glacigenic deposits at the edmonton convention centre, edmonton, alberta

Glacigenic deposits at the Edmonton Convention Centre, Edmonton, Alberta

N. R. CATTO Department of Geology, University of Alberta, Edmonton, Alta., Canada T6G 2E3

Received January 23, 1984 Revision accepted July 1 I , 1984

Four main stratigraphic units are present at the Edmonton Convention Centre site in downtown Edmonton. The lowermost sediment exposed is Cretaceous sandstone of the Horseshoe Canyon Formation. This is overlain by preglacial sands and gravels of uncertain age, which contain recumbent drag folds developed by glacial overriding of previously existing periglacial involutions. These sediments are surmounted by a glacigenic complex, consisting of subglacially modified sand and four till facies formed through passive melt out. The uppermost Quaternary sediment present is a proglacial lacustrine unit dominated by silts and clays with interbedded diamictons.

A single glacial episode is recorded in the sediments. Stratigraphic relationships with other exposures throughout the Edmonton region indicate that the glacigenic complex formed during the final ice advance. Variation in the orientation of glacially induced deformations in the preglacial sands and gravels and the basal glacigenic sand indicate that the direction of ice advance was not consistent throughout the final glacial event.

On a identifik quatre unites stratigraphiques dominantes sur le site du Centre des Congres d'Edmonton au centre ville d'Edmonton. Les sediments exposes B la base sont des gr&s du Crktace appartenant B la formation de Horseshoe Canyon. Ils sont recouverts par des sables et des graviers d'ige indetermink qui cornportent des plis d'entrainement renversks dCveloppks lors du charriage d'involutions pkriglaciaires prkalablement formkes. Ces sediments sont surmontCs par un complexe glacigknique incluant un sable sous-glaciaire perturb6 et quatre facies de till laissks par la fonte des glaces. A la partie supkrieure, les ddiments quaternaires appartiennent a une unit6 lacustre proglaciaire composee principalement de silts et d'argiles intercalks de diamictites.

Un seul Cpisode glaciaire est enregistre dans les sediments. Les relations stratigraphiques avec les autres formations exposees dans la rkgion d'Edmonton indiquent que le complexe glacigknique s'est form6 durant I'avancke finale du glacier. La variation de I'orientation des dkformations provoquCes par le glacier dans les sables et les graviers @riglaciaires et le sable glacigknique de la base rkvklent que la direction de I'avancCe n'etait pas uniforme durant tout le stade glaciaire final.

[Traduit par le journal]

Can. I. Eerth Sci. 21, 1473- 1441 119841

Introduction In recent years, extensive construction in Edmonton has

permitted the examination of several temporary exposures of glacigenic sediments. The most extensive and intriguing of these excavations was that required for the Edmonton Con- vention Centre, located along the north bank of the North Saskatchewan River (Fig. I).

Although the Grierson Hill site is conveniently adjacent to the downtown core. it posed several geotechnical problems. Solution of these problems required extensive geotechnical testing and necessitated the construction of a permanently an- chored tangent pile retaining wall on three sides of the excavation. The engineering aspects of the construction of the Con- vention Centre have been discussed by Balanko et al. (1982).

I Previous work Previous investigations of the Quatemary geology of the

Edmonton area have been mainly directed toward the identification of the basic sedimentary units. The initial researchers (e.g., Warren 1954; Bayrock and Hughes 1962; Westgate 1969) recognized the presence of a preglacial Pleistocene fluvial sand and gravel unit unconformably overlying Upper Cse- taceous strata of the Horseshc~ Canyon Formation (Green 1972). Th is unit was termed the "Saskatchewan Sands and Gravels" or "Saskatchewan Gravels" following the usage and criteria of Rutherford (1937) and Stalker (1968).

Overlying the Saskatchewan Sands and Gravels, two tills separated by a fluvial stratum were recognized by Warren (1954) and Westgate (1969). Westgate considered the more recent "upper till" to represent a glacial advance from the northeast during the late Wisconsin and the older "lower till" to

have been deposited by a glacier advancing from the northwest some time prior to or during the early Wisconsin. Bayrock and Berg (1964) recognized only a single till, ascribing the colour difference noted by Warren (1954) to oxidation of the upper portion of the stratum. These researchers also noted the presence of many stratified sand and gravel lenses throughout both portions of the t i[] . Kathol and MacPherson (1975) found no definitive evidence indicating that the deposition of the two till strata of Westgate (1969) was interrupted by a nonglacial event, but regarded the data supporting Bayrock and Berg's (1966) interpretation as inconclusive.

Shaw (1982) investigated exposures near Edmonton, at Villeneuve and Hugget. He concluded that the till at these localities was formed principally by passive melt out. Evidence for two glacial advances is present in the sections. The earlier advance originated from the northwest, whereas the flow direction during the more recent advance varied somewhat before the final establishment of a northeast-southwest trend. The chronological relationship between these two advances was not specified by Shaw, although he noted that stagnant ice from the initial advance may have been present during the deposition of the upper till. Two glacial advances have also been recognized in sections near Fort Assiniboine, north of Edmonton (St-Onge 1972b).

