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Hard‐bed streamlined landforms developed on Archaean basement rocks that underlie the Dwyka Group in the study area. (A) Roches moutonnées developed in Archaean mafic volcanic rocks. Ice flow was towards the south (towards the observer) as indicated by the plucked, lee side of the form. (B) Conspicuously asymmetric Roches moutonnées developed in Archaean quartzites of the Pongola Supergroup. Ice flow towards the south (left) is clearly evidenced by the asymmetry of the form, the up‐glacier side (right) being shallow and striated while the down‐glacier (left) side is devoid of striae (plucked) and steeper. Note the faintly stratified diamictite covering the striated pavement (white arrows) (C) A U‐shaped trough, 800 m wide and 100 m deep, carved into Archaean basement rocks of the Pongola Supergroup, filled with Dwyka sediments (dark strata, indicated by white arrows) and subsequently exhumed by sub‐modern erosion. This trough, oriented in a NNW‐SSE direction, is thought to have been carved by flowing ice during Dwyka times. Figure 1B for location.
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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 int...
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... The stratigraphically highest unit (Fig. 3C) is a feldspathic sandstone. The upward continuation of this sequence is concealed by Phanerozoic cover of the Dwyka Group of the Karoo Supergroup (Dietrich and Hofmann, 2019). ...
... The proximal valley and inner platform association (Visser, 1983a) attains comparatively small thickness (<300 m) and is best developed along the northern and northeastern basin margins (see also von Brunn, 1996;Visser, 1996;Cole, 1991;Dietrich and Hofmann, 2019). The outer platform association (Visser, 1983a), developed in the western Karoo Basin, exhibits thicknesses ranging from 300 to 800 m. ...
... Horizontally and obliquely displaced, intact brachiopod valves resulted from rapid transport and deposition (Martin et al., 1996). Crudely wavy bedding likely reflects periodic aggregation of uneven beds with erosive base (Isbell et al., 2013;Dietrich and Hofmann, 2019). The soft-sediment deformation structures indicate disturbance of sediments right after deposition (Owen, 1996), which may be caused their rapid sedimentation. ...
The Baoshan Block was part of the eastern Cimmerian continent along the northeastern margin of Gondwana during the late Paleozoic, and preserves continuous sedimentary records during the apex of the Early Permian glaciation. Although much work has been carried out on biostratigraphy and paleogeography, detailed sedimentological study of the glacial records in the Baoshan Block has gained little attention, which hampers the understanding of depositional processes along the northeastern margin of Gondwana. Here, we conduct a high-resolution sedimentological study on the Lower Permian Dingjiazhai Formation of the Baoshan Block, with the aim of reconstructing the evolution of sedimentary environments. Six major lithological facies and three facies associations are recognized from the Dongshanpo section. The diamictite facies association is composed of weakly stratified diamictite, stratified diamictite, and massive sandy siltstone facies, which are formed by proglacial outwash debris flows and settling of melt-water plumes. The lenticular gravelly sandstone facies association consists of lenticular gravelly sandstone, thin-bedded siltstone, and homogenous silty mudstone facies, suggesting a distal subaqueous proglacial offshore setting with episodic melt water plumes. The bioclastic rudstone facies association comprises of bioclastic rudstone and floatstone, and thin-bedded siltstone facies, which suggest an offshore depositional environment punctuated by bioclastic gravity flows. Accordingly, we reconstructed a gradually changed sedimentary model responding to the Early Permian deglaciation in the Baoshan Block, which probably represents a typical evolution of sedimentary environments along the northeastern margin of Gondwana during the Early Permian.
... A well-studied depositional environment that is created through this combination of processes is a grounding zone wedge (GZW). GZWs are asymmetric wedges of sediment that accumulate on continental shelves at the grounded, stable margins of marine-terminating ice sheets (Dowdeswell and Fugelli, 2012;Batchelor and Dowdeswell, 2015;Demet et al., 2019;Dietrich and Hofmann, 2019;Le Heron et al., 2022). The subglacial portions of GZWs are composed of upglacierdipping, planar "topsets", and most of the volume of the landform is deposited as downglacier-dipping "foresets" deposited as mass-transport deposits of mainly resedimented subglacial debris (Simkins et al., 2017;Prothro et al., 2018). ...
