Jacques Lanoie, Martin Fournier and Simon Laliberté are acknowledged for having explored during their master research a region poorly documented. They have provided the bases of the complex glacial history of western Charlevoix. Several students from UQAM, Université Laval and Université de Caen helped the authors during fieldwork and wrote short reports. The project was partly supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). Thanks are extended to the Centre d’Études Nordiques, Université Laval, for the logistical support. The authors are grateful to Venetia Bodycomb who carefully translated a large part of the manuscript and helped us clarify the text. The authors thank Bernard Hétu, William Shilts, Laurent Deschodt and Damase Mouralis for their thoughtful reviews that greatly improved the manuscript.

Introduction

1The lowlands of the brackish middle estuary (fig. 1) and the fluvial upper estuary (fig. 2) of the St. Lawrence River represent the threshold of the main drainage axis of the Great Lakes intracontinental watershed to the Atlantic Ocean during the Pleistocene. The lowlands are also the threshold for the invasion of epicontinental seas during the Late Wisconsinan (Champlain and Goldthwait seas), Lower Wisconsinan (Occhietti et al., 1996) and Sangamonian Interglacial (Occhietti et al., 1995). During the transitional englaciation and deglaciation phases of the Laurentide Ice Sheet (LIS), glacial lakes were dammed on the southwestern side of the central St. Lawrence Valley and in peripheral basins, while ice masses in the structural corridor control the extension of ocean waters downstream to the northeast. The threshold and adjacent landforms are located on the three geological domains of Québec (the Canadian Shield, St. Lawrence Sedimentary Platform and Appalachians) that form a territory whose topographic contrasts were responsible for complex and regionalized ice dynamics.

Serge Occhietti

2The deglaciation stages during the Late Glacial, particularly during the Allerød, have been well-documented on the southern shore of the middle Estuary (fig. 2), in the Appalachian piedmont (Martineau, 1977; LaSalle et al., 1977; Rappol, 1993; Veillette et al., 2017), in the Chaudière River region (Beauce) (Shilts, 1981; Blais, 1989; Occhietti et al., 2001a; Shilts et al., 2007; Normandeau, 2010), and in the Appalachian Uplands of southern Québec (McDonald,1969; McDonald & Shilts. 1971; Shilts, 1981; Prichonnet et al., 1982 ; Parent, 1987; Parent & Occhietti, 1988, 1999; LaRocque et al., 2003; Turgeon et al., 2003). In contrast, the deglaciation stages are not as well known in the St. Lawrence Lowlands and are restricted to observations around Québec City (LaSalle, 1978, 1984; Lamarche, 2011). On the north shore of the Estuary, the Saint-Narcisse and Mars-Batiscan moraines associated with the Younger Dryas (YD) have been the subject of numerous studies (e.g., Govare, 1994; Bolduc, 1995; Simard et al., 2003; Occhietti, 2007), whereas only a few examine the earlier stages of deglaciation (Rondot, 1974, 1989; Govare, 1994; Occhietti, 2007; Brouard et al., 2016).

Serge Occhietti

3The influence of the St. Lawrence Ice Stream (StLIS) is evident on both shores and on the islands of the estuary (Dionne, 1972; Rappol, 1993; Govare, 1994; Parent & Occhietti, 1999; Occhietti et al., 2001a; Veillette et al., 2017; Brouard et al., 2016; Légaré-Couture & Parent, 2017; Légaré-Couture, 2021). This ice stream represents the late intracontinental extension of the Laurentian Channel Ice Stream in the Gulf and the maritime lower Estuary (Margold et al., 2015).

4Several models have been proposed for the diachronic reconstruction of the glacial flow and retreat phases and the invasion of Atlantic waters into the middle Estuary (LaSalle et al. 1977; Thomas 1977; Parent et al. 1985; LaSalle & Chapdelaine 1990; Parent & Occhietti 1988, 1999; Bhiry et al., 2001; Occhietti et al., 2001a). The lack of precise chronology and the fragmentary nature of available data limit the contribution of these inter-regional syntheses. Québec-wide (Occhietti et al., 2011) and North American (Dyke et al., 2003; Dyke, 2004; Dalton et al., 2020) syntheses have long timeframes (typically 500 14C y or cal y) and cannot incorporate significant secular duration events.

5One event with a widespread impact serves as a chronological landmark in southern Québec: the seaward outflow of Glacial Lake Candona and the invasion of the Champlain Sea into the central St. Lawrence River Valley and the peripheral lowlands. The precise age of this event, referred to as the Lake Candona outflow/Champlain Sea invasion in this text, is in the range of 13-12.9 ± 0.1 ka (based on published terrestrial dating data) (Ridge et al., 2001; Occhietti & Richard, 2003; Richard & Occhietti, 2005; Rayburn et al., 2007; Occhietti, 2007; Cronin et al., 2008, 2012). Lake Candona (Parent & Occhietti, 1988; or Lake St. Lawrence, Pair & Rodrigues, 1993; or Lake Vermont, Rayburn et al., 2011) lasted over 100 years (Parent & Occhietti, 1999). It represents the confluence of the Lake Iroquois-Frontenac phase in the Lake Ontario basin with the Lake Vermont-Fort Ann phase in the Lake Champlain basin and the progressive inundation of glaciolacustrine waters into the St. Lawrence and Ottawa River lowlands and the valleys of the Appalachian foothills (Parent & Occhietti, 1999; Occhietti et al., 2001a) (fig. 2). We retain the name Lake Candona because it draws attention to the spatial evolution of this major lake, its paleontological characterization and its international extension. The opening of the St. Lawrence corridor to the Atlantic waters is diachronic. It started in the lower Estuary on the north shore of the Gaspé Peninsula (Richard et al., 1997; Hétu, 1998; Hétu & Gray, 2000; Veillette et al., 2017; see fig. 9 in Dalton et al., 2020), and then on the southern side of the middle and upper Estuary. In the central St. Lawrence Valley, the marine invasion preceded the accumulation of the Gabelle Glaciomarine Deposits to the north of the river (Occhietti, 2007), 70 km north of the Lake Candona outflow threshold along the Appalachian piedmont (fig 2). These deposits mark the beginning of the early YD episode that formed the Saint-Narcisse moraine complex (herein the Saint-Narcisse Moraine), which is used as a benchmark in this study.

6Several questions remain unanswered concerning deglaciation in the natural setting of the St. Lawrence Seaway during the Allerød. How can we strengthen the chronological framework and identify correlations in this area? Are the moraines a response to climatic fluctuations, or do they originate from variations in the properties and dynamics of the ice sheet margin? What were the patterns of opening to marine waters in the middle Estuary before the Lake Candona waters flowed into the Atlantic? What can we learn about the origin, duration, chronology and impact of the StLIS? These questions concern events in a key sector of the southeast margin of the LIS and their impact on global phenomena such as the influx of glacial meltwater into the North Atlantic Ocean.

7A small structural basin in the Laurentians and its extension into the lateral uplands on the north shore of the St. Lawrence middle estuary, referred to as the Beaupré Basin in the text (fig. 1), offers an exceptional amount of data on glacial retreat and the StLIS. This basin includes the lower reaches of the Sainte-Anne-du-Nord River and its tributaries, the downstream part of the Côte-de-Beaupré region, and the first highlands of the Charlevoix region. The objective of this paper is to provide preliminary answers to each of these questions and to readjust several elements of the paradigm applied to southern Québec based on a detailed study of this region and the surrounding areas on the north side of the estuary.

8The first chapters focus on the main indicators of Late Glacial events: striations and lineations, paleochannels and meltwater flows, moraines, lake deltas and outwashes, marine boundaries and shell dating, and chronological data. These data form the foundation of a new deglaciation paradigm for the region north of the estuary, with significant ice flows, a group of moraines attributed to the Intra-Allerød Cold Period (IACP) climatic oscillation, the long and late impact of the StLIS, and a major reassessment of the chronology.

1.1 - Study area

9The study area is located between 47° and 47°30’ north and 70°30’ and 71° west (fig. 1). It is bordered to the southeast by the middle estuary of the St. Lawrence Valley, to the east by the Gouffre River valley, to the west and northwest by the Côte de Beaupré region and the highlands of the Montmorency massif, and to the north by the highlands of the Lac des Neiges massif (at elevations over 1000 m). This is the transition area between the Québec City region and the Charlevoix astrobleme. The term ‘western Charlevoix’ is used to denote the territory that includes the Beaupré Basin and the Laurentian highlands to the north. The interpretation of digital elevation models (DEMs) derived from LiDAR surveys was extended to the west (Montmorency massif) and east (the astrobleme) to take into account the complex data documented during previous studies in the Beaupré Basin.

1.2 - Geology and physiographic subdivisions

10The Beaupré Basin is located at the convergence of the three geological domains. The Grenville Province of the Precambrian Canadian Shield makes up most of the bedrock in the territory, the rest being Ordovician rocks in the Lowlands. The islands of the Estuary are part of the Appalachians.

11The Grenville Province is represented by the Laurentians, which can be subdivided into several physiographic units (fig. 1): 1) the Ligouri Uplands between the Estuary and the Sainte-Anne Valley (Govare, 1994), which is topographically subdivided into the Ligouri Range that runs along the estuary by an escarpment, the inclined central plateau and a small western range that dominates the Sainte-Anne Valley;
2) the Ferréol Lowlands (Govare, 1994; Fournier, 1999), including the lower valleys of the Sainte-Anne, Jean-Larose, Mont-Saint-Étienne, and Brûlée rivers, the Saint-Tite Basin and the Lombrette Valley to the east; and 3) the Laurentian highlands to the west and north of the first two units, forming a major regional massif with many peaks above 1000 m. The study area is limited by the Gouffre Valley to the east, which is part of the semicircular depression of the Devonian astrobleme of Charlevoix (Rondot, 1989).

12The Ordovician limestone and shale of the St. Lawrence Sedimentary Platform border the middle Estuary (fig. 1) and form the substrate of a high terrace at an altitude of approximately 100 m and a lower Micmac/Mitis terrace at an altitude of 6 m (Goldthwait, 1911; Dionne, 1963). The narrow low Maillard terrace consists of thick Quaternary deposits on the Ordovician substrate (Dionne, 1996; Clet-Pellerin & Occhietti, 2000) and is dominated by the escarpment of the Precambrian Ligouri Range (fig. 1).

13The physiographic unit of the Ferréol Lowlands is located at an average altitude of 375 m, with some peaks reaching nearly 450 m. The Ligouri Uplands mainly form a tilted plane between 625 and 450 m, with some small lateral peaks at more than 775 m. This inclined plane ends to the southeast at the small basin of Saint-Tite, coalescing with the Ferréol Lowlands. To the north, the highlands of the Laurentians culminate in Mount Raoul Blanchard at 1166 m (fig. 1c). West of the Ferréol Lowlands, Mount Sainte-Anne has an elevation of 803 m. Most of the study area is drained by the Sainte-Anne River and its tributaries.

1.3 - Previous work

14This article incorporates recent original research and all available published and unpublished studies of the Côte de Beaupré and Charlevoix regions. The first author participated in the earliest studies of the region in 1968. Data from a thesis (Govare, 1994), master’s research (Lanoie, 1995; Fournier, 1999; Laliberté, 2005), student reports supervised by S. Occhietti and N. Bhiry, and excursion guides on the Quaternary of the region have already been included in synthetic articles on the Quaternary in the Appalachian area of southern Québec (Occhietti et al., 2001a) and in Charlevoix (Brouard et al., 2016). The analysis of DEMs and field surveys from 2017 to 2021 call into question some of the implicit or explicit models of deglaciation proposed prior to this study by LaSalle et al., 1977; Occhietti, 1989; Govare, 1994; Occhietti et al., 2001a, 2011; Brouard et al., 2016.