The glacigenic sediments are overlain by proglacial lacustrine silts and clays (Bayrock and Hughes 1962; St-Onge 1972~). Interbedded diamicton strata have been interpreted as subaqueous mass-movement deposits (Shaw 1975; Westgate et al. 1976; May 1977). Downcutting of the region by the modem North Saskatchewan River commenced approximately 11 000 years BP (Westgate 1969).

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CAN. J. EARTH SCI. VOL. 21, 1984

FIG. 1. Location of the Edmonton Convention Centre

Stratigraphy of the Convention Centre site In order to facilitate discussion of the sediments present at

the site, each of the major sedimentary units will be discussed separately. The stratigraphy of the site is illustrated in Fig. 2 a - c .

Bedrock Sandstone of the Horseshoe Canyon Formation is the domi-

nant lithology at the Convention Centre site. Minor amounts of interbedded siltstone, iron carbonates, coal, and bentonite are present.

In every location within the excavation site that has not been affected by mass movement of Quaternary material, the sandstone grades vertically upward into an intensely weathered regolith horizon. The thickness of this zone reaches a maximum of 90 cm in the northwest comer of the site and decreases irregularly to the south and east. The contact with the overlying preglacial gravels and sands is generally sharp and subplanar. The thickness of the regolith and the amount of feldspar de- pletion indicate that the horizon is pre-Quaternary, probably forming during the middle or late Tertiary.

In the upper metre of the unweathered zone, small-scale reverse faults are present. The thrust features are commonly less than 1 m in length. Evidence of the lack of disturbance of beds more than 1 m below the contact is provided by the undeformed nature of the incompetent siltstone, coal, and bentonite strata. The thrust planes do not exhibit any common orientation. No determination of the amount of disturbance the regolith had undergone was possible because of the absence of stratification planes in this unit.

A metres M) IW 120

R 0 20 40 89 I I

abrupt contact 1 --- gmdatioml contoct

(Northern W0ll) o oo- pebble layer ///////I reg~lith

I tonmhon centre (Central Section)

0-

C metres 100 1M

C' 0 20 40 60 80

I -- ~ c & m

hnds 8 Grad

0 I

Horseshoe Canym F m t l a n

Cowdon Cmtm { South Woll )

FIG. 2. Stratigraphy of the Edmonton Convention Centre site. (a) Northern wall. ( 6 ) Central section. (c) South wall.

Preglacial sands and gravels The thickness of preglacial sands and gravels varies from 1.3

to 5.8 m. The thickness of the unit is least in the southwest comer because of movements that have removed material. The upper contact, with glacigenic sediments, is mildly undulatory, with vertical variations of 15 cm/m common. Lithologic parameters for the unit are presented in Table l . The absence of minerals derived from igneous and metamorphic rocks indicates that the deposit was formed prior to the transport of Canadian Shield material to the area by glaciation.

Several contorted recumbent fold structures are present in the preglacial sediments. An example of these is illustrated in Fig. 3. Four fold structures with vertical dimensions in excess of 100 cm were noted. The width of these large folds varies

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TABLE 1. Lithologic properties of preglacial sediments and basal sands

Range and mean Range and mean* Range and mean Sediment type Texture (%) Mineralogy (%) Pebble lithology (%I

Preglacial sands Gravel 18-35 (26) Quartz 54-78 (66) Not analysed and gravels Sand 63-78 (71) Chert 9-35 (19) (15 samples) Silt 0- 6 (3) Feldspar 5-20 (12)

Heavy minerals 0 - 7 (2) Preglacial sands Gravel 65-90 (78) Not analysed Orthoquartzite 67-95 (79)

and gravels, Sand 10-35 (22) Chert 2- 18 (12) gravel beds Feldspathic sandstone 0- 1 l (6) (6 samples) Carbonates, shales 0-10 (2)

Basal glacigenic Sand 66-88 (78) Quartz 45-65 (54) Not applicable sand Silt 12- 3 1 (20) Feldspar 15-25 (20) (8 samples) Clay 0- 3 (2) Chert 5-10 (7)

Carbonate 0- 2 (1) Hornblende 3- 9 (7) Other heavy minerals 4-20 (1 1)

*Sand fraction between 2 and 46.

FIG. 3. Schematic diagram of recumbent fold in preglacial gravel.

from 45 to 115 cm. The fold axes plunge toward the north at angles between 3 and 8", and the noses are recumbent toward the east. Gravel forms the core of all the folds. Pebbles within the fold cores are arranged parallel to the disturbed gravel- sand contacts, and many are shattered or possess angular surfaces and edges. Small-scale low-angle reverse faults and lesser recumbent folds are present on the western sides of the large folds. The eastern margins and the areas beneath the recumbent overhangs are occupied by extensively contorted laminated preglacial sand units. Associated with the folds are small (12-38 cm long, 5-22 cm thick) lobate lenslike patches of preglacial gravel, which appear to have become detached from the lower stratum during deformation and were incorporated in the overlying basal glacigenic sediments. These gravel lenses truncate the bedding sf the overlying sand unit in some instances, but in other locations they are oriented con- formably with the sand stratification. Shattered and angular pebbles are abundant in these lenses.

