ABSTRACT: La terrasse Mitis a la pointe de Mille-Vaches (Peninsule de Portneuf), rive nord de l'estuaire maritime du Saint-Laurent : nature des depots et evolution du niveau marin relatif a la holocene.[The Mitis terrace at Pointe de Mille-Vaches (Portneuf Peninsula): Nature of deposits and evolution of the relative sea leve during the Holocene].

CGRG Bibliography of Canadian Geomorphology
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La basse terrasse a I'extremite sud-ouest de la peninsule de Portneuf, sur la Haute Cote-Nord du Saint-Laurent, correspond a la terrasse Mitis presente dans quelques autres localites de la rive nord de festuaire. Entierement composee de materiaux meubles detritiques, elle comprend plusieurs unites, la plupart sableuses, d'une 6paisseur superieure a 3 m. Mises en place en milieu littoral et inter-tidal, ces unites reposent sur un depot de sable fin gris, infratidal, luimeme susjacent au substrat limono-argileux depose dans la Mer de Goldthwait, il y a >9,5 ka. Un depot de limon sableux, gris, lamine, contenant des debris de plantes en place correspondant a un facies de schorre inferieur, coiffe la sequence sableuse. Par endroits, ce depot est lui-meme recouvert d'une couche de tourbe de 25-30 cm d'epaisseur. Lunite a la base de la falaise active a ete mise en place entre 1990 t 60 et 2740 ą 70 BF Les unites sableuses au-dessus datent de 1570 ą 60 a 1880 f 90 BP, alors que I'unit6 limoneuse a facies de schorre inferieur a donne un Age au '°C allant de 1570 t 50 a 1970 f 70 BP Lage median de la terrasse Mitis est de 1880 ą 90 BP, alors que si on prend en compte uniquement les dates pour I'unite limoneuse a facies de schorre inferieur, il est de 1830 ą 60 BP. L6dification de la terrasse Mitis a la pointe de Mille-Vaches est donc contemporaine de la plupart des sites des deux rives de I'estuaire. Le substrat argileux de la vaste batture en face de la terrasse, qui se prolonge sous celle-ci, a ete erode lors d'un bas niveau marin pendant I'Holocene moyen qui fut suivi d'une remontee du niveau de quelques metres. Un glissement de terrain historique, survenu vraisemblableŹment lors du seisme de 1663, caracterise la partie superieure de I'estran en face de la falaise active. [The Mitis terrace at Pointe de Mille-Uaches (Portneuf Peninsula), North Shore of the Lower St. Lawrence estuary: nature of deposits and evolution of the relative sea level during the Holocene. The low terrace at the SW extremity of the Portneuf Peninsula, on the north shore of the Lower St. Lawrence estuary, is an equivalent of the Mitis terrace occurring at a few other localities. The terrace is entirely made of detritic sediments. The various units, mostly fine to coarse sand deposited in the shore zone, are over 3 m in thickness. These sediments appear to unconformably overlie a subtidal dark grey fine sand resting on a surface eroded into a Goldthwait Sea clay deposit older than 9.5 ka. At the surface of the terrace, there is a grey, laminated sandy silt deposit with plant remains in situ (a lower marsh facies), locally covered by a thin (25-30 cm) layer of peat. The sand unit at the base of the studied exposure has been dated 1990 ą 60 to 2740 ą 70 BP. The overlying sandy units are dated 1570 ą 60 to 1880 ą 90 BP, whereas the silt unit at the surface is dated 1570 ą 50 to 1970 ą 70 BP. The median age of the terrace is 1880 ą 90 BP, whereas if we use only the '°C dates for the silt unit, it is 1830 ą 60 BP At Pointe de Mille-Vaches, the Mitis terrace is thus contemporaneous with most localities on both shores of the estuary. The substrate of the wide tidal flat in front of the Mitis terrace (an erosion surface underlying the terrace) was cut into the marine clay during a mid-Holocene lowstand, which was followed by a rising sea level of a few meters. An historical landslide, which possibly occurred during the 1663 seismic event, characterizes the upper part of the tidal zone in front of the active cliff].

Bibliography of Canadian Geomorphology