Subglacial meltwater origin and subaerial meltwater modifications of drumlins near Morley, Alberta, Canada

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary Geology 91 (1994) 285-298

Subglacial meltwater origin and subaerial meltwater modifications of drumlins near Morley, Alberta, Canada Timothy G. Fisher a, Ian Spooner b '~ Department of Geography, University of Calgary, Calgary, Alberta T2N 1N4, Canada b Department of Geology and Geophystcs, University of Calgary, Calgary, Alberta T2N 1N4, Canada Received December 10, 1991; revised version accepted March 24, 1993

Abstract

Drumlins in the Bow valley west of Calgary, Alberta, are interpreted as the product of subglacial and subaerial meltwater erosion. On the sides of the Bow valley highly elongated, streamlined, first-generation drumlins are aligned in an en-echelon fashion and are not significantly modified by post-glacial processes. In the valley bottom, scalloped, stubby, irregularly spaced second-generation drumlins are found in close association with valley fill. The drumlins are composed of bedrock and diamicton, and some are overlain by a thin gravel veneer. Stratified gravel is found on the leeward flanks of some second-generation drumlins. Meltwater erosional forms (s-forms) found on the limestone bedrock up-flow of the drumlins, and crescentic furrows around their proximal ends, indicate that subglacial meltwater beneath the Bow valley ice was probably the main erosive agent responsible for the shaping of the drumlins. The second-generation drumlins are believed to be the remnants of the first-generation drumlins that were dissected and reworked by postglacial, subaerial meltwater flow(s). Meltwater drainage from ice-dammed glacial Lake Kananaskis and possibly other glacially dammed tributary valley lakes, resulted in the deposition of megaripples (3.5 m high, 40 m wavelength) 8 km down-flow from the drumlins and reverse eddy deposits on the flanks of the drumlins. Post-glacial Bow River incision and terrace development has continued to further sculpt these forms.

I. Introduction

The Bow valley (Fig. 1) has been the focus of much research on the deglaciation of the western Canadian Cordillera. Any modification of existing models of Bow valley deglaciation would significantly alter regional models of deglaciation for western Canada. We have observed a continuum of features ranging from s-forms to drumlins to giant current ripples within the Bow valley. We show that this suite of geomorphic features is

consistent with the flood hypothesis for erosional drumlins (Shaw and Sharpe, 1987). The study area incorporates many diverse physiographic regions which include the Bow and Kananaskis river valleys which drain the Canadian Rocky Mountain Front Ranges (Fig. 1). The Front Ranges form the eastern edge of the Canadian Cordillera and are dominated by Paleozoic carbonate strata. The study area is the Morley Flats region of the Bow valley, a broad alluvial plain located about 65 km west of Calgary where

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the Bow River emerges from the Rocky Mountain Front Ranges (Fig. 2). The braided gravel outwash is termed the Morley gravel (Walker, 1971). Clusters of drumlinoid ridges are found over much of the Morley Flats and were previously interpreted as crag and tail erosional (valley bottom) or depositional (along valley wall) drumlins (Tharin, 1960), crevasse infillings (Nelson, 1963), and as products of basal accretion (Walker, 1971, 1973).

limited ground truthing. Most drumlins are located on the Stony Indian Reserve to which access was denied. Discontinuous exposures along 4 km of the Canadian Pacific Railroad right-of-way east of Seebe were examined (Fig. 3). Fig. 3 was produced by mapping the drumlin forms on aerial photographs and 1:50,000 map sheets. We use the non-genetic term "s-form" (Kor et al., 1991) to classify exposed bedrock erosional forms along Highways la and 1 in the Exshaw region and drumlin morphology in the Morley region (Fig. 2).

2. Methodology The Morley Flat drumlin field was examined by aerial photographs, low-altitude flights and

3. S-forms

3.1. Observations

Fig. 1. Location of the study area. Modified from Walker (1971).

