GLACIAL AND NON-GLACIAL SEDIMENTARY PROCESSES IN BALSFJORD (NORTH NORWAY)
FORWICK, MATTHIAS . University of Tromsoe, Department of Geology.
Vorren, Tore O. . University of Tromsoe, Department of Geology.
The objective of our study was to reconstruct the deglaciation history of Balsfjord (North Norway), as well as to investigate postglacial sedimentary processes. Balsfjord is a 46 km long and maximum 5 km wide fjord, located about 10 km south of Tromsoe. It is surrounded by up to 1500 m high mountains. The most prominent feature on the fjord floor is the "Skjevelnes moraine", an end moraine that divides the fjord into an "Outer" and "Inner Basin" that reach water depths of 190 m and 130 m, respectively.
Two piston cores and high-resolution seismic data (3.5 kHz penetration echo sounder) provide the basis for our study. Core JM98-1018-PC (10.01 m long) was recovered from the Outer Basin, whereas core JM98-1014-PC (9.96 m long) was taken from the Inner Basin. Core JM98-1018-PC was divided into 10 lithostratigraphic units. The lowermost unit comprises cyclic glacimarine sediments. These are truncated by a coherent slump that is covered with suspension fall-out. Further upcore, open marine sedimentation is interrupted by gravity flow deposits in form of one high-density turbidity current and two cohesive debris flows. Core JM98-1014-PC comprises exclusively non-glacial marine sediments and it is divided into 7 lithostratigraphic units. A debris flow deposit and suspension fall-out sediments compose the lowermost two units of the core. Further upwards, open marine sedimentation is interrupted by gravity flow deposits, in this case including two low-density turbidites.
Based on radiocarbon dates, the following chronology of Balsfjord was established. The Balsfjord glacier retreated from the Tromsoe-Lyngen moraines (Younger Dryas) prior to 10.4 14C ka BP. Between 10.4 and 9.9 14C ka BP deposition of the Skjevelnes moraine in the central part of Balsfjord occurred. After 9.9, but before 9.7 14C ka years BP, deposition of two minor end moraines took place. The transition from glacimarine to open marine environment occurred around 9.7 14C ka BP.
Three gravity flow events, deposited between c. 9.5 and 8.8 14C ka BP, were correlated between the two cores. Since these events can be correlated across the moraine, they are suggested to indicate regional avalanches. This rather high frequency of mass movements in the early postglacial period may primarily be due to tectonic activity induced by rapid isostatic uplift. The occurrence of a single gravity flow event in the Outer Basin was estimated to 8.5 14C ka BP. For the time after 8.5 14C ka BP, no avalanche activity is archived in the cores.
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