Introduction

On Earth, meteorite impact structures are most frequently recognised in cratonic areas, mostly in Precambrian crystalline shields and surrounding platforms with sedimentary cover. The ~1.9 Ga Svecofennian Crustal Domain (SCD; Fig. 1), together with transitional zones to neighbouring domains, occupies part of the Fennoscandian Shield and the neighbouring NW part of the Russian Platform, where the sedimentary cover is usually <2 km thick (Gorbatschev and Bogdanova 1993). Data on 23 (~15 % of known terrestrial impact sites) confirmed and 5 possible impact structures (Table 1, Fig. 1) are used as basis for this analysis. Several authors (Henkel and Pesonen 1992; Puura et al. 1994; Pesonen et al. 2000; Abels et al. 2002) have previously collected and verified the Fennoscandian and Baltic cratering database in detail.

Several impact structures in the SCD are studied by geological, geophysical, and geochemical methods. However, many impact structures require further studies to establish their structure and exact age. Geological evolution of the region has been thoroughly studied (see, e.g., Gorbatschev and Bogdanova 1993; Lahtinen and Huhma 1997), as well as paleogeographic maps have been compiled for late Proterozoic to Mesozoic (Nikishin et al. 1996, Buchan et al. 2000, Torsvik et al. 2001). These data show that the present shield-platform configuration formed after the Tertiary erosion and Pleistocene glaciations whereas the sediments covered much larger areas all over the present shield before the Cenozoic. The thickness of sediments has unevenly grown and reduced through alternating sedimentation and erosion epochs, respectively.

In Europe, the meteorite impact origin was first confirmed for Kaalijarv crater field (Reinwald 1938), ten years after a hypothesis of its impact origin was published (Reinwald 1928, Kraus et al. 1928). In 1938, A. Luha suggested an impact origin for Ilumetsa craters in SE Estonia (Aaloe 1960). Since the 1960ies, the recognition and search for new impact structures in the whole Baltic Sea region became consistent (Abels et al. 2002).

The age of known meteorite impact structures in the region (see Table 1) ranges between 1.2 Ga (Iso-Naakkima; Tynni and Uutela 1984) and <4000 years (Kaalijarv; Veski et al. 2001). The present-day diameter ranges between 0.08 km (Ilumetsa main crater; Raukas et al. 2001) and 65 km (Siljan; Kenkmann and von Dalwigk 2000). However, the eight satellite craters of Kaalijarv crater are only 13 - 39 m in diameter (Aaloe 1960).

Fig. 1. Location of impact structures in the Svecofennian and Karelian Crustal Provinces. Light gray areas mark the marginal zones of the Svecofennian Province, where TTZ -Tornquist-Teysseire Zone. Dark gray indicates anorogenic massifs of rapakivi granites and anorthosites. Paleoproterozoic Transscandinavian Igneous Belt (TIB) is marked by vertical and Karelian-Svecofennian Transition Zone (KSTZ) by horizontal stripes. Boundary between the Precambrian basement exposure of the Fennoscandian Shield and sedimentary areas of the Russian Platform and Bothnian Sea depression is shown by the combination of solid and dotted lines. Contours of local remnants of Proterozoic sedimentary basins (Gulf of Bothnia and Lake Ladoga) are given by stipped lines. Black/white dots are meteorite impact structures penetrating / not penetrating into the crystalline basement.

Fig. 1. Location of impact structures in the Svecofennian and Karelian Crustal Provinces. Light gray areas mark the marginal zones of the Svecofennian Province, where TTZ -Tornquist-Teysseire Zone. Dark gray indicates anorogenic massifs of rapakivi granites and anorthosites. Paleoproterozoic Transscandinavian Igneous Belt (TIB) is marked by vertical and Karelian-Svecofennian Transition Zone (KSTZ) by horizontal stripes. Boundary between the Precambrian basement exposure of the Fennoscandian Shield and sedimentary areas of the Russian Platform and Bothnian Sea depression is shown by the combination of solid and dotted lines. Contours of local remnants of Proterozoic sedimentary basins (Gulf of Bothnia and Lake Ladoga) are given by stipped lines. Black/white dots are meteorite impact structures penetrating / not penetrating into the crystalline basement.

Table 1. General characteristics.of impact structures at the Svecofennian Crustal Domain

Name. Countrv. Coordinates ("Selected references 1

Diameter Present setting Age

Target

Primary fill

Recent cover

Stratigraphie Radiometnc Palaeamagnetic

Kaaliian-. Estonia. 57°24' N. 22°44'E (Remwald 1928. Veski etal. 20011

0.11 PIatfam exposed |B

Sedimentary (S3+Q)

B

Lake

Ilumetsa: Estonia: 57°57; N. 27°24' E (Aaloe 1969. Raukas et al. 2001V

0 OS Platform, exposed Qi

Sedimentary (p2+Q).

B

Bog

Loeoisk; Belarus; 54°12' N. 27°48'E (Glazovskava et al 1993. Veretennikov et al 197ffr

17 Platform, buried Pa2 42.3±1.1 (K/Ar)

Composite (Ä5+V+D)

B+M+S

Qsed.

