Beobachtungen zum Problem der jung-Präkambrischen glazialen Ablagerungen in Südwestafrika
The Late Precambrian Numees Formation contains besides very thick deposits of tillite-like habit also laminated siltstones resembling glacial varved rocks. These enclose coarser sand grains, pebbles and boulders of all sizes which have dropped from above into the soft sediment. As only floating ice can have transported these large components, the conclusion is inescapable that the laminated siltstones are true glacial varves and that the tillite-like rocks are true tillites.This evidence for Late Precambrien glacial conditions makes it highly probable that other formations of a similar age, which have been described as tillites, are also of glacial origin. The Buschmannsklippe Formation begins locally with a tillite which seems to be more or less contemporaneous with the Numees Formation. A high percentage of facetted and deeply striated pebbles and boulders makes it probable that this tillite originated as a basal moraine.The Nama tillite in the Klein Karas Mountains is stratigraphically not very far removed and in time probably not very much younger than the Numees tillite. Striated and grooved floors are associated with this tillite.The Chuos tillite and the Otavi tillite of the Damara System are contemporaneous deposits of Late Precambrian age. Their age relative to the Numees tillite is not known. The Chuos- and the Otavi tillite are probably glaciomarine drifts. For these a glacial origin cannot be proved with the same degree of conclusiveness as for the above described deposits. However, the existence of glacial conditions in the Late Praecambrian having been proved for an adjoining area, a glacial origin may reasonably be assumed for the tillite-like rocks of the Damara System too.Both the Numees tillite and the Otavi tillite are intimately associated with sedimentary iron ore deposits. Oxygen deficiency in stagnating bottom waters, caused by an ice cover, is thought to be responsible for this peculiar combination of sediments.The excellent evidence for the existence of glacial conditions during parts of the Late Precambrian, found in South West Africa, strongly supports the assumption that the many tillite-like deposits of a similar age, which have been described from Central and West Africa are also of glacial origin.