Ecca Group

The Ecca Group is the second of the main subdivisions of the Karoo Supergroup of geological strata in southern Africa. It mainly follows conformably after the Dwyka Group in some sections, but in some localities overlying unconformably over much older basement rocks. It underlies the Beaufort Group in all known outcrops and exposures. Based on stratigraphic position, lithostratigraphic correlation, palynological analyses, and other means of geological dating, the Ecca Group ranges between Early to earliest Middle Permian (Asselian - Roadian) in age.[1][2][3]

Ecca Group
Stratigraphic range: Late Late Carboniferous-Early Permian
~303–290 Ma
TypeGeologic group
Sub-unitsWestern/Northwestern facies: Prince Albert Formation, Whitehill Formation, Collingham Formation, Tierberg Formation, Skoorsteenberg Formation, Kookfontein Formation, Waterford Formation.Southern facies: Prince Albert Formation, Whitehill Formation, Collingham Formation, Vischkuil Formation, Laingsburg Formation, Fort Brown Formation, Waterford Formation. Northeastern facies: Pietermaritzburg Formation, Vryheid Formation, Volksrust Formation
UnderliesBeaufort Group
OverliesDwyka Group
Lithology
PrimaryShale, mudstone, claystone, siltstone, chert, dolomite, coal
OtherQuartzite, pyrite
Location
RegionWestern & Eastern Cape
Country South Africa  Eswatini  Namibia  Botswana  Zimbabwe

A simplified geological map of the outcrops of Karoo Supergroup rocks in Southern Africa. The Ecca Group is represented by the orange key on the map.

Background

During the time of the deposition of the Ecca Group, the depositional environment, with some exceptions, was predominantly marine. The Ecca sea was vast but shallow, reaching only around 500 m at its deepest in its west/northwestern and southern facies where the Tanqua and Laingsburg Depocenters are situated respectively. The marine environment ranged from deep pelagic, submarine fan systems in the lower deposits which grade steadily north-eastwards to shallow marine deposits including shelf (continental) marine and marginal marine facies, and finally to beach deposits in younger successions. Coal-bearing fluvial-deltaic, and peatbog settings are also well known from the Ecca Group.[3][4][5][6]

The Ecca Group was deposited in a vast retroarc foreland basin. This foreland system was caused by crustal uplift (orogenesis) 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 mountain-building and erosion caused by the growing Gondwanide mountain range was the initial subsidence mechanism acting on the Karoo Basin. Flexural tectonics partitioned the Karoo Basin into the foredeep, forebulge, and backbulge flexural provinces. This resulted in deposition of the Karoo Basin.[7][8][9][10][11]

Geographic extent

A mountain in the Tanqua Karoo, South Africa, with multiple layers of turbidites formed in the south-western portion of the Karoo Sea about 300 million years

The rocks of the Ecca Group first appear near Sutherland in its westernmost deposits, and continues east through Laingsburg, Prince Albert, Jansenville, Grahamstown, and up until the coast near Port Alfred. In the central north deposits are found near Britstown, running along the Orange River between Petrusville and Hopetown. In the extreme northeast deposits are found east of Johannesburg past Vryheid, Durban, Pietermaritzburg and all the way down to Port St. Johns in the southeast.[12][13]

Stratigraphic units

The Ecca Group comprises sixteen recognized geological formations.[14] These individual formations have been grouped into three geographical areas, which are the southern, western/northwestern, and northeastern facies successions. In the east of South Africa there are deposits of as of yet undifferentiated mudstone sequences attributed to the Ecca Group.[15][16][17][18][19]

With the exception of the Prince Albert, Whitehill, Collingham, and the uppermost Waterford Formations which are found in both the southern and western/northwestern facies, the geological formations of the Ecca Group can only be found in one of the three previously aforementioned facies successions. This is because each of these facies successions represents differing preserved environments that can be observed in their diagnostic geological features. The facies successions, along with their geological formations, are described below:[20][21][22][23]

Western/Northwestern Ecca facies

This facies succession is purely marine. The rocks contain a complete transition, grading laterally into one other, from basin-floor marine deposits through to channelized submarine slope to shelf, pro-delta and beach environment deposits. The deposits of the western/northwestern facies fall within the Tanqua Depocenter, one of the vast submarine fan systems known from the marine Ecca.[24] Associated formations are listed below (from oldest to youngest):