1.4 - Methodology

15The rate and pattern of deglaciation in the Beaupré Basin are established by a detailed analysis of deposits and their stratigraphy, the search for glacial flow marks, mapping, and the study of the arrangement of constructed forms initially identified by stereoscopic analysis of aerial photos. The DEMs published by the Ministère des Forêts, de la Faune et des Parcs du Québec are now freely accessible and were used to integrate previous data and identify landforms not detectable by other methods, which were then verified in the field because DEMs only provide morphological clues and some forms may appear identical even though they are actually different. Shaded DEMs with a resolution of 2 m can give a misleading view of some shapes, so it was necessary to conduct a high-resolution (1 m) analysis. The 1-m DEMs make it possible to measure altitudes very accurately (± 1 m), as verified on topographic maps at 1:20,000. A preliminary DEM analysis was extended to the Laurentian areas adjacent to the Beaupré Basin (fig. 3) to understand the regional context. This initial analysis was then compared with field data from Lanoie (1995), Fournier (1999), Laliberté (2005), unpublished documents from Govare, original studies by Occhietti and Bhiry, and public data from Rondot (1974), Cousineau et al. (2014) and Brouard et al. (2016).

16Given the multiplicity of glacial flows (Lanoie, 1995; Fournier, 1999; Occhietti et al., 2001a), it is essential to establish their position relative to ice-marginal landforms such as moraines. Several criteria were used to establish the proximal side of moraines: the transverse asymmetry of the moraine, where the proximal ice side usually is steeper than the distal flank; the preferential clustering of large boulders on the proximal side, whereas the distal side commonly is mantled by proglacial waterlaid sediments; the convexity of arcuate ridges in valleys and conversely the concavity of the ice front features around reliefs; and marginal meltwater channels oriented along or perpendicular to the ice front.

17Depending on the publications and the laboratories used, the ages reported in this article are given in conventional years 14C BP (BP), or ka calibrated BP (ka), or calibrated years before 2000 (ka b2k). Marine shell ages are calculated in years 14C BP with δ13C = 0‰, not standardized to δ13C= -25‰, and in calibrated years with a regional ΔR = 175 years. The chronological frame of reference refers to the Time scale GICC05 of 20-year average values of δ18O by Rasmussen et al. (2014) for ages in ka b2k, and the Greenland INTIMATE event stratigraphy from Björck et al. (1998), Rasmussen et al. (2008) and Lowe et al. (2008) for ka. The reconstruction of late glacial phases is often hampered by the lack of precise ages, while glacial, lacustrine or transitional events in southern Québec during the Allerød often have short multi-year or multi-decennial durations, but rarely hundreds of years durations. For this reason, the sequence of events proposed in this article is based on the logical succession of phenomena in relative and sometimes absolute chronology. In the text, all altitudes shown are the present-day altitudes above sea level in the current state of glacio-isostatic compensation.

2 - Ice flow data: glacial erosion microforms, lithostratigraphic indicators and megascale glacial lineations

2.1 - Glacial erosion microforms: northward ice overflow (NIO) in the Beaupré Basin

18In western Charlevoix, 200 oriented glacial erosion microforms have been identified by Rondot (1989), Lanoie (1995), Fournier (1999), Occhietti et al. (2001a) and in this study (fig. 3). These microforms comprise unevenly distributed directional groups. Striations of the same orientation are not necessarily synchronous, but they are oriented toward the south (168°-217°), east, southeast (124°-165°), and east-southeast (081°-120°). The northward sets ENE-NE (040°-074°) and NNE (014°-034°) are limited to the lowlands and the Ligouri Uplands and their periphery. In the latter area, the NNE direction assigned to several striations is the opposite of the direction indicated on earlier maps. Striations can record fleeting glacial flows, but they are very sensitive to local topography. They may also record the same flow direction in different locations but are related to flows of different ages.

19Cross striations and grooves provide a relative chronology of ice flows (fig. 3). In the Laurentians, some W-E striations are intersected by the NNW-SSE striations from the major Montmorency flow. In the Beaupré Basin, the locally major flow to the north is subsequent to the W-E flow and moves laterally toward the NNE on the Ligouri Uplands, where the striations intersect those of the flow toward the NE.

20In the Québec City area, northward striations are limited to the lower foothills of the Laurentians (Paradis & Bolduc, 1999). We postulate that they are penecontemporaneous to those of the glacial overflow of the Beaupré Basin, as they succeed the main flow to the E and ENE and several subsequent flows oriented NE to SE (fig. 3). Our observations in this region confirm the presence of W-E or E-facing grooves, intersected by striations oriented toward the SE and the ESE. Along the Côte de Beaupré, late striations parallel to the Estuary are more recent than the striations to the north.

2.2 - Lithostratigraphic indicators of NIO in the Ferreol Lowlands and Ligouri Uplands

21The Quaternary stratigraphic sequence in the Beaupré Basin includes at least one unit of rhythmites and a carbonate till (the Saint-Tite Formation) below the interstadial peat unit of the Lombrette Beds dated at 14C age ˃ 50,800 BP (ULA-8033) (fig. 4a; Fournier, 1999; Bhiry et al., 2001). The presence of a glacial lake dammed by an ice stream in the Estuary and the invasion by a glacial flow to the north is therefore recurrent in the Beaupré Basin. In Late Glacial time, at the Martineau quarry on the right bank of the Sainte-Anne River (fig. 4d), the ice contact deposits are at least 3.5 m thick and contain boulders, pebbles and clasts of Ordovician sandstone and limestone. These deposits underlie 60 m thick deltaic deposits with a north-south dip (fig. 4d). The coarse deposits are evidence of a glacial contribution to the north, 10 km beyond the limit of the Canadian Shield and prior to the formation of the lake delta. In other sites, limestone, sandstone and shale boulders and pebbles have been identified in surface deposits as far as 13 km north of the Ordovician boundary (Fournier, 1999).

2.3 - Megascale glacial lineations

22To integrate the Beaupré Basin data into the appropriate context, the DEM analysis was extended to most of the territory on the northern margin of the St. Lawrence middle Estuary, from the Jacques Cartier Valley upstream to the edge of the Saint-Narcisse Moraine (downstream of La Malbaie) (fig. 3). The megascale streamlined forms or lineations printed in the soft bed of subglacial debris are clearly identifiable throughout almost all of this territory. These axially asymmetric forms are often true drumlinoids behind rocky hills. Their dimensions vary from approximately 100 m to more than 1 km long and from 10-20 m to 250 m wide. In most cases, they are clustered in unidirectional fields. These fields are attributed to rapid flows corresponding to late ice flows (Spagnolo et al., 2014) within the LIS margin, as reconstituted in Charlevoix (Brouard et al., 2016) and Ontario (Eyles & Doughty, 2016).

23The lineations correspond to the last dynamic glacial phase and involve subglacial materials saturated with water. South of the Saint-Narcisse moraine complex, the territory can be subdivided into several sectors (fig. 3):

• In the Beaupré Basin, the Ferréol Lowlands sector and its extension on the Ligouri Uplands, with lineations to the north and the NNE. This group of lineations is attributed to a glacial overflow to the north during a phase of the StLIS, called the Northward Ice Overflow (NIO) that occurred in the middle Estuary. It corresponds to the glacial striations oriented south-north and SSW-NNE.

• At the limit of the Beaupré Basin, in a very small area of valleys on the estuarine edge of the Ligouri Uplands, with short lineations to the west-northwest. The latter are associated with the late phase of the StLIS.

• To the west, on the Montmorency massif, numerous lineations and asymmetrical molded forms oriented NNW-SSE are attributed to a very fast and late flow in the center and north of the massif. This Montmorency Flow is significant and most of the north-south glacial striations are associated with it. The northwestern part of this area is intersected by the Saint-Narcisse Moraine, as shown by palimpsests lying under the deposits of the moraine complex (fig. 5). To the north, the highlands of the Lac des Neiges massif are devoid of lineations, except for a sector overlain by a field of NW-SE lineations (fig. 3).

Serge Ochietti

• To the west of Mont Ferréol (figs. 1 & 3), beyond the western margin of the Beaupré Basin, west-east lineations represent a small and isolated sector, truncated to the west by the Montmorency Flow. Some glacial striations outside this area indicate that the initial range of this west-east flow was much larger before the expansion of the Montmorency Flow and the NIO.

• To the north-east of the Beaupré Basin, the Ligouri Uplands are bordered by the Gouffre Valley. This valley is part of the Charlevoix astrobleme and is bounded on each side by NNW-SSE oriented lineations that mark the late phase of the Gouffre lobe identified by Govare (1994). Northwest of the astrobleme, the lineations and striations show a convergence inflection of glacial flow from the highlands of the Laurentians. Palimpsests of lineations oriented along the main Gouffre Flow were observed north of the sector and on the western edge of the Éboulements massif (fig. 3). The initial lineations were overthrusted by the major west-east Jean-Noël Flow.

• In the region of the Charlevoix astrobleme, Rondot (1974) demonstrated that the Saint-Narcisse Moraine corresponds to an overriding and cross-oriented readvance compared to an earlier glacial flow identified by a field of striations, lineations (crag and tails) and dispersal trains oriented west-eastward. Govare (1994) calls this flow the Jean-Noël Lobe, a term taken up by Brouard et al. (2016) who associate it with many lineations. The inventory of lineations shows that this sector is extensive, beginning below the highlands of the Laurentians and running along the northern edge of the Éboulements massif. The northern part of the sector is clearly truncated by the Charlevoix Lobe (in particular by the Gouffre and Malbaie lobes) (Govare, 1994) of the Saint-Narcisse readvance. The lineations at the eastern end of the sector toward La Malbaie do not reach the Estuary (Govare, 1994; Brouard et al., 2016). It appears that either the termination of the Jean-Noël Flow merged with the ice of the Estuary and/or a reactivation of the StLIS overlapped the easternmost area of the Jean-Noël Flow.

• On the southeast edge of the Éboulements massif, the SSW-NNE lineation field is already largely identified by Brouard et al. (2016). The striations oriented 030º near Saint-Joseph-de-la-Rive confirm the StLIS overflow to the north and northeast. Rare lineations from east to west in the Jean-Noël River Valley evoke a late overflow of estuarian ice.

2.4 - Preliminary conclusion

24Based on directional indicators, the Beaupré Basin provides evidence of two ice flows: the major overflow (NIO) to the north and northeast of the StLIS in the Ferréol Lowlands and Ligouri Uplands and the major flow of the StLIS along the SW-NE axis of the Estuary. The latter ice flow is also associated with secondary flows in small side valleys along the edge of the Ligouri Range and, further downstream, on the southern and southeastern edge of the Éboulements massif (fig. 3).

25A preceding W-E ice flow is also inferred based on a limited group of lineations at the western edge of the Beaupré Basin and from striations that are more widely distributed.

26A major NNW-SSE flow covered the Montmorency massif and the Gouffre Valley. As far as we know, this dynamic phase of the LIS margin is not recorded in the Beaupré Basin or the north side of the Éboulements massif.

27The Jean-Noël Flow is a major late flow that originated in an area in the Laurentians that lies outside the study area. The ice flowed over highlands and through corridors (fig. 3), extending into the northern part of western Charlevoix and onto the northern side of the Éboulements massif before the YD Saint-Narcisse Moraine readvance.

3 - Erosional channels: indicators of meltwater flows on ice margins and gradients

28Using the available DEMs, we were able to document several features that fall under the term ‘erosional channels’ (fig. 6): paleochannels at ice margins, subglacial and submarginal lateral meltwater channels, subvertical sides of moulins, channels related to threshold overflows at glacial margins, and subglacial channels incised in unconsolidated deposits. With the exception of dry and oversized valleys and gullies, which have also been documented, these channels denote temporary ice boundaries.

Serge Ochietti

3.1 - Erosional channels at the front of retreating ice margins

29On the tilted plane of the Ligouri Uplands, several channels and incised streams are oriented to the east or west, perpendicular to the general slope (fig. 6). These channels are parallel to the morainic ridges and mark stages of glacial retreat. Channels parallel to the ice flow indicate either drainage to the outside of the glacial front (spillways) or channelized subglacial meltwater flows from temporary frontal lakes.

