The Quaternary geology of the Narssaq area, South Greenland

Abstract

The topography and glacial striations in the Narssaq area indicate that the ice age glacial regime in this part of Greenland was characterized by ice movement constrained by the local topography, and a shallow depth of the ice cover. Erratics observed 1200 m above sea level provide a minimum estimate for the ice thickness. The most widespread type of glacial deposit consists of scattered boulders Iying on the glacially abraded bedrock surface with loose fillings of sand and gravet washed into depressions; this deposit probably reflects the debris content of the ice cover in a final short lasting phase of stagnation and melting. Counting of the boulders and stones shows that the travelling distances generally are short: rock types which are exposed as bedrock less than 1 km away comprise on average 72 % of the material. Some rock types, i.e. the syenites of the Precambrian Ilimaussaq Intrusion, decrease very rapidly in their frequencies as erratics downstream from their exposures, indicating that they were selectively crushed during transportation. The large spatial variation in the 'boulder communities' supports the idea of ice movement being directed by the topography, and poor mixing in the ice sheet. The low altitudes of the marine limits (47-60 m above sea level) also may be interpreted to reflect shallow ice thicknesses, while the few available dates for the timing of the isostatic upheaval, indicate that the Narssaq area was free of ice c. 11 000 years ago.