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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 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 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 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. 5; St-Onge et al. 1981). glaciolacustrine sediments ranging syndepositional and probably resulted from deep water sandy silt rhythmites, from the gradual melting of buried gla- through shallow water sand bodies to cier ice. These rhythmites were likely deltas of coarse sand and gravel (Fig. GEOSCIENCE CANADA Volume 39 2012 135

ness from 1 to 50 cm, the thinner ‘win- ter’ layers of silty clay rarely exceed 2 cm. Counting indicate that in excess of 500 individual doublets are exposed in the thicker sections. A crossbedded sandy unit, typically between 2 and 3 m thick (Fig. 5), is deposited on top of the rhyth- mite sequence. This remnant of an alluvial sand plain is commonly associ- ated with coarse-grained, valley-side deltas and marks the minimum level of infilling of the Coppermine River val- ley up to the level of 320 m asl, 50 to 70 m above present valley floor. During the downcutting phase of the ancestral Coppermine River numerous terraces were formed. The oldest preserved are at levels of 32, 25 and 20 m above present river level (Fig. 5). All these are erosional terraces carved into the rhythmite layers. They represent downcutting between the Kamut outlet phase at 370 m asl and the Dismal Lakes outlet phase at 285 m asl (Fig. 3). A lower terrace between 12 and 14 m above present river level is the result of aggradation (Fig. 5). A log found in this unit was dated at 4370 ± 60 14C years BP (GSC-3135); the cause for the aggradation and later erosion has not been determined but could be due to partial blocking of the river by a landslide farther down- stream.

Zone III: Big Bend to Rocky Defile Rapids Section To the west of the Coppermine River Figure 3. Deglaciation phases. valley between Big Bend and Kendall River, a 600-m high plateau of moder- 5). In places, glaciofluvial sand and mass, the eastern and northern deltas ate relief is breached by two broad val- gravel protrude above the lacustrine were constructed by more powerful leys draining east into the Coppermine sediments, indicating that in the very streams originating from a proximal, River valley: the Quicksand Creek and early stages of deglaciation, meltwater and still active, ice front to the east. Qingaluk Lake valleys (Fig. 1) in which was flowing in ice tunnels at the base It has not been possible to are concentrated thick accumulations of the present valley. determine if the absence of deltas of glacial drift (Figs. 2 and 6). A nar- Deltas (Figs. 4 and 5) with between 330 and 370 m (Fig. 5) was row morainic ridge crosses the plateau well-preserved braided channels on originally infilled in part, or not at all: between these two valleys (Fig. 3). West their surface are located in the valleys in other words it has not been possible of the ridge a large body of glacioflu- of numerous tributaries on either side to determine how much, if any, of the vial sand and gravel occupies the of the Coppermine River valley. The rhythmite layers were removed in the trough of Qingaluk Lake valley. This altitudes are consistently between 360 early phase of downcutting by the sedimentary unit was emplaced by and 370 m asl. Deltas on the east and river. west-flowing meltwater streams run- north side of the valley are composed Silty sand rhythmite units (Fig. ning from an ice mass with a frontal of coarser gravel than those on the 5) are exposed in numerous sections; position marked by the morainic ridge. south and west side, which tend to be their buff-white colour makes them Minor recession of the ice front sandier. While the deltas to the west highly visible both on air photographs allowed the drainage to change from and south were constructed by meltwa- and in the field. The ‘summer’ layers, west to south and finally to its present ter from a disintegrating residual ice composed of sandy silt, vary in thick- easterly course into the Coppermine 136

Figure 4. Glaciolacustrine delta, north side of the Copper- mine River Valley 16 km upstream of Big Bend (Fig. 3).

Figure 6. This air photograph (A13609-79) shows the area of Big Bend, Qingaluk Lake and Quicksand Creek. The section shown in Figs. 7 and 8 is on the north side of a patch of on the southern tributary of Quicksand Creek. Qin- galuk Lake is within an extensive coarse sand and fine gravel plain within a broad bedrock depression. Well developed ter- races are found along the Coppermine River.

Figure 5. Composite stratigraphic section, Glacial Lake Cop- permine; not to scale.

River (St-Onge and Geurts 1982). This sequence is Qingaluk Lake occupies a well exposed (Fig. depression in a large sand plain that 7) on the north- partly fills a broad arc-shaped depres- west side of a sion in the bedrock plateau west of the small southerly Coppermine River valley (Fig. 6). The branch of Quick- depression probably represents a for- sand Creek (St- mer course of the ancestral Copper- Onge 1980). A mine River valley which was earlier former channel Figure 7. Quicksand Creek section showing organic rich fill abandoned. As is the case for the valley carved in the sand in paleochannel in 310 m asl. Glacial Lake Coppermine delta upstream of Big Bend, the sandy silt unit was later filled (Fig. 6). rhythmite of the Qingaluk Lake valley with organic resulted from infilling up to 320 m asl debris (Fig. 8). A by Glacial Lake Coppermine sediment. piece of wood at the base of this fill sequence has yielded two dates; the This sand plain has a surface elevation sequence has yielded a date of 8400 ± basal layer is 7180 ± 100 14C years BP of 300–320 m asl and is underlain by a 80 14C years BP (GSC–2959), whereas a (GSC-4495) and the top of the peat is thick sequence of sandy silt rhythmites. ~90 cm layer of peat at the top of the 870 ± 90 14C years BP (GSC-4486). GEOSCIENCE CANADA Volume 39 2012 137

Figure 8. Quicksand Creek section, palynology and 14C dates.