3.2 - Forms of lateral erosion by meltwaters on steep rock slopes: lateral ice paleochannels (ravinelles and moulins)

30Subhorizontal channels in the form of a notch or meander are cut into the slopes of the highlands that surround the Beaupré Basin. They are located between 950 and 350 m above sea level and provide evidence of lateral erosion by meltwater between ice and rock. These channels can be of two types: subglacial or lateral meltwater channels (Benn & Evans, 1998; Bennett & Glasser, 2009). They have been described in Montana (Capps, 2004) and the Alps (the lateral meltwater ravines of Beaudevin, 2000). These forms of edge erosion do not necessarily indicate the altitude of the glacial margin. When they correspond to subglacial paleochannels, meltwaters circulate down to 150 m below the ice surface (Reynaud, 1989) above the intraglacial impermeable surface (Beaudevin & Pisano, 2020). We propose the term ravinelle for this type of encased and perched ravine, which is also observed on the slopes of the Ligouri escarpment and the Éboulements massif. On the western margin of the Ferréol Lowlands, ravinelles are intercalated between the lateral moraines of Mount Ferréol and Mount Sainte-Anne (figs. 6 & 7), which indicates the circulation of meltwater between the slopes and the glacier with a gradient of meltwater flow to the north, from the Estuary to the Laurentians.

31Subvertical spillways notched in the rocky slopes confirm the existence of former moulins on the edge of the ice. According to Reynaud (1989), the base of active moulins can reach a depth of 70 m below the top of the ice.

3.3 - Thresholds and discharge channels on highlands

32On the highlands of the Laurentians, particularly near the northern limit of the NIO, incised channels perpendicular to thresholds (fig. 6) indicate that meltwater lateral to the ice tongue in the valley overflowed into side valleys. These channels of meltwater overflow, such as the edge ravines or lateral furrows of Beaudevin (2000), provide evidence of the progressive melting of the ice, particularly on the slopes south of Mount Blanchard.

33On the eastern slope of the upper valley of the Du Sot River and below the thresholds of the Ligouri range at 756 and 730 m above sea level (fig. 8), a series of overflow channels correspond to discharges of meltwater from the estuary ice (StLIS) to the west. They indicate that the upper Du Sot Valley was at least partially deglaciated and consequently that the ice retreat in the central part of the Ligouri Uplands preceded or is contemporary with a phase of reactivation of the StLIS.

3.4 - Channels associated with the Larose polyphase debris fan

34Below Mount Sainte-Anne, paleochannels incised at elevations between 500 and 350 m are associated with a vast polyphase debris spreading complex, located in the upper basin of the Jean-Larose River (fig. 9). An initial small outwash was established between 500 and 450 m, bordered by paleochannels facing north. This upper part of the complex was deposited between Mount Sainte-Anne on the west side and the NIO ice on the east side. A second outflow of debris spread over 6 km between 450 and 350 m toward the northeast. A meander-shaped ravinelle lies at 450 to 445 m and is encased in the slope of Mount Sainte-Anne (fig. 9). This outflow indicates a major spreading of debris. The Larose debris Fan is deeply incised by paleochannels, which indicates the dispersal of successive influxes of debris to the north and north-northeast, including clasts of sedimentary rocks that spread at the lateral edge of the NIO ice. These south-north debris spills infer a large body of water or melting ice which is retained between the foothills of the Montmorency massif and the StLIS.

35Fournier (1999) recorded the dip of many sediments in the Beaupré Basin (fig. 6), which made it possible to discover the flow of meltwater to the north in the Beaupré Basin. Pebbles of sedimentary origin from deposits located beyond the limit of the Sedimentary Platform corroborate the northward flow of meltwater from the StLIS.

3.5 - Preliminary conclusion

36The thresholds, ravinelles and notches of the western Charlevoix and their altitudes have been reported on a projection along a meridian plane (fig. 10). The ravinelles represent a marker of the glacier edge in the highlands, as observed in alpine morphology. They make it possible to establish the limits reached by the NIO up to a minimum altitude of 800 m north of the Beaupré Basin.

37Paleochannels associated with moraines are a marker of the polarity of glacial margins. The relative age of the ice retreat in different regions can be inferred from meltwater overflows.

38The paleochannels are partly overtaken by the current stream network, but they highlight a gradient of meltwater to the north in the Beaupré Basin. This gradient indicates the continuous presence of an ice stream in the middle Estuary and temporary glacial lakes in the lateral basins during deglaciation. The Larose polyphase cone represents a series of high-energy influxes of glacial debris from the edge of the StLIS to a lateral basin during the ice retreat process.

4 - Moraines and ice margin landforms

39The DEM analysis reveals several types of glacial margins in western Charlevoix (tab. 1, fig. 11) and can be used to reinterpret the data from other sectors north of the middle Estuary (fig. 3). Typical edge moraines have been identified on the slopes of Mount Blanchard (figs. 10 & 11). Locally, the mounds of decaying moraine at an altitude of 950 m correspond to the convergence of the south-north overflow and the Gouffre Flow. On the Ligouri Uplands, the ridges are situated at an interval of 50 to 200 m and are composed of sandy ablation till with boulders. The concave ridges are indicative of glacial halts at the head of several valleys. Some of the quasi-rectilinear ridges (fig. 7) formed between active or passive ice masses; these moraines are related to converging ice flows (Eyles & Doughty, 2016) rather than interlobe moraines. Fringe moraines include mixed forms, such as lateral moraines and narrow flats, and include lateral paleochannels and strandlines of a lateral lake. In the northern part of the Ferréol Lowlands, the first transverse moraines appear to have undergone significant reworking when the waters of the lake extending to the north flowed over these moraines. These were followed by a series of small parallel ridges of metric height (De Geer moraines?) deposited at the edge of the lake that flooded the deglaciated land. In the context of the Beaupré Basin and its surrounding region, moraine formation was strongly linked to the topographical setting and the position relative to the StLIS. The moraines form an exceptionally dense network (fig. 3, tab. 1) as the regional ice margin was very sensitive to climate changes.

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4.1 - A case of convergence: the Saint-Cassien moraine complex

40The Ligouri Uplands are traversed by an ENE-WSW depression, which leads eastward toward the Estuary and westward to the Bras du Nord-Ouest Valley. In this setting (fig. 11), the moraine complex of Saint-Cassien was deposited transversally and laterally to the depression at an altitude of approximately 460 m. This is a late ice-margin complex at the convergence of overflow ice from the StLIS into the lower Du Sot Valley, ice from the Gouffre Lobe on the north and east sides, and the NIO ice in the south. Downstream, in the valley of the Bras-du-Nord-Ouest River, a mound of glacial margin, at an altitude between 477 m and 390 m, demarcates the lateral halt of the Gouffre Lobe, which was in the process of downwasting (Govare & Gangloff, 1991; Govare, 1994).

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4.2 - Glacial retreat in the Sainte-Anne valley and the Laurentians

41The meltwater flows reshaped transversal moraines in the upper part of this major valley. Irregular terraces correspond to several types of mostly reworked landforms, including deltas from the adjacent valleys, morainic ridges, outwash fans, distal fluvioglacial terraces, and rock glaciers (fig. 11; Lanoie, 1995; Hydro-Québec, 1980). The Sainte-Anne Valley remained glaciated until a later date while the adjacent highlands were rapidly deglaciated, except for the depressions. The valley exhibits the discontinuity between the Ligouri Moraines and those of the Ferréol Lowlands. The two clusters were largely contemporaneous with one another.

42The deglaciation of the Laurentians, north of the NIO area, was highly dependant on topography (figs. 3 & 11). To the northwest, on the Lac des Neiges massif, the absence of frontal moraines indicates rapid deglaciation due to thinning ice, followed by the melting of stagnant ice in depressions and valleys. To the north, the glacial retreat was punctuated (figs. 11 & 12, tab. 1) by the ridges seen at Lake Louis and the group of moraines and fans of the Lac Savane region. Eskers (fig. 3) confirm the presence of late ice tongues in the major valleys. The Saint-Narcisse Moraine lies 60 km from the shore of the St. Lawrence and about 20 km from the Savane Moraine, and overrides the edges of the NW-SE lineation field to the north and the NNW-SSE Montmorency lineations area to the northwest and west (fig. 3).

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4.3 - Preliminary inventory of ice margin deposits on the southern part of the Montmorency massif and re-examination of the moraines in the Charlevoix astrobleme

43On the southern margin of the Montmorency massif, the DEM analysis reveals different types of landforms that include ridges transversal to valleys, arched ridges at the top of small valleys, and fans with kettles (fig. 3). These landforms confirm the overflow of the StLIS into some of the valleys and a greater frequency of glacial contact deposits in the southern half of the massif. They may or may not be linked to the IACP and further study is required.

44The DEM land survey provides new findings in the astrobleme region. The moraines on the edge of the Gouffre Valley (fig. 11) had been previously identified by Govare (1994). This study demonstrates that these deposits represent the late stages of the Gouffre Flow, when the ice began to thin and valley-sided accumulations lowered to altitudes of 360, 260, and 230 m. Glacial striations and ice-margin deposits were also found on the southern slopes of the Éboulements massif (fig. 3), outside of the lineation area mapped by Brouard et al. (2016). These forms mark the overflow of the StLIS (fig. 3) and are distinct from the Rochette Moraine, which is composed of deposits from the glacial margin on the northern slopes of the Éboulements massif (Rondot (1974). These deposits run along the edge of the southern Jean-Noël Flow, but we cannot exclude the possibility of a polygenic origin for the till mounds at this edge.

45On the south slope of the Grands Fonds massif, at the far northeastern edge of the mapped region (figs. 1 & 2), a group of moraines indicates an ice retreat from the north to the south (fig. 13). The moraines situated at altitudes lower than 450 m may be a part of the Malbaie Lobe (Govare, 1994) of the Saint-Narcisse Moraine, which overflowed onto the lands on the right side of the Malbaie Valley up to an altitude of 450 m. The highest moraines on the south slope are located at altitudes ranging from 660 m to 580 m. These high elevations (relative to 450 m) could therefore be evidence of ice-margin deposition prior to the Saint-Narcisse readvance. They indicate a major overflow to the north that should be studied.

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4.4 - Preliminary conclusion

46The moraines of western Charlevoix were partially mapped by Rondot (1989), Govare (1994), Fournier (1999) and Brouard et al. (2016). They were initially linked to a south to north glacial retreat, contrary to the conclusions of our study. Given the steep slopes of the secondary valleys that overhang the St. Lawrence Estuary, it seemed likely that the ice also flowed into these valleys toward the Estuary. However, this study shows that the glacial front retreated from north to south and that the ice flowed upward from the Estuary along secondary valleys in the Ligouri Range.

47A group of moraines in western Charlevoix are located at high altitudes (between 950 and 650 m) and are associated with ravinelles. These moraines form a ring around the NIO zone and demarcate the ice overflow limit. A meltwater spillover ravine at an elevation of 850 m indicates that the Laurentian highlands to the north of the overflow site were partially deglaciated.

48The NIO ice in the Beaupré Basin represents a glacial entity that is disconnected dynamically from the Québec-Labrador dome but fed by the StLIS, and downwasting occurred at different rates over time.

49After an initial phase in which the overflow ice thinned, three moraine clusters (Ligouri, Ferréol Lowlands and Mounts) provide evidence of the same phase of glacial retreat, collectively referred to as the Beaupré Group of Moraines in this text (tab. 1). In the context of the Beaupré Basin, they provide detailed evidence of the slowdown of the glacial retreat. A more rapid deglaciation followed, interrupted by the minor halt of the Des Roches-Joachim moraines (figs. 3, 10 & 11) before the marine invasion.

50During the slowdown phase, the cluster of small discontinuous moraines above the estuary, referred to herein as the Escarpment Moraine, provide evidence of a late overflow of the StLIS.