Numerous small-scale recumbent folds and low-angle reverse faults not associated with the large deformations were also observed in the preglacial sediments. These folds were more prevalent in the sand members of the unit. Two distinct

groups of fold orientations are present: an eastwardly recumbent population, which encompasses the four largest folds and approximately 60% of the smaller ones; and a southerly recumbent group in the western portion of the excavation. The fold axes of the latter group plunge generally easterly at low (less than 5") angles, although horizontal and west-plunging fold axes were present.

Interpretation The large-scale deformations in the pregIacia1 sand and

gravel unit are interpreted as the resuIt of glacial overriding of preexisting large involutions developed in the gravel strata. The smaller folds are interpreted to have been produced by glacial motion. The cohesion of the gravel strata would be sufficient for folding and brittle deformation only if the gravel were frozen at the time of overriding.

Large involutions in preglacial sediments have been observed in the Edmonton area (Westgate et al. 1976). The involutions developed in gravel strata observed by the author at other localities in Edmonton consist of spherical to oblate structures 50-150 cm in height and 90-220 cm in width and breadth, with the granules and pebbles arranged concentrically. Shattered pebbles are common. Sand strata overlying the deformed units contain small reverse and normal faults along the contact with the involution. These structures have been previously interpreted to represent cold, moist climatic conditions (Westgate et al. 1976).

The original involutions must have formed subaerially, because of the climatic conditions required. The presence of reverse faults and small folds on the western sides of the struc- lures at the Convention Centre site and the morphology of the deformations suggest that the original spherical or oblate involutions were altered by the overriding of an eastwardly moving glacier, which exerted a tractive force. Thus, the original involurions were altered to form recumbent drag-fold structures, as shown diagrammatically in Fig. 4. The stratification in the up-force direction was partially preserved, whereas the structures on the distal sides were completely de- stroyed. No evidence to indicate whether the initial drag folds were formed ice-marginally or subglacially is present at the site.

Continued application of force resulted in the brittle deformation of some of the involutions, producing the disaggregated

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1476 CAN. J . EARTH SCI. VOL. 21, 1984

ice motion e

2 ice motion e

FIG. 4. Method of formation of recumbent drag folds, Convention Centre site. ( I ) Initial development of involution in gravel, caused by cryoturbation. (2) Ice override distorts involution in direction of ice motion, creating recumbent drag fold. Note distortion of laminations in overlying sediment. (3) Continued ice motion produces boudinage of drag fold, with frozen fragments incorporated in basal sands.

lenses of gravel noted in the overlying unit. Some of these lenses remained frozen during the extensive subglacial reworking of the overlying sand, as described below. Others were thawed and reworked into lenses paralleling the stratification of the sand unit. This indicates that the deposition of the overlying sand did not occur as a single episode of constant sedimentation, but was interrupted by periods of erosion.

Glacigenic sediments Glacially produced and derived sediments present at the

Convention Centre site include basal sands, diamictons, and sand lenses.

(i) Basal sands The thickness of the basal sands is 150- 170 cm along the

northern wall of the excavation. The unit thins rapidly to the south, reaching zero thickness approximately along the line of exposure of the south wall (Fig. 2c). The body of sediment is wedge shaped, with a curved upper surface. he upper contact, with the lower diamictons, is gradational in some locations along the northern face of the excavation. However, in general the contact is extremely sharp and is often marked by a band of pebbles and granules one clast thick that is imbedded in the diamictons. Laminations in the sands are deflected around these pebbles and granules.

Lithologic parameters for the basal glacigenic sands are presented in Table 1. No clasts larger than coarse sand size other than those contained in detached lobate lenses of preglacial grave1 are present. The silt and clay are dispersed throughout the unit; no lenses of texturally distinct sediment are present, with the exceptions of the lenses of preglacial gravel and irregular flame-shaped and lens-shaped siity diamicton structures 5- 19 crn in length. 2-6 crn thick, and 2-8 crn broad.

In addition to the flame-shaped diamicton lenses, the basal sand contains horizontal laminations 1-3 mm in thickness, imbricated clasts, and recumbent folds. The laminations are marked by coal. biotite, md chlorite and are irregularly spaced. The frequency of laminations tends to increase towards the base of the unit. Many of these laminations have been contorted subsequent to their deposition. The laminations are also dis- turkd irregularly in zones overlying the diamicton lenses.

Imbrication of the sand particles in undisturbed beds indicates that no consistent flow direction existed during deposition of the stratum. However, the current directions are approximately coincident with the preferred orientation of the granule and pebble clasts within the overlying diamicton unit in the immediate vicinity of each sample (Table 2) except in the areas surrounding the upper 30-50 cm portions of the large folds developed in the preglacial gravels. In these areas, the imbrication patterns indicate that the folds served as obstacles that deflected the water flow (Fig. 5).

Contorted small recumbent folds are present in the sands, but are uncommon. No faults or ioints were noted. The orientation of the axes of these folds and the direction of recumbence are often at variance to those of folds in the immediately underlying preglacial sediments. Although many orientations were noted, the modal direction of recumbence is east-southeast and the modal plunge direction is north.

Interpretation The basal sand unit is interpreted to be a subglacial glacio-

fluvial deposit. formed by the combined actions of fluvial reworking of preexisting proglacial sediment and deposition of sediment transported through the internal hydrologic system of the glacier. The subglacial stream probably flowed under a high confining glaciohydrostatic pressure between the lower frozen sediments beneath the ice and the debris-rich sole.

