ABSTRACT: Subglacial landsystems in the southern Rocky Mountain Trench (British Columbia, Canada): Toward understanding subglacial processes beneath the Cordilleran Ice Sheet.

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Abstract

Few studies have directly addressed the processes responsible for forming subglacial landsystems associated with the last (Fraser) glaciation of the Cordilleran Ice Sheet (CIS), yet an understanding of these processes is fundamental to inferring past ice sheet dynamics and thermal regime, and is critical if geologic data is to be used to constrain or test numerical ice sheet models. It has been suggested that the CIS may have been drained by ice streams based on the identification of deformation till, overdeepened basins, whalebacks and rock drumlins. Drumlins are common within the footprint of the Cordilleran Ice Sheet and, based on existing hypotheses, may record persistent ice streams that rode on a deforming bed, or landscape unconformities eroded by enormous meltwater floods (underbursts). Here we test the validity of these hypotheses of subglacial landsystem formation against observations from the southern Rocky Mountain Trench, British Columbia, Canada. The southern Rocky Mountain Trench is a major NW-SE trending topographic depression, up to 20 km wide and floored by drumlins. Drumlins occur in an en echelon, space-filling arrangement along a ~500 km-long tract terminating around Flathead Lake, toward the southern terminus of the Flathead Lobe. Individual drumlins are 0.5-2 km long, 0.1-1 km wide and ~30 m high. They range in morphology from classical inverted teaspoons and spindles with crescentic scours around their upflow noses, to downflow widening forms. Larger drumlins often appear fluted. We mainly report on drumlin sedimentology from two transects (each ~ 3 km in length) transverse to drumlin long axes. Drumlins are mainly composed of matrix-supported silty-sand diamicton containing up to 40% clasts, or bedrock. Diamictons are either massive, or stratified and interbedded with sand and gravel. Clast fabrics taken at vertical and lateral intervals within diamictic drumlins are highly variable and mainly exhibit spread bimodal to multimodal distributions. While some fabrics show alignment with drumlin long axes, most fabrics show no clear relationship to local drumlin form. Diamicton beds dipping toward the Trench axis, clast fabrics and clast morphological characteristics (orientations of striae and plucked ends on clasts, disposition of keels) suggest that most diamictons result from gravity flow and deformed lodgement processes. In places, diamicton beds are interstratified with sand and gravel and appear to be truncated by the drumlin form. This, in combination with juxtaposed bedrock drumlins, suggests an erosional origin for drumlins in the Trench. Given drumlin morphology and the presence of tunnel channels toward the terminus of the Flathead Lobe, we suggest that the southern Rocky Mountain Trench drumlin tract may record a landscape unconformity eroded by a meltwater underburst.

Bibliography of Canadian Geomorphology