A fluvioglacial and glaciolacustrine deltaic depositional model for permo-carboniferous coals of the Northeastern Karoo Basin, South Africa

Abstract

With the northward retreat of the late Palaeozoic Gondwana ice sheet a series of glacial valleys, partially filled with diamictite, dominated the landscape along the northern edge of the Karoo basin in South Africa. Consequent outwash sediments accumulated as fluvioglacial and glaciolacustrine deltaic deposits. Density underflow generated turbidity currents from which bottomset sediments were deposited. These comprise distal varved siltstones and shales, stratified pebbly-mudstones with dropstones, and proximal ripple-drift cross-laminated sandstones and siltstones. Delta foresets comprise large-scale accretionary cross-beds up to 5 m in thickness. Overlying outwash plain conglomerates and sandstones constitute the topset deposits. These are structured mainly by planar cross-beds which resulted from linguoid bar migration at high discharge. During falling-water stages plane-bedded and ripple-cross-laminated sandstones formed on bar surfaces under variable flow regime conditions. Upon abandonment of the outwash plain, shallow-rooted Arctic vegetation developed. Resulting peats exceeded 10 m in thickness and constituted precursors to coal seams in which variations in ash content are attributed to overbank splaying from recognisable anastomosing channels within the coal swamps. This study has illustrated a characteristic paraglacial sedimentation sequence; maximum depositional rates occur immediately after glacial retreat followed by decelerating sedimentation rates through time, leading finally to the development of extensive peats.