The periglacial involutions in the underlying gravel required support from surrounding sediments during their formation. Since the sediments that immediately overlie the involuted portions of the gravel and sand strata contain clasts derived from the Canadian Shield, the involutions must have developed when glaciofluvial outwash was being deposited from a glacier that was blocking the regional drainage routes to the northeast at some time during the Quaternary. The original sand unit must therefore have been ice-marginal. Several characteristics of the deposit, however, suggest that the sediment was sub- sequently reworked extensively in a subglacial environment. These are: texture, coincidence of flow directions with the directions of glacial motion as inferred from clast fabric analy- ses, nature of the contact with the overlying diamictons, and the presence of lenses of diamicton and preglacial gravel within the sand stratum.

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TABLE 2. Comparison of clast orientations in basal sand and overlying diamicton facies

Vertical Clast orientation Clast orientation separation

in basal sand in diamicton (cm)

NOTES: The similarity between the two groups of orientations suggests that they were produced as a result of the same event. Clast orientations given for the basal sand include both laboratory determinations on oriented blocks (I) and field observations (0.

The textural homogeneity of the sediment indicates that the amount of material larger than coarse sand available for fluvial transport was small. Since the deposit overlies sediments con- taining gravel strata and is overlain by glacial diamictons con- taining granules and pebbles, the unit must have formed under conditions that prevented the underlying gravels from being reworked and also prevented the segregation of large amounts of pebbles and granules from the glacial mass. The minor amounts of pebbles and granules eroded from the ice were probably transported over the beds of medium-grained sand in the manner described by Foley (1977) and Allen (1983). A water-filled tunnel beneath the ice, with the base consisting of frozen preglacial sediments, satisfies the textural conditions required for the depositional environment.

Current directions throughout the sand unit are approximately coincident with the preferred orientation of the large clasts within the overlying diamictons (Table 2). This coincidence suggests that the flow direction was controlled by the direction of ice motion. Subglacial tunnels would be expected to be oriented generally in the direction of ice motion (Weertman 1972), and this has been documented in topo- graphically confined glaciers (Collins 1979; Walder and Hallet 1979). Although the coincidence of flow directions in the sand and diamictons does not constitute definitive evidence, it is not incompatible with the subglacial tunnel hypothesis. The current directional indicators also show that the flow diverged around the large drag folds that protrude from the preglacial strata, indicating that these structures formed prior to the development of the stream.

The contact between the sand and the overlying diamictons is generally sharp and is often marked by a band of pebbles and granules one clast thick. These large clasts are imbedded in the diamictons. This relationship, combined with the deflection of laminations around the large clasts, suggests that the clasts served as obstacles to the flow and were not transported by the stream. The stream was thus confined from above, indicating a subglacial origin. A similar relationship between boulders and intratill sands was documented by Shaw (1982). The wedge-shaped nature of the sand body suggests that the direction of erosion was upwards into the ice and that the lower sediments were not eroded.

field measurements

4 laboratory measurements

FIG. 5. Divergence of clast imbrications around recumbent fold in preglacial gravel. Laboratory measurements of oriented blocks and field measurements combined indicate that the fold acted as an ob- stacle to water flow.

The sand contains discrete, unreworked, irregular lenses of diamictons and preglacial gravel. The geometry and lack of reworking of the majority of these lenses indicate that they were incorporated in a frozen state. Some preglacial gravel lenses were thawed during deposition of the sand, and these clasts have been reoriented parallel to the sand stratification. Boudinaged preglacial gravel lenses within the sand unit indicate that the ice flowed over the sediment, but do not provide information in regard to subglacial reworking.

The diamicton lenses, however, are more significant. The mineralogy and texture of the lenses are identical to those of diamicton facies I and la, which overlie the sand. No strati- graphically lower diamictons are present at the site. Con- sequently, it is necessary to assume that either (1) the diamicton fragments come from a lower unit, identical in texture and mineralogy to the overlying diamicton, that has been completely eroded away; or (2) the diamicton lenses formed as the result of subglacial stoping or calving and subsequent melting of ice blocks from the roof of a subglacial tunnel.

No evidence to support the first hypothesis exists in the vicinity of the site. Calving of blocks from the roofs of subglacial tunnels has been observed in alpine glaciers (Vivian and Bocquet 1973), and unsorted diamicton lenses interbedded in subglacially formed eskers have also been reported (Lundqvist 1969). A block of debris-rich ice that fell into a subglacial tunnel and was rapidly covered with sediment would produce an unsorted, unstratified diamicton lens upon ablation with an orientation unrelated to the stream direction. The resultant loss of volume would cause the overlying sands to collapse, producing disturbed laminations similar to those observed at the site. Since the blocks ablated beneath the water-sediment in- terface, no reworking of the sediment could occur, and the lenses remained texturally identical to the overlying diamictons. The volumes and dimensions of the lenses are more consistent with the volume of till that would be produced upon the ablation of discrete blocks than that expected from periodic ablation of the entire roof of the subglacial tunnel.

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1478 CAN. 1. EARTH SCI. VOL. 21, 1984

The suggested subglacial tunnel hypothesis can thus account for the textures, flow directions, upper contact, and included lenses of the sand unit.

