Between 7000 and 6000 yr B.P., relative sea level was as much as 5 m lower than today in the St. Lawrence estuary, Québec. A small transgression (Laurentian transgression) occurred between 5800 and 4400 yr B.P., which resulted in the construction of an 8- to 10-m terrace. About 3000 yr B.P., relative sea level was similar to present, and then a stillstand or a slight rise occurred during which a cliff (Micmac cliff) was cut into the emerged terrace. During a subsequent lowering of relative sea level (coastal emergence), a low aggradational terrace (Mitis terrace) was built between 2300 and 1500 yr B.P. at the base of the Micmac cliff. A new emergence curve for the south shore of the St. Lawrence estuary showing a mid-Holocene high stand of relative sea level is therefore proposed.
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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.
The Québec North Shore Moraine System, extending over 800 km between Manicouagan River and Kenamu River (Labrador), is the longest system known in that part of the Canadian Shield. It is lobated and well defined by a series of parallel till ridges, areas of hummocky ice-contact deposits, proglacia! deltas and outwash plains. It is located inland from a few kilometers to about 200 km from the North Shore of the Gulf of St. Lawrence. Between the Manicouagan and Romaine Rivers, the front is at a distance of 25 to 40 km from the southern margin of the Shield except in the Sept-IIes area, where it is less than 10 km inland. From the Romaine River, the front curves northeastward to the Mecatina River, 160 km northwest from the coastline. From there, it trends to the north and then to the northwest at which point it is more than 180 km from the St. Lawrence and over 275 km from the Labrador coast.
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