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Geology, stratigraphy and structure of the Lappajärvi meteorite crater, western Finland: preliminary results of deep drilling
91
Tectonophysics, 216 (1992) 91-97
Elsevier Science Publishers B.V.. Amsterdam
Extended Abstract Geology, stratigraphy and structure of the Lappajgrvi meteorite crater, western Finland: preliminary results of deep drilling Fredrik Pipping a and Martti Lehtinen b a Geological Suruey of Finland, SF-02150 Espoo, Finland h Geological Museum, University of Helsinki, PO Box 115, SF-001 71 Helsinki, Finland
(Received November 30, 1990; revised version accepted December 18, 1991)
In the 1920s Lake Lappajlrvi in western Finland was interpreted as a volcanic neck (e.g., Eskola, 1921). Later, however, it was recognized as a meteorite impact crater (Svensson, 1968; Lehtinen, 1970; 1976). To date, a geological map (Pipping, 19791, geophysical interpretations of the area (Elo, 19761, a geochemical study (Reimold, 1982), isotope dates (Jessberger and Reimold, 1980) and palaeomagnetic studies (Pesonen et al., 1984; Pesonen et al., this issue) on the impactites cropping out within the crater, have been published. The crater is about 23 km in diameter and the maximum difference in elevation between the topographic rim and the deepest troughs of the lake is 140 m. Plate I gives an oblique view of Lake Lappajarvi from the west. The vertical scale is exaggerated about 20 times compared with the horizontal scale. The gravimetric expression of the crater-a negative Bouguer anomaly-has a diameter of about 17 km (Elo, 1976). A generalized geological map of the crater and the surrounding bedrock is given in Figure 1. The bedrock, that is, the target rocks of the impact, consists of Palaeoproterozoic mica schist and pegmatitic granite about 1900-1950 Ma in age (Fig. 1). Peneplanation and deep weathering
Correspondence
to: F. Pipping, Geological Survey of Finland, SF-02150 Espoo, Finland.
0040-1951/92/$05.00
was followed by deposition of thin layers of Mesoproterozoic siltstones and sandstones (Pipping, 1991). There are indications that muddy to silty sediments and limestones of Cambrian age were later deposited on top of these layers. The meteorite impact has been dated by 40Ar-3’Ar measurements to 77.3 Ma (i.e., to the Late Cretaceous) (Jessberger and Reimold, 1980). The geochemistry of the Lake Lappajarvi impact rocks, from outcrops in and around Lake Lappajarvi, has been thoroughly studied by Reimold (1982), who paid particular attention to the possible target rocks, comparing their composition with that of the melt breccias and suevitic breccias. Giibel et al. (1980) have studied the siderophile elements in the impact melt of Lake Lappajarvi, concluding that the projectile was most probably of chondritic composition. Core drilling has been carried out within the crater: three drill holes have been completed and a fourth is planned. The first drill hole (DH 301, Fig. 11, was started on an outcrop of karnaite (impact melt breccia), about 3.6 km northwest of the assumed centre of the crater. The second hole (DH 302, Fig. 1) was placed 7.6 km southeast of the centre, the aim being to penetrate the crater fill close to the rim. The third hole (DH 303, Fig. 1) was drilled only 1.2 km away from the second one to check a local anomaly in the seismic, gravimetric and wideband electromagnetic sounding (SAMPO: see also Elo et al., this volume) profiles.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
Lake Lappajarvi seen at an oblique angle from the west (north is thus at the bottom of the page). A combination of a Landsat image and digital topographic height data. The Picture covers an area of 50X50 km. The vertical scale is exaggerated about 20 times compared with the horizontal scale. North is to the left in the picture, geographical coordinates as in Fig. 1. Landsat TM image, Finnish National Board of Survey, Remote Sensing unit.
PLATE I 7 3 :: 2 0 2
GEOLOGY.
STRATIGRAPHY
AND STRUCTURE
OF LAPPAJARVI
METEORITE
CRATER
___-=?_ -@ + -----= --++ =---_=_ -------_-I +fci?l z--z ---_------------------
--__-: - ------_-__ ------------zzzz--___ : + ----__-_ --_ - - -----_ _ _ __
z_
=--z-lrz------
--------
-_-_
--_--_ -------_--_
-
-
__
=:_-
0
0
-=x.zz-~-~-
0
0
0
0
,J
0
----------
0
---_---_--
---
-
----
-
------_ ----
-------
---------_.