(ii) Diamictons The unit overlying the basal sand and preglacial sediments is

composed of four diamicton facies, defined by structural and lithological criteria. Contacts between these facies are exclu- sively gradational. The thickness of the unit at the site is 10.1 m. Correlative sediments in the downtown area vary in thickness from 9.6 to 22 m. The variations in thickness appear to be largely controlled by preexisting topography. The upper contact, with lacustrine sediments, is characteristically gradational.

Diamicton facies 1 and la are similar in most respects, with the exception being the secondary jointing present in facies 1. Facies 1, present in the eastern and central portions of the excavation, varies in thickness from 6.2 to 7.9 m. Facies la is confined to the northwest comer of the site and ranges in thickness from 5.8 to 8.0 m. Other lithologic characteristics of the diamicton facies are presented in Table 3.

In the basal 2 m of both facies, tabular clasts of sandstone, siltstone, and coal (maximum dimensions 42 cm x 20 cm x 9 cm) were observed. The percentage of granules and pebbles decreases upwards, from 15% in samples above the basal contact to 6-8% in samples in the upper portion. Analysis of the large clasts in several horizons of both diamicton facies 1 and la indicated that the preferred orientation of the clasts was north-northeast-south-southwest (Fig. 6 a and b).

Diamicton facies 1 sediments in the northeastern and central portions of the site display two sets of secondary joints ortho- gonal to each other, oriented at approximately azimuth 210 and 120. The joints are all inclined at angles less than 15" from the vertical. The sediments of facies la are massive, save for the presence of minor randomly oriented desiccation joints.

Babcock (1977) attributed the development of similar jointing in the Edmonton region to slow downslope creep of prede- posited sediment from bedrock ridges and highs. Valley cutting by the North Saskatchewan River was cited as one of the major factors responsible for creating these topographic variations. In the excavation site, the azimuth 210 trend parallels almost exactly the course of the river valley in the vicinity. Near the site, the river executes a 90" meander bend and flows easterly along azimuth 115. The alignment of these two valley trends with the jointing pattern and the absence of joints in the sediments farthest from the river valley suggest that valley cutting of preexisting sediment is responsible for the development of the jointing pattern. The northwestern portion of the site has apparently not been subjected to any form of creeping motion, probably because of its distance from the valley wall prior to excavation.

Diamicton facies 2 is present in the northern portion of the site and varies in thickness from 1.9 to 3.7 m. ~ i l the contacts of this facies are gradational. Lithologic data are presented in Table 3.

Pebbles in diamicton facies 2 are generally oriented north- northeast-south-southwest, although the degree of alignment is much less than that shown by pebbles in facies 1 and la (Fig. 6c). Minor deviations from this pattern were noted.

Diamicton facies 3 is present along the northern border of the site and reaches a maximum thickness of 1.2 m. The contacts of this facies, including the upper contact with lacustrine sediments, are all gradational. No preferred direction of orientation of the pebbles was observed (Fig. 64.

Investigation of diamicton facies 2 and 3 was limited by the difficulty of accessibility. However, additional excavations throughout downtown Edmonton have produced temporary exposures of these diamictons.

Random sand lenses are present throughout all of the diamicton facies exposed at the site. The irregularly shaped and nonoriented lenses vary in thickness from 10 to 60 cm and in maximum width from 26 to 142 cm. The contacts between the sand lenses and the surrounding diamictons are distinct. Litho- logic data for the lenses are presented in Table 3. The only indication of current directions within the sand lenses is provided by the imbrication of the sand clasts. No consistent pattern of flow exists among the sand lenses or between the lenses and the preferred orientation of the fabric of the diamictons.

Interpretation Diamicton facies 1, la, and 2 are interpreted as tills de-

posited subglacially through basal melt out. The sediments of facies I and la were transported basally, whereas facies 2 is composed of englacially or basally transported sediments.

The diagnostic criteria for deposition of sediment by basal melt out are: (1) the presence of unlithified, sorted, stratified sediments interbedded with diamictons (Harrison 1957; Shaw 1979; Haldorsen and Shaw 1982); (2) the presence of structures, texturally differentiated subunits, and lithological dis- tributions that reflect those present in englacial sediments (Elson 1961; Haldorsen and Shaw 1982); (3) the presence of incorporated angular clasts of preexisting, friable sediment (Shaw 1982); and (4) the presence of clast fabric with a preferred orientation related to ice-flow direction (Harrison 1957; Lawson 1979; Haldorsen and Shaw 1982). Formulation of these criteria permits the evaluation of the basal melt out hypothesis.

The sand lenses present throughout the diamicton facies are unlithified and well sorted. Although the lenses do not show graded bedding or horizontal stratification, the clasts within them are imbricated. The directions of imbrication are consistent within individual lenses, but form no pattern throughout the site.

The variety of orientations of the lenslike bodies, especially those with sides inclined at steep angles, suggests that the streams that formed them were not free to flow under the sole influence of gravity, but were constrained by confining hydro- static pressure. Streams formed under these conditions are common in the englacial zones of ablating glaciers (Okko 1955; Boulton 1972). Although this point cannot be taken as conclu- sive evidence of deposition in the basal zone of the glacier during ablation, it is compatible with the subglacial hypothesis.