Numerous s-forms (see Kor et al., 1991 for specific s-form terminology) were examined at road-cuts along Highways 1 and la (Fig. 2) where surfaces had been protected from chemical dissolution by a covering of sand, gravel or diamicton. Rat tails (Prest, 1983), residual ridges similar to the scour remnant ridges of Allen (1982) (see Shaw and Sharpe, 1987 for a discussion), up to 1 dm in length and 2 cm in height were observed in the lee of a limestone bedrock ridge near Exshaw. Rat tail trends were variable over a distance of 4 m but always aligned subparallel to the axis of the Bow valley. Other well-formed rat tails were observed in the same local area but are now buried due to road construction (L.V. Hills, pers. commun., 1992; see Prest, 1983, fig. 21c). Sichelwaanen (crescentic erosion forms) have been described from glaciated environments (Bernard, 1971; Shaw and Sharpe, 1987; Murray, 1988; Sharpe and Shaw, 1989). South of Exshaw they are carved into limestone bedrock and are associated with facetted undulating surfaces. The average median ridge is 40 cm high, 10 m long and 2 m wide. Grooved and polished bedrock with diverging striae lightly inscribed on sichelwaanen surfaces were also observed. At the eastern-most location (Fig. 2) dm- to m-scale cavettos (Dahl, 1965) and grooves are overlain by a well-in-

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4. Drumlin morphology

durated grey, silty clay diamicton that is rich in striated clasts.

4.1. Observations 3.2. Interpretation Within the Morley Flats two distinct drumlin forms (referred to as generations) were observed. The first-generation forms are found at higher elevations above 1265 m (in the middle of the field), along the north and south Bow valley walls and are not surrounded by Morley gravel. The second-generation forms are on the valley bottom and are surrounded by Morley gravel (Fig. 3). The drumlin field expands in width down-flow from the mouth of the Bow valley trending 245 °65 ° (Fig. 3). North of Seebe some drumlins trend 190°-10 ° over a low bedrock divide into the Ghost River valley (Figs. 2, 3). At the southern edge of the study area the drumlins trend parallel to the 12 km long bedrock ridge (Fig. 3, outlined by the 1372 m contour interval north of Chiniki Lake), with the best developed forms on its northern side. Along the valley bottom there is a paucity of

S-forms are evidence for abrasive erosion by turbulent water (Allen, 1982; Sharpe and Shaw, 1989). Within glacial environments these forms have been used as evidence for subglacial meltwater events (Prest, 1983, fig. 21d; Shaw and Kvill, 1984; Murray, 1988; Shaw and Sharpe, 1987; Shaw et al., 1989; Fisher, 1989; Kor et al., 1991). The s-forms in the Exshaw region were observed at varying elevations above the valley bottom and on most non-weathered bedrock exposures, which suggests that while the Bow Glacier occupied the valley, a valley-wide erosive subglacial meltwater flow(s) occurred. The formation of the esker complex in the valley floor (Fig. 3) must postdate the s-forms, as the meltwater flow that sculpted the s-forms would have eroded or modified the esker complex.

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drumlin forms, and those that do exist are associated with the Morley gravel.

First-generationforms The first-generation forms are found at a higher elevation than the Morley gravel. They range in shape from classical elliptical forms to complexes of nested and en-echelon arranged forms (Figs. 3-5). To the northwest of Chiniki Lake the drumlins are stubby, widen distally and end abruptly. Drumlins with similar morphology have been described in drift from Wollaston Peninsula, N.W.T. (Sharpe, 1984, 1987) and in bedrock at Beverly Lake, N.W.T. (Shaw and Sharpe, 1987). The concentration of elongated drumlins south of Morley on both sides of Highway 1 are complex and strongly en echelon in form (Fig. 3). These drumlins are composed of till, some with a veneer of sand and gravel (Walker, 1971, 1973). Numerous ridges have crescentic scours (CS in

289

Fig. 5) around their proximal ends a n d / o r longitudinal furrows (SL in Fig. 5) parallel to the ridge long axis, many of which are occupied by lakes or bogs. The most complex forms are often stubby, subparallel or transverse to flow with superimposed linear ridges (Figs. 3, 5). Often, up flow from these forms, a transverse trough is proximal to numerous stoss side furrows (Fig. 5).

