Buried preglacial fluvial gorges and valleys preserved through Quaternary glaciations beneath the eastern Laurentide Ice Sheet

Abstract

The geomorphology, geometry, and sedimentary infi ll of buried gorges and V-shaped valleys observed at the base of major river valleys in the formerly glaciated southeastern Canadian Shield region have been revealed from excavation and drilling data acquired during the construction of hydroelectric dams and seismic data collected on lakes and offshore. Compilation of these previously published and unpublished data provided an exceptional opportunity to examine the morphology and spatial distribution of buried bedrock gorges and the mechanical processes responsible for their erosion. In some valleys, detailed observations of deep gorges have been allowed by their exhumation over large areas. Archive photographs show deep and large potholes, natural pillars, furrows, fl utes, and scallops on the wellpolished bedrock walls of the gorges. They also reveal that gorges and valleys have a sharp-ending V shape and very narrow base and are superimposed by a U-shaped valley, forming a buried valley-within-valley topography. The narrow and deep cross-profi le of these gorges, their well-polished slopes, and the type of bedforms observed within them are typical features of fl uvial bedrock channels. Drilling operations at many sites have also provided data on bedrock topography of these gorges within valleys and on the nature of their sedimentary infi ll. The different lines of evidence presented in this paper indicate that gorges and V-shaped valleys of the region were not eroded by the Laurentide Ice Sheet during Quaternary glaciations but are relics of a preserved preglacial fl uvial system eroded during a lower base level. This paleofl uvial system is interpreted to be linked to fl uvially cut channels observed on the seafl oor of the Estuary and Gulf of St. Lawrence.

From outwash to coastal systems in the Portneuf–Forestville deltaic complex (Québec North Shore): Anatomy of a forced regressive deglacial sequence

“…B). Bedrock thresholds suggest that the structural valley depths may not be significantly greater than the observed flank heights with the possible exception of inner gorges (Lajeunesse, ). Marine sediments preserved in terraces along the lower reaches of the valleys lying below 140 m.a.s.l.…”

Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

“…The presence of vertical walls undoubtedly indicates that the structural heritage had a preponderant influence in the shaping of the basement. Moreover, a pre‐existing network of fluvial valleys was potentially reworked and amplified by direct glacial abrasion (von Brunn, , ), as is the case for some Quaternary high‐latitude valleys that originated from the combination of both pre‐glacial fluvial and glacial erosion processes (Lajeunesse, ; Livingstone et al , ). Indeed, the major hiatus that separates the basement from the Dwyka Group suggests that the study area constituted an emerged domain during pre‐Dwyka times as a result of crustal uplift preceding the inception of the Karoo Basin and the onset of glaciation (Visser, ; Tankard et al , ) and onto which a fluvial system may well have developed.…”

The pattern and style of deglaciation at the Late Wisconsinan Laurentide and Cordilleran ice sheet limits in northeastern British Columbia

“…Similar to much of North America and Eurasia, Late Pleistocene ice sheets advanced over permafrost-rich deposits and deranged preglacial fluvial landscapes in northern Canada (Dyke and Prest 1987;Johnson 1990;Ó Cofaigh et al 2005;Hickin et al 2008;Lajeunesse 2014). The relatively subdued topography of northeastern British Columbia does not reflect the irregular, buried bedrock surfaces.…”

“…8A) are indicative of initial advance of ice margins over sand and clay-rich bed materials containing permafrost (cf. Dyke and Prest 1987;Johnson 1990;Kleman and Borgström 1994;Bennett et al 1998;Boulton et al 1999;Ó Cofaigh et al 2005;Bednarski 2008;Evans et al 2008;Hartman and Clague 2008;Hickin et al 2008;Trommelen and Levson 2008;Benediktsson et al 2009;Lajeunesse 2014;Hickin et al 2016).…”