Lithostratigraphy, depositional environments and sedimentology of the Permian Vryheid Formation (Karoo Supergroup), Arnot North, Witbank Coalfield, South Africa
- Authors: Uys, Joanne
- Date: 2009-04-30T09:28:39Z
- Subjects: Stratigraphic geology , Sedimentology , Facies (Geology) , Lithofacies , Karoo Supergroup , Mpumalanga (South Africa)
- Type: Thesis
- Identifier: uj:8338 , http://hdl.handle.net/10210/2464
- Description: M.Sc. , This work documents the lithostratigraphy and interpreted depositional environments of the Permian Vryheid Formation in the most northern proximal setting yet studied in the Witbank Coalfield. Data from 924 boreholes from two mining companies (Anglo Operations Ltd. and Xstrata Coal Ltd.) drilled over 50 years, covering an area of 910km2 revealed a 35m sequence of terrigenous clastic sedimentary rocks containing two coal seams. These seams are numbered No. 1 at the base and No. 2 at the top. Delineation of facies type, facies assemblages, lateral facies distributions and computer-based three-dimensional modeling facilitated the interpretation of the palaeodepositional environments. Eleven lithofacies are defined and interpreted hydrodynamically. Facies classification is based primarily on grain size and sedimentary structures. The modeling of the borehole information uses the finite element method to interpolate the thickness, roof and floor surfaces and trend of each seam and inter-seam parting between boreholes. The spatial position of the boreholes is defined using a digital terrain model that represents the current surface topography. Lateral distributions were correlated by repositioning the boreholes using the base of the No. 2 seam as a datum. Glaciofluvial, glaciolacustrine, bed-load (braided) fluvial and constructive progradational deltaic environments are interpreted in the study area. Fluvial channel sequences are dominant and cause the thinning of the coal seams below channel axes as well as splitting of both the No. 1 and No. 2 seams. Glaciofluvial influences also affect the lower portion of the No. 1 seam. Basement palaeotopography restricts the distribution of the lower splits of the No. 1 seam. The coals either ‘pinch-out’ or are absent above basement highs but blanket the adjacent low-lying areas. In contrast to the greater Witbank Coalfield, but concurrent with other studies in the more northern proximal regions, fluvial systems dominate over deltaic systems in the study area. Glaciodeltaic, fluviodeltaic and anastomosed channel fluvial systems recognized in the remainder of the Karoo Basin were fed by the braided fluvial systems in the study area. The close proximity of the study area to the northern edge of the basin accounts for the subtle differences in lithostratigraphy and interpreted depositional environments when compared with more distal sites to the south. For example, glaciofluvial clastic sediment input in the lower portions of the No. 1 seam and post-Karoo erosion that has removed the overlying seams; the deltaic progradational sequence, above the No. 2 seam, occurs twice in succession and the bioturbation, that has become characteristic of sedimentary sequence of the Vryheid Formation above the No. 2 seam in the central and southern parts of the Karoo Basin, is not as identifiable. These differences are explained by the extreme proximal location of the study area on the northern basin margin relative to the remainder of the Karoo Basin.
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Geological controls on no. 4 seam roof conditions at New Denmark Colliery, Highveld Coal Field, Karoo Basin, South Africa
- Authors: Stanimirovic, Jasmina
- Date: 2009-01-28T09:43:30Z
- Subjects: Facies (Geology) , Coal , Stratigraphic geology , Sedimentology , Mine roof control , Karoo Supergroup , Mpumalanga (South Africa)
- Type: Thesis
- Identifier: uj:14849 , http://hdl.handle.net/10210/1971
- Description: M.Sc. , The coal-bearing Permian Vryheid Formation of the Ecca Group (Karoo Supergroup) was investigated at New Denmark Colliery, situated in the north east section of the Karoo Basin, South Africa. The lithostratigraphy of the sequence is defined in terms of conventional lithostratigraphic terminology but also by applying detailed genetic stratigraphic schemes that have previously been proposed for the adjacent coalfields. The succession is divided up into depositional sequences named after the underlying and overlying coal seams, the No. 2, 3, 4 and 5 seam sequences. The sedimentary succession was divided up into five facies, namely: conglomerate facies, sandstone facies, interlaminated sandstone-siltstone facies, siltstone facies and coal facies. These were interpreted hydrodynamically. Facies assemblages were then interpreted palaeoenvironmentally. Glacial, fluvial, deltaic and transgressive marine sequences were responsible for forming this sedimentary succession. Attention was then focussed on the main economic No. 4 seam, which is mined underground at the colliery. Detailed subsurface geological cross-sections, core sequences and isopach maps of the No. 4 seam coal and the lithologies above, were used to determine specific aspects of the depositional environment that could contribute to unstable roof conditions above No. 4 seam. Coarsening-upward deltaic cycles, fining-upward bedload fluvial cycles, glauconite sandstone marine transgressions and crevasse-splay deposits are recognized in the overlying strata. Poor roof conditions occur parallel to palaeochannel margins because the interbedded channel sandstone and adjacent flood plain argillites cause collapsing along bedding plane surfaces. Rider coals overlying thin crevasse-splay sequences in close proximity to the No. 4 seam, create one of the most serious roof conditions; complete collapse occurs along the rider coal contact with the underlying splay deposits. Differential compaction of mudrock/shale/siltstone over more competent sandstone causes slickensided surfaces that weaken the roof lithologies. Correct identification of these sedimentological features will enable the prediction of potential poor roof conditions during mining operations and mine planning.
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