ELSEVIER International Journalof Coal Geology29 (1996) 147-186 The Grootegeluk Formation in the Waterberg Coalfield, South Africa: facies, palaeoenvironment and thermal history evidence from organic and clastic matter Kevin Faure t a,., James P. Willis a, j. Claris Dreyer b a Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa b Department of Geology, Grootegeluk Coal Mine, Ellisras 0555, South Africa Received 14 September 1994;accepted 17 August 1995 Abstract The Grootegeluk Formation of the Waterberg Coalfield is about 70 m thick and consists of relatively thin coal beds interbedded with numerous mudstone and carbonaceous mudstone layers. Maceral analyses of the coal seams and the interbedded carbonaceous mudstones show that vitrinite is the dominant maceral (up to 90 vol%), whereas inertinite, liptinite and reactive semifusinite generally occur in minor proportions. The vitrinitecontent increases upward in the formation, with a concomitant decrease in inertinite (including reactive semifusinite) from around 60% inertinite at the base of the formation. The upward increase in the vitrinite concentration is associated with an increase in the energy of the depositional environment, which is also considered to have been instrumental in the enhanced preservation of the vitrinite. The repetitive nature of the coal-mudstone succession, the lateral persistence of cycles, and the maceral and ash proportions, indicate that the formation may have resulted from a combination of allogenic controls (tectonism and climate) and authigenic controls (environmentsof deposition). The vitrinite reflectance (13-,m,x) of the Grootegeluk Formation (mean 0.72%) and palynological evidence indicate that the Grootegeluk Formation was subjected to maximum post-depositional temperatures of about 100°C. The lower portion of the Grootegeluk Formation is dominated by kaolinite, quartz and minor amounts of apatite, and the upper parts by quartz, kaolinite and minor amounts of montmorillonite, illite and microcline. The base of the Grootegeluk Formation is considered to have been relatively more distal and the upper portions more proximal to the source of the sediments. At the base of the Grootegeluk Formation a ~ 2 m thick tonstein zone occurs, which is dominated by well-crystallised kaolinite, organic matter ( ~ 40 wt%), siderite, calcite and minor proportions of apatite. The tonstein zone has mineralogical, maceral and chemical characteristics distinctive from the other mudstones. * Correspondingauthor. ' Present address: Department of Geochemistry, GeologicalSurveyof Japan, Tsukuba,Ibaraki305, Japan. 0166-5162/96/$15.00 © 1996ElsevierScienceB.V. All rights reserved SSDIO 166-5 162(95)00029-1