Fort Assiniboine area to the north of the study area. Bayrock & Berg (1966) and Kathol & McPherson. (1975) recognised only one till. Shaw (1982) recognised.
Sedimentology (1987) 34, 103-1 16
Glacial sedimentary processes and environmental reconstruction based on lithofacies JOHN SHAW Department of Geography, Queen’s University, Kingston, Ontario, Canada K7L 3N6
ABSTRACT Glacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially-induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass-movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass-movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass-movement.
INTRODUCTION A variety of sedimentary processes takes place in
glacial environments resulting in generally complex sedimentary sequences. Release of debris from ice, reworking and deposition by mass-movement processes, and deposition in fast flowing streams and in lakes may all occur within a few tens of metres of an ice margin. Temporal variability is added to this spatial variability, with diurnal, seasonal and other scales of change in weather conditions influencing sedimentary processes. In addition to the continuous change in position of ice margins, meltwater streams and marginal lakes further complicate the issue of glacigenic sedimentation. Even though we might understand the sedimentary processes that occur beneath, within, upon and beyond glaciers, the large number of potential sedimentary sequences that could plausibly be produced by a single glacial event makes their prediction all but impossible. Observations on modern glaciers are of limited use in this respect for they represent only a synoptic view that does not include the information on the evolution of environments and distribution of processes over
time and space that is required if sedimentary successions and their lateral variations are to be reconstructed. This paper presents an approach by which detailed study of the sediments themselves is used to infer sedimentary processes and reconstruct environments of deposition. To this end a number of facies have been identified at sites around Villeneuve, near Edmonton, Alberta (Fig. 1). Following the advice of Walker (1979), the facies were only identified after careful documentation of the sediments to be interpreted. It follows that the facies created specifically for this study are highly site dependent. Such facies may or may not be useful elsewhere. In this sense the facies are different from those of the facies code of Eyles, Eyles & Miall (1983). They used a predetermined facies code to describe vertical sequences. The facies used here are designed more for the interpretation of cross sections than vertical sequences. The regional setting of the deposits and their stratigraphic context will be outlined before embarking on a detailed discussion of the facies. 103
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‘L 10 km
0
200
krn.
Fig. 1. Location map showing the major pits and faces in which sedimentary sequences were recorded.
REGIONAL SETTING The Edmonton area is situated on the Western Plains of Canada with regional drainage being towards the northeast. The major stream in the area is the North Saskatchewan River which flows to the south of the study sites. The Laurentide Late Wisconsinan Ice Sheet retreated towards the northeast. Consequently the regional drainage was blocked to create large icemarginal lakes in the area (Bayrock & Hughes, 1962;
St-Onge, 1972a; Shaw, 1975; May, 1977). Cordilleran ice flowed out from the mountains and was probably confluent with the Laurentide ice to the west but never extended as far as the study area. Preglacial fluvial gravels in the area, the Saskatchewan Gravels of Westgate (1969), contain mainly quartzitic clasts where they were derived from the mountains and mainly quartz sand, coal and clay ironstone where they were derived from the local Cretaceous rocks. The onset of glaciation in the area
Glacial processes and environmental reconstruction
is clearly registered by the appearance of granites and gneisses from the Canadian Shield to the northeast. Sands associated with the glacial rocks are feldspathic and their pink colour contrasts with the ‘salt and pepper’ appearance of the sands derived from the Cretaceous sandstone. This strong lithological and mineralogical contrast between preglacial and glacial sediment makes for easy field recognition of the important stratigraphic level that marks the onset of glacial deposition. As already mentioned, the end of glaciation in the area was marked by the existence of a widespread glacial lake, Glacial Lake Edmonton. The deposits of this lake are ubiquitous in the area with the minor exception of areas of postglacial fluvial erosion. Consequently the base of the lacustrine deposits forms a readily indentifiable stratigraphic level. The sediment described and interpreted in this paper occurs between the stratigraphic level that marks the onset of glacial deposition and that marking sedimentation in glacial Lake Edmonton, beyond the direct influence of processes at the ice margin. Emphasis is placed on the detailed environments and processes that gave rise to these sediments.
FACIES D E S C R I P T I O N S All of the facies described here were exposed in a series of pits excavated between 1975 and 1984 in the Villeneuve area. Gravel was extracted from the preglacial deposits and the full glacial sequence was exposed. The method of quarrying involved clearing the overburden, leaving a series of vertical faces in the glacial deposits. The sediment described here was exposed in some of these faces (Fig. 1).
