International Geologiical Congress - Oslo 2008

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CGC-01 General contributions to climate change

 

New OSL datings in the rautuvaara stratigraphical key section in northern Finland

 

Pertti Sarala, Geological Survey of Finland (Finland)
Pertti Hakala, Geological Survey of Finland (Finland)
Heikki Hirvas, Geological Survey of Finland (Finland)
Pekka Huhta, Geological Survey of Finland (Finland)
Peter Johansson, Geological Survey of Finland (Finland)
Keijo Nenonen, Geological Survey of Finland (Finland)
Jorma Valkama, Geological Survey of Finland (Finland)
 

 

In 2006, the Geological Survey of Finland (GTK) carried out OSL dating campaign in Finland and one of the targets was the stratigraphical key section in the Rautuvaara iron mine (active 1974-1988) in Kolari commune, western Finnish Lapland. A part of the ore body was mined as an open pit which needed great amounts of land mass movement due to thick Quaternary glacigenic deposits above the bedrock surface. The deepest section was almost 25 m high including several till units and stratified interlayers. The length of walls in the studied open pit was about one kilometre. The open pit was studied detailed by GTK in the middle the 1970's and as a result five separate till beds with glaciolacustrine and -fluvial interlayers were recognised (Hirvas 1991). Later, after the mining activity ended, ground water filled the open pit with 10-meters-deep water body.
The idea was to take samples by hands from the stratified interlayers above the water table and by the percussion drill below water table. The OSL samples were taken into black plastic tube (diameter about 7 cm) from the upper part after cleaning and describing the sections. The samples deeper in the section were drilled by the percussion drilling equipment with flow-through bit into plastic tube which diameter was about 4 cm and length 1 m. In total 9 dating samples were taken from the stratified interlayers between the till units. OSL dating was done in the Luminescence Research Laboratory of the Radiation Research Department at Risø National Laboratory, Denmark.
OSL method gives interesting new results of the age of sediments in the section. Ages range from 70 ka to 171 ka in the whole section indicating the deposition during the Eemian interglacial or the Early Weichselian interstadials. An important result is that age distribution is not logic, i.e. the ages are not growing towards the bottom of the section. Actually, the oldest age is measured from the uppermost sands. Based on the dating results it can be assumed that glaciers have existed in western Finnish Lapland during the Early Weichselian, and at least the uppermost till units and stratified interlayers have been deposited during gold-warm climate cycles in that period. The ages of deeper interlayers may not be representative, because the dating results can be erroneous for several reasons, e.g. due to high palaeodose rates, and sampling problems related to percussion drilling.

 

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