Ice-marginal deposits of a surging tide-water glacier - a study of terrestrial and subsea landforms

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Kristensen, Lene۱; Benn, Douglas۱; Humlum, Ole۲; Ottesen, Dag۳; Christiansen, Hanne۱
Holding Date 11 October 2008

Landsystem assemblages deposited by surging glaciers have been described from terrestrial and marine environments, while combined studies from both environments are rare. A major surge of a Svalbard tidewater glacier around 1300 AD left behind a moraine system both on land as the rim around the fjord, and at the fjord bottom. Here we present a study of the ice-marginal landforms as these are found both at the fjord bottom and on land. A bathymetric survey of the fjord together with seismic profiles and previously published sediment cores allow us to determine the origin of the subsea landforms. The terrestrial moraines have been studied using aerial photographs, 2D resistivity surveying and section logging. The 1300AD surge of the Paulabreen Glacier System was the most extensive Neoglacial advance in the catchment. At present, Paulabreen is calving into Rindersbukta - a branch of Van Mijenfjorden, SW Spitsbergen. Since the first historic observations (1898) several of the tributary glaciers have surged - the latest being Skobreen in 2003-2005. This surge caused a 1.5 km advance of the glacier front into shallow waters. The landforms we observed to form during this surge are used as analogues for interpreting the older moraine system.
Three ice-marginal landforms are identified at the fjord bottom and on land:
1) An ice-proximal hummocky zone with a chaotic appearance. On land the hummocky zone is ice-cored - glacier ice is melting out in niches and the ice is easily identifiable in the geophysical profiles.
2) Glacial thrusting: Evidence on land of glacial thrusting is seen as stacked, folded and overridden beach sediments in a section. At the seabed thrusting is seen as an irregular ridge at the former glacier margin.
3) Debris flow: Around the entire margin of the surge limit a debris flow is found. The width is up to 1.5 km long on land and 3.5 km long in the fjord. The sediment salinity is that of sea water and it contains numerous unbroken bivalve shells, including Chlamys islandica and Mya truncata. Several examples of paired vales were found, but none in living position.
Glacial thrusting and a debris flow were formed at the 2005 surge. A hummocky moraine is under formation as the debris of the very dirty glacier front is melting out.
Debris flows have previously been described as a feature of glacier surges in the Svalbard fjords, but this type of deposit has as far as we are aware only been described on land at Coraholmen, in Isfjorden, Svalbard. At Paulabreen we are able to follow this unit for 25 km both on land and in the fjord. The consistency of the unit together with the unbroken nature of the large and fragile bivalve shells enable us to reject the hypothesis that the sediment was a deforming layer extruded from beneath the glacier as the surge terminated. Rather we believe that the entire mass was scraped off the seabed and pushed in front of the glacier and then deposited at the surge termination.