COULD THE NÄRKE-PLAIN BE AN
|Fig. 1: Red gneiss north of Lockhyttan, 145730E/658845N, (2497)
|Fig. 2: Red gneiss north of Lockhyttan, 145730E/658884N, (2516)|
|Fig. 3: Red gneiss at Garphytte mines, 145063E/657730N, (2525)|
The part of the dust cloud that falls back to the slope (outer crater) does not stay there for a longer time. In half-consolidated shape it slides down like a snow avalanche, different part of it mix with one another. Pictures Fig. 4, Fig. 6 and Fig. 7 show samples from the foot of the slope at Garphyttan (Pos B). The figures show a mixture of different layers; the wider layer in Fig. 5, Fig. 6 and Fig. 7 are much darker, finer in grain and hold more mica. The lighter layers consist only of quarts and feldspar; grain size 5 mm.
|Fig. 4: Slide-material of red gneiss at the foot of the cliff at Garphyttan. 145191E/657706N (2485)|
|Fig. 5: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2486)|
|Fig. 6: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2488)|
|Fig. 7: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2489)|
From the same site other samples are shown: Fig. 8, Fig. 9 and Fig. 10. The first picture shows the sawed and dyed with lacquer part; Fig. 9 shows the rough rear side and Fig. 10 shows a cut through this sample. We see crystalline quarts, red microcline and balls and fragments of a black mineral. This mineral is seen in Fig. 9 as fragment, too. Due to a series of parallel cleavage surfaces this mineral is recognized as diopside, hedenbergite or augite. How can these minerals occur in the same specimen together with microcline, when they normally do not occur together? The answer is: By a catastrophic event. A dark diopside occurs in the small iron-mines near the top of Klacken.
|Fig. 8: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2540)|
|Fig. 9: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2542)|
|Fig. 10: Slide-material of red gneiss at the foot of the cliff at Garphyttan 145091E/657706N (2544)|
Not the whole gneiss area is layered: Near (Pos C) there are extended slabs of stone, which have the same colour and the same grain size, but no layering, see Fig. 11; they could have been called aplite. May be this area has been covered by a homogeneous mixture of the powder.
|Fig. 11: Aplitic texture in a ‘red gneiss’-area, Stenarstugan, 145830E/658675N (2518)|
Gneiss lying on leptite or granite cannot be particularly thick. Core drilling would disclose the thickness of the layer and so the mechanism, how that raw material has reached its present position.
On the map of the bedrock named Örebro SV there is south of Örebro a large window right through the Cambrian sediment to the underlying bedrock. Along its southern side limited by a fault the base rock consists of younger granite and leptite. The granite has invaded the leptite: Thus the supposed impact has to be older than the granite and younger than the Lower Cambrian. The nearest age-determined granite is that from Fellingsbro (by I.S. Oen /8/), the result is 1715±50 million years. Thus the age of the astrobleme has to be between 1715 and 570 million years. This author knows nothing on the age of the Örebro-granite.
One could ask the question if it is possible to find PDF (Planar Deformation Features) in samples from this granite; these are formed at very high pressure, only, are a safe proof for an astrobleme. However, there are some problems:
Therefore lack of PDF is no proof for that the Närke-depression is not an astrobleme. This is similar to Criminology: One knows that a murder has happened but one has not the body!
Prepaleozoic carbonate rocks are part of the ‘leptite series’ of rocks, age around 1800 millions of years; they are relatively common in Central Sweden. They have nothing to do with the carbonate rocks from Ordovicium.
From the villages Axberg and Dylta there is a steep dipping horizon of calcium carbonate toward Garphyttan. Near Garphyttan there are minor outcrops of that stone. A large outcrop – now emptied of carbonate is about 2 km South of Garphyttan. In the forest touching upon Upper Garphyttan blocks of this primary carbonate are found, containing irregular inclusions of a dark diopside (Pos F).
In the same area (Pos G) irregular large dark-brown blocks are found, which evidently have come down from the steep above. These blocks would never been recognized as carbonate rock if they were not surrounded by a sea of hepatica. Hepatica is an extremely sensitive indicator for carbonate rock. Within a granite area one never would find Hepatica unless a carbonate rock has been transported to there by one or anther means.
The carbonate blocks in Upper Garphyttan are full of cracks, which are filled with limonite. Also after sawing the exposed surface looks dark. Inspected by a stereomicroscope things look different: One sees clear 2 mm large crystals that allow looking deep into the sawed surface. Everywhere very thin and spherically bent lamellae are seen, which makes the dark impression. To understand this written text a similarity may be used: When roasted peanuts are opened one sees the ‘nut’ covered with a very thin red-brown skin. The relation of the thickness of this ‘skin’ to the nut itself is about the same as the relation of our spherically bent lamellae to the crystallites in our sample. After dissolving the sample in hydrochloric acid the calcite grains disappear; concerning their mass the residue is a totally insignificant heap of lamellae.
