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Footprint of a Late Carboniferous Ice Sheet D.P https://doi.org/10.1130/G46590.1 Manuscript received 3 June 2019 Revised manuscript received 31 July 2019 Manuscript accepted 7 August 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 23 September 2019 Scratching the surface: Footprint of a late Carboniferous ice sheet D.P. Le Heron1, P. Dietrich2,3, M.E. Busfield4, C. Kettler1, S. Bermanschläger1 and B. Grasemann1 1 Department für Geodynamik und Sedimentologie, Althanstraße 14, Universität Wien, 1190 Vienna, Austria 2 Department of Geology, University of Johannesburg, Auckland Park Kingsway Campus, Johannesburg 2092, South Africa 3 Géosciences Rennes, UMR6118, Université de Rennes 1, 263 Avenue du Général Leclerc, Bâtiment 15, Campus de Beaulieu, 35042 Rennes Cedex, France 4 Department of Geography and Earth Sciences, Aberystwyth University, Llandinam Building, Aberystwyth SY23 3DB, UK ABSTRACT records deglaciation punctuated by short-term Field observations in conjunction with aerial images from an unmanned aerial vehicle stillstands and minor readvances (Dietrich and were used to create the first map of a glacial unconformity underlying the late Carboniferous Hofmann, 2019). Such basin-margin locali- Dwyka Group of South Africa. Crosscutting relationships reveal that the glacial unconformity ties record glacially striated pavements of two at Oorlogskloof, in which flutes, grooves, and striae were ploughed into unconsolidated sand, types: (1) hard-bedrock pavements, recording formed in a three-phased process charting a periodic shift in the locus of subglacial erosion. the direct abrasion of LPIA ice sheets onto The unconformity formed by a periodically decoupled ice sheet in a probable tidewater set- hard bedrock material (Du Toit, 1954; Visser ting. This model contrasts with earlier views that the structures simply record progressive and Loock, 1988; Bussert, 2010), and (2) soft- ice-margin liftoff during transgression, and they provide unique insight into the complex sediment pavements (e.g., Visser, 1990). The lat- temporal development of a 300 Ma subglacial environment. ter pavements, on which we focus herein, were first described in Oorlogskloof and surrounding INTRODUCTION and 1990s (Visser, 1983, 1987, 1989, 1990, area by Rust (1963). Unconformities are increasingly understood 1997; Visser and Kingsley, 1982). New insights Precisely how the sub-Dwyka unconformity as recording complex, evolving processes during from satellite image interpretation (Le Heron, was cut remains unclear. Highly complex ice basin evolution rather than simply stasis (Davies 2018; Andrews et al., 2019) together with a flows, including trunk glaciers, ice streams, and and Shillito, 2018). Where cut into soft sedi- new wave of field work on LPIA strata (Vor- outlet glaciers (e.g., Visser and Kingsley, 1982; ments, glacial unconformities may record the ster et al., 2016; Linol et al., 2016; Belica et al., Visser, 1997), have been interpreted. Deep paleo- degree of basal coupling or changes in ice-flow 2017; Griffis et al., 2018; Dietrich and Hofmann, valleys, some up to 160 km long, record differing velocity (Le Heron et al., 2005; Vesely and Ass- 2019) have sharpened the need to understand the depths of incision into underlying sedimentary ine, 2014). So-called “soft-sediment striated sur- subglacial unconformities, and the role of ice rocks and basement granite and gneiss (Visser faces” are extremely common in the glaciogenic streaming and surging behaviors. In this study, and Kingsley, 1982). Across the Karoo, glaciers Late Ordovician (Deynoux and Ghienne, 2004; we produced the first detailed map of a LPIA are thought to have coalesced from ice centers ini- Le Heron et al., 2005; Denis et al., 2010; Girard glacial unconformity from Oorlogskloof, North- tially located to the north, east, and south (Visser, et al., 2015; Tofaif et al., 2019) and Carbonifer- ern Cape Province, South Africa (Fig. 1), inte- 1987, 1989). Crucially, ice-margin disintegration ous–Permian (Visser, 1987, 1990; Assine et al., grating data from an unmanned aerial vehicle models are strongly influenced by interpretation 2018; Dietrich and Hofmann, 2019) records (UAV) and field observations. of the Oorlogskloof pavement in which a phase alike. Although widely used to inform regional of ice-margin liftoff culminated in collapse and ice-sheet flow models (Ghienne et al., 2007; Le STUDY AREA AND GEOLOGIC proglacial sedimentation (Visser, 1990). Heron, 2018; Visser, 1997), subglacial features BACKGROUND in soft sediment are prone to later deformation The Oorlogskloof area lies at the present- METHODOLOGY and fluidization (e.g., Le Heron et al., 2005). day western flank of the Karoo Basin of South Combining traditional