SEVERAL RECENTLY DISCOVERED SUPPOSED ASTROBLEMES IN DALECARLIA, SWEDEN

Version 3.0

SUMMARY

During the last years this author has identified several round structures of about 10 km diameter south of Lake Siljan in Dalecarlia (Sweden) in a band trending from SSW to NNE. Narrow lakes of circular shape, completed by circular trenches, often limit these structures. These structures very well could be astroblemes of their own, contemporary with the large Lake Siljan-astrobleme.

In favour of this interpretation are about hundred finds of impact-generated rock fragments in the moraine like: Shatter cones, impact melts, suevites, pseudotachylites and secondary impact-generated rocks. These finds are very similar to those from the Siljan-astrobleme. The possibility has to be admitted, that the impact-generated fragments could have been transported to the round structures by Holocene ice flow. However, there is a counter-argument: The impact-generated rocks are rather fragile! No such rock has been discovered in the very large (0,5 km²) gravel pit at Gräv (Pos. Am), which is at 45 km south of the centre of the Siljan-impact, in the direction of the ice-flow. On the other hand, plenty of impact-generated samples have been discovered at the same distance in the Lake Stora Flatten-astrobleme, which lies aside – far to the west – of the Sijan ice flow. This should be a proof of their local origin!

A further indication for the assumption that the round structures are astroblemes of their own, are large tilted sheets of rock in several of these, similar to tilted or vertical sheets of Ordovician carbonate within the Lake Siljan-astrobleme. In addition, there are two quarries within the supposed astroblemes, showing the shattered rock in situ at depth. Particularly the quarry in the Lake Långsjö-atrobleme (Pos. Aa) is important in this context. There the blasted rocks break along old cracks filled with a solidified melt, i.e. the old cracks are pseudotachylites. The position of the quarry is far too distant from the Lake Siljan-astrobleme to be affected by that. This strongly indicates, that the supposed Lake Långsjö-astrobleme really is an astrobleme of its own.

The final proof that at least some of these rings are astroblemes came on end of June 2008. It is at Lake Hummeln, described last in this report. There a completely different melt shows, that the found blocks could not have been transported there by the Holocene ice-flow, but must be of local origin from an impact at that site.

1
PREFACE

An astrobleme is the crater made on the Earth by a falling meteorite or comet. Strange enough, Europe’s largest astrobleme – the Lake Siljan astrobleme in central Sweden – has not been known before the 1960:s. Actually, the strange geology and botany of this region has been known for a long time, but not the reason for the ring dike of carbonate rock, which creates the fertile environment for ex. Orchides. Not before the mid 1960’s – mainly due to the research done by Professor Thorslund – the geological community recognised, that the Siljan ring dike was created by a very large meteorite, falling around 377 ± 2 million years ago /1/. This ring has a diameter of roughly 40 km; the towns most nearby are Leksand, Rättvik and Mora.

Satellite pictures over Dalecarlia disclose, that the regional tectonics of this landscape consist of parallel straight ridges and valleys, trending NNW to SSE. Due to this, round structures are easily recognised.

A series of 10 such round structures has been located south of Lake Siljan, trending about 67˚ from SW to NE. Due to finds in the field, this author is convinced, that most of them are astroblemes and has given them the name of the nearest town or lake (all containing ring trenches filled with lakes). With the exception of Fjärden he has visited all, see Fig. 0.

Fig. 0: Map of the region. Note the 10 blue rings south of Lake Siljan as sites of possible astroblemes. Grid size 25 x 25 km.

For the rings Stora Flaten-Snesen, Flosjön, Långsjön and Almosjön there exists strong evidence, that these really are astroblemes; within the supposed Långsjö-astrobleme there exist an active quarry, in which pseudotachylites and shattered bedrock is seen. Due to a possible transport of rocks from the nearby Siljan astrobleme by ice sheets during the Holocene, the evidence for the proposed Vådsjö-astrobleme is not as strong. The large Leksands- Insjö ring structure will be commented upon later on in this report.

2
WHY SO MANY RINGS?

Falling asteroids create large impact structures. Asteroids were the raw material for the formation of a planet to be between Mars and Jupiter, which, however, never coagulated to a single body like the Earth. To the best of our knowledge they can be regarded as a collection of cosmic “sand” or dust from exploded supernovas. The asteroids are large, but not large enough so that their own gravity should have compressed them to make them melt through; this limit occurs at about 1/10000 of the mass of the Earth, still at a formidable 5·10²⁰ kg.

When entering the atmosphere of the Earth at cosmic speed (≥ 11km/s) these bodies easily break up into pieces, which have the same speed and reach the surface of the Earth nearby. The probability that these ten bodies - lying so near by - are belonging to different falls is negligible. Since several of the supposed impacts – particularly the smaller ones – lie SW of the Siljan-astrobleme, we suppose, that the original meteorite approached the target from the SW.

The energy dissipated by a fall is (1/2)·mv². We neither know the mass, nor the velocity of an object; however, we can make a relative estimate of the total mass of the approaching asteroid in relation to the (unknown) mass of the Lake Siljan impact.

