Evidence for catastrophic subglacial meltwater sheetflood events on the Bruce Peninsula, Ontario

Abstract

The Bruce Peninsula, a carbonate bedrock escarpment, lies "downflow" from a sculpted bedrock terrain at the French River. The sculpted forms are attributed to a hypothesis of erosion by regional-scale, subglacial meltwater flooding. This paper presents new data from the Bruce Peninsula that tests the meltwater outburst hypothesis in a downflow direction of the predicted flood path. The bedrock surface of the Bruce Peninsula shows extensive development of sculpted features that bear a striking resemblance to s-forms at the mouth of the French River. They are self-similar and hierarchical in scale, ranging in dimensions from a few centimetres to several kilometres. Remarkable concentrations of potholes are located near the brow of the escarpment. The Bruce Peninsula lacks a pervasive cover of unconsolidated sediment. What little sediment exists has been modified into long, narrow drumlins. The Niagara Escarpment on the peninsula has been back wasted into the edge of the Paleozoic Michigan Basin. Along its east-facing slope, the escarpment is marked by more overdeepened reentrant valleys and intervening promontories than is normal for the rest of the escarpment. Clusters of rounded, percussion-marked boulders of exotic origin are concentrated at the heads of the reentrant valleys. Taken together, these features are inferred to support the hypothesis that subglacial outburst floods beneath the Laurentide ice sheet crossed Georgian Bay and strongly sculpted the Bruce Peninsula. The consistent orientation of the reentrant valleys, aligned with the French River sculpting across the basin to the northeast, and the backwasting of its caprock attest to the power and directional stability of the sheetfloods.

Stratigraphic Architecture and Sediment Facies of the Western Oak Ridges Moraine, Humber River Watershed, Southern Ontario*

“…This unconformity has been traced beneath the ORM on seismic data (Pugin et al, 1999) and in continuous core (Russell et al, 2003a). A variety of erosional forms along the unconformity suggest that during the Late Wisconsinan a subglacial outburst flood swept southward from the Canadian Shield and extensively eroded the terrain of this part of southern Ontario , the Niagara Escarpment (Kor and Cowell, 1998), and probably beyond (Shaw and Gilbert, 1990;Munro-Stasiuk et al, 2005). Drumlins were eroded by this regional sheet flood (Fig.…”

The pre-Late Wisconsin stratigraphy of southern Simcoe County, Ontario: implications for ice sheet buildup, decay, and Great Lakes drainage evolution

“…The escarpment formed as a result of differential erosion of resistive Silurian dolostones capping the cuesta versus softer, underlying Ordovician shale and limestone to the east (Armstrong and Carter 2010). The position and general form of the escarpment predate Quaternary glaciations, although its surface and the form of prominent reentrant valleys cut into the face of the escarpment have been significantly altered by glacial and glaciofluvial activity during the Wisconsin Episode (Straw 1968;Kor and Cowell 1998;Gao 2011;Eyles 2012;Mulligan 2015). Unlike the Niagara Escarpment, the geometry and form of the Laurentian Valley is far less well understood.…”

The Devil Lake pothole (Ontario): Evidence of subglacial fluvial processes

“…In the Great Lakes region these features have been documented by, for example, Shaw (1988), Sharpe and Shaw (1989), Gilbert (1990), Shaw and Gilbert (1990), Kor et al (1991), Gilbert andShaw (1992, 1994), Brennand and Shaw (1994), Pair (1997), Barnett et al (1998), and Kor and Cowell (1998). This paper describes a large pothole on a ridge crest in southeastern Ontario in the context of the regional topography, and proposes its origin by subglacial fluvial processes.…”