These dates indicate that the initial ly defined ripple infill of the channel with sand and sets. Large trough organic debris was completed in about crossbeds have a 1200 years followed by over 6000 years 050° current direc- of peat accumulation in a marshy envi- tion. Laminated ronment. Detailed study of the organic sand beds are material in this channel indicates a truncated by number of climatic fluctuations trough crossbeds inferred from variations in pollen that dip 25° to the assemblages (Fig. 8). Within the Cop- west. Ripples and permine River valley, broad terraces are planar cross-strati- found on either side of the river at 300 fication indicate m asl and mark a phase in the down- paleocurrent direc- cutting by the river. Like terraces at tions of 255 to similar altitudes upstream, they are 265°. Near the top related to the outlet of Dismal Lakes, of unit B, sand which is at approximately 285 m asl grades into sandy (Fig. 3).The Rocky Defile Rapids (Fig. silt and occasional 1) were first described by John clay layers, up to Richardson (Franklin 1823, p.153). An 5-cm thick, which abandoned section of the Coppermine appear convoluted. River valley is filled, in part, by a 110- Laminated silt Figure 9. Rocky Defile Rapids Section. m thick deposit of coarse sand, boul- beds are also ders and gravel (Fig. 9). Basal unit A is found near the top a massive structureless, high sand con- of unit B (Fig. 9). Unit C is a sand This sequence is interpreted as tent diamicton. Grain size ranges from matrix diamicton having clasts up to 1 an ice-contact complex deposited by a a silty sand matrix to boulders over 1 m in diameter that overlies unit B. This stream flowing west-southwest from m in diameter. Large clasts tend to be 12-m thick unit, grades upward into a glacier ice into Glacial Lake Copper- well rounded and lithologically diverse. coarse sand which, in turn, is overlain mine; it is here that northward The upper contact with unit B is quite by silty clay lenses intercalated with a drainage was dammed during the 365- abrupt. The base of 25-m thick unit B 50-cm thick medium sand bed (Unit m high level (Kamut Lake outlet consists of 0.5 m of massive silty sand D).The uppermost unit, exposed at phase) of Glacial Lake Coppermine grading into a laminated sand and silty Rocky Defile section, is a 15-m thick, (Ice front 2, Fig.3). To the north of sand layer 2 to 10 cm thick. Scattered massive, gravelly sand and cobbles up this ridge, a channel that cuts across a coarse-sand beds containing rare peb- to 30 cm in diameter; coarse sand and bedrock spur connects the Copper- bles near the base are overlain by poor- granules predominate. mine River valley with the Kendall 138

Figure 10. Air photograph of lower reaches of Kendall River and Mouse River basins. This air photograph (A22414-88) of the area just north of Rocky Defile Rapids shows a complex distribution of valley side terraces and perched deltaic above drumlinized or washed till surfaces. The Coppermine River has cut flights of terraces into large delta- shaped gravel deposits. These alluvial deposits generally lie below the upper surface of glaciolacustrine rhythmites that are overlain by coarse sand. A puzzling feature of the gravel deposit south of the Kendall River is the occurrence of numerous ridges of boulders fringing -like lakes.