At the same speed of all the objects it is reasonable to assume, that the mass of the excavated volume of rock a·r³ is proportional to the mass of the colliding object (r = radius of the astrobleme, in km). The calculation indicates, that for the Lake Siljan astrobleme we obtain for a·r³ = a·8490 and for the sum of all the other ten supposed astroblemes a·r³ = a·2061. Thus the mass of the Siljan astrobleme still is more than 4 times larger than the added mass of all the other supposed astroblemes.

3
BEDROCK OF THE TARGET REGION

The present surface bedrock is as follows: The central part of the Siljan uplift and of all astroblemes to the west of Siljan and of the Ljugaren region consist of younger serorogenic granites.

Leksands and Balungen astroblemes: Serorogenic granites and leptites (metamorphosed ashes from volcanic eruptions 1700 million years ago).

At the time of the “fall” an Ordovician carbonate layer covered the present bedrock, overlain by several kilometres of soft Devonian sediments. This carbonate can be found here and there as inclusions in remobilised granite. Currently this carbonate sheet is eroded away with the exception of xenolithes, preserved in the ring dike of the Siljan-astrobleme under “fallback” breccia.

4
EFFECT OF THE IMPACT ON THE SURFACE

The falling meteorite has very many effects on the at that time existing bedrock, which help us to identify a site as an astrobleme. However, there is one overall effect, which should be discussed on its own right.

During the impact, shock pressures of >100 GPa = 10⁶ bar and temperatures >3000˚C are generated, the latter mainly due to adiabatic compression /2/. This is the same effect as compressing air in a bike pump. When compressed, an ideal body is heated – but also cooled, when decompressed. Unfortunately bedrock is not an ideal body: It is heated by adiabatic compression (a reversible process), but also by friction between blocks of rock, sliding along one another. During this process heat is produced, also, but not in a reversible way: This heat stays within the rock and is relieved by conduction, only. This latter is a slow process.

Assume a cylinder of granite, with a diameter and height of 1 km, which had been heated to 1000˚C right through. The cylinder perimeter is artificially held at 0˚C at all time. Still after 580 years a central spherical part of 250 m diameter will be at >800˚C /3/. The boundary conditions used here are completely artificial; at more reasonable boundary conditions (determined by an in time decreasing heat flux at the perimeter) the cylinder will cool much more slowly. High temperatures near 1000˚C will prevail for ten thousands of years, which implies, that there is plenty of time at high temperature in a water saturated environment to form new minerals and new rocks from the debris of old ones. This author has found 5 cm large euhedral microcline crystals in reconstructed granite in the supposed Leksand astrobleme. In the original granite the microclines are ahedral and only some millimetres large.

5
METHODS OF FINDING SITES OF EVIDENCE

The topography often gives the first hint for the presence of an astrobleme. Lakes are not straight, but they appear as a boudin following the isohypses on the map, which latter often perpetuate the curved form of the lake. In the Flosjö astrobleme the lakes (two in series) and a dry valley encircle the astrobleme by about 220 degree.

In a forested landscape like Dalecarlia there is plenty of boulders in the forests. However, often they are heavily pitted or weathered and/or covered by moss and lichen. Thus it is hard to see, what is below the cover of the weathered surface and plant cover. In this case it is easier to check amelioration cairns in the fields. There the farmers have collected boulders discovered during ploughing and you get some feeling for the relative occurrence of the samples, you are looking for.

At granite sites the carbonate-prefering hepatica does not occur. If you nevertheless find hepatica and other carbonate-preferring plants, it is a good sign for the presence of carbonate, which in Dalecarlia means, that boulders, containing carbonate, have been cast there by the impact or later transported there by ice from a nearby astrobleme.

“Look for impact-related samples in gravel pits and in quarries, too.” The Flosjö-astrobleme has been found by this method.

6
RECOGNITION OF ASTROBLEMES

Astroblemes are disclosed by the physical and chemical traces of the impact. In the case of the Dalecarlia astroblemes we have to consider that the meteorites have fallen upon a ground consisting of carbonate, which may have given rise to particular reconstructed minerals in the debris. In the following a collection of identification features is given:

Numerous textures, structures and formations of different rocks have been found in many astroblemes; for this reason this author would like to suggest the following generic subdivision, exceeding that from /2/:

The rock type 2.3 has to be explained: We can assume, that at the site of the impact there was plenty of water present. Large amount of water, powdered rock and rock fragments have been cast out of the new crater. Immediately afterwards, together with the sediments, the water flowed down to sites at lower level. Subsequently later, slumps in the instable sediment formed rocks, similar to those found in subaquatic slumps. This type of rock has been found in the Lake Flosjön-astrobleme.

7
DESCRIPTION OF THE INDIVIDUAL ASTROBLEMES

7.1
THE LAKE SILJAN ASTROBLEME

To obtain tokens for the recognition of all the other minor astroblemes we first must discuss the finds from the Siljan- astrobleme. If these are accepted as consequences of a meteorite impact, similar rocks and features at other places have to be accepted as a proof of an astrobleme for that site, too. Here we have to be aware of the fact, that blocks of Siljan-material may have been transported by Holocene ice to the present site.

