The Harricana glaciofluvial complex, Abitibi region, Quebec: its genesis and implications for meltwater regime and ice-sheet dynamics

Abstract

Interlobate moraines have been defined by their relationshio to adjacent landforms and sediments, with minimal reference to any genesis suggested from their own geomorphology and sedimentology. A case in point is the “Harricana interlobate moraine”. The Harricana complex, a relatively continuous, linear accumulation of glaciofluvial sediments, is investigated in terms of its geomorphology, sedimentology, stratigraphic context, and landform associations for the portion between latitudes 48°N and 50°N. The complex is narrower to the north than to the south. In the narrower northern part, better rounding and poorer preservation of less resistant clasts suggest more vigorous transport in a narrower conduit; the inverse in the wider southern part favours less vigorous flows and higher deposition rates. These inferences, together with unidirectional paleoflows towards the ice margin and the relatively continuous upslope path of the southward-widening complex, favour synchronous formation of the Harricana complex in a continuous closed conduit, beneath an ice sheet which thinned southward. Subaqueous fans and grounding-line deposits with fine gravel and sandy in-phase wave structures in the south, may have been deposited in subglacial cavities adjacent to the complex, or later deposited in reentrants in calving ice fronts. The basic building blocks of the Harricana complex are gravel facies which indicate generally high energy, but unsteady, transport. These facies are arranged into macroforms. Composite and oblique-accretion, avalanche-bed (OAAB) macroforms are attributed to deposition by unsteady and non-uniform flows in conduit enlargements. Pseudoanticlinal macroforms were deposited in relatively narrow conduit segments of uniform width. Gravel-sand couplets register flow unsteadiness, perhaps related to variations in supraglacial metlwater supply. The origin of the Harricana complex as a mainly synchronous subglacial landform is linked to formation of adjacent streamlined bedforms and bedrock erosional marks by meltwater outburst floods. The complex is located where the orientation of these subglacial bedforms indicate strong flow convergence. Post-flood, ice sheet collapse along this convergence zone initiated redirection of ice flow, resulting in cross-cutting striae. Later subglacial meltwater systems followed new hydraulic gradients toward this trough of thinner ice, forming a major conduit and depositing the Harricana glaciofluvial complex.