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A Framework for the Investigation of Medial Moraine Formation: Austerdalsbreen, Norway, and Berendon Glacier, British Columbia, Canada

  • N. Eyles (a1) and R. J. Rogerson (a1)

Abstract

Morphology of medial moraines on Austerdalsbreen, Norway, and Berendon Glacier, British Columbia, depends upon englacial debris supply. Major sub-types of this "ablation-dominant" model are related to the zone of debris entrainment relative to the firn line, and the manner of entrainment.

On Austerdalsbreen, debris derived from extraglacial bedrock slopes is entrained via crevasses at the confluence of two ice-cap outlet glaciers below the firn line. Revelation of crevasse-bound debris generates a distinct ice-cored morphology which is destroyed as crevasse bottoms are revealed down-glacier.

On Berendon Glacier ice streams coalesce above and below the firn line. Above the firn line, debris from extraglacial rock outcrops, subnival and subglacial zones, undergoes seasonal sedimentation with snowfall, and extends throughout the ice depth. Distinct moraine morphology in the terminal zone is related to continuing debris supply. Most debris is transported at depth near the glacier base.

An "ice-stream interaction" model where medial moraines formed below the firn line from the confluence of ice streams with large lateral moraine load are morphologically controlled by flow, explains morphology on the Berendon Glacier in the main confluence zone only. Down-glacier, this moraine becomes "ablation dominant". A minor "avalanche-type" model is also recognized.

Résumé

La morphologie des moraines médianes de l'Austerdalsbre en Norvège et du Berendon Glacier en Colombie Britannique dépendent de l'approvisionnement en sédiments intra-glaciaires. Les principaux sous-types de ce modèle "à ablation dominante" se distinguent à partir de la situation de la zone d'entrainement des sédiments par rapport à la ligne des névés et du mode d'entrainement.

Sur l'Austerdalsbre les sédiments issus des pentes rocheuses non englacées sont entraînés à travers les crevasses vers la confluence des glaciers exutoires des deux calottes, en dessous de la ligne des névés. La réapparition de moraines liées à des crevasses engendre une morphologie particulière à coeur de glace qui est détruite lorsque les fonds de crevasses ressurgissent en bas du glacier.

Sur le Berendon Glacier, les courants de glace se réunissent au-dessus et en-dessous de la ligne des névés. Au-dessus de cette ligne, les matériaux issus des rochers extérieurs au glacier et ceux des zones sous-glaciales et sous-nivales se trouvent enfouis sous la sédimentation saisonnière duc aux chutes de neige et se répartissent dans l'épaisseur de la glace. On peut mettre en relation dans la zone terminale une morphologie distincte de la moraine selon le type d'apport de matériaux. La plus grande partie des matériaux sont transportés près du fond du glacier.

Un modèle "d'interaction des courants glaciaires" où les moraines médianes formées sous la ligne des névés à partir de la confluence des flux glaciaires avec une forte charge morainique latérale sont contrôlées morphologiquement par le courant, explique la morphologie du Berendon Glacier uniquement dans la principale zone de confluence. Plus bas sur le glacier cette moraine devient du type "ablation dominante". On a reconnu également un "type à avalanches" de moindre importance.

Zusammenfassung

Die Morphologie der Mittelmoränen am Austerdalsbre in Norwegen und am Berendon Glacier in British-Columbia, Kanada, hängt vom Nachschub an Schutt aus dem Gletscher ab. Wichtige Untertypen dieses "ablationsbedingten" Modells stehen in Beziehung zur Zone der Schuttauf-nahme relativ zur Firnlinie und zur Art der Aufnahme.

Am Austerdalsbre wird Schutt, der von Felshängen ausserhalb des Gletschers stammt, über Spalten am Zusammenfluss zweier Auslassgletscher der Eiskappe unterhalb der Firnlinie aufgenommen. Der Austritt spaltengebundenen Schutts erzeugt bestimmte Formen mit Eiskernen, die sich auf lösen, sobald die Spaltensohlen gletscherabwärts herauskommen.

Am Berendon Glacier vereinigen sich Eisströme ober- und unterhalb der Firnlinie. Über die Firnlinie wird Schutt aus Felszonen ausserhalb des Gletschers sowie aus schnee- und eisbedeckten Zonen im Wechsel mit Schneefallschichten sedimentiert; er durchsetzt den Gletscher in seiner ganzen Tiefe. Bestimmte Moränenformen im Zungengebiet stehen in Beziehung zum andauernden Schuttnachschub. Der meiste Schutt wird in der Tïefe nahe der Gletschersohle transportiert.

Ein Modell der "Wechselwirkung zwischen Eisströmen", worin die Morphologie von Mittelmoränen, gebildet unter der Firnlinie aus dem Zusammenfluss von Eisströmen mit starken Seitenmoränen, von den Fliessverhältnissen bestimmt wird, genügt am Berendon Glacier nur zur Erklärung der Morphologie in der Hauptzone des Zusammenflusses. Diese Moräne wird gletscherabwärts "ablationsbedingt". Ein untergeordnetes Modell vom "Lawinentyp" lässt sich ausserdem feststellen.

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References

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A Framework for the Investigation of Medial Moraine Formation: Austerdalsbreen, Norway, and Berendon Glacier, British Columbia, Canada

  • N. Eyles (a1) and R. J. Rogerson (a1)

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