... The subglacial portions of GZWs are composed of upglacierdipping, planar "topsets", and most of the volume of the landform is deposited as downglacier-dipping "foresets" deposited as mass-transport deposits of mainly resedimented subglacial debris (Simkins et al., 2017;Prothro et al., 2018). The "foresets" of GZWs are often massive and homogenous, both in seismic (e.g., Dowdeswell and Fugelli, 2012) and in outcrop (e.g., Dietrich and Hofmann, 2019). GZWs grow through the progradation and aggradation of these systems, a process that can take anywhere from decades to centuries (Batchelor and Dowdeswell, 2015;Simkins et al., 2017). ...
Glaciation during the Late Paleozoic Ice Age climatic interval (~ 370–260 Ma) was likely dynamic; consisting of numerous ice centers that grew and shrank asynchronously through time. Improvements in understanding of the spatial and temporal depositional complexity of glaciomarine sedimentary systems have shown that in order to characterize the conditions of Late Paleozoic glaciation, glaciogenic depositional systems and their stratigraphy must fundamentally be understood at the local level. To that end, this study reexamines the physical sedimentology and sequence stratigraphy of the type-section of the Permo-Carboniferous, glaciomarine Wynyard Formation (Wynyard Tillite) of the Tasmanian Basin by describing a 415 m thick interval of the formation, beginning at its basal erosional unconformity with the Proterozoic Burnie (Oonah) Formation.
The glacigenic nature of the Wynyard Formation stratigraphy is indicated by the characteristics of clasts throughout the succession (striated, faceted, angular, and variable lithologies and grain sizes). Three glacial depositional environments were interpreted: a grounding zone wedge deposited in an ice-contact setting, glacier-proximal portion of a grounding line fan and/or morainal bank, and cyclopelites deposited in a glacier-intermediate to glacier-distal setting. Mass transport and turbidite deposits are common throughout all facies associations, as is soft sediment deformation likely caused by slumping. Several boulder pavements occur throughout the succession, indicating periodic glacier grounding. Together, these facies associations indicate that this Wynyard Fm succession is composed of glacigenic sediments that were deposited in sub-aqueous, marine, predominantly proglacial environments. The sequence stratigraphic analysis indicates that this entire succession of the Wynyard Fm was likely deposited during the single retreat phase of the “Wynyard Glacier”, with one notable readvance over the area.
The interpretations made by this work enhance the understanding of glaciation of the Tasmanian Basin during the LPIA through facies analyses and a sequence stratigraphic approach, which allowed for detailed subdivision of lithologies that enabled inferences regarding the type, timing, and extent of the “Wynyard glacier”. This Wynyard Fm succession was likely deposited during a single glacier retreat with some minor readvances that may have occurred on the order of decades to years. Additionally, the glacier's grounding line was likely never more than a few kilometers upglacier (south) of this location during deposition.
These finding are significant, because the massive diamictites that comprise this succession had previously been considered homogeneous and therefore resisted detailed interpretations. Constraining the often-complex depositional histories of glacigenic strata in similar fashions across the Tasmanian Basin allow us to better understand how these glacigenic deposits fit into the global climate system of the Late Paleozoic Ice Age.
... The measurements of the proposed GZW in the Ennedi are consistent with data from modern high-latitude GZWs, which are generally <15 km long, between 4 and 280 km wide, and 15 to 100 m thick (Batchelor and Dowdeswell, 2015). Although interpretations of pre-Cenozoic GZWs have been attempted from a purely sedimentological perspective (Dietrich and Hofmann, 2019;Le Heron et al., 2022), no geomorphological evidence for these has been published. Therefore, in this study, we present the first geomorphological description of an exposed GZW in association with large-scale glacial lineations situated at the margin of an ice stream. ...
... This might attest to a re-advance of the ice stream after the deposition of the GZW material. Interpretations of GZWs in the ancient geological record are rare (Dietrich and Hofmann, 2019). Modern examples, on the other hand, are quite common (Batchelor and Dowdeswell, 2015;Dowdeswell et al., 2016;Fig. ...
... This is in contrast to southern Africa where well preserved glacial deposits are widespread (e.g. Visser, 1997;Dietrich and Hofmann, 2019), but probably younger (Latest Carboniferous; Griffis et al., 2021). Well-exposed, shallow-dipping outcrops extend hundreds of kilometres along strike and are ideal for satellite image mapping. ...