During exploration for Natural Gas twenty years ago in the centre of the Siljan ring an outcrop has been discovered (Pos. A), consisting of the solidified impact-melt. Unfortunately this author does not know, who has made this discovery and if it is published. For this reason it is here unsuitable to mention the coordinates of that outcrop.

Fig. 1: Impact-melt breccia from (Pos. A).

In its most homogeneous part this rock resembles a dark-brown Dalecarlia-porphyry, which should have been overlooked, when found as a loose boulder in the till. Of course this outcrop should not be touched. However, there exist plenty of boulders of nearby origin in the local till. Some can be regarded as suevites 6.12, others as impact-melt breccias 6.11. In these latter, thermally and mechanically damaged granite is floating within the brown melt (sometimes glass is used for melt). Here we have a large difference to porphyry: Phenocrysts in porphyries are monomineralic; here the melt contains fragments of the target rock, consisting of several minerals (Fig.1). This is the final proof that the Siljan ring is an astrobleme.

Fig. 2: Gliding in sheared granite (Pos. B).

At (Pos. B), a gravel pit on the road to the lake edge at Garsås, one can collect all types of impact-created rocks: Melt, completely crushed and reconstructed granite (Fig.2), pseudotachylites in granite (Fig.3) and an impact rock, metamorphosed by steam (Fig.4).

Fig. 3: Peudotachylites in sheared granite (Pos. B).

During the 1960:s the only evidence for the astrobleme hypothesis was the ring trench, shattered and tilted sheets of carbonate rock from Ordovician and shatter cones in the centre of the structure. Since then it has been shown, that shatter cones are not at all so rare, but can be found here and there, e.g. in the till at the edge of the Lake Siljan near the village Stumsnäs, there at a stone pier (Pos. D). Some 100 m NE of that point there is an outcrop on the shoreline (visible only at low water level), consisting of crushed granite in carbonate-silicate slurry.

Fig. 4: Weathered melt, probably exposed to steam (Pos. B).

In addition, there is a new site of crushed and quartz-annealed granite at the Lake Siljan edge, belonging to the village Garsås, (Pos. C). Annealing of meteorite-damage by quartz differs from that of structural movement, since in the first case the boulders are criss-crossed by the quartz-veins, whence in the other case there is only one (1) direction of quartz annealing. At (Pos. C) there are so many boulders that the outcrop has to be close; best it is seen at low-level water. At this site one can find:

• completely smashed granite
• a reconstructed rock, consisting of quartz
• quartz formed in situ with asphaltenes in cracks
• shatter cones

In reference /4/ Liljequist gives a further compilation of structures/textures, created by the impact of a larger meteorite.

Near (Pos. C) is a pier at (Pos. Aq), built up by boulders, dragged from the nearby waters. Here boulders up to 500 kg of pure white quartz are seen, showing an unusual rectangular cleavage. Since in the local bedrock pegmatites are not known, the origin of the quartz and their cleavage is not known.

Another, easily accessible site is the shore of Lake Siljan at the church of Rättvik, (Pos. Ar). There a cross is raised on an outcrop of the preserved Ordovician limestone. However, here the sedimentary limestone has been crushed and welded together again, so that all traces of its sedimentary origin (fossils, discontinuity surfaces) have disappeared. The shore below the cementary contains a lot of other impact- damaged rocks, too.

Here this author would like to add additional information on particular sites according to:

Ring-shaped trenches, waterfilled and/or dry (6.1):

• Lake Flosjö

Tilted plates (6.3):

• In the Siljan astrobleme: Carbonate quarries at Amtjärn, Skålberg and Osmundsberg
• In the Leksands astrobleme: Granite at Käringberget and several similar sheets north of Käringberget
• In the Almo astrobleme: The island Storön in the Almo lake

Note: Tilted plates not necessarily occur at the rim of the crater, only. For example, there is a limestone quarry at (Pos. An), with the former horizontal carbonate beds now upraised to vertical position. This site is only 12 km distant from the centre of the Lake Siljan-astrobleme.

There is no report from the Siljan astrobleme of large fields of local giant boulders (6.5) in the moraine; however, these are found in the Stora Flaten- and the Flosjön-astrobleme.

7.2
THE LAKE STORA FLATEN-ASTROBLEME

During the spring 2005 this author has found evidence for, that the ring north of the village Dala-Järna, including the lakes Stora Flaten and Stora Snesen could be an astrobleme. This ring has a diameter of 10,5 km, measured between the outer rims of the ring dike. Its centre is at (Pos. E). To reach interesting sites take the forest road between the lakes Stora Snesen and Åskaken, which starts from the public road between Leksand and Dala-Järna at (Pos. F) and reaches after 3 km the south end of the minor lake Långtjärn. East of the road you are within a large area, where one or two years ago the forest has been cut down and the undergrowth burned away. The fire has removed all brush and cleaned the boulders. Here again it is easy to find similar samples as in the Lake Siljan-astrobleme like:

• Shattered granite, cured by criss-cross of later quartz veins (Fig. 5)
• Impact-melt breccias, within which fragments of granite float in the melt (Fig. 6)
• Boulders, resembling pyroclasts (Fig. 8)
• Shatter cones in moraine boulders

Fig. 5: Crushed granite with innumerable cracks, cured by quartz filling.

