SEVERAL RECENTLY DISCOVERED SUPPOSED ASTROBLEMES IN DALECARLIA, SWEDEN

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SUMMARY

After having studied the well-known Siljan-astrobleme for several years and collected samples at a lot of up to date unknown sites, this author has identified several round structures of about 10 km diameter south of Lake Siljan in Dalecarlia (Sweden), in a belt trending from SSW to NNE. Narrow lakes of lunar 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. Stony meteorites have the tendency to break up when passing the Earth’ atmosphere. In this case they fall down in a belt, the smallest first, larger ones later down the road.

In favour of this interpretation are about several hundreds of finds of impact-generated rock fragments 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, this is not possible: The facts for this statement are discussed in Chapter 8.

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-astrobleme (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 two completely different melts (different from Lake Siljan melts) show that the discovered blocks could not have been transported there by the Holocene ice-flow, but must be of local origin from an impact at that site.

In samples from at least two supposed astroblemes several cubic-centimetres-large pieces of Ordovician limestone have been found within reconstructed granite; these are pieces of calcite, mechanically thrown into the slurry, which later formed the reconstructed granite. These limestone pieces are not later veins of dissolved calcite. In normal granite calcite never can occur.

All these round structures are worthwhile a thorough investigation. A strew-field of possibly ten large astroblemes is to the authors’ knowledge not known and would be a geological sensation. Up to date the Swedish Geological Community like universities, museums, the Swedish Geological Service and others have not shown any interest to follow up these finds. Hopefully foreign universities or researcher will show more interest.

The several years lasting work with the Siljan astrobleme and related astroblemes has led to understanding of the origin and forming of the Orsa sandstone. This is made up of the mater, ejected from the astrobleme crater.

This author will be pleased to guide foreign researchers to the sites.

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 ± 2million 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 thereexist 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 Leksand-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 supposedastroblemes 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 an unknown thickness of Silurian and 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.

The impact of the Siljan meteorite occurred 377 milj. years ago on Devonian sediments. Below these Silurian and Ordovician sediments have been deposited. The thickness of the lowest layer – from Ordovicium - has by prof. Thurslund been estimated to 130 m; in total the whole stack of sediments must have been at least 200 m thick, probably much more. Thus the rim of the Siljan impact crater mainly consisted of soft sediments that to day are gone. Also the undamaged layers (outside the impact area) have up to date disappeared.

The conclusion of this statement is that we today are looking on a deep level of the crater, probably at its deepest point, where large quantities of melt have collected. There exist estimates of the thickness of the former Ordivician, Silurian and Devonian sediments at the instant of the impact between 130 meters to 2 kilometers /5/, /6/, /7/, /8/, /9/ and /10/.

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 reasonableboundary conditions (determined by an in time decreasing heat flux at theperimeter) 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-preferring 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:

  6.1 Ring-shaped trenches often filled with lakes.
  6.2 Shatter cones: Conical striae on broken pieces of the basement rock, emerging from the apex of the cone, length up to 0,5 m.
  6.3 Within the crater there exists large tilted sheet ridges of the basement rock, with dimensions in the range 100 to 200 m along the cliff and 30 m above present ground, dip of sheet 20 to 80 degree.
  6.4 Microscopic damage to grains of quartz and feldspar (PDF = Planar Deformation Structures) /2/. Micromounts and the petrographic microscope are needed for inspection.
  6.5 Particularly for the Dalecarlian astroblemes: Large fields (0,5 km2) of boulders of local shattered bedrock, not transported by the Holocene ice, not rounded. These fields are shown in the local topographic map. It is believed, that during the impact the granite cracked at more distal positions; however, these cracks have been cured later on. During the recent ice age these cracks broke up again; the blocks are lying more or less in situ.
  6.6 Particularly for the Dalecarlian astroblemes: Mechanical inclusions of Ordovician carbonate (isolated grains up to 3 cm width) within from shattered bedrock reconstructed granite (Note: These inclusions are not later fillings of pre-existing cracks!)
  6.7 Particularly for the Dalecarlian astroblemes, within pure granite areas: Boulders, containing fragments of previous rocks and glasses, obviously established as sediments in water and later deformed by mudslides.
  6.8 Solidified mud, containing fragments of the previous bedrock.
  6.9 Intensively shattered bedrock (shattered in all dimensions), cured by quartz-impregnation.
  6.10 Shattered and reconstructed bedrock with large (up to 5 cm) euhedral or anhedral microcline crystals. These are dark-red, like in mylonites. Note: The crystals do not contain perthite.
  6.11 Impact-melt breccias: They consist of a melt matrix, containing damaged fragments of the target rock. Definition by /2/.
  6.12 Suevite: Polymict breccias with a clastic matrix, containing lithic and mineral clasts and cogenic impact melt fragments. Definition by /2/.
  6.13 Boulders of granite of low strength and heavy weathering, not found in solid rock. These seem to be fragments of reconstructed rock.
  6.14 High-pressure polymorphs of quartz (coesite and stishovite) in the debris, mainly in suevite. Micromounts and the petrographic microscope are needed for inspection.
  6.15 Pseudotachylites: These are thin sheets of melt (normal thickness 1 mm, but up to 10 mm) between blocks of rock, which during the instant of impact have sledded along one another, definition in /2/. However, here it is not obvious, whether the melt has been created locally by friction or has been intruded in from a distant source. Since we have to assume, that the bedrock near the centre of an impact has been at a very high temperature, melts could propagate far away from their source without solidifying. These thin sheets do not have fragments of the bedrock, but show gas bubbles. Real pseudotachylites form thin sheets without branching; sometimes the lateral displacement between the upper and the lower block can be seen. Pressed-in melt of distant origin often shows branching into minor cracks; these very thin branches often fade out. The rock-units on the upper and lower side are not displaced towards one another.
  6.16 Boulders of the type 6.11 and 6.12, which give the impression to have been metamorphosed by steam or hot water. Some of their constituents are transformed into clay minerals. This resembles dark basic lava near fumaroles, which is transformed to kaolin (Furnas, Island of Sao Miguel, Azores).
  6.17 Since large meteorites (kilometre in size diameter) vaporize during the instant of impact they may spread their mater over large distances, sometimes around half the Earth. If they contain odd elements like those of the platinum group (PGE), these may be used to ascertain the fact of an impact. However, not all meteorites contain such odd elements.
  6.18 Applicable not for all, but certainly for the Dalecarlian astroblemes: There sometimes carbonate-preferring plants can be found in a pure granite environment, where they should not occur. The reason for this is: Carbonate from the Ordovician occurs mechanically included in reconstructed granite or conveyed there by the Holocene ice.

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/:

  1. Primary impactites (melts formed during the impact from the impacting body and/or from local bedrock)
  1.1 Impact-melt breccias
  1.2 Suevites
  1.3 Pseudotachylites
  1.4 Reconstructed granite with bulbs of clear glass
  2. Secondary impactites (these are formed after the impact)
  2.1 Quartz-cured original crushed rock
  2.2 New rock, mainly consisting of coloured quartz
  2.3 In water settled material, possibly subaquatic slumps, which has been metamorphosed
  2.4 Hardened slurry of mainly carbonate dust, converted to carbonate-siliceous rock
  2.5 Mylonites, similar to red-brown porphyries, consisting of potassium feldspar, free from the glass phase of a porphyry. The “best” samples consist of intergrown microcline crystals, without any free quartz (=syenites), see 7.10.
  2.6 Reconstructed granite

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.

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© 2014-07-21 Erich Spicar