51On the Laurentian highlands, the Lake Louis and Savane moraines indicate a slowing glacial retreat on the edge of the LIS. A deglaciated inter-marginal zone stretched between these moraines and the Beaupré Group (figs. 12 & 15). Deglaciation then occurred rapidly on the highlands, with a glacial flow toward the east before the Saint-Narcisse readvance.

52The major phases of western Charlevoix apply to the northern shore of the middle and a part of the fluvial estuary of the St. Lawrence. With some variations in intensity, late northward ice flows stretched from the Québec City area to the Malbaie River and are documented by glacial striations, lineations, ravinelles and ice-margin landforms.

53In the astrobleme region, the discontinuous elements of the South-Éboulements Moraine are associated with a lineation field that is attributed to the StLIS. These elements are correlated to the Escarpment Moraine, but are distinct from the Rochette Moraine.

5 - Glacial lakes, deltas, outwash fans and glaciofluvial valley trains

54In western Charlevoix, signs of temporary glacial lakes are evident (tab. 2, fig. 6) in steeply sloping delta cones, raised deltas beside large valleys, stepped lateral deltas, outlet fans, valley delta fans and, in the Sainte-Anne Valley, by terraces of a distal fluvioglacial valley train and a deltaic termination. On the left side of this valley (fig. 14), narrow flats appear to be lateral benches of a narrow water body retained between the residual ice tongue of the valley and the western range of the Ligouri Uplands. Other discontinuous and irregular flats run along the slopes of mounts Sainte-Anne and Ferréol. They are erosive and constructed landforms composed of lateral channel fillings, lakeside flats and lateral moraines (figs. 7 & 9). Several stages of ice retreat in the Beaupré Basin are inferred from these data (tab. 2) in combination with moraines and paleochannels:

• The first delta cones and lateral deltas, from 800 to 660 m above sea level, provide evidence of the rapid and continuous deglaciation of the highlands and the presence of temporary lakes on the edge of the main valley ice tongues. A glacial tongue persisted in the Sainte-Anne Valley that was fed by the StLIS and the Laurentian ice.

• A relative stabilization of the level of the lakes bordering the glacial margin at around 650 m is associated with the first moraines of the Ligouri and Escarpment groups.

• A lake stabilization phase between 580 and 500 m indicates that the ice of the NIO melted on its periphery and was disconnected from the ice sheet, but this did not occur in the Sainte-Anne Valley (fig. 15). Meltwater from the Montmorency massif drained to the lowlands of Beaupré Basin to form a lake with changing boundaries (Lake Beaupré).

• The encased channel at an altitude of 460 m from the Larose polyphase fan marks the beginning of a series of glacial debris flows from the edge of the StLIS. These debris flows imply that the valleys of the southern margin of the Montmorency massif, upstream of the Beaupré Basin, were still glaciated up to this altitude. The 460 m lake phase is concomitant with the formation of the last ridges of the Brûlée and Mounts moraines.

• At an altitude of 370 m, the deltaic termination of the fluvioglacial valley train at Saint-Léon in the Sainte-Anne Valley in glacial Lake Beaupré correlates to flats along the southern slope of Mount Sainte-Anne at an altitude of 373 to 369 m. The deltaic termination also correlates to the small moraines of Des Roches and Joachim (fig. 10). Some residual ice was probably buried under the deltaic sands of the Sainte-Anne Valley during this last significant halt of the retreating overflow ice.

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55The ice front retreat on the tilted plane of the Ligouri Uplands is characterized by short-lived lakes (fig. 15). Several lakes in the study region indicate a lack of sedimentary filling in the depressions at the front of the glacier, which is related to short lived meltwater flows and the brevity of ice-margin lakes. At the head of the Bras du Nord-Ouest Valley, a strongly gullied delta situated between 560 and 530 m indicates a meltwater spill toward this valley and the Gouffre Ice Flow. Subhorizontal flats at an altitude of 515 m at the southern limit of the Moraine Brûlée and at an altitude of 420 m near Lake of Saint-Joachim, in addition to gently sloping fans (fig. 14), confirm the presence of short-lived water bodies at the front of the retreating ice margin. The main flats were mapped by Govare (1994) as lake embankment surfaces with ice south of the depressions.

56Despite evidence of glacial lake episodes in the Beaupré Basin, the absence or rarity of fine deposits in depressions and valleys is surprising. It is attributed mostly to the ephemeral nature of the water bodies and the high energy of meltwaters. The very strong post-glacial erosion in the valleys alone cannot explain this absence.

57In valleys on the north shore of the Estuary, upstream and downstream of the Beaupré Basin, deltas, outwash fans with kettles and ice contact deposits are related to lateral lakes to the StLIS. South of the Montmorency massif, Lamarche (2011) identified two deltas at 335 m, across the Montmorency River Valley and in the upper valley of the Petit-Pré River (fig. 3), and small deltas at lower elevations. She found fine glaciolacustrine deposits below the delta sands. Downstream, from the DEMs (fig. 3), small delta fans are strongly incised in the upper part of the Cazeau River and its tributaries between 340 and 310 m and below. In the Sault-à-la-Puce Valley, two ice-margin fans, riddled with kettles, at 350 and 325 m, are related to meltwater flow from the glaciated Montmorency massif over lateral dead ice of the StLIS. A delta at 270 m is incised by the river. At the confluence of the aux Chiens River and its main tributary, a bumpy ice-margin landform, between 360 and 330 m overlies a complex sequence of tills, stratified clay and ice-margin deposits. South of the Éboulements massif, at the head of valleys, overhanging ice-margin deposits are dipping toward the estuary at altitudes around 350 m (Govare, 1994; Occhietti & Dionne, 2008).

5.1 - Preliminary discussion and conclusion

58A glacial lake reconstructed from the Saint-Léon delta and outwash cones had previously been attributed to the presence of close ice south of the delta (Fournier, 1999; Bhiry et al., 2001; Brouard, 2016). However, our new observations change the understanding of the Saint-Léon delta. The mounds downstream of the delta are sandy but are not ice contact deposits; they represent a strongly gullied extension of the delta. In fact, the Saint-Léon delta is a part of the deltaic termination of the distal fluvioglacial deposits of the Sainte-Anne Valley in the marginal glacial Lake Beaupré. Underlying StLIS dead ice below the delta deposits might explain the deep gully erosion of the delta termination.

59The moraines, paleochannels and paleodeltas of the Beaupré Basin are related to the gradual disconnection of the NIO ice from the margin of the ice sheet over the Laurentians, while the NIO ice remained an expansion of the StLIS. Meltwater from the Montmorency massif converged with that of the deglaciated lowlands to form a diachronic lake that extended southward, Lake Beaupré, whose level was determined by the StLIS.

60In the Beaupré Basin, three lake-level episodes occurred at around 550 m, 460 m and 370-350 m and are associated with slowdowns in the glacial decay of the StLIS. Along the north shore, upstream and downstream the basin, several ice-margin and delta deposits at an elevation of circa 350 m, and sometimes close to 230 m, indicate temporary drainage levels of meltwaters along the north side of the StLIS, which could be related to contemporaneous relative sea-levels downstream in the lower estuary. They raise the question of how meltwaters from the ice sheet were evacuated via the middle Estuary to the Gulf of St. Lawrence.

6 - The Goldthwait sea

6.1 - Marine limit

61In Beaupré, the present-day tides of the St. Lawrence River reach a height of 5 m. Using DEMs, the post-glacial marine limit may be determined on the palaeocliff of the structural coast along the fault line of Cape Tourmente, between the Saint-Anne marine delta and Cape Tourmente (fig. 6). On the palaeocliff, a quasi-horizontal discontinuous band is situated at an altitude between 205 and 180 m with abrupt rocky outcrops. Strips of the small deltaic cone of the Friponne stream (fig. 6 & 10) are raised, situated 198 m above the estuary. At the base of Mount Sainte-Anne, the Jean-Larose River flows into a deltaic cone located at an altitude of between 250 and 217 m, deposited in glaciolacustrine waters. The end of this delta is truncated, with stepped flats from 200 to 196 m and from 192 to 189 m. These delta flats are associated with the start of marine invasion. At the exit of the du Sot River (fig. 8), a strip of the delta cone establishes the boundary of the local marine maximum at an altitude of between 200 and 197 m.

62The morphological data support a post-glacial marine limit at 198 m, with a high marine water level (high tides, storms) of up to 205 m. The hypothesis of a rapid lowering of the level from 205 to 198 m during the final phase of glacial Lake Beaupré cannot be excluded (fig. 10).

63The beginning of the marine transgression represents the baseline for local chronology. The field data indicate a continuous reduction in the thickness of the StLIS in this part of the middle Estuary prior to marine transgression. The data provide evidence for a model of marine invasion along a sea arm parallel to the coast instead of an instantaneous opening of the entire Estuary by a calving bay. This model accords with the model from the south shore of the middle and lower Estuary, in which Lake Chaudière-Etchemin precedes the arrival of marine waters (Occhietti et al., 2001a; Normandeau, 2010).

6.2 - The Beaupré marine delta

64The marine delta of the lower Sainte-Anne River rises to 181 m, below the marine limit. The delta is sandy but laterally includes, to the east, a large structure with coarse deposits and boulders encased in the sands of the delta. This lateral coarse unit is below a large ravine located on the rocky slope overlooking the delta. It implies a very local and late origin and questions the presence of a coarse summit unit over the entire delta proposed previously (Occhietti et al., 2008). Upstream of the delta, the river crosses a canyon encased in a threshold of 230 m of altitude capped with sub-metric boulders and erosion channels. Downstream of the delta (Section du Barrage, Laliberté, 2005; fig. 4b), marine clays overlain by silt and sand cover a lower sandy prodeltaic unit. From this dataset, the valley currently a canyon was obstructed by glacial margin deposits from the residual St. Lawrence Ice Current. The meltwaters from the Sainte-Anne watershed and the final phase of Lac Beaupré are at the origin of the channels incised in the threshold and the prodelta sands deposited downstream the threshold before the deposition of marine clays. Then, the main channel of the Sainte-Anne River was encased in the threshold to adjust to the relative sea level, in an environment of continuous glacio-isostatic rebound.

6.3 - ¹⁴C dating of marine shells

65The non-standardized calculated 14C ages (δ13C = 0‰) of marine shell fragments in the basal position of the clay-silts under the marine delta deposits of Sainte-Anne (Laliberté, 2005; tab. 3), are respectively 12,200 ± 100 BP (TO-10,930) at the Barrage site (fig. 4b), 11,940 ± 70 BP (Beta-180457) at the base of the Mount Sainte-Anne site (fig. 4c), and 11,890 ± 90 BP (TO-10929) at the Jean-Larose section. In calibrated years using Calib 8.2 (Stuiver et al., 2021), these marine shell-ages are between 14.25 and 12.92 ka.

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66They are older than the maximum ages known on the north shore of the Central St. Lawrence Valley, which vary between 11,600 and 11,300 BP (Occhietti, 2007). Younger shells ages, in a more recent stratigraphic position, validate the relative value of these ages (tab. 3).

67In the Beaupré Basin, these ages at first suggest that the penetration of marine waters by a northern local arm of the Goldthwait Sea precedes by several centuries the widespread marine invasion further upstream of the central St. Lawrence Valley at around 13.0-12.9 ± 0.1 ka.

68However, this difference remains theoretical given the margins of error, the high variability of the total reservoir effects observed in the epicontinental marine waters of the St. Lawrence Lowlands, and the many variables that cause it (Hillaire-Marcel, 1981; Rodrigues, 1988 &1992, Occhietti & Richard, 2003; Richard & Occhietti, 2005).

7 - Chronological elements: facts and interpretation

69Prior to the marine invasion, the Late Glacial in western Charlevoix is characterized by several other chronological and climatic landmarks, presented below in chronological order.

