AVERAGE DEPTH OF GLACIAL EROSION,. CANADIAN SHlELD. C.A. Kaszycki. Atomic Energy of Canada Limited, c/o Geological Survey of Canada, Ottawa ...
SCIENTIFIC AND TECHNICAL NOTES - NOTES SCIENTIFIQLIES ET TECHNIQUES AVERAGE DEPTH OF GLACIAL EROSION, CANADIAN SHlELD
C.A. Kaszycki Atomic Energy of Canada Limited, c / o Geological Survey of Canada, Ottawa, Ontario. K I A OE8 W.W. Shilts Terrain Sciences Division
The e x t e n t of glaciation' in Canada is such t h a t considerable e f f o r t has been devoted t o the study of t h e distribution and c h a r a c t e r of glacial and related sediments.
h o w e v e r , relatively l i t t l e research based on field observation and analysis of glacial sediments has been directed towards evaluating t h e e x t e n t of continental erosion a t t r i b u t a b l e t o glaciation. This l a t t e r question has particular relevance t o consideration of t h e use of t h e subsurface for disposal of toxic wastes including those arising from t h e generation of nuclear power.
A review of t h e l i t e r a t u r e presented many conflicting viewpoints concerning t h e amount of glacial erosion t h a t has occurred on t h e Canadian Shield. E s t i m a t e s vary from thousands of f e e t (White, 1972) t o a f e w t e n s of f e e t (Flint, 1971). In an a t t e m p t t o resolve this disparity, samples
Figure 1. Dispersal of Dubawnt Group erratics. Isopleths represent weight per cent Dubawnt clasts in the 2 to 6 mm fraction of till (Dubawnt outcrops modified from Wright, 1967; LeCheminant et al., 1977; and Donaldson, 1966, 1969). Inset map shows location of study areaFrom: Scientific and Technical Notes in Current Research, Part B; Geol. Surv. Can., Paper 79-18.
already collected by t h e Geological Survey of C a n a d a have been used t o e s t i m a t e a v e r a g e a m o u n t s of glacial erosion through a study of glacial dispersal of distinctive rock types, specifically t h e lithologically distinctive rocks of t h e Dubawnt Group (Donaldson, 1965) in t h e District of Keewatin (Fig. I). I t was hypothesized t h a t by evaluating t h e volume of Dubawnt d e t r i t u s in till located down-ice from Dubawnt outcrops, and by calculating t h e a r e a of outcrop traversed by i c e flow, i t would be possible t o e s t i m a t e t h e a v e r a g e depth of glacial erosion t h a t has occurred in t h e region. This exercise was designed t o develop t h e t e c h n i q u e s of performing such calculations a s well a s t o derive s o m e figures f o r erosion of a specific rock type. T h e Dubawnt Group is composed of relatively f l a t lying Proterozoic sedimentary and felsic volcanic rocks with c h a r a c t e r i s t i c red t o purple hues. Their distinctive colour a n d lithology permit reliable identification in t h e coarse till fractions. The a r e a of o u t c r o p extends f r o m t h e e a s t e r n end of Baker L a k e southwestward f o r approximately 320 km and is traversed by t h e c e n t r a l portion of t h e Keewatin Ice Divide. The existence of t h e dispersal train was first noted a s a strip of red coloured till extending f r o m t h e a r e a of Dubawnt outcrop southeastward t o t h e c o a s t of Hudson Bay (Shilts and Boydell, 1974). The distinctive colour of t h e till is imparted by glacially ground specular hematite, a derivative of t h e Dubawnt Group rocks. In order t o quantify t h e Dubawnt component, t h e granule-fine gravel fraction of approximately 800 till samples was examined. Each sample w a s sieved t o obtain particles in t h e 2 t o 6 m m s i z e range, and t h e percentages of Dubawnt Group rocks w e r e determined on a weight per c e n t basis. T h e fine sand and clay f r a c t i o n s of s o m e samples also were examined in order t o e s t i m a t e t h e Dubawnt component of t h e till throughout various size fractions. In t h e vicinity of t h e outcrop t h e Dubawnt component is found t o compose approximately 70 t o 80 per c e n t of t h e granule-fine gravel fraction of t h e till. This value decreases exponentially with distance in t h e direction of i c e flow (southeastward) until t h e dispersal train is artificially t r u n c a t e d a t t h e c o a s t of Hudson Bay. A t this point Dubawnt e r r a t i c s comprise 5 t o 10 per c e n t of t h e 2 t o 6 m m s i z e fraction. Mathematical extrapolation of t h e dispersal curve i n t o Hudson Bay indicates t h a t dispersal could continue f o r a t least 200 km past t h e coast. Preliminary calculations of depth of erosion have been based upon t h e assumption t h a t t h e Dubawnt component of t h e granule-fine gravel s i z e fraction is a reasonable approximation of. t h e Dubawnt component of t h e till a s a whole. Knowing t h e a r e a of dispersal, plus t h e average depth of till throughout t h e region (-3 m), t h e amount or depth of glacial erosion of Dubawnt