Under opbygning

2.2.6 Lake Saaksjarvi meteorite crater Lake Saaksjarvi, northeast of the Satakunta sandstone, hosts about 4.5 km wide impact crater of early Cambrian age (ea. 560 Ma). Below water level it contains unexposed suevite breccia and impact melt breccia (Papunen 1973, Muller et al. 1990, Elo et al. 1992, Pihlaja & Kujala 2000). The deep drilling in the centre of the gravity low penetrated 180 m of suevites and breccias before reaching the deformed Svecofennian mica gneiss (Pihlaja & Kujala 2000). No coherent impact melt layer was found in drilling. The suevite breccia (Fig. 2.2-14) contains mineral and rock fragments of the Svecofennian rocks, as well as fluidal glass fragments and vesicles filled with clay minerals and zeolites (Pihlaja & Kujala 2000). The impact melt rocks show characteristic shock effects for quartz, feldspars and biotite, including planar deformation features, diapletic quartz glass, ballen-quartz, checkerboard plagioclase and kink bands in biotite (Papunen 1973, Muller et al. 1990, Elo et al. 1992, Pihlaja & Kujala 2000). Both impacts melts and suevites are strongly enriched in lr, Pt, Pd, Ni and Cr. Of the major elements, Na and Ca are depleted, and Mg and especially K enriched in impact rocks compared with target rocks, as a consequence of replacement of structurally damaged shocked plagioclase with a secondary potassium feldspar and clay minerals (Pihlaja & Kujala 2000).

3.2.9 Stop No 8: Lake Sääksjärvi, Kokemäki Location Cape Kotkanniemi, ca. 10 km north-east of Harjavalta along road 2460, then ca. 1 km to the right along road 2475, and finally ca. 7 km to the left. X = 6814025, Y = 1573455 (Finnish National Coordinate System, Zone 1), WGS84: N 61°25'38.143", E 22°22'21.608". Introduction Lake Sääksjärvi, northeast of the Satakunta sandstone, overlies approximately 5 km wide impact structure of early Cambrian age (ca. 560 Ma). The impact structure is characterised by a circular gravity low of – 6.5 mGal (Fig. 26A). Furthermore, the structure is discernible on aerogeophysical maps as a magnetic and resistivity minimum. According to the petrophysics, rather low resistivity and density values, as well as high porosities, are attributed to the uppermost part of the structure (Elo et al. 1992). Below water level it contains unexposed suevite breccia and impact melt breccia (Papunen 1973, Műller et al. 1990, Elo et al. 1992, Pihlaja & Kujala 2000). The deep drilling in the centre of the gravity low penetrated 180 m of suevites and breccias before reaching the deformed Svecofennian mica gneiss (Pihlaja & Kujala 2000). No coherent impact melt layer was found in drilling. Plenty of small boulders of impact rocks occur on the shore of cape Kotkanniemi in the northern part of the lake. Description The suevite breccia (Fig. 26B) contains mineral and rock fragments of the Svecofennian rocks, as well as fluidal glass fragments and vesicles filled with clay minerals and zeolites (Pihlaja & Kujala 2000). The impact melt rocks show characteristic shock effects for quartz, feldspars and biotite, including planar deformation features, diapletic quartz glass, ballen-quartz, checkerboard plagioclase and kink bands in biotite (Papunen 1973, Műller et al. 1990, Elo et al. 1992, Pihlaja & Kujala 2000). Both impacts melts and suevites are strongly enriched in Ir, Pt, Pd, Ni and Cr. Of the major elements, Na and Ca are depleted, and Mg and especially K enriched in the impact rocks compared with target rocks, as a consequence of replacement of structurally damaged shocked plagioclase with secondary potassium feldspar and clay minerals (Pihlaja & Kujala 2000).

Meteoritics & Planetary Science:

THE SAAKSJARVI IMPACT STRUCTURE The Finnish bedrock represents a fairly typical Precambrian shield area. The northern and eastern part of Finland is largely of late Archaean age, while much of the bedrock of southern Finland formed during the Svecofennian orogeny -1900 Ma ago (Vaasjoki, 1996). The SMksjtirvi impact structure in southwestern Finland (Fig. 1) lies within the Svecofennian tonalitic migmatite zone, which consists mainly of intermediate plutonic rocks and mica schists and gneisses (Koistinen, 1996). This Pre-Vendian (ie., -0.65 Ga) peneplained Paleoproterozoic bedrock is overlain by tills formed during and after the Weichselian glaciations (Nenonen, 1993, recording two main transport directions, an older 325-335" trend and a younger 290- 295" trend (Salonen and Aumo, 1984). The length of boulder fans in Finland is normally -1-5 km but may occasionally reach 50-100 km (Salonen, 1986). As in the case of Sa&sjBrvi, subsequent glaciofluvial transport has significantly increased transport distances. The SWsjWi impact structure has a diameter of 5 km and was found on the basis of agate pebbles occurring in glaciofluvial material (Papunen, 1969). Agates (Fig. 2), observed in gravel pits in the Huittinen and Kokemiiki area some 30 km southeast of the lake Satiksjarvi by local collectors in the late 196Os, were originally identified by Mr. Y. Vuorelainen (Outokumpu Mining Company), who first believed them to be flint, which was a common component of ballast material dumped by ships in old Finnish harbours. It was later discovered, however, that some agates were embedded in vesicles of impactite-like lava boulders in gravel pits within the Karhiniemi esker at Huittinen (Papunen, 1969). These boulders were tentatively interpreted as impactite rocks by Papunen (1969, 1973). Detailed studies at the Max Planck Institute in Germany verified the impact origin, and an age of 5 14 2 12 Ma (40Ar-39Ar) was obtained for three whole rock melt samples (Muller et al., 1990). The boulder train of agates and impact melt rocks occurs mainly to the southeast of the lake Saiiksjavi and has been mapped by Papunen (1973), Halme (1974) and Salonen and Aumo (1984). Regional gravity and lowaltitude aerogeophysical measurements over the lake SWsjlrvi site revealed a circular Bouguer anomaly of 4.5 mGal -5 km in diameter (Elo et al., 1992). Microbrecciation found in deep drill holes verified that the SZiksjiirvi area was the boulder source (Elo et al., 1992). Later deep drillings revealed impact melt, suevite and impact breccia. The concentrations of siderophilic elements from impact melt rocks suggest that the Ssksjarvi projectile was most probably a chondritic stone meteorite (Palme et al., 1980; Schmidt et al., 1995). Boulders of impact melt breccias and suevites were found in great numbers on the shore line of lake Saiiksjarvi and particularly to the southeast of the lake (Salonen and Aumo, 1984). The petrography and chemical composition of the impact melt breccias (all boulders) have been studied by Papunen (1 969, 1973), Maerz (1979), Mutanen (1979) and Muller et al. (1990). Papunen (1969, 1973) classified the boulders into four types: (I) impact melt, dark greenish gray, similar to andesite in composition, and small dark green amygdales occasionally filled with silica, (2) vesicular impact melt breccia with crystalline matrix divided to greenish and light brown subtypes, the latter frequently containing agates, (3) impact melt breccia with aphanitic matrix lacking open vesicles, and (4) impact breccia. \duller et al. (1 990) describe their sample used for isotope determinations (and corresponding to Papunen's type 1) as a cryptocrystalline melt breccia, in which the matrix (composed of plagioclase laths, prismatic pyroxene and a mesostasis with opaques) comprises 68 vol%, vesicles, 7 vol% and partially melted mineral clasts and rock fragments, the remainder of the rock. Detailed petrographic descriptions of other impact types have not been published. MA