LAURENTIDE ESKER TYPES: THEIR GEOMORPHIC ELEMENTS AND SEDIMENTARY ASSEMBLAGES

A review of subglacial-subaqueous Laurentide eskers reveals that they are composed of at least four geomorphic elements, each containing different sedimentary assemblages. Ridges (cores) occur in a range of sizes, at the landsurface or buried, with single sinuous or anabranched planforms. They are mainly composed of coarse gravel facies recording powerful fluidal and hyperconcentrated flows, alternating with thinner or discontinuous sand units. They have low paleoflow variability, and downflow trends in clast characteristics. Lithofacies are arranged into macroforms (ridge-scale bedforms) recording powerful flows down non-uniform conduits, macroform style controlled by conduit geometry. Composite macroforms exhibit modest downflow and lateral fining. Terminal subaqueous fans are dominated by massive, diffusely graded, plane bedded, cross-bedded, cross laminated and draped sand, silt and clay with flame and water escape structures and high paleoflow variability; gravel facies are mainly limited to basal and proximal locations. They exhibit highly variable downflow and vertical sedimentary structures and textures; they may exhibit weak distal and/or vertical fining. Lateral beads may be connected to some esker ridges by short distributary ridges. They exhibit many of the sedimentary characteristics of subaqueous fans, yet with more frequent and extensive folding and faulting. Hummocky extended zones “straddle” some esker ridges. They exhibit intensely folded and faulted gravel and sand lithofacies. Sand lithofacies thicken away from the esker core.

Three esker types are identified on the basis of drainage pattern. Long dendritic eskers are dominated by ridges, terminate in subaqueous fans, and may exhibit lateral beads and hummocky extended zones. Short subparallel eskers are dominated by ridges and subaqueous fans that may alternate down the esker length. Short deranged eskers contain two subtypes: (1) longer eskers connect lake basins and are dominated by ridges and terminate in subaqueous fans, and (2) shorter eskers are dominated by intensely faulted ridge gravel. Variability in the frequency and arrangement of geomorphic elements and their associated lithofacies between esker types suggests that each may have different hydrogeologic implications.