Second-generation forms The second-generation (reworked) forms were described by Walker (1973, p. 1345) as "post-depositionally deformed by meltwater issuing from the Bow and Kananaskis valleys". On Fig. 3, the second-generation drumlins are surrounded by the light stippling which represents braided outwash, and are distinguishable from the first-generation forms by concave scours, truncated ends, straight sides in the Seebe and Morley areas and a more northerly orientation (2250-45 ° as compared to 245o-65 °) in the Seebe area (Figs. 3, 6).

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Fig. 4. Low-angle oblique photograph of Morley Flats. First-generation forms are in the foreground and left (south) of major highway. Arrow denotes Bow valley gap in the Rocky Mountain Front Ranges. From bottom of photo to the mountains is 20 kin.

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The Morley gravel (braided outwash) above the uppermost Bow River terrace abuts against, and infills along, the second-generation forms in the Seebe area. In the Morley region, braided outwash and terraces of a large alluvial fan can be traced from just east of Chiniki Lake down through the drumlins to the highest Bow River terrace east of Morley (Fig. 3). 4.2. Interpretation

The variable and often complex forms of the first-generation Morley drumlins are explained by subglacial meltwater flow(s). Although some forms resemble constructional drumlins described

by Shaw (1983) and Shaw and Kvill (1984) from northern Saskatchewan, the lateral and crescentic proximal furrows associated with most ridges are constituent features of scour remnant ridges (cf. Allen, 1982) or the medial ridges of Shaw and Sharpe (1987) and Kor et al. (1991). Drumlins in the Morley swarm composed of bedrock and till further support the erosional remnant hypothesis. This interpretation is consistent with the origin of the s-forms found up flow from the drumlins. The second-generation forms are first-generation forms truncated by subaerial floods. The terraces, braided outwash and fan morphology near Morley relate to a water source from the Chiniki valley, while the truncated forms and

Fig. 5. Stereo-pair of first- and second-generation drumlins. Note the crescentic scours (CS) around the proximal ends of the drumlins and scour lakes (SL) in the lee of numerous ridges and parallel to an en-echelon ridge. Note also the truncated first-generation forms in the lower left corner (dashed line) by Morley gravel braided outwash. Flow was from right to left.

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braided outwash in the Seebe area relate to flow from the Bow and possibly Kananaskis valleys (Walker, 1971).

5. Drumlin sedimentology

5.1. Observations The drumlins east of Morley are between 300 and 425 m long, 60-275 m wide and up to 23 m high. They are composed of till and bedrock, with the bedrock at either the proximal end (crag and tail drumlin), middle or lee of the drumlin (Tharin, 1960). Commonly there is a lee-side gravel veneer (Tharin, 1960; Walker 1971). Tharin (1960) described the till as a highly calcareous, pebble to silt diamicton with rounded limestone and quartzite clasts and shale and coal fragments. Two shallow excavations in the lee of a second-generation drumlin ridge 4 km northeast of

291

Seebe were examined (Fig. 3). The upper cut exposes 1.2 m of grey matrix-supported diamicton with a fissile structure (Fig. 8A). The matrix is homogeneous. Clasts sizes were polymodal, ranging from granules to small boulders. Clast shape varied from rounded (quartzite) to angular (carbonates). Clasts were not imbricated, but some (ca. 10%) were striated. The Seebe drumlin is separated from an adjacent drumlin running parallel to it by a 130 m wide channel containing shallow ponds (Fig. 7). The lower cut, level with the railroad tracks about 4 m above the valley bottom, exposes stratified gravel which grades laterally into a gravel-rich diamicton (Fig. 8B) overlain by colluvium. The matrix of the gravel-rich diamicton is silty sand varying to either silt or clean sand. Most clasts were round but some striated, angular limestone clasts and rare intraclasts of mud were observed. Clast size was variable, ranging from granules to cobbles. The stratified gravels were clast supported with a sandy matrix and pebble to cobble

Fig. 6. Aerial photograph of first- (south of dashed line) and second-generation (north of dashed line) drumlins. Note the truncation and sculpting of second-generation complex forms. Flow was from left to right.

