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Paul F. Hoffman Series 132 PAUL F. HOFFMAN SERIES which emplaced a wedge of massive marins postglaciaires. Cette séquence diamicton in the postglacial marine sédimentaire, composée d’une masse sediments. This sedimentary sequence, chaotique de matériaux hétérogènes composed of a chaotic mass of het- intimement mélangés accumulée sous erogeneous material that is intimately forme semi-fluide en milieu marin, est mixed, and is accumulated in the form peut-être le meilleur affleurement of a semi-fluid body in a marine envi- d’olistostrome quaternaire. ronment, is possibly the best exposed Un coup d’œil sur l’histoire du Quaternary olistostrome. Quaternaire de la région à l’est du A general overview of the Grand lac de l’Ours et au nord du lac Quaternary history of the area east of Point, montre : a) que la déglaciation Late Wisconsinan Great Bear Lake and north of Point s’est déroulée par fonte du glacier sans Lake shows: a) deglaciation proceeded réavancée discernable, b) que la vallée Morphosedimentary by large-scale downwasting with no de la rivière Coppermine entre le lac Sequences of the Lower discernable readvance pulses, b) that Point et les rapides de Rocky Defile the Coppermine River valley between était libre de glace vers 10 250 14C BP, Coppermine River Valley, Point Lake and Rocky Defile Rapids et c) que les mécanismes d’érosion ont Nunavut and Northwest was ice-free by 10 250 14C years BP, été intenses pendant une courte péri- and, c) that earth-shaping processes ode au moment de la déglaciation; Territories were intensely active for a short period depuis lors, le paysage est demeuré rel- at the time of deglaciation; since then ativement statique. Denis A. St-Onge the landscape has been mostly quies- Emeritus Scientist cent. INTRODUCTION Geological Survey of Canada On July 8, 1821, John Richardson, MD, 601 Booth St. #548, Ottawa, ON, SOMMAIRE a member of John Franklin’s party that Canada, K1A 0E8 Des séquences sédimentaires de la fin was on its way to the Polar Sea, made E-mail: [email protected] du Wisconsinien qui occupent des sec- the following observations from a tions de la vallée de la rivière Copper- point above Rocky Defile Rapids: “The SUMMARY mine définissent cinq zones mor- river contracting to the width of 120 yards, at A series of late Wisconsinan sedimen- phosédimentaires représentant quatre length forces itself through the rocky defile, a tary sequences occupy parts of the environnements différents du retrait narrow channel which it has cut during a Coppermine River valley, and are glaciaire, soit les milieux glaciaire, lapse of ages in the shelving foot of a hill. grouped into five morphosedimentary paraglaciaire, lacustre et marin. Ces The channel is bounded by perpendicular zones representing four environments séquences qui s’interdigitent souvent, rocky walls, varying in height from 100 to at the time of last ice retreat: glacial, documentent des épisodes sédimen- 150 feet, above which there is an immense paraglacial, lacustrine and marine. The taires de milieux qui varient avec le body of fine sand. The form of the land sequences commonly interfinger, and temps et les changements de position would lead one to suppose, that the river at document episodic deposition in time- du front glaciaire. Les sédiments some distant period, pent by the rock, formed transgressive environments related to glaciaires et paraglaciaires sont en par- a long narrow lake, whose superfluous waters ice frontal positions. Glacial and tie remaniés dans des milieux glaciola- were discharged by a magnificent cascade - an paraglacial sediments are, in part, custres allant des lits topset deltaïques opinion which is countenanced by the figures of reworked in a glaciolacustrine environ- de gravier à blocs jusqu’à des rythmites the sandy ridges, which rise immediately above ment extending from delta topsets of de limon-argile. Les rythmites glacio- the rapid to the height of 500 or 600 feet” bouldery gravel to bottomset rhyth- lacustres constituent la matrice d’une (Franklin 1823, p. 527). The lake mites of silt-clay. The glacial-lake séquence subséquente de lave torren- inferred by a very astute observer of rhythmites formed the matrix of a tielle, qui a déposé un énorme prisme nature was named, 150 years later, Gla- later sequence of debris-flow events, de diamictons dans les sédiments cial Lake Coppermine (St-Onge 1980). GEOSCIENCE CANADA Volume 39 2012 133 The late Wisconsinan ice sheet was centred on the Canadian Shield and covered most of the Canadian land mass east of the Cordillera and south of the Queen Elizabeth Islands. Retreat from its maximum is now rea- sonably well understood and con- strained (Dyke 2004). Starting 12 500 14C years BP, the northwestern part of the ice sheet melted back, generally west to east. As land was being exposed, large lakes were formed at, or near, the ice front. By 10 250 14C years BP the Coppermine