Architecture and structural evolution of an early Little Ice Age terminal moraine at the surge‐type glacier Múlajökull, Iceland

Abstract

The internal architecture and structural evolution of the Arnarfellsmúlar terminal moraine at Múlajökull, a surge-type glacier in central Iceland, is described in order to demonstrate submarginal and proglacial glaciotectonic processes during glacier surging, as well as constraining the age of the maximum extent of the glacier. The moraine is 4-7 m high, 50-100 m wide, and composed of a highly deformed sequence of loess, peat, and tephra that is draped by till up to the crest. The internal architecture is dominated by steep, high-amplitude overturned folds and thrusts in the crest zone but open, low-amplitude folds on the distal slope. Section balancing suggests a basal detachment (décollement) depth of 1.4 m and a total horizontal shortening of around 59%. This implies that the glacier coupled to the foreland about 70 m up glacier from its terminal position to initiate the formation of the moraine. The structural evolution is polyphase in that the formation commenced with low-amplitude open folding of the foreland, followed by overfolding and piggyback thrusting. Radiocarbon dating and analysis of tephra layers, along with historical references, indicate that the most likely time of moraine formation was between A.D. 1717 and 1760, which suggests that Múlajökull had its Little Ice Age maximum and most extensive surge earlier than many other surge-type glaciers in Iceland.

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

“…More drumlins continue to be exposed as the margin recedes (Benediktsson et al, ; Johnson et al, ; Jónsson et al, ), and ground penetrating radar reveals that drumlins extend upglacier from the margin several hundred meters (Lamsters et al, ). Exposed drumlins were shaped by ice no farther than ~2 km from the former glacier margin at its maximum, as indicated by the position of the Little Ice Age terminal moraine (Figure b; e.g., Benediktsson et al, ; Jónsson et al, ). Most drumlins are 100–400 m in length, 50–150 m in width, and 2–14 m in height with swales between them commonly occupied by lakes that reduce the apparent dimensions of the drumlins (Benediktsson et al, ).…”

“…Evidence of earlier surges in the form of multiple end moraines is ubiquitous, including the glaciotectonic Little Ice Age terminal moraine that formed between 1717 and 1760 (Benediktsson et al, ; Jónsson et al, ). Most of the recent surges have resulted in glacier advances of 200–400 m, each leaving end moraines (Figure b; Benediktsson et al, ; Johnson et al, ). The most recent surge in 2008 caused only a ~20 m advance but left a prominent glaciotectonic moraine (Benediktsson et al, ).…”

Progressive formation of modern drumlins at Múlajökull, Iceland: stratigraphical and morphological evidence

“…; Benediktsson et al . ; Fig. ), and the ice front was located about 760 m inside the 1992 surge position.…”

“…Thus, they hypothesized that the distal drumlins had experienced fewer surges than the drumlins closer to the glacier. Both Johnson et al (2010) andJ onsson et al (2014) stressed that their hypotheses need to be tested with more field data. Herein, we test the hypothesis proposed by Johnson et al (2010) and its predictions of spatial validity within the M ulaj€ okull drumlin field with new data and more detail on drumlin structure, sedimentology and stratigraphy from four large additional exposures.…”

Subglacial drumlins and englacial fractures at the surge‐type glacier, Múlajökull, Iceland

“…The two most recent surges were in 1992 and in 2008, when the glacier advanced beyond the current margin by ≤800 m and ≤200 m, respectively (Benediktsson et al. ; Jónsson et al. )…”