Abstract

Thin horizons (< 0.5 m) of black mudrock are present in the Dwyka Group (< 25 m thick), which overlies the floor of a palaeovalley near the northern margin of the Karoo Basin, South Africa. This palaeovalley was excavated by southward-moving glaciers from the mid-Namurian until the late Sakmarian. The Dwyka Group was deposited during deglaciation (late Sakmarian–early Artinskian), when a tidewater glacier retreated northwards up the valley.

The black mudrock forms a minor component of an argillaceous sequence up to 10 m thick that was deposited in the northern part of the palaeovalley in a subaqueous proglacial environment proximal to the glacier ice-front. The underlying lithofacies are dominated by diamictite that represent debris flow and minor debris rain deposits. Overlying strata comprise sandstone (± 2 m thick), which is attributed to sedimentation on an ice-proximal, prograding, subaqueous outwash fan, and a thin (< 0.3 m) conglomerate-diamictite couplet that represents sediment gravity flows associated with failure of unstable material, when rapid glacial melting caused a sudden rise in water-level.

Presence of Botryococcus braunii in the black mudrock indicates freshwater that was derived from the nearby melting glacier and also small icebergs. The marine setting of the Karoo Basin during the Dwyka glaciation implies a brackish-water environment in the palaeovalley due to mixing with freshwater near the glacier ice-front.