0
0
0
oooo_
--------
=-z_-_ ------_--
----0000
0
0
0
o
-
-- -
000
--_-
--
PALAEOPROTEROZOIC
MESOPROTEROZOIC
Granodiorite
Siltstone
Mica schist
UPPER CRETACEOUS
Pegmatite
granite
Suevitic
Basic volcanics
L
and sandstone
and fragmental
Melt breccia
’
1
breccias
(= ktirnlite)
Drill hole
5 km
Fig. 1. Simplified
geological
map of the LappajGvi autochthonous
region,
bedrock
after
Pipping
(1979) and Vaarma
with weak shock-metamorphic
(1991). The outermost
features.
ring denotes
F. PIPPING
Ni ppm 0
40
60
Pt METALS
100
740
180
0
2
4
6
8
AND
M. LEHTINEN
ppb 10
12
14
60 100 140 180 220
220
SiO2 %
LITHOLOGY 60
Fig. 2. Profile
of drill
hole 301, with
64
iithology
68
+Ru
+Pd
Ok
-Pt
K20% 72
in the middle, percentages
A fourth hole will be drilled within the central part of the crater, about 2.8 km southwest of the centre. The intention is to penetrate the sequence of rocks generated by the impact outside the known extension of karnsiite. Short accounts of the early results of deep drilling, begun in 1988, have already been published (Pipping, 1989; Lehtinen, 1990; Pipping and Vaarma, 1990; Elo et al., this issue; Kukkonen et al., this issue; Pesonen and Marcos, this issue). Drill hole 301 was drilled with normal diamond core drilling equipment (42 mm core diameter). From a depth of about 170 m, in the fragmental breccia, core loss was heavy. Drilling had to be discontinued at 218 m due to mechanicaf problems. A profile of the drill hole is given in Figure 2. The upper 145 m consist of karnaite (fragment-laden impact melt rock), divided into three layers (klrnaite 1, 2 and 3, Fig. 2) each one slightly different from the other in texture and colour. These are followed by a vesiculated melt breccia only about 0.3 m thick. Beneath it is a 5 m thick layer of suevite, followed by a fragmental
74
Ni and Pt metals content
on the left,
SO,
and K,O
weight
on the right.
breccia laden with shock-metamorphic rock, mineral fragments and glass shards. Down to about 170 m, planar elements in quartz are common, indicating shock pressures of up to 30 GPa (300 kbar), see, for example, Grieve and Head (1983) and Grieve (1990). Forty XRF analyses on the major elements were made from selected samples of the drill core (Fig. 2) The a na 1yses show that the three karnlite layers are very similar and chemically homogeneous (Lehtinen, 19901. The small variations noted in composition seem to correlate with texture and/or porosity. Thus, for instance, the middle klrnaite layer is characterized by fine perlitic fissuring and the total H,O and Cl contents are twice what they are in the overlying and underlying layers of karnaite. Only nine analyses are available (NiS fire assay, ICP/MS) of the Pt group metals Ru, Pd, Ir and Pt. Note the 2.5 to lo-fold enrichment in the karnaite compared with the suevite and the underlying fragmental breccia (Fig. 2). A similar enrichment of Ni, as seen in Figure 2, was also noted by Lehtinen (1976) and by Fregerslev and
GEOLOGY,
STRATIGRAPHY
AND
STRUCTURE
CiF LAPPAJARVI
METEORITE
Carstens (1976), and of Ni and Ir by Reimold (1982) when they compared outcropping klrnaite with the inferred target rocks. The present results support the interpretation that these elements in the impact melt breccia have a meteoritic source. Microscopic investigations confirm the very homogeneous nature of the middle layer of karnaite (karnaite 2, Fig. 2). Microlites of pyroxene, plagioclase and twinned cordierite are common, and the perlitic glass is fresh and clear (“Type I karnaite”; Lehtinen (1976)). In the overlying and underlying layers of karnaite the glass has recrystallized to feldspar and quartz, pyroxene microlites are mostly altered into brownish biotite and common pseudomorphs after cordierite consist of a mixture of chlorite and biotite (“Type II karnaite”; Lehtinen (1976)). The second hole (302) was drilled near the crater rim in order to penetrate an annular shallow graben revealed by the Bouguer anomaly pattern (Elo, this issue). Wireline drilling was used (core diameter 56 mm) to ensure good core