Relative sea-level changes in the St. Lawrence Estuary from deglaciation to present day

Abstract

The St. Lawrence estuary was deglaciated between 13 and 12 ka. The south shore was ice free by 12 ka, whereas some areas of the north shore (Baie-St-Paul, La Malbaie, Tadoussac, and Baie-Comeau) were partly ice covered until 11.5-11 ka. On the south shore, however, the northern margin of the remaining Appalachian ice was generally situated close to (2-3 km) the present-day shoreline. Depressed coastal areas were progressively submerged by the Goldthwait Sea. On the south shore, its maximum level ranges between 110 m at Matane and 180 m at Lévis, although the altitude does not increase upstream in a regular fashion as previously thought. A similar pattern characterizes the north shore. The maximum postglacial sea level is 170 m at Baie-Comeau, 140-150 m at Tadoussac, 175 m at Baie-St-Paul, and 190 m at Québec City. Because the relative sea level and the isostatic recovery were not synchronous on both shores, geologic events should be examined separately at least for the first half of the Holocene. On both shores, however, the isostatic recovery was relatively rapid; more than 85% occurred during the first 3 k.y. after deglaciation. On the south shore, the relative sea level was close to present day ca. 8-7 ka; a lowstand (ؓ10 m) occurred between 7 and 6 ka and was followed by the 10-12 m Laurentian transgression between 5.8 and 3.5 ka. Evidence for these two events has been collected from 11 localities between Québec City and Matane. Field work in progress on the north shore between Québec City and Baie-Comeau also provides evidence for these two events. The last important relative sea level fluctuation is related to the Micmac shoreline (ؓ6 m level), an event dated as 2.5-2 ka on both shores except at Tadoussac where it is slightly younger (1.4 ka). The lowstand and the Laurentian transgression, which occurred during the Holocene, are possibly related to the northward migration of the forebulge and to the disintegration of the Laurentide ice sheet remnant in central Québec during the hypsithermal period in southeastern Canada, but may also be related to tectonic activity in an area of high seismicity.

La terrasse Mitis à la pointe de Mille-Vaches (péninsule de Portneuf), rive nord de l’estuaire maritime du Saint-Laurent : nature des dépôts et évolution du niveau marin relatif à l’holocène

“…Entre 6 et 6,5 ka, le NMR était vraisemblablement voisin du niveau actuel, du moins si on se base sur les dates au 14 C obtenues sur des myes (Mya arenaria) en position de vie dans une couche de vase argileuse au-dessus du substrat argileux érodé, le long de l'estuaire de Petites-Bergeronnes (Dionne, 1996b). À la pointe de Mille-Vaches, les indices d'une remontée du NMR après le bas niveau de l'Holocène moyen mis en évidence dans plusieurs sites de la rive sud de l'estuaire (Dionne 1988(Dionne , 2001(Dionne , 2002bDionne et Coll, 1995 ;Dionne et Pfalzgraf, 2001) sont rares. Seule la date de 3870 ± 60 BP (UL-1920) sur des valves de clovisses arctiques (Mesodesma arctatum), trouvées dans une poche sableuse entre des couches déformées du substrat argileux du glissement de terrain relique, permet d'affirmer la présence, quelque part dans la péninsule de Portneuf, d'un dépôt sableux infratidal, en autant que ce dépôt repose bien en discontinuité sur la surface d'érosion correspondant au bas niveau de l'Holocène moyen.…”

“…Néanmoins, cette coupe renseigne sur l'édification et l'âge de la terrasse Mitis. Bien que la surface d'érosion dans l'argile de la Mer de Goldthwait ne soit pas adéquatement datée, nous croyons qu'elle a été taillée lors d'un bas niveau survenu au milieu de l'Holocène et que, à l'instar des événements survenus sur la rive sud (Dionne, 1998(Dionne, , 2001, le bas niveau a été suivi d'une remontée. Les données obtenues à partir des coupes de Ragueneau et de la péninsule de Manicouagan-Outardes (Bernatchez, 2003) devraient permettre sous peu de retracer les principales étapes du NMR sur la Haute Côte Nord.…”

Shore Platform Processes in Eastern Canada

“…4). Present changes in relative sea level are poorly known in Gaspé, and the data derived from tidal gauges in different locations are often contradictory, although geomorphological evidence suggests that relative sea level is rising slowly today (Dionne, 2001).…”

“…Historical and tidal gauge records indicate that relative sea level has risen by 40 cm in the last 100 years. Dionne (2001) reasserted that glacio-isostatic uplift has generally The difference between higher high water (HHWLT) and lower low water (LLWLT) at large tides…”

Propriétés magnétiques et sédimentologiques de séquences holocènes de l'estuaire du St-Laurent et du fjord du Saguenay, Canada /

“…During the last millennium, glacio-isostatic uplift of the land averaged a few millimeters per century, whereas twentieth century tide-gauge data show no consistent trends in sea-level (cf. Dionne, 2001 and references therein). core and with a 14 C production rate record ) indicate that core 2220 chronology is in good agreement with the GISP2 chronology and the tree-ring chronology from .…”