A critical assessment of subglacial mega-floods: A case study of glacial sediments and landforms in south-central Alberta, Canada

Abstract

The existing subglacial megaflood explanation of glacial sediment deposition and landform production in south-central Alberta relies upon four sets of observations and associated interpretations: (1) a subglacial genesis for all glacilacustrine sediments lying stratigraphically below a local melt-out till or “ice stagnation facies”; (2) a melt-out genesis for the “ice stagnation facies”; (3) a syn-depositional status for the glacitectonic disturbance visible in most exposures due to periodic ice–bed re-coupling; and (4) a subglacial megaflood origin for a “truncation surface” and clast lag that occurs at the top of the sedimentary exposures and the carving of hummocky terrain and flutes in the area. We provide alternative interpretations of the sediment-landform assemblages of the area and demonstrate that alternative, simpler interpretations of the evidence can be proposed. Specifically, we propose: (1) a proglacial rather than subglacial origin of glacilacustrine sediments; (2) a glacilacustrine origin for the “melt-out till” and its subsequent glacitectonic disturbance by minor glacier readvances; (3) postdepositional rather than syn-depositional glacitectonic disturbance due to a significant advance by glacier ice after the accumulation of a thick sequence of glacilacustrine sediments; (4) that some linear chains of hummocks are the morphological expression of glacitectonic folding and thrusting; (5) that the use of the truncation surfaces to propose a subglacial fluvial origin for hummocky moraine is flawed logic, because they do not constitute the tops of hummocks but rather occur on the interfluves that separate the numerous small coulees incised into the valley sides during postglacial time. Truncation surfaces and clast lags occur locally on the beds of spillway tracts and can be explained as the products of erosion by subaerial spillway water. The mega-flood explanation of flutings and hummocky moraine in the region is also questioned because these landforms record a regional ice flow imprint of mis-aligned and cross-cutting flow sets terminating at series of inset and locally cross-cutting and lobate transverse ridges (moraines). These landform associations preclude a subglacial flood origin for the flutings and demonstrate that the transverse ridges are more appropriately interpreted as recessional push moraines. The similarity of the glacial landforms of this region to the active, temperate glacial landsystem produced by historical glacier recession in modern glacierized catchments, where sheetfloods have not been influential in subglacial landform production, allows us to apply a modern analogue to landscape interpretation that does not include subglacial sheetflooding.