Late Winconsinan glacial dynamics, deglaciation, and marine invasion in southern Quebec

Abstract

Deglaciation patterns of the Laurentide ice sheet in southern Québec were related to climatic and nonclimatic factors. Thinning of the ice sheet and thermolatitudinal ice retreat are directly linked to the global warming at the end of late Wisconsinan time, between 17 ka and 11 ka. However, correlations between regional deglacial events and global climatic oscillations during that period have yet to be established, except for the St. Narcisse Moraine event, which has been assigned to the Dryas III, and perhaps for the reactivation of Laurentide ice in the middle Chaudiere Valley area during an older cold event. Nonclimatic factors also played a major role on the deglaciation of the region. After the last glacial maximum, the Laurentide ice sheet began to decrease, and the St. Lawrence corridor channelized a major ice stream, the St. Lawrence ice stream, which became a major feature of the southeast sector of the ice sheet. The St. Lawrence ice stream is a flow convergence zone caused by a combination of ice dynamics and topographic factors and rapid ablation at its terminus. The head of the flow convergence migrated deeply into the Laurentide ice sheet and caused thinning of adjacent ice masses. As a consequence of this accelerated ablation, an Appalachian sector became differentiated from the main ice sheet. Regionally, the terminus of the ice stream was a calving bay that retreated along the Laurentian channel to the mouth of the Saguenay fjord. The ice stream and the deglaciated estuary generated the well-known flow reversal along much of the northern margin of the Appalachian sector. In addition to these generalized deglaciation processes, local and regional topographic features influenced the ice dynamics and the final deglaciation patterns.

Ice-Flow Chronology and Palimpsest, Long-Distance Dispersal of Indicator Clasts, North of the St. Lawrence River Valley, Quebec

“…Striations were measured at 11 sites along the shoreline. The most common pattern shows a first ice movement toward 160°-170°crossed by a younger one toward 140°-150°, suggesting a late shift toward the east due to drawdown (Occhietti et al, 2001) as one approaches the St. Lawrence River Valley (Fig. 8).…”

The Saint-Narcisse morainic complex and early Younger Dryas events on the southeastern margin of the Laurentide Ice Sheet

“…mostly to a previously increased ablation process resulting from converging ice flow towards the St. Lawrence Estuary. The pre-Saint-Narcisse west-east ice flow in Charlevoix (Rondot, 1974;Govare, 1995;Lanoie, 1995;Fournier, 1998;Occhietti, 2001) represented the northern converging side of the St. Lawrence Ice Stream which drained the LIS ice in the St. Lawrence Estuary and Gulf (Parent and Occhietti, 1999;Occhietti et al, 2001b).…”

Late Wisconsinan Sedimentation in the Québec City Region: Evidence for Energetic Subaqueous Fan Deposition During Initial Deglaciation

“…), northwest paleoflows in the high-energy subaqueous fan facies suggest that both the grounding line and the meltwater source were located updip to the south. Candidate meltwater sources include Lake Candona, or Lake Chaudière-Etchemin, which was located in the Chaudière-Etchemin Valley (see Occhietti et al, 2001b), or possibly even the Champlain Sea after it invaded the Lake Candona basin. Development of a body of water in the valley axis west of Québec City would have effectively severed the pressure gradient that maintained southward flow in subglacial streams that fed these ice-ponded glacial lakes (see esker data in Warwick area on Fig.…”

“…Deglaciation east of Québec City was marked by the development of a northeast flowing ice stream in the valley axis. Between ~13 500 and ~12 000 14 C BP, an arm of marine water, possibly in the form of a calving bay (Thomas, 1977), migrated inland against the ice stream from the Atlantic Ocean to at least the Saguenay River and possibly to a position near Québec City, forming the Goldthwait Sea (Stea et al, 1998;Occhietti et al, 2001b). This effectively isolated a large ice mass over the Appalachians from the Laurentide Ice Sheet (LaSalle et al, 1977;Chauvin et al, 1985).…”

“…The deglacial history of the Québec City region is poorly understood, and has proven difficult to integrate into larger basin-scale deglacial models of the St. Lawrence Valley (LaSalle et al, 1977;Chauvin et al, 1985;Occhietti, 1988, 1999;Occhietti et al, 2001b). During deglaciation, the greater Québec City region acted as a gateway for major meltwater discharges into the Atlantic Ocean Occhietti, 1988, 1999;Rodrigues and Vilks, 1994), experienced complex ice margin fluctuations (Blais and Shilts, 1989;LaSalle and Shilts, 1993;Fournier, 1998;Paradis and Bolduc, 1999;Bhiry et al, 2001), was transgressed by the Champlain Sea (Occhietti et al, 2001a), and underwent a subsequent episode of isostatic rebound and base level fall that forced shorelines to regress to their present day position.…”