<sup>40</sup>K-<sup>40</sup>Ar dating of the Söderfjärden crater, Vaasa, western Finland

Lehtovaara, Jyrki J.

doi:10.1080/11035898509452604

Cited by 7 publications

(14 citation statements)

References 15 publications

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“…The age of the formation is determined to be 1200-1300 Ma (Lehtovaara, 1984a), but if as indicated by Talvitie et al (1975) it is a graben its origin may be connected with the Svecokarelidic bedrock development. If wholly independent of its geological environment the formation of the circular structure gives rise to more problems with regard to its good fitting to the area1 tectonic systems and the lack of clear impact features.…”

Section: Discussionmentioning

confidence: 99%

The bedrock of the Vaasa circular structure, Western Finland

Raitala

1985

Earth Moon Planet

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The Vaasa circular structure (sometimes called the Soderfjarden basin) consists of roughly circular or polygonal flat cultivated ground with a diameter of 5-6 km and a surrounding bedrock ring rising 50 m above the centre of the basin and the sea. The maximum basin depth is about 300 m and is filled with Lower Cambrian sedimentary rocks with the uppermost levels being loose overburden of silts with a depth of 30 to 70 m.The surrounding bedrock consists of Precambrian mica and quartz-fieldspar gneisses, quartz-and granodiorites, and porphyritic granites. Some specimens collected from the glacial deposits depict the rocks in the centre of the Vaasa circular structure. A petrographic description of bedrock types and some glacial boulders are presented.The origin of the Vaasa circular structure is not yet solved but there are some existing theories: (1) depression formation due to tectonic movements, (2) impact cratering, and (3) volcanic cryptoexplosion. There is not any conclusive evidence found for any one of these theories and both endoand exogenic causes must be taken into account in the form of a volcano-tectonic collapsing caldera or impact crater, respectively.

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Section: Discussionmentioning

confidence: 99%

The bedrock of the Vaasa circular structure, Western Finland

Raitala

1985

Earth Moon Planet

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“…The contact between the lowermost Cambrian greywacke and the Paleoproterozoic migmatitic mica gneiss and granodiorite is sharp, and no basal conglomerate or breccia of any type exists (Lehtovaara 1982). Lehtovaara (, ) interpreted this drilling to have reached the true floor of the crater, whereas Abels () considered that the drilling sampled only a slumped megablock or a terrace, and that the true floor of the crater remains unsampled and lies farther below.…”

Section: Geologic Background and Previous Studiesmentioning

confidence: 99%

“…Two opposing views have been presented about whether Söderfjärden was a marine impact or not. A marine setting was favored by Lehtovaara (1982, , , ). In contrast, Puura et al.…”

Section: Geologic Background and Previous Studiesmentioning

confidence: 99%

“…Problems arise when the temporal relationship between the impact event and the sedimentation is not clear, i.e., if the sedimentary rocks represent pre‐ or postimpact lithologies (Kohonen and Vaarma , ). However, even a more profound question of whether or not there is actual evidence for an impact origin has in many cases, like Saarijärvi and Söderfjärden in Finland, been addressed only tentatively; although evidence for shock metamorphism has been presented (e.g., Lehtovaara ; Pesonen et al. ), more detailed analysis has so far been lacking.…”

Section: Introductionmentioning

confidence: 99%

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Shock metamorphic features in quartz grains from the Saarijärvi and Söderfjärden impact structures, Finland

Öhman¹,

Preeden²

2013

Meteorit & Planetary Scien

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Abstract-Shock metamorphic features at the Saarija¨rvi (D > 2 km) and So¨derfja¨rden (D = 6.5 km) structures in Finland have so far only been studied tentatively, although both are considered to be proven impact structures. This work presents the first detailed universal stage study of planar deformation features (PDFs), feather feature lamellae (FFL), and planar fractures (PFs) in quartz grains from a polymict impact breccia dike from So¨derfja¨rden, and from sedimentary crater-fill rocks from Saarija¨rvi. Planar microstructures, particularly PDFs, are very rare and poorly developed or preserved in Saarija¨rvi, whereas in So¨derfja¨rden they are much more common and well defined. Miller-Bravais indices of the planar microstructures in both Saarija¨rvi and So¨derfja¨rden are indicative of relatively lowshock pressure but high shear conditions, only compatible with an impact origin for these structures. Although a Proterozoic age for Saarija¨rvi cannot be ruled out, the observations of shock features throughout the sedimentary crater-fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a Lower Cambrian (or younger) impact age.

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“…Thus far, nine meteorite impact structures have been recorded in Finland: six of them are filled with Pre-Quaternary sedimentary rocks containing fossils, i.e. Lappajärvi (age 77 Ma) (Svensson 1968, Lehtinen 1976, Jessberg & Reimold 1980, Uutela 1990, 1998 and Söderfjärden (age 530-510 Ma) (Laurén et al 1978, Tynni 1978, 1982a, Lehtovaara 1984, 1985, Hagenfeldt 1989a, b, Abels et al 2000 in western Finland, Iso-Naakkima (age 1000-650 Ma) (Elo et al 1993) in central Finland, Lumparn in Åland (age 600-443Ma) (Tynni 1982b, Svensson 1993, Abels et al 2000 in south-western Finland, Saarijärvi in northern Finland (unknown age between 2450-500 Ma) (Tynni & Uutela 1985, Öhman et al 2000, and the Karikkoselkä impact crater -the subject of this paper.…”

Section: Introductionmentioning

confidence: 99%

Proterozoic and early Palaeozoic microfossils in the Karikkoselkä impact crater, central Finland

Uutela¹

2001

Bull Geol Soc Finland

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The Karikkoselkä impact crater is located at Petäjävesi (Lat. 62°13.3' N, Long. 25°14.7' E), in central Finland. The crater is filled with impact-generated breccias and redeposited sedimentary rock yielding microfossils. The assemblage consists of Proterozoic, Cambrian and Ordovician acritarchs, cyanobacteria and green algae thoroughly mixed in the deposit. The late Ordovician acritarch Diexallophasis striatum indicates the maximum age of the impact event in the Keila Regional Stage, middle Caradocian in British Series, 458-449 Ma or later. A till sample overlying the sediments that infill the crater yields only Quaternary pollen and spores, indicating that the impact event occurred prior to the Fennoscandian Ice Age. The most likely palaeomagnetic age of 260-230 Ma (late Permian to early Triassic) is neither excluded nor supported by the microfossil results. However, other palaeomagnetic ages are excluded leaving this the most likely age. This article presents new evidence of Proterozoic and early Palaeozoic deposits that covered central Finland.

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⁴⁰K-⁴⁰Ar dating of the Söderfjärden crater, Vaasa, western Finland

Cited by 7 publications

References 15 publications

The bedrock of the Vaasa circular structure, Western Finland

The bedrock of the Vaasa circular structure, Western Finland

Shock metamorphic features in quartz grains from the Saarijärvi and Söderfjärden impact structures, Finland

Proterozoic and early Palaeozoic microfossils in the Karikkoselkä impact crater, central Finland

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40K-40Ar dating of the Söderfjärden crater, Vaasa, western Finland

Cited by 7 publications

References 15 publications

The bedrock of the Vaasa circular structure, Western Finland

The bedrock of the Vaasa circular structure, Western Finland

Shock metamorphic features in quartz grains from the Saarijärvi and Söderfjärden impact structures, Finland

Proterozoic and early Palaeozoic microfossils in the Karikkoselkä impact crater, central Finland

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⁴⁰K-⁴⁰Ar dating of the Söderfjärden crater, Vaasa, western Finland