River valley. This abandoned channel to the southeast has a sill elevation of 300 m. and drains a large number of lakes. Figure 11. Section 3 km downstream from the mouth of Zone IV: Rocky Defile Rapids to In the Mouse River. Melville Creek Section Dismal Lakes– North of Rocky Defile Rapids (Figs. 1 Kendall River and 3), the Coppermine River crosses a depression, a field with indi- distinct depressions now occupied by broad NW–SE bedrock depression vidual up to 2000 m long and Dismal Lakes were buried under a bounded on the south by the rolling trending between 335 and 310o indi- stagnant ice mass. Fluvioglacial and upland of the Big Bend region cate that the last active ice flowed debris-flow processes emplaced (600–800 m asl) and to the north by toward the northwest. Adjacent to numerous kame and minor morainic the Coppermine and September moun- these drumlin field sequences of nest- ridges on both sides of the valley with tains (400–600 m asl). The Dismal ed lateral moraines, valley-side kame steeper ice-contact slopes towards the Lakes occupy the northwestern part of terraces and ice-contact deltaic land- basins. When the northwest basin the bedrock depression and drain to forms suggest that the downwasting of became ice free, a large kame delta was the Coppermine River valley by the a remnant ice mass in the valleys took deposited. At 285 m asl, this chan- rocky and turbulent Kendall River. The place. nelled flat-top deposit now stands 30 Mouse River occupies the depression During deglaciation, the two m above present lake level and sepa- GEOSCIENCE CANADA Volume 39 2012 139 rates the ‘two’ Dismal Lakes. This ter- rain marks the water level of a lake which drained through the now aban- doned spillway at the northwestern end of the Dismal Lakes, at 285 m asl into a glacial lake farther west (St-Onge and Dredge 1985). The basin east of the Copper- mine River now drained by Mouse River contains a varied suite of sedi- ments reflecting a somewhat complex history of deglaciation (Fig. 10). A sequence of channels, deltas, and part- Figure 12. Diagrammatic stratigraphic section of the Coppermine River valley ly collapsed was likely formed at from September Mountains to Coronation Gulf (Fig. 1). the margin of an ice mass which occu- pied the Mouse River lowlands. As the the river (Fig. 10, Tg 225). interpreted as a debris flow complex highlands to the south became ice-free, North of the Dismal Lakes resulting from rapid downcutting by water flowed towards the lower ground and the Kendall and Mouse River the Coppermine River, following the and deposited sediments in a narrow basins, the Coppermine River enters a disintegration of glacier ice north of embayment at the margin of the ice. In narrow, steep-sided bedrock valley that September and Coppermine moun- time, an ice-marginal lake was formed cuts through the high ridges of Cop- tains. The relationship of this diamic- into which deltas were built. The disap- permine and September mountains. ton to the Sleigh Creek Formation pearance of the ice resulted in the col- Erratics are strewn over bedrock (below) is not known. lapse of the kames and in the deltas uplands. Terraces on either side of the being perched at 300 m asl on the river are covered with generally Zone V: Melville Creek to south side of the basin (Fig. 10 Dg). unmodified ice-contact sediments and Coronation Gulf Section By the time glacier ice had landforms including esker ridges. The Melville Creek to Coronation Gulf nearly completely disappeared from the The 27-m high section on the section (Figs. 1 and 2) contains a com- area, a lake still occupied part of the north side of the Coppermine River plex sedimentary sequence which yields Kendall River–Mouse River basins, (Fig. 11) has two poorly stratified sandy an abundance of information on the which was dammed by glacial ice in the diamictons containing clasts up to 1.5 postglacial history of this part of the valley north of September Mountains m in diameter. Boulders are generally river valley (Fig. 12). The valley of (Ice front 3, Fig. 3). At least 17 m of well rounded and of varied lithology; a Melville Creek is partly filled with rhythmically bedded sandy silt and clay well-stratified coarse sand unit sepa- glaciofluvial sand and gravel including were deposited in this lake which was rates the two diamictons. Lower unit A a large multi-crested esker complex and the last remnant of Glacial Lake Cop- is a 10.25 m thick diamicton, com- numerous terraces. The esker follows permine (Fig. 3). The bedded silt posed of a sand matrix supporting the east side of the Coppermine River grades upward into coarse sand and clasts ranging in size from pebbles to Valley as far downstream as Willow planar beds, gently dipping at 330°. boulders over 1 m in diameter. Boul- Creek (Fig. 1) where, after a short gap, Large crossbeds and ripple trough der–cobble concentrations identify it continues on the west side of the crossbeds indicate a high-energy envi- poorly defined beds 1 to 2 m thick and valley. Just above Willow Creek, both ronment established during Dismal having a slight dip to the NW. Unit B sides of the esker complex are flanked Lakes’ outlet phase of Glacial Lake is a 3.75 m thick layer of coarse sand. by a flat gravel surface scarred by Coppermine. The lake level was main- Laminated fine to medium sand with numerous abandoned channels and a tained by glacial ice which damned the lenses of coarse sand form an abrupt few lakes (Fig. 13, Ls 190). The large narrow Coppermine River valley to the lower contact. The sequence coarsens delta at 160 to170 m asl on the east north of Larrigan Creek. At this site upward to pebbly sand with gravel and side of the Coppermine River (Fig. 13) (Fig. 3) an outwash delta built into the cobbles. The upper part of the unit is is the highest of a series constructed lake now forms terraces culminating at marked by planar cross-stratification; into the postglacial sea (St-Onge and approximately 300 m asl. interbedded sandy silt and pebbly sand Bruneau 1982). The river eventually removed layers dip at 15° toward 340°. The Sediment sequences logged in most of the coarsening upward upper contact at the base of a boulder paleo-bedrock valleys (Fig. 14) and sequence emplaced in the Mouse River lag is a pebbly sand matrix with ripples. exposed in numerous sections along basin. The remnants now stand as ter- A boulder and cobble concentration the modern Coppermine River valley races 50 m above river level. In the forms the abrupt basal contact of unit have made it possible to establish a area that was incised by the Copper- C, which is 13-m thick composed of a lithostratigraphic diagram extending mine River, an elongated fan-shaped series of sandy diamicton layers 1 to 4 from Muskox Rapids to Coronation body of coarse gravel was deposited. m thick, separated by cobbly lags, in Gulf (Fig. 12). This represents an This channelled surface now forms a place overlain by draped laminations. offlap marine sediment sequence 15-m high terrace on the east side of These poorly sorted sediments are coarsening upward from fine 140

Figure 14. View looking east of the Sleigh Creek paleo-valley filled with debris flow diamicton covered by sand and gravel of the 165–170 m asl marine delta (Fig. 13).

