- Email: [email protected]
Söderfjärden: a Cambrian impact crater in western Finland
157
Tecronophysics, 216 (1992) 157-161 Elsevier Science Publishers B.V., Amsterdam
Extended Abstract SGderfjBrden: a Cambrian impact crater in western Finland Jyrki J. Lehtovaara Department of Geology, Unicersity of Turku, SF-20500 Turku, Finland
(Received January 16, 1991; revised version accepted December 18, 1991)
In the mid 197Os, geological interest was aroused in Finland by an exceptionally circular, or hexagonal, geomorphic basin called Stiderfjkden. Located on the west coast of Finland (Fig. l), the basin is clearly visible on almost any map and to all satellite imagery. Local Proterozoic Svecofennian bedrock is not exposed inside the clay covered S~er~~rden depression, which has a diameter of 5 km, but it is occasionally met along the encircling, discontinuous rim of hills, 30-40 m in height (Plate I). The bedrock consists of Vaasa granite, a heterogeneous porphyritic granitoid with some migmatitic character; no shock effects were found in the rim (LaurCn et al., 1978). The SGderfjirden depression was first thought (Talvitie et al., 1975) to be a graben formed by a subsided hexagonal block of bedrock that resistivity measurements suggested was overlain by a considerable cover of markedly less resistive material. In further studies by LaurCn et al. (1978), a circular gravity anomaly of up to -6 mgal was found around its centre (Figs. 2 and 3), and seismic measurements revealed a maximum thickness of 350 m for the seismically slower cover rocks encircling the central uplift. Weakly consolidated sedimentary rocks, palaeontologi~ally dated to the Cambrian by Tynni (1978), were first found as glacial boulders and then in holes drilled
Correspondence
to: 3.X Lehtovaara,
Department University of Turku, SF-20500 Turku, Finland.
HO-1951/92/$05.~
of Geology,
through the 80 m thick glacial overburden into the deepest seismically revealed ‘moat’ (Fig. 4; La&n et al. (1978)). When later intersected at a site with maximum thickness (Lehtovaara, 1982), the Cambrian rocks appeared to have a sharp contact with the basin floor without any basal conglomerate. The rock found in the central uplift, 40 m below the glacial overburden (Fig. 4; LaurCn et al. (1978)), consists of bands varying from slightly mechanically altered bedrock to unidentifiable finely ground breccia that appears to be locally pseudotachyl~i~. No indisputable traces of a melt
t
f
21”E
I
29%
Fig. 1. Location of SGderfjiirden and two other impact sites in Finland. Coordinates for the centre of Siiderfjarden are: latitude 63”01’N, longitude 21”35’E.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
I
LANDSAT
PLATE
TM 870720+
Finnish
National
Board
GT DEM image looking
-
.
F. Pipping,
crater
--
_, .
Survey of Finland,
..?\”
Board
exaggeration National
with vertical and Finnish
(light basin in the centre),
-
Geological
_ _,*-
across the SGderfjiirden
of Survey. Courtesy:
eastwards
of X 20 added of Survey.
by computer
at the
E 03
s
S6DERFJiiRDEN:
A CAMBRIAN
’
IMPACT
SODERFJXRDEN 10
159
CRATER
BOUGUER
ANOMALY
0
10
10
A
mgal
0
-5 0
IO0
0
b
SBDERFJXRDEN 10
0
HOR.
GRADIENT
OF
BA
0
10
0
10
10
0
Fig. 2. (a) Bouguer gravity anomaly of the Siiderfiiiden basin. (b) The horizontal gradient anomaly. The data are based on those of Laurin et al. (1978). Digitized at the Geological Survey of Finland.
J.J. LEHTOVAARA
160
Fig. 3. Original gravity profile across the Siiderfjiirden depression, based on a density contrast of -350 kgm-“, according to Lauren et al. (1978).
