Notes on the Geology and Physiography of the Lake Saint-Jean Area, Québec Peter Clibbon Et Robert Bergeron
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Document généré le 2 oct. 2021 05:19 Cahiers de géographie du Québec Notes on the Geology and Physiography of the Lake Saint-Jean Area, Québec Peter Clibbon et Robert Bergeron Volume 7, numéro 13, 1962 Résumé de l'article Le lac Saint-Jean est situé à l'intérieur du Bouclier aux sources du Sague-nay. Il URI : https://id.erudit.org/iderudit/020420ar occupe la partie sud-est d'une dépression structurale d'environ 65 milles de DOI : https://doi.org/10.7202/020420ar long par 35 milles de large, limitée sur trois côtés par des escarpements de faille. Le sous-sol de ces basses terres est constitué de calcaires et de schistes Aller au sommaire du numéro paléozoïques recouverts d'épais dépôts d'argile et de sable. Trois séries de terrasses marines bien définies encerclent la cuvette. Les hautes terres environnantes sont plus ou moins accidentées selon les Éditeur(s) endroits, avec une altitude relative moyenne d'environ 500 pieds. Des paragneiss, des gneiss mélangés, des anorthosites, des roches charnockitiques Département de géographie de l'Université Laval et des granits, tous d'âge précambrien, constituent le sous-sol de ce plateau. Les zones les plus accidentées correspondent en grande partie aux roches ISSN charnockitiques, aux granités et aux anorthosites massives, tandis que les zones gneissiques ne sont que légèrement ondulées. Des tills d'origine locale 0007-9766 (imprimé) tapissent les pentes et d'épais dépôts fluvioglaciaires remplissent les vallées 1708-8968 (numérique) principales. Découvrir la revue Citer cet article Clibbon, P. & Bergeron, R. (1962). Notes on the Geology and Physiography of the Lake Saint-Jean Area, Québec. Cahiers de géographie du Québec, 7(13), 81–100. https://doi.org/10.7202/020420ar Tous droits réservés © Cahiers de géographie du Québec, 1962 Ce document est protégé par la loi sur le droit d’auteur. L’utilisation des services d’Érudit (y compris la reproduction) est assujettie à sa politique d’utilisation que vous pouvez consulter en ligne. https://apropos.erudit.org/fr/usagers/politique-dutilisation/ Cet article est diffusé et préservé par Érudit. Érudit est un consortium interuniversitaire sans but lucratif composé de l’Université de Montréal, l’Université Laval et l’Université du Québec à Montréal. Il a pour mission la promotion et la valorisation de la recherche. https://www.erudit.org/fr/ NOTES ON THE GEOLOGY AND PHYSIOGRAPHY OF THE LAKE SAINT-JEAN AREA, QUÉBEC by Peter CLIBBON Institut de géographie, université Laval, Québec and Robert BERGERON * Institut de géographie, université Laval, and ministère des Richesses naturelles de la province de Québec. RÉSUMÉ Le lac Saint-Jean est situé à l'intérieur du Bouclier aux sources du Sague- nay. Il occupe la partie sud-est d'une dépression structurale d'environ 65 milles de long par 35 milles de large, limitée sur trois côtés par des escarpements de faille. Le sous-sol de ces basses terres est constitué de calcaires et de schistes paléozoïques recouverts d'épais dépôts d'argile et de sable. Trois séries de terrasses marines bien définies encerclent la cuvette. Les hautes terres environnantes sont plus ou moins accidentées selon les en droits, avec une altitude relative moyenne d'environ 500 pieds. Des paragneiss, des gneiss mélangés, des anorthosites, des roches charnockitiques et des granits, tous d'âge précambrien, constituent le sous-sol de ce plateau. Les zones les plus accidentées correspondent en grande partie aux roches charnockitiques, aux granités et aux anorthosites massives, tandis que les zones gneissiques ne sont que légère ment ondulées. Des tills d'origine locale tapissent les pentes et d'épais dépôts fluvioglaciaires remplissent les vallées principales. Lake Saint-Jean is north of the Saint Lawrence at the head of Saguenay river. It lies in the southeast part of the extensive Lake Saint-Jean Iowland or basin. This Iowland is approximately 65 by 35 miles, its long dimension being northwest-southeast ; its élévation above sea-Ievel ranges between 320 and 700 feet but much of it lies between 350 and 500 feet. The surrounding Lauren- tian highland stands between 200 and 700 feet above the surface of the Iowland. DESCRIPTION OF THE AREA The Iowland area Lake Saint-Jean is the dominant feature of the Iowland. The lake mea- sures approximately 29 miles by 18 miles and is subcircular in form ; it covers 375 square miles. Like the Iowland its long axis is oriented northwest-southeast. Its depth does not exceed 200 feet. The lake is ringed by a séries of terraces eut in marine and fluviatile sédiments ; thèse terraces extend in low steps almost to * Published with the permission of the Deputy-Minister, Québec Department of Natural Resources. Cahiers de Géographie de Québec. — 6 82 CAHIERS DE GEOGRAPHIE the margins of the lowland. The