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flow direction

1

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Fig. 7. Schematic diagram of the Seebe drumlin. (A) Stratigraphy of the Seebe drumlin. Till is overlain by an eddy deposit and capped by colluvium. The fabric analysis of the poorly sorted gravel on the drumlin flanks indicates a paleocurrent opposite to the regional trend of the drumlins. Key: d = drumlin trend; n = number of observations; S~ = mean normalized strength of clustering for the principal eigenvector; ~ = mean orientation; 2 = mean dip). (B) Location of the upper and lower exposures in the Seebe drumlin. Note scalloped flanks and abrupt contact of the drumlin flanks with braided outwash.

clast f r a m e w o r k . A p a l e o f l o w d i r e c t i o n o f 30 ° was r e c o r d e d in t h e gravels ( m e a n e i g e n v e c t o r statistically significant at t h e 1% significance level [ M a r d i a , 1972]) while t h e d r u m l i n long axis t r e n d s 222 ° . A t a r a i l r o a d cut n e a r S e e b e Village ( A on Fig. 3) a grey d i a m i c t o n i d e n t i c a l to t h a t o b s e r v e d at the S e e b e d r u m l i n is u n c o n f o r m a b l y o v e r l a i n by a b o u l d e r lag a n d w e l l - s o r t e d a n d s t r a t i f i e d gravel (Fig. 8C). T h e 1.8 m thick gravel has a p l a n a r t a b u l a r s t r u c t u r e with c r o s s - b e d s o f clast-supp o r t e d large c o b b l e s n o r m a l l y g r a d e d to p e b b l e s i n d i c a t i n g flow from 225 ° . T e n m e t r e s to t h e east the gravel is 1.4 m thick a n d consists o f o p e n a n d

c l o s e d work, h o r i z o n t a l l y b e d d e d cobbles.

pebbles and

5.2. Interpretation The combination of bedrock, diamicton, sand a n d gravel m a k i n g up t h e M o r l e y d r u m l i n s nullifies any single p r o c e s s to explain t h e d r u m l i n s e d i m e n t o l o g y a n d final form. T h e grey d i a m i c t o n is i n t e r p r e t e d as a till b e c a u s e it displays massive fissile s t r u c t u r e , c o n t a i n s s t r i a t e d clasts, a n d is similar to s e d i m e n t i n t e r p r e t e d as till by T h a r i n (1960), W a l k e r (1971) a n d R u t t e r (1972). T h e i n t i m a t e a s s o c i a t i o n of till a n d b e d r o c k o b s e r v e d

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203

Fig. 8. (A) Grey, matrix-supported diamict with fissile structure in the upper exposure of the Seebe drumlin. (B) Poorly sorted stratified gravel which merges laterally into a gravel-rich diamicton exposed in the lower exposure of the Seebe drumlin. (C) Grey till (in lower right corner) unconformably overlain by well sorted and stratified gravel. Figure is standing on colluvium.

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Fig. 8 (continued).

in many drumlins reflects subglacial deposition (lodgement?) under the Bow valley ice. The firstgeneration forms consist of different combinations of sediment and bedrock, and are morphologically similar to the erosional remnant drumlins of subglacial meltwater floods (Shaw and Sharpe, 1987). We submit that drumlin form rather than internal sedimentology be used to support formation by meltwater erosion. Lee-side stratification has been observed in Irish drumlins (Dardis et al., 1984; Hanvey, 1987, 1989), while in extraglacial environments, Baker (1973) described pendant bars as streamlined mounds of flood gravel in the lee of bedrock outcrops. Komar (1983) described streamlined forms that developed by erosion of a form with deposition in its wake. Sand and gravel deposits on the first-generation drumlins are restricted to their lee sides, an observation inconsistent with pervasive sedimentation expected in an ice-marginal environment. The sand and gravel veneer on the firstgeneration drumlins may be interpreted as a sub-

