Palaeochannels (stone-rolls) in coal seams: Modern analogues from fluvial deposits of the Okavango Delta, Botswana, southern Africa

Two varieties of fluvial sandstones are associated with coal seams in the Permian Witbank Coalfield of South Africa. The first comprises lenticular channel-fill sandstone, minor conglomerate and siltstone encased entirely within the coal seam. Medium- to coarse-grained arkosic sandstones are structured by planar cross-bedding and fine upwards into carbonaceous siltstone and overlying coal. These palaeochannel-fill deposits are 5 km wide in proximal basin areas and narrow down palaeoslope to less than 1 km in width. The second variety of channel fill consists of medium- to coarse-grained sandstone which occurs below the floor of the coal seam. These deposits form undulating ridges that occur either in sub-parallel groups or as isolated shoe-string type bodies at the coal-floor rock contact. Both features are referred to as stone-rolls in colliery terminology. These ancient deposits are compared with active and abandoned fluvial systems from the Okavango Delta in Botswana, southern Africa. Active channels are flanked by extensive peat swamps which effectively confine and stabilize the channel margins. Clastic sedimentation is therefore completely confined to the channels by the vegetation and deposition occurs by vertical aggradation. Recently abandoned channels in the Okavango Delta display a variety of well preserved bed-forms which alternate with scoured depressions along the thalweg producing an undulating topography to the channel floor. These features provide a modern analogue for the stone-rolls in the coalfield deposits. The subsequent channel abandonment, vegetation encroachment and peat formation over the sand bodies would form a coal seam superimposed on the irregular surface, provided channel abandonment was followed by subsidence, overlying sediment deposition, compaction and coalification of the peat. Although the Okavango rivers are narrower and transport finer-grained sand than their Permian counterparts, the mechanism of formation and style of deposition was similar for both systems.