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28-668: The Ellisras Basin is a geological basin that spans the border between South Africa and Botswana , extending west from the town of Lephalale (formerly Ellisras) in Limpopo province. Basin fill consists of sedimentary rocks of the Karoo Supergroup , 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

56-477: A distinctive glittering appearance. Finer-grained sandstones occur throughout the lower deposits of the Molteno Formation, growing coarser toward the upper sections. Sporadic coal seams populate the Molteno Formation, and less common quartz pebble and cobble conglomerates occur in the lower sections. These geological features form part of six stacked fining upward cycles where repeating patterns of

84-453: Is a large-scale structural formation of rock strata formed by tectonic warping ( folding ) of previously flat-lying strata into a syncline fold. They are geological depressions , the inverse of domes . Elongated structural basins are a type of geological trough . Some structural basins are sedimentary basins , aggregations of sediment that filled up a depression or accumulated in an area; others were formed by tectonic events long after

112-614: Is also being done in the Botswana portion of the basin, where the basin hosts the Mmamabula 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

140-401: Is predominantly mudstone derived from a mafic source. It includes some coal. A 2-m-thick tonstein 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

168-715: Is the lowermost geological formation of the Stormberg Group . It overlies the Burgersdorp Formation of the Beaufort Group , and underlies the lower Elliot Formation (LEF). Containing alternating finely-grained, medium-grained, and coarse-grained sandstones , the formation features greyish mudstone layers with the coarser sandstones exhibiting trough cross-bedding structures. The mudstones also contain siltstones . The sandstones contain secondary quartz over growths and clasts , giving them

196-508: The Equisetum but appear to have been far less common, as only a few specimens have been recovered. Lycopods , bryophytes , Ginkgoales , and 50 species of fern have also been found, as well as associated plant frutifications, organs, and pollens . Fossil leaf impressions and other soft vegetative material of these species appear commonly in the low-energy mudstone-rich depositional environments. The vegetative material occurs near to where

224-809: The Karoo glaciation 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

252-646: The Mmamabula coalfields in Botswana . The Ellisras Basin is about 320 kilometres (200 mi) north of the main Karoo Basin . It extends east from the Kalahari Basin that underlies much of Botswana. The South African portion of the Ellisras basin is about 90 kilometres (56 mi) from east to west and 35 kilometres (22 mi) from north to south. The portion in Botswana extends much further to

280-1021: The Molteno Formation 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 . The most recent cover in

308-605: The insect fauna particularly important, because well-preserved fossil remains of insects are very rare. The plant and insect fossils had unique ecological co-associations, and occupied distinct habitats . The dominant fossil flora is associated with seven recognized habitat types. The first two habitats include arboreal species of Dicroidium , an extinct genus of seed fern that grew in either lush, riparian forests or more temperate woodlands . Herbaceous forms of Dicroidium are also found but did not dominate. Nineteen species of Dicroidium alone have been recovered from

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336-465: The Earth's climate when the Molteno Formation rocks were deposited. Local and international paleobotanists and entomologists revere the Molteno Formation for its richly diverse plant and insect fossils. The Molteno Formation fossils include 204 plant species and 333 insect species, making it one of the richest Triassic -age plant and insect assemblages ever discovered. Entomologists consider

364-507: The Molteno Formation. The next habitat is also temperate woodland , but a different seed fern species dominates it: Sphernobaiera . Another habitat contains thickets of the conifer species Heidiphyllum . Only two other species of conifer are known from the Molteno Formation. Finally, there are Equisetum (horsetail) marshes , comprising 21 species in five genera, and Ginkgophytopsis fern meadows. The seven habitat types contain various cycad species. Cycads were as diverse as

392-621: 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 Stormberg Group is represented in the Ellisras Basin by the Greenwich Formation (about 30 m thick; possibly equivalent to

420-499: The area could restrict activities that require large amounts of water, including mining and the processing of high-ash-content coal. There is interest in the potential for coalbed methane 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 300 metres (980 ft). The high vitrinite content of

448-494: The basin floor. Mafic rocks of the Limpopo Belt underlie the basin floor in the northeast. The Bushveld Complex underlies the basin in the southeast. 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

476-550: 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. Citations Sources Geological basin A structural basin

504-486: The coarser-grained rocks (conglomerates, coarse-grained sandstones) grade upwards into finer-grained rocks (medium to fine-grained sandstone, mudrocks , and coal seams). Sandstones are more common in the lower deposits while mudstones are more dominant in the upper deposits. However, the mudstones are part of repeating fining upward cycles with coarser-grained sandstones and thin coal seams. The Molteno Formation has five members, from oldest to youngest: The sediments of

532-450: The different Molteno Formation members preserve the different environments in which they were first deposited. The conglomerates and coarser-grained sandstones were deposited in high-energy braided fluvial systems. The more medium to fine-grained sandstones were deposited in mixed load meandering river channels. The mudrocks were deposited in distal floodplain deposits , and the coal seams in localized peat bogs . The Molteno Formation

560-612: The eastern portion of the basin, where the Karoo Supergroup is as much as 1,500 metres (4,900 ft) thick; thickness decreases to the west. In the Ellisras Basin, the Dwyka Group ( late Carboniferous to Permian ) at the base of the Karoo System is represented by the Waterkloof Formation, diamictite and other rocks of colluvial and glaciofluvial origin, associated with meltwater or floating ice from

588-596: 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 mudstones in the Waterberg Group . 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

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616-448: The most commonly found fossil insect . However, beetles are the most rich in species , with 161 species documented. Other insect fossils include dragonflies , dragonfly aquatic nymphs , and clam shrimp species. Insect and other invertebrate trace fossils have been found in the fine sandstones and mudstone deposits. Dinosaur trackways have been found in one locality, however, no vertebrate remains have yet been yielded from

644-494: The north and south. The basin formed at the same time as the Karoo Supergroup , and is around 310 to 180 million years old. It has the asymmetrical cross-section typical of a half-graben , 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 faulting in

672-683: 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. 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 was operational. Extensive exploration

700-468: The plants had originally grown. On the other hand, floods often swept woody material far away from its original habitat. Petrified wood fragments, cones , and other woody material predominantly lie in the high-energy depositional environments dominated by the coarser sandstones. The Molteno Formation's insects are considered its most important fossils because of their high diversity. The insect fossils include 333 species and 117 genera. Cockroaches are

728-512: The sedimentary layers were deposited. Basins may appear on a geologic map as roughly circular or elliptical, with concentric layers. Because the strata dip toward the center, the exposed strata in a basin are progressively younger from the outside in, with the youngest rocks in the center. Basins are often large in areal extent, often hundreds of kilometers across. Structural basins are often important sources of coal , petroleum , and groundwater . Molteno Formation The Molteno Formation

756-414: The time. While the different depositional environments were temperate overall, they experienced extreme seasonal differences. Winters were near-freezing with moderate rainfall, and midsummer temperatures were harsh. The Permo-Triassic extinction event occurred a few million years before the Molteno Formation rock sediments were deposited. However, its lingering effects continued to influence the stability of

784-524: Was part of a greater inland basin, which the Gondwanide mountain range bordered in the south. Braided rivers flowed down from the mountains northwards, steadily joining meandering river channels and eventually meeting with floodplains and marshes. These depositional environments each have associated fossil flora and fauna that are unique to them. The fossil flora and fauna co-associations reveal different, distinctive habitats that were present during

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