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Geology of the Early Palaeozoic Lockne impact structure, Central Sweden
169
Tectonophysics, 216 (1992) 169-185 Elsevier
Science
Publishers
B.V., Amsterdam
Geology of the Early Palaeozoic Lockne impact structure, Central Sweden Maurits Lindstriim and Erik F.F. Sturkell Department of Geology and Geochemistv, (Received
December
Stockholm Unil,ersity, S-106 91 Stockholm, Sweden
3, 1990; revised version
accepted
January
12, 1992)
ABSTRACT Lindstriim, M. and Sturkell, E.F.F., 1992. Geology of the Early Palaeozoic Pesonen and H. Henkel (Editors), Terrestrial impact Craters and Fennoscandia. Tectonophysics, 216: 169-185.
Lockne impact structure, Central Sweden. In: L.J. Craterform Structures with a Special Focus on
The diameter of the crater at Lockne as seen on topographic maps is 7-8 km. The Tandsbyn Breccia. the principal evidence of impact, consists of strongly crushed local basement. Its distribution follows the crater margin. On the north margin, basement granite rests on an inverted sub-Cambrian erosion surface on the lowermost Middle Cambrian. Tangential faults occur at the periphery of the structure. After the impact occurred under the sea during the middle Ordovician, there was a resurgence of ejecta-loaded sea water which deposited a chaotic breccia (Lockne Breccia), which has the appearance of a debris flow. The Lockne Breccia, together with a less coarsely grained turbidite (“Loftarsten”), which lies immediately above it, predominantly consist of clasts of lower to middle Ordovician limestone and, mainly subordinate, inclusions of Tandsbyn Breccia and isolated clasts of crystalline basement. Fragments of impact melt are important components of the turbidite. A protective cover of sediments formed after the impact, is, in turn, overlain by an outlier of a Caledonian overthrust nappe, which occupies the central and topographically deepest part of the crater. Shocked quartz has been identified in the Loftarsten.
impacts, it is also well preserved spects.
Introduction Only about 10% of the more than 120 known meteoritic impact structures on the earth are Ordovician or older (Grieve, 1987). The great majority of the known impacts took place on land: marine impacts are scarce. Whereas small terrestrial impact craters (diameters less than 2 km in sedimentary bedrock and less than 4 km in crystalline basement) are fairly well understood, larger structures require a great deal more study. Here we describe a recently identified impact structure that is unusual because it evidently formed in an Ordovician environment, and has a diameter of at least 7 km. Contrary to many old
Correspondence
to: M. LindstrGm,
and Geochemistry, holm, Sweden.
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Stockholm
Department
University,
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The Lockne area: research history
The Lockne area (Fig. 1) is situated 20 km south of 6stersund at the boundary between Caledonian overthrust rocks to the west and Proterozoic to Lower Palaeozoic rocks (with less evidence of Caledonian deformation) to the east. The Proterozoic forms the crystalline basement to the Palaeozoic, which consists of marine sedimentary deposits of standard Baltoscandian platform type. The Lockne area has been known to be interesting but difficult to interpret, for more than a century. Wiman (1900) reported the occurrence of veIy coarse elastic deposits of Ordovician age, and Thorslund (1940) elaborated the stratigraphy in a major, now classic, monograph. The au-
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M. LINDSTRhM
170
270 L
285
300 I
315
325
310
355
I
370
385
100
I
I
115
L30
AND E.F.F. STURKELL
LL5
I 5km
Plate I Topography
of the area around Lake Locknesjiin.
The surface covered by water is coloured contours approximately
5 m apart.
either blue or black. Altitude
in metres,
GEOLOGY
Fig.
OF THE
I. General
mentioned.
EARLY
reference
PALEOZOIC
LOCKNE
1MPACT
map with several of the localities
inset map shows outline of Sweden with location
of the btersund
area. National
coordinate
171
STRUCTlJRE
system is used.
tochthonous succession contains two extensively exposed breccias. The older breccia was regarded as arkose-like by Thorsiund (1940). The name “arkose-breccia” has stuck, although it is inadequate. Here we introduce the new name Tundsbyn Breccia for it and this name is used throughout this paper. The younger breccia has been named the Lockne Breccia (Jaanusson et al., 1982; LindstrGm et al., 1983). Its clasts, of various sizes, consist mainly of Ordovician Orthoceratite limestone but fragments of Tandsbyn Breccia may also be abundant. Thorslund (1940) found that major outcrops of the Tandsbyn Breccia were surrounded by younger sedimentary rocks, including the Lockne
Breccia, and interpreted this relationship as evidence that the Tandsbyn Breccia had formed islets around which the other rocks had been deposited. LindstrGm et al. (1983) and Simon (1987a) demonstrated that the Lockne Breccia has the structure of debris flows, and that the “islets” of Tandsbyn Breccia are, in fact, large inclusions in the Lockne Breccia. Simon (1987a) found that the Lockne Breccia and an immediately superjacent, greywacke-like unit, called Loftarsten, contained sand-sized clasts of melted rock that are too large to have been transported as atmospheric suspension. Since no Ordovician volcanism is known in the vicinity, he was unable to explain its occurrence. However, Wickman (1988) suggested that these clasts are impact melt and reinterpreted the Lo&e Breccia as an impact allogenic breccia. Since the Lockne Breccia is known to be of early Caradoc (middle Ordovician) age, Wickman’s intkrpretation even provided the impact with a precise date. Wickman sought the centre of the impact well to the northwest of the Lockne area, which he regarded as a marginal part of a fairly large structure. Lindstr~m et al. (1991) defined an impact crater with a diameter of 7-8 km based on the occurrence of the Tandsbyn Breccia. They dated it to the Middle Cambrian, partly because crushed basement rests, in inverted position, on Middle Cambrian sediments at the northern margin of the structure. Geology and geomorphology
of the impact area
The area is cut by a regional Iineament that continues northwestwards into the Caledonian structure (see Striimberg, 1976). Lake L.ocknesjGn is part of this lineament. We have identified the Lockne crater with a topographic depression (Plate I) to the west of the northern part of Locknesjiin. This depression has a north-south diameter of about 8 km west of the lake and continues eastwards under the lake. Its northern, western and southern margins consist of hills that rise as much as 140 m above the lake. The eastern margin of the depression is a till-covered plain on the eastern shore of the lake; scanty
M. LINDSTRZ)M
172
r I
AND E.F.F. STURKELL
Geological map of the Lockne area ,
lkm
I
Overthrust fault Sulphide &a
deposii
Shale
Dalby Dalby
limestone/ mudmound
Loftarsten Lockne
Breccia
Orthocerntite limestone TGyen Shale alum
shale
Tandsbyn Asby
Breccic
Dolerite
granite voicanite
cl
Fig.4
6985
-I-+++++
Fig. 2. Geological map of Lockne area.
CXOLOGY
OI; THE
EARLY
PALEOZOlC
LOCKNL:
IMPACT
173
STRUC-IXJRE
exposures indicate that the plain is underlain by Palaeozoic sedimentary rocks, whereas its eastern margin consists of raised Proterozoic basement. The structurally uppermost rocks of the area are overthrust lower Ordovician Tijyen Shale and lower to middle Ordovician Orthoceratite limestone which occupy the centre of the topographic depression. They rest on autochthonous Dalby Limestone of Caradoc age; that is, a distinctly younger sediment (Figs. 2, 3). The base of the overthrust nappe is exposed along the road to Lake Stor-Handsjon (6987.1f 1446.9; these and other coordinate figures given in this paper are to the nearest 100 m and refer to the Swedish National Grid, indicated in the maps), where well preserved Dalby Limestone dips northeastward under strongly sheared Tiiyen Shale. The same structura1 relations occur to the west of Grlstjarnen (6986.5/1448.2), although without exposure of the thrust contact. The Orthoceratite limestone, overlying the Tiiyen Shale is strongly folded and sheared. It is exposed, for instance, at the road east of Tandsbyn (69~8.~/144~.2) and on both sides of Musviken (6989.6/1449.0 and 6989.9/1449.2X Exposures have existed at various localities in the vicinity of Bleka (6987.5/1~8.2). An autochthonous Lower Pataeozoic succession of standard Baltoscandian type occurs along the Caledonian margin, to the south and east of Gstersund (Fig. 3). It begins with gravelly sandstone with Lower Cambrian fossils, a couple of metres thick at the most. The Middle and Upper Cambrian consists of a more than 20 m thick, black, bituminous shale (“alum shale”). Close to the base of the Ordovician succession there is a grey, graptolitic shale (Toyen Shale; Lower Ordovician) that can be about 5 m thick. This is overlain by a few tens of metres of bedded Orthoceratite limestone of Arenig to Llandeilo age. In the lowermost Caradoc we find the Lockne Breccia, which is overlain by a turbiditic, greywackelike sandstone (~ftarsten) with a ciast composition which resembles that of the underlying breccia. The widely distributed Dalby Limestone is mainly developed as a light grey catcareous shale with nodules which range from infrequent to abundant. of fossiliferous limestone; however, lo-
Normal succession
Crater rim
Ashgill
Carada
hi&lo
Llanvin
Arenig
Tremadot upper
Middle l
**t*****
Lower
ora Shale lzizzzl R!zzl
-:-I-I-T Toyen Shale lzz
Dalby Limestone
alum shale l
b++e Tandsbyn Breccia
@
Lockne Breccia
I
Kzl
Orthoceratite ~imcstone
**I*** .. .., . sandstone D
Fig. 3. Lower Paleozoic outline stratigraphy of Lockne area. The relative thicknesses of the units are not represented.
tally it forms reef-like mud mounds. The Caradoc to Ashgill succession above it only occurs sporadicaIly and will not concern us here. The lower part of the standard succession is not represented in the impact area. The Dalby Limestone and underlying Loftarsten and Lockne Breccia occupy the flanks of the hills that surround the lower ground. They dip towards the
M. LINDSTRGM
174
centre of the topographic depression. In the upper slopes the Tandsbyn Breccia is exposed. The occurrence of the geological units thus contributes to the saucer-like shape of the structure. If the stratigraphic units do not vary excessively in thickness, the Tandsbyn Breccia should occur at a depth of more than 70 m below the level of the lake in the central parts of the structure, whereas its outcrop reaches up to 140 m above the lake in surrounding hills. The eastern segment of the structure is either covered by the lake (as well as by the thick till on the eastern shore), or has been removed by faulting. The second alternative is plausible in view of geomorphological, geological and geophysical evidence for a major fault along the length of the lake, which trends NNW-SSE.