Figure 13. Air photograph of Sleigh Creek area. This air photograph, part of A-23634-97, shows the regional setting of the section between Fockler and Sleigh creeks on the east side of the Coppermine River (Figs. 15 and 18). A large esker complex which originates in Melville Creek south of the pho- tograph follows the east side of the river before heading in a northwest direction. On the west side of the Coppermine River glaciolacustrine fine sand and silt mantle hummocky topography up to 190 m asl. Below 170 m asl a large delta built into the post glacial sea. Numerous terraces have been carved in this deposit by the Coppermine River. sand–silt–clay rhythmite deposits to variable thickness- bouldery gravel deltas. A 30-m thick es; the lower diamicton complex is located in the rhythmite middle of these marine sediments and sequence is up to was named the Sleigh Creek Formation 14 m thick (Fig. by St-Onge and Lajoie (1986). 12; Bruneau 1985), The Sleigh Creek Formation whereas the upper (Figs. 13 to 18) is bracketed by sequence has a Figure 15. Contact between units A and B in the Sleigh sequences of very fine sand, silt and maximum exposed Creek section (Fig. 18); Dr. Paul Hoffman in foreground. clay in horizontal beds 5 to 50 cm thickness of 3 m. thick that have sharp upper and lower Sections 1.5 to 2 km upstream of ning wedge and that the flows respon- contacts, and are normally graded. In Bloody Fall (Figs. 1 and 12) show up sible for their transportation did not at least two 2-m horizons, the beds to 70 m of rhythmites on boulder-cov- reach this far north. Marine shells have have been intensely contorted by sub- ered bedrock. In these sections, the been found in the upper rhythmite aqueous mass flow events. The lower diamicton sequence is not present sug- sequence, below 110 m asl, and also in and upper rhythmite sequences have gesting that it forms a northward-thin- the sand layers overlying it. The rhyth- GEOSCIENCE CANADA Volume 39 2012 141

Figure 16. Sand and silt layer in the upper unit B (Fig. 18) of Figure 17. Sand and silt layer shown in Figure 16 resting on the Sleigh Creek section. stony diamicton.

the buried channel are well exposed at both ends of the section. Erosion by the Coppermine River along the entire length of the section has provided excellent exposures of the channel fill. Total thickness of the section is 45 m. The lowermost 8.5 m is largely talus although bedrock is sporadically exposed. Two exposed diamicton units have a minimum thickness of 23 m. The upper 13.5 m is coarse sand and the bouldery talus of the uppermost delta built into the postglacial sea (St- Onge and Bruneau 1982). At the south end of the section, fine sand and silt rhythmite layers are draped over the few large clasts protruding from the uppermost diamicton bed. The Escape Rapids section (Figs. 19 to 22) extends for over 500 m on the east side of the valley. Roughly 70 m of channel fill are exposed and consist of two diamicton units having a combined thickness of 50 m. This sequence is overlain by 10 m of coarse sand and bouldery gravel, part of a Figure 18. Stratigraphy of the Sleigh Creek section with variations in bed thickness postglacial marine delta. In a gully at (BTh), maximum grain size (MGS) and granulometry of the matrix. the north end of the section, thick silt and fine-sand rhythmite deposits are mite sequences form a package that include all of the units and sedimenta- exposed at the same level as the extends to the present sea coast and, ry characteristics of the Sleigh Creek diamictons. Individual beds are up to presumably, below Coronation Gulf. Formation. Several other sections 1.5 m thick suggesting that these are The sedimentary characteristics and the along the Lower Coppermine River proximal turbidites. Small fragments of presence of marine shells near the top also show one or more of the units marine shells were found, but none in of the stratigraphic column strongly described. life position. In the same part of the suggest that the sediments of the The Sleigh Creek section (Figs. section, a poorly exposed unit of sandy rhythmite sequences were emplaced by 13 to 18) extends for 1.5 km on the silt rhythmite occurs between the turbidity currents in a marine basin east side of the Coppermine River just diamicton sequence and the overlying (Lajoie and St-Onge 1985). upstream from the mouth of Sleigh deltaic sediments. The sections described below Creek (Fig. 13). The bedrock walls of 142

Figure 20. Section on the east side of the Coppermine River valley downstream from Escape Rapids (Fig. 19) showing numerous diamicton layers. Marine nearshore sand and silt.

Figure 19. Air photograph of Escape Rapids area. This Escape Rapids area, where the section described in Figure 15 is located, is illustrated in this air photograph, part of A- 21510-92. The rapids developed over a gabbro sill which forms a cuesta east of the river. A large marine delta of boul- dery gravel lies lower in elevation than the adjacent, older Figure 21. Some of the diamicton layers separated by pebbly marine sediments of sand and silt. A network of braided sand layers at the base of Unit B (Fig. 22) of the Escape paleochannels is well preserved on the delta surface. Rapids Section.