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Fig. 2. Facies A, Low angle plane beds in sand and gravel with till balls. The erosional margin of the channel containing these beds is seen to the right. Selected till balls and the channel margin are enhanced. Scale bar 0.5 m.
only in the Consolidated Concrete pit (Fig. 1) where it occurs below facies B. Facies B: homogeneous diamicton Homogeneous diamicton is invariably tough and difficult to penetrate with a knife and is usuallyjointed. Joints are either columnar, extending into overlying deposits, or sub-horizontal and intersecting, bounding lenses of the diamicton 20-30 mm thick and 0.1-0.2 m wide. The basal few millimetres of this facies may contain streaked-out inclusions of the underlying beds. A striking feature of this diamicton is the abrupt, planar lower contact that truncates underlying crossstratified, cross-laminated and plane beds (Fig. 3). Two departures occur from this planar contact. A variety of groove marks, some with stones at their downflow end, were formed at the base of the diamicton by dragging stones through the sand substrate, and by lodging stones on the bed which
Facies A: plane-bedded sand and gravel with till balls This facies occupies broad channels cut into the underlying preglacial sands and gravels. The maximum thickness of this facies is about 3 m. It shows low angle plane bedding with the beds dipping at right angles to the axes of the channels in which they are contained. These plane beds are in places cut by scours, themselves filled with plane beds and clusters of till balls. The sand and gravel is well sorted and contains ‘till balls’ or diamicton clasts up to 0.15 m in diameter (Fig. 2). The plane bedding, gravel and till balls indicate deposition in the upper flow regime (Fahnestock & Haushild, 1962). This facies is noted
Fig. 3. Preglacial sand and gravel at the base of the sequence is erosionally overlain by diamicton facies B which displays prominent jointing. Faulted intradiamicton sand, outlined in black, is contained in facies B. Scale 0.32 m.
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Fig. 4. The underside of facies B showing a linear fluted ridge and associated groove, at the top of the photograph, produced by the dragging of a clast through the substrate. The trailing ridges and prow associated with the clast show that it lodged and these bed features represent positive forms on the ice bed caused by flow around the clast. Scale bar 50 mm.
grooved the base of the overriding glacier (Fig. 4). Larger scours up to 0.5 m in depth, also occur at the base of this diamicton in association with boulders (Shaw, 1983). Diamicton facies B contains faceted and striated clasts, many of which are of Laurentian Shield origin. It also contains angular, unconsolidated clasts of the underlying non-glacial sediment. Intradiamict beds of glacial sand and gravel are common and show intense faulting (Fig. 3), whereas ribbons of glacial sand and gravel at the base of this diamicton are unfaulted The upper surface of this diamicton may be planar and marked by a boulder lag where fluvial erosion has taken place subsequent to the deposition of the till. Elsewhere the surface is irregular and the diamicton has been injected into overlying sediment (Shaw, 1982). The long axes of clasts in this diamicton show strongly preferred orientation parallel to the regional direction of ice-flow determined from sole markings and surface fluting (Fig. 5).
mud. Upwards through the thickness of any one unit of this diamicton, there is a transition in which stratified mud and sand become increasingly admixed with diamicton. The impression is given that the diamicton is made up of stratified sediment that has been intermixed by a combination of faulting, folding and attenuation during mass flow. However, this explanation is not complete for it is common to find a larger proportion of stones in the diamicton than is found in the associated stratified sediment. This occurs despite the fact that the matrix of the diamicton is mostly sand and silt. No characteristic jointing pattern is associated with this diamicton which often appears massive. It is also soft and can be easily excavated in contrast to the tough diamicton of facies B. Facies D: faulted, current-beddedsand and gravel This facies is volumetrically the most important in some sections where it reaches a maximum thickness of 3.9m. It contains beds of sand and gravel with minor units of silt. Sedimentary structures include
Facies C: heterogeneousdiamictonand stratified sediment This facies consists of a common association in which a heterogeneous diamicton, containing bodies of stratified sediment that show various stages of intermixing with the diamicton, is intimately related to beds of contorted and/or faulted stratified sediment (Fig. 6). Thicknesses of this facies range from about 20 mm to 2.5 m. The lower boundaries of the facies are almost always gradational into stratified sand and
Fig. 5. Scmidt net plot of fault poles and Eigenvector of clast long axes attitudes. Trends of sole marks and azimuths of dip of cross-lamination and stratification.
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Fig. 7. Cross-stratified sand of facies D with scour around an ice-rafted clast. Scraper 0.2 m long.
plane bedding, tabular and trough cross-bedding in sand and gravel, cross-lamination in sand, usually with low angles of climb, and some graded bedding in fine sand and silt. The graded beds are draped over underlying bedforms. Deposition was mainly in the upper part of the lower flow regime, indicating strong currents. Paleocurrent estimates show these currents to have been unidirectional, although some sets of cross-lamination indicate regressive ripples and reverse flows (Fig. 5). The sand units include a few oversized stones (Fig. 7). Thrust faults that cross-cut facies D are steeper to ) to the south (