Fig. 6: Impact-melt breccias. The elongated sample is from (Pos. G), the round one from the centre of the Siljan-astrobleme.

Fig. 8: The sample resembles sintered volcanic ash (Pos. G).

At (Pos. G), near a dead birch, there lie two boulders. One of them is very similar to the impact-melt breccias from the Siljan region (Fig. 6). The elongated sample is from the Stora Flaten-astrobleme, the rounder from the Siljan region at (Pos. A).

Fig. 7 shows a block found in a small gravel pit north of the northern end of Lake Långtjärn (Pos. H). The melt in this block is darker then that from the Siljan region. The white and grey inclusions in this sample are calcite (Note: This is not a late filling of cracks by a solution, but mechanically agglomerated calcite grains). In other samples, not shown here, there is more calcite. How can calcite enter a “granite”, if not mechanically?

Fig. 7: The sample contains a darker melt than that from Lake Siljan and some agglomerated calcite (Pos. H).

In the same pit there is a larger – not transportable - boulder with quartz curing and dark glass. Note that the everywhere present boulders of the local granite are quite different from our astrobleme samples.

The other boulder from the dead birch (Fig. 8) has another character, not found in the Siljan-region. It resembles a pyroclast like one from the Azores or from south of Rome. It consists of small granite fragments in a yellow ash and probably is a suevite.

Please leave these bolder untouched as examples for other visitors. There is no problem to find similar samples on the burned spot.

Another site for finds of impactites is a recently built forest road, which starts at (Pos. I) and leads to the west. At the end of that road (Pos. J) you can find impact-melt breccias.

One can wonder, why impactites have not been found within the ring yet, but outside the ring at Långtjärn. There are some low hills, may be of 30 m height above lake level, to day completely covered by till. We may assume, that these hills contain a core of impactite-material, from which the small boulders are derived.

Some readers may suggest, that the samples found at the Lake Långtjärn may have come there by icedrift from the Siljan-ring. This cannot be the case: The ice flow direction from the centre of the Siljan astrobleme passes Långtjärn about 40 km east. Besides, the impactites are very fragile and could never stand such a long journey.

Lake Stora Flaten forms the northern branch of this supposed astrobleme and Lake Stora Snesen its eastern part. Inside this ring, but also outside, there are extended fields of large local moraine boulders. These are very little worked by transport. Most of the boulders are several m³ large; the largest one at is about 50 m³ large. We believe, that the bedrock has been broken up during the impact; the fractures certainly have been annealed by quartz. 377 million years later, under the Holocene ice cover, the old fractures have broken up again and the boulders form these large fields, see (Pos. K).

Another site for finds from this astrobleme is an old gravel pit at the northern end of Lake St.Baggbod-Öradstjärn at (Pos. L). This place is south of the Lake Stora Flaten astrobleme; most probably the impact specimen have come there transported by ice.

A further site is a gravel pit at (Pos. As). There reconstructed granite, sealed by a dark melt, is to be found.

7.3
THE LAKE FLOSJÖN ASTROBLEME

If you should study the map sheet 13E SO Vansbro more carefully, you would discover that Lake Flosjön is a part of a ring trench with a perimeter of about 220˚ with its centre at (Pos. M) and a radius of 5 km. In the east this ring passes the Harpick-island in the Lake Flosjön and the hill Tutberget on that island (Pos. R). Tutberget is nothing else than a tilted sheet, similar to Käringberget in the town of Leksand. It rises about 27 m above the level of the lake, with strike 20˚E and a dip of 60 to 80˚E. Another indication for an astrobleme is the accumulation of large local moraine boulders in the centre of the ring at a location named Trolldalen and at other locations near the ring.

With the previous knowledge from the Stora Flaten-astrobleme this author has investigated a nearby location outside of the ring, i.e. the foot of the hill Bodberget near the Alpine pasture Forsbodarna (Pos. N). According to rumours hepatica should grow there. Indeed, it was found there in a granite region on a saddle surface between Budberget and Forsbodarna. In the amelioration cairns -besides normal granite - boulders were found, very similar to impact-melt breccias. These contained isolated clusters of free calcium carbonate (no late filling of cracks), which explains the occurrence of hepatica and other carbonate-prefering plants (Fig. 9).

Fig. 9: Plenty of dark glass in this sample. The white dots are calcite and a calcium-silicate (Pos. N).

Along a recently built forest road N of the summit of Budberget in the valley of the brook Nordanbergsängs-bäcken (Pos. O), there are further boulders with cracks, filled with pseudotachylite.

However, the richest site for finds is quite near at (Pos. P), which is a recently ploughed area, where the trees have been cut down. It is about 100 m south of the turning for Forsbodarna. There almost every block or boulder is affected by an impact; mostly you will find secondary impactites of type 2.3 of our classification. Fine-grained material seems to have been collected at a particular site and later sledded downhill, forming a rock, similar to subaquatic turbidities, see Fig. 10, 11 and 12.

