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Reconsidering the glaciogenic origin of Gondwana diamictites of the Dwyka Group, South Africa

Molén Mats O., Smit J. Johan

Geologos

2022

Vol. 28, No. 2

83--113

The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, 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 have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreter to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by Continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture 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.

The Late Devonian (Frasnian/Famenian) mass extinction : a proposed test of the glaciation hypothesis

Mc-Ghee G. R.

Geological Quarterly

2014

Vol. 58, No. 2

263--268

It is argued in this paper that late Frasnian global cooling was the first step in the onset of the Late Palaeozoic Ice Age, and that the late Frasnian extinctions are analogous to the early Oligocene (Oi-1) extinctions that took place in the first step in the onset of the Cenozoic Ice Age. It is argued that the physical evidence for the existence of late Frasnian glaciation, like the Oi-1 glaciation, is largely geochemical: the sharp increases in δ18O values and positive δ13C anomalies that occurred in the late Frasnian and earliest Famennian. In addition to the geochemical evidence, also like the Oi-1 glaciation, stratigraphic calculations indicate a major sea-level fall occurred during the late Frasnian and early Famennian, a sea-level fall that is argued to have been glacially produced. It is here proposed that the best possible independent physical evidence for the existence of late Frasnian glaciation, other than the geochemical and sea-level evidence, would be the discovery of ice-rafted debris in marine sediments of late Frasnian age similar to the ice-rafted debris found in Oi-1 marine sediments (Zachos et al., 1992; Ehrmann and Mackensen, 1992).