recovery. A profile of the drill hole is given in Figure 3. Beneath 74.3 m of Quaternary sediments (mainly layers of till) there was an 18.2 m thick layer of silty-sandy sediments, characterized by disturbed primary sedimentary structures; for example, highly inclined bedding (60” dip) and small-scale step faulting in sub-horizontal finegrained and well graded beds. The sediments are not fully lithified. The sediments are underlain by a saprolite of mica schist down to about 130 m and beneath that by fresh mica schist down to 165.7 m, where drilling was discontinued. It is believed that the sediments rest unconformably on the mica schist saprolite, emplaced by sliding along listric, lowangle faults during the final stages of structural readjustment in the crater. Examination of the microfossils in the sedimentary layer revealed acritarchs, for example, Leiosphaeridia and Synsphaeridia. This dates the sediments to the Mesoproterozoic (around 1200 Ma) (Uutela, 1990). Similar acritarchs, albeit thermally altered and deformed, were found in the uppermost parts of the saprolite. The changes were probably caused by friction and heat as the
95
CRATER
Depth
Mesoproterozoic
Palaeoproterozoi Mica schist saprolite
Fresh mica schist
166.75 Fig. 3. Profile of drill hole 302, lithology. The upper contact of the Mesoproterozoic sediments is erosional, the lower one is probably tectonic in origin.
sediments slid into their present allochthonous position. The saprolite of mica schist is almost 40 m thick and has an average density of 2340 kg/m3. In contrast, the fresh mica schist underlying the saprolite has a density of 2695 kg/m3. This difference is due to the replacement of plagioclase by kaolinite in the saprolite rock. The saprolite predates the allochthonous sediments and should thus be of early Mesoproterozoic age. Drill hole 303 was drilled to check geophysical data that showed a thin overburden underlain by rather fresh bedrock. The bedrock intersected is a pegmatitic granite with signs of weak shock metamorphism. The hole was only drilled to a depth of 35 m. The interpretation of geophysical profiles suggests that the pegmatitic granite represents a threshold within the system of annular faulting in the crater.
96
To summarize, the three holes drilled within the Lappajarvi impact crater have provided the following new information on the geology and structure of the Lake Lappajtirvi crater: (1) The impact breccia fill near the centre of the crater is more than 200 m thick. There is a sequence of impact breccia layers (2) in which the karnaite melt breccia is followed by a thin layer of suevite and allochthonous fragmental breccias. (3) The k&n%te is also petrologically and chemically homogeneous in a vertical section, corroborating the observations of Reimold (1982) on a horizontal profile. (4) The existence of an annular graben structure with pre-impact sedimentary rocks, as indicated by gravity profiles, is confirmed by the Mesoproterozoic sediments intersected. (5) An almost 40 m thick layer of Proterozoic saprolite is preserved in an autochthonous position inside the crater. The drilling data make it possible to compare the structure of the Lappajarvi crater to that of other complex, high velocity, impact formations, such as Siljan (Rondot, 1975; 19901, Manicouagan (Grieve and Head, 19831, Charlevoix and Clearwater (Rondot, 19701, especially as to their annular graben structure. The preserved Mesoproterozoic saprolite is an unusual feature on the glaciated Fennoscandian shield.
The authors wish to thank Lit. Phil. M. Vaarma for technical assistance, for many fruitful discussions and for placing his geological map of the northern parts of the Lappajarvi area at our disposal. Our warmest thanks are also due to Ritva Forsman and Liisa Siren who drew Figures 1 and 2. Critical comments by W.U. Reimold and an anon~ous referee are gratefully acknowledged. References Elo, S., 1976. Tiheysmallien kiiyt&ti painovoima-anomalioiden tulkinnassa. Geologi, 28: 6.5-69 (in Finnish with English abstract).