A fossil Mesosaurus tenuidens (syn.Mesosaurus brasiliensis), Permian (270 myo)
  • Collingham Formation: Is composed of thin beds of siliceous, dark grey mudstone alternated with softer, yellow-grey tuff beds. The fine-grained nature of the sediments indicate that the depositional environment was a low energy suspension setting in a submarine fan environment. The presence of tuff beds indicates that volcanic activity took place during the time of sediment deposition.
  • Tierberg Formation: Predominantly composed of blackish, planar, argillaceous shales. Tuff beds are found in the lower sections of this formation while calcareous concretions and clay pellet conglomerates are found in the upper sections. Upwards-coarsening sequences of mudstones, siltstones, and sandstones that exhibit ball-and-pillow structures are also found in the uppermost sections. The presence of the shales in the lower Tierberg indicate a low energy marine environment that transitioned to pro-delta to distal delta depositional environments with the appearance of the upward coarsening sequences and other associated geological features. Various invertebrate fossils have been recovered, namely fish scales, sponge spiracles, and trace fossils of Planolites and Nereites.
  • Skoorsteenberg Formation: Only found in the southernmost area of the western facies. A lens-shaped formation composed of five sandstone-rich units that are interbedded with shale layers. These contain bouma turbidite sequences. The depositional environment is thought to have been an unstable delta-front slope of a fluvial delta system. Trace fossils of worm feeding trails and Glossopteris fossils are common.
  • Kookfontein Formation: The lower section of this formation is composed of horizontally laminated dark-grey shales that are interbedded with clastic rhythmites. Minor upward thickening cycles are observed here which grow more prominent in the upper sections. The rock type changes in the upper sections to alternating siltstone and fine-grained sandstone beds. This formation is a continuation of the environmental facies of the Skoorsteenberg Formation where sediments were deposited in a pro-delta setting.
  • Waterford Formation: The western/northwestern deposits of this formation are composed of a mix of shale, siltstone, rhythmite, and fine to medium-grained sandstones. The sandstones and siltstones are horizontally laminated and often exhibit wave-rippled surfaces. Alternating slump and slide and coarsening upward cycles are observed in the lower sections while in the upper sections ball-and-pillow structures and channel-fill deposits are observed. These features indicate that the depositional environment constituted delta front deposits in the lower sections which transitioned to a fluvial delta-plain. Fossilized tree logs are frequently found in the sandstone layers, and in the other rock layers Cruziana and Skolithos burrow traces are found. Together with the underlying Tierberg, Skoorsteenberg, and Kookfontein Formation, the Waterford Formation forms an upward-coarsening deltaic megacycle.[30][31][32]

Southern Ecca facies

This facies succession is the largest of the three facies succession. Its lowermost formations are deep marine comprising basin floor pelagic sediments and submarine fan systems that grade upwards into channelized submarine slopes to shelf marine and beach environments. The Laingsburg Depocenter is found in this facies succession and include the following formations (from oldest to youngest):

Northeastern Ecca facies

The northeastern facies is shallow marine in its lowermost and uppermost sections, and then changes to coal-bearing fluvial-deltaic peat swamp settings in its central deposits. The northern facies often overlies unconformably on much older basement rocks unlike the other facies of the Ecca Group. It comprises three geological formations (from oldest to youngest):

Example of a peat swamp environment, similar to the depositional environment preserved in the Vryheid Formation rocks. Plant material would have sunk below surface to the bottom of the swamp, being preserved in the anoxic environment. Over time plant material would be compacted and buried by sediment. Over millions of years the plant material mineralized, becoming coal

Correlation

Biostratigraphic correlation of fossils in the greater Gondwana across present-day South America, southern Africa, Antarctica and Australia

The lower geological formations of the Ecca Group, particularly the rocks of the Whitehill Formation, correlate in age with the Huab Basin of northwestern Namibia, and lower formations of the Kalahari Basin found in Namibia, Botswana, and Zimbabwe. Near the small town of Khorixas in Namibia there is a locally well-known national monument called the Petrified Forest. Petrified logs were brought into the area and are considered to have been sourced from the nearby deposits of the Huab Basin.[43][44][45]

Fossil tree from the Permian Ecca Group of the Karoo Sequence at the petrified forest, east of Doro !Nawas, Namibia

Abroad, Ecca-aged deposits are known from the Paraná Basin of Brazil and the Petolas Basin of both Brazil and Uruguay where fossils of Mesosaurus and Glossopteris have also been recovered. Finally, geological dating has also proven the lower Ecca formations to correlate with the Barnett Shale and Marcellus Formation of the United States.

See also

References

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  2. R.M.H. Smith, P.G.Eriksson, W.J.Botha (1993-01-01). "A review of the stratigraphy and sedimentary environments of the Karoo-aged basins of Southern Africa". Journal of African Earth Sciences (And the Middle East). 16 (1–2): 143–169. Bibcode:1993JAfES..16..143S. doi:10.1016/0899-5362(93)90164-L. ISSN 0899-5362.CS1 maint: multiple names: authors list (link)
  3. Jirah, Sifelani; McPhee, Blair W.; Viglietti, Pia A.; Bamford, Marion K.; Choiniere, Jonah N.; Hancox, P. John; Barbolini, Natasha; Day, Michael O.; Rubidge, Bruce S. (2016), "Advances in Nonmarine Karoo Biostratigraphy: Significance for Understanding Basin Development", Origin and Evolution of the Cape Mountains and Karoo Basin, Regional Geology Reviews, Springer, Cham, pp. 141–149, doi:10.1007/978-3-319-40859-0_14, ISBN 9783319408583
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  5. Richard G.Vos, David K.Hobday (1977-01-01). "Storm beach deposits in the late Palaeozoic Ecca Group of South Africa". Sedimentary Geology. 19: 217–232. Bibcode:1977SedG...19..217V. doi:10.1016/0037-0738(77)90032-X. ISSN 0037-0738.
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