Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

Abstract

In the eastern part of the Karoo Basin of South Africa, the sedimentary record of the Late Palaeozoic Ice Age, the Dwyka Group, consists of an up to 200 m thick accumulation of massive to crudely stratified diamictite occasionally interstratified with siltstone, sandstone and conglomerate horizons. Three distinct sedimentary units, separated by intervening glacial erosion surfaces, are viewed as ice‐margin fluctuation sequences. The lowermost one, resting on highly uneven, glacially abraded Archaean basement, has been interpreted as a grounding zone wedge deposited after the retreat and stabilization of the ice margin after the inundation of the Karoo Basin. The grounding zone wedge interpretation is based on its thickness (up to 100 m), the dominance of diamictite, and its facies assemblage and inferred depositional processes (rain‐out of debris, dropstone dumping, mass and debris flow, till). Overlying the grounding zone wedge, deposits are sedimentary units interpreted as glaciofluvial or ice‐contact delta and grounding zone wedges, respectively. By analogy with Quaternary sedimentary sequences, deposition of the Dwyka Group in the study area might have been very rapid (tens to hundreds of thousand years) and may hence correlate with the ultimate deglacial sequence of the Western Karoo Basin, as both successions are covered by the postglacial Ecca Group. Although commonly observed and imaged on modern, high‐latitude continental shelves, grounding zone wedges have never been interpreted in the ancient geological record. This paper therefore outlines a model defining criteria necessary for their identification.

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

“…The Late Palaeozoic Ice Age (Fielding et al ., 2010; Montañez & Poulsen, 2013) resulted in the expansion of ice sheets across Gondwana, with a general diachronous trend charting the drift of South Gondwana over the South Pole. A rich archive of glacigenic deposits and associated sand‐rich outwash complexes is recorded for example from Devonian basins in Bolivia (Bache et al ., 2012), Late Carboniferous to Permian basins of Argentina, Brazil and Uruguay (Vesely et al ., 2015; Aquino et al ., 2016; Alonso‐Muruaga et al ., 2018; Assine et al ., 2018), roughly coeval successions in South Africa (Dietrich & Hofmann, 2019) and Ethiopia (Bussert, 2014), and Permian deposits in Oman (Martin et al ., 2012) and Australia (Fielding et al ., 2010). With the sole exception of the eastern Karoo Basin, South Africa (Dietrich et al ., 2019), there is no specific interpretation of upper‐flow‐regime bedforms in Late Palaeozoic glacigenic strata.…”

Scratching the surface: Footprint of a late Carboniferous ice sheet

“…At the northeastern flanks, diamictites accumulated in a restricted glacial valley setting (Visser and Kingsley, 1982). In the eastern Karoo Basin, a complex, condensed signature records deglaciation punctuated by short-term stillstands and minor readvances (Dietrich and Hofmann, 2019). Such basin-margin localities record glacially striated pavements of two types: (1) hard-bedrock pavements, recording the direct abrasion of LPIA ice sheets onto hard bedrock material (Du Toit, 1954;Visser and Loock, 1988;Bussert, 2010), and (2) softsediment pavements (e.g., Visser, 1990).…”

“…Where cut into soft sediments, glacial unconformities may record the degree of basal coupling or changes in ice-flow velocity (Le Vesely and Assine, 2014). So-called "soft-sediment striated surfaces" are extremely common in the glaciogenic Late Ordovician (Deynoux and Ghienne, 2004;Le Heron et al, 2005;Denis et al, 2010;Girard et al, 2015;Tofaif et al, 2019) and Carboniferous-Permian (Visser, 1987(Visser, , 1990Assine et al, 2018;Dietrich and Hofmann, 2019) records alike. Although widely used to inform regional ice-sheet flow models (Ghienne et al, 2007;Le Heron, 2018;Visser, 1997), subglacial features in soft sediment are prone to later deformation and fluidization (e.g., Le Heron et al, 2005).…”

“…In the Karoo Basin of South Africa, there has been a tradition of investigation of late Paleozoic ice age (LPIA) deposits of the Dwyka Group stretching back a century (Du Toit, 1921), with groundbreaking work on paleogeographic reconstructions and facies analysis in the 1980s and 1990s (Visser, 1983(Visser, , 1987(Visser, , 1989(Visser, , 1990(Visser, , 1997Visser and Kingsley, 1982). New insights from satellite image interpretation (Le Heron, 2018;Andrews et al, 2019) together with a new wave of field work on LPIA strata (Vorster et al, 2016;Linol et al, 2016;Belica et al, 2017;Griffis et al, 2018;Dietrich and Hofmann, 2019) have sharpened the need to understand the subglacial unconformities, and the role of ice streaming and surging behaviors. In this study, we produced the first detailed map of a LPIA glacial unconformity from Oorlogskloof, Northern Cape Province, South Africa (Fig.…”

A conceptual model for glaciogenic reservoirs: From landsystems to reservoir architecture

“…The stratigraphic change in deposition from ice marginal to proglacial may be gradual up-section if the ice margin retreat is slow (continuous annual retreat at a similar rate), or abrupt, if retreat is rapid/catastrophic and it occurs over a significant distance (Sejrup et al, 2016;Stokes et al, 2015). If the retreat is gradual Ice-contact deltas may transitions into glacier-fed deltas (Table 3 and Figure 4, Figure 5 andFigure 6) as the ice sheet retreats and the ice margin emerges from the water (Dietrich et al, 2017(Dietrich et al, , 2016Dietrich and Hofmann, 2019). Glacier-fed deltas are one of the most prospective reservoir candidates as the sediments are commonly sand dominated and the depositional processes are efficient at sorting and portioning the different grainsizes, resulting in thick, laterally extensive packages with good reservoir properties.…”