The explanation is the following: During the impact of a meteorite this and the bedrock beneath the impact site is pulverized, partly to vapour, partly to rock powder. During the upward flight the vapour condenses on the tiny particles. The tiny calcite particles with their coating fall down later, sinter together and burst their coating, which now lies like rubbish in between these. This carbonate stone has no similarity at all with the original primary carbonate. Here it is called ‘reconstructed’ carbonate.
There is a second particularity in these ‘reconstructed’ carbonate stones: On a sawed surface – but even on a broken surface – up to 5 mm large balls of a red-brown iron oxide are seen. It is assumed that these balls are fragment of an oxide (not magnetite) that have been thrown up and during their way up and down grinded to a spherical form.There is a third particularity: Some parts of this sample in polished shape show red flaws (like turbidities in water), without distinct limits. At high magnification one sees that the flaws consist of thousands of red needles, oriented in all directions. These are exsolutions from the reconstructing carbonate rock; because of colour and form colloidal hematite seems to be plausible:
4 Fe3O4 + O2 = 6 Fe2O3
Here below samples from original primary calcite are shown as Fig. 12 and two samples of reconstructed calcite as Fig. 13 and Fig. 14. The first sample is a non-polished sawed surface; the other sample is a broken surface (note the accumulation of iron-oxide in the upper right corner). The samples are from Upper Garphyttan and were near one another (Pos G).
|Fig. 12: Old (original) carbonate stone, Garphyttan (2506)|
|Fig. 13: Reconstructed carbonate stone, sawed surface (2551)|
|Fig. 14: Reconstructed carbonate stone, natural surface (2552)|
On Garphytte Klack there are several minor iron-ore mines (Pos E) on a course. The ore mineral is magnetite that is located in a dark diopside; no other skarn-minerals exist there. The ore is a steep layer. Since diopside/hedenbergite has been found at the base of the slope (Fig. 9) it seems to be abundant. This gives the impression that this ore is not a normal skarn-ore, which originates from exhalations on the Sea floor, but is an impregnation ore, with a feeder in a radial fissure in the rim of the outer crater of the astrobleme, see Fig. 15. The magnetite is hardly seen in the photograph.
|Fig. 15: Magnetite ore (the main part is diopside), Garphytte mines 145063E/657730N (2533)|
In the mining area another rock as boulders is seen too; this type I up to date have not seen as solid rock. It may have been a cover on the read gneiss that had mainly eroded. The rock may be described as a fine aplite and consists of quarts and feldspar, only. The grain size is below 0,5 mm, a diffuse layering can be suspected, see Fig 16.
|Fig. 16: Aplite in boulder at Garphytte mines, 145063E/657730N, (2531)|
A further dark-brown rock is found at the base of the slope to Garphytte Klack. The rock has a visible layering, but there is no system in it. The mineral grains are round, without sharp edges like in granite, merge in one another. As a guess this is an early fraction of the downfall from the plume of powdered rock. The banding one can infer goes across the whole sample and indicates that in the sample is an ’up’ and a ‘down’. The colour could be that from goethite (Pos H); see Fig. 17.
|Fig. 17: Dark, quarts-rich rock, probably deposit from the impacts dust-cloud, Garphyttan, (2521)|
Outside the northern wing of the supposed impact parabola there is at Skärmarboda (Pos I) a wide area of caves established by piled up granite boulders, large as homes. During the retreat of the Quaternary Ice sheet 10 000 years ago the surges of the retreating Yoldia Sea have washed off the detritus below the boulders and thus created the caves. The following is just a speculation: Probably a fault younger than the supposed astrobleme has created a cliff, from which later these blocks broke off. However, it is the granite as such, which demands attention. The microcline in it is not as usually dyed pale rose, but it is brick red. The rare occurring plagioklas are crystals are located within the microcline crystals. The latter are about a centimetre wide, have grown at the expense of the previous minor crystals by cannibalism. The quarts grains, too, are dyed antique-rose, red-brown or lilac.
The most obvious fact is the weathering of this granite. Other granite, lying on the ground for millions of years, gets a rounded and smooth shape; here samples of the local Örebro-granite are rough on its surface, consist of loosely ‘glued’ crystals, which are full of edges and corners.
The granite is in consistency comparable with a Rappakivi-granite, but no cover of the microcline-crystals by plagioklas exists.