+
l
+
+
+
+*+
Fig. 4. Geological
map of Nordanbergsberget.
E.F.F. STURKELL
The chain of hills surrounding the topographic depression is interrupted by cross-cutting valleys at Tandsbyn and in the area to the east of Hlllniiset (Fig. 2). The Tandsbyn gap must have existed in the middle Ordovician because its south flank was the site of reef-like build-ups during the time that the Dalby Limestone was being deposited (Thorslund, 1940). The build-ups contain numerous geopetal fabrics which indicate that they are of Ordovician age and that the present topography then existed (our observation). The gap at Hallnaset evidently existed in the middle Ordovician, because both the Loftarsten and the Lockne Breccia thicken from the upper slopes to the lower ground near the lake. The crystalline bedrock of the higher ground outside the topographic depression is cut by sub-
+++++
++++++ +++++++ ++++++++ +++++++++
AND
250 m
+++++
For location
major slab of granite
turned
of map area see upper part of Fig. 2. Thick line shows the boundary upside down just outside
the transient
crater.
of a
GEOLOGY
OF THE
EARLY
PALEOZOIC
LOCKNE
IMPACT
175
STKVCTURE
vertical faults, some of which run tangentially to the periphery of the assumed crater (Fig. 2). We have found two locahties at which tangential faults form graben-like structures in which Cambrian basal conglomerate and alum shale are offset against crystalline basement rocks (6984.5/1449.5 and 6984.5/1448.6).
Structures occurring at Nordanbergsberget (Fig. 4) to the north of Locknesjon are regarded as particularly important to the geological interpretation of the impact. Simon (1987a) was the first geologist to observe that the Proterozoic granite of the Skanska quarry on the northeast flank of the hill Nordanbergsberget rests on Cam-
Fig. 5. Skanska quarry at Nordanber~b~rget. Granite resting with inverted erosive contact on Lower and Middle Cambrian sedimentary rocks. (a) General view from southwest. Cambrian sediments are exposed at the foot of the northeast quarry wail. (b) Detail of contact. Granite, partly weathered, partly fresh, in upper part and as boulders within the Cambrian. Cambrian alum shale in lower part.
176
brian alum shale. His interpretation, that the structure represented Caledonian overthrusting (Simon, 1987b), can be challenged on three principal grounds: (1) The alum shale immediately under the granite is too well preserved compared with other alum shales that have suffered overthrusting.
M. LINDSTRBM
AND
E.F.F. STURKELL
Bedding planes show no signs of shear and contain delicate organic and early diagenetic structures that are mechanically intact. (2) The vergence, and hence direction of movement indicated by the structure is towards the northeast, that is, at right angles to the local direction of Caledonian overthrusting. Lindstrijm
Fig. 6. Tandsbyn Breccia. (a) Specimen with granite fragments in partly weathered carbonate-rich matrix from 6989.3/1443.3. Scale line = 5 cm. (b) Sawed specimen from 600 m northwest of Groffelglrde (6989.7/1446.6). Scale line = 2 cm.
GEOLOGY
OF THE
tARLY
PALEOZOIC
LOCKNE
IMPACT
STRUCI’URE
et al. (1991) argued on the basis of these two observations that the granite must have collapsed on top of the alum shale after having been uprooted by an impact to the south of the hill. With this origin for the granite cover, the alum shale would at least have escaped much of the horizontal shear caused by overthrusting of a thick rock pile. (3) Subsequent quarrying operations, have brought to light the third principal evidence against the overthrust hypothesis. The contact between the granite and the underlying Cambrian is a weathered transgression surface with major, well rounded clasts of granite turned upside down along its entire length (Fig. 5). This inversion could not have originated from overthrusting but is a characteristic feature of the margins of impact craters (see, for instance, Melosh (19891, see also discussion below). The upper part of the hill has outcrops of undisturbed layers of varying thickness, all less than 1 m, of basal Cambrian gravelly sandstone, resting on weathered Proterozoic granite and dolerite, and overlain by alum shale with the same features as observed in the quarry. The weathering of the dolerite left rounded cores of less altered rock between which sediment has penetrated downwards. The alum shale is, in turn, overlain by the same granite that rests on alum shale in the quarry (Fig. 4). We can see no viable alternative to the explanation that the flap of granite of Nordanbergsberget was overturned by impact. Tandsbyn Breccia The Tandsbyn Breccia, consists of intensely fractured but otherwise little altered fragments of local Proterozoic rocks (Fig. 6). The thin Holocene weathering crust can be light grey to whitish in colour, especially in the area of the protolith, but in fresh outcrop it can be almost black. Where the protoiith is dolerite or metavolcanics the weathered colour can be ochreous and the colour of fresh fractures dark greyish green to almost black. Thin sections show the typical Breccia to consist of angular fragments, mostly of quartz and
177
feldspar in a fine-gra;ned and slightly recrystallized, micaceous matrix. The great force that went into crushing it is most evident from plagioclase crystals in which the twin lamellae are bent, offset, and broken with great frequency and irregular orientation. A good description of the rock was given by Simon (1987a). The type locality of the Tandsbyn Breccia is the crest of the slope to the south of the railway southwest of the village of Tandsbyn and its former railway station (6987.4/1445.7). At this locality, the breccia consists of light grey, plagioclaserich granite. A large spectacular boulder of identical breccia occurs at Grubban a couple of hundred metres southwest of the type locality. The outcrop area of Tandsbyn Breccia forms a belt along Highway 81 from northeast of the church of Lockne to the side road to Tandsbyn ~6990.1/1445.7) which continues through a wide area to the north and northeast of Tandsbyn. At Tandsbyn the continuous presence of the breccia is interrupted by the sedimentary rocks of the Tandsbyn gap, to the south of which the higher ground consists of Tandsbyn Breccia from the type locality to west of HZlln%set. Occurrences even farther south are scattered and form tectonically delimited enclaves within areas of less disturbed Proterozoic rocks. Good outcrops of the breccia occur, for example, at Highway 81 to the north of the church of Lockne (6992.4/1449.6) and 100 m east of Bgcksved (6991.3/1447.2), to the west of the raiiway, 600 m northwest of Groffelg~rde (6989.7,’ 1446.6) and along the private road towards Lake Stor-HandsjGn, 1.5-1.7 km south of the gate (6986.5/1447.0 to 6986.3/1447.1), as well as west of H%In%et (6986.7/1450.4 to 6984.6/ 1450.4). The two last-mentioned occurrences present greater lithologic variation than the others, which are of predominantly granite composition. The outcrops along the Stor-Handsjbn road consist, to a great extent, of granitic breccia, but a variety also occurs which has a very dark grey, dense matrix and abundant subordinate clasts of a lighter hue. In thin section the matrix is seen to be composed of mainly chlorite, muscovite, quartz and plagioclase. Sutured grain contacts without preferred orientation identify it as a devitrified
M. LINDSI‘K~M
178
interstitial melt. Similar breccias with devitrified melt occur west of HIllnGet. The rock fragments consist of dolerite and intermediate metavolcanics. An instance of well preserved melt rock with a porphyritic texture and affinity to the Tandsbyn Breccia occurs 500 m south of Hlllnlset
Fig 7. Lamellar fro1m
100m
quartz in thin sections from Tandsbyn
east of Biicksved (6991.3/1447.2).
interpreted
as diaplectic
E.F.F. STURKELL
(6985.0/ 1451.5). The outcrop is contiguous with one of the more spectacular occurrences of Tandsbyn Breccia. In contrast to all Proterozoic supracrustal rocks in the area, it lacks any fabric evidence of tectonization and must, therefore, be related to the younger geological events in the area.
Breccla. The scale line in both photographs
The irregular
AND
grain in the upper right quadrant
is 100 pm. (a) Specimen
extinguishes
LO 16
in all positions and is
glass. (b) Specimen LO 1 from the railway section between LQngHnge and StensjG (6993.0/1450.25).
