Ívar Örn Benediktsson

Active drumlin field revealed at the margin of Múlajökull, Iceland: A surge-type glacier

ABSTRACT Recent marginal retreat of Mulajokull, a surge-type outlet glacier of Hofsjokull, Iceland, has revealed a drumlin fi eld consisting of more than 50 drumlins. The drumlins are 90–320 m long, 30–105 m wide, 5–10 m in relief, and composed of multiple beds of till deposited by lodgment and bed deformation. The youngest till layer truncates the older units with an erosion surface that parallels the drumlin form. Thus, the drumlins are built up and formed by a combination of subglacial depositional and erosional processes. Field evidence suggests each till bed to be associated with individual recent surges. We consider the fi eld to be active in the sense that the drumlins are shaped by the current glacial regime. The Mulajokull fi eld is the only known active drumlin fi eld, and is therefore a unique analogue to Pleistocene drumlin fi elds.

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

The internal architecture and structural evolution of the Arnarfellsmular terminal moraine at Mulajokull, 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 4-7m high, 50-100m 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 (decollement) depth of 1.4m and a total horizontal shortening of around 59%. This implies that the glacier coupled to the foreland about 70m 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 Radiocarbon dating and analysis of tephra layers, along with historical references, indicate that the most likely time of moraine formation was between A.D. 1717 and 1760, which suggests that Mulajokull had its Little Ice Age maximum and most extensive surge earlier than many other surge-type glaciers in Iceland.

6 Ice-Marginal Environments: Geomorphic and Structural Genesis of Marginal Moraines at Mýrdalsjökull

Abstract Ridge-shaped ice-marginal moraines paralleling the glacier margin are produced during glacier advances or stillstands, or they are formed by limited winter re-advances during overall glacier retreat. As ice-marginal moraines outline the configuration of glaciers, they are useful when interpreting modern glacial landsystems or reconstructing ancient glacial environments. At Mýrdalsjokull, glacier fluctuations allowed studies of ice-marginal moraine formation during the glacier advance in the 1980s. Ice-marginal moraines display wide variety of geomorphic and structural types reflecting the glacier dynamics and different sedimentary and topographic conditions of the forefields. It is intriguing that without having observed the active ice-marginal moraine genesis year by year, most of the resulting moraine ridges now found in the glacier forefield would have been misinterpreted. We review processes and implications of ice-marginal moraine formation along the margins of Kotlujokull and Slettjokull, two major outlets from the Mýrdalsjokull ice cap. In addition, we focus on some of the most prominent neoglacial ice-marginal moraines in glacier forefields surrounding Mýrdalsjokull.

Submarginal drumlin formation and late Holocene history of Fláajökull, southeast Iceland

ABSTRACT Fláajökull is a non-surging outlet glacier draining the south-eastern part of the Vatnajökull, southeast Iceland. Fláajökull was stationary or advanced slightly between 1966 and 1995 and formed a prominent end moraine. Glacial retreat since then has revealed a cluster of 15 drumlins. This study focuses on the morphology and sedimentology of the drumlins. They are 100–600 m long, 40–130 m wide, and have cores of glaciofluvial sediment or till. The drumlins are draped by ~1 m thick, massive subglacial traction till. The glacier forefield is characterized by a number of arcuate and saw-tooth, terminal and recessional moraine ridges, overridden moraines with fluted surfaces, and glaciofluvial outwash. Some of the drumlins extend towards the 1995 end moraine but terminate abruptly at the moraine and are not observed in front of it. This suggests that they were formed sub-marginally during the 1966–1995 terminal position. The sedimentary structure of the drumlins is best explained by the sticky spot model. Dating and dendrochronological analyses of birch logs found on the surface of one of the drumlins indicate that the valley was forested about 2100 calendar year BP, after which the glacier started to reform, possibly due to an abrupt change in climate.

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

U.S. National Science Foundation The Icelandic Research Fund The University of Iceland Research Fund The Energy Research Fund of Landsvirkjun The Carlsberg Foundation The Royal Physiographic Society in Lund, Sweden The Fulbright Foundation

Production and preservation of the smallest drumlins

Abstract Few very small drumlins are typically mapped in previously glaciated landscapes, which might be an important signature of subglacial processes or an observational artefact. One hundred and forty-three newly emergent drumlins, recently sculpted by the Múlajökull glacier, have been mapped using high-resolution LiDAR and aerial photographs in addition to field surveying. In this paper, these are used as evidence that few small drumlins (e.g., height H 4 m, width W 40 m, length L 100 m) are produced; at least, few survive to pass outside the ice margin in this actively forming drumlin field. Specifically, the lack of a multitude of small features seen in other landforms (e.g., volcanoes) is argued not to be due to (i) Digital Elevation Model resolution or quality, (ii) mapper ability in complex (i.e., anthropogenically cluttered or vegetated) landscapes or (iii) post-glacial degradation at this site. So, whilst detection ability must still be at least acknowledged in drumlin mapping, and ideally corrected for in quantitative analyses, this observation can now be firmly taken as a constraint upon drumlin formation models (i.e., statistical, conceptual or numerical ice flow). Our preferred explanation for the scarcity of small drumlins, at least at sites similar to Múlajökull (i.e., ice lobes with near-margin drumlin genesis), is that they form stochastically during multiple surge cycles, evolving from wide and gentle pre-existing undulations by increasing rapidly in amplitude before significant streamlining occurs.

Subglacial drumlins and englacial fractures at the surge-type glacier, Múlajökull, Iceland: SUBGLACIAL DRUMLIMS AND ENGLACIAL FRACTURES

1 The interaction between drumlins and overriding glacier ice is not well studied, 2 largely due to the difficulty of identifying and accessing suitable active subglacial en3 vironments. The surge-type glacier Múlajökull, in central Iceland, overlies a known 4 field of actively forming drumlins and therefore provides a rare opportunity to inves5 tigate the englacial structures that have developed in association with ice flow over 6 the subglacial drumlins. In this study detailed ground penetrating radar surveys are 7 combined with field observations to identify clear sets of up-glacier and down-glacier 8 dipping fractures at Múlajökull’s margin. These are interpreted as conjugate shear 9 planes or Pand R-type Reidel shears that developed and filled with saturated sedi10 ment derived from the glacier bed, during a previous surge. The fracture sets exhibit 11 focused spatial distributions that are influenced by the subglacial topography. In 12 particular, down-glacier dipping fractures are strongly focused over drumlin stoss 13 slopes. These fractures, although well developed at depth, were mostly unable to 14