rock were found and the finely ground varieties are chemically indistinguishable from the Vaasa granite. A search for mineralogical or other features indicative of a possible meteorite impact yielded only some sets of shock-induced lamehae in some quartz grains found in the central uplift and in a few glacial boulders. There was also less heavy brecciation and kink band formation. On these grounds, Lauren et al. (1978) cautiously proposed that, even if S~de~~rden definitely was an explosion crater, there was not enough evidence to prove whether it was volcanic or astroblemic in origin. After drilling into the crater bottom, 13001400 m off-centre, at the deepest site of the sedimentary infill (Fig. 4; Lehtovaara, 1982) and re-evaluating the general data (as given above), the present author concluded (Lehtovaara, 1984; 1985) that a meteorite impact origin is the most likeiy in view of the geophysical and minerafogical evidence from this geological setting. The
0
I
Acknowledgement
300 m
cl B La
crater probably formed on the bottom of a shallow Cambrian sea, the existence of which is inferred from the presence of marine sediments and which may have reduced the shock effects of the impact. Thus the age of crater formation could be set at approximately 5.50 Ma, as the present (Cambrian) sedimentary infill of the crater immediately subsided to the bottom of the crater, and is present in slumps and other deformed structures. Trials to date fragments of the impact breccia radiometrically using the K/Ar wholerock method (Lehtovaara, 1985) only yielded dates of around 1200-1300 Ma, indicating that, although these dates are younger than the conventional Svecokarehan U/Pb age span of 1800-1900 Ma, this can only be attributed to a small extent to the meteorite impact. In comparison with the better investigated Lappajarvi impact site (Lehtinen, 1976; 1990; Pipping and Lehtinen, this issue), 100 km to the east (Fig. 11, which, although relatively young at 77 Ma (Jessberger and Reimold, 1980) has a rather down worn rim, the Soderfjarden crater, which is considerably older, is relatively well preserved as there has been less defo~ation and it was instantly infilled with sediment. With a diameter of 5 km, Soderfjarden is much smaller than the Lappajsirvi crater, which has a diameter of 17 km. However, due to its central uplift, Soderfjlrden is classed as a complex crater. The S~der~~rden event occurred much closer in time to that of the third Finnish impact site, Lake Stiaksjarvi (Papunen, 1969; Elo et al., this issue), which is 200 km to the south (Fig. I> and dated by 40Ar/ “9Ar to 514 Ma (Grieve and Robertson, 1987).
Ouaternary Cambnan
sedtmenls
Precambrian
bedrock/impact
Precambrian
bedrock.
breccla
waNered
e = 2 160
2290
kQm’3
Q- 2410
2500
kQ”,.3
p= 2610
2670kQm.’
Fig. 4. Generalized east-west cross-section (exact location shown as profile A-B in Fig. 2) of the S%erGiirden crater based mainly on drilling data (hole numbers i-4) and geophysics, modified from Lehtovaara (1985). The rock types are shown schematically.
I want to thank Seppo Elo of the Geological Survey of Finland warmly for the processing and print of Figure 2. References Grieve, R.A.F. and Robertson, P.B., 1987. Terrestrial impact structures. Geol. Surv. Can. Map 1658A, scale 1:63 000 000. Episodes, 10(3) (supplement).
SSDERFJ.&RDEN:
A CAMBRIAN
IMPACT
CRATER
Jessberger, E.K. and Reimold, W.U., 1980. A Late Cretaceous 4”Ar-“yAr age for the Lappajirvi impact crater, Finland. J. Geophys., 48: 57-59. Lauren, L., Lehtovaara, J. and Bostrom, R., 1978. On the geology of the circular depression at SoderfjLrden, western Finland. Geol. Sutv. Finl. Bull., 297: 5-38. Lehtinen, M., 1976. Lake Lappajarvi, a meteorite impact site ion western Finland. Geol. Surv. Finl. Bull., 282: l-92. Lehtinen, M., 1990. Petrology and mineralogy of the impact melt and breccias in the deep drill core, Lake Lappajirvi, western Finland. In: L.J. Pesonen and H. Niemisara (Editors), Symp. Fennoscandian Impact Structures (Espoo and Lappajarvi, Finland, May 29-31, 19901, Prog, and Abstr., Geol. Surv. Finl., p. 19. Lehtovaara, J.J.. 1982. Stratigraphi~al section through Lower Cambrian at Soderfjarden, Vaasa, western Finland. Bull. Geol. Sot. Finl., 54: 36-43.
IhI
Lehtovaara, J.J., 1984. Vaasan Soderfjarden-sedimenttiaineksen suojaama kambrikaudella muodostunut kraatteri. Terra, 96: 23-33 (with English abstract). Lehtovaara, J.J., 1985. 4”K-40Ar dating of the S~derfj~rden crater, Vaasa, western Finland. Geol. For-en. Stockholm Wrh. 107: 1-6. Papunen, H., 1969. Possible impact metamorphic textures in the erratics of the Lake SIiiksjarvi area in southwestern Finland. Geol. Sot. Fin]. Bull., 41: 151-155. Talvitie, J., Pernu, T. and Raitala, J., 1975. The circular Vaasa structure in the Baltic Shield, western Finland. Dep. Geophys., Univ. Oulu Contrib., 59: l-15. Tynni, R., 1978. Lower Cambrian fossils and acritarchs in the sedimentary rocks of Siiderfjarden. western Finland. Bull. Geol. Surv. Fin).. 297: 39-81.