GEOLOGY
OF THE EARLY
PALEOZOIC
LOCKNE
IMPACT
179
STRUCTURE
Outcrops of Tandsbyn Breccia occur within the mapped area of the Lockne Breccia, where most of them are underlain by Ordovician sedimentary rocks. These outcrops are thus demonstrably inclusions in the Lockne Breccia. However, the major occurrence of Tandsbyn Breccia west of the railway, about 1 km west of Grubban (6987.4/1443.9) continues under the surrounding Lockne Breccia and possibly rests on Proterozoic basement. Most inclusions of Tandsbyn Breccia, consist of granite, but 150 m north-northwest of Klockslsen (6984.6/1444.3) there is an inclusion consisting of fragments of Proterozoic Asby Dolerite in a matrix of recrystallized melt rock. The Asby Dolerite has an ophitic texture. The devitrified interstitial melt contains abundant fragments of inclusionrich melt rock and itself consists mainly of a felt-like, and structurally isotropic, mass of micro-crystalline chlorite. A rock of this kind might have formed in connection with the intrusion of the Asby Dolerite but we connect it with the Tandsbyn Breccia because it shows evidence of only one event of penetrative brecciation. Since all other inclusions of Proterozoic derivation in the Lockne Breccia bear evidence of the cataclasis that formed the Tandsbyn Breccia, we do not believe that a major body of rock found so close to the crater could bear the imprint of brecciation in connection with Proterozoic intrusion without any evidence of the younger brecciation event. It is because we do not find the evidence of a second brecciation that we attribute the breccia and its interstitial melt rock to the event that formed the Tandsbyn Breccia. Shock metamorphism
Lindstrom et al. (1991) found shocked quartz in the Lockne Breccia and interpreted it as being reworked from impact deposits. However, shock metamorphism should also be present in the Tandsbyn Breccia. A total of 30 thin sections of the Tandsbyn Breccia were searched for features of shock metamorphism. All showed different degrees of recrystallization. This circumstance may be explained by the fact that the area was subjected to
raised temperatures after nappe emplacement during the Caledonian orogeny, as well as to hydrothermal activity immediately after the impact. Nine specimens of Tandsbyn Breccia from eight localities (Fig. 7) contained quartz with a lamellar structure reminiscent of shocked quartz (Von Engelhardt and Bertsch, 1969; Alexopoulos et al., 1988). The lamellae are straight and parallel and run from margin to margin of the grains; most frequently the same set continues in adjacent grains. The interval between lamellae is 5-20 pm. The lamellae are hardly ever parallel to the undulose extinction which is an omnipresent feature of the quartz. Although the structure described is reminiscent of shocked quartz, it is by no means as distinct as the examples given by Stoffler (1972). Samples of shocked quartz kindly provided by D. Stijffler show much stronger lamellae. R.A.F. Grieve, who kindly examined thin sections of the Loftarsten and Tandsbyn Breccia, verified the presence of shocked quartz in the Loftarsten but not in the Tandsbyn Breccia. Lockne Breccia and Loftarsten Turbidite Simon (1987a) gave a detailed description and analysis of the elastic units known as Lockne Breccia and Loftarsten. The predominant components of the Lockne Breccia are clasts, ranging in size from sand to hundreds of cubic metres, of lower to middle Ordovician limestone in a calcareous-argillaceous matrix. There are variable amounts (from none to abundant) of angular clasts derived from the local basement and inclusions of Tandsbyn Breccia, which consist of strongly shattered local basement. Lenses of Cambrian bituminous limestone occur sporadically. The structure of the unit is, as a rule, chaotic. The greatest thickness can be estimated to be less than 30 m in the outcrop area. Within the assumed crater the Lockne Breccia rests on the Tandsbyn Breccia, but further away it rests on the members of the Palaeozoic succession, from Cambrian alum shale to the uppermost part of the Ordovician Orthoceratite limestone. It is overlain by the Loftarsten. Its age, inferred from
M. LlNVSTRZiM
that of the youngest underlying beds, the youngest inclusions and the oldest overlying beds, is within the conodont sub-zone of ~a~to~~~d~s gerdae. The huge size of some of the inclusions has rendered geologic mapping and stratigraphic work in the Lockne area especially difficult because it has not always been obvious that outcrops represent inclusions in a breccia, rather than autochthonous bedrock. The Loftarsten was named by the local population. It is a graywacke-like, mostly arenitic rock of roughly the same composition as the Lockne Breccia, on which it frequently rests. Simon (1987a) found in it a high percentage of lava-like melt rock. The deposit is graded, with gravel frequently dominating in the basal part, whereas the top can be siltstone or mudstone. It is either massive or planar bedded. The boundary against the Lockne Breccia is either sharp or transitional. The Loftarsten is overlain by the fossiliferous Dalby Limestone. The northernmost known outcrop of Lockne Breccia and Loftarsten is at Torvalla, 12 km north of Lockne (Thorslund, 1940); the southernmost known outcrop is at Hallen 50 km south of Lockne (Thorslund, 1940; Simon, 1987a). In both places the thickness is around 1 m. Arenitic rock, lithologically indistinguishable from the Loftarsten, occurs in numerous elastic dikes in strongly fractured basement at the raiiway north of the church of Tandsbyn (Simon, 1987a), as well as in the high ground southwest of the Skanska quarry at Nordanbergsberget (our observation). Geophysics Regional air magnetometry was carried out for the Swedish Geological Survey in 1978. The resulting maps are not public but as far as we are informed they do not contain any independent evidence concerning the Lockne impact. Geophysical investigations specifically aimed at the elucidation of the Lockne impact structure are in their reconnaissance stage. ~agnetomet~ and reflection seismics (Floden et al,, 1990) carried out on Lake Locknesjon indicate that the local magnetic field within the assumed impact
AND
E.F.F. STURKELL
structure is practically without contrasts, whereas there is considerable local variation in intensity outside it. The seismic study on Locknesjiin indicates that compacted, boulder-rich till may be as thick as 80 m or more, and that it may be almost indistinguishable from rocks such as the Lockne Breccia. Within the assumed impact structure bedrock reflectors may be over 100 m below the surface of the lake, whereas outside it the bedrock reaches the surface. The seismic results thus confirm that the bedrock surface forms a closed basin the shape of which appears to be subcircufar. We identify this basin with the apparent crater of an impact structure. A pilot study of resistivity has been carried out by Herbert Henkel, at the Geological Survey of Sweden, who has kindly informed us about the results. They indicate strong brecciation of bedrock along a transect from Nordanbergsberget in the direction of Lake Stor-Handsjon. The brecciation apparently fades out about 1.6 km north of Stor-Handsjon, appro~mately where we expect the boundary of the impact structure to be. Furthermore, the granite of Nordanbergsberget is strongly fractured according to resistivity measurements. Discussion When the senior author (M.L.) began detailed geological mapping and stratigraphic studies in the Lockne area in 1978, the principal question concerned the origin of the very coarse elastics contained in the Lockne Breccia. The ruling interpretation was that they were derived by shoreline erosion of the local sedimentary succession, The working hypothesis of M.L. was that this interpretation was wrong, that the Lockne Breccia was not a littoral conglomerate but a debris flow deposited, most probably, at some considerable distance from any shore, and that the Loftarsten unit above it was a turbidite. The initial studies were reported in a series of diploma theses and one doctoral dissertation (Simon, 1987a) at the University of Marburg, in which the working hypothesis appeared to be confirmed by ample evidence. Simon’s dissertation contains three important discoveries that were carefully documented al-
GEOLOGY OF THE EARLY PALEOZOIC LOCKNE IMPACT STRUCTURE
though they could not be adequately explained. They were: (1) Fragments of melt rock of volcanic appearance were common in the Lockne Breccia and the Loftarsten unit. Their size precluded long distance transport by air. However, volcanism was very unlikely to have occurred in the area in the Ordovician. (2) The Tandsbyn Breccia, hitherto known as “arkose-breccia” or “arkose-like breccia”, was a crush rock, although how the cataclasis originated remained an enigma. (3) Fractured, but not excessively crushed, granite rested on essentially well-preserved Cambrian alum shale at Nordanbergsberget. This structure was explained as a Caledonian overthrust in a later publication by Simon (1987b). The senior author initially encouraged this interpretation. Soon afterwards, Wickman (1988) claimed that the observations of Simon, in particular those concerning the Lockne Breccia, could be best explained by relating them to impact. The senior author was initially sceptical of this idea, but it soon became evident that Simon’s three critical discoveries can indeed be explained by a major impact but probably by nothing else. The discovery of shocked quartz (Lindstriim et al., 1991) was taken as confirmation. The idea that the Lockne area has been affected by a major impact raises the following questions: (1) site and size of the impact; (2) resultant structure; (3) age, in particular with regard to the Tandsbyn and Lockne Breccias; (4) other circumstances, such as the environment in which the impact occurred, the subsequent history of the structure and its state of preservation. Since the structures and rocks related to the impact are concentrated in the Lockne area we are convinced that this area contains the impact site. Wickman’s (1988) assumption of a very large structure, with its centre 20 km northwest of Lockne, is founded on the impression that the Lockne Breccia in the Lockne area (and at Brunflo, 5 km north of Lockne) represents only a
181
minor sector of the marginal wall of the structure. We reject this concept for two reasons: (1) Breccia containing fragmented basement (Tandsbyn Breccia) does not occur outside the Lockne area. (2) The sedimentary succession underlying the lateral equivalent of the Lockne Breccia has remained stratigraphically undisturbed at localities that would have belonged to the final crater of the large impact envisaged by Wickman (1988). This is the case,for instance, at Torvalla (Thorslund, 1940). The impact hypothesis offers what appears to be the only acceptable explanation of the Tandsbyn Breccia. Due to the fact that this breccia consists of fragments of the local Proterozoic bedrock, from which it cannot everywhere be sharply delimited, the impact structure has to include parts of the Lockne area. The mapped distribution of the Tandsbyn Breccia, topography, dips of undisturbed beds resting on the Tandsbyn Breccia, concentric faults, and geophysical data indicate that the topographic low at Lockne represents the final crater, the diameter of which, in this case, is 7-8 km. The centre is covered by the nappe remnant on both sides of Musviken and some 4 km southwards. The occurrence on the northeast flank of Nordanbergsberget of an overturned sub-Cambrian erosion surface on granite, resting on Lower to lowermost Middle Cambrian sediments, belongs to the structural inventory of craters formed by impact (Melosh, 1989) and indicates that the crater margin was just south of Nordanbergsberget. With a diameter of 7-8 km the crater exceeds the normal dimensions of small, simple craters (Grieve, 1987; Melosh, 1989). Due to the fact that the centre is occupied by a nappe remnant it is, however, impossible as yet to tell, whether a central uplift is present, or not. Although the stratigraphy of the Lower Palaeozoic of Lockne and its neighbouring areas is fairly well known, the age of the impact is debatable. The youngest possible age is given by the lower Caradoc Lockne Breccia. This dating, originally suggested by Wickman (1988), has two attractive features: most of the major geological
182
anomalies of the Lockne area can be explained by a single event and the regional distribution of Lockne Breccia and Loftarsten provides the impact with a signature that can be identified outside the Lockne area. However, this interpretation raises questions that are not readily answered. For example: (1) Why are the Tandsbyn and Lockne Breccias everywhere sharply delimited from one another, if they formed through a single event? (2) Why is only lowermost Middle Cambrian present under the flap of overturned granitic basement at Nordanbergsberget? (3) Why are the basement-derived components of the Tandsbyn Breccia intensely fractured, whereas Ordovician limestone components of the Lockne Breccia are well preserved? We have been very much impressed by the circumstance that the Tandsbyn Breccia lacks any clasts of the sedimentary rocks that must have been present at the impact site, if the impact was Ordovician. Bodies of monomictic Tandsbyn Breccia, occurring as inclusions within the Lockne Breccia, strengthen the impression that the Tandsbyn Breccia formed well before the Lockne Breccia. Furthermore, we fail to see how a flap of granite could descend upon a Middle Cambrian surface without any younger sediment trapped in between, if the impact was younger than Middle Cambrian. The answer to the third question raised above seemed to us to be that the Ordovician limestone must be younger than the impact. The only dates compatible with the regional stratigraphy are earliest Middle Cambrian and middle Ordovician. These considerations convinced us (Lindstrom, 1991; Lindstrijm et al., 1991) that the impact was indeed early Middle Cambrian. The date given by the structure at Nordanbergsberget. However, we are now positive that this age assignment was wrong and that the middle Ordovician age initially suggested by Wickman (1988) and others (in particular H. Henkel), with whom we have discussed, is correct. The reason for this change of conviction follows below. In October 1991, Tom Floden, of the Department of Geology and Geochemistry of the Stockholm University, brought home a 140 m long drill
M. LINDSTROM
AND
E.F.F.
