Bulletin of the Geological Society of Finland (Jun 1986)
The Central Puolanka Group - a Precambrian regressive metasedimentary sequence in northern Finland
Abstract
Quite recently, an amphibolite-facies metasedimentary sequence at least 2500 m thick, named the Central Puolanka Group (CPG) was recognized underlying the traditional Jatuli-type sequences at the western margin of the Early Proterozoic Kainuu Schist Belt in Central Puolanka. To the west, towards the Archaean basement of the Pudasjärvi ‒ Vaala area, the primary sedimentary and stratigraphic features of the metasediments have been destroyed by severe metamorphism and migmatization to such a degree that the lowermost and the western parts of the CPG, have inverted into gneisses, called collectively the West Puolanka Paragneiss (WPP), within which it is no longer possible to map the rocks in terms of paleosedimentology and whose sedimentation basement has not yet been found. The zone between the CPG proper and the WPP is underlain by the Kettukangas paragneiss which may locally show turbidite features and which grades stratigraphically upwards into the CPG. The CPG itself consists of three formations called, from the oldest to the youngest, the Puolankajärvi Formation (PjF), the Akanvaara Formation (AvF) and the Pärekangas Formation (PkF). In terms of primary sediments, the lower and middle part of the PjF is made up of turbidite sands and muds deposited either at the outer parts of a shelf or in a deep water fan of an ancient continental margin. The upper part consists of cross-bedded or rippled silts and fine sands with interbeds of cross-bedded sands whose amount increases upwards towards the AvF. The latter is made up almost entirely of cross-bedded sands deposited either in an inner shelf or in deltaic environments. They are overlain abruptly but conformably by the dominant muds and silts of the PkF showing evidence of a tidal environment. This regressive CPG sequence is overlain unconformably by the fluvial gravels and sands of the first of the transgressive Jatuli-type sequences which form the bulk of the Kainuu Schist Belt. The CPG can be mapped as a coherent lithostratigraphic unit from Puolanka to Paltamo. The preliminary mapping of the paragneiss areas west of the Kainuu Schist Belt indicates, however, that the sediments of this sequence once covered a wide area which extended from Puolanka to at least Koillismaa in the north and to Otanmäki, at least, in the south and they may represent the same sedimentaryvolcanic cycle which produced the Middle and Upper Lapponian sediments and volcanics of Lapland.
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