7.1 - Relationship between the ice overflow to the north of the estuary and the Appalachian ice-flow reversal in Québec and New England during the Bølling-Allerød

70The NIO in the Beaupré Basin was a catastrophic event on the margin of the LIS in the St. Lawrence axis. It has been associated (Occhietti et al., 2001a) with the Appalachian ice-flow reversal along the Québec Ice Divide (Shilts, 1981), after the thinning of the ice stream in the lower and middle St. Lawrence Estuary. This reversal has been identified by Chalmers (1898); Lamarche (1971, 1974), Lortie (1975, 1976), Dionne (1972), Martineau (1977), Shilts (1981); Lortie & Martineau (1987); Blais (1989); Rappol (1993) and Shilts et al. (2007), and in Maine by Lowell (1985). All published data shows that the Appalachian ice-flow reversal is diachronic. Using data from Maine and models of ice sheet dynamics, Lowell et al. (1990) suggest that this Appalachian reversal predates the marine invasion of the middle Estuary lowlands by at least 1000 years, between the end of the Bølling and the beginning of the Allerød (figs. 16 & 17). It was correlated with Dryas I, before Bølling (Occhietti et al., 2001a), however, this now seems to be far too old.

7.2 - Climatic significance of the Beaupré Group of Moraines: correlation with the New Brunswick Killarney episode and the Intra-Allerød Cold Period (IACP)

71Under the previous model of south-to-north deglaciation, the 60 km distance between the Estuary and the Saint-Narcisse Moraine implied a long withdrawal period. It was therefore consistent to correlate the Brûlée Moraine with the Older Dryas (Dryas II) cold episode (Occhietti in Bhiry et al., 2001). The deglaciation model recognized in this study, with north-to-south deglaciation of overflow ice, and a south-to-north deglaciation of Laurentide ice, represents a much shorter glacial retreat.

72Despite the context of rapid Allerød deglaciation, the Beaupré and Savane moraines in western Charlevoix represent a pronounced slowdown of the ice retreat, while the StLIS is still very active (Escarpment Moraines). This slowdown follows a thinning of the ice, from the highest moraines at 950-750 m to 650 m (fig. 12), and precedes the rapid retreat prior to the marine invasion (fig. 10). The slowdown episode occurs (figs. 16 & 17) shortly before the marine invasion and the early YD, and is therefore likely related to the Killarney Oscillation defined in New Brunswick and correlated with the IACP (Levesque et al., 1993; Levesque et al., 1994; Yu & Eicher, 1998; Yu & Wright, 2001).

73The IACP is recognized within the USA (Yu & Eicher, 1998; Yu, 2007), and in Europe (Switzerland and Germany) as Oscillation Gerzensee 3 (Lotter et al., 1992; Merkt & Müller, 1999; van Raden et al., 2013) which is observed also at Lake Ammersee (von Grafenstein et al., 1999). It corresponds to the GL-1b stadial of the Greenland Ice Sheet according to the NGRIP Greenland Ice Core (GICCO5) 18O chronology (Rasmussen et al., 2014) as well as that of the INTIMATE group (Lowe et al., 2008) (figs. 16 & 17).

74The IACP is attributed to a disturbance of the North Atlantic thermohaline circulation (Donnelly et al., 2005; Thornalley et al., 2010), also observed in the Norwegian Sea (Karpuz & Jansen, 1992) and in the Cariaco Basin in northern Venezuela (Yu & Eicher, 2001). The decrease in thermohaline circulation is attributed to the catastrophic discharge into the Atlantic of the waters of glacial Lake Iroquois (Donnelly et al., 2005). The level of Lake Iroquois, which flooded the Lake Ontario basin, dropped 100 m between the Frontenac and Belleville levels, thus draining the corresponding waters to the Hudson Valley via glacial Lake Vermont (Pair & Rodrigues, 1993).

75The oscillation duration is of the order of 2 to 3 centuries. The INTIMATE group (Lowe et al., 2008) timed the chronological boundaries of the IACP between 13,331 and 13,099 b2k cal years, i.e., between 13,261 and 13,049 cal years BP. Van Raden et al. (2013) readjusted these ages between 13,274 and 12,989 cal years BP. Considering the temporal spread of the glacial response, observed for example during the study of the Dryas II moraines of New Hampshire (Thompson et al., 2017), the arbitrary chronological limits of 13.3-13 ka are used (figs. 16 & 17).

76Given its North Atlantic magnitude and duration, the IACP is a major oscillation that should be expressed during the deglaciation of the SE margin of the LIS. According to annual mean temperature curves reconstituted from the Greenland ice cores (Rasmussen et al., 2006) and from the INTIMATE data (Lowe et al., 2008), the duration and extent of cooling of the IACP are greater than those of the Dryas II, in a cooler Allerød context than that of Bølling. These characteristics would explain partly the spread over 11 km of the moraine ridges of the Ligouri Moraines in western Charlevoix. The IACP is followed, at the end of the Allerød, by a brief return to the milder temperatures of the GI-1a before the YD cold chron (fig. 16). Locally in western Charlevoix, the Killarney Oscillation predates the deglaciation of the Petit Lac à l’Ange sector (see below) and therefore cannot be recorded in the pollen diagram of this site. At the centre of the Charlevoix astrobleme, the Rochette and South-Éboulements Moraines are likely associated with parts of the IACP.

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7.3 - Postglacial vegetational sequence: Petit Lac à l’Ange sediments and minimum age for western Charlevoix moraines

77Petit Lac à l’Ange is located 20 km south of the Saint-Narcisse Moraine, ENE of the Savane Moraine. Consequently, the lake basin was deglaciated after the emplacement of the latter moraine. The pollen diagram of a 5.67 m long sediment core (diameter 5 cm) delivered a complete postglacial vegetational sequence representative of the regional southern Laurentian Highlands (Labelle & Richard, 1981; Fréchette et al., 2020).

78Sediment type, chronology, pollen zonation and vegetational inferences are depicted for the basal sediments (567-400 cm) (fig. 18). Silty sand sediments (567-555 cm) underly a silty clay (555-512 cm). They are topped by an increasingly organic silty gyttja up to 485 cm, after which the detrital, inorganic fraction is negligeable throughout the core up to the sediment-water interface. Loss on ignition runs from 1-2‰ at the base to about 30% (dry weight) over 500 cm depth. The transition occurs between 515-500 cm, a segment over which diatoms streaks are evident in the interval between 512 and 509 cm.

79The basal pollen assemblages (567 to 503 cm) witnessed a progressive establishment of herbs then dwarf shrubs sparse enough to allow pellicular erosion of the surroundings, as shown by the sandy silt (fig. 18). An initial quasi desertic tundra (Td, sub-zones 1a and 1b) was succeeded by an herb-dominated tundra (Th, sub-zone 1c with plentiful Poaceae, Cyperaceae, Rosaceae, Fabaceae, Caryophyllaceae, Ranunculaceae, Compositae, Ericaceae) with many species of present-day arctic or alpine distribution (Salix herbacea, Oxyria digyna, Saxifraga oppositifolia, Koenigia islandica, Oxytropis maydelliana, Armeria labradorica, Polygonum viviparum and Dryas integrifolia). A shrub tundra dominated by willows and dwarf birches was then emplaced, however admitting some aspen trees (Ts, sub-zone 1d). Eventually, the afforestation proceeded with the development of aspen (sub-zone 2a) succeeded by a dominance of green alder (Alnus alnobetula subsp. crispa) and the establishment of the rest of the boreal tree flora (sub-zone 2b). Finally, a closed forest cover was attained around 10.150 ka (fig. 18).

80Being located 20 km south of the Saint-Narcisse Moraine, Petit Lac à l’Ange could have witnessed the environmental conditions before and during the readvance of the ice front. A radiocarbon date of 10,710 ± 215 BP (12,590 cal yr BP, i.e. 12.59 ka) was obtained at the average depth of 503 cm for the beginning of the afforestation stage (tab. 4, fig. 18, subzones 1d/2a). It would almost correspond to the beginning of the YD. While this is the oldest date for the sediments of Petit Lac à l’Ange, it is considered much too old because it dates the beginning of the afforestation phase and not the onset of a cold climatic episode. The date should correspond to the end of the YD, not to its beginning. Potential old carbon effect is not considered as a contamination possibility in the local geological context.

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81Obviously, the chronology of the Petit Lac à l’Ange must be revisited. Moreover, an unknown length of time spanning the development of the tundra (pollen zone 1, fig. 18) is represented by the 64 cm-long organic-poor sediments located below the 503 cm depth. The corresponding time span must be assessed.

82Calibration of conventional radiocarbon dates and AMS dating of terrestrial plant macrofossils were not available to Labelle and Richard (1981). A sedimentary interval 7 to 10 cm thick had to be used then for dating purposes. The organic-poor sediments between 567 and 503 cm could not be radiocarbon-dated at the time. In order to assess the length of time involved in this basal sediment interval, we first calibrated the existing dates (tab. 4). The large standard error of the radiocarbon dates (± 160 to ± 350 conv. 14C yr) due to the thick samples resulted in a very wide range of calibrated ages. The result for the date centered on 495.5 cm is similar to the one at 488.5 cm, although located 7 cm higher in the core. An older date centered on 495.0 cm lies between the two. The age inversion caused by the date at 495.5 cm was probably caused by sampling 2-3 parallel cores correlated by their sedimentology, in order to get sufficient material. The date at 495.0 cm is consequently rejected.

83We then plotted an age/depth curve of the remaining calibrated dates for the entire core (fig. 18, insert) and examined it along with the sedimentology and the pollen stratigraphy from 400 to 567 cm (fig. 18, main plot). The duration of the 64 cm of undated sediments at the base of the core (567 to 503 cm) can be evaluated based on the pollen concentration content (number of pollen grains per cubic centimeter of fresh sediment) measured during pollen analysis (Richard et al., 1992). One simply needs to divide the pollen concentration at the levels analyzed palynologically by appropriate pollen influx values (number of grains per square centimeter per year) for the various pollen sub-zones (various vegetation types) and to apply the resulting value (yr) to the corresponding sedimentary interval (pollen sub-zone). The sum of the durations obtained in this way is then added to the oldest reliable calibrated radiocarbon date of the profile in order to get an age for the beginning of the sedimentation in the basin. Pollen influx values for each subzone or tundra stages (pollen zone 1, sub-zones 1d to 1a, fig. 18) were estimated from the literature pertaining to the pollen influx for the various vegetation types.

84An average pollen concentration (n grains.cm3) was derived for each pollen zone and subzone of Petit Lac à l’Ange, based on the measurements made by Claude Labelle (Labelle & Richard, 1981). The successive values for the tundra sub-stages/sub-zones are 3000 grains.cm3 (1a and 1b), 35 000 grains cm3 (1c) and 47 000 grains cm3 (1d); the afforestation sub-zones record higher values: 75 000 grains.cm3 (2a) and 250 000 grains cm3 (2b); and the final forest stage (zone 3) is set to an average of 400 000 grains.cm3 (tab. 5). The average pollen influx (pollen accumulation rate: n grains.cm2.yr) over the undated inorganic sediments was recalculated for pollen sub-zone 2a (592-491 cm) and the overlying pollen sub-zone 2b (491-472 cm) of the afforestation stage by interpolation of their lower and upper boundaries on the calibrated age/depth curve (fig. 18). They correspond to the establishment of the trees around the lake.

Serge Ochietti

85To estimate the duration of the underlying inorganic sediments during the tundra subzones (1a to 1d), two scenarios of average pollen influx values were applied (tab. 5). The exercise applied to the sediments underlying pollen subzone 2a leads to unrealistic basal (567 cm) ages of 20.8 or 13.76 ka (cal kyr BP) according to the chosen scenario. This is confirmation that the basal date at 503 cm is too old and must be rejected (tab. 4). Applied to the sediments underlying pollen subzone 2b, the exercise leads basal age (567 cm) of 20.8 and 12.715 ka according to the chosen scenario (tab. 5). The last date is acceptable and could be kept as the most probable oldest age for the sediment sequence of Petit Lac à l’Ange. Moreover, this last scenario leads to an age of about 11.8 ka for the onset of the afforestation stage (base of sub-zone 2a, fig. 18), which corresponds nicely to the end of the cold YD episode around 11.6 ka (Rasmussen et al., 2014; Mangerud, 2020), taking into account the uncertainties for the estimations of pollen influxes during the tundra phase.

