Abstract We identify late Paleozoic ice age (LPIA) subglacial megalineations from field and geospatial imagery of the Twyfelfontein area of northern Namibia, and present the results of a geomorphometric analysis of those data. Asymmetric 0.1–1.5 km-long megawhalebacks indicate a paleo-ice flow to the northwest. We infer that an ice stream draining the LPIA Kaokoveld ice sheet existed within the proto-Huab River valley and that was comparable to ice streams in modern Antarctica. Recognition of a paleo-ice stream in northern Namibia supports interpretations of glaciogenic sedimentary successions (Itararé Group) in southern Brazil that suggest the presence of major, terrestrial glacial outlet systems in southern Africa during the LPIA.

Citation: Andrews GD, McGrady AT, Brown SR, Maynard SM (2019) First description of subglacial megalineations from the late Paleozoic ice age in southern Africa. PLoS ONE 14(1): e0210673. https://doi.org/10.1371/journal.pone.0210673 Editor: Shuan-Hong Zhang, Chinese Academy of Geological Sciences, CHINA Received: October 19, 2018; Accepted: December 28, 2018; Published: January 30, 2019 Copyright: © 2019 Andrews et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the paper and its Supporting Information file. Funding: This work was supported by the West Virginia Space Grant Consortium (awarded to Andrew McGrady). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction We infer the presence of a late Paleozoic ice age (LPIA) paleo-ice stream in the Huab River valley based on the occurrence, shape, and size of elongate bedforms exposed on a Carboniferous-Permian glacial erosional surface. Many parts of Gondwana exhibit bedrock features including striations and glacial valleys formed during the LPIA, including Argentina [1], southern Australia [2], southern Brazil [3], Ethiopia, [4], Namibia [5, 6], South Africa [7, 8], and Uruguay [9]. Evidence for ice streams draining the LPIA ice sheets, however, is reported only rarely and mostly from the peripheries (Chad [10], Ethiopia [4]), but never before from southern Africa despite being at or close to the South Pole throughout the Carboniferous and early Permian [11]. Ice streams are channels filled with ice flowing faster than the surrounding ice sheet, and are the principle arteries for ice flow from the sheet’s center to the margin [12], often forming flow networks ≤103 km in length and with catchments of ≤106 km2 [13]. ‘Megalineations’ result from deposition, deformation, or erosion by the overriding ice stream where the degree of elongation is proportional to the velocity of the overriding ice mass (e.g., [14]). Mega-scale subglacial lineations (MSGLs) are the largest and most elongate megalineations, and are closely associated with modern fast-flowing (>102 m a-1) ice streams over soft-sediment in Antarctica (e.g., [15]) and Greenland. Megalineations form a size-shape continuum from meter-scale roche moutonées, through rock drumlins and megawhalebacks, to kilometer-scale MSGLs [16]. Because they are bedrock features, they have a high preservation potential through repeated glacial advances and retreats [17], and are reliable indicators of paleo-ice flow directions. Pleistocene megalineations have been used extensively to reconstruct paleo-ice streams in Antarctica, Canada, and Scotland [18], and provide a wealth of understanding about ancient ice dynamics and ice extents.