STURKELL
core through the fill of the Tvlren impact crater south of Stockholm (FlodCn et al., 1986). The age of the impact, as we can now ascertain, is either very close to or identical to that of the Lockne Breccia. The succession drilled at Tvaren has at its base a breccia consisting exclusively of fragments of the local crystalline basement. In position and composition this is analogous to the Tandsbyn Breccia. Above this unit there is a sharp lower boundary a single bed, almost 60 m thick, continuously graded from a lowermost portion of coarse limestone rubble with numerous clasts of basement rocks and basal breccia, through gravel, coarse sand, medium and fine sand, to massive mudstone at the top. The limestone breccia is structurally the precise counterpart of the Lockne Breccia, whereas the gravelly through sandy to muddy portions of this graded bed have their counterpart in the Loftarsten turbidite at Lockne. The limestone clasts that have been examined in the graded deposit contain as little evidence of shattering as most of those found in the Lockne Breccia. In both structures the sedimentary breccia and the more finegrained turbiditic deposit following upon it are directly overlain by fossiliferous marine sediments belonging to the middle Ordovician (lower Caradoc) Dalby Limestone. The Tvaren succession is interpreted as having formed after impact at sea; the graded bed apparently consists of ejecta brought back into the crater by resurgent water immediately after the impact. The same interpretation will apply at Lockne, in the case of the Lockne Breccia and Loftarsten. Both impact structures are dated by the youngest limestone occurring in the breccias and the oldest non-disturbed post-impact sediments within the crater. One of the weaknesses of our previous dating of the Lockne structure was that the evidence for it in regional stratigraphy was negative. For want of continuity and homogeneity of sedimentation in the earliest Middle Cambrian it could not be demonstrated that no major impact had occurred in the Gstersund region at that time. In view of what appeared to be positive evidence inside the structure itself, this was regarded as satisfactory. With the revised dating as middle Ordovician the
GEOLOGY OFTHEEARLYPALEOZOICL~CKNE
183
IMPACX STRUG~URE
situation is entirely different. The Lockne Breccia has been identified 30 km from the impact centre, although at this distance it is fairly thin, and the clasts are of very modest size (Simon, 1987aI. At Brunflo and the artillery range of Grytan, lo-12 km from the centre of the Lockne structure, we have, however, observed the Lockne Breccia to be both thick and extremely coarse. The physics of involvement of water in terrestrial impact cratering has not been discussed as much as interaction with the atmosphere (Melosh, 1989). Water movements must have been complex but would have begun with the instantaneous expulsion of a great volume of water from the impact site. The back-surge of the sea would have had a considerable capacity for erosion, transport, and deposition. To the solid load belonged ejecta and material eroded from the periphery, rim wall and inner slopes of the crater. The sea water back-surge which filled the crater, added to another volume that covered the site before the impact and was chased away during the moment of cratering. Because it settled from a mixed and dense suspension, most of the backsurge deposit would have been without lamination or sedimentary features other than grading by size. However, the deposit could also have been complex. The latest suspensoid to settle would have comprised the fine-granted fraction, as in the case of the Loftarsten. The breccia lens of crushed crystalline basement (Tandsbyn Breccia) that existed on the crater floor (Melosh, 1989) was, however, largely spared by the backsurge. In a crater of the size of Lockne, the mass of impact melt is only a few percent of the total mass displaced by cratering (Melosh, 1989). If it was preserved as a continuous body, this body must be sought under the nappe remnant. Its evolution would have been greatly influenced by interaction with sea water. Strong thermal convection within the Tandsbyn Breccia and within fractures developing in the solidifying melt brought about rapid cooling at the same time as hydrothermal mineralization systems were set up. We have some evidence of hydrothe~al activity in the vicinity of Nordanbergsberget (widespread fluorite mineralization, sulphide, a minor occur-
rence of low-grade silver ore), but the main activity would have occurred within the area now covered by the nappe outlier. Most of the recrystallization of impact products most probably took place during the stage of hydrothermal activity that must have been intensive just after the impact and ended (in geological terms) very soon after it. However, it is likely that the very low thermal metamorphism that occurred after Caledonian overthrusting was responsible for a recrystallization that was at least equally extensive because the process may have lasted over a hundred thousand times longer. The definition of the crater roughly coincides with the outer boundary of the autochthonous Tandsbyn Breccia. At the southern margin, Ha&u&et (Fig. 21, with outcrops of relatively little fractured basement, is just outside the crater. The faults at Miirttjtirnen (6984.5/1448.6) are situated close to the south margin. To the southwest (6985.5/1445.0), the Djupdalen fracture zone appro~mately coincides with the boundary. The occurrences of Cambrian bituminous shale at Nordanbergsberget must be outside the crater, although still quite close. These points together define a circle with a diameter of about 8 km and its centre between Bleka and Musviken (Fig. 21. Summary The Lockne area, situated 20 km south of dstersund on the Caledonide boundary of central Sweden, contains an impact structure with a diameter of 7-8 km. The impact occurred at sea in an area consisting of Proterozoic crystalline basement covered by several tens of metres of Lower Palaeozoic sedimentary rocks. The lower 20-30 m of the cover rocks were mainly Cambrian black shale that did not play any significant role in connection with the impact, except for the circumstance that its lowermost beds, of earliest Middle Cambrian age, became covered by an overturned flap of basement rock at Nordanbergsberget. This circumstance contributed to the erroneous dating of the impact as Middle Cambrian. The upper part of the pre-impact sediments consisted of lower to middle Ordovician lime-
M. LINDSTRijM
184
stone that was a few tens of metres thick. This limestone was an important component of the ejecta blanket. Since the impact occurred at sea, much of the ejecta blanket was thrown back into the crater by back-surging water. Other parts are preserved in the sedimentary succession of adjacent areas, where they help to date the impact as middle Ordovician. In the crater and its immediate vicinity the back-surge sediments form two discrete units, the lower of which has been named the Lockne Breccia. It is very coarse and has lately been interpreted as a debris flow. The upper unit, called Loftarsten, is a turbidite consisting of gravel, sand and mud. The base of the crater fill consists of crush rock that covers much geological map area in a belt extending along the margin of the apparent crater. This rock has been known as “arkosebreccia”, but we have renamed it the Tandsbyn Breccia. It is composed entirely of fragments of the local crystalline basement. Caledonian overthrust nappes once covered the area, but only a minor nappe outlier has been left by erosion; this outlier occupies the centre of the impact structure. Impact melt is preserved in several places; we attribute its devitrification to Caledonian regional metamorphism. The backsurge after the impact eroded parts of the rim wall, but enough was left standing to form topographic highs. During subsequent marine deposition the slopes of some of these highs became the sites for the growth of reef-like mud mounds. The rest of the area became covered by argillaceous and nodular Dalby Limestone. Acknowledgements
Maurits LindstrGm initiated and supervised the project and is responsible for the stratigraphic and petrologic interpretations. Erik F.F. Sturkell coordinated and carried out geological and geophysical mapping and is responsible for the geophysical interpretations. We thank the municipality of bstersund for substantial support to our research, and Herbert Henkel, S.G.U., for data, discussion, and critical review. Benno Kathol, University of Stockholm, helped decisively in the field and carried out thin-section photography.
AND
E.F.F.
STURKELL
Tom FlodCn, in co-operation with whom the study of the Tvlren drill core is in progress, supplied information that was essential for our interpretation of the Lockne structure. References Alexopoulos, J.S., Grieve, R A.F. and Robertson,P.B., 1988. Microscopic lamellar deformation features in quartz: Discriminative characteristics of shock-generated varieties. Geology, 16: 796-799. Flodtn, T., Tunander, P. and Wickman, F.E., 1986. The TvIren Bay structure, an astrobleme in southeastern Sweden. Geol. FGren. Stockholm Fiirh., 108: 225-234. Flodtn, T., Sturkell, E. and WannBs, K., 1990. Morfologi och Berggrund i LocknesjGn-ett bidrag till undersiikningen av Meteoritkratern vid Lockne. Rep. to City of &tersund (unpubl.). Grieve, R.A.F., 1987. Terrestrial impact structures. Annu Rev. Earth Planet. Sci., 15: 93-102. Jaanusson, V., Larsson, K. and Karis, L., 1982. The sequence in the Autochthon of JZmtland. In: D.L. Bruton and SW. Williams (Editors), Field Excursion Guide IV Int. Symp. Ordovician System. Paleont. Contrib. Univ. Oslo, 279: l-10. LindstrGm, M., 1991. Den yngre berggrunden. In: M. LindstrGm, J. Lundqvist and T. Lundqvist (Editors), Sveriges Geologi FrHn Urti Till Nutid. Studentlitteratur, Lund, pp. 123-229. LindstrGm, M., Simon S., Paul, B. and Kessler, K., 1983. The Ordovician and its mass movements in the Lockne area near the Caledonian margin, central Sweden. Geol. Palaeontol., 17: 17-27. LindstrGm, M., Ekvall, J., Hagenfeldt, S., Siiwe, B. and Sturkell, E.F.F, 1991. Well-preserved Cambrian impact exposed in Central Sweden. Geol. Rundsch., 80: 201-204. Melosh, H.J., 1989. Impact Cratering. A Geologic Process. Oxford Univ. Press, New York, 245 pp. Simon, S., 1987a. Stratigraphie, Petrographie und Entstehungsbedingungen von Grobklastika in der autochthonen, ordovizischen Schichtenfolge JImtlands (Schweden). Sver. Geol. Unders. Ser. C, 815: l-156. Simon. S., 1987b. Caledonian deformation of basement in the Lockne area, Jiimtland, central Sweden. Geol. F&en. Stockholm Fiirh., 109: 269-273. StGffler, D., 1972. Deformation and transformation of rockforming minerals bv natural and experimental shock processes. 1. Behavior of minerals under shock compression. Fortschr. Miner., 49: 50-l 13. StrGmberg, A., 1976. A pattern of tectonic zones in the western part of the East European Plattform. Geol. Fiiren. Stockholm F&h., 98: 227-243. Thorslund, P., 1940. On the Chasmops Series of Jemtland and SGdermanland (Tvlren). Sver. Geol. Unders. Ser. C, 136: l-191.
GEOLOGY
OF THE
EARLY
PALEOZOIC
LOCKNE
IMPACT
STRW-I’URE
Von Engelhardt, W. and Bertsch, W., 1969. Shock induced planar deformation structures in quartz from the Ries crater, Germany. Contrib. Mineral. Petrol., 20: 203-234. Wickman, F.E., 1988. Possible impact structures in Sweden. In: A. Boden and K.G. Eriksson (Editors), Deep Drilling in Crystalline Bedrock. I. The Deep Gas Drilling in Siljan
185 Impact Structure, Sweden and Astroblemes. Springer, Berlin, pp. 298-327. Wiman, C., 1900. Eine untersilurische Litoralfacies bei Locknesjiin in Jemtland. Bull. Geol. Inst. Univ. Uppsala, 4: 133-151.
