Basal ice facies and supraglacial melt-out till of the Laurentide Ice Sheet, Tuktoyaktuk Coastlands, western Arctic Canada☆
Introduction
Glaciated lowlands in western Arctic Canada and northwestern Siberia contain many bodies of massive ice and icy sediments. Icy sediments contain excess ice, usually in the form of multiple ice lenses (Rampton and Mackay, 1971), and have a gravimetric ice content of less than 250%, whereas massive ice usually forms large, tabular bodies and has an ice content exceeding 250% (Harris et al., 1988). The distinction between massive ice and icy sediments, however, can be difficult to apply to complex stratigraphic sequences whose ice contents vary greatly over horizontal and vertical distances of a few metres or less as a result of glacial disturbance. Such glacially-deformed sequences can be described generally as massive ice and icy sediments (MI–IS) and specifically in terms of ice facies (cryofacies), as outlined in this paper.
MI–IS within glacial limits in Canada and Siberia have usually been interpreted either as buried glacier ice or as intrasedimental ice (ice formed within pre-existing sediment; Mackay and Dallimore, 1992). Distinguishing between them is fundamental to understanding the origin of massive ice, the interactions between glaciers and permafrost, and the history of Pleistocene ice sheets. However, interpretation of MI–IS is often problematic (Vtyurin and Glazovskiy, 1986, French and Harry, 1990), because intrasedimental ice may be difficult to distinguish from basal glacier ice, since both ice types can form by the same freezing processes (cf. Mackay, 1989). Thus the extent and relative importance of buried ice and intrasedimental ice remain uncertain. In addition, the possibility of widespread preservation of basal ice from the Laurentide Ice Sheet (LIS) merits scrutiny given (1) the growing consensus, albeit based on limited ice exposures, that buried Laurentide ice is widespread within large moraine belts, hummocky till and glaciofluvial deposits in western Arctic Canada (Sharpe, 1992, Dredge et al., 1999, St-Onge and McMartin, 1999, Dyke and Savelle, 2000) and (2) the suggestions that buried ice from the Barents-Kara Ice Sheet has been widely preserved for c. 80–90 ka in northwestern Siberia (Kaplyanskaya and Tarnogradskiy, 1986, Astakhov and Isayeva, 1988, Svendsen et al., 2004) and that buried ice of Miocene age has been preserved in southern Victoria Land, Antarctica (Marchant et al., 2002).
A new approach to describing MI–IS and interpreting their origin is to distinguish individual ice facies, based on the physical characteristics of the ice and sediment, and compare them with facies from intrasedimental massive ice and contemporary basal glacier ice. Hubbard and Sharp (1995), for example, have proposed a genetic classification for the basal ice facies of Alpine glaciers, each facies being attributed to distinctive basal conditions and processes. In contrast, classification and genetic interpretation of MI–IS facies have not yet been attempted.
This paper discusses glacially-deformed MI–IS inside the limit of the LIS in the Tuktoyaktuk Coastlands of western Arctic Canada (Fig. 1). The objectives are (1) to distinguish the individual ice facies of MI–IS; (2) to compare them with contemporary basal ice and englacial ice from Greenland, Alaska and Iceland, and with intrasedimental massive ice from Canada, in order to interpret their origin; and (3) to examine the cryostratigraphic, sedimentological and genetic relationships between MI–IS and overlying till. We conclude that basal Laurentide ice buried by supraglacial melt-out till is widely preserved in the Tuktoyaktuk Coastlands.
Section snippets
The study area
The Tuktoyaktuk Coastlands form part of the Arctic Coastal Plain between the Mackenzie Delta and Amundsen Gulf, Northwest Territories (Fig. 1). They are in the zone of continuous permafrost.
Methods
Sedimentological logging and sketching of large stratigraphic sections were carried out to determine the cryostratigraphic setting of the MI–IS. Particular attention was given to establishing (1) the cryostructures and cryofacies (Murton and French, 1994), (2) the nature and origin of the upper and lower contacts of the MI–IS, and (3) the stratigraphic and sedimentological relationships between diamicton dispersed within the MI–IS and the diamicton above it. Volumetric ice contents were
Description of MI–IS facies
Seven facies of glacially-deformed MI–IS are distinguished according to field estimates of volumetric ice content and sediment texture (Table 1).
Melt-out till
Several lines of evidence indicate that the Toker Point till above MI–IS in the study area formed by melt-out at the ice surface, and therefore is a supraglacial melt-out till. First, the till overlies an angular unconformity formed by thaw or erosion (Murton and French, 1993, Murton and French, 1994), as discussed below. Second, the till is present above MI–IS containing dispersed diamicton, but it is absent above massive ice that is debris-free.
Third, the till has a similar texture (Fig. 5),
Basal ice layer
The widespread MI–IS in the study area show, in the same stratigraphic sequences, features common to both basal and intrasedimental ice, but differ significantly from the massive intrasedimental ice at Peninsula Point. Any explanation of these facts must therefore accommodate both ice types and apply to a regional scale.
We interpret the glacially-deformed MI–IS as remnants of the basal ice layer of the LIS. This interpretation resolves the apparent paradox of co-existing basal and
Conclusions
- 1.
Complex sequences of glacially-deformed massive ice and icy sediments in the study area can be described systematically in terms of cryofacies (Table 1). This description facilitates comparison with other ice-rich sequences in permafrost and glacial environments and so assists with interpreting the origin of the ice.
- 2.
A basal-ice origin for the MI–IS in this study is indicated by (a) ice facies and facies groupings similar to those from the basal ice layers of contemporary glaciers and ice sheets
Acknowledgements
The Natural Sciences and Engineering Research Council of Canada and the Geological Survey of Canada (GSC) supported JBM's fieldwork between 1989 and 1993 by grants to Professor H.M. French. The Leverhulme Trust and the Tyrell Fund of the Geological Society supported the project between 1998 and 2000. CW is grateful to the University of Brighton Research Support Fund, and RIW's participation was supported by the University of Greenwich. The Polar Continental Shelf Project, Natural Resources
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Polar Continental Shelf Contribution Number 00904