91 Introduction the Southern Part of Ostrobothnia, the Suupohja Region
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Quaternary studies in the northern and Arctic regions of Finland Edited by Antti E.K. Ojala Geological Survey of Finland, Special Paper 40, 91– 95,2005. A PROPOSAL FOR FORMAL LITHOSTRATIGRAPHICAL NAMES IN THE SUUPOHJA REGION, WESTERN FINLAND by Reijo Pitkäranta Department of Geology, 20014, University of Turku, FINLAND E-mail: reijo.pitkaranta@utu.fi Pitkäranta, Reijo, 2005. Aproposal for formal lithostratigraphical names in the Suu- pohja region, western Finland. Geological Survey of Finland, Special Paper 40 , 91–95, 2 figures and 1 table. The Suupohja region in southern Ostrobothnia, western Finland, has diverse Qua- ternary lithostratigraphy. Former and recent observations of the lithostratigraphical units have been compiled and formal names for them proposed. Altogether seventeen identified units (excluding the Holocene clay, peat and eolian deposits) are included to the Suupohja Group. It consists of the following formations, from bottom to top: Karhukangas Formation (nine different lithofacies observed), Kankalo Sand (two or three different lithofacies), Kariluoma Till, Harrinkangas Formation, Kauhajoki Till and Lumikangas Formation. In addition, to the Paleozoic lithified sandstone, which locally has been observed at the bottom above the crystalline Precambrian bedrock, the name “Lauhanvuori Sandstone” is proposed. Majority of the Quaternary sediments have been depositedinglacial(till)orice-marginalenvironment(glaciofluvialandglaciolacustrine/- marine sediments). In the upper parts also shore and eolian deposits can be observed. The sediments below the surficial Kauhajoki Till are older than the Middle Weichselian Substage, and the oldest sediments in the deeper parts may originate from the Middle or even Early Pleistocene Epoch. Key words (GeoRef Thesaurus, AGI): glacial geology, sediments, lithostratigraphy, stratigraphic units, lithofacies, Quaternary, Pleistocene, South Ostrobothnia, Finland Introduction Formerstudies havebeen concentrated almost exclusively on eskers, which usually have distinct The southern part of Ostrobothnia, the Suupohja geomorphology and where abundant open sections region, has diverseQuaternarylithostratigraphy are observable. During the inventory of sand and withseveral clearlydistinguishable sedimentary gravel resources in the Kauhajoki district (Kurkinen units. These lithostratigraphical features have been et al. 1994), also other types of accumulations with described in several papers since the 1970’s (see e.g. multiple stratigraphy were found, and which do not Niemelä & Tynni 1979, Saarnisto & Salonen 1995, havethe geomorphological form of an esker.In Donner 1995, Nenonen 1995). The Suupohja region these accumulations different types of till units and consists of the municipalities of Isojoki, Jurva, Kari- both coarse- and fine-grained sorted deposits can joki,Kauhajoki,Teuva,Kaskinen,Kristiinankaupunki be observed superimposing upon each other. The and Närpiö (Fig. 1). drilling made by the Geological Survey of Finland 91 Geological Survey of Finland, Special Paper 40 Reijo Pitkäranta Fig. 1. The Suupohja region with the thick multilayer accumulations (grey areas). The “till-covered eskers” (see Niemelä 1978) are marked as black and eskers without till cover as white. Sites described in previous papers are also indicated. at Karhukangas penetrated 95.3 m thick Quaternary All the observed sedimentary units in the Suupo- sediment sequence and ended into sandstone (Huhta hja region have been grouped to form the Suupohja 1997). The drill core revealed twelve distinguishable Group, which consists of six Quaternary formations. lithofacies, which obviously have been deposited The sedimentary units have been described recently in different sedimentary environments. Up to five in Pitkäranta (submitted) and in previous papers (see glacial-deglacial phases can be constructed accord- e.g. Niemelä & Tynni 1979, Donner 1988, Gibbard et ing to the drilling, although it is possible that some al. 1989, Bouchard et al. 1990, Nenonen 1995, Huhta of the multiple layers have been deposited during a 1997, Kujansuu & Uutela 1997). Four new and two single glaciation due to glacier oscillation or from previousformationnameproposalsarepresentedinthe different parts of a glacier (see e.g. Brodzikowski & following. In addition, formal name for the Paleozoic Van Loon 1991). Different till layers, glaciofluvial, sandstone (see Simonen & Kouvo 1955, Lehtovaara shore and eolian sediments, permafrost structures and 1982) is proposed. paleosoils refer to two separate glacial and deglacial phases (before the Holocene) interpretable even in the Formal lithostratigraphy in glaciated areas uppermost few metres (Fig. 2). The interpretation of different