Tecronophysics, 216 (1992) 157-161 Elsevier Science Publishers B.V., Amsterdam
Extended Abstract SGderfjBrden: a Cambrian impact crater in western Finland Jyrki J. Lehtovaara Department of Geology, Unicersity of Turku, SF-20500 Turku, Finland
(Received January 16, 1991; revised version accepted December 18, 1991)
In the mid 197Os, geological interest was aroused in Finland by an exceptionally circular, or hexagonal, geomorphic basin called Stiderfjkden. Located on the west coast of Finland (Fig. l), the basin is clearly visible on almost any map and to all satellite imagery. Local Proterozoic Svecofennian bedrock is not exposed inside the clay covered S~er~~rden depression, which has a diameter of 5 km, but it is occasionally met along the encircling, discontinuous rim of hills, 30-40 m in height (Plate I). The bedrock consists of Vaasa granite, a heterogeneous porphyritic granitoid with some migmatitic character; no shock effects were found in the rim (LaurCn et al., 1978). The SGderfjirden depression was first thought (Talvitie et al., 1975) to be a graben formed by a subsided hexagonal block of bedrock that resistivity measurements suggested was overlain by a considerable cover of markedly less resistive material. In further studies by LaurCn et al. (1978), a circular gravity anomaly of up to -6 mgal was found around its centre (Figs. 2 and 3), and seismic measurements revealed a maximum thickness of 350 m for the seismically slower cover rocks encircling the central uplift. Weakly consolidated sedimentary rocks, palaeontologi~ally dated to the Cambrian by Tynni (1978), were first found as glacial boulders and then in holes drilled
Correspondence
to: 3.X Lehtovaara,
Department University of Turku, SF-20500 Turku, Finland.
HO-1951/92/$05.~
of Geology,
through the 80 m thick glacial overburden into the deepest seismically revealed ‘moat’ (Fig. 4; La&n et al. (1978)). When later intersected at a site with maximum thickness (Lehtovaara, 1982), the Cambrian rocks appeared to have a sharp contact with the basin floor without any basal conglomerate. The rock found in the central uplift, 40 m below the glacial overburden (Fig. 4; LaurCn et al. (1978)), consists of bands varying from slightly mechanically altered bedrock to unidentifiable finely ground breccia that appears to be locally pseudotachyl~i~. No indisputable traces of a melt
t
f
21”E
I
29%
Fig. 1. Location of SGderfjiirden and two other impact sites in Finland. Coordinates for the centre of Siiderfjarden are: latitude 63”01’N, longitude 21”35’E.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
I
LANDSAT
PLATE
TM 870720+
Finnish
National
Board
GT DEM image looking
-
.
F. Pipping,
crater
--
_, .
Survey of Finland,
..?\”
Board
exaggeration National
with vertical and Finnish
(light basin in the centre),
-
Geological
_ _,*-
across the SGderfjiirden
of Survey. Courtesy:
eastwards
of X 20 added of Survey.
by computer
at the
E 03
s
S6DERFJiiRDEN:
A CAMBRIAN
’
IMPACT
SODERFJXRDEN 10
159
CRATER
BOUGUER
ANOMALY
0
10
10
A
mgal
0
-5 0
IO0
0
b
SBDERFJXRDEN 10
0
HOR.
GRADIENT
OF
BA
0
10
0
10
10
0
Fig. 2. (a) Bouguer gravity anomaly of the Siiderfiiiden basin. (b) The horizontal gradient anomaly. The data are based on those of Laurin et al. (1978). Digitized at the Geological Survey of Finland.
J.J. LEHTOVAARA
160
Fig. 3. Original gravity profile across the Siiderfjiirden depression, based on a density contrast of -350 kgm-“, according to Lauren et al. (1978).
