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

Abstract

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, interpreted 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.

Comment to: Detecting upland glaciation in Earth’s pre-pleistocene record

“…It is the combination and appearance of surface microtextures which is important. The conclusion in the paper by Molén (2014) is: "1 A glacigenic grain typically exhibits largescale fractures (F1) and irregular abrasion (A1)," Later work has further specified this combination of surface microtextures, i.e., fresh fractures that are irregularly abraded (e.g., Molén and Smit, 2022). Soreghan et al (2022) refer to a paper by Sweet and Brannan (2016) to indicate that surface microtextures can be preserved during long fluvial transport.…”

“…However, the sand grains pictured by Sweet and Brannan (2016), their fig. 3 display different fractures, or actually fractured grains, and not any abrasion, neither regular abrasion from fluvial transport nor irregular abrasion from glacial grinding (e.g., Mahaney, 2002;Molén, 2014;Molén and Smit, 2022). Their pictured grains are all similar to grains that have only been fractured, by any high-stress process (e.g., this also occur in strong rivers displaying clasts that are tumbling around) and these grains are also similar to the Late Paleozoic grains pictured by Soreghan et al (2022, their fig.…”