CONTRASTING GLACIAL GEOLOGY OF THE STRANDIR AND ISAFJARDARDJUP COASTLINES, NW PENINSULA, ICELAND
PRINCIPATO, SARAH M. INSTAAR and Dept. of Geological Sciences; University of Colorado.
Geirsdottir, Aslaug . University of Iceland.
Andrews, John T. INSTAAR and Dept. of Geological Sciences; University of Colorado.
Terrestrial fieldwork is in progress to address questions regarding ice extent and glacial history for part of the Northwest Peninsula (NWP) of Iceland during the last glacial maximum and through the deglaciation and the Holocene. Radiocarbon dates on foraminifera above diamicton units in marine cores offshore in Hunafloaall suggest that ice retreated from the shelf by approximately 13ka, and there is abundant ice-rafted debris in marine cores from the Reykjarfjordur region until approx. 9.1-9.5ka (Andrews et al. 2000; Castaneda, 2001). This suggests that ice was calving into the fjord until the early Holocene. Detailed geomorphic mapping is being completed along the Strandir coast and the eastern side of the Isafjardardjup coast to determine the Late Quaternary and deglacial history on land, and correlate it with the marine core chronology. Cosmogenic exposure dating, radiocarbon dating, tephrachronology, and morphostratigraphic relationships are combined to determine the terrestrial chronology.
The glacial geology of the Strandir coast is dominated by erosive glacial landforms. Steep peaks, horns, and arÍtes, have elevations up to approximately 800m asl. Striated bedrock surfaces are abundant along the Strandir coast, but glacial sediment deposits are minimal. In contrast, the eastern Isfjardardjup coast contains moraines in front of active outlet glaciers of the Drangajokull Icecap in Kaldalon and Leirufjordur, as well as a large moraine in the Sela river valley. The ages of these moraines are unknown, and a large moraine at the head of Kaldalon is especially dubious. Boulders on the crest of these moraines were collected for cosmogenic exposure dating, and dates are pending. Behind the large moraine in Kaldalon, at least 4 younger moraines are present. A stratigraphic section is located in a stream cutbank eroding through a grassy, subdued moraine approx. 1km behind the large moraine at the head of Kaldalon. Alternating layers of peat, diamicton, and sand present in this section may be interpreted as fluctuations between landscape stability and glacial advances. Buried wood found in the section at a depth of 265cm has a radiocarbon date of 3328+/- 45 14C years, and the base of the peat section has a date of 2503+/-59 14C years. Subsequent peat samples have been submitted to confirm the minimum age of this moraine. Behind this subdued, vegetated moraine a steeper, gravely moraine is present, which is probably Little Ice Age. Lichenometry measurements of Rhizocarpon gr. geographicum on the less well vegetated moraines in Kaldalon provide relative ages of the glacier fluctuations during the past few hundred years.
The marine limit in the study area varies from approximately 30m a.s.l. in the northwest to approximately 45m. in the southeast. The large moraine at the head of Kaldalon lies behind/ up valley from the marine limit, so it is possibly early Holocene in age. However, there is no direct stratigraphic overlap between the moraine and marine/beach sediments in this valley. In the next valley south of Kaldalon, gravels drape over a small ridge of diamicton, but no organic material was found for dating. Mollusk shells were present in a raised beach terrace north of Kaldalon, approximately 7m above sea level, and yield a radiocarbon date of 3612+/- 40 14C years.
An 8cm thick basaltic ash unit surrounded by peat layers is located in the southwestern portion of the study area (Principato, 2000). Geochemistry, color, grain size, and a radiocarbon date of 9081+/- 68 14C years on peat below the ash confirm that this unit is the Saksunarvatn Ash. This tephra facilitates a link between the terrestrial and marine environments, as it is present in several marine cores from NW Iceland. This tephra was not found along the eastern Isafjardardjup coastline, although it is present in marine cores in Isafjardardjup and lake cores in Efstadalsvatn (Geirsdottir et al. in subm). It is possible that the tephra deposits have been eroded away from the eastern Isafjardardjup coast or covered by colluvium, but it also might mean that this area was still ice covered when the Saksunarvatn was deposited. Future field investigations are needed to determine the extent of this tephra in the northern part of the study area.
Andrews, J. T., Hardardottir, J., Helgadottir, G., Jennings, A. E., Geirsdottir, A., Sveinbjornsdottir, A. E., Schoolfield, S., Kristjansdottir, G. B., Smith, L. M., Thors, K., and Syvitski, J. P. M., 2000, The N and W Iceland Shelf: Insights into Last Glacial Maximum Ice Extent and Deglaciation based on Acoustic Stratigraphy and Basal Radiocarbon AMS dates. Quaternary Science Reviews, v. 19, p. 619-631.
Castaneda, I., 2001., Holocene paleoceanographic and climatic variations of the inner north Iceland continental shelf, Reykjarfjordur area, MSc Thesis, Univ. of Colorado, Boulder.
Geirsdottir, A., Andrews, J.T., Olafsdottir, S., Helgadottir, G., and Hardardottir, J., in subm, A 35ka record of iceberg rafting from NW Iceland: following the ice retreat from shelf to land. Polar Research.
Principato, S.M., 2000, The glacial geology of Reykjarfjordur, NW Iceland. Geological Society of America Program with Abstracts, vol. 32, no. 7.
Figure 1. Location of study area.
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Copyright © 2001 INSTAAR, Univ. of Colorado