Early Proterozoic stratigraphy of the Salahmi-Pyhäntä area, central Finland, with an emphasis on applying the principles of lithodemic stratigraphy to a complexly deformed and metamorphosed bedrock

Abstract

The medium-highly metamorphozed and intensely deformed early Proterozoic supracrustal rocks of the study area, located at the boundary zone between the Karelian and Svecofennian zones in central Finland, are classified following the recommendations of the North American Stratigraphic Code into 16 lithostratigraphic and lithodemic units. The lithostratigraphic procedures can be applied only to the southeastern corner of the area where the late Archaean basement complex is rimmed by less than 100 metres thin fluvial metapsammite formations locally underlain by a thin palaeosol of sericite and chloritoid schists. These formations, which are correlated with the Kainuu tectofacies, are overlain by at least 1 km thick sequence of turbiditic metapsephites and metapsammites of the Haajainen and Rotimojoki formations of the western Kaleva tectofacies. The major central part of the area is cored by two gneiss complexes, earlier considered to be the Archaean basement, but now classified mostly as lithodemic derivatives of the Kaleva tectofacies. These complexes are mantled by gneisses and schists which represent western, more metamorphosed distal equivalents of the Haajainen and Rotimojoki formations. The western margin of the area is composed of gneisses and schists part of which show Svecofennian affinities. Excluding the Archaean basement, the area is intruded and migmatized by Svecokarelidic intrusive rocks.

Keywords

• Stratigraphy
• metasedimentary rocks
• gneisses
• schists
• Proterozoic
• Vieremä
• Kiuruvesi
• Pyhäntä
• Finland