The textural composition of the till resembles that observed in englacial sediments (Boulton 1972; Lawson 1981). How- ever, the friable nature of the Cretaceous bedrock does not permit many sand-sized or larger clasts to survive extensive glacial transport. Consequently, the coarse fraction of all the tills in the Edmonton area is dominated by distally transported metamorphic and igneous rocks and minerals and resistent or- thoquartzites. Thus, lithology and mineralogy of tills cannot be used in the Edmonton area to assess the modal transport distance, unless the sediments were derived from the immediate vicinity of the site.

No angular clasts of preexisting unlithified sediment were observed. The presence of tabular clasts of weakly consoli- dated Horseshoe Canyon sandstones, siltstones, and coals, however, indicates that deposition of the till was by passive

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Bas

ic i

gneo

us

0-12

(7

) Si

lt 20

-50

(36)

C

arbo

nate

0-

6

(2)

Met

amor

phic

0-

6

(2)

Cla

y 12

-48

(30)

H

ornb

lend

e 4-

9

(6)

Che

rt

0-

8 (2

) O

ther

hea

vy m

iner

als

5-21

(1

1)

Iron

car

bona

te,

shal

e, c

oal

0-

4 (2

) D

iam

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n G

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les

and

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35-4

5 (4

0)

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36-5

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4)

faci

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rger

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2-

5 (3

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ar

35-4

5 (3

9)

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4 (4

3)

(5 sa

mpl

es)

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2

1 -30

(2

7)

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rt

4-

6 (5

) B

asic

ign

eous

4-

14

(7)

Silt

32-5

7 (4

1)

Car

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te

0-

2 (I

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orph

ic

0-

6 (3

) C

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7 (3

0)

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8(

6)

C

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(2)

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min

eral

s 4-

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0-

3 (2

) D

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n G

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les

and

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rtz

35

(35)

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rtho

quar

tzite

37

-40

(39)

fa

cies

3

larg

er c

last

s 3-

5

(4)

Feld

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40

-45

(42)

A

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c ig

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-56

(52)

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nd

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-67

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) M

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ic

3-

5 (4

) C

lay

11 - 19

(1 5)

H

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5

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1 - 8

(5

) O

ther

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als

10-

1 1 (

I 1)

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lens

es

Sand

75

-90

(86)

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-60

(53)

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ot a

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lt 10

-23

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17 - 22

(20

) C

lay

0-

3 (I

) C

hert

2-

5

(4)

Car

bona

te

0-

2 (I

) H

ornb

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e 3-

6

(4)

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min

eral

s 12

-27

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*San

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ctio

n be

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n 2

and

4+.

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1480 CAN. I. EARTH SCI. VOL. 21, 1984

FIG. 6. Typical clast orientation fabrics, diamicton facies. (a) Facies I . ( b ) Facies la. ( c ) Facies 2. (d) Facies 3.

melt out, either supraglacially or subglacially. These sediments are incapable of resisting the comminution involved in the lodgment process. The local origin of the clasts indicates that the transport distance was small, suggesting basal transport and deposition.

Diamicton facies I , la, and 2 all display clast fabrics that are strongly oriented north-northeast - south-southwest . The fabric patterns are similar to those observed from melt-out tills in the Edmonton area (Shaw 1982) and are comparable in form to fabric patterns from other sediments interpreted as melt-out tills (Harrison 1957; Lawson 1979; Haldorsen 1982).

When supraglacial melt out occurs, the fabric generated during glacial flow tends to be altered as the result of mass movement, fluvial reworking, and collapse caused by ablation of underlying ice. The resultant fabrics are usually chaotic (Kruger and Marcussen 1976; Lawson 1981), although fan- shaped and unimodal fabrics may be produced by individual

or combined mass-movement events (Niewiarowski 1969; Marcussen 1975). However, oriented fabric patterns formed by local mass movements are unlikely to correspond consistently to the regional direction of ice flow.

At the Convention Centre site, the preferred orientation of the fabric does correspond to the inferred direction of advance on a regional scale of the most recent glacier to affect Edmon- ton, as determined by Westgate (1969). Thus, the nature of the fabric pattern and the regional glacial geology of the Edmonton area indicate that deposition by melt out from a glacier that advanced from the north-northeast is a hypothesis capable of explaining the characteristics of the till. The sedimentary characteristics of diamicton facies 1, la, and 2 are therefore compatible with all four of the criteria indicated for basal melt out, although no one criterion is absolutely diagnostic.

Diamicton facies 3 can be interpreted as a supraglacial melt out till, fonned in proximity to or within a supraglacial or

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proglacial lake. The random fabric is characteristic of till deposits deformed by mass movements (Lawson 1981), and the gradational contact with lacustrine sediments indicates that the two sediment types formed penecontemporaneously.

Lacustrine sediments Lacustrine sediments are present in the northern portion of

the Convention Centre site (Fig. 2). The lower contact is generally gradational with the diamicton units, although locally sharp contacts do occur where the sediments are underlain by facies 2.