glacial lee-side deposit at a hydraulic jump where flow expansion led to sediment deposition. The general stratigraphy of the second-generation Seebe drumlin consists of lee-side stratified gravel and gravel-rich diamicton draping the till core of the drumlin (Fig. 7). The stratified and imbricate gravel fabric indicates a flow direction nearly the opposite to that of the proposed subglacial meltwater flood (Fig. 7A). Pardee (1942) described reverse paleocurrents within poorly sorted sediments (unsorted clasts which ranged in size from pebbles to boulders with a matrix of sand and silt) which were interpreted as eddy bar deposits in valleys flooded by the catastrophic drainage of glacial Lake Missoula and deposited as flood debris slurries in the channel margin recesses.

Based upon the above discussion, and the similarity between sediments described by Pardee (1942) and those on the flank of the Seebe drumlin, a similar erosion and depositional interpretation is used to explain the fine-grained, lee-side

T.G. Fisher, L Spooner/ Sedimentary Geology 91 (1994) 285-298

sediments at the base of the Seebe drumlin. A subaerial flow reworked the first-generation drumlins in the valley bottom, eroded the channel and scour lakes between the drumlins and deposited the stratified gravel and gravel-rich diamicton as a back eddy deposit in the drumlin lee.

6. Giant current ripples

6.1. Observations Giant current ripples found on the highest (1232 m a.s.1.) alluvial surface northeast of the Ghost Reservoir (Fig. 2) are down-flow of and elevationally accordant with the braided outwash associated with the second-generation drumlins. Ridge wavelength averages 40 m (ca. 55 ridges, ranging from 22 to 57 m in wavelength) with an average amplitude of 3.5 m (range 2 to 5 m). The ripples are composed of planar tabular cross-bedded pebbles and cobbles with rare small boulders. Individual cross-beds dip at 20 ° to 35 °, and fine upwards from open-work c o b b l e / b o u l d e r s to closed-work pebbles. The ripple gravel surface is pitted with kettles > 200 m in diameter and up to 15 m in depth.

6.2. Interpretation Giant current ripples are often associated with rapid drainages of glacial lakes (Theil, 1932; Pardee, 1942; Bretz et al., 1956; Fahnestock et al., 1969; Baker, 1973), subglacial floods (Shaw and Gorrel, 1991) and other floods (Moore and Moore, 1988). By association the Ghost Reservoir ripples also suggest a high-magnitude flood.

7. Discussion and conclusions

Menzies and Rose (1989) suggest that drumlins are formed by either: (1) deformation of the glacial substrate; or (2) catastrophic subglacial meltwater floods. We propose that the continuum of erosive forms in bedrock and sediment observed within the Bow valley is consistent with a

295

subglacial meltwater origin for drumlin formation. We explain the Morley drumlin swarm through a two stage model. (1) Subglacial meltwater flow sculpts the carbonate bedrock where relief is great and the Bow River valley is confined (Exshaw region), leaving behind erosional remnant s-forms in the bedrock. Removal and modification of till and bedrock by the same subglacial meltwater flow, east of the Rocky Mountain Front (Seebe-Morley regions), result in the formation of drumlins as remnant ridges, some with a leeside sand and gravel veneer. (2) Subaerial flooding reworks the lowest elevation drumlins, forms the giant current ripples along the Ghost Reservoir and results in back eddy deposits in the lee of some drumlins. As the first-generation forms do not cross-cut each other a simultaneous origin of all drumlins as opposed to a piecemeal formation is indicated. Forms do not cross-cut one another but they diverge. The divergence of the Morley drumlin swarm to the north portrays a different pattern than the Peterborough, Ontario drumlin swarms in Paleozoic lowland areas (Shaw and Sharpe, 1987, fig. 9) and the Beverly Lake, N.W.T. drumlin field on the Canadian Shield (Prest, 1983, fig. 37), composed of non-diverging, parallel ridges. The divergence of the Morley Flats drumlins may be explained by lateral flow of a lobate ice mass expanding into the Bow valley, a n d / o r the influence of the bedrock topography on concentration of flow. The subglacial meltwater erosional model (Shaw and Sharpe, 1987) did not explain how, in a physiographic region of low relief, the vast quantities of water (e.g. 84.1 x 103 km 3, Shaw et al., 1989) needed to produce extensive erosive drumlin swarms might have been stored and suddenly released. The smaller Morley drumlin swarm would require a proportionally smaller volume of meltwater (an order of magnitude smaller; 10.5 km width as compared to > 100 km width of the Peterborough or Livingstone Lake drumlin fields; Shaw and Sharpe, 1987, and Shaw et al., 1989, respectively) to create similar forms. In the Cordillera it is conceivable that high-relief basins and tributary valleys feeding the Bow val-