field work (photo- Careful analysis is therefore required to reveal Africa (Fig. 1). The late Carboniferous–Perm- graphs, descriptions, measurements) with UAV their true origin (e.g., subglacial vs. iceberg keel ian Dwyka Group was deposited by oscillating, imagery, we mapped the unconformity beneath generated; Woodworth-Lynas and Dowdeswell, high-latitude ice masses (Visser, 1989), with the Dwyka Formation in Oorlogskloof (Fig. 1) 1994; Vesely and Assine, 2014). up to 800-m-thick diamictites and intergla- to document its geomorphology. Using a DJI In the Karoo Basin of South Africa, there cial mudstones accumulating during up to four Mavic Pro drone, high-resolution aerial pho- has been a tradition of investigation of late Pa- glacial cycles in the basin depocenter (Visser, tographs were obtained from multiple eleva- leozoic ice age (LPIA) deposits of the Dwyka 1997). At the northeastern flanks, diamictites tions (8–100 m) and stitched together in Agi- Group stretching back a century (Du Toit, 1921), accumulated in a restricted glacial valley set- soft Metashape software (www.agisoft.com). A with groundbreaking work on paleogeographic ting (Visser and Kingsley, 1982). In the eastern digital elevation model (DEM) together with a reconstructions and facies analysis in the 1980s Karoo Basin, a complex, condensed signature mosaicked orthophoto were exported to QGIS CITATION: Le Heron, D.P., et al., 2019, Scratching the surface: Footprint of a late Carboniferous ice sheet: Geology, v. 47, p. 1034–1038, https://doi.org/10.1130/ G46590.1 1034 www.gsapubs.org | Volume 47 | Number 11 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1034/4852061/1034.pdf by guest on 30 September 2021 RESULTS Gondwana Glacial Botswana Arabia sediment Johannesburg Description Africa South 0 100 km 360 Ma Pole The study section consists of medium- to 340 Ma 2000 km coarse-grained sandstone and pebbly sand- India N Swaziland 360 Ma stone of the Silurian- to Devonian-aged Nar- South 320 Ma Australia S O U T H A F R douw Subgroup (Table Mountain Group, Cape America Antarctica I C A Supergroup; Thamm and Johnson, 2006), onto Main map 280 Ma which series of glacially related landforms tied Namibia to the LPIA were cut. The surface dips gently Lesotho east at 4°. Analysis of the composite aerial im- age (Fig. 2) demonstrated that the glacial land- forms are spatially organized into three distinct Durban packages (Figs. 3 and 4), and that these packages show crosscutting relationships. Oorlogskloof 31°S Package 1 exposes two distinct sets of (study area) streamlined features, namely (1) flutes, and (2) T grooves and striae. Both sets of features trend H E E Paleo-highlands east-west to ESE-WNW (270 to N285; Fig. 3; E S I C Glacial deposits see also Visser, 1990, his figure 6). Flutes are Ice sheet sharp-crested, 5–20-cm-amplitude, and 50-cm- Cape CAPE FOLD BELT wide features. Asymmetric in transverse profile, Town Ice margin Port they exhibit steep (25°–40°) northward sides Elisabeth direction and less steeply dipping (10°–25°) southward 18°E 20°E 22°E 24°E 26°E 28°E sides. Miniature grain-flow lobes occur on southward slopes, downlapping the troughs Figure 1. Paleogeographic map of South Africa during the late Paleozoic ice age, and outline between the flutes (Fig. 3B). Striations also of ice-sheet flow character, based on Visser (1997) and Lopez-Gamundi and Buatois (2010). Study area at Oorlogskloof is situated at the western margin of the Karoo Basin. occur on the surfaces of the flutes Fig. 3B( ). Flutes are distributed into sets of one to five that lie on a planar striated and grooved surface (www.qgis.com). Applying the methodology of aerial image. The resultant image, which served also characterized by smaller-scale (a few mil- Le Heron et al. (2019), a transparency algorithm as the foundation for mapping (Fig. 2), enhances limeters to centimeters in amplitude) flutes. The was applied to the latter, allowing the two im- geological features that may be present on the flute sets, separated by the striated and grooved ages to be combined and producing a composite orthophoto and absent on the DEM. surface devoid of large flutes, thus define a lin- 0 652043 Figure 2. Composite aerial image and corre- sponding interpretation of Oorlogskloof glacial 6520410 pavement (South Africa). Colors correspond to three different packages 323690 323710 323730 323750 323770 323790 of subglacial bed form, which can be distin- guished on the basis of slightly differing orien- tations at crosscutting G relationships. Color-cod- ing also corresponds to 3 Figures 3 and 4. Locations of photos B–G shown in Figure 3 are indicated. E C 1 B F 2 D Cross cutting relationships: 3 Package 3 2 Package 2 N 1 Package 1 Ice flow
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