F. PIPPING
AND
M. LEHTINEN
Eskola, P., 1921. On volcanic necks in Lake Jinisjiirvi in Eastern Finland. Bull. Comm. Geol. Finl., 55: I-13. Fregerslev, S. and Carstens, H., 1976. FeNi metal in impact melt rocks of Lake Lappaj%vi, Finland. Contrib. Mineral. Petrol., 55: 255-263. Giibel, E., Reimold, U., Baddenhausen, H. and Palme, H., 1980. The projectile of the Lappajarvi Impact Crater. Z. Naturforsch., 35 (a): 197-203. Grieve, R.A.F., 1990. Impact cratering on the Earth. Sci. Am., 262 (4): 44-51. Grieve, R.A.F. and Head, J.W., III., 1983. The Manicouagan impact structure: an analysis of its original dimensions and form. In: W.V. Boynton and T. Ahrens (Editors), Proc. 13th Lunar Planet. Sci. Conf., 2. Geophys. Res. Supple., 88: A807-A818. Jessberger, E.K. and Reimold, W.U., 1980. A late Cretaceous 40Ar-“9Ar age for the Lappaj&vi impact crater, Finland. J. Geophys., 48: 57-59. Lehtinen, M., 1970. New evidence for an impact origin of Lake Lappajarvi, western Finland. Bull. Geol. Sot. Finl., 42: 89-93. Lehtinen, M., 1976. Lake Lappajarvi, a meteorite impact site in 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 Lappajiirvi, western Finland. In: L.J. Pesonen and H. Niemisara (Editors), Symp. Fennoscandian Impact Structures (Espoo and Lappajlrvi, Finland, May 29-311, Programme and Abstracts. Geol. Surv. Finl., p. 19. Pesonen, L.J., Pipping, F. and Halls, H.C., 1984. Pateomagnetic study of the Lake LappajSrvi impact crater, western Finland. Terra Cognita, 4: 371. Pipping, F., 1979. Pre-Quaternary rocks. Sheet 2313, Alajirvi. Geological map of Finland 1:loO 000. Geol. Surv. Finl. Pipping, F., 1989. Deep drilling within the Lappajlrvi impact crater. Geol. Surv. Finl. Spec. Pap., 10: 9-10. Pipping, F. 1991. Lappajiirven meteoriittikraatterin kairausten tuloksista. Geologi, 43: 14-19 (in Finnish with English abstract). Pipping, F. and Vaarma, M., 1990. LappajIrvi impact crater, geology and structure. In: L.J. Pesonen and H. Niemisara (Editors), Symp. Fennoscandian Impact Structures (Espoo and Lappajlirvi, Finland, May 29-311, Programme and Abstracts. Geol. Surv. Fin]., p. 18. Reimold, W.U., 1982. The Lappajatvi meteorite crater, Finland: petrography, Rb-Sr, major and trace element geochemistry of the impact melt and basement rocks. Geochim. Cosmochim. Acta, 46: 1203-1225. Rondot, J., 1970. La structure de Charlevoix comparee ia d’autres impacts miteoritiques. Can. J. Earth Sci., 7: 1194-1202. Rondot, J., 1975. Comparaison entre les astroblemes de Siljan, Suede, et de Charlevoix, Quebec. Bull. GeoI. Inst. Uppsala, 6: 85-92. Rondot, J., 1990. Siljan before erosion. In: L.J. Pesonen and H. Niemisara (Editors), Symp. Fennoscandian Impact
GEOLOGY,
STRATIGRAPHY
AND
STRUCTURE
OF LAPPAJiiRVl
METEORITE
Structures (Espoo and Lappajlrvi, Finland, May 29-31), Programme and Abstracts. Geol. Surv. Finl., p. 54. Svensson, N.-B., 1968. Lake Lappajarvi, Central Finland: a possible meteorite impact structure. Nature, 217 (5127): 438.
CRATER
91
Uutela, A., 1990. Proterozoic microfossils from the sedimentary rocks of the Lappajarvi impact crater. Bull. Geol. Sot. Finl., 62 (2): 115-121. Vaarma, M., 1991. Pre-Quaternary rocks. Sheet 2314, Evijirvi. Geological map of Finland 1: 100 000. Geol. Surv. Finl.
