Morphological analysis and evolution of buried tunnel valleys in northeast Alberta, Canada

Tunnel valleys are large elongated depressions eroded into unconsolidated sediments and bedrock. Tunnel valleys are believed to have been efficient drainage pathways for large volumes of subglacial meltwater, and reflect the interplay between groundwater flow and variations in the hydraulic conductivity of the substrate, and basal meltwater production and associated water pressure variations at the ice-bed interface. Tunnel valleys are therefore an important component of the subglacial hydrological system. Three-dimensional modelling of geophysical and lithological data has revealed numerous buried valleys eroded into the bedrock unconformity in northeast Alberta, many of which are interpreted to be tunnel valleys. Due to the very high data density used in this modelling, the morphology, orientation and internal architecture of several of these tunnel valleys have been determined. The northeast Alberta buried tunnel valleys are similar to the open tunnel valleys described along the former margins of the southern Laurentide Ice Sheet. They have high depth to width ratios, with undulating, low gradient longitudinal profiles. Many valleys start and end abruptly, and occur as solitary, straight to slightly sinuous incisions, or form widespread anastomosing networks. Typically, these valleys are between 0.5 and 3 km wide and 10 and 30 m deep, although the depth of incision along some thalwegs exceeds 100 m. Several valleys extend for up to 60 km, but most are between 10 and 30 km long. Valley fills comprise a range of lithofacies, including stacked sequences of diamict, glaciofluvial sands and gravels and glaciolacustrine silts and clays. Displaced bedrock, presumably of glaciotectonic origin, also occurs within several anastomosing valleys. Several channel bodies are exposed along a number of valley sections suggesting progressive valley development through repeated cycles of sediment discharge. Cut-and-fill structures that are capped by fine-grained sequences of rippled sand and mud-rich drapes within these channel bodies suggest unstable flow regimes within the valley and the discharge of sediment-laden basal meltwater under flood-like conditions followed by wane flow events or periods of lower meltwater discharge, likely concomitant with localized modification by glacial ice. Basal meltwater is inferred to have been released as episodic jökulhlaups beneath the western Laurentide Ice Sheet, which at times re-used existing valley systems, which were spatially and temporally stable features, and at other times incised new valleys.