The sediments are primarily silt and clay. The lower 1.5 m of the lacustrine material is structureless, with the exception of interbedded diamicton. These lenslike bodies vary irregularly in thickness from 5 to 32 cm and are wedge or spoon shaped. The matrix is predominantly silty, and granules and pebbles constitute 1-5% of the sediment. These clasts are subangular and generally subequant, although the proportion of elongate clasts is larger than for diamicton facies 1, la, and 2. These lenses are interpreted as supraglacial or proglacial flow deposits.

The upper portion of the lacustrine silt and clay is weakly laminated and devoid of diamicton lenses. It extends to the surface, except where overlain by colluvium.

direction shifted gradually to the south-southwest, and subglacial modification of the basal sands was intensified. Since the till was deposited by passive melt out, not lodgment, its fabric reflects the final direction of ice advance and fails to preserve any earlier orientations. Thus, variance exists between the ice-flow directions recorded by the earliest sediments to be affected and the till deposited during the ablation of the glacier.

Regional significance A single advance is recorded in the glacigenic sediments

at the Convention Centre site. The till grades upwards into lacustrine sediments correlated with the uppermost glacially influenced sedimentary unit in the Edmonton region and is therefore correlated with the most recent glacial advance recognized by Westgate (1969) and Shaw (1982).

The absence of till formed by earlier glacial advances, such as that reported from the Fort Assiniboine area by St-Onge (1972b), is probably due either to erosion of previously deposited sediment between glaciations, or to local nondeposition during the earlier glaciations. However, the possibility that the site remained covered by stagnant ice during an interstadial of indefinite length, in the manner suggested for other sites in the Edmonton region by Shaw (1982), cannot be eliminated at present. In this case, any supraglacial till or other sediments produced by the stagnating ice must have been extensively

Colluvium reworked during the subsequent advance, since no material that Colluvium from several mass-movement events overlies the could be identified as supraglacial was observed in the sedi-

sediments over the southern portion of the site. The material is ments of facies and la. a structureless silt with scattered pebbles, reaching a maximum The Convention Centre site exposure thus does not resolve thickness of 87 cm' The genera' form of the is wedge the stratigraphic debate over the late Quaternary of the Edmon- shaped, increasing in thickness towards the south. ton region. It has, however. provided additional information

Summary The evidence available indicates that the glacigenic deposits

at the Convention Centre site were formed as a result of a single glacial advance. lnitial deformation of the preglacial unit, re- presented by the recumbent drag folds, was followed after an interval of unknown duration by the deposition of four till facies, representing subglacial and supraglacial environments, and the formation of a basal sand unit by subglacial meltwater flowing under high pressure. No unconformities could be recognized in the sequence. The general ice-flow direction was from the north-northeast, although the initial drag folds were formed by eastward-moving ice.

The diversity of orientations in the fold structures present in the preglacial sediments and basal sands could be caused either by multiple glacial advances or by fluctuations within a single glacier. The presence of lobate lenses of preglacial sediment within the subglacial sand indicates that deformation continued after the initial glacial overriding. Variations in fold orientation similar to those observed in the till fabrics would thus be expected to occur. However, the possibility that an advance that preceded the deposition of the basal sand was responsible for some of the deformation of the preglacial sediments cannot be excluded entirely.

The major recumbent drag folds in the preglacial gravel trend easterly and were formed by ice moving in that direction. However, they are imbedded in a subglacially modified sand unit that formed penecontemporaneously with at least the latter stages of deformation and that is conformable with the overlying till. These facts can be reconciled by postulating an initial eastwardly glacial advance, possibly a very local deflection of the main ice flow, that deformed the preglacial sediments either subglacially or proglacially. As overriding continued, the flow

.2

about the sedimentology of the region's glacigenic deposits and the genesis of glaciotectonic structures.

Acknowledgments The advice, guidance, and assistance of Dr. Stan Thomson,

Department of Civil Engineering, and Dr. N. W. Rutter, De- partment of Geology, University of Alberta, and Dr. J . Shaw, Department of Geography, Queen's University, Kingston, Ontario, are acknowledged and greatly appreciated. The paper was substantially improved through the comments of two anonymous reviewers. Financial support for this research was provided by the Department of Civil Engineering, University of Alberta, through Natural Sciences and Engineering Research Council of Canada (NSERC) grant 3559. ALLEN, J. R. L. 1983. Gravel overpassing on humpback bars supplied

with mixed sediment: examples from the lower Old Red Sandstone, southern Britain. Sedimentology, 30, pp. 285-294.

BABCOCK, E. A. 1977. A comparison of joints in bedrock and frac- tures in overlying Pleistocene lacustrine deposits, central Alberta. Canadian Geotechnical Journal, 14, pp. 357-366.

BALANKO, L. A,, MORGENSTERN, N. R., and YACYSHYN, R. 1982. Tangent pile wall, Edmonton Convention Centre. EBA Engineering Consultants Ltd., American Society of Civil Engineers National Convention, Las Vegas, NV, Technical Paper 82-1, 19 p.

BAYROCK, L. A., and BERG, T. E. 1966. Geology of the city of Edmonton. Alberta Research Council, Report 66- 1, 30 p.

BAYROCK, L. A., and HUGHES, G. M. 1962. Surficial geology of the Edmonton District, Alberta. Alberta Research Council, Preliminary Report 62-6, 40 p.