Fig. 10: Nice sample of water-transported secondary impactite (Pos. P).

Fig. 11: Weathered and water-transported fine stuff (Pos. P).

Fig. 12: Layer of tumbled granite pieces on fine stuff (Pos. P). On downside weathered melt.

Likewise there is a dark red metamorphic rock, consisting mainly of red microcline and small amounts of epidote, almost free from quartz, Fig. 13. Despite this rock is similar to red porphyry, it is free from a glass phase. Similar red microcline is formed in fracture zones from the stone powder. Here and there on the sites there are boulders of the similar Garberg granite or Garberg porphyry from a site about 75 km NNW, and for this reason the red metamorphic rock could be a fraction from Garberg. However, this red metamorphic rock is found at all sites of impactites, only, which makes the Garberg origin somewhat questionable. One can imagine that a hot-water fluid had leached powdered granite. The fluid has removed all quartz and left behind the potash-feldspar powder, which later on metamorphosed to the red rock.

Fig. 13: Very dark red microcline like that formed from mylonite in a slip cracks. The dark dots are rounded grains of quartz.

A further location for finds is the centre of the Lake Flosjö-astrobleme at Trolldalen. There, in addition to giant scale boulders, at (Pos. Q) an old gravel pit with heaps of discarded stones and boulders occurs. This material – as everywhere in gravel pits - is water-transported to the very site, therefore not typical for the bedrock just below. There exist stones of the local granite, which evidently have been crushed to debris, but cured by glass to a new resilient stone, se Fig. 14 and 15. The glass content varies between 10 to 90%. The rock is hard, not equally disintegrated as the impacted granite from the Lake Siljan astrobleme. At this site a stone was found containing two balls of glass, of hen’s egg size, see Fig. 16a. Since before the impact, there are no cavities in the granite, which could be filled with the liquid glass, the granite during the impact must have been disintegrated to grains, around which the glass has been pressed in; subsequently this mass has solidified to the resilient rock, we find to day.

At the same site (Pos.Q) another stone has been found, which – after deliberate crushing – showed grains of granite floating in a dark-brown melt, see Fig. 16b. There are white grains in the mixture, too, which proved to be calcite. How can calcite enter reconstructed “granite”, if not mechanically after the original granite has been shattered to small fragments?

This site is near the centre of the impact: Its force there has acted vertically downwards and had only a small component of sheer aside, which is found in more peripheral locations. This could be the explanation for the resilient glass-impregnated granite.

Fig. 14: Sawed plate of restructured granite from Trolldalarna (Pos. Q). The dark filling is glass; these fillings are interconnected.

Fig. 15: Glass in hard rock from Trolldalarna (Pos. Q).

Fig. 16a: Egg-size drops of glass on reconstructed granite, Trolldalarna (Pos. Q).

Fig. 16b: Mechanical calcite inclusions in reconstructed granite, Trolldalarna (Pos. Q).

Another site of rich occurrence s is a slope at (Pos. V). There and even at (Pos. G) two very strange blocks have been found; without doubt these are conglomerates.

In the Siljan-area there exist conglomerates at at least two locations: At Lake Djurssön and at the village Söderås, 2 km S of Rättvik. These conglomerates are ordinary conglomerates, containing rounded stones of different nature in a sandy sediment as matrix.

The conglomerate from (Pos. V and G) is completely different from the ordinary conglomerates: The round inclusions consist of an amorphous material like flint, but contain round cavities like gas-bubbles. These are partly filled with crystals. One gets the impression of a solidified, devitrified glass with gas inclusions, Fig. 17 and 18. Fig. 18 is the rear side of the right sample in Fig. 17.

Fig. 17: Conglomerate consisting of balls of flint-like stuff in a matrix similar to impact-melt.

Fig. 18: Rear side of right-hand sample in Fig. 17 showing the flint-like stuff.

The matrix of the conglomerate is the brown stuff like in Fig. 4. Could this be some late agglomeration of original impactite material? Ordinary conglomerates do not contain balls of amorphous material with gas bubbles!

Two kilometres south of (Pos. V) there is another one (Pos. W) within a small, protected area – Björberget - that is extremely rich in hepatica. This site is so far away from the Lake Siljan astrobleme that in this granite a local source of carbonate must exist, either a residue of the original cover of carbonate or boulders of carbonate, transported there. Transport from the Siljan-astrobleme is extremely improbable. A thick cover of soil makes the excavation difficult.

Tachylites are found, too, along a forest road in a very fine-grained granite at (Pos. Ao).

All these sites and occurrences, also the tilted granite sheet on the Harpick-island, makes me completely convinced of the astrobleme origin for this ring.

7.4
THE LAKE LÅNGSJÖ-ASTROBLEME

The Lake Långsjön-astrobleme has its centre at (Pos. X) and a radius of 4,25 km. Roughly 180˚of its perimeter consists of the lake or of lowland with parallel isohypses. The whole area is covered by forest; for this reason searching for impactites is very difficult. Up to now nothing has been found.

