A Regional Scale 3-D Model of the Witbank Coalfield, Northern Karoo Basin, South Africa

Abstract

Geological studies of the sedimentary strata of the Witbank Coalfield form a spectrum of increasingly detailed and sophisticated models as more information has become available. This study establishes a regional, three-dimensional sedimentological model of the clastic strata of the Vryheid Formation and pre-existing basin floor topography of the Witbank Coalfield. This is achieved by visualizing the three-dimensional distribution of the interburden and overburden sedimentary rocks within the study area. By producing a 3-D model, multiple scenarios can be examined to determine the best fit to both the data and geological understanding, resulting in a better-tested and more representative model. The data used in the construction of the model were borehole logs from across the Witbank Coalfield. The correctly positioned boreholes are displayed in three dimensions, using a viewer written in Tcl script. Within this environment the coal seams, basement and tillite horizons are automatically extracted. The interburden rocks are manually picked to form 3-D triangular wedges representing a particular rock type. The sedimentary strata of the Witbank Coalfield represent a continuous, conformable basin fill sequence deposited after post-glacial warming. Several regressive sequences capped by coal are recognized, overlain by transgressive marine sedimentary rocks. As the 3-D model delineates the distribution of the rocks and the facies association in both space and time, the evolution of these various sedimentary environments through time can be defined. Most noticeable is the strong northeast-southwest trend of the fluvial axes. It is proposed that these axes represent the underlying glacial valleys carved out prior to the deposition of the Vryheid Formation sedimentary rocks. In addition, the various axes are superimposed on one another through time, the notable exception being the fluvial products of the upper part of the sequence. This may be due to the majority of the glacial valleys being filled by the time the sediment of this sequence was deposited. Deltaic processes were more predominant in the western portion of the study area, with the distal (southerly) portions of the deltas more prone to change in position, than the proximal portions. The distribution of the coal is controlled by the influx of clastic sediment into the basin, rather than only rises and falls in the sea level. When the clastic sediment supply was high, no coal-forming material was preserved. However when this dropped, the rate of sediment supply was matched by the rise in sea-level. This produced stable conditions, which allowed the growth of plants that would eventually form the peat and coal. When the sediment supply rates dropped further, the continued, gradual marine transgression resulted in the inundation of the areas by marine sediments. With the novel scientific approach of the examination of the three-dimensional characteristics of the depositional environments of the sequences, a user-friendly geological model is developed. In this case the model confirmed the previous interpretations, giving greater confidence in the geological assumptions and thus producing a more robust model.