Geomorphometric analysis We measured to maximum lengths and widths of 93 megalineations around Twyfelfontein using freely available satellite imagery in Google Earth Pro (Fig 1, S1 Dataset). Axial lengths were measured in GE Pro using the built-in line measuring tool. The elongation ratio (E) is calculated as the length of the long axis divided by the perpendicular short axis length. To test our methodology and the veracity of GE Pro-sourced data, we conducted a blind analysis of different types of megalineations sculpted into sedimentary or metasedimentary rocks reported in Krabbendam et al. [18]. These include the Cree Lake drumlin field (northwest Saskatchewan), as well as ‘megagrooves’ at Amadjuak Lake (southern Baffin Island) and near Ullapool (northern Scotland). The bedforms at Twyfelfontein inhabit a narrow range of lengths (75–1,523 m; Fig 3A), widths (28–499 m), and E values (1.1–5.3; Fig 3B). The median long axis length is 226 m, the median width is 117 m, and the median E value is 2.0. Using the classification scheme of Krabbendam et al. [18] 88 of the Twyfelfontein bedforms are ‘megawhalebacks’ (long axis >100 m, E <10; Fig 3B) and five are ‘whalebacks’ (long axis <100 m, E <10). PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 3. Geomorphometric data. (A) Cross-plot of MSGLs axes from Twyfelfontein (blue), Cree Lake, Saskatchewan (orange), Amadjuak Lake, Baffin Island (green), and Ullapool, Scotland (red). (B) Cross-plot of long axis length and elongation ratio. Fields for (I) megaridges, (II) rock drumlins and whalebacks, and (III) megawhalebacks follow Krabbendam et al. [18]. (C) Violin plot of the distribution of megalineation long axes between Twyfelfontein and the three comparison sites. The white dot is the median value, the heavy black line is the interquartile range, and the ‘violin’ is a mirror-imaged kernel density plot. (D) Violin plot of the distribution of megalineation elongation ratios for Twyfelfontein and the three comparison sites. https://doi.org/10.1371/journal.pone.0210673.g003 The bedforms in Saskatchewan, Baffin Island, and Scotland are generally longer (Fig 3C) and more elongate (Fig 3D) than at Twyfelfontein; although the Namibian examples are part-buried in modern sand, and therefore their dimensions are minima. Those in the Canadian examples are classified as megawhalebacks, whereas those at Ullapool are more elongate ‘megaridges’. This range of morphologies is typical of megalineations where there is a continuum between different bedforms (e.g., [16]), and especially between drumlins and whalebacks (<100 m long and E <0.1), megawhalebacks (>100 m; E <10), and megaridges (>100 m; E >10).

