the Ellisras Basin by the Greenwich Formation (about 30 m thick; possibly equivalent to the
Molteno Formation The Molteno Formation is a geological formation found in several localities in Lesotho and South Africa. It lies mainly south of Maseru, near Burgersdorp, Aliwal North, Dordrecht, Molteno, and Elliot. It extends as far north as Matatiele in the ...
in the Karoo Basin), Lisbon Formation (about 100 m thick; equivalent to the Elliot Formation), and the Clarens Formation (about 120 m thick). In the Ellisras Basin, the Drakensberg flood basalts that cap the Karoo stratigraphic sequence are represented by a 75-m basalt section with a potassium-argon age of 179 million (+/–5 million) years. The basalt is of a low Ti- Zr type, similar to basalts found in Springbok Flats, central Botswana and
Lesotho Lesotho ( ), officially the Kingdom of Lesotho, is a country landlocked country, landlocked as an Enclave and exclave, enclave in South Africa. It is situated in the Maloti Mountains and contains the Thabana Ntlenyana, highest mountains in Sou ...
. The most recent cover in the north comes from weathering of the gneiss of the Limpopo Mobile Belt and the Karoo rock, and in the south comes from weathering of the Waterberg sandstones.


Resource potential

Ellisras basin is of economic interest due to the presence of the Waterberg Coalfield, which is expected to become the most important coal resource in South Africa, although as of 2009 only the
Grootegeluk Coal Mine The Grootegeluk Coal Mine is an open cast coal mine within the Waterberg Coalfield of the Limpopo province of South Africa. It is operated by Exxaro. The mine is from Lephalale, and employs 2,000 people. Discovery In 1920, coal was found in t ...
was operational. Extensive exploration is also being done in the Botswana portion of the basin, where the basin hosts the
Mmamabula Mmamabula is a planned coal mine and coal-fired power station to the east of the main road and rail corridor in Botswana between Gaborone and Francistown and south of the Serorome River. The power station would be near to the village of Mmaphash ...
coalfield. The Waterberg Coalfield deposits are contained in the Grootegeluk Formation, and may account for over 50% of South Africa's coal reserves. Mining and utilisation of the coal is expected to be constrained by its depth, high ash content, and structural complexity. Additionally, severe limitations on water availability in the area could restrict activities that require large amounts of water, including mining and the
processing Processing is a free graphical library and integrated development environment (IDE) built for the electronic arts, new media art, and visual design communities with the purpose of teaching non-programmers the fundamentals of computer programming ...
of high-ash-content coal. There is interest in the potential for
coalbed methane Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Au ...
development from deep coal beds in the Ellisras Basin, particularly in the northeastern parts of the basin where the Grootgeluk Formation coal is found at depths greater than . The high vitrinite content of the coal indicates a significant potential for coalbed methane production. The methane resource in the basin has been estimated to total one trillion cubic feet (28 billion cubic meters) of gas, but this estimate is based on very little data. As of 2011, the Anglo Operations mining group had begun investigating the basin's methane potential by drilling more than 70 wells and conducting production tests.


References

Citations Sources * * * * * * * * * * {{refend Geology of South Africa Basins