7.4 - The Saint-Narcisse moraine and the beginning of the Younger Dryas: a chronological landmark in the northern Charlevoix region

86The Saint-Narcisse Moraine is associated with an LIS readvance of varying magnitude at the beginning of the YD (Occhietti, 2007). The readvance process is confirmed by the cross-direction relationship between the lineations of previous flows and the pattern of the moraines in the northern parts of western (Grands Jardins Lobe) and central Charlevoix (Charlevoix and Malbaie lobes) (Rondot, 1989; Govare, 1994; Brouard et al., 2016; fig. 3). The age of the end of the Allerød/beginning of the YD is estimated at 12.86 ka, based on data from Greenland (Rasmussen et al., 2014; figs. 16 & 17). In Norway, cirque glaciers precede the emplacement of continental moraines by one to two centuries, due to the inertia of the ice sheet’s readvance (Larsen et al., 1984). The response of flora to cooling in the Eiffel, Germany, begins after a delay of 185 years compared to the age of the onset of YD in Greenland (Obreht et al., 2020). A delay of the same order of magnitude is proposed for the age of the external front of the Saint-Narcisse Moraine, dated around 12.7 ka (Occhietti, 2007). In western Charlevoix, the Savane Moraine, located 20 km to the south, is therefore a few centuries earlier. The period of time during late Allerød between the end of this moraine cluster (end of the IACP) and the beginning of the Saint-Narcisse Moraine corresponds to a series of episodes of short duration: W-E flow and the Jean-Noël Flow, rapid deglaciation, and inertia time of the YD readvance.

Serge Ochietti

8 - Synthesis of regional deglaciation events

87The events in western Charlevoix and the neighbouring regions are placed in the general context of the St. Lawrence middle Estuary during the Bølling-Allerød. The glacial dynamics of the LIS margin in this region depend on topographic factors and glacial components. Constraints to glacial dynamics as the ice sheet thinned include the structural corridor of the St. Lawrence Platform accentuated by the 400-m-deep Laurentian Channel downstream of the middle Estuary (fig. 2), the Lac des Neiges and Mount Blanchard massifs with altitudes of more than 1000 m in the Laurentians (figs. 1 & 3), the Beaupré Basin at the base of these massifs, and the semicircular depression of the astrobleme. The thickness and state of the ice (cold or close to the melting point with a water-saturated base), the position of the centres or axes of glacial dispersal on the southern part of the Québec-Labrador Dome, the separation of Appalachian ice caps, and the presence of ablation fronts accelerated by calving bays downstream of the estuary all played a role in determining the presence and location of ice streams and flows. Over time, the thickness and state of the ice depended on the climatic factors of temperature and precipitation.

88Based on the available data, we have reconstructed a series of stages defining the Late Glacial history of the St. Lawrence middle Estuary (figs. 16 & 17). The proposed correlation of events on the northern and southern edges of the middle Estuary remains to be verified by univocal chronological data. These correlations are based on the consistency of morphological data, lithostratigraphy and glacial dynamics.

8.1 - Major eastward flow

89The erosion marks of a major eastward glacial flow transverse to the Estuary (figs. 2, 19A & 19B) are observed along the entire southern edge of the Estuary (Lortie & Martineau 1987; Rappol, 1993; Lowell et al., 1990; Veillette et al., 2017) but rarely on the northern side, which was heavily eroded during subsequent glacial flows. The initial eastward ice flow shifted obliquely toward the SE in the area on the southern side of the middle Estuary (Veillette et al., 2017) and the initial flow to the SE shifted toward the ESE in the Chaudière area (Shilts et al., 2007).

90Contrary to the hypothesis of Occhietti et al. (2001a), the eastward flow does not correspond to a convergence of ice toward the calving bay located downstream in the lower Estuary or the Gulf of St. Lawrence, but is instead related to ice movement from a glacial dispersal zone of the Québec-Labrador Dome in the Lake Mistassini area, as documented by Veillette (2004).

8.2 - Phase of progressive flow convergence toward the lower St. Lawrence Estuary

91Striations to the northeast (060°) on the Ligouri Uplands (figs. 1 & 3) confirm that ice flows converged along the St. Lawrence axis, driven by topographic influences and the channeling of thick ice toward a calving bay in the lower Estuary or Gulf. The active NE flow is evidenced by a thin till cover overlying the striations, and is attributed to the arrival in the middle Estuary of the head of ice convergence toward the calving bay (figs. 19C & D) following the gradual retreat of this convergence from the Gulf toward the continental interior (Parent & Occhietti, 1999). The maritime part of the Estuary was necessarily partly deglaciated and a shelf probably developed in the downstream part of the Laurentian Channel Ice Stream. As a result, the ice cap on the Appalachian highlands of the Gaspé Peninsula (Veillette & Cloutier, 1993; Olejczyk & Gray, 2007) was gradually disconnected from this ice stream. According to Rondot (1974), Lanoie (1995) and Govare (1994), the calving bay would have gradually displaced the glacial dispersal centre of the Québec-Labrador Dome toward the highlands of the Laurentians further southwest. The expansion stages of an arm of the early Goldthwait Sea, from the Gulf to the middle Estuary, are illustrated in Dalton et al. (2020). The ice at the head of the convergence was thick. Evidence of this flow may lie in the high moraines on the southern slope of the Grands Fonds massif (fig. 2). Most of the striations identified on the rocky surfaces of islands in the middle Estuary (Dionne, 1972 and unpublished data; Légaré-Couture & Parent, 2017; Légaré-Couture, 2021) and on its southern shore (same authors; Rappol, 1993; Veillette et al., 2017) confirm the persistent glacial flow in the NE axis.

8.3 - Thinning of the ice sheet margin in the middle estuary axis and the Appalachian ice mass disequilibrium

92Accelerated ablation results in a pronounced thinning of the LIS margin at the scale of the entire St. Lawrence corridor and its margins. The Lac des Neiges and Blanchard massifs had formed a natural barrier to the flow of thinned ice from the LIS to the Charlevoix lowlands (fig. 19E). Due to the lack of ice input, these lowlands underwent rapid melting, probably toward the end of the Bølling (figs. 16 & 17). The presence of temporarily deglaciated areas is suggested by intercalations of sandy beds in the upper till of the Beaupré Basin. In the sections being studied south of the Éboulements massif, stratified sands and silts below the till cover are of undetermined age and could precede the last northward flow.

Serge Ochietti

93The thinning ice throughout the middle Estuary and on the southern edge of the Charlevoix region created a void relative to the Appalachian ice mass disequilibrium. Consequently, the northward ice flow initiated in the Gaspé peninsula extended upstream into the Appalachians of southern Québec, beyond the Chaudière Valley (fig. 2). Gradually, the ice divide moved southeastward into the northern part of Maine (Lowell et al., 1990; Shilts et al., 2007; figs. 19D, E & F).

8.4 - NIO on the northern edge of the middle estuary

94The NIO, a major regional episode, was the result of the Appalachian ice-flow reversal and the relative thinness of the ice in the Estuary. The overflow reached altitudes of up to 950 m on the periphery of Mount Blanchard and moved up the ramp of the Ligouri Uplands beyond 700 m (figs. 10 & 12). Ice invaded the lower Sainte-Anne Valley. To the west, on the Montmorency massif side, the border moraines culminated at 650 m on Mount Ferréol and Mount Sainte-Anne. Based on the associated lineations, this flow was initially rapid and represented an extension of the StLIS. The initial boundaries to the north were probably indistinct at the time of the overflow, at its contact with the ice sheet margin. The W-E ice flow area adjacent to the western limit of the Beaupré Basin (fig. 3) is related to a persistent eastward ice flow on the Montmorency massif at the initial stages of the NIO.

8.5 - Thinning during the Allerød: beginning of NIO downwasting on the margins of the Beaupré Basin

95In western Charlevoix, new or reinterpreted data show that the NIO ice separated from the ice sheet margin and dissipated from the north or northeast to the south or southwest (depending on the local context), while remaining connected to the StLIS in the south. This phase is attributed to continuous deglaciation, over 10 km, progressing from the establishment of border moraines and ravinelles to the first ridges of the La Flippe Moraine, without any significant halts in the ice front (fig. 11).

96Convergence moraines and meltwater outflows into the northern sector, at the boundary between the two types of ice, demonstrate that the ice front was also retreating northward in the Laurentians. On the margin of this thinning ice sheet, topographic highs are the first to be deglaciated, with late lobes remaining in the valleys (fig. 15). The rapid pace of the northward retreat, initially to the ridges of Lake Louis then to the first ridges of the Savane Moraine (figs. 11 & 12), is supported by the scarcity of frontal accumulations and the position of present-day lakes on topographic highs, surrounded by glacial deposits. The ice probably stagnated or remained buried in the northern part of the Sainte-Anne Valley. A deglaciated inter-marginal zone opened up, stretching between the estuary and laurentian ices (figs. 12 & 15).

97This thinning phase logically generalized on all margins of the middle Estuary. It corresponds to a mild episode of the Allerød, during the fairly long interval of the GI-1c (Greenland chronology) (figs. 16 & 17).

8.6 - Events during the Intra-Allerød Cold Period (IACP)

8.6.1 - Reactivation of the ice sheet margin: Montmorency and Gouffre SSE flows

98A major phase of flow to the SSE on the Montmorency massif circumvented the highlands of the Lac des Neiges massif, but did not cover the protected W-E ice flow area and the Beaupré Basin (figs. 3 & 19F). The Gouffre Flow appears to be contemporaneous. Logically, the vast flow to the SSE should have covered the entire northern slope of the Éboulements massif and the lower Malbaie Valley. If this was the case, it is likely that the Jean-Noël Flow and the Saint-Narcisse readvance (Gouffre and La Malbaie lobes) erased any evidence of this flow, except for a few palimpsests west of the Éboulements massif (figs. 3 & 19F).

99The SSEward Laurentide ice flow was an extension of the major late flow identified by Veillette (2004) in the Lake Mistassini region, which was reconstructed using the dispersal of erratic boulders by the Québec-Labrador Dome. It was probably active on the northern margin of the middle Estuary at the beginning of the IACP, around 13.25 ka. It marks the reequilibration of the dome, probably due to both the change in the state of the basal ice and the consequences of climatic cooling.

100It should be noted that following this study, most of the south-facing striations documented in the figures of Occhietti et al. (2001a) are now assigned to the late NNW- SSE Montmorency and southward Gouffre flows.

8.6.2 - Slowdown of the ice retreat in the Beaupré Basin and the Laurentians

101The moraines of the Beaupré Basin record a pronounced slowdown in the retreat of the overflow ice as it continued to be fed by the StLIS. Secondary lobes of the StLIS extend into the valleys along the structural edge of the Ligouri Range and on the southern slope of the Éboulements massif. This slowdown, amplified by the topography, is attributed to the IACP between about 13.3 and 13 ka (figs. 16 & 17). During this time, the overflow ice was thinning in the west, as evidenced by the presence of en echelon lateral moraines and the remnants of lake shores on the slopes of mounts Sainte-Anne and Ferréol (the Mounts Moraine). Further north, in the lower Mont-Saint-Étienne River area, a series of ridges (De Geer-type moraine landscape) suggests a proglacial lake to the north, barred by the overflow ice. In the Sainte-Anne Valley, the moraines on the right bank and the strandlines on the left bank are records of the thinning ice and the lowering of associated lateral water bodies.

