Erosion of bedrock by subglacial meltwater, Georgian Bay, Ontario: a regional view

Abstract

A spectacular series of sculpted erosional forms (s-forms) is mapped and described from a 70 km wide area along the shore of Georgian Bay, Ontario, which, except for a scattered boulder lag, has been swept clean of sediment. A great variety of sculpted forms is described and illustrated and grouped into three classes: transverse, longitudinal, and nondirectional forms. Transverse forms comprise transverse troughs, muschelbrüche, sichelwannen, and comma forms; longitudinal forms comprise spindle flutes, cavettos, and furrows; and nondirectional forms consist of undulating surfaces and potholes. Transverse forms are preferentially located on stoss slopes, and longitudinal forms on lee slopes of rock rises. Undulating, nondirectional forms are found on distal slopes, and potholes at major breaks in slope. This correlation of form and bed topography suggests that relief exerts considerable control on both form and location. Form geometry is also inferred to be related to coherent flow structures and their interaction with the bed. Flow scale, vorticity, separation, bifurcation, strength, and direction are inferred from erosional-mark properties. In some cases, erosional forms appear to have caused the flow structure by which they were perpetuated. Sculpted forms occur at different scales, and the inferred flow structures are thought to have operated over the same scale range.Attributes of the forms, boulder lags, and inferred flow structures clearly reflect erosion by powerful, turbulent, subglacial meltwater flows. The erosional forms are observed over an area 70 km wide, which, taken together with a strongly uniform paleoflow direction, indicates regional-scale flow. The Georgian Bay floods were comparable in discharge (~107 m3/s) with floods from glacial Lake Missoula, Livingstone Lake drumlins, and Sable Island tunnel valleys. The most likely site for the storage of meltwater that drained catastrophically to form the erosional-mark field was the lowland stretching north from the Abitibi Highlands to Hudson Bay.

The instability theory of drumlin formation applied to Newtonian viscous ice of finite depth

“…The latter idea may be more readily consistent with some observations of stratified drumlin interiors (for example, in the Puget Sound), although others may be constructional, as in northern Saskatchewan (Shaw 1996); the former may be consistent with drumlins formed largely of till, though some of these may also be erosional (e.g. Kor et al 1991;Sharpe et al 2004).…”

Subglacial deforming bed conditions recorded by late Quaternary sediments exposed in Vineland Quarry, Ontario, Canada

“…South-facing (down-ice) bedrock forms are sharply truncated along joint surfaces and show similar characteristics to the 'plucked' ends of roches moutonnées (Sugden et al 1992). Comparable erosional features have been reported on Paleozoic bedrock exposures elsewhere (Kor et al, 1991;Tinkler 1993;Rea et al 2000).…”

Lobal affinity of Late Wisconsin tills at St. Marys in southwestern Ontario, Canada

“…The large sizes of the boulders in transport, combined with the size of the bedform, indicate that a large discharge of water must have transported and deposited them. Combined with the meltwater erosion marks on the bedrock surface (Kor et al 1991) beneath the gravels and on the large boulders within the gravel, it indicates turbulent meltwater flow was involved and supports a subglacial origin for this unit. Unit 2 may represent deposition within an ice-walled subglacial conduit, an esker, or possibly part of a broader subglacial meltwater flow, although the observed areal extent of Unit 2 is fairly restricted and would tend to support a conduit setting or esker for deposition.…”