... Differences between glaciogenic and mass flow features often can be revealed by comparing data from different geological disciplines (compare Shanmugam et al., 1994;Major et al., 2005;Talling et al., 2007Talling et al., , 2012; Dakin et al., 2013;Shanmugam, 2016;Molén, 2017Molén, , 2021Molén, , 2022aMolén, , 2022bDietrich & Hofmann, 2019;Peakall et al., 2020;Cardona et al., 2020). Geological features which are commonly interpreted as glaciogenic, for example, striated, grooved and polished bedrock, including all kinds of chevron structures/crescentic gouges/chattermarks, grooves and nailhead striations, can form as a result of different kinds of mass movements, such as avalanches, slides and different kinds of sediment gravity flows (Draganits et al., 2008;Dakin et al., 2013;Molén, 2017Molén, , 2021Molén, , 2022aMolén, , 2022bKennedy & Eyles, 2021). ...
... In the southern part of South Africa, the Dwyka Group was tectonically deformed during uplift and partially metamorphosed (Fagereng, 2014). In conclusion, the Karoo Basin shows evidence of downwarping as a lithospheric deflection (Dietrich & Hofmann, 2019). The basin may be seen as a retroarc foreland basin, and the Dwyka Group forms its basal part (Johnson et al., 1997;Catuneanu et al., 2005;Barbolini et al., 2018;Hansen et al., 2019;Dietrich & Hofmann, 2019). ...
... In conclusion, the Karoo Basin shows evidence of downwarping as a lithospheric deflection (Dietrich & Hofmann, 2019). The basin may be seen as a retroarc foreland basin, and the Dwyka Group forms its basal part (Johnson et al., 1997;Catuneanu et al., 2005;Barbolini et al., 2018;Hansen et al., 2019;Dietrich & Hofmann, 2019). ...
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.
... However, whilst GZWs can form in the absence of an ice shelf (Powell and Alley, 1997;Bart et al., 2018), their sedimentary character can be quite different at tidewater termini due to the availability of meltwater. Greater meltwater reworking permits the development of more heterogeneous ice-contact fans or mixed influenced GZW-fan systems (e.g., Demet et al., 2019;Dietrich and Hofmann, 2019). The predominance of poorly sorted, highly concentrated glaciomarine deposits in the Makganyene Formation outcrops studied herein, the limited evidence for meltwater reworking (e.g., no unidirectional cross-stratification, rare and isolated examples of turbulent sorting), and the oxygenating subglacial brine model of iron formation all point to deposition from a relatively cold glacier, which would be consistent with an ice shelf setting. ...
The Makganyene Formation is a Siderian (2.45–2.22 Ga) diamictite-dominated succession, with both outcrop and subcrop in the Griqualand West Basin of the Transvaal Group of South Africa. We provide new outcrop and core descriptions from this succession, supplemented by microscopic analyses, to present an updated depositional model for a classic Palaeoproterozoic diamictite. Although internal correlation of core and outcrop successions is not possible, a recurring pattern is observed where diamictites are organised into coarsening-upward motifs at the tens of metres scale. With additional finds of striated clasts, and evidence for dropstones both at the core scale and at the microscopic scale, earlier interpretations of glacial control on sedimentation can be substantiated, with modification of glacial diamictites by mass flow processes also recognised. Overall, given the characteristic progradational stratigraphic architecture, we propose a new model for the Makganyene Formation which is considered to represent deposition of a grounding zone wedge at an ancient, oscillating ice margin.
... Visser (1997) summarised the LPIA ice flow record of South Africa as a series of highland valley glaciers flowing northward and southward into the Karoo Basin, thereby joining trunk ice streams that flowed westward into southern Brazil and Argentina ( Figure 5A). In the Karoo Basin, hard bedrock striated surfaces are recorded in multiple basin margin locations, such as in Douglas (du Toit, 1954) at Nooitgedacht near Kimberley (Visser and Loock, 1988) and in KwaZulu-Natal (Dietrich and Hofmann, 2019). However, soft-sediment striated surfaces are also recorded (e.g., in Oorlogskloof; Visser, 1990) and most likely represent basin-marginal and intra-basinal equivalents, respectively. ...
The deep time (pre-Quaternary) glacial record is an important means to understand the growth, development, and recession of the global cryosphere on very long timescales (10⁶–10⁸ Myr). Sedimentological description and interpretation of outcrops has traditionally played an important role. Whilst such data remain vital, new insights are now possible thanks to freely accessible aerial and satellite imagery, the widespread availability and affordability of Uncrewed Aerial Vehicles, and accessibility to 3D rendering software. In this paper, we showcase examples of glaciated landscapes from the Cryogenian, Ediacaran, Late Ordovician and Late Carboniferous where this approach is revolutionizing our understanding of deep time glaciation. Although some problems cannot be overcome (erosion or dissolution of the evidence), robust interpretations in terms of the evolving subglacial environment can be made. Citing examples from Australia (Cryogenian), China (Ediacaran), North and South Africa (Late Ordovician, Late Carboniferous), and Namibia (Late Carboniferous), we illustrate how the power of glacial geomorphology can be harnessed to interpret Earth’s ancient glacial record.