Laotjaiäri'i. Finland. 63°10'' W. 23°40' E Qessbere and Reimold 1980. Mänttäri and

Koivisto 2001. Picmna and Lehtmen 19921

23 Shield, exposed K2 77 3±0.4 (Ar/Ar) Composite (PP+Cbn+O)

Dellen. Sweden: 61°50' W. J 6°45' E (Deutsch et al. 1992. Henke! 1992. Müller et al 1990")

B+M+S

Lake+Q sed

19 Shield, exposed K2 g9.0±2.7 (Rb/Sr)

Crystalline (PP)

B+M+S

Lake+Q sed

Mien: Sweden: 56°25' N. 14°52'E (Bottomlev etal. 1987. Henkei 19831

6.5 Shield, exposed Ki 121±2.3 (Ar/Ar)

Crystalline (MP)

B+M+S

Lake+Q sed.

Dob el» Latvia. 56°35' ]i. 23°15' E (Paura et al. 19941

4.5 Platform, exposed C2-Pi

Sedimentary (S+D+C)

B

Q sed.

Vepriai. Lithuania: 55°05 ' IT. 24°34' E (Motuza and G-aiiius 197S. Motuza 19941

7.5 Platform, buried C-Pi

Sedimentary (Cbn+0+S+D?j)

B

Qsed.

Mishina Gora. Russia: 58°43' N, 28°03' E (Masaitis 19991 2,5.^-4 0 Platform, exposed C-P

Composite (PPi-V+Cbn+O+D)

B

Si lian; Sweden; 61 "05 ' I-T. 15°00'E ÍÁberg et al. i989. Bottom! ev et al. 1987. Coiiini 19SS. Grieve 198?. Keitmann and vonDalwi^k 2000)

Shield, exposed D-Ci

Karikkoselka; Finland. 62°] 5' N. 25°15'E fLehtmen el al 1996. Pesonen et al 1999a-)

1.3 Shield, exposed 0? 260-230 Composite (PP+V+Cbn+O) B

Lockne. Sweden; 63°00' H. 14°49' E (Lindstrom et al. 1996. Ormo and Miyamoto 2002. Ormo etal. 2002. Themault and Lmdstnim bake+Q sed

Lake+Q sed 19951

7.5 Shield, exposed Q>3 Composite (PP+Cbn+O+wat)

Kardia; Estonia; 58°5-9' M. 22°40'E (Ainsaar et a! 2002. Plado et al. 1996. Puura and Suuroja 19921, 4.0 Platform, buried 03 Composite (PP+V+Cbn+O+wat) ' B

Tvaren. Sweden. 5S°46' N. 17°25' E frloden et al 1986. Frednksson and Wiokman 1963. Ormo andBlomgvist 19961

2.0 Shield, exposed O3 Composite (PF+Cbn+O)

Hummeln*; Sweden; 57°22' H. 16° 19' E iTloden etal. 1988. Lindström et al. 1999. Orrnö et al 1999)

1.2 Shield, exposed O2 Composite (PP+Cbn+Q)

Granbv*. Sweden; 58°26' N. 14°56' E iGrahn et al. 1996. Henkel and Pesonen 1992)

3.2 Shield, exposed Or Composite (MP?+Cbn+0)

Sääksiárvi; Finland; 61°25;' H. 22°23' E (El o et al 1992. K.innunen and Lmdqvist 1998, Mülle:- et ai. 1990. Papunen 19921

4.5 Shieid, exposed Cbn3 514±12 (Ar/Ar) Mizarai; Lithuania; 54"01' N. 23°54' E iMotuza and Gailius 19781

5.0 Platform, buried Cbn^

Soderfiarden; Finland. 62°41' H. 21°35' E (Abels et al. 2000. Lehtovaara 1 9821

6.6 Shieid, exposed Cbn

Neuamnd; Estonia, 5-9°20' N, 23°31' E (Suuroja and Suuroja 2Q001 7.0 Platform, exposed Cbni

Composite (PP?+V+Cbn)

Crystalline (PP+wat)

Lake+Q sed Q marine sed Bay, Q-.sed Lake+Q sed Q sed.

B+M+S Lake + Q manne sed. Sediments

Name. Country. Coordinates (Selected references )

Diameter Present setting Age

Target

Str atigr ap hi c E adiometrie_Palaeomagnetic--

Shield, exposed NP

Paasselka. Finland. 62°09' N. 29°24' E ("Oilman et al. 2003. Pesonen et al. 1999b~l 10.5 Shield, exposed NP?

Biorko*-, Sweden. 59°24' N. 17°35~ E CFloden et al. 1993.2002)

8.0 Shield, exposed MP

Iso-Naakkima, Finland;. 62°11' M. 27°09' E (Elo et al. 1993. Tynm arid Uutela 2.0 Shieid, exposed MP 1200±100 Composite? (PP+Sed?)

