Active retreat of a Late Weichselian marine‐terminating glacier: an example from Melasveit, western Iceland

Abstract

Large and complete glaciotectonic sequences formed by marine‐terminating glaciers are rarely observed on land, hampering our understanding of the behaviour of such glaciers and the processes operating at their margins. During the Late Weichselian in western Iceland, an actively retreating marine‐terminating glacier resulted in the large‐scale deformation of a sequence of glaciomarine sediments. Due to isostatic rebound since the deglaciation, these formations are now exposed in the coastal cliffs of Belgsholt and Melabakkar‐Ásbakkar in the Melasveit district, and provide a detailed record of past glacier dynamics and the inter‐relationships between glaciotectonic and sedimentary processes at the margin of this marine‐terminating glacier. A comprehensive study of the sedimentology and glaciotectonic architecture of the coastal cliffs reveals a series of subaquatic moraines formed by a glacier advancing from Borgarfjörður to the north of the study area. Analyses of the style of deformation within each of the moraines demonstrate that they were primarily built up by ice‐marginal/proglacial thrusting and folding of marine sediments, as well as deposition and subsequent deformation of ice‐marginal subaquatic fans. The largest of the moraines exposed in the Melabakkar‐Ásbakkar section is over 1.5 km wide and 30 m high and indicates the maximum extent of the Borgarfjörður glacier. Generally, the other moraines in the series become progressively younger towards the north, each designating an advance or stillstand position as the glacier oscillated during its overall northward retreat. During this active retreat, glaciomarine sediments rapidly accumulated in front of the glacier providing material for new moraines. As the glacier finally receded from the area, the depressions between the moraines were infilled by continued glaciomarine sedimentation. This study highlights the dynamics of marine‐terminating glaciers and may have implications for the interpretation of their sedimentological and geomorphological records.

Refining the history of Younger Dryas and Early Holocene glacier oscillations in the Borgarfjörður region, western Iceland

“…The lowermost glaciomarine sediments were defined by Ingólfsson (, ) as a single unit that was informally named ‘the Ásbakkar diamicton’ but is referred to as ‘Unit A’ in Sigfúsdóttir et al . () and in this paper. Previously, shells and shell fragments identified to the Macoma calcarea community have been radiocarbon dated revealing ages between 14.6 and 13.7 cal.…”

“…The Melasveit area contains thick successions of Late Weichselian glacial and glaciomarine deposits that have largely undergone deformation by an outlet glacier from Borgarfjörður and possibly other outlet glaciers during the Late Weichselian (Ingólfsson ; Hart & Roberts ; Sigfúsdóttir et al . ). These deposits are mostly overlain by coarse littoral sand and gravel deposited during the isostatic readjustment of the area in the Early Holocene (Ingólfsson ).…”

“…The extensively deformed marine sediments in Melasveit have provided clear indication that glacier(s) re‐advanced into the lowland areas of Borgarfjörður during the Late Weichselian (Ingólfsson , ; Sigfúsdóttir et al . ). Furthermore, the configuration of the Skorholtsmelar moraine ridge, distribution of erratic boulders and lithology of surface deposits indicate that the moraine formed by a glacier advancing southwards from Borgarfjörður (Figs , B; Ingólfsson , ; Sigfúsdóttir et al .…”

“…Furthermore, the configuration of the Skorholtsmelar moraine ridge, distribution of erratic boulders and lithology of surface deposits indicate that the moraine formed by a glacier advancing southwards from Borgarfjörður (Figs , B; Ingólfsson , ; Sigfúsdóttir et al . ). The eastern side (ice‐distal) is partly flanked by the Skorholt delta possibly formed by subglacial outwash when the glacier was standing at the moraine ridge (Fig.…”

“…; Sigfúsdóttir et al . ). The area accommodates well‐preserved glacial sediments and landforms from the deglaciation that record glacier fluctuations and sea level changes during that time.…”