Tectonophysics, 216 (1992) 169-185 Elsevier
Science
Publishers
B.V., Amsterdam
Geology of the Early Palaeozoic Lockne impact structure, Central Sweden Maurits Lindstriim and Erik F.F. Sturkell Department of Geology and Geochemistv, (Received
December
Stockholm Unil,ersity, S-106 91 Stockholm, Sweden
3, 1990; revised version
accepted
January
12, 1992)
ABSTRACT Lindstriim, M. and Sturkell, E.F.F., 1992. Geology of the Early Palaeozoic Pesonen and H. Henkel (Editors), Terrestrial impact Craters and Fennoscandia. Tectonophysics, 216: 169-185.
Lockne impact structure, Central Sweden. In: L.J. Craterform Structures with a Special Focus on
The diameter of the crater at Lockne as seen on topographic maps is 7-8 km. The Tandsbyn Breccia. the principal evidence of impact, consists of strongly crushed local basement. Its distribution follows the crater margin. On the north margin, basement granite rests on an inverted sub-Cambrian erosion surface on the lowermost Middle Cambrian. Tangential faults occur at the periphery of the structure. After the impact occurred under the sea during the middle Ordovician, there was a resurgence of ejecta-loaded sea water which deposited a chaotic breccia (Lockne Breccia), which has the appearance of a debris flow. The Lockne Breccia, together with a less coarsely grained turbidite (“Loftarsten”), which lies immediately above it, predominantly consist of clasts of lower to middle Ordovician limestone and, mainly subordinate, inclusions of Tandsbyn Breccia and isolated clasts of crystalline basement. Fragments of impact melt are important components of the turbidite. A protective cover of sediments formed after the impact, is, in turn, overlain by an outlier of a Caledonian overthrust nappe, which occupies the central and topographically deepest part of the crater. Shocked quartz has been identified in the Loftarsten.
impacts, it is also well preserved spects.
Introduction Only about 10% of the more than 120 known meteoritic impact structures on the earth are Ordovician or older (Grieve, 1987). The great majority of the known impacts took place on land: marine impacts are scarce. Whereas small terrestrial impact craters (diameters less than 2 km in sedimentary bedrock and less than 4 km in crystalline basement) are fairly well understood, larger structures require a great deal more study. Here we describe a recently identified impact structure that is unusual because it evidently formed in an Ordovician environment, and has a diameter of at least 7 km. Contrary to many old
Correspondence
to: M. LindstrGm,
and Geochemistry, holm, Sweden.
0040-1951/92/$0_5.00
Stockholm
Department
University,
0 1992 - Elsevier
S-106
Science
of Geology 91 Stock-
Publishers
in several re-
The Lockne area: research history
The Lockne area (Fig. 1) is situated 20 km south of 6stersund at the boundary between Caledonian overthrust rocks to the west and Proterozoic to Lower Palaeozoic rocks (with less evidence of Caledonian deformation) to the east. The Proterozoic forms the crystalline basement to the Palaeozoic, which consists of marine sedimentary deposits of standard Baltoscandian platform type. The Lockne area has been known to be interesting but difficult to interpret, for more than a century. Wiman (1900) reported the occurrence of veIy coarse elastic deposits of Ordovician age, and Thorslund (1940) elaborated the stratigraphy in a major, now classic, monograph. The au-
B.V. All rights reserved
M. LINDSTRhM
170
270 L
285
300 I
315
325
310
355
I
370
385
100
I
I
115
L30
AND E.F.F. STURKELL
LL5
I 5km
Plate I Topography
of the area around Lake Locknesjiin.
The surface covered by water is coloured contours approximately
5 m apart.
either blue or black. Altitude
in metres,
GEOLOGY
Fig.
OF THE
I. General
mentioned.
EARLY
reference
PALEOZOIC
LOCKNE
1MPACT
map with several of the localities
inset map shows outline of Sweden with location
of the btersund
area. National
coordinate
171
STRUCTlJRE
system is used.
tochthonous succession contains two extensively exposed breccias. The older breccia was regarded as arkose-like by Thorsiund (1940). The name “arkose-breccia” has stuck, although it is inadequate. Here we introduce the new name Tundsbyn Breccia for it and this name is used throughout this paper. The younger breccia has been named the Lockne Breccia (Jaanusson et al., 1982; LindstrGm et al., 1983). Its clasts, of various sizes, consist mainly of Ordovician Orthoceratite limestone but fragments of Tandsbyn Breccia may also be abundant. Thorslund (1940) found that major outcrops of the Tandsbyn Breccia were surrounded by younger sedimentary rocks, including the Lockne
Breccia, and interpreted this relationship as evidence that the Tandsbyn Breccia had formed islets around which the other rocks had been deposited. LindstrGm et al. (1983) and Simon (1987a) demonstrated that the Lockne Breccia has the structure of debris flows, and that the “islets” of Tandsbyn Breccia are, in fact, large inclusions in the Lockne Breccia. Simon (1987a) found that the Lockne Breccia and an immediately superjacent, greywacke-like unit, called Loftarsten, contained sand-sized clasts of melted rock that are too large to have been transported as atmospheric suspension. Since no Ordovician volcanism is known in the vicinity, he was unable to explain its occurrence. However, Wickman (1988) suggested that these clasts are impact melt and reinterpreted the Lo&e Breccia as an impact allogenic breccia. Since the Lockne Breccia is known to be of early Caradoc (middle Ordovician) age, Wickman’s intkrpretation even provided the impact with a precise date. Wickman sought the centre of the impact well to the northwest of the Lockne area, which he regarded as a marginal part of a fairly large structure. Lindstr~m et al. (1991) defined an impact crater with a diameter of 7-8 km based on the occurrence of the Tandsbyn Breccia. They dated it to the Middle Cambrian, partly because crushed basement rests, in inverted position, on Middle Cambrian sediments at the northern margin of the structure. Geology and geomorphology
of the impact area
The area is cut by a regional Iineament that continues northwestwards into the Caledonian structure (see Striimberg, 1976). Lake L.ocknesjGn is part of this lineament. We have identified the Lockne crater with a topographic depression (Plate I) to the west of the northern part of Locknesjiin. This depression has a north-south diameter of about 8 km west of the lake and continues eastwards under the lake. Its northern, western and southern margins consist of hills that rise as much as 140 m above the lake. The eastern margin of the depression is a till-covered plain on the eastern shore of the lake; scanty
M. LINDSTRZ)M
172
r I
AND E.F.F. STURKELL
Geological map of the Lockne area ,
lkm
I
Overthrust fault Sulphide &a
deposii
Shale
Dalby Dalby
limestone/ mudmound
Loftarsten Lockne
Breccia
Orthocerntite limestone TGyen Shale alum
shale
Tandsbyn Asby
Breccic
Dolerite
granite voicanite
cl
Fig.4
6985
-I-+++++
Fig. 2. Geological map of Lockne area.
CXOLOGY
OI; THE
EARLY
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LOCKNL:
IMPACT
173
STRUC-IXJRE
exposures indicate that the plain is underlain by Palaeozoic sedimentary rocks, whereas its eastern margin consists of raised Proterozoic basement. The structurally uppermost rocks of the area are overthrust lower Ordovician Tijyen Shale and lower to middle Ordovician Orthoceratite limestone which occupy the centre of the topographic depression. They rest on autochthonous Dalby Limestone of Caradoc age; that is, a distinctly younger sediment (Figs. 2, 3). The base of the overthrust nappe is exposed along the road to Lake Stor-Handsjon (6987.1f 1446.9; these and other coordinate figures given in this paper are to the nearest 100 m and refer to the Swedish National Grid, indicated in the maps), where well preserved Dalby Limestone dips northeastward under strongly sheared Tiiyen Shale. The same structura1 relations occur to the west of Grlstjarnen (6986.5/1448.2), although without exposure of the thrust contact. The Orthoceratite limestone, overlying the Tiiyen Shale is strongly folded and sheared. It is exposed, for instance, at the road east of Tandsbyn (69~8.~/144~.2) and on both sides of Musviken (6989.6/1449.0 and 6989.9/1449.2X Exposures have existed at various localities in the vicinity of Bleka (6987.5/1~8.2). An autochthonous Lower Pataeozoic succession of standard Baltoscandian type occurs along the Caledonian margin, to the south and east of Gstersund (Fig. 3). It begins with gravelly sandstone with Lower Cambrian fossils, a couple of metres thick at the most. The Middle and Upper Cambrian consists of a more than 20 m thick, black, bituminous shale (“alum shale”). Close to the base of the Ordovician succession there is a grey, graptolitic shale (Toyen Shale; Lower Ordovician) that can be about 5 m thick. This is overlain by a few tens of metres of bedded Orthoceratite limestone of Arenig to Llandeilo age. In the lowermost Caradoc we find the Lockne Breccia, which is overlain by a turbiditic, greywackelike sandstone (~ftarsten) with a ciast composition which resembles that of the underlying breccia. The widely distributed Dalby Limestone is mainly developed as a light grey catcareous shale with nodules which range from infrequent to abundant. of fossiliferous limestone; however, lo-
Normal succession
Crater rim
Ashgill
Carada
hi&lo
Llanvin
Arenig
Tremadot upper
Middle l
**t*****
Lower
ora Shale lzizzzl R!zzl
-:-I-I-T Toyen Shale lzz
Dalby Limestone
alum shale l
b++e Tandsbyn Breccia
@
Lockne Breccia
I
Kzl
Orthoceratite ~imcstone
**I*** .. .., . sandstone D
Fig. 3. Lower Paleozoic outline stratigraphy of Lockne area. The relative thicknesses of the units are not represented.
tally it forms reef-like mud mounds. The Caradoc to Ashgill succession above it only occurs sporadicaIly and will not concern us here. The lower part of the standard succession is not represented in the impact area. The Dalby Limestone and underlying Loftarsten and Lockne Breccia occupy the flanks of the hills that surround the lower ground. They dip towards the
M. LINDSTRGM
174
centre of the topographic depression. In the upper slopes the Tandsbyn Breccia is exposed. The occurrence of the geological units thus contributes to the saucer-like shape of the structure. If the stratigraphic units do not vary excessively in thickness, the Tandsbyn Breccia should occur at a depth of more than 70 m below the level of the lake in the central parts of the structure, whereas its outcrop reaches up to 140 m above the lake in surrounding hills. The eastern segment of the structure is either covered by the lake (as well as by the thick till on the eastern shore), or has been removed by faulting. The second alternative is plausible in view of geomorphological, geological and geophysical evidence for a major fault along the length of the lake, which trends NNW-SSE.
