Geological Survey of Finland Bulletin 369 Sedimentation of the Jotnian Satakunta sandstone, western Finland by J . Kohonen , P . Pihlaja, H. Kujala and J. Marmo Geologi an tulkimuskeskus Espoo 1993 Geological Survey of Finland, Bulletin 369 ~~EDIMEN1'ATION OF THE JOTNIAl'~ SATAKUNTA SA:NDSTONE, WESTERN FIl'JLAND by J. KOHONEN, P. PIHLAJA, H. KUJALA and J. MARMO with 20 figures, 2 tables and 1 appendix GEOLOGIAN TUTKIMUSKESKUS ESPOO 1993 Kohonen, J., Pihlaja, P., Kujala, H. & Marmo, JIl 1993. Sedimentation of the Jotnian Satakunta sandstone, western Finland. Geological Survey of Finland, Bulletin 369, 34 pages, 20 figures, 2 tables and 1 appendix. Sedimentology of the Mesoproterozoic (' Jotnian') Satakunta sandstone has been examined by methods of facies and grain size distribution analysis. The unmetamor­ phosed sandstone covers an NW-SE trending area about 15 x 100 km 2 in size, as an extension of a wide submarine sandstone area. The thickness of the arkosic sand­ stone formation is more than 600 meters and has been estimated to reach 1800 m. Cross-bedding structures indicate NW directed transport of detritus. Accord­ ing both the paleocurrents and the grain size distribution, the proximal part of basin was situated in the SE. At the southwestern contact, supply tranverse to the basin axis is evident, but the northeastern, steep border of the sandstone seems to be con­ trolled by younger faults, not by the primary basin edge. All primary features observed are typical of fluvial sediments. The only indica­ tion of marine contribution is the presence of glauconite in the distal part of the basin. The infilling of the depository was the result of an alluvial plain and, possi­ bly, sandy fan delta. The sedimentation was dominated by poorly cyclic, low sinuosity streams and, in proximal areas, by debris floods. Observations from a drill core (591 m long) indicate that major part of the Satakunta sandstone consists of medi­ um grained arkoses and subarkoses dissimilar to the coarse, pebbly sandstones typical of the present level of erosion. It seems that the alluvial type sediments offlapped more fine grained sandstones as the system prograded towards NW. The precise age and original extent of the basin are not possible to evaluate, but the development may have begun during the emplacement of rapakivi granites and associated mafic dykes. The final preservation seems to be controlled by the subsidence related to the eruption of 'Postjotnian' dykes and sills, which show a clear spatial connection with the sandstone. Key word~ (GeoRef Thesaurus, AGI):paleosedimentology, sandstone, lithofacies, grain size, paleocurrents, deposition, grabens, Proterozoic, Mesoproterozoic, Jotnian, Satakunta, Finland The authors' adress: Geological Survey ofFinland Betonimiehenkuja 4 SF-02150 Espoo Finland ISBN 951-690-503-X ISSN 0367-522X Vammala 1993 Vammalan Kirjapaino Oy CONTEl'JTS Introduction 0000•000000000000000000000000000000000000•000000•000•00 5 00. 0000000000•00 Geological setting .000 0000000000000000000000000000000 7 The basin 0000000•000000000000000000000000000000000000•000000•000•00 8 Composition and provenance of the sandstone 00000000000•0000000000•00 9 Grain size distribution 00000000000000000•000000000000000•0000000000•00 10 Litofac:ies descriptions 000000000000000000000000000000000•00000000•0•00 12 Methods and sites of observation 00000000000000000000•00000000•0•00 12 The Harjavalta section 00000000000000000000000000000•00000000•0•00 12 Gravelly lithofacies (Gm and Gp) 00000000000000000•0000000000•00 12 00 0 0 • 0 • 0 0 'Trough cross-bedded facies (St 1, St2 •and St3) 0000000•0000 14 'Planar crossbedded facies (Sp) 0000000000000000000D00000000•0000 15 •• 0 0 Horizontally bedded or laminated sandstone facies (Sh) 0 0 0 0 0 0 0 16 Horizontally laminated or rippled sandy mudstone/siltstone facies (Fl) 16 The Knapernummi section 0 0 0 0 0 0 0 0 0 0 o. 000000000000000•00000•00•0•00 18 Trough cross-bedded facies (St) 000000000000000000•00000000•0•00 18 0.0 00000000000000•000•0000•0•00 Planar cross-bedded facies (Sp) 0 0 0 18 •• 0000•00•0•00 Horizontally bedded facies (Sh) 000000000000000000 18 Mudstone facies (Fm) 000000000000000000000000000 .. 0 0 0 0 0 0 0 0 0 0 0 0 0 18 Sedimentary structures in other observation sites 000 0 0 0 .. 0 0 0 0 0 0 0 • 0 0 0 0 0 19 00 .. 0 0 0 0 0 • 0 • 0 0 0 0 0 The: drill core »Pori 1» 000000000000000000000000000 21 Facies sequences 00•00000000•0000000000000000000000000000000000•00000 22 Paleocurrents 0000•0000000000000000000 e . 00000000000000000000000•00000 24 Depositional models 0000000000000000000000000000000000000000000•00000 27 Summary and discussion 000000000000000 o. 00000000000000000000000D00000 29 •• 000000000000000000 0.0 0000000000000000000000D00000 Conclusions 0 0 0 0 0 0 31 Acknowledgements . 