Abstract
M.Sc.
The objective of this work is to establish a regional, three-dimensional sedimentological model of the clastic strata of the Vryheid Formation and pre-existing basin floor topography of the Witbank Coalfield, South Africa. This considers an area extending from 25°30’ S to 26°30’ S by 28°30’E to 30°E. This has been undertaken using computer modelling to examine the three-dimensional distribution of the sedimentary rocks. The basis for the geological model presented in this thesis is 1190 borehole logs collected from three mining companies involved in coal extraction in the study area, namely Anglo American Coal Corporation, Duiker Mining Limited and Ingwe Coal Corporation Limited. These borehole logs are converted to a common data format for inclusion into the 3-D model. These borehole logs are correctly positioned in space using a digital elevation model. The primary objective of this research was to visualize the interburden and overburden sedimentary rocks within the study area, so as to be able to understand the distribution and hence origin of these rocks. As commercially available modelling tools have several restrictions with respect to the current work, Geovision cc. was contracted to develop the tools needed for the geological visualization of the data. Using the three-dimensional model, the distribution of the various facies and facies associations can be defined, to understand the depositional history of the basin. These characteristics include the delineation of the general trend of the basement topography. The distribution of the lower glacial and paraglacial sedimentary rocks within the study in steep-sided palaeovalleys, marked by rapid facies and thickness changes from diamictite through argillite, and minor sandstones and conglomerates is shown in 3-D. Evidence of lobate deltas and bedload sandy river deposits between the No. 2 and No. 4 Seams is presented in 3-D. The 3-D characteristics of the rocks between the No. 4 and No. 5 Seams, proposed to represent a period of deltaic progradation during an overall marine transgression is described. By using a 3-D model it is possible to evaluate the distribution of sedimentary rocks, both through space and through time. These palaeoenvironmental interpretations facilitate a better understanding of the genesis of the Witbank Coalfield. Furthermore, this enhanced geological modelling can facilitate improved mine planning and mining techniques.