102To the north, in the Laurentians, the concentric ridges of the Savane Moraine indicate that the LIS front was stabilizing and the ice was flowing to the south and southeast. This complex along the glacial margin, 40 km from the Estuary, was contemporaneous, in whole or in part, with the Chenaux and Brûlée moraines of the Ligouri Group of moraines (fig. 12), respectively situated 18 to 10 km from the Estuary along their retreat axes. The relationship with the Appalachian moraines is discussed in more detail below.

8.6.3 - Late reactivation of the Saint-Lawrence Ice Stream

103The StLIS ice flowed into small valleys (figs. 3 & 6) and lowlands of the north shore of the middle and upper estuary from the Québec City region to the Malbaie Valley (glacial striations, the Escarpment and South-Éboulements moraines,). On the south shore of the fluvial estuary, this phase is inferred from numerous striations observed in the middle Chaudière Valley across the Appalachian piedmont (Blais, 1989; Occhietti et al., 2001a; Shilts et al., 2007) (fig. 19F).

8.7 - Rapid ice retreat south of the Brûlée moraine and north of the Savane Moraine: the last GI-1a downwasting of the Allerød

104South of the Brûlée Moraine, the scarcity of glacial retreat evidence indicates that deglaciation was rapid until the opening to marine waters (figs. 3, 6 & 11). The small morainic ridge of Des Roches–Joachim marks the last clearly identifiable halt in the retreat of overflow ice and is associated with the 370 m lacustrine phase. The final accumulations of ablation till, transverse to the Sainte-Anne Valley, represent the last deposits associated with the StLIS, at an altitude of 210 to 230 m. Evidence of westward meltwater flow in paleochannels (fig. 6) confirm that the StLIS was still in place during the deglaciation of the Ligouri Uplands.

105As the StLIS thins, the ice sheet margin on the Laurentians receded northward rapidly. The discharge of meltwater from west to east through the high valleys of the Brûlée and Mont-Saint-Étienne rivers (fig. 6) suggest gradual deglaciation on the Montmorency massif. The distal and late fluvioglacial procession in the middle Sainte-Anne Valley, terminating with the Saint-Léon Delta, show that most of the upper Sainte-Anne Valley was already deglaciated during the 370 m lacustrine phase of glacial Lake Beaupré. The glacial retreat north of the Savane Moraine to beyond the Saint-Narcisse readvance front, over a distance of about 20 km in the meridian axis, was rapid, without noticeable halts, despite the west-east and Jean-Noël flows episodes spreading of the ice at a temperature close to its melting point.

106The highlands of the astrobleme were rapidly deglaciated after the passage of the Jean-Noël Flow, while a tongue of ice remained in the Gouffre Valley in connection with the StLIS until marine flooding occurred.

9 - Overview of the St. Lawrence Ice Stream in the middle Estuary

107The StLIS in the middle Estuary represents the continental and late termination of the major LCIS recognized downstream (Margold et al., 2015; Dalton et al., 2020).

108The present-day middle Estuary of the St. Lawrence, 170 km long between the Saguenay and the eastern tip of Île d’Orléans, is composed of two main parts according to data from the St. Lawrence Council (Conseil du Saint-Laurent, 2017) and NONNA bathymetric data from the Canadian Hydrological Service: the North Channel, which is narrow, discontinuous and no more than 83 m deep, running along the coast, and the shallow southern component composed of two channels with a maximum depth of 30 m (fig. 11). A wide platform less than 10 m deep lies between the southern channels and the coast, extending the vast foreshore (Dionne, 1968, 1990, 2001). Upstream, the fluvial estuary begins with two shallow and narrow channels at Île d’Orléans. Even taking into account the accumulation of post-glacial sediments in the present-day middle Estuary, the thickness of the ice stream was asymmetrical between the two shores, and thinner at the location of Île d’Orléans. The altitudes of lateral moraines and lakes lateral to the StLIS can generally be used to estimate the thickness of the StLIS, adding 10 m to the altitude on the south edge and up to more than 100 m locally on the north shore. The ice of the middle Estuary was potentially anchored to the substrate for a long time.

109Légaré-Couture & Parent (2017) mapped thousands of ice stream features along the St. Lawrence valley using Lidar imagery. They stated the highly variable ice flow directions observed in the Charlevoix and upstream Québec City and concluded that «ice flow within the streaming zone was very dynamic». As the thesis of Légaré-Couture (2021) will be made public in 2023, the following synthesis is a first attempt based on all other available data. It is now possible to differentiate several fluctuations in the StLIS of the middle Estuary:

• Convergence at the head of the main ice stream, triggered by a very distal calving bay in the Gulf of St. Lawrence, the ice remained thick (see Dalton et al., 2020);

• Differentiation of an ice stream in the middle Estuary as the ice thinned; both Laurentide and Appalachian ice converged toward a distal calving bay in the maritime estuary off the coast of Gaspésie (fig. 19C);

• Intense thinning of the ice in the middle Estuary and its margins during an episode of generalized deglaciation (figs. 19D & E);

• Major overflow on the northern edge, triggered by the Appalachian ice-flow reversal toward the Estuary depression (fig. 19E);

• Resumption of flow convergence toward the calving bay as the ice sheet continued to thin during the phase of the over-100 m downwasting; based on striations observed in the middle Chaudière Valley across the Appalachian piedmont (Blais, 1989; Occhietti et al., 2001a; Shilts et al., 2007) (fig. 19F);

• Reactivation to the NE of the StLIS, during or shortly after the SSE main flow of the ice sheet, as evidenced by the Escarpment Moraine on the north shore and by striations on the south shore. The ice stream was buttressed by the SSE ice flow and/or a major stream was initiated in the central valley of St. Lawrence. It appears that this phase is likely synchronous to the Killarney episode-IACP. Lateral to the stagnant ice of the Bois-Francs (fig. 19F), the StLIS in the fluvial and middle Estuary was exclusively of Laurentide origin and it terminated downstream at a calving bay probably wedged along the south coast.

• Formally, when the ice stream in the middle Estuary is disconnected laterally from the LIS to the north and the Appalachian ices to the south, the resulting ice tongue becomes a lobe at the SE margin of the LIS.

110The mode by which the StLIS-middle Estuary lobe ice dissipated in the middle and fluvial estuary remains poorly understood due to the lack of reliable 14C dating and direct data. The ice became thinner as the level of glacial Lake Beaupré fell from 650 to 340 m. A stabilization phase around 340 ± 20 m seems generalized based on glacial contact deposits and proglacial landforms observed in the valleys adjacent to the Estuary: south of the Montmorency massif (fig. 3), in the Gouffre Valley (lateral margin deposits at 330 m) and south of the Éboulements massif (Govare, 1994). In the Chaudière Valley region, 50 km upstream from the middle Estuary, a temporary lake more than 50 to 70 km long between the ice stream and the piedmont of the Appalachians, Lake Chaudière-Etchemin, preceded the local marine invasion (Occhietti et al., 2001a) which was initiated further downstream by an arm of the sea (LaSalle et al., 1977; Chauvin et al., 1985). Normandeau (2011) distinguishes two phases for this lake: a first level between 375 and 360 m and a second between 305 and 290 m. Also noted on the piedmont of the Appalachian are traces of a late lacustrine episode between 240 and 210 m, which briefly preceded the opening to the Champlain Sea. This temporary lake was not connected to Lake Candona. These lacustrine phases in the piedmont confirm the late presence of Laurentide ice upstream from the middle Estuary and north of the foothills, and also corroborate the marine invasion from the south in the central St. Lawrence Valley. Based on the location alone, this means the sea arm reaching the Côte de Beaupré region on the north shore does not signify the middle Estuary was completely deglaciated at that time. This lends support to the hypothesis that late ice was anchored at the location of Île d’Orléans. Also plausible is the hypothesis of an early phase of marine invasion limited to the Québec City area (the Charlesbourg phase of the Champlain Sea), as proposed by Parent and Occhietti (1988) based on the age of 12,400 ± 160 BP (GSA-1583; 15.42-14.04 ka) determined from fossiliferous till discovered by LaSalle (1978), revised later to 10,880 ± 90 BP (Beta-115870; 13.14-12.62 ka; in Occhietti et al., 2001b). A new undated marine unit, identified in boreholes by Lamarche (2011), below a till unit and the upper clays commonly related to Champlain Sea, could be the evidence of this early marine phase or of an older one. If this new unit is late Allerød in age, the clay beds would have been obscured downstream of the Québec City area by the subsequent glacial readvance.

111Up to about 370-340 m thick, the StLIS remained active. During the late phase of the middle Estuary ice, the StLIS-middle Estuary lobe continued to be fed by Laurentide ice and to flow downstream, driven by an open calving front on the Goldthwait Sea. The configuration of this calving front remains to be established, but it was probably influenced by the asymmetry of the bottom of the Estuary and by the active ice in the Saguenay axis (Dionne & Occhietti, 1996). Near the end, given the glacio-isostatic depression, it probably had the appearance of a thin glacial tongue and a partially anchored shelf, before it became dismantled.

10 - Testing the correlation between southern Québec moraines and the IACP

112Assigning the moraines in the Beaupré Basin to the IACP presupposes that the ice sheet margin can record climatic fluctuations of this magnitude. This opens up new perspectives regarding the interpretation of the moraine belts in southern Québec and neighbouring states in the USA.

113The IACP correlation is based on the age of the moraines and the climatic signal. The oscillation recorded in Greenland has now been accurately dated between 13.274 and 12.989 ka (Van Raden et al., 2013). However, it is likely that the response of glacial margins to this oscillation was spread over a longer period of time by inertia. Thus, the age range of 13.35 to 13 ka will be retained here as a chronological reference (figs. 16 & 17). The IACP is not clearly identifiable in the North American Varve Chronology established from New England lakes: “[...] after 13,800 yr b2k, [...] the signal in the varve records is delayed and attenuated because of its mediation through landscape-forming processes.” (Ridge et al., 2012).

114Chronologically, the oscillation is part of the post-glacial tundra phase recorded on the pollen charts of southern Québec (Richard, 1994a,b; Elkadi, 2013; Chapdelaine & Richard, 2017), which explains why it cannot be clearly identified on the diagrams. The reactivation of Laurentide ice, represented by the south-southeastward Montmorency and southward Gouffre flows on the north shore of the Estuary and the Beauce Event on the south shore, is a major glacial signature with climatic impact.

115The southernmost moraine belts of Québec, Frontière and Dixville-Ditchfield, are thought to predate the IACP based on their currently accepted ages of 13.5 ka and 13.4 ka, respectively (Parent & Occhietti, 1999; Ridge, 2003; Richard & Occhietti, 2005; Ridge et al., 2012; Thompson et al. 2017; Chapdelaine & Richard, 2017) (figs. 16 & 17). The Sutton–Cherry River–East Angus–Mégantic Moraine Belt, with an estimated age of 13.3 ka, is the best developed of the regional belts (fig. 2). Associated with the Beauce Event (Occhietti et al., 2001a), it corresponds to a well-defined period of stabilization. It is locally composed of several concentric end moraines. A lake-bottom end moraine identified on the acoustic profiles of Lake Massawipi (Turgeon et al., 2003) is attributed to the position of the Lake Memphremagog Va glacial front of Boissonnault & Gwyn (1983), itself correlated with the Cherry River recurrence (Boissonnault & Gwyn, 1983; Parent, 1987). If a climatic origin is accepted for the Beauce Event and the SSEward flow of Laurentide ice, then the Sutton-Mégantic Belt would correspond to the beginning of the IACP. The 50-year gap between the estimated age and the start of the GI-1b interstadial phase remains negligible but should be verified.

116The Mont Ham–St-Ludger Moraine Belt (Parent, 1987), located 24 to 17 km further back to the northwest, likely represents a later phase of the IACP around 13.2 ka. It is discontinuous and less defined than the Sutton–Cherry River–East Angus–Mégantic belt. The northern part, identified as the Saint-Ludger Moraine, marks the stabilized boundary of the Bois-Franc residual Appalachian ice (Normandeau, 2010).

