Modelling of distribution and geometry of lithological complexes of the Ecca Group (the Karoo Supergroup) in SW Botswana

Marek Wendorff, Lechosław Radwański, Bartosz Papiernik

Abstract


The Ecca Group, a subdivision of the Karoo Supergroup (U. Carboniferous-L. Jurassic) in SW Botswana is a sequence deposited as marine deltaic bodies considered to have been supplied from a cratonic source elevated north of the basin. The Karoo strata in this region are covered unconformably by sands of the Kalahari Beds (Upper Cretaceous – Recent). Therefore the bedrock outcrops are extremely rare, limited in size and a low number of boreholes drilled in this vas area (ca. 340 x 540 km) provide the only insights into the succession of the Karoo Supergroup. Very long distances between individual boreholes make correlation, interpolation between localities and interpretations of geometry of lithological bodies that would provide clues supporting basin analysis by traditional means very problematic. Therefore, modelling of the Ecca Group lithofacies associations with the use of Petrel software was performed, which resulted in the first approximation of the space-time relations between lithological complexes of this unit. These relations in turn suggest evolutionary trends of the basin during deposition of the Ecca Group strata.
The model suggests two main zones of supply indicated by two distinctly different patterns of deltaic lithofacies associations, and their evolution controlled by post-Dwyka palaeotopography and its subsequent modifications by local subsidence in the centre of the depository. Initially rapid southward progradation of relatively fine-grained delta body located in the west of the area was followed by subsidence-induced aggradation interrupted by stages of abandonment and marine transgression. Such variations, emphasised by the presence of sandy clinoforms of the delta lobes separated by basinal ‘fines’, imply significant interplay between rates of supply and subsidence. On the other hand, the delta formed in the east contains relatively high proportion of coarse-grained sandstone facies overlying prodelta fines as laterally extensive tabular body formed most probably by lateral migration of distributary channels and delta-front mouthbars, and devoid of abandonment stages. Proximal litofacies of the “western delta” fill the subsiding depocentre and grade distally into synchronously deposited prodelta fines towards the south. By contrast, distal fine-grained prodelta facies fill basin depocentre in the eastern area and are overlain by proximal facies of the “eastern delta”.

Keywords


The Karoo Supergroup, the Ecca Group, deltaic sedimentation, modelling, SW Botswana

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References


Bordy E.M., Segwabe T. & Makuke B., 2010. Sedimentology of the Upper Triassic-Lower Jurassic (?) Mosolotsane Formation (Karroo Supergroup), Kalahari Karoo Basin, Botswana. Journal of African Earth Scienices, 58, 127-140.

Caers J., 2005. Petroleum Geostatistics. Society of Petroleum Engineers.

Carney J., Aldiss D. & Lock N., 1994. The Geology of Botswana. Geological Survey of Botswana Bulletin Series, 37, 1-113.

Catuneanu O., Wopfher H., Eriksson P.G., Cairncross B., Rubidge B.S., Smith R.M.H. & Hancox P.J., 2005. The Karoo basins of south-central Africa. Journal of African Earth Sciences, 43, 211-253.

Diskin S. & Wendorff M., 2011. Southern Botswana's Ecca and Beaufort Groups - where did their sediments come from? 23r Colloquium of African Geology (CAG23) Abstracts Volume, University of Johannesburg, Republic of South Africa, 8th -14th January 2011, 123.

Doyen P., 2007. Seismic Reservoir Characterization: An Earth Modelling Perspective. EAGE Publications, 1-255.

Dubrule O., 1998. Geostatistics in Petroleum Geology. AAPG Continuing Education Course Note Series, 38, 1-163.

Emery D. & Myers K. (Eds), 1996. Sequence stratigraphy. Blackwell Science, Oxford.

Isbell J.L., Cole D.I. & Catuneanu O., 2008. Carboniferous-Permian glaciation in the main Karoo Basin, South Africa: Stratigraphy, depositional controls, and glacial dynamics, [in:] Fielding C.R., Frank T.D. & Isbell J.L. (Eds), Resolving the Late Paleozoic Ice Age in Time and Space, Geological Society of America Special Paper, 441, 71-82.

Johnson M.R., Van Vuuren C.J., Hegenberger W.F., Key R. & Shoko U., 1996. Stratigraphy of the Karoo Supergroup in southern Africa: an overview. Journal of African Earth Sciences, 23, 3-15.

Key R.M., Tidi J., McGeorge I., Aitken G., Cadman A. & Anscombe J., 1998. The Lower Karoo Supergroup geology of the southeastern part of the Gemsbok Sub-basin in the Kalahari Basin, Botswana. South African Journal of Geology, 101, 225-236.

Meixner H.M. & Peart R.J., 1984. The Kalahari drilling project. Geological Survey of Botswana Bulletin Series, 27, 1—224.

Modie B.N. & Herisse Le A., 2009. Late Palaeozoic palynomorph assemblages from the Karoo Supergroup and their potential for biostratigraphic correlation, Kalahari Karoo Basin, Botswana. Bulletin of Geosciences, 84, 337-358.

Smith R.A., 1984. The Lithostratigraphy of the Karoo Supergroup in Botswana. Geological Survey of Botswana Bulletin Series, 26, 1-239.

Wendorff M., 2008. Stratigraphy of the Karoo Supergroup in Southern Africa and sedimen-tological criteria for palaeogeographieal transition between marine foreland and continental intracratonic basin in Botswana, [in:]: Haczewski G. (Ed.), 1st Polish Geological Congress, Polish Geological Society, Krakow, 26—28 June 2008, 128.

Wendorff M., Diskin S. & Letsholo M., 2012. Geochemical and mineralogical indicators of provenance and stratigraphy in the Ecca Group (Permian) of the Gemsbok Basin: A deltaic sequence in the Karoo Supergroup, SW Botswana, [in:]: Missoni S. & Gawlick HJ. (Eds), 29th IAS Meeting of Sedimentology, Abstracts, Schladming, Austria, 583.




DOI: https://doi.org/10.7494/geol.2013.39.1.55

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