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Stormberg Group

From Wikipedia, the free encyclopedia
Triassic/Jurassic geological group in the Karoo Supergroup in South Africa
Stormberg Group
Stratigraphic range:Late Triassic-Early Jurassic
~237–182 Ma
TypeGeologic group
Unit ofKaroo Supergroup
Sub-unitsMolteno,Elliot &Clarens Formations
UnderliesDrakensberg Group
OverliesBeaufort Group
Thicknessup to 4,593.176 feet (1,400 m)
Lithology
PrimaryMudstone,claystone,siltstone
OtherSandstone,shale,tuff
Location
RegionEastern Cape,KwaZulu-Natal,Free State,Lesotho,Matabeleland,Central District,Karas, andHardap
Country South Africa
 Lesotho
 Zimbabwe
 Botswana
ExtentKaroo Basin
Type section
Named forStormberg Mountains

Simplified geological map of the outcrops of Karoo Supergroup rocks in Southern Africa. The Stormberg Group is represented by the green key on the map.

TheStormberg Group is one of the four geological groups that comprises theKaroo Supergroup in South Africa. It is the uppermost geological group representing the final phase of preservedsedimentation of theKaroo Basin. The Stormberg Group rocks are considered to range between LowerTriassic (Olenekian) to Lower Jurassic (Pliensbachian) in age. These estimates are based on means of geological dating includingstratigraphic position,lithostratigraphic andbiostratigraphic correlations, andpalynological analyses.[1][2][3]

Background

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Sediment deposition of the Stormberg Group took place in a terrestrial environment that was seasonallyarid. Thedepositional environment in the lower sections of the Stormberg was similar to that of theKatberg Formation. Both places feature coarser-grainedsandstones that lack fining-upward sequences, thus pointing to analluvial fan andbraided river environment. Thedepositional environment changes towards the centre of the Stormberg asmudstones become more common, pointing to a change tofluvial-lacustrine deposits where sediments were deposited in low-energyfluvial settings. The upper Stormberg rocks changes back to beingsandstone-rich. Thesesandstones represent preserveddune fields deposited byaeolian processes in adesert environment.[4][5][6][7][8][9]

As the Stormberg Group is part of theKaroo Supergroup its associated rocks were deposited in aretroarcforeland basin. Afault-controlled crustal uplift (orogenesis) in the south influenced theforeland system at the beginning of the Stormberg deposition. This crustal uplift had been underway millions of years prior due to thesubduction of the Paleo-pacific plate beneath the Gondwanan Plate, which had also resulted in the creation of the Gondwanide mountain range. At this time a divergent plate boundary was forming theAtlantic Ocean, southwest ofGondwana, heralding the earliest stages of the break-up of the Gondwanansupercontinent.[10][11][12]

There are nooutcrops or exposures of the Stormberg Group West of 24ºE. This was because orogenic loading in the south by theGondwanide mountains from the earlyTriassic caused changes in position of theforebulge andforedeep in theforeland basin system. This resulted in the deposition zones shifting to the eastern and northeastern regions of theKaroo Basin from theEarly Triassic until theEarly Jurassic, when theDrakensberg Groupvolcanics commenced.[13]

Geographic extent

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Outcrops and exposures of the Stormberg Group are found in several localities inLesotho, and in theFree State,KwaZulu-Natal, andEastern Cape provinces inSouth Africa.

Subdivision

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The Stormberg Group is composed of three maingeological formations that are found in numerous localities acrossLesotho and in theFree State,KwaZulu-Natal, andEastern Cape provinces inSouth Africa. These formations are listed below (from oldest to youngest):

Paleontology

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The Stormberg Group contains manyfossils. It is also an important geological group as the Stormberg rocks are the only rocks inSouth Africa wheredinosaurfossils have been discovered.[14][15][16][17][18][19][20] In the lower sections diverse fossil plants[21][22] are also found as well as preserved dinosaurtrackways.[23][24]

