Controlled moraine development and debris transport pathways in polythermal plateau icefields: examples from Tungnafellsjökull, Iceland

Abstract

AbstractA modern landsystems analogue for plateau icefield glaciation is presented, based upon Tungnafellsjökull in the arid, cold interior of Iceland. Landform mapping and intensive clast form analysis on three separate forelands occupied during the Little Ice Age reveal variability in the morphology of moraines and the degree of clast modification, which is interpreted as a function of the complex interplay between: (a) the efficacy of glacial erosion; (b) passive versus active debris transfer mechanisms; and (c) the volume of debris provision for ice‐marginal freeze‐on mechanisms. A steep outlet bounded by rockwalls produced a landform assemblage and debris transport signature typical of valley glaciers, whereas two unconfined outlets produced a landform‐sediment imprint typical of polythermal conditions. This comprises subglacial bedforms, relating to the initiation of warm‐based ice by strain heating below icefalls at the plateau edge, terminating at arcs of ice‐cored (controlled) moraine representative of frozen snout conditions. Patterns in clast form data on the unrestricted forelands reflect the characteristics of glacier dynamics and thermal regimes at the margin of a plateau icefield, where bedrock steps of sufficient size to initiate strain heating and bedrock abrasion and plucking are located only a short distance from the marginal freeze‐on conditions of a snout creating debris‐rich folia and controlled moraine. Additional englacial debris may well have been carried to the glacier margin through the development of ogives and englacial thrusting at the base of the icefall, thereby explaining the occurrence of striae on clasts that have otherwise been little modified by glacial abrasion. Copyright © 2010 John Wiley & Sons, Ltd.