Possible ice-margin fluctuations in the Dwyka series in Rhodesia
Abstract The succession of Lower Karroo rocks in a horehole in the mid-Zambezi valley in Rhodesia is described. The Dwyka Series is 334 ft. thick and contains three cycles of coarse to fine sediments. These are interpreted as being controlled by the periodic advance and retreat of mountain glaciers situated on the east side of a fresh water lake. Each cycle consists of a coarse lower part, comprising tillites and/or fluvioglacial outwash gravels, followed by fine limnoglacial shales. The limnoglacial varvites give an indication of the length of time between a glacial retreat and readvance, which amounts to 15,000–20,000 years. It is considered that these intervals are too short for a tectonic control of sedimentation and that periodic advances and retreats of ice margins account better for the observed changes in the grade of the sediments. This fresh water Upper Carboniferous lake was marginally situated in relation to the main glaciated region of this part of Gondwanaland.