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ley served as water reservoirs. For example Hazard Lake, Yukon drains regularly by a j6kulhlaup (Clarke, 1982; Liverman, 1987). J6kulhlaups would have channelled subglacial meltwater down the Bow valley, eroding bedrock and sediment. The s-forms at Exshaw appear subdued compared to many forms on the Canadian Shield, and could be explained by lower-magnitude floods, lack of exposure, and poor preservation due to weathering of carbonate bedrock. At the Rocky Mountain Front, high relief rapidly gives way to wider valleys and subdued relief to the east. Here, meltwater spread over a broader area and less sediment was eroded. Residual drumlins resulted. Concave scours, truncated drumlins, giant current ripples and outwash restricted to the valley bottom, are all associated with the second-generation drumlins and are evidence for a younger subaerial flood. There are two separate areas of second-generation forms, one near Morley, the other near Seebe. Those south of Morley (Fig. 3) were eroded by meltwater issuing from the valley occupied by Chiniki Creek. Glacial Lake Kananaskis formed between the southward retreating Kananaskis Glacier and the Bow Glacier to the north (Walker, 1971). As the Bow Glacier retreated, successively lower outlets were uncovered. The Chiniki outlet was either the last or second last outlet to open. The eroded drumlins with the inter-ridge braided outwash and alluvial fan distal to the eroded ridges (east of Morley) are attributed to the glacial Lake Kananaskis drainage. The floodwater which formed the second-generation drumlin forms in the Seebe region originated from either the opening of the last outlet of glacial Lake Kananaskis as the Bow valley ice continued to retreat, or from the Bow valley. A suggested catastrophic outburst flood in the Banff region (Eyles et al., 1988, 1990; but opposed by Mandryk and Rutter, 1990) or from other glacially dammed tributary valleys is a likely erosional mechanism for the formation of the second-generation drumlins in the Bow valley west of Exshaw. A similar occurrence of drumlins fanning out from the mouth of the North Saskatchewan River where it exits in the Cordillera (Smith, 1990, p.

42) can be found 100 km north of the Morley site. There, a drumlin field similar in size, extent and morphology is hypothesised to have resulted from a sequence of events similar to those proposed for the Morley area. The formation of the first- and second-generation drumlins would require a minimum of two short-duration (1 subglacial, 1 subaerial)j6kulhlaup-style floods. Though the erosional results of the floods are described we have not discussed the depositional expression of these events. About 5 km east of the study area, and within the Bow valley, lacustrine sediments of glacial Lake Calgary (Tharin, 1960; Wilson, 1983; Harris, 1985; Moran, 1986) are first encountered. We have observed massive to highly contorted lacustrine silt, dewatering structures and stoss-depositional climbing ripples indicative of rapid sedimentation, which could be related to the subglacial/ subaerial meltwater erosion of till at the Morley Flats. Both the age and depositional environments of the glacial Lake Calgary silts is poorly constrained (Harris and Ciccone, 1983, 1986; Jackson, 1986); it is conceivable that the Bow valley may be a region in which all components of the subglacial erosional meltwater model for drumlin formation are found.

Acknowledgements We would like to thank Gerald Osborn and Stuart Harris for a helpful review of the original manuscript and two anonymous referees for many constructive comments. We are also grateful to Derald Smith who pointed out the giant current ripples to us. Partial funding was provided by the University of Calgary's Graduate Student Association, Academic Project Fund.

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