Refining the pattern and style of Cordilleran Ice Sheet retreat: palaeogeography, evolution and implications of lateglacial ice‐dammed lake systems on the southern Fraser Plateau, British Columbia, Canada

Abstract

Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conceptualized as being dominated by passive downwasting (stagnation), in part because of the lack of large recessional moraines. Yet, multiple lines of evidence, including reconstructions of glacio‐isostatic rebound from palaeoglacial lake shoreline deformation suggest a sloping ice surface and a more systematic pattern of ice‐margin retreat. Here we reconstructed ice‐marginal lake evolution across the subdued topography of the southern Fraser Plateau in order to elucidate the pattern and style of lateglacial CIS decay. Lake stage extent was reconstructed using primary and secondary palaeo‐water‐plane indicators: deltas, spillways, ice‐marginal channels, subaqueous fans and lake‐bottom sediments identified from aerial photograph and digital elevation model interpretation combined with field observations of geomorphology and sedimentology, and ground‐penetrating radar surveys. Ice‐contact indicators, such as ice‐marginal channels, and grounding‐line moraines were used to refine and constrain ice‐margin positions. The results show that ice‐dammed lakes were extensive (average 27 km2; max. 116 km2) and relatively shallow (average 18 m). Within basins successive lake stages appear to have evolved by expansion, decanting or drainage (glacial lake outburst flood, outburst flood or lake maintenance) from southeast to northwest, implicating a systematic northwestward retreating ice margin (rather than chaotic stagnation) back toward the Coast Mountains, similar in style and pattern to that proposed for the Fennoscandian Ice Sheet. This pattern is confirmed by cross‐cutting drainage networks between lake basins and is in agreement with numerical models of North American ice‐sheet retreat and recent hypotheses on lateglacial CIS reorganization during decay. Reconstructed lake systems are dynamic and transitory and probably had significant effects on the dynamics of ice‐marginal retreat, the importance of which is currently being recognized in the modern context of the Greenland Ice Sheet, where >35% of meltwater streams from land‐terminating portions of the ice sheet end in ice‐contact lakes.

Calving Seasonality Associated With Melt‐Undercutting and Lake Ice Cover

“…• We present the first year round record of calving activity at a lacustrine ice-margin and classify 216 calving events by mechanism and volume • Melt-undercutting and the presence and absence of lake ice cover are associated with seasonality in calving rates and processes • Lengthening summer seasons and thinning ice-margins are likely to increase calving rates and thus rates of mass loss at lacustrine margins Brennand, 2015;Stokes & Clark, 2003;Stokes & Clark, 2004). Consequently, a detailed understanding of the lacustrine margins of the GrIS is necessary to accurately forecast its dynamic response to ongoing climate change.…”

Deglaciation of the Cordillera of Western Canada at the end of the Pleistocene

“…However, even in interior regions dominated by 'top-down' wasting, the ice sheet retreated back toward high montane source areas, mainly the Coast Mountains (Clague, 1987;Huntley and Broster, 1997;Perkins et al, 2013;Perkins and Brennand, 2015;Brennand and Perkins, 2017). For example, the active front of the ice sheet in southern British Columbia retreated to the north-northwest (Fig.…”

A Neoproterozoic ice advance sequence, Sperry Wash, California

“…Moreover, the vast preserved sediment thicknesses, >600 m in this study and >400 m in the Kingston Range (Le Heron et al ., ), far exceed recorded examples from glacilacustrine systems (e.g. Johnsen & Brennand, , ; Etienne et al ., ; Winsemann et al ., ; Livingstone et al ., ; Perkins & Brennand, ).…”