BOULTON, G. S. 1972. Modem arctic glaciers as depositional models for former ice sheets. Journal of the Geological Society, London, 128, pp. 361-393.

COLLINS, D. N. 1979. Quantitative determination of the subglacial hydrology of two alpine glaciers. Journal of Glaciology, 23,

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1482 CAN. J. EARTH SCI. VOL. 21, 1984

pp. 347-362. Commission Geologiska Finlandi, Bulletin 170, 133 p. ELSON, J. E. 1961. The geology of tills. In 14th Canadian Soil RUTHERFORD, R. L. 1937. Saskatchewan gravels and sands in central

Mechanics Conference, National Research Council of Canada, Alberta. Transactions of the Royal Society of Canada Transactions, Associate Committee on Soil and Snow Mechanics. Edited by E. Section 4, Series 3, 31, pp. 81 -95. Penner and J. Butler. National Research Council of Canada, Tech- SHAW, J . 1975. Sedimentary successions in Pleistocene ice-marginal nical Memorandum 69, pp. 5 - 36. lakes. In Glaciofluvial and glaciolacustrine sedimentation. Edited

FOLEY, M. G. 1977. Gravel-lens formation in antidune-regime flow- by A. Jopling and B. McDonald. Society of Economic Paleon- a quantitative indicator of hydrodynamic regime. Journal of Sedi- tologists and Mineralogists, Tulsa, OK, Special Publication 23, mentary Petrology, 47, pp. 738-746. pp. 281-303.

GREEN, R. M. 1972. Geological map of Alberta. Alberta Research - 1979. Genesis of the Sveg Till and Rogen moraines of central Council, Map 35. Sweden: a model of basal melt-out. Boreas, 8, pp. 409-426.

HALDORSEN, S. 1982. The genesis of tills from Astadalen, south- - 1982. Melt-out till in the Edmonton area, Alberta, Canada. eastern Norway. Norsk geologisk tidsskrift, 62, pp. 17-38. Canadian Journal of Earth Sciences, 19, 1548- 1569.

HALDORSEN, S., and SHAW, J. 1982. The problem of recognizing STALKER, A. MAcS. 1968. Identification of Saskatchewan gravels melt-out till. Boreas, 11, pp. 261-277. and sands. Canadian Journal of Earth Sciences, 5, pp. 155- 163.

HARRISON, P. W. 1957. A clay-till fabric, its character and origin. ST-ONCE, D. A. 1972a. Sequence of glacial lakes in north-central Journal of Geology, 65, pp. 275-308. Alberta. Geological Survey of Canada, Bulletin 213.

KATHOL, C. P., and MCPHERSON, R. A. 1975. Urban geology of - 19726. La stratigraphie du Quaternaire des environs de Fort- Edmonton. Alberta Research Council, Bulletin 32, 61 p. Assiniboine, Alberta, Canada. Revue de geographie de Montrkal,

KRUGER, J., and MARCUSSEN, 1. 1976. Lodgment till and flow till: a 26, pp. 153-163. discussion. Boreas, 5, pp. 61 -64. VIVIAN, R., and BOCQUET, G. 1973. Subglacial cavitation phenomena

LAWSON, D. E. 1979. A comparison of the pebble orientations in ice under the glacier d'Argentikre, Mont Blanc, France. Journal of and deposits of the Matanuska Glacier, Alaska. Journal of Geology, Glaciology, 12, pp. 439 -45 1. 87, pp. 629-645. WALDER, J., and HALLET, B. 1979. Geometry of former subglacial

198 1. Distinguishing characteristics of diamictons at the mar- channels and cavities. Journal of Glaciology, 23, pp. 335-346. gin of the Matanuska Glacier, Alaska. Annals of Glaciology, 2, WARREN, P. S. 1954. Some glacial features of central Alberta. pp. 78-84. Transactions of the Royal Society of Canada, Section 4, Series 3,

LUNDQVIST, J. 1969. Morphogenetic classification of glaciofluvial 48, pp. 75-86. deposits. Sveriges geologiska undersokning, Serie C, 767, pp. WEERTMAN, J. 1972. General theory of water flow at the base of a 1-72. glacier or ice sheet. Reviews of Geophysics and Space Physics, 10,

MARCUSSEN, I. 1975. Distinguishing between lodgment till and flow pp. 287-333. - till in Weichselian deposits. Boreas, 4, pp. 1 13 - 123. WESTGATE, J. A. 1969. The Quaternary geology of the Edmonton MAY, R. W. 1977. Facies models for sedimentation in the glacio- area. In Pedology and Quaternary research. Edited by S. Pawluk.

lacustrine environment. Boreas, 6, pp. 175 - 180. University of Alberta Press, Edmonton, Alta., pp. 129- 15 1. NIEWIAROWSKI, W. 1969. Stone arrangement in till of the last inland WESTGATE, J. A., KALAS, M., and EVANS, R. 1976. Field trip guide-

ice in the lower Vistula region. Geografia, 8, pp. 137- 148. book C-8. Geological Association of Canada - Mineralogical OKKO, V. 1955. Glacial drift in Iceland: its origin and morphology. Association of Canada, Annual Meeting, Edmonton, Alta., 49 p.

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glacigenic deposits at the edmonton convention centre, edmonton, alberta

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