+
l
+
+
+
+*+
Fig. 4. Geological
map of Nordanbergsberget.
E.F.F. STURKELL
The chain of hills surrounding the topographic depression is interrupted by cross-cutting valleys at Tandsbyn and in the area to the east of Hlllniiset (Fig. 2). The Tandsbyn gap must have existed in the middle Ordovician because its south flank was the site of reef-like build-ups during the time that the Dalby Limestone was being deposited (Thorslund, 1940). The build-ups contain numerous geopetal fabrics which indicate that they are of Ordovician age and that the present topography then existed (our observation). The gap at Hallnaset evidently existed in the middle Ordovician, because both the Loftarsten and the Lockne Breccia thicken from the upper slopes to the lower ground near the lake. The crystalline bedrock of the higher ground outside the topographic depression is cut by sub-
+++++
++++++ +++++++ ++++++++ +++++++++
AND
250 m
+++++
For location
major slab of granite
turned
of map area see upper part of Fig. 2. Thick line shows the boundary upside down just outside
the transient
crater.
of a
GEOLOGY
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LOCKNE
IMPACT
175
STKVCTURE
vertical faults, some of which run tangentially to the periphery of the assumed crater (Fig. 2). We have found two locahties at which tangential faults form graben-like structures in which Cambrian basal conglomerate and alum shale are offset against crystalline basement rocks (6984.5/1449.5 and 6984.5/1448.6).
Structures occurring at Nordanbergsberget (Fig. 4) to the north of Locknesjon are regarded as particularly important to the geological interpretation of the impact. Simon (1987a) was the first geologist to observe that the Proterozoic granite of the Skanska quarry on the northeast flank of the hill Nordanbergsberget rests on Cam-
Fig. 5. Skanska quarry at Nordanber~b~rget. Granite resting with inverted erosive contact on Lower and Middle Cambrian sedimentary rocks. (a) General view from southwest. Cambrian sediments are exposed at the foot of the northeast quarry wail. (b) Detail of contact. Granite, partly weathered, partly fresh, in upper part and as boulders within the Cambrian. Cambrian alum shale in lower part.
176
brian alum shale. His interpretation, that the structure represented Caledonian overthrusting (Simon, 1987b), can be challenged on three principal grounds: (1) The alum shale immediately under the granite is too well preserved compared with other alum shales that have suffered overthrusting.
M. LINDSTRBM
AND
E.F.F. STURKELL
Bedding planes show no signs of shear and contain delicate organic and early diagenetic structures that are mechanically intact. (2) The vergence, and hence direction of movement indicated by the structure is towards the northeast, that is, at right angles to the local direction of Caledonian overthrusting. Lindstrijm
Fig. 6. Tandsbyn Breccia. (a) Specimen with granite fragments in partly weathered carbonate-rich matrix from 6989.3/1443.3. Scale line = 5 cm. (b) Sawed specimen from 600 m northwest of Groffelglrde (6989.7/1446.6). Scale line = 2 cm.
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et al. (1991) argued on the basis of these two observations that the granite must have collapsed on top of the alum shale after having been uprooted by an impact to the south of the hill. With this origin for the granite cover, the alum shale would at least have escaped much of the horizontal shear caused by overthrusting of a thick rock pile. (3) Subsequent quarrying operations, have brought to light the third principal evidence against the overthrust hypothesis. The contact between the granite and the underlying Cambrian is a weathered transgression surface with major, well rounded clasts of granite turned upside down along its entire length (Fig. 5). This inversion could not have originated from overthrusting but is a characteristic feature of the margins of impact craters (see, for instance, Melosh (19891, see also discussion below). The upper part of the hill has outcrops of undisturbed layers of varying thickness, all less than 1 m, of basal Cambrian gravelly sandstone, resting on weathered Proterozoic granite and dolerite, and overlain by alum shale with the same features as observed in the quarry. The weathering of the dolerite left rounded cores of less altered rock between which sediment has penetrated downwards. The alum shale is, in turn, overlain by the same granite that rests on alum shale in the quarry (Fig. 4). We can see no viable alternative to the explanation that the flap of granite of Nordanbergsberget was overturned by impact. Tandsbyn Breccia The Tandsbyn Breccia, consists of intensely fractured but otherwise little altered fragments of local Proterozoic rocks (Fig. 6). The thin Holocene weathering crust can be light grey to whitish in colour, especially in the area of the protolith, but in fresh outcrop it can be almost black. Where the protoiith is dolerite or metavolcanics the weathered colour can be ochreous and the colour of fresh fractures dark greyish green to almost black. Thin sections show the typical Breccia to consist of angular fragments, mostly of quartz and
177
feldspar in a fine-gra;ned and slightly recrystallized, micaceous matrix. The great force that went into crushing it is most evident from plagioclase crystals in which the twin lamellae are bent, offset, and broken with great frequency and irregular orientation. A good description of the rock was given by Simon (1987a). The type locality of the Tandsbyn Breccia is the crest of the slope to the south of the railway southwest of the village of Tandsbyn and its former railway station (6987.4/1445.7). At this locality, the breccia consists of light grey, plagioclaserich granite. A large spectacular boulder of identical breccia occurs at Grubban a couple of hundred metres southwest of the type locality. The outcrop area of Tandsbyn Breccia forms a belt along Highway 81 from northeast of the church of Lockne to the side road to Tandsbyn ~6990.1/1445.7) which continues through a wide area to the north and northeast of Tandsbyn. At Tandsbyn the continuous presence of the breccia is interrupted by the sedimentary rocks of the Tandsbyn gap, to the south of which the higher ground consists of Tandsbyn Breccia from the type locality to west of HZlln%set. Occurrences even farther south are scattered and form tectonically delimited enclaves within areas of less disturbed Proterozoic rocks. Good outcrops of the breccia occur, for example, at Highway 81 to the north of the church of Lockne (6992.4/1449.6) and 100 m east of Bgcksved (6991.3/1447.2), to the west of the raiiway, 600 m northwest of Groffelg~rde (6989.7,’ 1446.6) and along the private road towards Lake Stor-HandsjGn, 1.5-1.7 km south of the gate (6986.5/1447.0 to 6986.3/1447.1), as well as west of H%In%et (6986.7/1450.4 to 6984.6/ 1450.4). The two last-mentioned occurrences present greater lithologic variation than the others, which are of predominantly granite composition. The outcrops along the Stor-Handsjbn road consist, to a great extent, of granitic breccia, but a variety also occurs which has a very dark grey, dense matrix and abundant subordinate clasts of a lighter hue. In thin section the matrix is seen to be composed of mainly chlorite, muscovite, quartz and plagioclase. Sutured grain contacts without preferred orientation identify it as a devitrified
M. LINDSI‘K~M
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interstitial melt. Similar breccias with devitrified melt occur west of HIllnGet. The rock fragments consist of dolerite and intermediate metavolcanics. An instance of well preserved melt rock with a porphyritic texture and affinity to the Tandsbyn Breccia occurs 500 m south of Hlllnlset
Fig 7. Lamellar fro1m
100m
quartz in thin sections from Tandsbyn
east of Biicksved (6991.3/1447.2).
interpreted
as diaplectic
E.F.F. STURKELL
(6985.0/ 1451.5). The outcrop is contiguous with one of the more spectacular occurrences of Tandsbyn Breccia. In contrast to all Proterozoic supracrustal rocks in the area, it lacks any fabric evidence of tectonization and must, therefore, be related to the younger geological events in the area.
Breccla. The scale line in both photographs
The irregular
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grain in the upper right quadrant
is 100 pm. (a) Specimen
extinguishes
LO 16
in all positions and is
glass. (b) Specimen LO 1 from the railway section between LQngHnge and StensjG (6993.0/1450.25).
