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Glacial Erosion by the Laurentide Ice Sheet

  • D. E. Sugden (a1)

Abstract

The aim of the paper is to analyse landscapes of glacial erosion associated with the Laurentide ice sheet at its maximum and to relate them lo the three main variables affecting glacial erosion, namely former basal thermal regime of the ice sheet, the topography of the bed, and the geology of the bed. The key to the analysis is the comparison of the distribution of landscape types with the simulated pattern of the basal thermal regime of the former ice sheet.

Landscapes of areal scouring are found to be associated with zones of basal melting and occur beneath much of the former ice-sheet centre and in those places where the topography favoured converging ice flow. The landscape type may also have formed beneath cold-based ice when it was carrying debris inherited from an up-stream zone of regelation. Areas with little or no sign of glacial erosion occur primarily in the north in the Queen Elizabeth Islands but they also occur on uplands associated with diverging ice flow; they coincide with areas calculated to have been covered by cold-based ice devoid of debris. Landscapes of selective linear erosion are common on uplands near the eastern periphery of the ice sheet. In these situations, pre-existing valleys channelled ice flow and created a situation where there was warm-based ice over the valleys and cold-based protective ice over the intervening plateaux. Variations in the permeability of the bedrock base have modified the landscape pattern, mainly in those areas where there was a change from one basal thermal regime to another. In general, permeable rocks tend to have experienced less erosion than impermeable rocks.

Using lake-basin density as an indication of the intensity of glacial erosion, a zone of maximum erosion is identified and this forms a ring between the centre of the former ice sheet and its periphery. This ring coincides with a zone where melt water from the ice-sheet centre is calculated to have frozen on to the bottom of the ice sheet. This regelation incorporated basal debris into the ice, forming a basal layer 20-50 m thick and afforded an efficient means of debris evacuation.

A conceptual model is developed and hangs round the following postulates:

  • (1)Landscapes of glacial erosion are related primarily to the basal thermal regime of the ice sheet.
  • (2)Landscapes of glacial erosion are equilibrium forms related to maximum glacial conditions. This implies that at some stage in the Pleistocene the Laurentide ice sheet was in a stable maximum condition for a long period of time.
  • (3)Mechanisms allowing evacuation of debris rather than those of abrasion or fracture may be the most important in influencing the amount of erosion achieved by an ice sheet.
  • (4)Cold-based ice may accomplish erosion if it contains debris.

Résumé

Le but de cet article est d'analyser les paysages dus à l'érosion glaciaire au maximum de la glaciation Laurentide et de les rattacher aux trois principales variables agissant sur l'érosion glaciaire: le régime thermique qui régnait autrefois à la base de la calotte, la topographie du lit et la géologie du lit. La clé pour une telle analyse est la comparaison entre la distribution des types de paysages et le comportement simulé du régime thermique à la base de l'ancienne calotte.

Le décapage en nappe est associé à des zones de fusion à la base sous la majeure partie du centre de l'ancienne calotte et ces zones se situent aux endroits où la topographie favorise la convergence des courants de glace. Ce type de paysage peut également se former sous un glacier froid à la base lorsqu'il charrie des débris morainiques hérités d'une zone de regel à l'amont. Les zones présentant peu ou pas de traces d'érosion glaciaire se trouvent au nord des Queen Elizabeth Islands, mais se rencontrent également sur les hautes terres liées à une divergence des courants de glace; elles coïncident avec des surfaces que les calculs montrent avoir été couvertes par de la glace froide dépourvue de débris morainiques. Les paysages d'érnsinn linéaire selective sont communs sur les hauts plateaux au voisinage de la périphérie orientale de la calotte. Dans ces sites, des vallées préexistantes ont canalisé le courant de glace et une glace froide protectrice s'est installée sur les plateaux intermédiaires. Des variations dans la perméabilité de la base du bedrock peuvent modifier le type de paysage sur une échelle locale surtout dans les secteurs de transition entre un régime thermique et un autre. En général, les roches perméables tendent à avoir connu moins d'érosion que les roches imperméables.

En utilisant l'abondance des lacs comme un indice de l'intensité de l'érosion glaciaire, on peut repérer une zone d'érosion maximale qui forme un anneau entre le centre de l'ancienne calotte et sa périphérie. Cet anneau coïncide avec une zone où l'eau de fusion issue du centre de la calotte regelait sur ses bords. Ce regel incorporait des débris morainiques dans la glace formant un niveau de base de$$$ 20 à 50 m d'épaisseur et fournissait un moyen efficace d'évacuation des matériaux.

