Glaciation-induced features or sediment gravity flows – An analytic review

patrimonio de origen glaciar de la Serra da Capelada (Geoparque mundial de la Unesco Cabo Ortegal, Galicia, Península Ibérica)

“…En relación con lo anterior, surge un segundo aspecto a discutir: el de si las rocas estriadas descritas antes son claramente glaciares o se han producido por un movimiento en masa. Algunas investigaciones recientes (Hu y McSaveney, 2018;Molén, 2023) describen la presencia de estrías asociadas a avalanchas de rocas. Sin embargo, los ejemplos que se presentan en las publicaciones muestran el clásico desprendimiento sobre una topografía suavemente inclinada.…”

Patterns, processes and models - an analytical review of current ambiguous interpretations of the evidence for pre-Pleistocene glaciations

“…Even on land the most important process of moving material is mass wasting (Shanmugam, 2020), while other processes may be dominant in confined areas (i.e., areally and/or environmentally restricted) and/or during shorter time intervals. Quaternary tills are commonly thin (2-15 m, more often on the lower end), except in confined areas with general thicker tills (e.g., 10-52 m in a 300-km-long band at the southern border of the North American inland ice sheet; Molén, 2023a), and therefore their preservation potential for deep time would be low. These observations indicate that, whatever the climate was during Earth history, there would be few preserved features originating from earlier glaciations on former higher, stable bedrock.…”

“…In the Neoproterozoic, diamictites are commonly preserved in thick sequences in tectonically unstable subsiding basins at the edges of cratons, and would therefore not easily qualify as subglacial sediments in situ (Eyles, 1993). Furthermore, geological features from the Late Paleozoic Ice Age and the Hirnantian (Ordovician) glaciation, in most places, are preserved in areas of subsiding basins and/or areas affected by transgressions (Ghienne, 2003;Buatois et al, 2010;López-Gamundí, 2010;Schatz et al, 2011;Molén & Smit, 2022;Molén, 2023a). The progress in knowledge of SGFs has changed interpretations of most ancient diamictites which had formerly been interpreted to be commonly primarily subglacial, to instead be (to a large part) reworked SGFs, especially cohesive debris flows.…”

“…At first glance, diamictites formed by cohesive SGFs and till deposited by glaciers, may be difficult to distinguish (e.g., Lowe, 1982;Visser, 1983;Wright et al, 1983), and there have been many mistakes of interpretation (e.g., Dufresne et al, 2021;Molén, 2023a). Cohesive debris flow deposits may contain all particle sizes, including a large fraction of clay, similar to tills (e.g., Molén & Smit, 2022), and therefore these may be difficult to distinguish by particle size analyses, contrary to other sediments which are more easily separated, e.g., outwash, sheet flow and loess (Blott & Pye, 2012).…”

“…These influential early papers were published before detailed documentations from studies of sediment gravity flows (SGFs) became well known amongst geologists, and these early interpretations have prevailed and often directed later interpretations and research questions of diamictites worldwide. Papers describing deposits interpreted to have been produced by former glaciations have been published for thousands of sites, from all geological periods (e.g., five episods in the Cretaceous), in the Precambrian interpreted to have covered probably the complete Earth one or many times (Snowball Earth or Slushball Earth), including major glaciations during the Hirnantian (Late Ordovician) and the Late Palaeozoic (Hambrey & Harland, 1981a;Deynoux, 1985a;Deynoux et al, 1994;Molén, 2023a).…”