Tectonophysics, 216 (1992) 91-97
Elsevier Science Publishers B.V.. Amsterdam
Extended Abstract Geology, stratigraphy and structure of the Lappajgrvi meteorite crater, western Finland: preliminary results of deep drilling Fredrik Pipping a and Martti Lehtinen b a Geological Suruey of Finland, SF-02150 Espoo, Finland h Geological Museum, University of Helsinki, PO Box 115, SF-001 71 Helsinki, Finland
(Received November 30, 1990; revised version accepted December 18, 1991)
In the 1920s Lake Lappajlrvi in western Finland was interpreted as a volcanic neck (e.g., Eskola, 1921). Later, however, it was recognized as a meteorite impact crater (Svensson, 1968; Lehtinen, 1970; 1976). To date, a geological map (Pipping, 19791, geophysical interpretations of the area (Elo, 19761, a geochemical study (Reimold, 1982), isotope dates (Jessberger and Reimold, 1980) and palaeomagnetic studies (Pesonen et al., 1984; Pesonen et al., this issue) on the impactites cropping out within the crater, have been published. The crater is about 23 km in diameter and the maximum difference in elevation between the topographic rim and the deepest troughs of the lake is 140 m. Plate I gives an oblique view of Lake Lappajarvi from the west. The vertical scale is exaggerated about 20 times compared with the horizontal scale. The gravimetric expression of the crater-a negative Bouguer anomaly-has a diameter of about 17 km (Elo, 1976). A generalized geological map of the crater and the surrounding bedrock is given in Figure 1. The bedrock, that is, the target rocks of the impact, consists of Palaeoproterozoic mica schist and pegmatitic granite about 1900-1950 Ma in age (Fig. 1). Peneplanation and deep weathering
Correspondence
to: F. Pipping, Geological Survey of Finland, SF-02150 Espoo, Finland.
0040-1951/92/$05.00
was followed by deposition of thin layers of Mesoproterozoic siltstones and sandstones (Pipping, 1991). There are indications that muddy to silty sediments and limestones of Cambrian age were later deposited on top of these layers. The meteorite impact has been dated by 40Ar-3’Ar measurements to 77.3 Ma (i.e., to the Late Cretaceous) (Jessberger and Reimold, 1980). The geochemistry of the Lake Lappajarvi impact rocks, from outcrops in and around Lake Lappajarvi, has been thoroughly studied by Reimold (1982), who paid particular attention to the possible target rocks, comparing their composition with that of the melt breccias and suevitic breccias. Giibel et al. (1980) have studied the siderophile elements in the impact melt of Lake Lappajarvi, concluding that the projectile was most probably of chondritic composition. Core drilling has been carried out within the crater: three drill holes have been completed and a fourth is planned. The first drill hole (DH 301, Fig. 11, was started on an outcrop of karnaite (impact melt breccia), about 3.6 km northwest of the assumed centre of the crater. The second hole (DH 302, Fig. 1) was placed 7.6 km southeast of the centre, the aim being to penetrate the crater fill close to the rim. The third hole (DH 303, Fig. 1) was drilled only 1.2 km away from the second one to check a local anomaly in the seismic, gravimetric and wideband electromagnetic sounding (SAMPO: see also Elo et al., this volume) profiles.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
Lake Lappajarvi seen at an oblique angle from the west (north is thus at the bottom of the page). A combination of a Landsat image and digital topographic height data. The Picture covers an area of 50X50 km. The vertical scale is exaggerated about 20 times compared with the horizontal scale. North is to the left in the picture, geographical coordinates as in Fig. 1. Landsat TM image, Finnish National Board of Survey, Remote Sensing unit.
PLATE I 7 3 :: 2 0 2
GEOLOGY.
STRATIGRAPHY
AND STRUCTURE
OF LAPPAJARVI
METEORITE
CRATER
___-=?_ -@ + -----= --++ =---_=_ -------_-I +fci?l z--z ---_------------------
--__-: - ------_-__ ------------zzzz--___ : + ----__-_ --_ - - -----_ _ _ __
z_
=--z-lrz------
--------
-_-_
--_--_ -------_--_
-
-
__
=:_-
0
0
-=x.zz-~-~-
0
0
0
0
,J
0
----------
0
---_---_--
---
-
----
-
------_ ----
-------
---------_.
