In defence of the meltwater (megaflood) hypothesis for the formation of subglacial bedform fields

Abstract

Shaw, J. 2010. In defence of the meltwater (megaflood) hypothesis for the formation of subglacial bedform fields.ABSTRACT: Characteristics of large-scale fluting and hummocky terrain on the Canadian Prairies test glacial and meltwater hypotheses for landform genesis. These tests defend the meltwater model. Neither sedimentary nor glaciotectonic processes can fully explain such erosional landforms. Province-scale flow paths, which mark palaeo-ice streams and subglacial flood routes, contain large-scale fluting with flanking hummock terrain. Antecedent relief causes these paths to differ from other flood landscapes such as the Scablands. Proponents of the glacial hypothesis use an invalid analogy between Icelandic and Prairie landsystems. They suggest that groove-ploughing formed largescale fluting, and that ice pushing created hummocky terrain. However, landform location, form, and extent, surface lags, truncated architecture, and landform associations favour the meltwater hypothesis. A simple thought experiment and clear understanding of the principle of least number of assumptions answer the criticisms that meltwater forms cannot cross-cut and that the meltwater hypothesis disregards proper hypothesis testing. An example of cross-cutting erosional marks supports this theory. No narrow tract of smoothed terrain with fluting terminates at the glacially thrust Neutral Hills, negating an important point in the glacial hypothesis. While neither the glacial hypothesis nor postglacial winnowing explain boulder and cobble lags with percussion marks, meltwater processes explain them well.

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

“…However, as noted, other processes can, of course, cause erosion or aggradation (e.g., Alley et al, ). Potential erosion by water in subglacial channels is neglected and could be substantial; observations at Múlajökull rule out fluvial erosion only on the grand scale envisioned by Shaw (; Johnson et al, ).…”

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…The physical continuity between the basal glacial erosion surface marking the km-scale depression (GES5) on the one hand, and its subaerial counterpart to the north corresponding to the basal erosion surface of deeply-incised channels on the other hand, indicate contemporaneous development and connexion of the subglacial depression with fluvioglacial channels and the corresponding proglacial deltaic system lying downslope (see Girard et al, 2012aGirard et al, , 2012b. In this interpretation the network of deeply incised channels forms a system of glacial spillways that might be compared to the 'channelled scablands' draining ice fronts of the Laurentian (Bretz, 1969;Shaw et al, 1999;Baker, 2009;Shaw, 2009), Icelandic (Waitt, 2002) and Siberian (Rudoy, 2002) ice sheets in the late Quaternary, although the hard-rock substrate of the latter admittedly differs from the end-Ordovician, possibly frozen, sandy substrate.…”

A comparative case study of subglacial bedforms in northern Lithuania and south-eastern Iceland

“…These were summarised into five classes by J. Menzies in 1984, but there are currently two key competing hypotheses (Benn, Evans 1998;Clark et al 2009;Shaw 2010). Boulton's (1987) glacial hypothesis (direct erosion and/or deposition by ice or by a subglacial deforming bed create glacial bedforms) and Shaw's (2002) meltwater (megaflood) hypothesis (erosion and/or deposition by subglacial sheet floods create meltwater bedforms).…”