ABSTRACT: Les rythmites tidales de la transgression Laurentienne, moyen estuaire du Saint Laurent (Quebec) [ bout the mid-Holocene muddy tidalites with no coarse debris, middle St. Lawrence estuary (Quebec)].

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Mises en place lors de la Transgression laurentienne (6 a 5 ka BP) dans le secteur amont du moyen estuaire, les rythmites inter¬tidales limono-sableuses exposees dans la partie superieure des diverses coupes examinees ne contiennent pas de sediments detri¬tiques plus grossiers que du sable fin a moyen, alors que la batture vaseuse et argileuse actuelle est couverte de cailloux de tailles variees allant du galet au mega-bloc. Bien que peu abondants, on trouve, par contre, des cailloux clans les trois fades intertidaux plus recents (0,1 a 0,9 ka) exposes dans la micro-falaise du schorre superieur en erosion, a Montmagny et a Sainte-Anne-de-Beaupre. Labsence de cailloux dans les rythmites intertidales de 1'Halocene moyen demeure difficile a comprendre et a expliquer. Lhypothese d'une periode plus chaude sans couvert glaciel hivernal est seduisante mais peu plausible. On pense plutot que, lors de la Transgression laurentienne, la batture argi¬leuse actuelle etait recouverte d'un depdt semblable a celui observe a la base de la falaise vive de la terrasse de 8-10 m a Montmagny, date de 8 a 7 ka BP. Si tel etait le cas, les cailioux residuels reposant sur la surface d'erosion taillee dans I'argile de la Mer de Goldthwait, entre 9 et 8 ka, n'etaient pas exposes a I'action des glaces d'estran ni a celle des vagues et des courants de maree. Posterieurement a femersion des terres (apres 4 ka BP), I'erosion aurait erode le depot limono-argi¬leux au-dessus de la surface argileuse et expose les cailloux concen¬tres sur la surface d'erosion taillee dans le substrat argileux. Les cailloux actuellement visibles sur la batture comprennent donc ceux mis on place avant la Transgression laurentienne et ceux qui se sont ajoutes au cours des derniers millenaires. [ABSTRACT: About the mid~Holocene muddy tidalites with no coarse debris, middle St. Lawrence estuary (Quebec). Deposited during the Laurentian Transgression (6 to 5 ka BP), the tidal rhythmites, exposed in various localities of the upstream area of the middle St. Lawrence estuary do not contain any debris coarser than fine to medium sand, whereas the modern muddy and clayey tidal flat is covered by abun¬dant clasts of various size ranging from pebbles to mega-boulders. Although less frequent, there is coarse debris in the three recent (0.1 to 0.9 ka) intertidal facies exposed in the microcliff of the high marsh at Montmagny and Sainte-Anne-de-Beaupre. The absence of coarse debris in the mid-Holocence tidalites remains difficult to understand and explain. The existence of a warmer climate without an ice cover in winter at the time is interesting but unlikely. It is suggested instead that when the Laurentian Transgression occurred, the clay substrate of the modern tidal flat was covered by a fine grained deposit similar to the silty-clay deposit exposed at the base of the cliff cut into the 8-10 m terrace at Montmagny which is dated 8 to 7 ka. In this case the coarse debris lag characterizing the erosion surface cut into the Goldthwait Sea clay was not exposed; consequently, coarse debris were not available for reworking by shore ice, waves and tide cur¬rents. Following land emergence after 4 ka BP, erosion of the silty¬clay deposit exposed the former coarse debris lag whereas additional coarse material was most likely added during the last millennium. Consequently, there is a great contrast between the mid-Holocene tidalites and the modern tidal flat deposit.].

Bibliography of Canadian Geomorphology