117The Ulverton-Tingwick Moraine, another 16 to 20 km further back from the Mont Ham–St-Ludger belt, was deposited approximately 100 years before the Lake Candona outflow / Champlain Sea invasion into the central St. Lawrence River Valley (Parent & Occhietti, 1999) dated 13-12.9 ± 0.1 ka. This moraine, located 70 km south of the YD Saint-Narcisse Moraine, may correspond chronologically to the end of the IACP or later, depending on the mode of deglaciation in the St. Lawrence Valley. Turgeon et al. (2003) estimate the gap between the Cherry River and Ulverton-Tingwick moraines to be at least 250 years.

118To the northeast of these moraine belts, the stagnant Bois-Francs ice (Parent & Occhietti, 1988; Occhietti et al., 2001a), disconnected from the ice sheet and without dynamic input from lateral Appalachian ice, persisted while the moraine belts were deposited. The influence of a cold phase, such as the IACP, would explain this longevity.

119In the middle and lower valley of the Chaudière River, the last moraines on the piedmont of the Appalachians immediately precede or follow the path of the marine invasion and thus seem relative to the IACP or postdate it. Downstream, on the south shore of the middle Estuary, in the Bas-du-Fleuve area facing the Charlevoix region, and further downstream along the coast of the lower Estuary, moraines are dated by marine shells (Dionne 1972; LaSalle et al., 1977; Martineau, 1977; Rappol, 1993; Hétu, 1998; Hétu & Gray, 2000; Veillette et al., 2017). Some predate the IACP, but detailed analysis is needed. Further along the north coast, in the Gaspé Peninsula, several marine deltas at the termination of glacier tongues from the Gaspé ice cap have been dated at 11,700 BP and are cautiously associated with the IACP (Hétu & Gray, 2000).

120In Maine, loss-on-ignition diagrams for several lakes indicate an episode of lower organic matter content that precedes the YD episode and is cautiously attributed to the IACP. The 14C ages between 11,790 ± 395 BP (12.91-14.97 ka) and 11,620 ± 100 BP (13.3-13.62 ka) frame the episode (Dorion et al., 2001).

121Correlations still need to be verified in the Lake Champlain lowlands of New York and Vermont where there is a lack of continuity with the moraines of Québec. The glacial margins of Ellenburg-Plattsburg and Enosburg Falls (Franzi et al., 2016) may correspond in whole or in part to the IACP, based on their relative position and the proposed age of 13.2-13.1 ka for the Enosburg Falls margin.

122In the western part of New York State, continental material (wood and bone) from the late-glacial readvance of the Fowlerville Moraine Complex in the Genesee Valley has been dated at 13.5-12.9 ka (Young & Owen, 2017). Taking into account the distance of this moraine to the paleo-shores of glacial Lake Iroquois in the Ontario Basin, the readvance corresponds to either the IACP or a glacial flood without a climatic cause.

11 - Remaining issues

123The change in the deglaciation model whereby the StLIS becomes the main component warrants a review of all the data from the northern edge of the middle Estuary.

124Downstream, in the Charlevoix astrobleme region, the extent of the SSEward ice flow, the nature of the Rochette and Grands Fonds high moraines, the interpretation of the undated lithostratigraphic units of the southern edge of the Éboulements massif (Occhietti & Dionne, 2008), and the chronology and extent of the marine invasion are aspects that need further study to complete the overall portrait of the middle Estuary.

125Upstream, the extension of the northward flow in the Québec City region and on the edge of the Montmorency massif has not yet been defined. The ice-margin deposits, identified on the DEMs, are subsequent to the NIO and probably contemporaneous with the Beaupré Group of Moraines, but further study is required.

126In the Laurentian highlands of the Charlevoix region, the presence of rock glaciers on the slopes of the upper Sainte-Anne Valley and small moraines of cirque glaciers (unpublished text of J. Rondot and the first author; Govare, 1994) support the hypothesis that ice or snow and associated late deposit accumulated during the YD. The presence of such glaciers has been documented in Maine (Dorion, 2002; Dorion et al., 2001) and Nova Scotia (Mott et al., 1986; Stea et al., 1998; Stea, 2004; Stea & Mott, 2005). In Charlevoix, the cirque valleys correspond to a long-term glacial heritage and indicate a repeated erosional process during past englaciations and transitional phases. An inventory and a study of lake-bottom sediments are still needed.

127On a larger scale, assigning moraines to the IACP (both north and south of the Estuary) paves the way for correlations with neighbouring regions in the Atlantic Provinces, New England, New York State, and Ontario. A vast program of dating by applicable methods is essential (14C of basal vegetal fragments, OSL, cosmogenic nucleides), in addition to detailed pollen analyses of late glacial units. Was the SE margin of the LIS sensitive to the oscillation? The chronology of the Appalachian ice-flow reversal, the final ablation of the middle Estuary lobe, and the displacement of the ice divide onto the Appalachian Mountains still need to be addressed. Was the Ontario Ice Stream (Ross et al., 2006) in the upper fluvial valley of the St. Lawrence synchronous (Occhietti et al., 2007) with the NIO in the middle Estuary or the late reactivation of the StLIS? How did the meltwaters circulate compared to those of the StLIS? Further study into these issues would strengthen the relationship between these events and global phenomena such as the IACP, the YD, the influx of cold water and debris into the North Atlantic, and fluctuations in Greenland.

CONCLUSION AND PARADIGM REVISION

128The Beaupré Basin on the north shore of the St. Lawrence middle Estuary is located between the Laurentian highlands, where maximum altitudes exceed 1000 m, and the St. Lawrence structural corridor. DEM analysis has revealed that this setting favoured the preservation of exceptionally detailed records of fluctuations in the LIS margin during the Bølling-Allerød. The Late Glacial in this basin and its periphery is characterized by:

• The LIS margin composed of two types of ice: estuarian StLIS ice and Laurentide ice on the highlands and the periphery;

• The differentiation and then prolonged persistence of the active ice stream in the axis of the St. Lawrence Estuary, until the marine invasion;

• An early northward overflow from this ice stream (the NIO of the middle Estuary) locally attaining altitudes of 950 and 750 m along the steep structural slope of the Laurentians;

• Major flow to the SSE of the ice sheet (Montmorency and Gouffre flows) around the Beaupré Basin without invading it;

• Gradual north-to-south retreat of the overflow ice, synchronous with the persistence of the StLIS, with a slowdown phase recorded by the Beaupré Group of Moraines (the Ligouri, Ferréol Lowlands and Mounts moraine clusters);

• A concomitant general gradual retreat of ice in the Laurentians from south to north and a thinning of the western (Montmorency massif) and eastern (Gouffre Valley) LIS margin;

• An early deglaciated inter-marginal zone, isolated in the SE margin of the LIS before the dissipation of the StLIS and the marine invasion;

• At the scale of the northern margin of the middle Estuary, ice intrusions into the secondary valleys along the Ligouri range (Escarpment Moraine), late northward ice flow on the astrobleme (South-Éboulements Moraine which is distinct from the Rochette Moraine) and later ice margin features south of the Montmorency massif imply very active ice stream dynamics;

• In the Laurentians, late NW-SE flow and the eastward Jean-Noël Flow, north of the deglaciated inter-marginal zone, preceding the Saint-Narcisse readvance.

129The new regional chronology is based on Greenland Ice Core Chronology 2005 (GICC05) and the New England Varve Chronology. It challenges that of Occhietti et al. (2001a), which was based on an estimated age of the Champlain Sea, now known to be erroneous. Events in the fluvial and middle Estuary region appear to be much more recent and of shorter duration. In the Beaupré Basin and its periphery, the re-estimated age of the basal sediments of Petit Lac à l’Ange (ca. 12.75 ka) serves as a chronological reference point for local deglaciation. It predates the beginning of the regional YD (Saint-Narcisse Moraine) to the north and is later than the cold phase of the Beaupré Group of Moraines to the south and the Savane Moraine to the west. The latter formed a few centuries earlier and thus correlate with the IACP (episode GI-1b, between 13.26 and 13.050 ka). The first ridges of this group, at the beginning of the IACP, are most likely contemporaneous with the Montmorency Flow to the SSE and the Cherry River–East-Angus Moraine Belt. This younger framework makes it possible to better assess the age of the regional NIO and the Appalachian ice-flow reversal that causes it. The Appalachian reversal is diachronic from the Gaspé ice cap to the level of the fluvial estuary. As this reversal is the consequence of pronounced thinning of middle Estuary ice attributed to the Bølling, the age of the early NIO in the middle and fluvial estuary can be reasonably placed between the middle Dryas (Dryas II) and the beginning of the Allerød, i.e. a maximum of 800 cal y before the start of the IACP.

130When integrated with directional and deglaciation data from the south shore of the middle and fluvial estuary, the original data from this study provide a new portrait of glacial dynamics from the Last Glacial Maximum to the Lake Candona outflow/Champlain Sea invasion, shedding new light on some of the interpretations of Occhietti et al. (2001a). The glacial dynamics of the SE ice sheet margin in the fluvial and middle estuary region are complex. They are influenced by several factors: the position of ice dispersal areas on the Québec-Labrador Dome to the north and northwest, accelerated ablation caused by the migration of a calving bay from the Gulf to the Estuary in the northeast, the later gradual separation of Appalachian ice in the south, and, finally, the steering influence of topography as a function of the thinning ice. The main episodes of glacial dynamics and retreat as they relate to the phases of the StLIS in the middle Estuary are as follows:

• A major flow, southeastward and transverse to the St. Lawrence corridor, moves onto the Appalachians of Québec and Maine. This flow is not associated with convergence toward the calving front. The age of this episode is usually related to the LGM or later and precedes the Bølling;

• The retreat of the head of convergence related to the calving bay leads to a gradual thinning of the ice, followed by the differentiation of a wide stream in the middle Estuary axis (flow phase toward the NE in the Ligouri Highlands). This episode, which takes place in a still-thick ice sheet margin, is probably pre-Bølling or very early Bølling;

• A major thinning of the ice sheet margin, most probably occurring during the Bølling optimum, leads to Appalachian ice imbalance and lower inputs of Laurentide ice in the Beaupré Basin and the lowlands of the Charlevoix astrobleme;

• The gradual upstream migration of Appalachian outflow to the fluvial estuary, eventually triggers an overflow on the northern margin of the middle Estuary, probably between late Bølling or the middle Dryas (Dryas II) and the beginning of the Allerød. The Beaupré Basin is invaded by this NIO;

• A major SSE ice sheet flow (Montmorency and Gouffre flows) causes the Beauce Event to occur on the south shore. The ice stream in the middle Estuary is strongly reactivated (Escarpment and South-Éboulements moraines);

• The glacial retreat is marked by significant halts, represented by the Beaupré Group of Moraines and the Sutton–Cherry River–East Angus–Mégantic and Mont Ham moraine belts. These halts are attributed to the IACP, between about 13.3 and 13 ka. Concurrently, the ice stream in the middle Estuary thins, as defined by strandline phases 650 m, 550 m, 440 m and 370-340 m of glacial Lake Beaupré to the north and two levels of Lake Chaudière-Etchemin to the south, while staying fed by Laurentide ice. To the north, in the Laurentians, the deglaciation zone between the NIO and Laurentide ices widens; the Savane Moraine corresponds to the IACP or a part of it;

• The incursion by Atlantic waters is late compared to regional deglaciation, due to the persistence of the StLIS becoming a lobe of the LIS margin.

131Assigning the Beaupré and Savane moraine groups to the IACP opens up a new perspective for the interpretation and dating of the moraine belts in southern Québec and in the northern parts of the neighbouring states of New York and Vermont. The SE margin of the LIS is sensitive to fluctuations, which introduces the prospect of inter-regional correlations of isochronous events and dynamic readjustments. The ice stream of the St. Lawrence middle Estuary was a major entity on the SE margin of the LIS during the Bølling-Allerød.