The Diamicton Sequence the upper limit of Unit A. In the there is no sign of channelling within In both sections studied the diamicton Sleigh Creek section (Figs. 16 and 18) the two diamicton sequences. Beds are sequence is composed of a lower Unit the top of Unit A is a 2.8 m bed of easily identified by the presence of thin A and an upper Unit B (Figs. 18 and gravel in a silty matrix; large clasts are interbeds of generally massive, but 22). Colour distinguishes the diamicton rare. locally crossbedded, fine to very coarse units; Unit A is light buff whereas Unit Upper Unit B is a sequence of sand; climbing ripples were seen at B is a light brown. In both sections, dark brown sandy diamicton beds. In only one location. Within the diamic- the matrix of Unit A in the diamicton the Escape Rapids section (Fig. 21), the ton sequences, the beds are generally is siltier (30% sand, 65% silt, 5% clay) basal bed of the unit is a bouldery massive; traction structures are absent, than Unit B (55% sand, 40% silt, 5% diamicton with individual clasts up to and reverse grading was observed in clay). On average, clast size is smaller 60 cm in diameter. The bed in the two beds. The large clasts commonly in Unit A than in Unit B. Clasts are same stratigraphic position in Sleigh have long axes both parallel and per- less abundant in Unit A averaging 25 Creek section (Figs. 17 and 18) is a pendicular to bedding. The clasts in the to 30%; in Unit B they make up to very pebbly diamicton with individual diamicton range up to boulder size. 50% of the deposit. clasts seldom larger than 5 cm. They vary considerably in size both In the Escape Rapids section The diamicton beds range in from bed to bed and laterally within (Figs. 20, 21 and 22) a 4-m thick band thickness from 1 to 7 m. The basal individual beds. Well rounded cobbles of interfingering silty sand lenses and contact with the underlying sediments and boulders make up variable propor- irregular thin beds of diamicton mark is sharp and not obviously erosive; tions of the sediments. GEOSCIENCE CANADA Volume 39 2012 143

Lack of imbrication and grading in the Sleigh Creek diamicton indicates a lack of clast to clast movement during flow. The high degree of roundness of the gravels is an inherited feature from previous depositional cycles, and is characteristic of gravel and till in the Wisconsinan drift of the region. The rhythmite sequences that underlie the diamicton are thought to have been emplaced by turbidity cur- rents generated by the massive sedi- ment flux at the outlet of the Copper- mine River. The overlying rhythmite sequences also accumulated from tur- bidity currents, but originated in deep- er water at the front of the rapidly building bouldery delta at 165 m asl Since the diamicton sequence is deposited between marine rhythmite layers it is likely that deposition occurred in a subaqueous environment. The Sleigh Creek Formation is an exceptionally well exposed, early post- glacial olistostrome.

DEGLACIATION PHASES Figure 22. Stratigraphy of the Escape Rapids section with variations in bed thick- The information required to construct ness (BTh), maximum grain size (MGS) and granulometry of the matrix. a time-constrained model of ice retreat in the Coppermine River valley region, To evaluate the provenance of guish the channel fill from the till: a) extending from Point Lake to Corona- the diamicton, clasts larger than 5 cm the diamictons of the fill were deposit- tion Gulf, was gathered through sys- in diameter were systematically sam- ed in distinct layers interbedded with tematic geological mapping in the area pled along a vertical profile in the sand and gravel, and b) the clasts, with- by the Geological Survey of Canada Sleigh Creek section. In the lower in the fill, average 40 to 50% of far- between 1979 and 1984 and, from diamicton (Unit A), 58% of the sample travelled erratic material. In contrast, 1985 to 1989, for regions to the north has local provenance, the other clasts although exposures of till are poor, the and northwest. From the distribution are derived from sources outside the deposit appears massive with an erratic of sediments (described in the first Coppermine Homocline (Baragar and content of less than 25%. These data part of this paper) it is now possible to Donaldson 1973), mostly from the area indicate that the diamicton sequence propose a general model of Late Wis- of Wopmay Orogen to the south and the local till have distinct rock consinan ice retreat. The four ice front (Hoffman 1984). In the upper diamic- types and provenance. phases described are not defined by ton (Unit B), 56% of the sample is of continuous ice-contact deposits. Most- local provenance, and the balance from Discussion ly, the margins are surmised from the other sources. As a whole, 40 to 45% The model proposed for the accumula- configuration of glacial lakes in the of the fragments in the Sleigh Creek tion of the Sleigh Creek Formation is Coppermine, Kendall and Richardson diamicton represent rock types that are summarized in Figure 23, which repre- river valleys, and are based on the rea- not present in the Coppermine Homo- sents a section along the fill in a soning that lake levels determined cline. In comparison, the study of frag- bedrock valley that cuts across a cuesta from sedimentary deposits require that ment composition in the till suggests a landscape. The coarse fraction of the regions lying below those levels were much higher percentage of locally Sleigh Creek Formation is matrix sup- occupied by glacier ice. Also, both derived rock types. ported; the beds are massive or location and elevation of deltas, built The most striking difference inversely graded, and have sharp non- in glacial lakes, can be used to discern between the diamicton of the Sleigh erosive contacts and traction structures ice-front positions. Another criterion is Creek Formation and the regional till is are absent. These characteristics sug- side-hill or lateral channels and kames, colour. The Sleigh Creek Formation is gest that the coarse fraction was which could only have been formed by light buff to brown whereas the till deposited by a debris flow. In this type a stream confined on one side by gla- matrix has a distinctly reddish tinge of deposit each bed results from a sin- cier ice. derived from red sandstone and shale. gle flow event in which matrix strength The ice fronts define signifi- Other fundamental properties distin- is the main clast support mechanism. cant events in the course of ice retreat 144

