Glacial geomorphology of the Neutral Hills Uplands, southeast Alberta, Canada: The process-form imprints of dynamic ice streams and surging ice lobes

Abstract

Emrys (2020) 'Glacial geomorphology of the Neutral Hills Uplands, southeast Alberta, Canada : the process-form imprints of dynamic ice streams and surging ice lobes.', Geomorphology., 350 .

Formation of ribbed bedforms below shear margins and lobes of palaeo-ice streams

“…Four categories of formation processes have been proposed: (i) deformation or reshaping of pre-existing sedimentary mounds, such as former streamlined or marginal landforms, by overriding ice (Boulton, 1987;Lundqvist, 1989;Möller, 2006); (ii) fracturing and extension of frozen beds along transitions from warm-to-cold ice bases, where tensional stresses increase (Hättestrand and Kleman, 1999;Sarala, 2006); (iii) subglacial meltwater floods responsible for the formation of inverted erosional marks at the ice base, infilled by sediments (Shaw, 2002); and (iv) till deformation, in response to the flow of ice over bed heterogeneities resulting from variations in pore water pressure, the basal thermal regime and bed strength (Terzaghi, 1931;Shaw, 1979;Bouchard, 1989;Kamb, 1991;Tulaczyk et al, 2000;Lindén et al, 2008;Stokes et al, 2008). The last process is consistent with physically based mathematical models demonstrating that ribbed bedforms naturally arise from wavy instabilities in the combined flow of ice, basal meltwater and viscoplastic till (Hindmarsh, 1998a, b;Fowler, 2000;Schoof, 2007;Clark, 2010;Chapwanya et al, 2011;Sergienko and Hindmarsh, 2013;Fowler and Chapwanya, 2014;Fannon et al, 2017).…”

“…In conclusion, ribbed bedforms occur in a highly varied distribution within ice sheets and ice streams and multiple hypotheses have been proposed for their formation. A size and shape continuum has been suggested between ribbed, hummocky and streamlined bedforms, suggesting that these subglacial bedforms could form in response to the same governing processes modulated by either ice flow velocity or ice flow duration (Aario, 1977;Rose, 1987;Dunlop and Clark, 2006;Stokes et al, 2013b;Ely et al, 2016;Fannon et al, 2017). Moreover, ribbed bedforms are frequently overprinted by drumlins and embedded within polygenetic land systems, corresponding to multiphase stories, which complicate their interpretation (Cowan, 1968;Aylsworth and Shilts, 1989;Stokes et al, 2008).…”

Ribbed bedforms in palaeo-ice streams reveal shear margin positions, lobe shutdown and the interaction of meltwater drainage and ice velocity patterns

“…Sets of parallel mega-grooves (furrows within the substratum), carved by boulders trapped and transported at the ice base, are sometimes described in paleo-ice stream trunks. They are 95 typically 1 km in length, a few tens of metres in depth and frequently show a mega-raft at their downstream terminations and bordering ridges (Atkinson et al, 2016;Newton et al, 2018;Evans et al, 2020). Streamlined bedforms and grooves provide a relevant proxy for reconstructing ice flow velocity and direction (Dyke and Morris, 1988;Boyce and Eyles, 1991;Stokes and Clark, 2002a;Briner, 2007).…”

“…If groove distributions and elongations reproduced in the experiments are easily comparable to MSGLs along ice stream beds, the process of formation is somehow different since MSGLs are believed to result from the ploughing and carving of ice keels into soft substratum while grooves are formed by ploughing and carving of larger-size particles into the soft bed (Smith, 1948). Groove ploughing, initiated by the subglacial displacement of bedrock mega rafts across a softer bed, has however been proposed to explain the formation of parallel lineations 630 beneath palaeo-ice streams in Alberta (Atkinson et al, 2018;Evans et al, 2020).…”

“…14). Glaciotectonic thrust masses and crevasse-squeezed 685 ridges have notably been observed in southeast Alberta and linked to the dynamics of surging lobes during melting and retreat of the Laurentide Ice Sheet (Evans et al, 2020).…”