0
0
0
oooo_
--------
=-z_-_ ------_--
----0000
0
0
0
o
-
-- -
000
--_-
--
PALAEOPROTEROZOIC
MESOPROTEROZOIC
Granodiorite
Siltstone
Mica schist
UPPER CRETACEOUS
Pegmatite
granite
Suevitic
Basic volcanics
L
and sandstone
and fragmental
Melt breccia
’
1
breccias
(= ktirnlite)
Drill hole
5 km
Fig. 1. Simplified
geological
map of the LappajGvi autochthonous
region,
bedrock
after
Pipping
(1979) and Vaarma
with weak shock-metamorphic
(1991). The outermost
features.
ring denotes
F. PIPPING
Ni ppm 0
40
60
Pt METALS
100
740
180
0
2
4
6
8
AND
M. LEHTINEN
ppb 10
12
14
60 100 140 180 220
220
SiO2 %
LITHOLOGY 60
Fig. 2. Profile
of drill
hole 301, with
64
iithology
68
+Ru
+Pd
Ok
-Pt
K20% 72
in the middle, percentages
A fourth hole will be drilled within the central part of the crater, about 2.8 km southwest of the centre. The intention is to penetrate the sequence of rocks generated by the impact outside the known extension of karnsiite. Short accounts of the early results of deep drilling, begun in 1988, have already been published (Pipping, 1989; Lehtinen, 1990; Pipping and Vaarma, 1990; Elo et al., this issue; Kukkonen et al., this issue; Pesonen and Marcos, this issue). Drill hole 301 was drilled with normal diamond core drilling equipment (42 mm core diameter). From a depth of about 170 m, in the fragmental breccia, core loss was heavy. Drilling had to be discontinued at 218 m due to mechanicaf problems. A profile of the drill hole is given in Figure 2. The upper 145 m consist of karnaite (fragment-laden impact melt rock), divided into three layers (klrnaite 1, 2 and 3, Fig. 2) each one slightly different from the other in texture and colour. These are followed by a vesiculated melt breccia only about 0.3 m thick. Beneath it is a 5 m thick layer of suevite, followed by a fragmental
74
Ni and Pt metals content
on the left,
SO,
and K,O
weight
on the right.
breccia laden with shock-metamorphic rock, mineral fragments and glass shards. Down to about 170 m, planar elements in quartz are common, indicating shock pressures of up to 30 GPa (300 kbar), see, for example, Grieve and Head (1983) and Grieve (1990). Forty XRF analyses on the major elements were made from selected samples of the drill core (Fig. 2) The a na 1yses show that the three karnlite layers are very similar and chemically homogeneous (Lehtinen, 19901. The small variations noted in composition seem to correlate with texture and/or porosity. Thus, for instance, the middle klrnaite layer is characterized by fine perlitic fissuring and the total H,O and Cl contents are twice what they are in the overlying and underlying layers of karnaite. Only nine analyses are available (NiS fire assay, ICP/MS) of the Pt group metals Ru, Pd, Ir and Pt. Note the 2.5 to lo-fold enrichment in the karnaite compared with the suevite and the underlying fragmental breccia (Fig. 2). A similar enrichment of Ni, as seen in Figure 2, was also noted by Lehtinen (1976) and by Fregerslev and
GEOLOGY,
STRATIGRAPHY
AND
STRUCTURE
CiF LAPPAJARVI
METEORITE
Carstens (1976), and of Ni and Ir by Reimold (1982) when they compared outcropping klrnaite with the inferred target rocks. The present results support the interpretation that these elements in the impact melt breccia have a meteoritic source. Microscopic investigations confirm the very homogeneous nature of the middle layer of karnaite (karnaite 2, Fig. 2). Microlites of pyroxene, plagioclase and twinned cordierite are common, and the perlitic glass is fresh and clear (“Type I karnaite”; Lehtinen (1976)). In the overlying and underlying layers of karnaite the glass has recrystallized to feldspar and quartz, pyroxene microlites are mostly altered into brownish biotite and common pseudomorphs after cordierite consist of a mixture of chlorite and biotite (“Type II karnaite”; Lehtinen (1976)). The second hole (302) was drilled near the crater rim in order to penetrate an annular shallow graben revealed by the Bouguer anomaly pattern (Elo, this issue). Wireline drilling was used (core diameter 56 mm) to ensure good core recovery. A profile of the drill