Similar samples are found at Budberget on the northern end of Lake Flosjön in Dalecarlia/9/. The granite at Skärmarboda can very well have been affected by an astrobleme, probably by ‘our’ astrobleme. Under these conditions – long exposure to high temperature - the brick-red and large microcline crystals are created. See Fig. 18.
|Fig. 18: Granite from Skärmarboda, 146190E/659295N (2511)|
During the course of this investigation this author has from SGU/Libraray received a paper by P. J. Holmquist from 1935 /10/. Holmquist has investigated the granite – particularly the Örebro-granite – between Örebro and Kilsbergen. At that time this granite still was exposed near the suburb Hageby, to the North of the centre. At Hageby there was a minor quarry for granite; there he recognized that the granite was heavily affected by pressure. Large sheets – only 0,25 m thick – could be lifted off the bedrock. The extreme pressure within this complex astrobleme can very well have created these sheets.
Holmquist also has studied all available exposures between Örebro and Garphyttan and has found one at the northern end of Lake Tysslingen at Älgesta (Pos J), where the bedrock is neither granite nor leptite. The hillock consists of two different melts that are intercalated in one another, see Fig. 19.
Holmquist suggests that an “exogene” process has affected the local bedrock, but without any suggestion what this could have been.
|Fig. 19: Two melts intermixed, Älgesta, 145645E/657970N (2519)|
During this authors recent visit in June 2014 to Garphyttan he has found other samples of reconstructed carbonate rock that deviate much from samples show in Fig. 13 and 14. These samples are so typical and unique that he anticipates the final result of this investigation: These samples have been formed by sedimentation out of a dust cloud from the impact of a meteorite.
The cast-up material consist i.a. of quarts from the local granite, now rounded and polished in the dust cloud. These quarts grains have sedimented together with the carbonate dust and have been ‘baked in’, forming a reconstructed carbonate stone. Due to weathering of the carbonate, the grains (1 to 3 mm large) are now uplifted and clearly visible. Sedimentation from a water stream never can establish this disparate mixture.
In Rättvik, near the graveyard, at the beach at low water level, there is a similar mixture of fine-grained carbonate and sand. This has been created by the impact of the Siljan meteorite, which formed a large plume of debris. Here the ‘sand’ is very fine grained, consisting of ‘sand’ particles of some few tens of a millimetre. The sand fraction has formed the s.c. Orsa-sandstone. Three years ago the graveyard in Rättvik has been increased; the existing stony ground had to be removed and replaced by soil. The stones have been of the type reconstructed calcite, with plenty of fine quarts grains baked in. These balls of conglomerate of about half a metre size are now spread around the town in curves as ‘fenders’. All mixtures of sand and carbonate are present; outside the public library there is a bolder of about 2m3 size to be seen.
The difference between this reconstructed carbonate (called “calcrete”) and the original Ordovician carbonate is
Back to Garphyttan: Starting from a large boulder at coordinate 145060E/
657686N (a large block in the forest East of Upper Garphytta) and following the bearing 35˚ west up the hill, one notes here and there overgrown holes in the ground, up to 5 m wide. These have a collar of Hepatica round it; have contained carbonate stone (Pos K). Since the nearest smelting furnace is not further than 200 m away, the assumption that the carbonate had been fetched for the smelting of ore, is not wrong. These particular stones were original calcium carbonate like in Fig. 12. or were reconstructed carbonate like in Fig. 13 and 14.
This time our author at (Pos K) has found samples of reconstructed carbonate, similar to those from Rättvik. See Fig.20, Fig. 21 and Fig. 22. All figures belong to the same sample. It is 16 cm long; the ‘baked in’ quarts grains are easily seen.
|Fig. 20: Reconstructed, from air sedimented carbonate stone, with quarts- grains (2582)|
|Fig. 21: Same sample as Fig.20. The brown band in the right part is a melt, containing quarts-grains, too (2586)|
|Fig. 22: Backside of Fig.20. A brown melt is clearly visible in the right part,
However, there is another phenomenon, which is harder to understand. In the sample there are several cracks, filled with the former melt. This melt contains grains of quarts, too, absorbed under its way through the sample. According to our picture of Kilsbergens creation the rim of Kilsbergen is identical with the outer crater of the astrobleme. In a round astrobleme (nearly vertical impact) we do expect the melt in the centre, but not in the periphery. Here in Garphyttan the unconsolidated sediment has been created within few hours after the impact; however, it has to have some rigidity – which is achieved first after some years – to accept a melt without other damage.
It may be that in case of a very low flight angle the centre of all what is happening is much nearer the outer crater rim than at vertical impact? May be seismic measurement along a radial line inwards could give some information.
|Fig. 23: The white stuff is recrystallized carbonate stone (recrystallized from the astroblemes’ dust cloud). The black dots are often hollow drops of a former melt (2593)|
Another sample, taken some few meters from (Pos K), holds another mystery: Fig. 23 shows this sample; the picture is in reality 6 cm wide. We see black dots, sitting in depressions, about some tens of a millimetre wide. By the stereomicroscope one recognizes these dots to be hollow semi-spheres, consisting of some glass. The dots sit in reconstructed carbonate stone. We can imagine, that during blowout a very high speed of many km/s prevails and the melt rock forms spheres. These spheres certainly contain residues of air or water vapour. The high blowout velocity transports rapidly the plume to high altitude; there the outer pressure is low, the spheres burst by the large pressure difference. These spheres or their coating may be the dark background material we have commented upon in description of the reconstructed carbonate stone of Fig. 13.