lithofacies in the drillings, open sections and Utilizing formal lithostratigraphy helps in commu- test pits is based on grain-size composition, texture, nication. It is a tool when dealing with more or less color, organic content, sedimentary structures and complexlithostratigraphy,andalsoclarifiescorrelation directionalelements(whenobservable/measureable). between units in different areas.The presupposition is In addition, refraction seismic and ground-penetrat- thattheunits,whicharegoingtobenamed,willbeduly ing radar measurements were utilized (see Pitkäranta defined, characterized, described and proposed (see 1996, Pitkäranta submitted). instructions in the International Stratigraphic Guide, 92 Geological Survey of Finland, Special Paper 40 A proposal for formal lithostratigraphical names in the Suupohja region, western Finland Hedberg 1976 [1st edition] and Salvador 1994 [2nd One problem in the procedure of formal naming is edition]). Using formal names diminishes confusing, the designation of stratotypes (type sections). Natural subjective and nonconsistent descriptive terms. sections, such as river terraces and seaside cliffs, are Assigning formal names tolithostr atigraphical rare in these areas. Observations have been made units has been rarely used in Finland and in Fenno- usually at artificially made open sections (e.g. quar- scandia as a whole (cf. Gibbard 1992). Reasons for ries, construction sites), where excavation is going this are probably lack of tradition to name units and on and the original description sites will easily be the fact that Quaternary deposits in Fennoscandia destroyed. On the other hand, where excavation has are relatively thin and heterogenous, and particular ceased, section walls will collapse or they have been lithofacies associations occur usually in only limited filled. Consequently, designated stratotypes lose their area. This may lead to a jungle of lithostratigraphic information/position as a representative observation names, because both litho- and chronostratigraphic site for particular lithostratigraphic units. Thus, in ad- correlation between different areas is difficult, and dition to the designation ofthe original primary strato- hence combining units from different areas is often type (holostratotype), also supplementary stratotypes impossible. Deposits from the same time interval and (parastratotypes) should be indicated if possible (see even the same sedimentary event may vary a lot from Salvador 1994). One possibility is also to establish one place to another.Also sediments originating from rules, how to preserve valuable geological sections. a single sedimentary event can vary a lot in a vertical section (see e.g. Eyles et al. 1983, Johnson & Hansel Proposed formal names for the Suupohja Group 1990,Brodzikowski&VanLoon1991).Furthermore, the lack of organic sediments and fossils make the Up to seven different till units, three glaciomar- correlation uncertain. If they are present, possible ine/-lacustrine units, three glaciofluvial, two wave- redeposition must be considered. Finally, detailed de- generated and three eolian sediment units (including scriptions of the deposits have concerned only limited Holocene sediments) have been interpreted in the areas,andeventhenthedescriptionsareheterogenous. Suupohjaregion.Alltheobservedlithofaciesandtheir They don’t necessarily fill the requirements, which formal names are summarized inTable 1. Generalized should be taken into account when assigning formal cross-section from Karhukangas to Lumikangas with names (see Salvador 1994). identified lithofacies is illustrated in Figure 2. Five Table 1. Summaryof the lithostratigraphy,brief description of the depositsand tentativechronostratigraphyof the Suupohja Groupsediments. The locations of the type sections are indicated in the Finnish co-ordinatesystem, Zone 1. Elevation (Z) in meters abovepresentsea level. Lithostratigraphic Observable unitsShortdescription of the sediments Chronostratigra- Type section names withinterpretation phy or type area Lumikangas Stratified sand and gravel, shore depositHolocene X=6910.00 Formation Y=1559.80 Z=165 Kauhajoki Till Twodifferent Dark massivebasal till, high contentof Middle or Late X=6915.50 types (see Bou- fines (35–60 %<0.0625mm) Weichselian Y=1560.45 chard etal. 1990) Z=140 Harrinkangas Severaldifferent Glaciofluvial gravel, sand and silt,com- LateSaalian X=6915.50 Formation units(see Gib- pact,deformed, permafroststructures –Eem Y=1560.45 bard etal. 1989) Z=140 Kariluoma Till Atleasttwo Lightcompactsandylodgementtill, 10 LatePleistocene X=6909.71 differenttypes –30 %<0.0625mm (LateSaalian) Y=1549.02 Z=140* KankaloSand Three unitsStratified, cross-bedded and ripple cross- Middle or Late X=6914.42 laminated medium-grained glacioflu- Pleistocene (Mid- Y=1547.56 vial, shore and eolian sands, compact, dle