Genesis of an esker-like ridge over the southern Fraser Plateau, British Columbia: Implications for paleo-ice sheet reconstruction based on geomorphic inversion

Abstract

Robust interpretations of meltwater systems operating during ice sheet decay are integral to reconstructing deglacial patterns and style. Yet over reliance on meltwater landform morphology with limited attention to morpho-sedimentary relationships, and basin-scale geomorphic and stratigraphic context can lead to unreliable geomorphic inversion-based paleo-ice sheet reconstructions. This problem is illustrated by the evolution of Young Lake esker-like ridge (YLER) formed in the Young Lake basin (YLB) on BC's southern Fraser Plateau during decay of the last Cordilleran Ice Sheet (CIS). We integrate data from digital elevation models, aerial photographs, sedimentary outcrops, water wells and shallow geophysics (ground-penetrating radar, electrical resistivity tomography). Previous interpretations of YLER as both an esker and an ice-contact, poorly-sorted, stratified deposit emplaced by westerly flowing meltwater, imply an eastward retreating ice margin. Geophysical data from a flat-topped component of YLER reveal slipface and planar-bedded sand and gravel overlying lacustrine sediments, characteristic of a Hjulstrom delta. Eastward-dipping foresets in a Gilbert delta exist at the eastern terminus. Contextually our observations suggest, despite esker-like morphology, YLER was not deposited within a subglacial ice tunnel. Instead, it formed through deposition of subaerial outwash between and/or on dead ice in front of a regionally backwasting ice margin. The complex deglacial evolution of YLB, including a drainage reversal and formation of two glacial lakes, supports northwestward backwasting of the CIS and dead ice within YLB. We conclude that accurate geomorphic inversion of meltwater landforms for deglacial paleo-ice sheet reconstruction requires knowledge of landform-scale morpho-sedimentary relationships and basin-scale geomorphic and stratigraphic context.