1. PALEOENVIRONMENTAL RECONSTRUCTION OF THE KOOKFONTEIN FORMATION, TANQUA, KAROO BASIN, SOUTH AFRICA
Malikah Patel1, Daniel Mikeš2
Department of Geology, Geography and Environmental Science, Stellenbosch University, South Africa,
Department of Geology, Geography and Environmental Science, Stellenbosch University, South Africa,
Introduction and background
Aims & Methodology
The main Karoo Basin is considered to be a retroarc foreland basin formed as a result of subsidence solely due to orogenic loading along the southern and south western margin of the basin [2,3,4,5]. However, petrographic and geochemical studies of the Southwest Karoo turbidites [6] and tectonic models/structural reconstructions of the Cape Fold Belt (CFB) [7] suggest that the fold belt was not substantially emerged at the time of deposition of the submarine fans. The most likely source for the early Karoo Basin sediments would then not be the Cape Fold Belt, but the North Patagonian Massif.
Focusing on the marine environment and Ecca Group, the Permian Tanqua depocenter consists of a progradational siliciclastic succession from submarine fan, prodelta, delta front to delta plain systems. The submarine fan system consists of five sand units separated by (hemi) pelagic shales [1]. The deltaic system consists of a coarsening-up and thickening-up sequence of mudstones in the prodelta, intercalated muds and sands in the delta front and sands in the delta plain. As opposed to the submarine fan system, the deltaic system has only been studied by few and then only regionally. This project therefore focuses on the nature of the deltaic depositional environment and its evolution through time in response to the tectonic setting of the Tanqua Basin.
The aim of this study is to use facies and facies association relationships to determine the depositional environment of the deltas and determine the factors which deposited these sediments namely, sea level or tectonics.
The excellent exposure of the more resistant benches enable visual outcrop correlation of the stratigraphic units over long distances. The physical shape of Pienaarsfontein Se Berge and its orientation oblique to the estimated depostional dip permits measured sections to be correlated in both strike and dip directions allowing for a partial three-dimensional analysis of the depositional system
Figure 2. Slope system of the Tanqua depocentre
Study Area
Figure 1. Location of study area in South Africa. Landsat image:
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Sedimentary Log and Facies
Tanqua Delta System
Facies E: amalgamated fine-med sandstone beds with plane parralel laminated at the bases and wave rippled tops
Facies B: soft-sediment deformed very fine-grained sandstone and siltstone
Coarser grained sandstones
Facies D: very fine- to fine- grained sandstone with very low-angle cross-stratification and plane-parallel laminations
Heterolithics and slumping
Finer grained silts and muds
Facies C: bioturbated very fine grained sandstone
Fig 5. Helicopter image of 13 cycles of the Kookfontein Formation, Pienaarsfontein Se Berge (Wilds, 2005)
Figure 6. Model of the system as the coarser-grained sediments progrades over the distail fine-grained sediments
Facies A: Heterolithic current ripple and plane-parralel laminated interbedded very fine-grained sandstone, siltstone and mudstone
The depositional style of the Permian Tanqua Depocentre is progradational; hence the proximal coarse grained sediments progrades over the distal fine grained ones, and the boundary between them migrates and creates a lithological boundary. The stratigraphic units are defined by their coarsening-upward character and record deposition from a combination of river processes, waves and tides during an overall relative sea-level rise. The sediments are interpreted to be deposited through processes which include high and low density turbidite flows.
Slumping and sliding processes recorded in the Kookfontein have been documented on the slope and are caused by sediments which are fed to the slope via short-headed, over extended shelf edge deltas during low realtive sea level; sand piled up at the end of a channel; rapid deposition of that sand made it unstable which resulted in sliding away (slumping).
Fig 3. Stratigraphic section and photographs of cycle 5 of Kookfontein Formation
Stratigraphic unit 5 is a representative log which consists of aggradational, coarsening up packages dominated by fluvial processes with minimal wave and tide influence. The river-dominated prodelta and distal delta front predominately consist of soft-sediment deformed sandstones and siltstones and heterolithic current-ripple and plane-parallel laminated sandstone and siltstones. Progradation of the deltaic system resulted in increased soft-sediment deformation that primarily occurred in situ to debris flows. .
Future work and recommendations
References
A detailed outcrop-study involving master cross-sections of the study area would be done to determine the geometry and sediment-fill. Combining this with the stratigraphic sections/thicknesses and paleocurrent directions a facies model based on tectonic influence would also be obtained.
Johnson, S. D., Flint, S., Hinds, D., and Wickens, H. D., (2001). Sedimentology , 48:
Vissers, J. N. J., (1993). Sedimentary Geology, v, 83, p
Cole, D. I. (1992). In Inversion Tectonics of the Cape Fold Belt Karoo and Cretaceous Basins of Southern Africa (Eds: M. J. De Wit & I. G. D. Ransome ),pp 87–99.
Veevers, J.J, Cole, D.I. and Cowan, E.J., (1994). Southern Africa: Karoo Basin and Cape Foldbelt. In: Permian-Triassic Pangean Basins and Foldbelts along the Panthalassan Margin of Gondwanaland. 236: p 242.
van Lente, B. (2004). Ph.D. Thesis, University of Stellenbosch, South Africa.
Wild, R. J., (2005). Ph.D. Thesis, University of Liverpool.
King, R.C. (2005). Ph.D. Thesis, University of Liverpool, England, UK.
Understanding the facies architecture and spatial distributions of tide and wave-influenced deltas may have important implications for predicting and mapping reservoir quality. The results would be comparised with other analogues deltaic systems and existing outcrop, to emphasise petroleum related applications.