A Preliminary Report on the Geology of the Koli Area

A PRELIMINARY REPORT ON THE GEOLOGY OF THE KOLI AREA TAUNO PIIRAINEN, MIKKO HONKAMO and SEPPO ROSSI PIIRAINEN, TAUNO; HONKAMO, MIKKO and Rossi Seppo 1974: A preliminary report on the geology of the Koli area. Bull. Geol. Soc. Finland 46, 161—166. The bedrock of the Koli area originates from two separate otogenic cycles. Of these only the younger one, the Svecokarelian orogenic cycle, is dealt with in detail in this account. The sedimentary rock units of this cycle were deposited at the edges of the karel<an geosyncl:nal basin. The origin of the sedimentary formations and that of the basic magmatism of the area is closely associated with the development of this basin which was characterized by the subsidence of its bottom, and by sudden releases of the thus generated strain field. Tauno Piirainen and Seppo Rossi, Dept. of Geology, University of Oulu, SF-90100 Oulu 10, Finland. Mikko Honkamo, Geological Survey of Finland, SF-02150 Otaniemi, Finland. Introduction geological data were gathered during this work, and these data serve as a base for the inter- Extensive geological investigations were car- pretation of the geology of the Koli area. ried out in the north-eastern parts of the North Karelian schist belt at the end of the 1950's and Stratigraphy in the beginning of the 1960's in connection with uranium ore explorations conducted by The stratigraphy of the Koli area is summar- the Atomienergia, and Outokumpu companies. ized in the map in Fig. 1. As can be seen, there At that time the quartzite belt from Koli to are two main stratigraphie units: the basement Kaltimo was intensely explored. At the end of complex, and the Svecokarelian formations. the 1960's, when prospecting for uranium began Both of these units have stratigraphical problems again, the examination of this quartzite belt was of their own. extended by Outokumpu Oy from Koli through The characteristic rock types of the basement Hattusaari village to Nunnanlahti, and from complex are different gneisses. Of these, the there further west to the »Höytiäinen arc». At granite, granodiorite, and trondhjemite gneisses this time prospecting was not limited to the are the main types. Within the gneisses there are quartzites alone, but parts of the basement erratic schist inclusions which at Koli, are complex in the east, and the phyllites in the west represented by the Ipatti formation (Väyrynen were also covered. Considerable amounts of 1933). The same formation continues to Hattu- 162 Tauno Piirainen, Mikko Honkamo and Seppo Rossi SVECOKARELIAN Kalevian jS5=jGraywacke slate formation Jatulian lAlbjte diabases |and metadiabases I'.••;.;.vjOrthoquartzite formation! j^^^Arkose quartzite formation [:•:•'.•'.•'-VJ Orthoquartzite formation I u»«» - H Basal formations BASEMENT COMPLEX Serpentinites Ipatti formation Granite gneiss formation Stratification Schistosity / Fold axis and lineafion It- I i i i i 1 5km Fig. 1. Geological map of the Koli area. saari, and appears again at Nunnanlahti. These some locations graywackes are found in which schists are composed mainly of basic, inter- graded bedding is well preserved. The schists mediate, and acidic volcanics, and of sediments are cut by granites. Elsewhere they are replaced closely associated with volcanic activity. At without any sharp contacts by different gneisses A preliminary report on the geology of the Koli area 163 which, in a samples taken from the slope of the orthoquartzite I formation could not have been Koli hill, were determined to be 2.8 billion years very well consolidated before the development old (Kouvo 1958). of the hiatus because no quartzite pebbles are In comparison with those of the basement found in the immediate vicinity of the hiatus complex the Svecokarelian metasediments are level in the Koli—Nunnanlahti belt. well preserved. They are sub-devided into the The arkose quartzite formation is cross following formations, from the oldest to the bedded throughout. On the basis of this obser- youngest: basal, orthoquartzite I, arkose quartz- vation (Pasek 1971), and because of the continuity ite I, arkose quartzite, orthoquartzite II, and the of the arkose quartzite formation to the »Höy- graywacke schist formation. These sediments tiäinen arc», it is clear that the material has been have been described by Gaal (1964), and by transported from the east, northeast, and even Piirainen (1968) for the area South of Koli. northwest. The first four formations can be correlated with Another, more pronounced hiatus is found the Jatulian formations, and the fifth with the between the orthoquartzite II, and the gray- Kalevian formations as designated by Väyrynen wacke schist formations. At the bottom of the (1954). The Jatulian formations are cut by graywacke schist formation there is a conglo- tholeiitic and spilitic hypabyssic intrusions merate, the pebble material of which, (e.g. at (Piirainen 1969, Honkamo 1972) which