terraces are generally flat and featureless but some of those underlain by sand exhibit blowouts and dune colonies. In the vicinity of the lake the continuity of the plain is broken only by the valleys of major rivers that drain into the lake, such as the Ashuapmuchuan, the Mistassini and the Peribonca. Many of the vertical sections in the stratified sands and clays along the rivers are from 100 to 200 feet thick. The smaller water courses of the lowlands are also deeply incised. The extensive drift cover blanketing the lowland area is hère and there pierced by rounded bosses of crystailine rocks and tabular outcroppings of sedi- mentaries. For example, immediately to the east of the lake a séries of Iow whalebacks of anorthosite projects through great thicknesses of clays and other unconsolidated deposits, and in the Roberval area knolls of granité rise above the covering of drift and the Paleozoic iimestones of the lowland. A few eskers, kames and kettles are also scattered throughout the basin. The Lake Saint-Jean lowland is delimited on the south, west and east by bold escarpments. A Iittie-dissected scarp, which stands between 350 and 700 feet, defmes the southern boundary of the lowland between Kenogami ïake and Metabetchouan river. The scarp is oriented slightly north of west and has a rectiiinear trace. West of the Metabetchouan the scarp strikes west-northwest ; hère it is Iess bold, having an average height of only 200 to 300 feet above the lowland surface. A well-defmed, rectiiinear scarp delimits the lowland on the northwest ; it strikes due north and stands 200 to 400 feet above the surface of the basin. The scarp that delimits the basin on the southeast also strikes due north ; al- though it stands approximately 250 feet above the lowland it has been greatly dissected along its entire Iength. It is breached by two major east-west trenches, the northern containing Saguenay river and the southern containing Kenogami lake. To the north and northeast of Lake Saint-Jean the contact between highland and lowland is Iess clear-cut. Wide, north-striking valleys extend from the lowland deep into the Laurentian highland, which has a relatively Iow élévation in this area (figure 2, in pocket). The lowland and highland are thus interpenetrative and their contact is jagged and irregular. Along the entire Iength of the contact the highland stands more than 200 feet above the surface of the lowland. Denis (1933, p. 63) has effectively pointed out the contrast between the northern and southern boundaries of the lowland : (( Were the dépression once more invaded by an arm of the sea, the southern shore would be a well- defmed cliff with but few embayments, whereas to the north and northwest we should fmd an irregular coast with deep bays and promontories, and off-coast islands marking the site of outlying masses of rock that protrude above the level of the lowland. )) The Laurentide highland Encircling the Lake Saint-Jean lowland and separated from it by promi nent escarpments is a gently rolling to rugged upland termed the Laurentian highland. The highland exhibits average local relief of approximately 500 feet GEOLOGY AND PHYSIOGRAPHY OF THE LAKE SAINT-JEAN AREA 83 PHOTO I (R.C.A.F. Photo A-136S7-40) The Shield-Iowland contact at the western edge of the Lake Saint-Jean basin. Hère the contact is rectilinear and abrupt. It is bordered on the lowland side by fiuvioglacial deposits and a string of kettle-form Iakes. but in the northeast and southeast portions of the map-area relief ranges up to 2,500 feet. The area is underlain by crystalline rocks that are more or less résistant to contemporary processes of weathering and érosion, their durability being reflected in the rugged nature of the Iandscape. Throughout the area val- Ieys hâve been deeply dissected along joints and other Iines of structural weakness in the bedrock, resulting in a blocky Iandscape of steep slopes and cliffs. Many of thèse slopes are mantled with sandy till consisting of angular to subangular blocks of rock set in a matrix of coarse gravel and fmely ground material. The till is Iocally derived and its depth over bedrock varies from a few inches to more 84 CAHIERS DE GEOGRAPHIE than 30 feet ; the deposits hâve considerably softened the rugged highland topography. Fluvioglacial deposits are abundant on the Iower slopes of Hills and in the principal valleys of the area, occurring as well-sorted outwash plains and poorly sorted kames. DESCRIPTION AND EROSIONAL BEHAVIOR OF THE ROCKS Although well populated and close to the large industrialized zone of the upper Saguenay, the Lake Saint-Jean area was not well known in terms of régional geology until récent years. From 1857 to 1916 periodic investigations were carried out in the Iowland area by officers of the Geological Survey of Canada, and since 1932 the gênerai région has been under study by the Geological Surveys Branch of the Québec government.