Implications A LPIA paleo-ice stream? The megalineations at Twyfelfontein, and associated tillites of the Pennsylvanian-Permian Dwyka Group, record the existence of a significant body of ice that flowed to the (modern) northwest, probably during the LPIA. There were probably numerous advances and retreats of ice during the LPIA, and we do not attempt to ascribe these features to a specific glacial event. The overwhelming majority of Twyfelfontein bedforms are megawhalebacks [18] (Fig 3B) and would fall across the rock drumlin and MSGL fields of Ely et al. [16] and the rock drumlin field of Stokes et al. [24]. The bedforms described meet some or all of the criteria used in previous studies to classify MSGLs and to identify paleo-ice streams. MSGLs are typically differentiated from other elongate bedforms by E values of >10 although statistical analysis of very large geospatial datasets (n > 104 bedform features) suggests an elongation value >8 is more reasonable where a continuum exists with drumlins [16, 24]. In Stokes et al. [24]’s study only 23% of megalineations had E values >10. Krabbendam et al. [18] break-out highly (E >10) and medium (E <10) elongation megalineations, where medium elongation megalineations are transitional with and occur with typical drumlins. Recognizing that there is (1) a continuum in bedform shape and size [16], that (2) megalineations and drumlins often coexist, and (3) that the most elongate MSGLs are found in soft, marine sediments (e.g., [14]), an arbitrary definition of minimum MSGL shape and size is not useful in ancient, eroded, and poorly exposed glacial landscapes. We infer that the megalineations at Twyfelfontein can be considered MSGLs or as transitional between elongate drumlins and MSGLs. Therefore, they record passage of an ice stream of some significant ice flow velocity. The relationship between ice stream velocity and underlying bedforms is limited to geophysical studies of active ice streams where the velocity is known (e.g., [15]). All things being equal, drumlins and megawhalebacks (E <10) are interpreted to form at 102–103 m a-1, and MSGLs (E >10) at >103 m a-1 [24]; rates comparable to many Antarctic ice streams. Therefore, it is likely, that similar conditions have existed under comparably sized, long-lived ice sheet(s) in the LPIA of Gondwanaland, and that the megalineations at Twyfelfontein preserve a record of this ice streaming. Glacial outflow from the Kaokoveld ice sheet Northern Namibia and then-adjacent parts of what is now SE Brazil were located close to the South Pole in Early Mississippian and moved to about 60°S through the Pennsylvanian and Permian [11]. The region, along with the rest of southern Africa, was inundated by LPIA ice lobes in the Middle and Late Mississippian and again from the Early Pennsylvanian to the earliest Permian [6]. The ice sheet over northern Namibia (‘Kaokoveld ice sheet’ [3]; Fig 4) probably waxed and waned throughout the LPIA, and may sometimes have been contiguous with the adjacent Namaland and Transvaal ice sheets (e.g., [25]). Ice over northern Namibia and southern Angola flowed west towards outlets that supplied glaciogenic sediment to the Paraná Basin in Brazil (Fig 4; e.g., [26]) whereas ice over central and southern Namibia outlet to the Kalahari, and possibly the Karoo, basins to the south and southeast (e.g., [7]). PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 4. Paleogeographic reconstruction. Reconstruction of the SE Brazilian and southern African margins during the LPIA, adapted from [3] and [27], and with data from [19] and [20]. Known paleocurrent directions preserved in the Itararé Group and the paleo-ice flow directions reported here are shown. https://doi.org/10.1371/journal.pone.0210673.g004 There is abundant evidence of LPIA sediment supply from southern Africa in the Itararé Group, contemporaneous with the Dwyka Group, within the Paraná Basin. The Itararé Group records glacial advance and retreat at the ice margin followed by the onset of open marine conditions [26]. Northwest-trending paleochannels and paleovalleys, including sculpted bedrock surfaces, have been described throughout the Itararé Group [3], where paleocurrent directions are consistently northwesterly (Fig 4; [26]). The scale of the paleovalleys present in the Itararé Group (0.8–8 km wide, >65 km long; e.g., [28]) and the bedforms within them indicates a sustained, voluminous glacial discharge, suggesting that they were downstream of major ice streams. Plate reconstructions juxtapose SE Brazil and northern Namibia during the LPIA supporting links between the Kaokoveld ice sheet and its distal deposits in the Paraná Basin (e.g., [3, 23, 26]). Hitherto, the evidence of LPIA ice streams has come from downstream glaciogenic sedimentary records in the paleovalleys of the Itararé Group. Our observations from immediately below the contemporaneous Dwyka Group provide the first definitive evidence of ice streams from the Namibia highlands that likely fed the Itararé Group paleovalleys. If this is correct, then ice flowing through the paleo-Huab River valley at Twyfelfontein continued for at least another 200 km downstream before grounding and calving into the seas of the Paraná Basin (Fig 4).

Summary We have identified a field of large, subglacial sculpted bedforms in northern Namibia that lead us to infer the past existence of major ice stream draining a LPIA ice cap in southern Africa. The location and paleo-ice flow directions of the whalebacks and megawhalebacks indicate northwestward flow of ice along the paleo-Huab River valley that likely discharged into the shallow marine environment several hundred kilometers downstream in modern Brazil.

Supporting information S1 Dataset. MS Excel file containing megalineation length (m), elongation ratio, and azimuth from the Twyfelfontein area. https://doi.org/10.1371/journal.pone.0210673.s001 (XLSX)

Acknowledgments Thanks to Dougal Jerram, Clayton Grove, and Ansgar Wanke for introducing us to the geology of the Huab Basin and Twyfelfontein. The ASTER GDEM data product was retrieved from the online Data Pool, courtesy of the NASA Land Processes Distributed Active Archive Center (LP DAAC), USGS/Earth Resources Observation and Science (EROS) Center, Sioux Falls, South Dakota, https://lpdaac.usgs.gov/data_access/data_pool. ASTER GDEM is a product of NASA and METI.