The mountain chain Kilsbergen – West of Örebro – encloses the Närke-plain by a semicircle. Starting with this topography this author put forward the hypothesis that a meteorite, coming from the East under a very flat angle, might have created the Närke-plain. Since the probability to find other evidence is much larger at the brink of the Kilsbergen than within the plain, he has investigated the rim of Kilsbergen at Garphyttan and north of it. The rocks upon to the tens of kilometres long rim are in literature interpreted as red leptite gneisses. Another interpretation could be, that they have started as sediments from the dust cloud, created by the impact of a meteorite. At this occasion local Precambrian carbonate stone has been pulverised, too, cast up in the atmosphere and then fallen back to ground. The powder has sintered to a new reconstructed carbonate stone. This contains up to 5 mm large balls of a reddish iron oxide and flaws of tiny hematite needles.
Also other ‘impossible’ rocks are found at the foot of the Kilsbergen like a mixture of red-brown microcline, quarts and augite.
Also during a later visit to the brink of Kilsbergen at Garphyttan further two rocks have been found, which are generated from the powder in the eruptive cloud, formed by a meteorite impact:
The multitude of finds, previous publications from 1935 where the author Holmquist states, that the Precambrian bedrock within the Närke-plain must have been affected by exogene processes, makes this author believe, that the Närke-plain is a very old complex astrobleme, with Kilsbergen forming the outer crater rim.
There does not exist any previous publication, which might indicate, that the Närke-plain could be an astrobleme.
Many thanks to the librarian Lars Johansson at the SGU Library for lending this author many geological maps of the region and many copies of previous publications.
Also many thanks to the local guide in Garphyttan Ludwig Dalborg.
The author is physicist (PhD) from the university at Stuttgart and geologist (fil cand) from Uppsala University. He has studied the Siljan astrobleme and the other astroblemes, belonging to this fall, during the recent ten years and of course also the waste literature on astroblemes. A small part of all collected samples are to be seen at the Museum belonging to the local Geological Society at Ludvika with address Ludvika, Gammelgården.
|/1/||Ernest Magnusson: Description of the Geological Map Örebro NV, SGU Series Ae Nr 6, 1970|
|/2/||National atlas over Sweden: Rock and soil, p. 130|
|/3/||H. J. Melosh: Impact Cratering, Clarendon Press, Oxford 1989|
|/4/||Geological Map Örebro NV, Series Ae, Nr 6|
|/5/||Geological Map Örebro SV, Series Ae, Nr 5
Geological Map of the bedrock Örebro NV, Series Af, nr 102
Geological Map of the bedrock Karlskoga SO, Series Af, nr 183
|/6/||Geological Map ”Latorp” Nr 55, 1875|
|/7/||B. Bayly: Introduction to petrology, Prentice-Hall, Inc. Engle-wood Cliffs, New Jersey, sida 284|
|/8/||I. S. Oen: Isotopic age determinations in Bergslagen, Sweden: VII, Geologie en Mijnbouw, Vol.64(4), p.385-388, 1985|
|/9/||Erich Spicar: Several recently discovered supposed Astroblemes in Dalecarlia, Sweden. http://www.vbgf.se/rapporter.html|
|/10/||P. J. Holmquist: A bedrock reconnoitring tour within the Lekebergs area
GFF Volume 57, Folder 2, p. 275-286, 1935
|/11/||Erich Spicar: Ett försök att förstå Orsa-sandstenens tillkomst
|/12/||Chrystos Petalas, 1985: Sedimentary Petrology of the Orsa-sandstone, cental Sweden. UUDMP rsearch report 40, 1-138.
University of Uppsala, Institute of Geology, Department of mineralogy and petrology ÖV: Master Thesis
|(Pos A)||145103E/657721N||Gulhålet in Garphyttan|
|(Pos B)||145091E/657706N||At the fot of the brink in Garphyttan|
|(Pos C)||145830E/658675N||Stenarsstuga, south of Lockhyttan|
|(Pos E)||145063E/657730N||Mines on Garphytte Klack|
|(Pos F)||Ej återfunnen||Upper Garphyttan|
|(Pos G)||Ej återfunnen||Upper Garphyttan|
|(Pos H)||Ej återfunnen||Upper Garphyttan|
|(Pos K)||145060E/657691N||Upper Garphyttan|