Landform and sediment imprints of fast glacier flow in the southwest Laurentide Ice Sheet

Abstract

Evidence for former fast glacier flow (ice streaming) in the southwest Laurentide Ice Sheet is identified on the basis of regional glacial geomorphology and sedimentology, highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream. Preliminary mapping from a digital elevation model of Alberta identifies corridors of smoothed topography and corridor-parallel streamlined landforms (megaflutes to mega-lineations) that display high levels of spatial coherency. Ridges that lie transverse to the dominant streamlining patterns are interpreted as: (a) series of minor recessional push moraines; (b) thrust block moraines or composite ridges/hill-hole pairs constructed during readvances/surges; and (c) overridden moraines (cupola hills), apparently of thrust origin. Together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms. Localised cross-cutting and/or misalignment of flow sets indicates temporal separation and the overprinting of ice streams/lobes. The fast-flow tracks are separated by areas of interlobate or inter-stream terrain in which moraines have been constructed at the margins of neighbouring (competing) ice streams/outlet glaciers; this inter-stream terrain was covered by more sluggish, non-streaming ice during full glacial conditions. Thin tills at the centres of the fast-flow corridors, in many places unconformably overlying stratified sediments, suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams/outlet glaciers is consistent with subglacial deformation theory. In this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory, sometimes with fast-flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally. The occurrence of minor push moraines and flutings and associated landforms, because of their similarity to modern active temperate glacial landsystems, are interpreted as indicative of ice lobe marginal oscillations, possibly in response to seasonal climatic forcing, in locations where meltwater was more effectively drained from the glacier bed. Further north, the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging, possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice-bed decoupling and drawdown initiated by the development of ice-dammed lakes.

Deglaciation of the Laurentide Ice Sheet from the Last Glacial Maximum

“…These techniques, allied with the growth of remote sensing studies across large regions of the ice sheet bed enabled a large number of ice streams to be identified (e.g. De Angelis and Kleman, 2005Kleman, , 2007Evans et al, 2008;Ross et al, 2009;Stokes et al, 2009;Ó Cofaigh et al, 2010).…”

Crevasse-squeeze ridge corridors: Diagnostic features of late-stage palaeo-ice stream activity

“…These characteristics were used by Gravenor and Kupsch (1959) to link them to controlled disintegration, whereby ice structures dictated the accumulation of diamictic sediment; hence they became known as linear disintegration ridges. The spatial patterns and regional distributions of these ridge networks have more recently been employed by Evans et al ( , 2008Evans et al ( , 2014 to propose that they originate by the squeezing of subglacial till into basal crevasses formed during glacier surging, as identified on modern surging glaciers ( Fig. 2A; Sharp, 1985a, b;Rea, 1999, 2003;Evans et al, 2007;Schomacker and Kjaer, 2007).…”

“…Crevasse squeeze ridges (CSRs) have been unequivocally linked to surging glaciers based upon modern process-form regimes (Sharp, 1985a, b;Bennett et al, 1996;Rea, 1999, 2003;Evans et al, 2007), which have in turn been employed to identify glacier and ice stream surging in the palaeoglaciological record (Evans et al, , 2008. In these contemporary and ancient case M A N U S C R I P T ACCEPTED MANUSCRIPT 2 studies, the pattern of CSR development is distinctive in that they occur as cross-cutting, geometrical ridge networks composed of diamicton (basal till) and extend across large parts, if not all, of the glacier bed and form arcuate, ice flow-transverse, subparallel sets of conjugate paired ridges that cross-cut each other at a range of angles up to 90° (Sharp, 1985a;Rea, 1999, 2003).…”

“…Clearly, therefore, CSR patterns are critical to assessing surging versus nonsurging dynamics of former glaciers. Although large areas of CSRs have been associated with palaeo-surging in the SW Laurentide Ice Sheet, these occur as glacier-wide networks with arcuate, downflow-concave limbs (Evans et al, , 2008(Evans et al, , 2014. In contrast, we here report on a linear assemblage of diamicton-cored CSRs from the bed of a palaeoice stream that formerly operated within the southwest margin of the Laurentide Ice Sheet (Ross et al, 2009;Ó Cofaigh et al, 2010) and propose that the pattern and distribution of the ridge network is related to ice stream dynamics, specifically internal flow unit reorganization, immediately prior to its shutdown.…”

The Glacial Geomorphology of the North-West sector of the Laurentide Ice Sheet

“…The activity and differing dynamics of both marine and land-terminating ice streams is well documented across the LIS (Patterson, 1997;Stokes and Clark, 2004;Jansson, 2005;De Angelis and Kleman, 2008;Evans et al, 2008;Ross et al, 2009;Stokes et al, 2009;O'Cofaigh et al, 2010). In the north-western sector of the LIS, Beget (1987) suggested a low profile ice surface and streaming flow, based on drift limits along the western Cordillera.…”