River valley, south of Rocky Defile Rapids was freed by the receding ice sheet and, as a result, a large lake occupied the valley. (All dates in this paper are conventional 14C years BP). Meltwater from the ice sheet east of the valley carried enormous quantities of sediment into Glacial Lake Coppermine. Well exposed sedi- mentary sequences make it possible to document the history of this lake from its inception to its eventual drainage into Coronation Gulf. Five summers of systematic mapping (St-Onge 1988) provided the data required to recon- struct the complex history of this lake, which extended from Point Lake to Rocky Defile Rapids (Fig. 1) and, based on sedimentary facies, to propose a general model of the deglacial history of the Coppermine River region. The bedrock south of Big Bend (Fig. 1) is part of the Wopmay Orogen (Hoffman 1984) composed of five tectonic zones which include a great variety of metamorphic rocks Figure 1. Location map. and granite bodies. North of Big Bend, northward-dipping Precambrian Zone I: Point Lake – White Sandy paraglacial conduits, which entrained sedimentary rocks, mostly sandstone, River large amounts of debris that were are profusely intruded by Coronation Prior to glaciation, the Coppermine deposited as gravel and sand sediment Gabbro sills (Baragar and Donaldson River valley was well established in the bodies. In places, powerful meltwater 1973). rolling upland underlain by rocks of streams also removed all, but the This paper is dedicated to Dr. the Canadian Shield between Point largest boulders, in their path. Thus, Paul Hoffman who, in 1979, convinced Lake and Coronation Gulf (Fig. 1). corridors were created in which sec- me to go back to field work and map The Coppermine River valley is now tions of bedrock alternate with ice- the Quaternary Geology sediments in mantled by a discontinuous blanket of contact features, such as esker ridges, the Coppermine River region. glacial drift and is partly filled with gla- transverse and circular ridges of cial, glaciofluvial, glaciolacustrine and glaciofluvial gravel and minor morainic MORPHOSEDIMENTARY ZONES fluvial sediments (St-Onge 1984). ridges (Fig. 3). The concentration of Along a transect, from Point Lake to Ice originating from a disper- very coarse material at fairly regular Coronation Gulf, five major mor- sal centre to the east was responsible intervals along eskers that are, in phosedimentary zones are identified for the emplacement of the thick till in places, continuous for several tens of (Fig. 2). Each comprises several sedi- the Scotstoun Lake region (Fig. 3). The kilometres, suggests they were con- mentary facies reflecting environments melting of this ice mass produced structed as successive, comparatively of deposition, during deglaciation enormous amounts of water concen- short segments (St-Onge 1984). phases (Fig. 3). trated in subglacial, englacial and Deltas of coarse gravel and 134 Figure 2. Glacial Lake Coppermine – Morphosedimentary Zones. sand (Fig. 4) were constructed on both deposited in water bodies confined to Zone II: White Sandy River to Big sides of the river valley, but are more low areas between bedrock or till Bend common on the eastern side. The slopes, and large blocks of dead ice. Between the White Sandy River in the higher perched deltas lay at approxi- Boulders melting out from the margin south and the Big Bend in the north mately 370 to 365 m asl (Fig. 5), indi- of the ice dropped onto the rhythmite (Fig. 1), and to the east and north of cating that they are associated with the deposits. Final melting of the glacier the Coppermine River valley, thick similarly elevated oldest documented ice triggered extensive collapse and deposits of glacial drift are found in outlet of Glacial Lake Coppermine, deformation within the sediments. the Mouse River Graben between the through Kamut Lake (Fig. 1; St-Onge These deformed rhythmite layers indi- Coppermine River and the headwaters et al. 1981). cate that at some stage, the headwaters of the Mouse River (Fig. 2). Glacioflu- South of Rocknest Lake, of the Kamut Outlet phase of Glacial vial sand and gravel are locally abun- numerous lakes and streams are Lake Coppermine were in stagnant ice dant and occur typically as esker ridges fringed by silty clay rhythmite layers, from the disintegrating ice mass in the flanked by terraces up to one kilometre up to 6 m thick; these layers contain Scotstoun Lake area (Fig. 1; St-Onge wide. Ridges generally trend southeast numerous small stones and boulders 1984). to northwest. and are intensely deformed. Fresh From 32 m above the present Within the Coppermine River exposures of the lower unit are more level of the Coppermine River, exten- valley are abundant exposures of deformed than the upper unit, indicat- sive sand terraces mark stages in down- coarsening-upward sequences of ing that the deformation is, in part, cutting (Fig.
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