Group rocks has been e s t i m a t e d t o be 5.5 t o 8.0 m (a figure in a g r e e m e n t with Flint, 1971). Further, more refined calculations of erosion, taking i n t o a c c o u n t t h e variation of Dubawnt components throughout various s i z e fractions, and more precise estimations of variation in t h e depth of till throughout t h e dispersal a r e a , a r e n o t expected t o a l t e r this preliminary e s t i m a t e significantly. A knowledge of t h e r a t e of erosion is useful when a t t e m p t i n g t o e s t i m a t e t h e magnitude of erosion t o be e x p e c t e d a t one place from t h a t calculated a t another. If, f o r example, t h e amount of erosion calculated for t h e Dubawnt dispersal train resulted from only 4000 years of a c t i v e erosion, i t i s n o t necessarily valid t o a s s u m e t h a t t h e magnitude of .erosion elsewhere on t h e Canadian Shield, w h e r e a c t i v e erosion may h a v e spanned t e n s of thousands of years, will b e similar t o t h a t of t h e Dubawnt Group rocks. In o r d e r t o d e t e r m i n e erosion r a t e s f o r t h e Dubawnt Group it is necessary t o define t h e period of t i m e during which i c e was flowing towards Hudson Bay. The popular view t h a t i c e flow f e a t u r e s in Keewatin, indicating i c e flow
towards Hudson Bay, a r e of late-glacial origin c a n b e seriously challenged by a study of t h e Dubawnt dispersal train. T h e f a c t t h a t Dubawnt e r r a t i c s have been transported a t l e a s t 300 km towards Hudson Bay i s inconsistent with t h e t i m e constraints f o r debris transport (2000 t o 4000 years) imposed by defining t h e Keewatin Ice Divide a s a late-glacial f e a t u r e formed when rising s e a w a t e r in Hudson S t r a i t split an i c e dome centred on Hudson Bay. The minimum r a t e of debris transport in t h e basal i c e imposed by t h e s e constraints (100 t o 300 rn/year) is t o o high t o b e considered applicable t o a largely s t a g n a n t wasting i c e 'mass. If t h e Keewatin Ice Divide is considered t o have been a c t i v e throughout most of t h e period of Wisconsin Glaciation, t h e dispersal of Dubawnt e r r a t i c s c a n b e assumed t o have t a k e n place over a much longer p e F i ~ dof time, at a more realistic r a t e of flow. An a v e r a g e flow r a t e of 10 m l y e a r would allow 30 000 years f o r t h e transportation of Dubawnt e r r a t i c s over a distance of 300 km. It h a s been assumed, therefore, t h a t t h e Keewatin Ice Divide and/or o t h e r landbased c e n t r e s of outflow w e r e a c t i v e throughout m o s t of t h e Wisconsin Glaciation and t h a t r a t e s of glacial erosion with respect t o t h e Dubawnt dispersal train may b e calculated on this basis. It is evident t h a t t h e definition of t h e Dubawnt dispersal train has not only afforded an opportunity t o study glacial erosion of t h e Canadian Shield but has also e f f e c t e d a rethinking of t h e traditional c o n c e p t of t h e configuration of t h e Laurentide Ice S h e e t (Shilts et al., in press). References
Donaldson, J.A. 1965: T h e Dubawnt Group, Districts of Keewatin and Mackenzie; Geological Survey of Canada, P a p e r 64-20, 11 p. 1966:
Geology, Schultz Lake, District of Keewatin; Geological Survey of Canada, Map 7-1966, scale I inch t o 4 mile.
1969:
Descriptive notes (with particular r e f e r e n c e t o t h e L a t e P r o t e r o z o i c Dubawnt Group) t o accompany a geological m a p of c e n t r a l Thelon Plain, D i s t r i c t s of Keewatin and Mackenzie; Geological Survey of Canada, P a p e r 68-49, 4 p., Map 16-1968.
Flint, R.F. 1971: Glacial and Quaternary Geology; John Wiley and Sons Inc., New York, 892 p. LeCheminant, A.N., Blake, D.H., Leatherbarrow, R.W., and d e Bie, L. 1977: Geological studies: Thirty Mile L a k e and MacQuoid L a k e map areas, District of Keewatin; in R e p o r t of Activities, P a r t A, Geological Survey of Canada, P a p e r 77-IA, p. 205-208. Shilts, W.W. and Boydell, A.N. 1974: Terrain Mapping in t h e Churchill - Chesterfield Inlet Corridor, D i s t r i c t of Keewatin; R e p o r t of Activities, P a r t A, Geological Survey of Canada, Paper 74-I A, p. 253-256. Shilts, W.W., Cunningham, C.M., and Kaszycki, C.A. The Keewatin Ice Sheet - reevaluation of t h e traditional c o n c e p t of t h e Laurentide Ice Sheet; Geology. (in press) White, W.A. 1972: Deep erosion by continental i c e sheets; Geological Society of America Bulletin, v. 83, p. 1037-1056. Wright, G.M. 1967: Geology of t h e southeastern b a r r e n grounds, p a r t s of t h e Districts of Mackenzie a n d Keewatin ( o p e r a t i o n s Keewatin, Baker, Thelon); Geological Survey of Canada, Memoir 350, 9 1 p.
GEOLOGICAL SURVEY PAPER 79-1 B
CURRENT RESEARCH PART B RECHERCHES EN COURS PARTIE B
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