Correlation

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The Stormberg Group is correlated with several geological groups and formations in other parts ofsouthern Africa. Some examples include the Tuli Basin in the northern parts ofSouth Africa,Botswana, andZimbabwe, and theEtjo Sandstone ofNamibia. Abroad, rocks of the Stormberg Group correlate with theChinle Formation of theColorado Plateau inUtah, United States, theSanta Maria,Caturrita andCandelária Formations of the in the Triassic and Early Jurassic connectedParaná Basin in southeasternBrazil and theTriassic sequences of theArgentine Northwest; theChañares,Los Rastros,Ischigualasto andLos Colorados Formations of theIschigualasto-Villa Unión Basin, theQuebrada del Barro Formation of the Marayes-El Carrizal Basin and the Quebrada de los Fósiles andRío Seco de la Quebrada Formations of theSan Rafael Block. In southernmostGondwana, the group correlates with theFremouw Formation ofAntarctica.

References

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  1. ^Cairncross, B (22 September 2016).Depositional framework and control of coal distribution and quality, Van Dyks Drift area Northern Karoo basin (Masters Thesis).hdl:20.500.11892/83920.
  2. ^Christie, ADM (22 September 2016).Stratigraphy and sedimentology of the Molteno formation in the Elliot-Indwe area, Cape Province (Masters Thesis).hdl:20.500.11892/83933.
  3. ^Johnson, M.R. (1991). "Sandstone petrography, provenance and plate tectonic setting in Gondwana context of the southeastern Cape-Karoo Basin".South African Journal of Geology.94 (2–3):137–54.hdl:10520/AJA10120750_463.
  4. ^Bordy, E. M. (1 September 2005). "The contact of the Molteno and Elliot formations through the main Karoo Basin, South Africa: a second-order sequence boundary".South African Journal of Geology.108 (3):351–364.Bibcode:2005SAJG..108..351B.doi:10.2113/108.3.351.
  5. ^Turner, BR (22 September 2016).The stratigraphy and sedimentary history of the Molteno formation in the main Karroo basin of South Africa and Lesotho (PhD Thesis).hdl:20.500.11892/108799.
  6. ^Turner, B. R. (1 December 1977). "Fluviatile cross-bedding patterns in the Upper Triassic Molteno Formation of the Karoo (Gondwana) Supergroup in South Africa and Lesotho".South African Journal of Geology.80 (3):241–252.hdl:10520/AJA10120750_1265.
  7. ^Smith, R.M.H.; Eriksson, P.G.; Botha, W.J. (January 1993). "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.
  8. ^Bordy, Emese M.; John Hancox, P.; Rubidge, Bruce S. (March 2004). "Fluvial style variations in the Late Triassic–Early Jurassic Elliot formation, main Karoo Basin, South Africa".Journal of African Earth Sciences.38 (4):383–400.Bibcode:2004JAfES..38..383B.doi:10.1016/j.jafrearsci.2004.02.004.
  9. ^Bordy, E. M. (1 September 2005). "The contact of the Molteno and Elliot formations through the main Karoo Basin, South Africa: a second-order sequence boundary".South African Journal of Geology.108 (3):351–364.Bibcode:2005SAJG..108..351B.doi:10.2113/108.3.351.
  10. ^Catuneanu; Hancox; Rubidge (December 1998). "Reciprocal flexural behaviour and contrasting stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa".Basin Research.10 (4):417–439.Bibcode:1998BasR...10..417C.doi:10.1046/j.1365-2117.1998.00078.x.S2CID 56420970.
  11. ^Sciscio, Lara (2016).Position of the Triassic-Jurassic boundary in South Africa and Lesotho : a multidisciplinary approach aimed at improving the chronostratigraphy and biostratigraphy of the Elliot Formation, Stormberg Group (PhD Thesis). University of Cape Town.hdl:11427/20847.