rock were found and the finely ground varieties are chemically indistinguishable from the Vaasa granite. A search for mineralogical or other features indicative of a possible meteorite impact yielded only some sets of shock-induced lamehae in some quartz grains found in the central uplift and in a few glacial boulders. There was also less heavy brecciation and kink band formation. On these grounds, Lauren et al. (1978) cautiously proposed that, even if S~de~~rden definitely was an explosion crater, there was not enough evidence to prove whether it was volcanic or astroblemic in origin. After drilling into the crater bottom, 13001400 m off-centre, at the deepest site of the sedimentary infill (Fig. 4; Lehtovaara, 1982) and re-evaluating the general data (as given above), the present author concluded (Lehtovaara, 1984; 1985) that a meteorite impact origin is the most likeiy in view of the geophysical and minerafogical evidence from this geological setting. The
0
I
Acknowledgement
300 m
cl B La
crater probably formed on the bottom of a shallow Cambrian sea, the existence of which is inferred from the presence of marine sediments and which may have reduced the shock effects of the impact. Thus the age of crater formation could be set at approximately 5.50 Ma, as the present (Cambrian) sedimentary infill of the crater immediately subsided to the bottom of the crater, and is present in slumps and other deformed structures. Trials to date fragments of the impact breccia radiometrically using the K/Ar wholerock method (Lehtovaara, 1985) only yielded dates of around 1200-1300 Ma, indicating that, although these dates are younger than the conventional Svecokarehan U/Pb age span of 1800-1900 Ma, this can only be attributed to a small extent to the meteorite impact. In comparison with the better investigated Lappajarvi impact site (Lehtinen, 1976; 1990; Pipping and Lehtinen, this issue), 100 km to the east (Fig. 11, which, although relatively young at 77 Ma (Jessberger and Reimold, 1980) has a rather down worn rim, the Soderfjarden crater, which is considerably older, is relatively well preserved as there has been less defo~ation and it was instantly infilled with sediment. With a diameter of 5 km, Soderfjarden is much smaller than the Lappajsirvi crater, which has a diameter of 17 km. However, due to its central uplift, Soderfjlrden is classed as a complex crater. The S~der~~rden event occurred much closer in time to that of the third Finnish impact site, Lake Stiaksjarvi (Papunen, 1969; Elo et al., this issue), which is 200 km to the south (Fig. I> and dated by 40Ar/ “9Ar to 514 Ma (Grieve and Robertson, 1987).
Ouaternary Cambnan
sedtmenls
Precambrian
bedrock/impact
Precambrian
bedrock.
breccla
waNered
e = 2 160
2290
kQm’3
Q- 2410
2500
kQ”,.3
p= 2610
2670kQm.’
Fig. 4. Generalized east-west cross-section (exact location shown as profile A-B in Fig. 2) of the S%erGiirden crater based mainly on drilling data (hole numbers i-4) and geophysics, modified from Lehtovaara (1985). The rock types are shown schematically.
I want to thank Seppo Elo of the Geological Survey of Finland warmly for the processing and print of Figure 2. References Grieve, R.A.F. and Robertson, P.B., 1987. Terrestrial impact structures. Geol. Surv. Can. Map 1658A, scale 1:63 000 000. Episodes, 10(3) (supplement).
SSDERFJ.&RDEN:
A CAMBRIAN
IMPACT
CRATER
Jessberger, E.K. and Reimold, W.U., 1980. A Late Cretaceous 4”Ar-“yAr age for the Lappajirvi impact crater, Finland. J. Geophys., 48: 57-59. Lauren, L., Lehtovaara, J. and Bostrom, R., 1978. On the geology of the circular depression at SoderfjLrden, western Finland. Geol. Sutv. Finl. Bull., 297: 5-38. Lehtinen, M., 1976. Lake Lappajarvi, a meteorite impact site ion western Finland. Geol. Surv. Finl. Bull., 282: l-92. Lehtinen, M., 1990. Petrology and mineralogy of the impact melt and breccias in the deep drill core, Lake Lappajirvi, western Finland. In: L.J. Pesonen and H. Niemisara (Editors), Symp. Fennoscandian Impact Structures (Espoo and Lappajarvi, Finland, May 29-31, 19901, Prog, and Abstr., Geol. Surv. Finl., p. 19. Lehtovaara, J.J.. 1982. Stratigraphi~al section through Lower Cambrian at Soderfjarden, Vaasa, western Finland. Bull. Geol. Sot. Finl., 54: 36-43.
IhI
Lehtovaara, J.J., 1984. Vaasan Soderfjarden-sedimenttiaineksen suojaama kambrikaudella muodostunut kraatteri. Terra, 96: 23-33 (with English abstract). Lehtovaara, J.J., 1985. 4”K-40Ar dating of the S~derfj~rden crater, Vaasa, western Finland. Geol. For-en. Stockholm Wrh. 107: 1-6. Papunen, H., 1969. Possible impact metamorphic textures in the erratics of the Lake SIiiksjarvi area in southwestern Finland. Geol. Sot. Fin]. Bull., 41: 151-155. Talvitie, J., Pernu, T. and Raitala, J., 1975. The circular Vaasa structure in the Baltic Shield, western Finland. Dep. Geophys., Univ. Oulu Contrib., 59: l-15. Tynni, R., 1978. Lower Cambrian fossils and acritarchs in the sedimentary rocks of Siiderfjarden. western Finland. Bull. Geol. Surv. Fin).. 297: 39-81.