0000000000000000000000000000000000000000000D00000 32 References 0000000•0000000000000000000000000000000000000000000•00000 33 Appendix 1: Locations of sandstone outcrops and drilling sites 00000•00000 35 Geological Survey of Finland, Bulletin 369 5 INTRODU(:TION The Jotnian Satakunta sandstone, Mesopro­ Vaasjoki, 1977) is situated immediately to the terozoic (Riphean) in age, is located in western south of the sandstone and two coeval rapakivi Finland south of town Pori. The unmetamor­ stocks are present close to the northeastern flank phosed, poorly exposed sandstone covers a of the graben (Vaasjoki et aI., 1988). The 'Sub­ NW-SE trending, fault-bounded area about jotnian' diabase dykes are held to be slightly old­ 15 x 100 km 2 in size. According to Winterhalter er (c. 1650 Ma; Pihlaja, 1987) than the rapaki­ et al. (1981) and L. Bergman (1982) the sandstone vis. Both the rapakivi and the sediments are cut extends under the Gulf of Bothnia and links to­ by the 'Post-Jotnian' diabases (1260 ± 10 Ma; gether the red beds of Satakunta graben, Gavle Suominen, 1991), and the sandstone has been es­ valley and Nordingra area. timated 1300--1400 Ma old (Simonen, 1980). The term 'Jotnian' was introduced by J.J. Although detailed works concerning petrolo­ Sederholm (1897) for unmetamorphic sandstones gy, mineralogy and provenance of the Satakun­ and associated igneous rocks in the Precambri­ ta sandstone (e.g. Simonen and Kouvo, 1955; an of Fennoscandia. The Jotnian-type sediments, Marttila, 1969) are present, the depositional fea­ arkoses, siltstones, shales and conglomerates, are tures have not been investigated systematically. known to occur in some ten localities (Fig. 1). The aim of this paper is to study the All these occupy shallow basins, tectonic depres­ palaeosedimentology of the Satakunta sandstone sions or grabens bordered by fractures or fault by methods of facies analysis. Also palaeocur­ zones most of which are N'W-SE oriented. rent data from 14 localities is presented. The The Mesoproterozoic is in Satakunta repre­ grainsize variation within the sandstone was 0 b­ sented by rapakivi granite, sandstone and olivine served, not only from the outcrops but also from diabase. The Laitila rapakivi batholith (1570 Ma; 22 drill cores. 6 Geological Survey of Finland, Bulletin 369 o I \ " r~"- 1/ Archean rocks Rapakivi granite '::" 1'- mIIIIIII Svecokarelian rocks I:~~I Subjotnian dyke swarms [TI Sveconorwegian rocks __ Satakunta sandstone [«/J Caledonian rocks * Other Jotnian sediment localities [~ Phanerozoic platform cover D Area indicated in Fig.2 Fig. 1. Location of the Satakunta sandstone and other Jotnian occurences of the Fennoscandian Shield. Geologi cal Sur vey of Finland, Bulletin 369 7 GEOLOGICAL SETTING The bedrock of southwestern Finland is com­ rocks; the anorogenic Mesoproterozoic rapakivi posed mainly of Paleoproterozoic (Svecofennian, granites and diabases are the additional consti­ 1900-1800 Ma) felsic plutonic and supracrustal tuents (Fig. 2). 6860 6860 BOTHNIAN SEA o o to 6740 67 40 o, 1 ~1 4 1 -~ 1 5 ~-='-"41 [/1'-'1''-1 8 I ~ \ 'jjj 9 -- - 6 '-/\ 1\ Fig. 2. Geolog ical map of the Satakunta area . I = sandstone, 2 = ' Postjotnian' diabase, 3 = rapakivi, 4 = 'Subjotnian' dia­ base, 5 = migmat itic mica gneiss, 6 = metavolcanite, 7 = granite, 8 = quartz-diorit e and tonalite, 9 = diorit e, gabbro and ultrama fite. Map is simplified fro m I : 100 000 map sheets (par tly unpublished) of the Geological Map of Finland . 8 Geological Survey of Finland, Bulletin 369 The Svecofennian supracrustals consist mainly The Svecofennian rocks suffered a polyphase of medium-grade metamorphic mica schists and deformation. Besides the folding, major shear migmatitic mica gneisses. In places, the mica and fault zones divide the bedrock into blocks gneisses turn into high-grade metamorphic kin­ showing diverse metamorphic grades and indicat­ zigites containing garnet, cordierite and, local­ ing likely different erosional levels. The dominant ly, sillimanite. Intermediate to mafic pyroclastics direction of the Svecofennian ductile shear zones and lavas occur as interlayers within metasedi­ is I\lW-SE:. ments. The Laitila rapakivi batholith and three minor Synorogenic granodiorites, tonalites (some­ stocks (Eurajoki, Kokemaki and Reposaari) are times garnetiferous) and quartz diorites are the situated in proximity of the Satakunta sandstone. predominant plutonic rocks of the basement In addition, geophysical modelling (E10, 1976, area. SOJme variants of granodiorite contain 1982) suggests that the sandstone is underlain by microcline megacrysts. Gabbros, diorites and several kilometers of rapakivis. The olivine dia­ ultramafic rocks are restricted to some intrusive bases occur in form of sills and feeder