Figure 23. The Sleigh Creek Formation facies model. to which a tentative age can be the west side, indicates the Forcier this frontal zone. assigned, in large part due to a major Lake Moraine was constructed by synthesis of deglaciation of central and west-flowing ice. According to Dyke Phase 2: Glacial Lake Coppermine northern Canada by Dyke (2004). (2004), the Forcier Lake Moraine is During ice retreat between deglaciation correlative with a late phase of the phases 1 and 2 (Fig. 3), the Copper- Model of Ice Retreat Bluenose Lake Moraine (St-Onge and mine River valley from Rocknest Lake McMartin 1995) and would be part of to 10 km south of the Kendall River Phase 1: Forcier Lake Moraine the 10.25 14C ka BP phase (Dyke 2004). was ice-free and was occupied by Gla- The most westerly of the former ice- This would place the Forcier Lake cial Lake Coppermine. The outlet of front positions determined in this Moraine Phase within the Younger the lake was at 365 m asl through study were established during the Dryas Chronozone (11–10 ka BP). The Kamut Lake and Sloan River (St-Onge Forcier Lake Moraine phase (Fig. 3). Dismal Lakes basin was completely et al. 1981). Near Rocky Defile Rapids, The Forcier Lake Moraine is located occupied by ice at this time and melt- the former ice front is marked by a approximately halfway between water eroded a 4 km long and 30 to 50 massive deposit of sand, gravel and McTavish Arm of Great Bear Lake m deep channel at the western end of boulders filling a pre-glacial section of and the Coppermine River and is the lake basin. A large sand plain was the Coppermine River Valley (Fig. 9). described by St-Onge et al. (1981). deposited west of Dismal Lakes and In the Scotstoun Lake region, This moraine is a single 5 to 20 m high north of Dease River (St-Onge and meltwater eroded 30 to 40 m deep, box ridge that extends northward for nearly Dredge 1985). section gorges in slate that underlies 60 km. It is composed of boulders, up The ice front flowed around a the divide between Rocknest Lake and to 3 m in diameter, in a sand-gravel 550-m high plateau north of the west- the Hepburn and Coppermine rivers. matrix. West flowing meltwater ern part of Dismal Lakes. Ice-marginal At the downstream ends of these breached the ridge and deposited small lakes formed between this plateau and canyons, coarse sand and gravel deltas sand and gravel outwash fans; these are the ice lobe in the basin of Richardson were built at 365 m as1 into Glacial covered by string bogs in many places. and Rae rivers. Numerous kames and Lake Coppermine. This elevation clear- Its eastern slope is generally steeper deltas at approximately 270 m asl with ly indicates that the melting of the than the west; this, along with fans on collapse structures to the north mark Scotstoun Lake ice, the carving of the GEOSCIENCE CANADA Volume 39 2012 145 rock canyons and the construction of Glacial Lake Coppermine to drain into marine molluscs from calcareous sedi- the deltas were synchronous with the the Richardson River basin. There is ment as being too old. Furthermore, Kamut outlet phase of Glacial Lake no well defined channel in the narrow England et al. (2012) demonstrated Coppermine (St-Onge and Guay 1982). wind gap found there to substantiate that, “The age discrepancy between suspen- North of Rocknest Lake, the this, but this may be because the sion and deposit feeders in calcareous terrain ice front was located at the headwaters drainage was partly across dead ice, or is non-systematic in space and time thereby of White Sandy River (Fig. 3). Rhyth- because active scree accretion has invalidating the application of a correction.” mite deposits and sand terraces are buried the channel. Therefore, given that many of the ear- part of the Glacial Lake Coppermine Along the western tributary of lier dates can no longer be considered suite of sediments and indicate that the the river north of Hope Lake, a series reliable, indicators of the time when river valley was ice-free and well nour- of stepped deltas of bouldery gravel Coronation Gulf (Figs. 1 and 3) was ished with medium- and fine-grained testify to the rapid lowering of ice freed from glacial ice, must be based sediment. marginal lake levels as a consequence on the deglaciation reconstruction by North of the point where the of the rapid shrinking of the Richard- Dyke (2004), in which Coronation White Sandy River flows into the Cop- son–Rae rivers ice lobe (St-Onge and Gulf is shown as becoming ice-free permine River, numerous perched McMartin 1995; Dyke et al. 2003). between 10 000 and 9 600 14C BP. gravel deltas are found east and north However, for the lake to exist, glacier of the Coppermine River valley. Phase 4: Postglacial Sea ice must have blocked the valley north Because the deltas are large and consist During Phase 4 (Fig. 3) the ice in of Rocky Defile where the top of the of coarse bouldery gravel, their con- Coronation Gulf thinned rapidly as a paleo-valley infill is at 310 m asl (Fig. struction required a high energy flow result of calving into the Arctic Ocean 9). This implies that a substantial that could only have come from melt- to the northwest. West of Coronation amount of glacier ice existed north and water streams from glacier lying no far- Gulf, retreat of the ice front allowed northwest in the Richardson River ther away than the present divide, an the glacial lakes to drain by a series of basin and in Coronation Gulf (Phase average distance of less than 15 km. side-hill channels. 