Bulletin of the Geological Society of Finland (Jun 1989)
Lithostratigraphy of the early Proterozoic Kainuu Schist Belt in the Kurkikylä-Siikavaara area, northern Finland, with emphasis on the genetic approach
Abstract
The early Proterozoic (Karelian) supracrustal formations of the northeastern part of the Kainuu Schist Belt in the Kurkikylä-Siikavaara area in northern Finland are formally classified into the Kurkikylä, Korvuanjoki and East Puolanka Groups. The Kurkikylä Group (400‒700 m) represents a continental rift stage on the late Archaean cratonic basement and consists of two interfingering formations of immature conglomerates, diamictites and sandstones of alluvial fan and braided river origin. One of these terminates in the fine-grained rocks of a lacustrine basin associated with subaqueous mass flows and some oversized clasts suggesting glacially influenced deposition for this later stage. Active rifting is indicated by one formation with superimposed subaerial lava flows. The Korvuanjoki Group (600‒1200 m), separated by a chemical weathering crust from the Kurkikylä Group, consists of two formations of quartz pebble conglomerates and mature sandstones of braided alluvial plains suggesting deposition in half-grabens still in a cratonal setting. The group was produced by the reactivation of earlier rift faults in phases of crustal extension and corresponds to the incipient ocean/inland basin stage in the development of a continental margin before complete crustal breakup. The East Puolanka Group (2300‒2600 m) was deposited unconformably on the Korvuanjoki Group and consists of six quartzitic formations with minor finegrained rocks and one formation of volcanics, predominantly subaerial lavas. The group represents deposition on the proximal parts of a platformal shelf exhibiting records of three transitions from a fluvial to a shallow marine environment. The group belongs to the lower part of a large opening clastic wedge on a divergent continental margin which, in these early stages, was repeatedly rifted, as indicated by sporadic volcanism and the intrusion of diabases.
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