However, about 1 km inside the perimeter there is a quarry at (Pos. Aa), which is very rich of pseudotachylites (here thin layers of a brown melt or a brown precipitate between blocks of bedrock, that have sledded on one another), of pieces of shatter cones, of now exposed surfaces, where the sliding can be seen and of quartz-cured crushed granite, see Fig. 19a. Note the melt phase between the two quartz veins. Evidently later on the blocks aside the tachylite have separated somewhat allowing quartz-saturated water to penetrate the crack. Tectonic movement is far too slow to generate so much cracks and glass and does not generate crushed granite, which later on is sealed together by quartz solutions in water.

Fig. 19a: A melt phase between the two quartz veins.

During blasting in the quarry the rock sometimes separated at old cracks, thus freeing the tachylite. This is normally 1 mm thick, can reach up to 5 mm. In such tachylites small grains of other rocks are seen, indicating that this melt has been pressed in from a distant position. A prerequisite for this mode – distant transport – requires the rock to be hot, i.e. heated by adiabatic compression. See Fig. 19b, 19c and 19d.

Fig. 19b: Layer of pseudotachylite, quartz-rich side, 5 mm thick.

Fig. 19c: Same pseudotachylite as in figure 19b, the rear side rich in melt.

Fig. 19d: Two pseudotachylites about 5 mm thick.

There exists much evidence for that the bedrock in the quarry has been affected by an impact. This could be an impact by itself (the Långsjö-astrobleme) or due to the damage done by the Siljan-astrobleme. Concerning this latter hypothesis we have to look for positions of the same damage at similar distance from the Siljan-astrobleme. Disregarding the damage to the bedrock and its tilting inside the ring there is evidence of tilting in the exposition Kårgärdet, about 1,5 km NE of the Orsa railway station. Here the Ordovician limestones and the local porphyry are raised up to 70˚; the distance to the centre of the Siljan-astrobleme is 18,7 km. The next expositions to the east – the limestone quarry Kallholn – is at the same distance, but practically not affected. The last exposition at Kallmora is at a distance from the centre of 19,2 km. There, a fine-grained sandstone (Orsa-sandstone) from upper Silurian, is not affected at all.

The quarry at the supposed Långsjö-astrobleme is so far as 37,5 km from the centre of the Siljan-astrobleme. If we cannot find additional sites of damage at a similar distance due to the Siljan-astrobleme we have to assume, that the Långssjö impact is an astrobleme of its own. This in turn strengthens our hypothesis that several of the other investigated supposed astroblemes are in fact real astroblemes.

Within this supposed astrobleme there are two further sites of finds. The one is a forest road north of Gäddtjärn with Coordinates according to (Pos. At). There in the sand of the road filling stones affected by an astrobleme are found. Near the road very many giant boulders are seen.

The other site is a gravel pit (Pos. Au), which most probably has furnished the gravel to above road. There are so many boulders in the gravel, so they had to be blasted. These rocks look like reconstructed granite with constituents, not normally seen in granite.

7.5
THE LAKE VÅDSJÖ-ISRAELSSJÖ ASTROBLEME

The centre of this supposed astrobleme is at 13 km west of the church at Siljansnäs at (Pos. S); its radius is 3 km. During the first inspection this author reacted upon the rich vegetation along the small river Långsån, which indicates carbonate. This could be due to local residues of the former carbonate cover or material from the nearby Siljan-astrobleme, conveyed there by the Holocene ice. At (Pos. T), in a gravel pit, a large fraction of the boulders is affected by an impact. Pure melt is rare, as shown in Fig. 20; Fig. 21 shows the cracked and quartz-cured rock. However, in a nearby large gravel pit at (Pos. U), about 3,5 km south of (Pos. T), impact-affected boulders are missing. Therefore those from (Pos. T) can be local.

Fig. 20: Canal of melt through granite. The thin white band is a late quartz impregnation along the old fracture zone (Pos. T).

Fig. 21: In all directions cracked and quartz-cured granite (Pos. T).

A further site for finds is a forest road, leaving a larger forest road at (Pos. Y) and climbing up to its end at 340 m height. Along this road impact-affected material can be found. One find is a block of brown coloured (former) quartz jelly, which contains silicified pieces of the stem of a crinoidea, Fig. 22.

Fig. 22: Silicified pieces of stems of a crinoidea in quartz-rich matrix.

Fig. 23: Metamorphosed melt (Pos. Z).

A further site of finding is a gravel pit at (Pos. Z), where only one interesting piece of a metamorphosed melt has been found, Fig. 23. This piece could originate from the Lake Siljan-astrobleme.

At Lake Vådsjön there starts a canal for water transport to a hydropower plant; it is cut out of the rock. May be, samples of the local rock could be found there. Up to now the nature of this ring is not completely clear.