hole is given in Figure 3. Beneath 74.3 m of Quaternary sediments (mainly layers of till) there was an 18.2 m thick layer of silty-sandy sediments, characterized by disturbed primary sedimentary structures; for example, highly inclined bedding (60” dip) and small-scale step faulting in sub-horizontal finegrained and well graded beds. The sediments are not fully lithified. The sediments are underlain by a saprolite of mica schist down to about 130 m and beneath that by fresh mica schist down to 165.7 m, where drilling was discontinued. It is believed that the sediments rest unconformably on the mica schist saprolite, emplaced by sliding along listric, lowangle faults during the final stages of structural readjustment in the crater. Examination of the microfossils in the sedimentary layer revealed acritarchs, for example, Leiosphaeridia and Synsphaeridia. This dates the sediments to the Mesoproterozoic (around 1200 Ma) (Uutela, 1990). Similar acritarchs, albeit thermally altered and deformed, were found in the uppermost parts of the saprolite. The changes were probably caused by friction and heat as the
95
CRATER
Depth
Mesoproterozoic
Palaeoproterozoi Mica schist saprolite
Fresh mica schist
166.75 Fig. 3. Profile of drill hole 302, lithology. The upper contact of the Mesoproterozoic sediments is erosional, the lower one is probably tectonic in origin.
sediments slid into their present allochthonous position. The saprolite of mica schist is almost 40 m thick and has an average density of 2340 kg/m3. In contrast, the fresh mica schist underlying the saprolite has a density of 2695 kg/m3. This difference is due to the replacement of plagioclase by kaolinite in the saprolite rock. The saprolite predates the allochthonous sediments and should thus be of early Mesoproterozoic age. Drill hole 303 was drilled to check geophysical data that showed a thin overburden underlain by rather fresh bedrock. The bedrock intersected is a pegmatitic granite with signs of weak shock metamorphism. The hole was only drilled to a depth of 35 m. The interpretation of geophysical profiles suggests that the pegmatitic granite represents a threshold within the system of annular faulting in the crater.
96
To summarize, the three holes drilled within the Lappajarvi impact crater have provided the following new information on the geology and structure of the Lake Lappajtirvi crater: (1) The impact breccia fill near the centre of the crater is more than 200 m thick. There is a sequence of impact breccia layers (2) in which the karnaite melt breccia is followed by a thin layer of suevite and allochthonous fragmental breccias. (3) The k&n%te is also petrologically and chemically homogeneous in a vertical section, corroborating the observations of Reimold (1982) on a horizontal profile. (4) The existence of an annular graben structure with pre-impact sedimentary rocks, as indicated by gravity profiles, is confirmed by the Mesoproterozoic sediments intersected. (5) An almost 40 m thick layer of Proterozoic saprolite is preserved in an autochthonous position inside the crater. The drilling data make it possible to compare the structure of the Lappajarvi crater to that of other complex, high velocity, impact formations, such as Siljan (Rondot, 1975; 19901, Manicouagan (Grieve and Head, 19831, Charlevoix and Clearwater (Rondot, 19701, especially as to their annular graben structure. The preserved Mesoproterozoic saprolite is an unusual feature on the glaciated Fennoscandian shield.
The authors wish to thank Lit. Phil. M. Vaarma for technical assistance, for many fruitful discussions and for placing his geological map of the northern parts of the Lappajarvi area at our disposal. Our warmest thanks are also due to Ritva Forsman and Liisa Siren who drew Figures 1 and 2. Critical comments by W.U. Reimold and an anon~ous referee are gratefully acknowledged. References Elo, S., 1976. Tiheysmallien kiiyt&ti painovoima-anomalioiden tulkinnassa. Geologi, 28: 6.5-69 (in Finnish with English abstract).
F. PIPPING
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