The Dwyka Tillite of Carboniferous-Permian age lies at the base of the Gondwana sequence within the Karroo basin of South Africa. The formation consists of up to 1,000 m of massive diamictite and locally contains laminated interbeds (some with isolated stones interpreted as dropstones), irregularly bedded diamictites, and, at places, deformed sandstone bodies. Much of the Dwyka is interpreted as true tillite formed by wasting in place of debris-laden ice; other facies include subaqueous-mudflow deposits composed of mobilized till and subaqueous and outwash material.

Glacially striated basement floors abound; these, with rarer boulder pavements, clast studies, and the trends of glacial valleys, indicate that a continental ice sheet lay north of the Karroo basin, centered in Rhodesia and Zambia. The sheet extended southward to near the south coast of Africa, and in the east merged with another that lay over Swaziland, Mozambique, and then-attached Antarctica. Lobes from the latter reached into Natal and as far as the easternmost Cape Province from regions that now are occupied in part by the Indian Ocean. Another ice lobe apparently entered the Karroo basin on the northwest and extended southward from South-West Africa and on southeastward. Facies distributions suggest that an unfrozen arm of the sea reached as far as the coast of South-West Africa from near the Falkland Islands.

Published paleomagnetic data indicate that the Gondwana continents, grouped around southern Africa, moved across the South Pole during the late Paleozoic. Glaciation apparently ensued when broad continental areas reached near-polar positions and expanses of open water were sufficiently near at hand to provide evaporative moisture.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.