7.6
THE SILJANSNÄS-ASTROBLEME

South of the village Siljansnäs there is a lake communicating with Lake Siljan. It is almost divided in two by an island; the western side of the lake is called Alviken, the eastern side Byviken. Around this lake there is a chain of hills. If it wouldn’t be for the rich occurrence of carbonate-preferring plants one might have overlooked this supposed astrobleme and never investigated it. However, even this ring seems to be an astrobleme with radius 1,2 km. The evidence for this is the island, called Storön. This island is elongated, length about 2 km, heading 20˚W, forms a large sheet with strike 20˚W and dip 20˚E. May be one would not react, would it not be for the cliff at its east side. This is about 30 m high and absolutely vertical, called Digerberget. Here we have a tilted sheet, like “Käringberget” inside the town of Leksand.

Fig. 24: Pseudotachylites at (Pos. Ab).

At (Pos. Ab), in an amelioration cairn, there are stones with undeniable traces of pseudotachylite, Fig. 24. A further site for finds is near the village Siljansnäs in an amelioration cairn (Pos. Ac).

Since the large Siljan astrobleme is too far away to tilt a sheet, we have to assume, that even the Siljansnäs-ring is an astrobleme. Boulders with impactite marks can have been transported there to from the Siljan-astrobleme, but not the tilted sheet in the local lake.

7.7
THE LEKSAND-ASTROBLEME

From Lake Siljan there runs a narrow bay like an estuary to the town of Leksand. From the village Östanhol to Leksand this bay has a curved rim. Southwest of Leksand this depression in the terrain continues into the next Lake Insjön. During a late phase of the most recent Ice-age, when the level of Lake Siljan was about 8 m higher than to day, there was a continuous bay from Östanhol to the east end of the Lake Insjön. This corresponds to roughly 180˚ of the whole perimeter. This was the first indication, that there might exist a further astrobleme, here called the Leksand-astrobleme. Its centre is at (Pos. Ad) and the radius is 8,4 km.

The next indication is a series of tilted sheets in an area north of Leksand. The first and most impressing one is that within the town of Leksand, named Käringberget (Pos. Ae). The name is associated with the burning of witches in the 1670:s at that place. In fact, the place is unusual even from a geological point of view. From the former execution place (to day a parking lot) and 10 m to the west one reaches the brink of a formerly horizontal plate of granite, now dipping at 60˚W and striking 30˚W. The top of this plate is about 75 m above the level of the railway east of it, i.e. above the valley level; the foot of the plate disappears in moraine boulders, see Fig. 25.

Fig. 25: Tilted sheet of Käringberget, west side.

Another, similar tilted sheet, is at (Pos. Af), height about 25 m above surrounding ground. There are additional similar and minor cliffs north of Käringberget, all parallel to the supposed ring trough. It is hard to believe that these should have survived since the latest large folding epoch, which occurred during the Paleoproterozoikum. However, they could very well have been caused by a meteoric impact.

Therefore searching for impactites started. At South Torrberg several such have been found in an amelioration cairn at (Pos. Ag), pseudotachylites and a reconstructed boulder with 5 cm large potash feldspar crystals. At North Torrberg there is a larger boulder in an amelioration cairn, containing many thin pseudotachylites (a brown melt), (Pos. Ah).

Another site is east of the former railway station Slättberg. There exists older granite, only. Parallel to the public road there is a forest machinery track. These vehicles have crushed the granite by their load; the fracture is often along former pseudotachylites. Here these are very thin, around 1 mm, filled with brown melt. This granite is local, no other (ice-transported) rocks exist (Pos. Ai). Therefore the tachylites also have to be local.

The most interesting place is a quarry near the road Rättvik-Falun at (Pos. Aj). It is situated on the rim of the supposed astrobleme. In this quarry for macadam all types of rock exist: Granite, a rock consisting mainly of coloured quartz and a black dike, to day consisting of chlorite slate (might have been a dolerite dike). Also a few pseudotachylites occur, see Fig. 26. The distance to the centre of the Siljan-astrobleme is 25 km. The quartz is of a glassy variety, clear but dyed, hard and brittle like pegmatite quartz. Pseudotachylites are few, probably because they have been destroyed later during their life. There is a brown rock, looks like porphyry, with inclusions of cm-large fragments of the chlorite slate. To this author it appears, that the “porphyry” is not at all porphyry, but consists of brown-dyed quartz, which has absorbed fragments of the chlorite slate, see Fig. 27.

Fig. 26: Pseudotachylites at (Pos. Aj).

Fig. 27: Brown dyed quartz with inclusions of chlorite slate (Pos. Aj).

Most probably we are looking here inside the bedrock on a portion, which had been very hot for thousands of years and percolated by steam. This has broken down remainders of feldspar and changed the dolerite dike to chlorite. The question is, if this is due to a local astrobleme (the Leksand-astrobleme), or to the nearby Lake Siljan-astrobleme. The distance from the quarry to the centre of the Siljan-astrobleme – as we define it – is 25 km.

In summary: The sheets of rock, tilted near Leksand and the ring dike west of Leksand point towards an astrobleme of is own; the rocks in the above named quarry point very strongly on heat effects from an astrobleme, but not from which.