2, Fig. 3) and an ice front was continu- An ice tongue occupied the The disintegration of the ice ous with the Central Wollaston Penin- southeastern part of the Dismal Lakes in Coronation Gulf and in the low- sula phase (Dyke et al. 2002) dated at basin. Here, a moraine of bouldery till lands to the south resulted in marine 10.2 ±0.1 – 10.4 ±0.1 ka. Glacial Lake was deposited across the valley of the inundation of the isostatically Coppermine is estimated to have lasted Dismal Lakes on the distal side of depressed lowlands to a present eleva- a minimum of 450 years (St-Onge et which, a thick apron of coarse out- tion of 170 m asl (St-Onge and al. 1981) and was a significant feature wash gravel was deposited in a lake. Bruneau 1982). Within the Copper- in the deglaciation landscape during The altitude of the outlet was con- mine River valley a large bouldery delta the late Chronozone. trolled by the sill elevation at 285 m asl at 165 m asl (Figs. 13 and 23) marks This interpretation requires that the of the channel cut at the western end this limit as do numerous other deltas 10.25 14C ka BP paleogeographic map of the lake during Phase I. This east of the Coppermine River and (Dyke 2004) be modified so that the deglaciation phase is correlative with south of Coronation Gulf. All deltas Coppermine River Valley was ice free the 10 400–10 200 years BP Central show ice-contact features proximally at this time. Wollaston Peninsula phase (Dyke et al. indicating that dead ice was still pres- A calcareous concretion 2003). Glacial Lake Coppermine is ent at this stage. This marine transgres- washed out from the base of the varve thus a major feature of the late sion marks the termination of the Late sequence of Glacial Lake Coppermine Younger Dryas Chronozone. Wisconsinan glaciation in the Copper- has yielded a 14C age of 14 400 ± 500 mine River valley and the beginning of 14C years BP (UQ-814). This is ca. Phase 3: Mouse and Richardson the emplacement of an offlap sedi- 3 800 years older than the estimate for Rivers mentary sequence between Muskox the Glacial Lake Coppermine above In the Kendall River–Dismal Lakes val- Rapids and Coronation Gulf (Fig. 12). and appears old relative to all the gen- ley, a retreat of 1 to 5 km from ice erally accepted models of deglaciation position 2 opened a channel just north DATING (Dyke 2004). of Rocky Defile Rapids at 300 m asl The events discussed are rarely dated (Fig. 3). When this occurred, the directly and generally only minimum SUMMARY AND CONCLUSIONS Kamut Lake outlet was abandoned, ages can be assigned. Table 1 in St- The deglaciation model presented here and Glacial Lake Coppermine drained Onge and McMartin (1995), lists the explains the morphosedimentary units northwest, through this new, lower significant radiometric dates along Dol- of late Quaternary age, in the Copper- outlet. Following this, drainage may phin and Union Strait to the northwest mine River valley region, as a continu- have been through the 285 m asl chan- of Coronation Gulf (Fig. 1). In the um from glacial to glaciolacustrine to nel at the western extremity of Dismal Coronation Gulf area (Fig. 1) dates marine environments. Lakes, as was previously assumed (St- range from 9 880 ±90 to 10 530 ±260 The postglacial history clearly Onge 1980; St-Onge et al. 1981) or it 14C years BP (St-Onge 1987). However demonstrates that the transition period could have been across a saddle north Dyke (2004) argued for the exclusion from a glacial to a nonglacial environ- of Dismal Lakes at 275 m asl, allowing of dates based on deposit-feeding ment was a time of great environmen- 146 tal change involving multiple phases in Montréal, provided essential expertise the shore of the polar seas in the the activity of geological processes. in the study of the Sleigh Creek For- years 1819-1822: London, John Mur- Meltwater was moving vast amounts of mation. Thanks are also due to Dr. W. ray: Edmonton, Hurtig, 1969, 768 p. sediment that was deposited as out- Padgham, Chief Geologist, Depart- Hoffman, P.F., 1984, Geology, Northern wash complexes or in glaciolacustrine ment of Indian Affairs and Northern Internides of Wopmay Orogen, Dis- sequences. trict of Mackenzie, Northwest Territo- Development, Yellowknife, whose ries: Geological Survey of Canada, When the ice receded from office provided grants to students, Map 1567A, scale 1:250 000. the Coronation Gulf region, glacial expert radio expediting during the field Lajoie, J., and St-Onge, D.A., 1985, Charac- lakes drained and the sea inundated the season and very much appreciated teristics of two Pleistocene channel fill depressed land. In incising through the access to fixed wing aircraft. Pilots, deposits and their implication on the glacial lake sediments, the Coppermine mining camp managers and colleagues interpretation of megasequences in River mobilized enormous volumes of who helped in a variety of ways are ancient sediments: Sedimentology, v. unconsolidated glacial lake sediment assured of my deep gratitude. Com- 32, p. 59-67, that occupied the preglacial valley. The ments by Marc St-Onge, Art Dyke and http://dx.doi.org/10.1111/j.1365- resulting debris flows, rapidly filled a two anonymous reviewers which 3091.1985.tb00492.x. network of bedrock valleys that were helped improve an earlier version of St-Onge, D.A., 1980, Glacial Lake Copper- mine, north-central District of then below sea level. Very