are not Puso), is composed of quartzites and meta- encountered within the Kalevian formations in diabases. North of Nunnanlahti the corre- the area described here. sponding conglomerates are found in contact The above mentioned Jatulian quartzite for- with the basement complex, and in addition to mations continue without interruption from the above mentioned pebble materials they also Koli through Hattusaari village to Nunnanlahti. contain rock types found in the basement From Nunnanlahti they extend further west as complex. Some of the quartzite pebbles are the »Höytiäinen arc», but not contiguously, schistose indicating that the Jatulian formations being separated from the Koli—Nunnanlahti were deformed to some extent before the depo- belt by a dislocation zone (Fig. 1). The ortho- sition of the Kalevian formations took place. All quartzite I formation is missing in the »Höytiäi- these data suggest that the hiatus between the nen arc», and the Jatulian quartzites begin with Jatulian and the Kalevian formations covers a the arkose quartzite formation just as they do in considerable area, but it does not seem to the south, on the northern side of the Jakokoski continue very far to the south where basic dome (Piirainen 1968). In the »Höytiäinen arc» volcanics are found at the corresponding strati- the Jatulian quartzites begin with a conglom- graphical level (Nykänen 1971). In the Koli area erate, the pebble material of which is composed these volcanics seem to have been eroded, and mainly of gneisses, but also of meta diabases, only their hypabyssic vents remain as dykes. albite diabases, and quartzites. These pebbles, No radiometric age determinations have been however, are probably not Svecokarelian but made on the basic magmatic rocks in the Koli most likely originate from the Pre-Svecokarelian area, but it is evident that they are of the same Ipatti formation which, at Nunnanlahti, covers age as the basic volcanics to the south of the a wide area. area. Here, these volcanics, have been dated by The absence of the orthoquartzite I formation Kouvo at 2.15 billion years (oral communi- from the »Höytiäinen arc» indicates the existence cation). of a hiatus. Here the hiatus cuts the basement The Kalevian basal conglomerates gradually complex, but between Nunnanlahti and Koli it change upwards into graywacke schists, the cuts only the orthoquartzite I formation. The constituent minerals of which are quartz, plagio- 164 Tauno Piirainen, Mikko Honkamo and Seppo Rossi clase, potassic feldspar, biotite and muscovite, to this rule is the Hattusaari trough, which is with sulphides and graphite, and carbonates surrounded by steep angle bedding, and cut by as accessory minerals. At some locations the N—S striking vertical schistosity (Fig. 1). This amounts of these accessory minerals increase, is S2-schistosity which is diagonal to the direc- and the resulting rocks can be called black tion of the principal pressure. schists, or limestones, respectively. The grain To the west of Nunnanlahti the quartzites size in the graywacke schist formation is vari- form an arc (the »Höytiäinen arc», Fig. 1). Here able, e.g. at Romppala the coarsest fractions also the bedding is coincident with the schistos- reach the grain size of a conglomerate. These ity, and the strike follows that of the quartzite conglomerates are situated in the middle of the Höytiäinen syncline thus representing the upper- most stratigraphical horizon. STAGE 4 Structure fSSJ&^HiS^T^ y i *y y / # r J In the basement complex a plastic phase of >fj // / deformation is visible, followed by a separate rigid one. In connection with the plastic phase wide areas have been remobilized. The remo- STAGE 3 bilized parts seem to build up fold structures which can be traced in the schists of the Ipatti formation. Acidic intrusive rocks which cut all the other rock types are also encountered. The ipfÄs rigid phase of deformation has developed in > 'v> yy 1 J places into tectonic movements, which affect STAGE 2 the whole formation thus generating fold structures of its own. :: •• •: : '^{.v'-"' y' The structure of the Svecokarelian bedrock is far less complicated than that of the basement Ty/yy"ffy/v^y y Yyy V y y r,/ complex. The Svecokarelian formations have \y///yy ///yy w y // y 1 undergone only one phase of folding where the principal pressure has been from the southwest. STAGE 1 During the deformation the quartzites and the basic dykes that cut them have together formed . .ii,;;:.y y y v 7 y y y y y y T^tyTy », Vy Y * 1 a competent bed, and the graywacke schist V y ~ y y y y "VV formation an incompetent one. Consequently, i EZZF 2 Hh3 3 G22 4 E23 5 EMI 67 Sg the quartzites and the basic dykes are weakly I schistosed, and the strike and dip of the schistos- . Fig. 1. Schematic illustration of the geological divelope- ity usually follow those of the bedding. In con- ment of the Koli area.

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