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.[Keyser, A. W., & Smith, R. M. H. (1978). Vertebrate biozonation of the Beaufort Group with special reference to the western Karoo Basin. Geological Survey, Department of Mineral And Energy Affairs, Republic of South Africa.][Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.]
Background
During the period when
sedimentation of the Beaufort Group rocks took place, the Ecca sea had retreated to the northeastern
Karoo Supergroup. All
sediment deposition at this time took place in a terrestrial, although in a predominantly fluvial or
Alluvial plain environment that was seasonally
arid. This environment covered a vast area and deposition was influenced by a
Back-arc region foreland basin. This foreland system was caused by
Tectonic uplift (
Orogeny) that had previously begun to take course due to the
subduction of the Palaeo-pacific plate beneath the Gondwanan Plate. This resulted in the rise of the Gondwanide mountain range in what is known as the Gondwanide orogeny. The continuation of mountain-building and erosion from the growing Gondwanide mountain chain and associated subduction created accommodation space for sediment deposition in the Karoo Basin. Orogenic loading was the initial subsidence mechanism acting on the Karoo Basin and flexural tectonics partitioned the Karoo Basin into the
Foreland basin,
forebulge, and backbulge flexural provinces. Orogenic loading and unloading caused changes in position of the forebulge and foredeep. This resulted in the deposition zones shifting from the proximal or distal regions of the Karoo Basin.
The Beaufort Group rocks are predominantly mudstone-dominated up until the upper sections in the lower Tarkastad Subgroup. Tuffs are also found due to concurrent volcanic activity that took place with the foreland tectonics.
Geographic extent
The geological formations of the Beaufort Group are outcrop over approximately 145 000 km
2, attaining a total thickness of around 6000 m thick at its thickest outcrops. In the west, the lowermost Beaufort Group rocks are found east of Laingsburg and remain continuous eastward to East London. Deposits are also found in the central Karoo and continue north-north-eastwards to Gariep Dam, Colesberg, and up to Bloemfontein. In the extreme north and north-east, the Beaufort Group outcrops in Harrismith and northeastern KwaZulu-Natal.
Stratigraphic units
Adelaide Subgroup
The Adelaide Subgroup is the lower subgroup of the Beaufort Group and contains all Middle to Lopingian-aged rocks. This subgroup contains six geological formations in total, however, these formations are latitude specific. These formations are the Abrahaamskraal and Teekloof Formations, west of 24°E, Middleton and Balfour Formations east of 24°E, and the Normandien and Emakwezini Formations in the northern Free-State and KwaZulu-Natal provinces. Composing the lower Beaufort succession, the rocks are mudstone-dominated. The in the lower Beaufort Group are mainly greenish-grey to blueish-grey and gradually change to greyish-red, reddish-brown or purple in color. The dominant presence of mudstones in the lower Beaufort Group represent tranquil depositional settings such as overbank or floodplain facies associations. The formations here are listed below (from oldest to youngest):
West of 24 degrees
-
Abrahamskraal Formation: The Abrahamskraal Formation is found in both its western and eastern deposits. Previously, east of 24 degrees deposits of the Abrahaamskraal Formation were known as the Koonap Formation.
-
Teekloof Formation: Floodplain facies association comprising some greenish-grey mudstones. However, redder mudstones are dominant.
East of 24 degrees
-
Middleton Formation: Lies conformably over the Abrahamskraal Formation in the east. Semi-arid climate supported a lush flora and fauna that thrived along Meander and semi-permanent . The formation contains lenses of red mudstone which are likely to have been deposited in a subaerial fluvial environment.
-
Balfour Formation: Low-energy, fluvial environmental facies, most likely one that had Meander. Mudstone-rich.
-
Normadien Formation (northeastern region only): Mudstone-rich, but these are interbedded with diagnostic layers of very coarse-grained that exhibit coarsening upward cycles.
-
Emakwezini Formation (northeastern KwaZulu-Natal only): An extremely understudied geological formation that outcrops in a thin, faulted, and meridional belt in central Eswatini and the southern Lebombo Basin of northeastern KwaZulu-Natal. It is actively mined for its coal seams which are found intercalated with fine, greenish-grey to brown mudstones and . The mudstones contain layers of fining upward successions of coarse-grained, yellow-white sandstones. Rare limestone lenses are sometimes also found. The mudstones have yielded diverse fossils, mainly of plants such as Phyllotheca, Glossopteris and associated Dictyopteridium. Insect are also found, such as of Neoliomopterum picturatum, various Mollusca fossils, fish scales of Coelacanthus and unidentified ganoid fish scales and teeth. Being upper Permian in age, it correlates with the Normandien and Balfour Formations, but is differentiated from these two formations due to its unique Sedimentary rock facies structure. Unlike the rest of the Beaufort Group sequence, the Emakwezini Formation was deposited in a permanently wet, fluvial-Lake environment where peat were present.
[Jourbert, M.R., 1994. Emakwezini Formation. Lexicon of South African Stratigraphy: Part, 1.]
Tarkastad Subgroup
There are no equivalent deposits of the Tarkastad Subgroup west of 24°E. This is either due to erosion of the lowlands west of 24°E or sediment deposition in the western section of the Karoo Supergroup ceased at the end of the Permian. In the lower sections sandstones dominate, especially in the Katberg Formation. In the upper units the sandstone to mudstone ratio steadily evens out.
-
Katberg Formation: Dominated by fine to medium-grained sandstones. Alluvial fan and braided river environmental facies.
-
Burgersdorp Formation: Very mudstone-rich which are reddish or purple in color.
-
Verkykerskop Formation (extreme northeast only): Composed entirely of fine-grained sandstone interbedded with very coarse sandstones.
-
Driekoppen Formation (extreme northeast only): Nearly entirely composed of mudstone.
Paleontology
The Beaufort Group is internationally renowned for its diverse
fossil fauna,
in particular for its
therapsid fossils.
[Boonstra, L.D., 1969. The fauna of the Tapinocephalus Zone (Beaufort beds of the Karoo).] The entire expanse of this geological group has been categorized into eight fossil
or assemblage zones.
[Kitching, J. W. (1970). A short review of the Beaufort zoning in South Africa. In Second Gondwana Symposium Proceedings and Papers (Vol. 1, pp. 309-312).] These assemblage zones are listed below:
-
Cynognathus Assemblage Zone (Youngest)
-
Lystrosaurus Assemblage Zone
-
Daptocephalus Assemblage Zone
-
Cistecephalus Assemblage Zone
-
Tropidostoma Assemblage Zone
-
Pristerognathus Assemblage Zone
-
Tapinocephalus Assemblage Zone
-
Eodicynodon Assemblage Zone (Oldest)
The Beaufort Group deposits also yield numerous insect, plant, and trace fossils.
Correlation
The Beaufort Group rocks correlate chronologically with numerous other geological formations and groups within
southern Africa and abroad. Most notably from numerous localities in
Russia,
,
,
,
Madagascar,
India, and
Australia.
See also
