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 rocks are considered to range between Lower Triassic (Olenekian) to Lower Jurassic (Pliensbachian) in age. These estimates are based on means of geological dating including stratigraphic position, lithostratigraphic and biostratigraphic correlations, and palynological analyses.^[1][2][3]

Stormberg Group
Stratigraphic range: Late Triassic-Early Jurassic ~237–182 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geologic group
Unit of	Karoo Supergroup
Sub-units	Molteno, Elliot & Clarens Formations
Underlies	Drakensberg Group
Overlies	Beaufort Group
Thickness	up to 4,593.176 feet (1,400 m)
Lithology
Primary	Mudstone, claystone, siltstone
Other	Sandstone, shale, tuff
Location
Region	Eastern Cape, KwaZulu-Natal, Free State, and Lesotho
Country	South Africa  Lesotho
Extent	Karoo Basin
Type section
Named for	Stormberg 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.

Background

Sediment deposition of the Stormberg Group took place in a terrestrial environment that was seasonally arid. The depositional environment in the lower sections of the Stormberg was similar to that of the Katberg Formation. Both places feature coarser-grained sandstones that lack fining-upward sequences, thus pointing to an alluvial fan and braided river environment. The depositional environment changes towards the centre of the Stormberg as mudstones become more common, pointing to a change to fluvial-lacustrine deposits where sediments were deposited in low-energy fluvial settings. The upper Stormberg rocks changes back to being sandstone-rich. These sandstones represent preserved dune fields deposited by aeolian processes in a desert environment.^{[4][5][6][7][8][9]}

As the Stormberg Group is part of the Karoo Supergroup its associated rocks were deposited in a retroarc foreland basin. A fault-controlled crustal uplift (orogenesis) in the south influenced the foreland system at the beginning of the Stormberg deposition. This crustal uplift had been underway millions of years prior due to the subduction 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 the Atlantic Ocean, southwest of Gondwana, heralding the earliest stages of the break-up of the Gondwanan supercontinent.^[10][11][12]

There are no outcrops or exposures of the Stormberg Group West of 24ºE. This was because orogenic loading in the south by the Gondwanide mountains from the early Triassic caused changes in position of the forebulge and foredeep in the foreland basin system. This resulted in the deposition zones shifting to the eastern and northeastern regions of the Karoo Basin from the Early Triassic until the Early Jurassic, when the Drakensberg Group volcanics commenced.^[13]

Geographic extent

Outcrops and exposures of the Stormberg Group are found in several localities in Lesotho, and in the Free State, KwaZulu-Natal, and Eastern Cape provinces in South Africa.

Subdivision

The Stormberg Group is composed of three main geological formations that are found in numerous localities across Lesotho and in the Free State, KwaZulu-Natal, and Eastern Cape provinces in South Africa. These formations are listed below (from oldest to youngest):

• Molteno Formation
• Elliot Formation
• Clarens Formation

Paleontology

The Stormberg Group contains many fossils. It is also an important geological group as the Stormberg rocks are the only rocks in South Africa where dinosaur fossils 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 dinosaur trackways.^[23][24]

Correlation

The Stormberg Group is correlated with several geological groups and formations in other parts of southern Africa. Some examples include the Tuli Basin in the northern parts of South Africa, Botswana, and Zimbabwe, and the Etjo Sandstone of Namibia. Abroad, rocks of the Stormberg Group correlate with the Chinle Formation of the Colorado Plateau in Utah, United States, the Santa Maria, Caturrita and Candelária Formations of the in the Triassic and Early Jurassic connected Paraná Basin in southeastern Brazil and the Triassic sequences of the Argentine Northwest; the Chañares, Los Rastros, Ischigualasto and Los Colorados Formations of the Ischigualasto-Villa Unión Basin, the Quebrada del Barro Formation of the Marayes-El Carrizal Basin and the Quebrada de los Fósiles and Río Seco de la Quebrada Formations of the San Rafael Block. In southernmost Gondwana, the group correlates with the Fremouw Formation of Antarctica.

References

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. 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. 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. 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. 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. doi:10.1046/j.1365-2117.1998.00078.x.
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). 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. doi:10.1016/j.gr.2017.07.009.
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. 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. doi:10.1007/BF02987824.
16. Chinsamy, Anusuya (1993). "Bone histology and growth trajectory of the prosauropod dinosaur Massospondylus carinatus Owen". Modern Geology. 18: 319–29.
17. Knoll, Fabien (8 July 2002). "Nearly complete skull of Lesothosaurus (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.
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. 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. 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.