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STRUCTURE
Outcrops of Tandsbyn Breccia occur within the mapped area of the Lockne Breccia, where most of them are underlain by Ordovician sedimentary rocks. These outcrops are thus demonstrably inclusions in the Lockne Breccia. However, the major occurrence of Tandsbyn Breccia west of the railway, about 1 km west of Grubban (6987.4/1443.9) continues under the surrounding Lockne Breccia and possibly rests on Proterozoic basement. Most inclusions of Tandsbyn Breccia, consist of granite, but 150 m north-northwest of Klockslsen (6984.6/1444.3) there is an inclusion consisting of fragments of Proterozoic Asby Dolerite in a matrix of recrystallized melt rock. The Asby Dolerite has an ophitic texture. The devitrified interstitial melt contains abundant fragments of inclusionrich melt rock and itself consists mainly of a felt-like, and structurally isotropic, mass of micro-crystalline chlorite. A rock of this kind might have formed in connection with the intrusion of the Asby Dolerite but we connect it with the Tandsbyn Breccia because it shows evidence of only one event of penetrative brecciation. Since all other inclusions of Proterozoic derivation in the Lockne Breccia bear evidence of the cataclasis that formed the Tandsbyn Breccia, we do not believe that a major body of rock found so close to the crater could bear the imprint of brecciation in connection with Proterozoic intrusion without any evidence of the younger brecciation event. It is because we do not find the evidence of a second brecciation that we attribute the breccia and its interstitial melt rock to the event that formed the Tandsbyn Breccia. Shock metamorphism
Lindstrom et al. (1991) found shocked quartz in the Lockne Breccia and interpreted it as being reworked from impact deposits. However, shock metamorphism should also be present in the Tandsbyn Breccia. A total of 30 thin sections of the Tandsbyn Breccia were searched for features of shock metamorphism. All showed different degrees of recrystallization. This circumstance may be explained by the fact that the area was subjected to
raised temperatures after nappe emplacement during the Caledonian orogeny, as well as to hydrothermal activity immediately after the impact. Nine specimens of Tandsbyn Breccia from eight localities (Fig. 7) contained quartz with a lamellar structure reminiscent of shocked quartz (Von Engelhardt and Bertsch, 1969; Alexopoulos et al., 1988). The lamellae are straight and parallel and run from margin to margin of the grains; most frequently the same set continues in adjacent grains. The interval between lamellae is 5-20 pm. The lamellae are hardly ever parallel to the undulose extinction which is an omnipresent feature of the quartz. Although the structure described is reminiscent of shocked quartz, it is by no means as distinct as the examples given by Stoffler (1972). Samples of shocked quartz kindly provided by D. Stijffler show much stronger lamellae. R.A.F. Grieve, who kindly examined thin sections of the Loftarsten and Tandsbyn Breccia, verified the presence of shocked quartz in the Loftarsten but not in the Tandsbyn Breccia. Lockne Breccia and Loftarsten Turbidite Simon (1987a) gave a detailed description and analysis of the elastic units known as Lockne Breccia and Loftarsten. The predominant components of the Lockne Breccia are clasts, ranging in size from sand to hundreds of cubic metres, of lower to middle Ordovician limestone in a calcareous-argillaceous matrix. There are variable amounts (from none to abundant) of angular clasts derived from the local basement and inclusions of Tandsbyn Breccia, which consist of strongly shattered local basement. Lenses of Cambrian bituminous limestone occur sporadically. The structure of the unit is, as a rule, chaotic. The greatest thickness can be estimated to be less than 30 m in the outcrop area. Within the assumed crater the Lockne Breccia rests on the Tandsbyn Breccia, but further away it rests on the members of the Palaeozoic succession, from Cambrian alum shale to the uppermost part of the Ordovician Orthoceratite limestone. It is overlain by the Loftarsten. Its age, inferred from
M. LlNVSTRZiM
that of the youngest underlying beds, the youngest inclusions and the oldest overlying beds, is within the conodont sub-zone of ~a~to~~~d~s gerdae. The huge size of some of the inclusions has rendered geologic mapping and stratigraphic work in the Lockne area especially difficult because it has not always been obvious that outcrops represent inclusions in a breccia, rather than autochthonous bedrock. The Loftarsten was named by the local population. It is a graywacke-like, mostly arenitic rock of roughly the same composition as the Lockne Breccia, on which it frequently rests. Simon (1987a) found in it a high percentage of lava-like melt rock. The deposit is graded, with gravel frequently dominating in the basal part, whereas the top can be siltstone or mudstone. It is either massive or planar bedded. The boundary against the Lockne Breccia is either sharp or transitional. The Loftarsten is overlain by the fossiliferous Dalby Limestone. The northernmost known outcrop of Lockne Breccia and Loftarsten is at Torvalla, 12 km north of Lockne (Thorslund, 1940); the southernmost known outcrop is at Hallen 50 km south of Lockne (Thorslund, 1940; Simon, 1987a). In both places the thickness is around 1 m. Arenitic rock, lithologically indistinguishable from the Loftarsten, occurs in numerous elastic dikes in strongly fractured basement at the raiiway north of the church of Tandsbyn (Simon, 1987a), as well as in the high ground southwest of the Skanska quarry at Nordanbergsberget (our observation). Geophysics Regional air magnetometry was carried out for the Swedish Geological Survey in 1978. The resulting maps are not public but as far as we are informed they do not contain any independent evidence concerning the Lockne impact. Geophysical investigations specifically aimed at the elucidation of the Lockne impact structure are in their reconnaissance stage. ~agnetomet~ and reflection seismics (Floden et al,, 1990) carried out on Lake Locknesjon indicate that the local magnetic field within the assumed impact
AND
E.F.F. STURKELL
structure is practically without contrasts, whereas there is considerable local variation in intensity outside it. The seismic study on Locknesjiin indicates that compacted, boulder-rich till may be as thick as 80 m or more, and that it may be almost indistinguishable from rocks such as the Lockne Breccia. Within the assumed impact structure bedrock reflectors may be over 100 m below the surface of the lake, whereas outside it the bedrock reaches the surface. The seismic results thus confirm that the bedrock surface forms a closed basin the shape of which appears to be subcircufar. We identify this basin with the apparent crater of an impact structure. A pilot study of resistivity has been carried out by Herbert Henkel, at the Geological Survey of Sweden, who has kindly informed us about the results. They indicate strong brecciation of bedrock along a transect from Nordanbergsberget in the direction of Lake Stor-Handsjon. The brecciation apparently fades out about 1.6 km north of Stor-Handsjon, appro~mately where we expect the boundary of the impact structure to be. Furthermore, the granite of Nordanbergsberget is strongly fractured according to resistivity measurements. Discussion When the senior author (M.L.) began detailed geological mapping and stratigraphic studies in the Lockne area in 1978, the principal question concerned the origin of the very coarse elastics contained in the Lockne Breccia. The ruling interpretation was that they were derived by shoreline erosion of the local sedimentary succession, The working hypothesis of M.L. was that this interpretation was wrong, that the Lockne Breccia was not a littoral conglomerate but a debris flow deposited, most probably, at some considerable distance from any shore, and that the Loftarsten unit above it was a turbidite. The initial studies were reported in a series of diploma theses and one doctoral dissertation (Simon, 1987a) at the University of Marburg, in which the working hypothesis appeared to be confirmed by ample evidence. Simon’s dissertation contains three important discoveries that were carefully documented al-
GEOLOGY OF THE EARLY PALEOZOIC LOCKNE IMPACT STRUCTURE
though they could not be adequately explained. They were: (1) Fragments of melt rock of volcanic appearance were common in the Lockne Breccia and the Loftarsten unit. Their size precluded long distance transport by air. However, volcanism was very unlikely to have occurred in the area in the Ordovician. (2) The Tandsbyn Breccia, hitherto known as “arkose-breccia” or “arkose-like breccia”, was a crush rock, although how the cataclasis originated remained an enigma. (3) Fractured, but not excessively crushed, granite rested on essentially well-preserved Cambrian alum shale at Nordanbergsberget. This structure was explained as a Caledonian overthrust in a later publication by Simon (1987b). The senior author initially encouraged this interpretation. Soon afterwards, Wickman (1988) claimed that the observations of Simon, in particular those concerning the Lockne Breccia, could be best explained by relating them to impact. The senior author was initially sceptical of this idea, but it soon became evident that Simon’s three critical discoveries can indeed be explained by a major impact but probably by nothing else. The discovery of shocked quartz (Lindstriim et al., 1991) was taken as confirmation. The idea that the Lockne area has been affected by a major impact raises the following questions: (1) site and size of the impact; (2) resultant structure; (3) age, in particular with regard to the Tandsbyn and Lockne Breccias; (4) other circumstances, such as the environment in which the impact occurred, the subsequent history of the structure and its state of preservation. Since the structures and rocks related to the impact are concentrated in the Lockne area we are convinced that this area contains the impact site. Wickman’s (1988) assumption of a very large structure, with its centre 20 km northwest of Lockne, is founded on the impression that the Lockne Breccia in the Lockne area (and at Brunflo, 5 km north of Lockne) represents only a
181
minor sector of the marginal wall of the structure. We reject this concept for two reasons: (1) Breccia containing fragmented basement (Tandsbyn Breccia) does not occur outside the Lockne area. (2) The sedimentary succession underlying the lateral equivalent of the Lockne Breccia has remained stratigraphically undisturbed at localities that would have belonged to the final crater of the large impact envisaged by Wickman (1988). This is the case,for instance, at Torvalla (Thorslund, 1940). The impact hypothesis offers what appears to be the only acceptable explanation of the Tandsbyn Breccia. Due to the fact that this breccia consists of fragments of the local Proterozoic bedrock, from which it cannot everywhere be sharply delimited, the impact structure has to include parts of the Lockne area. The mapped distribution of the Tandsbyn Breccia, topography, dips of undisturbed beds resting on the Tandsbyn Breccia, concentric faults, and geophysical data indicate that the topographic low at Lockne represents the final crater, the diameter of which, in this case, is 7-8 km. The centre is covered by the nappe remnant on both sides of Musviken and some 4 km southwards. The occurrence on the northeast flank of Nordanbergsberget of an overturned sub-Cambrian erosion surface on granite, resting on Lower to lowermost Middle Cambrian sediments, belongs to the structural inventory of craters formed by impact (Melosh, 1989) and indicates that the crater margin was just south of Nordanbergsberget. With a diameter of 7-8 km the crater exceeds the normal dimensions of small, simple craters (Grieve, 1987; Melosh, 1989). Due to the fact that the centre is occupied by a nappe remnant it is, however, impossible as yet to tell, whether a central uplift is present, or not. Although the stratigraphy of the Lower Palaeozoic of Lockne and its neighbouring areas is fairly well known, the age of the impact is debatable. The youngest possible age is given by the lower Caradoc Lockne Breccia. This dating, originally suggested by Wickman (1988), has two attractive features: most of the major geological
182
anomalies of the Lockne area can be explained by a single event and the regional distribution of Lockne Breccia and Loftarsten provides the impact with a signature that can be identified outside the Lockne area. However, this interpretation raises questions that are not readily answered. For example: (1) Why are the Tandsbyn and Lockne Breccias everywhere sharply delimited from one another, if they formed through a single event? (2) Why is only lowermost Middle Cambrian present under the flap of overturned granitic basement at Nordanbergsberget? (3) Why are the basement-derived components of the Tandsbyn Breccia intensely fractured, whereas Ordovician limestone components of the Lockne Breccia are well preserved? We have been very much impressed by the circumstance that the Tandsbyn Breccia lacks any clasts of the sedimentary rocks that must have been present at the impact site, if the impact was Ordovician. Bodies of monomictic Tandsbyn Breccia, occurring as inclusions within the Lockne Breccia, strengthen the impression that the Tandsbyn Breccia formed well before the Lockne Breccia. Furthermore, we fail to see how a flap of granite could descend upon a Middle Cambrian surface without any younger sediment trapped in between, if the impact was younger than Middle Cambrian. The answer to the third question raised above seemed to us to be that the Ordovician limestone must be younger than the impact. The only dates compatible with the regional stratigraphy are earliest Middle Cambrian and middle Ordovician. These considerations convinced us (Lindstrom, 1991; Lindstrijm et al., 1991) that the impact was indeed early Middle Cambrian. The date given by the structure at Nordanbergsberget. However, we are now positive that this age assignment was wrong and that the middle Ordovician age initially suggested by Wickman (1988) and others (in particular H. Henkel), with whom we have discussed, is correct. The reason for this change of conviction follows below. In October 1991, Tom Floden, of the Department of Geology and Geochemistry of the Stockholm University, brought home a 140 m long drill
M. LINDSTROM
AND
E.F.F.