Crystalline (FP)

Composite? (PP+sed?). Crystalline (NP)

Primary Recent coper fill

Suvasvesi N; Finland. 62°41 [ M. 28°11 "E (Fesonen et al. 1996. Werner et al 2002')

4.0 Shield, exposed Cbm? ~250, ~565 or Crystalline (PP)

-775 Ma

Suvasvesi S*. Finland. 62'35' M. 28"] 4' E (Lehtinen et al. 2002. Ohman et al. 2003') 4 0 Shield, exposed Cbm? Crystalline (PP)

Lumpam, Finland; 60°09' N, 20°0g" E (Abels et al. 1998, 2000, Winterhalter 19-825

9.0 Shield, exposed NP -580 Ma Composite (MP+Jn)

Avike-bukten*; Sweden; 62°30' N. 17°41'E (Henkel 1991. Kenkel et.al. 2002)

10.0 Shield, exposed NP Composite (MP+PP)

Jarnsiarvi: Russia. 61°58' M. 30°55' E (Masaitis 1975. Muller et al 1990. Raitala and Halkoaho 19921 16.0

Index: the same is used in Fig. 1 Name: not proved, but in this papa' accepted as impact structure

Stratigraphy: Q, Quaternary, Pa, Paleogene, £ Cretaceous; PZ, Palaeozoic; P, Permian, C,-Carboniferous; D, Devonian; 2, Silurian; 0, Ordovician, Cbn, Cambrian; NP, Neoproterozoi'c; V, Vendian; MP, Mesoproterozoic, Jn, Jotnian, PP, Paleoproterozoic; AR, Archaean Primary fill: 3, impact breccia; M, imp act melt, S, suevite Recent cover: sed., sediments

Table 2. Preservation level of meteorite impact structures at the Svecofennian Crustal Domain.

PL Characteristics

Impact structure and age Additional signatures

Ejecta largely preserved Ejecta partly preserved

Kaalijarv Q2 Ilumetsa Q2 Kardla O3 Lockne O3 Neugrund Cbni

Ejecta removed, rim partly preserved

Dobele C2-P1 Vepriai C-P1

Rim largely eroded, crater-fill preserved Logoisk Pa2 Dellen K2

Mien K1

Mizarai Cbn3 Crater-fill products partly preserved

Lappajarvi K2

Mishina Gora C-P

Karikkoselka O? Tvaren O3 Hummeln* O2 Granby* O2 Saaksjarvi Cbn3 Soderfjjarden Cbn Suvasvesi N & S* Cbn1 Lumparn NP

Janisjarvi NP

Apparent crater Apparent crater

Buried, rim-related anticline in cover Incisions, partly hollowed Re-exposed, incisions, post-impact fill

Buried Buried

Planated, buried

Hollowed, allochthonous breccia and melt partlysurvived

Hollowed, allochthonous breccia and melt partly survived

Buried

Hollowed, breccia and melt partly survived

Planated, allochthonous breccia partly survived

Hollowed by glaciers

Hollowed by glaciers

Hollowed by glaciers

Hollowed by glaciers

Hollowed by glaciers

Hollowed by glaciers

Hollowed by glaciers

Incisions, partly hollowed by glaciers

Hollowed, allochthonous breccia and melt partly survived

6 Remnants of crater-fill preserved, crater floor exposed

Avike-bukten* NP Bjorko* NP Paasselka NP? Iso-Naakkima MP Crater floor exposed, substructure exposed Siljan D-C1

Hollowed by glaciers Hollowed by glaciers Hollowed by glaciers Hollowed by glaciers

Planated, ring graben hollowed by glaciers

* not proven

Only a few structures in the SCD have retained easily observable features, whereas the majority are deeply eroded (Table 2) and lost their superstructure. To quantify the rate of survival and erosion of impact structures, Dence (1972) proposed a scale of preservation levels (PL, Table 2). Recognition of roots of impact structures (PL = 6) and even deeply eroded (PL = 4 - 5) craters occurs often to be unsurpassed obstacle. Identification of the only PL = 7 (Siljan, Sweden) is more an exception than a usual practice as was recognised due to its large size and preservation of the outer moat with slumped target sedimentary rocks (Fredriksson and Wickman 1963). In the shield area, Pleistocene glaciations have played an extraordinary important role in destruction of superstructures of almost every crater, as well as buried structures. Therefore, several impact structures in the shield are at the PL = 5, but being completely or partially buried under a Quaternary blanket.

Conditions of post-impact survival or removal, and the structural reshaping, have changed through time, i.e., processes, causing crater preservation or destruction have recurrently replaced each other. This, and its influence to the post-impact development of impact structures in the Baltic Sea region is the main topic of the present paper. Scenarios of formation and post-impact evolution of crater are traced out. First, data on geological and physical environments of impact sites (shield or platform, land or sea), second, post-impact sequence of possible burial and erosion caused by epeirogenic and syn-orogenic tectonic processes or sea-level changes, and, third, Quaternary glacial erosion, accumulation, and flooding of the Baltic erosional depression by post-glacial water-bodies were taken into consideration.

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