Late Wisconsinan grounding zones of the Laurentide Ice Sheet margin off the Québec North Shore (NW Gulf of St Lawrence)

Abstract

Swath bathymetry data and seismic profiles collected in the NW Gulf of St Lawrence reveal a series of wedge-shaped depositional systems interpreted as grounding zone wedges (GZWs). Some segments of the GZWs change locally to form frontal moraines, or morainal banks, and subaqueous ice-contact fans, reflecting changes in either the nature of the ice margin or the rate of sediment input. These grounding zones (GZ) of the ice margin extend laterally along three isobaths at depths of 180 (GZ1), 120 (GZ2) and 80 (GZ3) m (±20 m) along the Québec North Shore shelf, the 120 m-deep GZ2 system being traceable over a distance of >300 km. Associated GZWs can occur in three geometries along a same isobath system: curvilinear, lobate and shelf-break. GZ systems were built during three distinct stages of stabilization of the marine-based southeastern margin of the Laurentide Ice Sheet following its rapid retreat across the deeper waters of the Laurentian Channel in the Gulf of St Lawrence after 14.8 cal ka BP. The occurrence of GZ along distinct isobaths indicates that bathymetry exerted a strong control on ice stabilization during deglaciation by reducing the relative water depth at the ice margin and thereby the buoyancy and rate of iceberg calving. However, fluctuations and readvances of the ice margin are also recorded by the overprinting of a portion of the GZ2 system by the younger GZ3 system, potentially suggesting an additional response to climate-driven forcing.

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 time span encompassed within each glacial erosion surfaces is, however, not constrained but as ice‐margin retreat phases and the deposition of associated sedimentary suites were rapid, correlative advances were arguably of the same frequency. In Quaternary series, GS stacked in a sedimentary succession ascribed to a deglacial trend and recording glacial stillstand or re‐advance represent short intervals of time (lower‐rank glacial retreat surface sensu Zecchin et al , , see also Occhietti, ; Lajeunesse et al , ; Dietrich et al , ). Hence, the duration of the deposition of the whole Dwyka Group in this area may be viewed as having taken place in a time range similar to Quaternary deglacial sequences, as already proposed by Haldorsen et al (), and being thus comparable to other ancient deglacial sequences (Girard et al , ; Dietrich et al , ).…”

“…The association or juxtaposition of recessional curve‐crested GZWs may strengthen such an interpretation. Bathymetric and seismic surveys are particularly well‐suited for the recognition of the geomorphological indicators (Dowdeswell et al , ). Architectural indicators, which can be unravelled by both seismic and outcrop studies, comprise the identification of the stacking pattern and the stratigraphic architecture of the GZW pointing to the active progradation of the GZWs (internal architecture showing seaward‐dipping clinoforms topped by horizontal sheets, truncation of horizons by erosion surfaces, landward‐dipping backsets; Dowdeswell and Fugelli, ; Lajeunesse et al , ). Sedimentological indicators are best emphasized by outcrop investigations and consist, on the one hand, of bedding and lamination patterns that show a predominance of thick, massive to crudely stratified strata, interstratified with cross‐stratified conglomeratic sandstones in which the presence of contorted beds and glaciotectonically deformed horizons is common (Demet et al , ). On the other hand, a wide range of grain sizes is supposed to characterize the largely unsorted material making up the bulk of GZWs (diamictite) while the abundance of exotic, outsized, faceted and/or striated clasts should be regarded as characteristic. Finally, biological indicators can aid in the interpretation of GZW as specific diatom and foraminifera assemblages have been found in the diamictic material forming GZWs (Prothro et al , ).…”

“…Bathymetric and seismic surveys are particularly well-suited for the recognition of the geomorphological indicators (Dowdeswell et al, 2016a). • Architectural indicators, which can be unravelled by both seismic and outcrop studies, comprise the identification of the stacking pattern and the stratigraphic architecture of the GZW pointing to the active progradation of the GZWs (internal architecture showing seaward-dipping clinoforms topped by horizontal sheets, truncation of horizons by erosion surfaces, landward-dipping backsets; Dowdeswell and Fugelli, 2012;Lajeunesse et al, 2018). • Sedimentological indicators are best emphasized by outcrop investigations and consist, on the one hand, of bedding and lamination patterns that show a predominance of thick, massive to crudely stratified strata, interstratified with cross-stratified conglomeratic sandstones in which the presence of contorted beds and glaciotectonically deformed horizons is common (Demet et al, 2019).…”

“…Collectively, the association of these deposits ( Figures 7 and 8) capped by a composite GS (Figure 8) enables this succession to be interpreted as a GZW (see also Visser, 1997). The GZW correspond to asymmetrical, wedge-shaped depocenters emplaced at the submarine grounding zone of fast-flowing or streaming ice by the continuous delivery and resedimentation through a variety of sedimentary processes of subglacial material (till) from up-glacier (von Brunn and Talbot, 1986;Powell and Domack, 2002;Dowdeswell and Fugelli, 2012;Bell et al, 2016;Dowdeswell et al, 2016aDowdeswell et al, , 2016bLajeunesse, ;Bart et al, 2017;Prothro et al, 2018;Lajeunesse et al, 2018;Batchelor et al, 2018; see also Demet et al, 2019 for some facies). Although generally interpreted as being deposited at the grounding line of a floating ice shelf (Batchelor and Dowdeswell, 2015), GZWs may also form at tidewater termini, depending on the availability of meltwater and/or deformability of subglacial till (Powell and Alley, 1997).…”

An introduction to glaciated margins: the sedimentary and geophysical archive

“…Major new insights into the glaciated margin architecture along the northern shore of the Gulf of St Lawrence are made in two closely related papers by Lajeunesse et al (2018) and Dietrich et al (2018) by combining detailed outcrop observations with shallow marine bathymetry data. In the only paper that explicitly links systems of completely different ages (Quaternary and Ordovician, c. 440 myr apart), Dietrich et al (2018) compare and contrast the architecture of deglacial sedimentary systems using traditional outcrop logging and sedimentary facies analysis.…”