STURKELL
core through the fill of the Tvlren impact crater south of Stockholm (FlodCn et al., 1986). The age of the impact, as we can now ascertain, is either very close to or identical to that of the Lockne Breccia. The succession drilled at Tvaren has at its base a breccia consisting exclusively of fragments of the local crystalline basement. In position and composition this is analogous to the Tandsbyn Breccia. Above this unit there is a sharp lower boundary a single bed, almost 60 m thick, continuously graded from a lowermost portion of coarse limestone rubble with numerous clasts of basement rocks and basal breccia, through gravel, coarse sand, medium and fine sand, to massive mudstone at the top. The limestone breccia is structurally the precise counterpart of the Lockne Breccia, whereas the gravelly through sandy to muddy portions of this graded bed have their counterpart in the Loftarsten turbidite at Lockne. The limestone clasts that have been examined in the graded deposit contain as little evidence of shattering as most of those found in the Lockne Breccia. In both structures the sedimentary breccia and the more finegrained turbiditic deposit following upon it are directly overlain by fossiliferous marine sediments belonging to the middle Ordovician (lower Caradoc) Dalby Limestone. The Tvaren succession is interpreted as having formed after impact at sea; the graded bed apparently consists of ejecta brought back into the crater by resurgent water immediately after the impact. The same interpretation will apply at Lockne, in the case of the Lockne Breccia and Loftarsten. Both impact structures are dated by the youngest limestone occurring in the breccias and the oldest non-disturbed post-impact sediments within the crater. One of the weaknesses of our previous dating of the Lockne structure was that the evidence for it in regional stratigraphy was negative. For want of continuity and homogeneity of sedimentation in the earliest Middle Cambrian it could not be demonstrated that no major impact had occurred in the Gstersund region at that time. In view of what appeared to be positive evidence inside the structure itself, this was regarded as satisfactory. With the revised dating as middle Ordovician the
GEOLOGY OFTHEEARLYPALEOZOICL~CKNE
183
IMPACX STRUG~URE
situation is entirely different. The Lockne Breccia has been identified 30 km from the impact centre, although at this distance it is fairly thin, and the clasts are of very modest size (Simon, 1987aI. At Brunflo and the artillery range of Grytan, lo-12 km from the centre of the Lockne structure, we have, however, observed the Lockne Breccia to be both thick and extremely coarse. The physics of involvement of water in terrestrial impact cratering has not been discussed as much as interaction with the atmosphere (Melosh, 1989). Water movements must have been complex but would have begun with the instantaneous expulsion of a great volume of water from the impact site. The back-surge of the sea would have had a considerable capacity for erosion, transport, and deposition. To the solid load belonged ejecta and material eroded from the periphery, rim wall and inner slopes of the crater. The sea water back-surge which filled the crater, added to another volume that covered the site before the impact and was chased away during the moment of cratering. Because it settled from a mixed and dense suspension, most of the backsurge deposit would have been without lamination or sedimentary features other than grading by size. However, the deposit could also have been complex. The latest suspensoid to settle would have comprised the fine-granted fraction, as in the case of the Loftarsten. The breccia lens of crushed crystalline basement (Tandsbyn Breccia) that existed on the crater floor (Melosh, 1989) was, however, largely spared by the backsurge. In a crater of the size of Lockne, the mass of impact melt is only a few percent of the total mass displaced by cratering (Melosh, 1989). If it was preserved as a continuous body, this body must be sought under the nappe remnant. Its evolution would have been greatly influenced by interaction with sea water. Strong thermal convection within the Tandsbyn Breccia and within fractures developing in the solidifying melt brought about rapid cooling at the same time as hydrothermal mineralization systems were set up. We have some evidence of hydrothe~al activity in the vicinity of Nordanbergsberget (widespread fluorite mineralization, sulphide, a minor occur-
rence of low-grade silver ore), but the main activity would have occurred within the area now covered by the nappe outlier. Most of the recrystallization of impact products most probably took place during the stage of hydrothermal activity that must have been intensive just after the impact and ended (in geological terms) very soon after it. However, it is likely that the very low thermal metamorphism that occurred after Caledonian overthrusting was responsible for a recrystallization that was at least equally extensive because the process may have lasted over a hundred thousand times longer. The definition of the crater roughly coincides with the outer boundary of the autochthonous Tandsbyn Breccia. At the southern margin, Ha&u&et (Fig. 21, with outcrops of relatively little fractured basement, is just outside the crater. The faults at Miirttjtirnen (6984.5/1448.6) are situated close to the south margin. To the southwest (6985.5/1445.0), the Djupdalen fracture zone appro~mately coincides with the boundary. The occurrences of Cambrian bituminous shale at Nordanbergsberget must be outside the crater, although still quite close. These points together define a circle with a diameter of about 8 km and its centre between Bleka and Musviken (Fig. 21. Summary The Lockne area, situated 20 km south of dstersund on the Caledonide boundary of central Sweden, contains an impact structure with a diameter of 7-8 km. The impact occurred at sea in an area consisting of Proterozoic crystalline basement covered by several tens of metres of Lower Palaeozoic sedimentary rocks. The lower 20-30 m of the cover rocks were mainly Cambrian black shale that did not play any significant role in connection with the impact, except for the circumstance that its lowermost beds, of earliest Middle Cambrian age, became covered by an overturned flap of basement rock at Nordanbergsberget. This circumstance contributed to the erroneous dating of the impact as Middle Cambrian. The upper part of the pre-impact sediments consisted of lower to middle Ordovician lime-
M. LINDSTRijM
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stone that was a few tens of metres thick. This limestone was an important component of the ejecta blanket. Since the impact occurred at sea, much of the ejecta blanket was thrown back into the crater by back-surging water. Other parts are preserved in the sedimentary succession of adjacent areas, where they help to date the impact as middle Ordovician. In the crater and its immediate vicinity the back-surge sediments form two discrete units, the lower of which has been named the Lockne Breccia. It is very coarse and has lately been interpreted as a debris flow. The upper unit, called Loftarsten, is a turbidite consisting of gravel, sand and mud. The base of the crater fill consists of crush rock that covers much geological map area in a belt extending along the margin of the apparent crater. This rock has been known as “arkosebreccia”, but we have renamed it the Tandsbyn Breccia. It is composed entirely of fragments of the local crystalline basement. Caledonian overthrust nappes once covered the area, but only a minor nappe outlier has been left by erosion; this outlier occupies the centre of the impact structure. Impact melt is preserved in several places; we attribute its devitrification to Caledonian regional metamorphism. The backsurge after the impact eroded parts of the rim wall, but enough was left standing to form topographic highs. During subsequent marine deposition the slopes of some of these highs became the sites for the growth of reef-like mud mounds. The rest of the area became covered by argillaceous and nodular Dalby Limestone. Acknowledgements
Maurits LindstrGm initiated and supervised the project and is responsible for the stratigraphic and petrologic interpretations. Erik F.F. Sturkell coordinated and carried out geological and geophysical mapping and is responsible for the geophysical interpretations. We thank the municipality of bstersund for substantial support to our research, and Herbert Henkel, S.G.U., for data, discussion, and critical review. Benno Kathol, University of Stockholm, helped decisively in the field and carried out thin-section photography.
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Tom FlodCn, in co-operation with whom the study of the Tvlren drill core is in progress, supplied information that was essential for our interpretation of the Lockne structure. References Alexopoulos, J.S., Grieve, R A.F. and Robertson,P.B., 1988. Microscopic lamellar deformation features in quartz: Discriminative characteristics of shock-generated varieties. Geology, 16: 796-799. Flodtn, T., Tunander, P. and Wickman, F.E., 1986. The TvIren Bay structure, an astrobleme in southeastern Sweden. Geol. FGren. Stockholm Fiirh., 108: 225-234. Flodtn, T., Sturkell, E. and WannBs, K., 1990. Morfologi och Berggrund i LocknesjGn-ett bidrag till undersiikningen av Meteoritkratern vid Lockne. Rep. to City of &tersund (unpubl.). Grieve, R.A.F., 1987. Terrestrial impact structures. Annu Rev. Earth Planet. Sci., 15: 93-102. Jaanusson, V., Larsson, K. and Karis, L., 1982. The sequence in the Autochthon of JZmtland. In: D.L. Bruton and SW. Williams (Editors), Field Excursion Guide IV Int. Symp. Ordovician System. Paleont. Contrib. Univ. Oslo, 279: l-10. LindstrGm, M., 1991. Den yngre berggrunden. In: M. LindstrGm, J. Lundqvist and T. Lundqvist (Editors), Sveriges Geologi FrHn Urti Till Nutid. Studentlitteratur, Lund, pp. 123-229. LindstrGm, M., Simon S., Paul, B. and Kessler, K., 1983. The Ordovician and its mass movements in the Lockne area near the Caledonian margin, central Sweden. Geol. Palaeontol., 17: 17-27. LindstrGm, M., Ekvall, J., Hagenfeldt, S., Siiwe, B. and Sturkell, E.F.F, 1991. Well-preserved Cambrian impact exposed in Central Sweden. Geol. Rundsch., 80: 201-204. Melosh, H.J., 1989. Impact Cratering. A Geologic Process. Oxford Univ. Press, New York, 245 pp. Simon, S., 1987a. Stratigraphie, Petrographie und Entstehungsbedingungen von Grobklastika in der autochthonen, ordovizischen Schichtenfolge JImtlands (Schweden). Sver. Geol. Unders. Ser. C, 815: l-156. Simon. S., 1987b. Caledonian deformation of basement in the Lockne area, Jiimtland, central Sweden. Geol. F&en. Stockholm Fiirh., 109: 269-273. StGffler, D., 1972. Deformation and transformation of rockforming minerals bv natural and experimental shock processes. 1. Behavior of minerals under shock compression. Fortschr. Miner., 49: 50-l 13. StrGmberg, A., 1976. A pattern of tectonic zones in the western part of the East European Plattform. Geol. Fiiren. Stockholm F&h., 98: 227-243. Thorslund, P., 1940. On the Chasmops Series of Jemtland and SGdermanland (Tvlren). Sver. Geol. Unders. Ser. C, 136: l-191.
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Von Engelhardt, W. and Bertsch, W., 1969. Shock induced planar deformation structures in quartz from the Ries crater, Germany. Contrib. Mineral. Petrol., 20: 203-234. Wickman, F.E., 1988. Possible impact structures in Sweden. In: A. Boden and K.G. Eriksson (Editors), Deep Drilling in Crystalline Bedrock. I. The Deep Gas Drilling in Siljan
185 Impact Structure, Sweden and Astroblemes. Springer, Berlin, pp. 298-327. Wiman, C., 1900. Eine untersilurische Litoralfacies bei Locknesjiin in Jemtland. Bull. Geol. Inst. Univ. Uppsala, 4: 133-151.