Reconsidering the glaciogenic origin of Gondwana diamictites, Dwyka
Group, South Africa
Abstract
The Gondwana Late Paleozoic Ice Age is probably best represented by the
Dwyka Group in South Africa. Striated and grooved surfaces or pavements
are commonly considered to be subglacially formed, as are diamictites
which have been interpreted as in situ or reworked tillites. These
interpretations were tested by investigation of outcrops in formerly
well studied areas, throughout South Africa. Detailed analyses focused
on striated surfaces/pavements and surface microtextures on quartz sand
grains in diamictites. The sedimentological context of four pavements,
interpreted to be glaciogenic, display features commonly associated with
sediment gravity flows, rather than glaciation. A total of 4271 quartz
sand grains were subsampled from outcrops that are mainly considered to
be tillites formed by continental glaciation. These grains, analyzed by
SEM, do not demonstrate the characteristic surface microtextures
combinations of fracturing and irregular abrasion associated with
Quaternary glacial deposits, but mainly a mix of surface microtextures
associated with multicyclical grains. The Dwyka Group diamictites
warrant reinterpretation as non-glacial sediment gravity flow deposits.