Observations géomorphologiques sur les méga-blocs d'un schorre à Spartina alterniflora, estuaire maritime du Saint-Laurent, Québec / Geomorphic observations on the mega-boulders in a Spartina alterniflora tidal marsh, Lower St. Lawrence estuary, Québec

摘要

Abstract At Isle-Verte, a locality on the south shore of the Lower St. Lawrence estuary, the clayey tidal flat is composed of a salt marsh characterized by a large number of erratics including boulders larger than 100 cm, most of which being a lag of an eroded Goldthwait Sea clay deposit dated ca. 10 to 12.5 ka. Over 80% of the mega-boulders observed are located in the low marsh area covered by Spartina alterniflora. Of the 308 boulders measured, Precambrian (igneous, metamorphic and volcanic) from the Canadian shield, located on the north shore of the estuary, about 30 km from Isle-Verte, represent 73% of all lithologies, whereas the remaining 27% are Appalachian local lithologies (sandstone, quartzite and conglomerate). Most mega-boulders are subrounded (56.5%) and subangular (37%) in shape. The largest and the smallest mega-boulders measured and weighed respectively 600 x 360 x 145 cm (58.7 tons) and 205 x 105 x 30 cm (1.2 ton). The median weight of mega-boulders is 7 tons, while those weighing between 5 and 10 tons account for 42.8% and those between 15 and 50 tons, for 10%. Over 11% of the mega-boulders were lying directly on the eroded surface in the intertidal zone, whereas about 73% were still partly buried in the clay substrate. In 1996, we observed 34 mega-boulders (12%) that have been displaced by shore ice on distances ranging from one to 68 m. The largest boulder moved by ice over 30 m weighed approximately 50 tons.