Un modèle théorique est imaginé qui repose sur les hypothèses suivantes: (1) (1)Les paysages d'érosion glaciaire sont liés au premier chef au régime thermique à la base de la calotte glaciaire. (2) (2)Les paysages d'érosion glaciaire sont des formes d'équilibre surtout au maximum de la glaciation. Cette dernière conclusion implique qu'à un certain stade du Pleistocene, la calotte glaciaire Laurentide a été dans des conditions de maximum stable pendant une longue période. (3) (3)Une érosion en nappe intense suppose des mécanismes efficaces d'évacuation des matériaux produits tout autant que des mécanismes d'érosion ou de fracture. (4) (4)Les glaciers froids à la base peuvent être des agents d'érosion s'ils contiennent des débris morainiques.

Zusammenfassung

Ziel dieser Arbeit ist es, Landsehaftcn glazialer Erosion aus dem Hochststand des Laurentinischen Eisschildes zu analysicrcn und sic mit den drei Hauptparamctern in Beziehung zu setzen, von der glaziale Erosion abhàtigt. nâmlich dem fruheren Wârmeumsatz am Untergrund des Eisschildes, der Topographie und der Geologie des Untergyundes. Der Schlussel zut Analyse ist der Vergleich zwischen der Verteilung von Landschaftstypen und dem simulierten Modell des basalen Wärmeumsatzes im früheren Eisschild.

Für Landschaften mit flàchenhafter Abschurfung lasst sich eine Verbindung zu Zonen mit Abschmelz-vorgangen feststellen; sie treten unter eincm Grosstcil des Zentrums des fruheren Eisschildes und zwar an solchen Stcllen, wo die Topographie konvergierenden Eishuss begûnstigle, Dersltbe Formtyp kann sich aber auch unter kaltem Eis gebildet haben, wenn dieses Schutt aus einer weiter oben gelegenen Zone der Regelation enthielt. Gebictc mit genngen oder gar keinen Anzeichen von glazialer Erosion kommen vor allem im Norden der Queen Elizabeth Islands vor. aber auch auf Hochflachen, wo der Eisfluss divergierte; sie decken sich mit jenen Gebicten, die rechnerisch mit kaltem, schuttfreiem Eis bedeckt waren. Landschaften selektiver Erosion sind auf Hochflachen nahe dem Ostrand des Eisschildes hàufîg. In diesen Lagen wurde der Eisstrom durch vorgegebene Tiiler gelenkt; dadurch ergab sich eine Situation, in der temperiertes Eis liber den Tâlcrn und kaltes, schïitzendes Eis iiber den dazwischen angeordneten Plateaus lag. Unterschiede in cler Durch-làssigkeit des Felsuntergrundes konnen zu lokalen Verànderungen des Formeumusteiï; fiihren, vor allem in solchen Gebielen, wo die thermischen Verhàltnisse am Untergrund von eincm zum andern Typ wechseln. Allgemein ist durchlâssiger Fels weniger erosionsanfâllig als uncturchlâssiger.

Zieht man die Dichte der Seenbccken als Kriterium für die Intensitàt der glazialen Erosion heran, so lasst sich eine Zone maximaler Erosion feststellen, die ringfeirnig zwischen dem Zentrum des fruheren Eisschildes und seiner Périphérie liegt. Dieser Ring fallt mit jener Zone zusammen, wo Schmelzwasser aus dem Zentrum des Eisschildes auf den Untergrund des Eisschildes auffror. Durch dièse Regelation wurde basaler Schutt in das Eis aufgenommen; in einer Schicht von 20-50 m Mâchtigkeit fand so ein sehr wirkungs-voller Abransport von Schult statt.

Die entwickelte Modellvorstellung beruht auf folgenden Postulaten: (1) (1)Für die Bildung von Landschaften mit glazialer Erosion sind vor allem die thermischen Verhàltnissen am Untergrund des Eisschildes ausschlaggebend. (2) (2)Landschaften mit glazialer Erosion sind Gleichgewichtsformen, die zu einem glazialen Hochststand gehôren. Dies bedeutet zugleich, dass in einem bestimmten Stadium des Pleistozàns der Laurentinische Eisschild sich für lange Zeit auf cinem stationarcn Hochststand befand. (3) (3)Mechanismen, bei denen der Schuttransport die Abrasion oder den Bruch uberwiegt, durften bestimmend für den Grad der Erosion durch einen Eisschild sein. (4) (4)Kaltes Untergrundeis kann Erosion bewirken, wenn es Schutt enthält.

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Glacial Erosion by the Laurentide Ice Sheet

  • D. E. Sugden (a1)

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