7.8
THE SUPPOSED LAKE LJUGAREN-ASTROBLEME

On the topographic map 14F SO Rättvik the lakes Dådran and Ljugaren form a part of a ring-depression. The bedrock map of the Kopparberg County shows – somewhat displaced to the north – a circle of the same size, containing rapakivi granite in the east and a very coarse syenite in the west. The rapakivi consists of several cm-wide potash-feldspar crystals, mantled by plagioclase. This rock weathers extremely easy to loose grains. The syenite consists of dark red potash feldspar of up to 5 cm grain size. No pegmatites have been found in any of the two rocks. At (Pos. Ak) there are large-scale shatter cones to be seen in the boulders on the beach and dark read pseudotachylites. Such large potash feldspar crystals have been recognized at other sites of supposed astroblemes, which might have been developed out of rock powder, like they develop in mylonites. Due to the combination of shatter cones and pseudotachylites this site is worth of further investigation.

7.9 THE SUPPOSED LAKE BALUNGEN-ASTROBLEME

Despite its evident ring shape, consisting of several lakes, up to date only one single find has been made which relates to a supposed astrobleme, viz. a piece of a soevite at (Pos. Al), unfortunately in an esker; this means, that the sample could emanate from anywhere. More research is necessary.

7.10 THE LAKE HUMMELN-ASTROBLEME

Due to very rich finds in the field the prefix ‘supposed’ has been avoided here. There is no doubt, that this astrobleme is a real one. This supports the assumption that several or all of the other astroblemes are real, too.

In Sweden large lakes have only one name, i.e. not the prefix ‘lake’. Examples are Vänern, Vättern, Siljan, Dellen, Mien and Hummeln. The last four names are well known astroblemes. Hummeln is a lake 15 km NW of Oscarshamn. Our Lake Hummeln (Hummelsjö) is a small lake 8 km SW of the town of Vansbro in Central Sweden and 75 km SW of the centre of the Lake Siljan-astrobleme. The similarity of the name and the fact that this lake with a round form lies straight on the line of impact of all the other here described suspected atroblemes made this author interested.

The lake is semicircular, with the round shore to the north. The diameter of the astrobleme appears to be about 1,3 km. It is reached from state-road 71, turning on a forest road, which starts 100 m west of a railway viaduct, before the village Rågsveden. The forest road heads to the east. After about 5 km at (Pos. Ap) one is in the middle of a large deforested area at the highest point of the road. There from the whole field of prospecting can be overlooked.

NE of that point is an area that has been cleared by fire from shrub and vegetation on ground. There all boulders are clean from lichen and mosses. Almost all boulders have a reddish-brown appearance, most of them are affected by the astrobleme. The unaffected granite there has a grain size of about 5 mm with pale-orange potash feldspar, white albite and clear quartz in same proportions. The affected stone shows pseudotachylites, which often consist of three layers: The middle layer consists of the brown melt, boarded by later layers of quartz. This damage is quite easily found.

Another type of damage is a rock looking like a yellow volcanic ash. Its constituents are mainly quartz grains of < 0.5 mm and some much larger rounded quartz grains. The rock is similar to that from the Lake Stora Flaten-astrobleme, there Fig. 8.

Another type of rock consists of potash feldspar and quartz. The feldspar is deep orange, does not show cleavage, looks like relatively fast-reconstructed granite. A further type of rock is not very common there. The same type has been found in the Lake Flosjö-astrobleme and is shown in Fig. 13. On the first sight it looks like red porphyry, but it is not a porphyry, because completely free from free quartz and therefore a syenite. Its texture looks by free eye as a melt; at 10 times magnification one sees that the matrix consist of inter-grown feldspar crystals with local cleavage. The colour is deep red-brown, like microcline from mylonite. A tentative explanation of this rock is the following: The impact of the falling meteorite has pulverised the bedrock and created high local temperatures. Steam leached this slurry, removed quartz (therefore so much quartz annealing at other sites) and left the pure feldspar slurry to recrystalize. This red-brown rock is found at sites of astroblemes, only. The hypothesis explains, too, the heavy occurrence of free quartz at (Pos. Aq) in the Siljan-astrobleme, where blocks of several hundreds of kilograms of pure milky quartz can be seen.

Fig. 28: Pink grey parts - melt, light parts - damaged original granite (Pos. Ap).

Fig. 29: Red-brown melt and reconstructed granite (Pos. Ap).

The most convincing evidence for an astrobleme are blocks rich in melt (see Fig. 28 and Fig. 29). They occur scanty, but can be found. The one this author has found contained very much melt, which is completely different from that at other sites. Due to a rich content of tiny black particles the melt is gray. The particles – size some tenths of a millimetre – look like hematite, but are magnetic (= maghemite). Some parts of the quartz layers are dyed pink like by erythrine. Black dikes appear, too. Having compared the melt from Lake Siljan och that from Lake Hummeln nobody can claim, that they are identical. Therefore the latter must be genuine and autochtonous, which proves, that Lake Hummeln is an astrobleme of its own. Possibly – due to its small size – the falling body did not evaporate completely and therefore the chemistry of the impact process is another here; we see here remains of the falling meteorite.