  12. ^Sciscio, Lara; de Kock, Michiel; Bordy, Emese; Knoll, Fabien (November 2017). "Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin".Gondwana Research.51:177–192.Bibcode:2017GondR..51..177S.doi:10.1016/j.gr.2017.07.009.hdl:10261/338607.
  13. ^Anderson, J. M.; Cairncross, B. (1 December 1995). "Palaeoecology of the Triassic Molteno Formation, Karoo Basin, South Africa – sedimentological and palaeontological evidence".South African Journal of Geology.98 (4):452–478.hdl:10520/EJC-943825b44.
  14. ^Smith, R.; Kitching, J. (June 1997). "Sedimentology and vertebrate taphonomy of the Tritylodon Acme Zone: a reworked palaeosol in the Lower Jurassic Elliot Formation, Karoo Supergroup, South Africa".Palaeogeography, Palaeoclimatology, Palaeoecology.131 (1–2):29–50.Bibcode:1997PPP...131...29S.doi:10.1016/S0031-0182(96)00143-5.
  15. ^Galton, Peter M.; Heerden, Jacques (April 1998). "Anatomy of the prosauropod dinosaurBlikanasaurus cromptoni (Upper Triassic, South Africa), with notes on the other tetrapods from the lower Elliot Formation".Paläontologische Zeitschrift.72 (1–2):163–177.Bibcode:1998PalZ...72..163G.doi:10.1007/BF02987824.S2CID 128464155.
  16. ^Chinsamy, Anusuya (1993)."Bone histology and growth trajectory of the prosauropod dinosaurMassospondylus carinatus Owen".Modern Geology.18:319–29.
  17. ^Knoll, Fabien (8 July 2002). "Nearly complete skull ofLesothosaurus (Dinosauria: Ornithischia) from the Upper Elliot Formation (Lower Jurassic: Hettangian) of Lesotho".Journal of Vertebrate Paleontology.22 (2):238–243.doi:10.1671/0272-4634(2002)022[0238:NCSOLD]2.0.CO;2.S2CID 85790601.
  18. ^Barrett, Paul M. (1 September 2004). "Sauropodomorph dinosaur diversity in the upper Elliot Formation (Massospondylus range zone : Lower Jurassic) of South Africa : research letter".South African Journal of Science.100 (9–10):501–503.hdl:10520/EJC96290.
  19. ^McPhee, Blair; Bordy, Emese; Sciscio, Lara; Choiniere, Jonah (2017)."The sauropodomorph biostratigraphy of the Elliot Formation of southern Africa: Tracking the evolution of Sauropodomorpha across the Triassic–Jurassic boundary".Acta Palaeontologica Polonica.62.doi:10.4202/app.00377.2017.
  20. ^McPhee, Blair W.; Benson, Roger B.J.; Botha-Brink, Jennifer; Bordy, Emese M.; Choiniere, Jonah N. (October 2018)."A Giant Dinosaur from the Earliest Jurassic of South Africa and the Transition to Quadrupedality in Early Sauropodomorphs".Current Biology.28 (19): 3143–3151.e7.Bibcode:2018CBio...28E3143M.doi:10.1016/j.cub.2018.07.063.PMID 30270189.
  21. ^Anderson, HM (1974). "A brief review of the flora of the Molteno 'formation' (Triassic), South Africa".Palaeontologia Africana.17:1–10.
  22. ^Bamford, Marion K. (January 2004). "Diversity of the Woody Vegetation of Gondwanan Southern Africa".Gondwana Research.7 (1):153–164.Bibcode:2004GondR...7..153B.doi:10.1016/S1342-937X(05)70314-2.
  23. ^Turner, Brian R. (1978). "Trace Fossils from the Upper Triassic Fluviatile Molteno Formation of the Karoo (Gondwana) Supergroup, Lesotho".Journal of Paleontology.52 (5):959–963.JSTOR 1303840.
  24. ^Rossouw, G. J.; Shone, Russell W.; Kitching, James W.; Raath, Michael A. (1990). "Dinosaur tracks in Triassic Molteno sediments: the earliest evidence of dinosaurs in South Africa?".Palaeontologia Africana.27:89–95.hdl:10539/16154.
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