large deltas, this paper are greatly appreciated. The the highest of which are at 170 m asl, Mackenzie, Northwest Territories: figures were drafted by Tracy Barry Canadian Journal of Earth Sciences, v. were built into the receding sea while and Kim Nguyen whose talents trans- 17, p. 1310-1315, , tens of metres of silty rhythmites were formed sketches into art. http://dx.doi.org/10.1139/e80-137. being deposited in the deeper part of St-Onge, D.A., 1984, Surficial deposits of the paleobasin. REFERENCES the Redrock Lake area, District of By comparison the present Baragar, W.R.A., and Donaldson, J.A., Mackenzie, in Current Research, Part day geological processes in the Cop- 1973, Coppermine and Dismal Lake A, Geological Survey of Canada, 84- permine River valley region are subtle map areas: Geological Survey of 1A, p. 271-278. and the landscape is essentially quies- Canada, 71-39, 20 p. and Maps 1337A St-Onge, D.A., 1987, Excursion Guide- cent. The roaring rapids, the pictur- and 1138A. book A-22, XII International INQUA esque falls, the minor landslides and Bruneau, H. C. 1985, Séquence sédimentai- Congress; Quaternary geology and the churning of surface material by re du secteur aval de la rivière Copper- of the Lower Cop- frost action do not significantly modify mine, Territoires du Nord-Ouest: permine River Valley, District of Géographie physique et Quaternaire, Mackenzie: National Research Council landforms inherited from the brief and v. XXXIX, No 3 p. 315-322 of Canada, 32p. turbulent period of ice retreat. Dyke, A.S., 2004, An outline of North St-Onge, D.A., 1988, Coppermine River, American deglaciation with emphasis District of Mackenzie, N.W.T., Com- ACKNOWLEDGEMENTS on central and northern Canada, in J. posite map of the following map- Fieldwork for this project was made Ehlers and P.L. Gibbard, eds., Quater- sheets: 83 G, Redrock Lake; 83 J, Hep- possible initially by a grant from the nary Glaciations–Extent and Chronol- burn Lake; 86 N, Dismal Lakes; 86 O Rector’s fund, University of Ottawa. ogy, Part II, North America: Amster- W1/2, Coppermine; Geological Sur- Later grants from Natural Sciences and dam, Elsevier, Developments in Qua- vey of Canada, Map 1645A. Engineering Research Council, Energy, ternary Science, v. 2b, p. 373-424. St-Onge, D.A., and Bruneau, H.C., 1982: Mines and Resources (now NRCan), Dyke, A.S., Andrews, J.T., Clark, P.U., Eng- Dépots meubles du secteur aval de la and Indian Affairs and Northern land, J.H., Miller, J.H., Shaw, J., and rivière Coppermine, Territories du Development now AAND), allowed Veillette, J.J., 2002, The Laurentide and Nord-Ouest, dans Recherches en Inuitian ice sheets during the last Gla- cours, partie B, Commission géolo- graduate students to become involved. cial Maximum: Quaternary Science gique du Canada, 82-1B, pp. 51-55. Of the many individuals who helped Reviews, v. 21, p. 9-31, St-Onge, D.A., and Dredge, L., 1985, the author and his students, Dr. Paul http://dx.doi.org/10.1016/S0277- Northeast extension of Glacial Lake Hoffman is certainly the one I am 3791(01)00095-6. McConnell in the Dease River basin, most indebted to; he and his senior Dyke, A.S., St-Onge, D.A., and Savelle, District of Mackenzie, in Current field assistant Dr. Marc St-Onge con- J.M., 2003, Deglaciation of South- Research, Part A, Geological Survey vinced me to go back to field work and western Victoria Island and adjacent of Canada, 85-1A, pp. 181-186. map the Quaternary sediments in the Arctic Mainland, Nunavut – North- St-Onge, D.A., and Geurts, M-A.,1982, Les Coppermine River region. One sum- west Territories: Geological Survey of formes d’effondrement et le mode de mer in the field with Paul’s group led Canada Map 2027A, scale 1:500 000. déglaciation de la région du lac Qinga- me, in 1982, to leave academic life and England, J., Dyke, A.S., Coulthard, R.D., luk, Territoires-du-Nord-Ouest, Cana- go back to the tundra, thus the rest of McNeely, R., and Aitken, A., 2012, da: Géographie Physique et Quater- The exaggerated radiocarbon age of naire, v. 36, p. 233-240, my career was defined. Their sustained deposit-feeding molluscs in calcareous http://dx.doi.org/10.7202/032479ar. help with field logistics made possible environments: Boreas, (Article first St-Onge, D.A. and Guay, F. 1982, Qua- the systematic mapping of the surficial published online: 27 APR 2012), ternary geology of upper Coppermine deposits of the Coppermine River http://dx.doi.org/10.1111/j.1502- River valley, District of Mackenzie, in basin. Dr. Jean Lajoie, retired from the 3885.2012.00256.x. Current Research, Part A, Geologi- département de géologie, université de Franklin, J., 1823, Narrative of a journey to cal Survey of Canada, Paper 82-1A, GEOSCIENCE CANADA Volume 39 2012 147

p. 127-129. St-Onge, D.A., and Lajoie, J., 1986, The late Wisconsinan olistostrome of the lower Coppermine River valley, Northwest-Territories: Canadian Jour- nal of Earth Sciences, v. 23, p. 1700- 1708, http://dx.doi.org/10.1139/e86- 157. St-Onge, D.A, and McMartin, I., 1995, Quaternary Geology of the Inman River Area, Northwest Territories: Geological Survey of Canada Bulletin 446, 59 p, Map 1846A. St-Onge, D.A., Geurts, M.A., Guay, F., Dewez, V., Landriault, F., and Léveillé, P., 1981, Aspects of the deglaciation of the Coppermine River region, Dis- trict of Mackenzie, in Current Research, Part A, Geological Survey of Canada, 81-1A, p. 327-331.

Received April 2012 Accepted as revised August 2012