... For instance, in near-coastal regions offshore southern Ireland, bedrock is outcropping at the seafloor and is in relatively shallow water, while it deepens towards the centre of the shelf (Figs 5.2A-C-D, 5.5D). Outcropping bedrock can impact stability of the grounding line by providing pinning points for ice anchorageDietrich and Hofmann, 2019) that is reflected by the deposition of GZW and moraines off south-west Ireland, in a similar scenario to that of the present-day Thwaites Glacier in Antarctica (e.g.Castleman et al., 2021).Fine laminated and massive muds overlying the till indicate glaciomarine deposition likely established following the retreat of the ice margin. Although the low concentration of biogenic material within these sediments was a challenge for radiocarbon dating, one date towards the top of this facies yielded an age of 12,381 ± 490 cal BP. ...
Palaeo ice-sheet reconstructions are considered a key approach to increase our understanding of past climate change and how this impacts on the cryosphere. These reconstructions have shown that ice sheets can have a relatively fast response to climate and ocean forcing mechanisms. This has raised concerns about the future stability of ice sheets in a warming world, especially those that are marine-based or marine-terminating, such as the Greenland and Antarctic Ice Sheets. However, predictions of ice sheets stability are complex and their long-term accuracy remains a major weakness in climate change science.
Palaeoglaciological reconstructions offer one critical approach to improve our understanding of how ice sheets respond to climatic drivers over full glacial cycles or through dynamic periods of ice sheet history. As such, palaeo-ice sheets act as useful analogues for helping to determine the important drivers that can influence ice sheets in the future. This study examines the southern margins of two former ice sheets: the British-Irish ice Sheet (BIIS) and the Newfoundland Ice Sheet (NIS). These are located on opposite sides of the North Atlantic Ocean but at similar latitudes. Both ice sheets were grounded below sea level, were drained by ice streams and had extensive marine margins potentially exposed to changes in large-scale ocean and atmospheric circulation. Therefore, they represent good analogues for modern marine-terminating ice sheets.
New marine geophysical and sediment data were analysed across the Celtic Sea shelf, between Ireland and the UK, which was occupied by the Irish Sea Ice Stream (ISIS), the largest outlet of the BIIS. Geomorphological mapping shows a large meltwater drainage system, including tunnel valleys, beneath the central axis of the former ISIS. This evidence implies significant and erosive meltwater release, potentially influencing the rapid retreat across the shelf. At this stage (~25 cal BP), the ISIS was also retreating close to the southern Irish coastline. Some 30 km off the coast, a relatively large grounding-zone wedge was formed, together with a sequence of morainic ridges, which are capped by glacimarine laminated and massive muds. These features show a stepped retreat of the ISIS margin towards to the coastline. The difference in behaviour of the retreating ice sheet near the present-day coast compared to that in the central axis was probably governed by topographical and geological controls, including variations in water depth and the presence of bedrock outcrops acting as pinning points.
On the other side of the Atlantic Ocean, new data on the southern shelf of Newfoundland were analysed. Here, fjords served as outlets for sediment-laden meltwater draining the former NIS. Intense and widespread calving occurred across the NIS marine margin following its extension to the shelf edge. When the ice sheet reached the present-day coastline, it stabilised at the mouth of the fjords and formed a series of moraines that record a still-stand of the ice margin. The combination of new and extant data suggest that the still-stand occurred between 16.3 ka cal BP and 15 ka cal BP. Stratified glacimarine sediments accumulated at the mouth of the fjords during a period of prevailing cold-water conditions when relative sea level was ~30 m higher than today.
Comparison between the two study areas shows different topographic settings and asynchronous ice-sheet behaviour during the last deglaciation. The onset of retreat between the two ice sheets is around 10 ka apart. A comparison of the results against existing proxy data from the North Atlantic Ocean highlighted that deglaciation of both shelves was initiated in the absence of ocean warming, when eustatic sea level was at a minimum. Internal glaciological factors were therefore most likely responsible for the demise of both marine sectors. This demonstrates that marine-terminating ice margins can internally trigger their own demise in very different glaciological settings and within overall cold conditions. Such information provides additional data for ice sheet numerical models that investigate links between rate and pattern of retreat and the drivers of ice sheet variability.
