Abstracts
Abstract
The shoreline between Les Lscoumins and Baic-des-Bacons. on the North Shore of the Lower St. Lawrence, is characterized by a wide clayey tidal flat (800 to 2 000 m) and a clifT cut into the 30-m tcnace made of (Quaternary unconsolidated deposits. The shore zone is covered hv thousands of boulders of various lithologies and sizes, including mega-bouldcrs scattered throughout the tidal flat, small boulders (25 to 40 cm) commonlv forming pavements at the surface of the tidal flat, and boulders of various sizes on the beach at the base of the cliff.
Most erratics are Precambrian igneous and metamorphic rocks from the adjacent Shield area, however, dolostonc erratics arc also a significant component. In an area 18 km long, 3661 dolostonc erratics were observed and measured There are various types of dolostonc including 30 slromaiolitic dolostonc erratics; most are grey (74%) and pink (20%). Among other characteristics, 28% of the erratics arc striated and about half are surrounded: small size erratics (I to 10 kg) account for about 59%. those 10 to 50 kg. for 30%, those 50 to 100 kg. 7%. while 4% weigh over UK) kg. the largest boulder weighing two tons. Die two largest slromaiolitic dolostonc erratics weigh respectively 28(1 and 175 kg.
There arc four areas in the Precambrian Shield were dolostonc formations occur: the sedimentary basin of Mistassini. the east coast of Hudson Bay. between Long Island and Inukjuak. the Labrador Trough, and a wide area in the Grenvillc between Hull and Lake Bastong. The most likely source of the dolostonc erratics observed at I es Lscoumins is the Mistassini sedimentary basin Ihis interpretation implies a major ice flow lo the SL during the Wisconsinan from an ice dome or an icedevide located to iheNW of Lake Mistassini The erratics occurring in the area ofl.es Lscoumins were likely first transported by a glacier via ihc Lake St. Jcan-Sagucnay corridor before being released in the Goldihwait Sea most probably by icebergs or possibly by a nearby ice shelf. Distances of displacement range from 400 lo 500 km Man> erratics may also have been moved locally by shore ice during the Holoccne
RESUME
l,c rivage cntrc Les Lscoumins et Baie-des-Bacons. sur la Gdte Nord du Saint-l.aurcni, est caractcrise par unc large batture (800 a 2000 m) argileuse et unc falaisc entail lam ta terrasse de 30 ni constiiuec de depots meubles quatemaircs. II est parscm*" de millicrs de cailloux. de lithologies et dc tail les varices, formant des dallages sur le bas est ran et des amas divers sur le haut du rivage. au pied dc la falaisc.
La presque totaliic des cailloux (plus dc 90%) appartient aux diverses lithologies ignees el metamorphiques du socle precambricn a proxtmite. Mais on trouvc aussi dc nombreux cailloux de dolomie. unc liihologic rare dans le BoucMcr l^urentidicn. Sur une distance dc 18 km. 3661 cailloux de dolomie ont etc" observes et mesurcs. II en exisle plusieurs varictcs; du point dc vuc dc la couleur. Ics dolomies grises el roses dominent. lotalisanl respectivemenl 74% et 20%. Parmi les autrcs caractcristiqucs. il y a une trcntainc dc blocs de dolomie a stromatolites, plus de 28% des cailloux dc dolomies observes sont slries et prcs de la moitii soni suban-ondis. Les cailloux de petite taille (I a 10 kg) compient pour environ 59%, ceux de 10 a 50 kg, pour 30%, ceux dc 50 a 100 kg. 7%. alors que 4% pesent plus de 100 kg. le plus gros alteignant deux tonnes. Les deux plus gros blocs de dolomie a stromatolites pesent respectivement 175 et 280 kg.
II existc quatre grandes aires de roches dolomiiiques dans Ic BoucMcr precambricn: le bassin sediment aire de Mistassini. le scctcur cntre la Grande lie el Inukjuak. sur la cote est de la mcr d'Hudson. Ic fosse du 1-abrador. ainsi que dans Ic Grenvillc, entre Hull et !e lac Baslong. l,a source la plus plausible des crraiiqucs de dolomie des Hscoumins demcure le bassin de Mistassini, ce qui impliquc un counuit dc glace important, au Wisconsinien. vers le SK a partir d'un dome ou d'une Mgnc de partage des glaccs situe au NO du lac Mistassini. Les erratiques de dolomie du rivage des Lscoumins onl d'abord ete transported par le glacier via la depression du l,ac-Saint-Jean et du Saguenay avant d'etre relaches par les icebergs ou peut-clre mime, en partie, par unc plate-forme dc glace localised en bordurc de la Mcr de Goldthwaii, Les deplaecmenis sont de I'ordre de 400 a 500 km. Par ailleurs. plusieurs blocs ont pu ctre dcplace's localemcni par les glaces flotiantes au coursde I'Holocene.
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