Palynofacies of the Lower No 4 Coal Seam (Highveld Coalfield South Africa)

Abstract

A palynofacies study of the lower number 4 coal seam in the Highveld coalfield of the Karoo Basin, South Africa, was conducted to get more insight into the palaeoenvironment. The lower number 4 coal seam was sampled near Secunda, approximately 200 km south east of Pretoria. The Main Karoo Basin is classified as a retro-arc foreland basin, which covers an area of approximately 700 000 km2 and can reach a maximum thickness of up to 12 km. The accumulation of coal in the Karoo Basin occurred during the Permian in the Vryheid Formation of the Ecca Group. There are two palynofacies assemblages representing different depositional environments. The first palynofacies assemblage identified in the bottom section of the coal seam is characterised by the presence of high amounts of spores and equidimensional phytoclasts. This indicates that it may have formed during wet cooler conditions with less transport. The second palynofacies assemblage and top section of the coal seam is characterised by an increase in bisaccate pollen grains and improved sorting of the phytoclasts. This signature may indicate a higher fluvial influx into the swamp with the higher amount of taeniate bisaccate pollen grains showing that the palaeoenvironment was warmer during deposition. The Protohaploxypinus sp. from the glossopterid plant was the dominant flora in the upland area while the lowland is indicative of fern wetland communities. The lower number 4 coal seam depositional environment therefore changed from swamp to increased fluvial input into the swamp. There are also barren samples that were due to the igneous intrusion in the proximity of the sampled area. The igneous intrusion formed a devolitalisation zone that reached a thickness of 17 to 19 m. The increase in temperature adjacent to the dolerite dyke caused the organic matter surrounding it to mature, which could possibly create gas pockets.