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Jeszcze jedna dyskusja na temat daty spadku meteorytu Morasko

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2017

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vol. 8

123-148

EN

It’s over 100 years after discovery of the first iron meteorite in the area of Morasko, but still we are not sure about date of the fall. Recent researches suggest that it happened about 5000 years ago, however there are some other facts proving, that event took place in the last millenium. Verification o 14C radio carbon dating results revealed, that it can give us only the maximum date (meaning: not older than). The main problem was that probes were taken from mixed types of sediments – some created before and some after the impact. Few years earlier there was another 14C examination of sediments lying beyond the floor of 2 small cavities and their age was estimated to be approximatelly 700 years BP. Similar results were received after verification of organic matter under charcoal in the weathering crust of the meteorite, which was found in the Morasko Restricted Area. Another proof was discovery of shrapnel in the root of old tree, which could not be older than 800–1000 years. At least 2 times it was discussed, if finding of Morasko iron meteorites couldn’t be matched with described observation A.D. 1301, mentioned in annales, hovever nobody was able to prove it. Analysis of historical documents lead to information of appearance of 3 suns, after sunrise, what happened in 14 Feb. 1271. Further investigation of other sources gave us even more facts. We can read in annales that in the same year, castles and cities were burned and destructed to the ground. There is also information that Poznan, the largest city in the area (approx 7.5 km South from the Morasko), was burned in (or shortly before) the year of 1274. What is more Annales from Poznan and from its parent town Wielkopolska were finished in A.D. 1271, just in the middle of the sentence. Finally, confirmation is given by archeological researches, which tell us that city walls and castle of Poznan were ruined shortly after they were constructed, but scientists found difficulty to answer, when exactly it could happen. The main problem was unknown weapon, able to leave such a large destruction, even when considered next centuries. As we can find in annales that Poznan in A.D. 1253 had castle and fortifications so only iron meteorite impact in 1271, producing craters in the area of Morasko and Umultowo, can explain the scale of the devastation discovered by archeologists.

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Wykorzystanie danych Numerycznego Modelu Terenu do poszukiwania i wstępnej identyfikacji struktur impaktowych

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2020

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vol. 11

110-124

EN

Is it possible that in Poland, instead of Morasko and still unconfirmed structures Porzadzie, Jaszczulty and Ochudno, may exist some other impact craters? To answer this question, investigations were performed, which were based on the digital elevation models. There are plenty of cavities, having different origin (e.g. a result of past glaciations, human activities or the Second World War), where some criteria may be applied, which could help to distinguish one from another. The main assumption was that during oblique impact (low angle in relation to the surface) there may develop special type of craters having cardioid (heart-shaped) ejecta pattern (as well as shape of cavity), with axis of symmetry parallel to the initial trajectory. Such structures were observed many times during experiments as well as can be easily found on other celestial bodies. Having preserved rims, even if they are much shallower than typical craters (because of secondary changes), such cavities are perfect candidates to be suspected of the impact origin. As a result of research between 2015 and 2017 there were discovered 3 locations (instead of Ochudno) with heart-shaped structures, which may represent combined effect of ejection angles, velocities and directions, known from experiments and confirmed craters. In all cases there are noticeable rims. The largest “spoon-like” structure, discovered near the Rowista village, is almost perfect copy of a very unusual elongated crater located north of the Acheron Fossae on Mars.

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Analiza cech impaktu ukośnego na przykładzie struktur Porządzie, Jaszczułty i Ochudno

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2016

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vol. 7

151-176

EN

It’s considered that one half of all projectiles are striking surface of planets at the angle lower than 45°. Our knowledge of oblique impact events is based on computer modelling, laboratory experiments and observation of craters on the Moon, Mars, Venus and other celestial bodies. Recent discovery in northeast Poland, reveals at least four structures with features reserved for low-angle impacts. In this article we show and explain them using high-quality laser air scanning data (LiDAR) in comparison to results of experiments and similar real known objects on other planets. Also we try to verify by analyzing historical sources if multiple impact event could occur in our area and, if it was noticed and reported in the papers.

4

Przegląd najważniejszych prac dotyczących próby ustalenia daty spadku meteorytów oraz wieku kraterów Morasko

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2021

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vol. 12

129-148

EN

One of criteria suggesting impact origin may be recognition of extraterrestrial matter inside or around suspected cavities. In case of Morasko some dating results throw doubt on link between craters and meteorites. Conclusion of some past research papers was that cavities were formed about 5 ka BP (e.g. palynological investigation, luminescence dating), while more or less facts testify fall in the Middle Ages (e.g. “young” charcoal pieces in crust of meteorites or shrapnel stuck in the roots of old tree). In this paper we perform comprehensive analysis of each result and check if there exists alternative explanation. During past palynological investigation, there were examined two craters. It was concluded that beginning of accumulation of sediments in smaller cavity started 5500–5000 BP. However, pollen spectrum for largest basin was different suggesting younger age. The problem is that according to bathymetric maps, samples could be collected from inner uplift (similar feature was observed inside Porzadzie and Jaszczulty, unconfirmed impact structures yet). If we compare profiles from craters with recent research (palynological study supported by radiocarbon dating) on sediments in Lake Strzeszynskie (6 km SW from Morasko) pollen spectrum seems to be similar more to results dated to <1000 BP. Especially percentage of non-arboreal pollen for both examined Morasko craters is much higher (5 times greater than it was for layer dated to 5 ka BP in Lake Strzeszynskie). Possibility of short disturbance (related to impact) cannot be excluded, but in such case any time of event should be taken into account. For small lakes (like those filling the craters) also local conditions could play important role. Analysis of luminescence dating may also discuss past conclusions. OSL method applied for samples taken from the thin layer of sand in the bottom of largest structure (under 3,9 m of organic sediments filling the crater) suggested age 5–10 ka BP for 24 aliquots. Same number of samples (24) revealed age 0–5 ka BP (13 samples with age <3 ka BP including several younger than 1 ka BP). Older dates can be explained by partial or even no zeroing, but last contact with light (zeroing signal) of sand grains (excavated from depth of almost 4 meters under organic matter) seems that might occur only during (or shortly after) the impact. It is difficult to find convincing arguments, which can undermine initial radiocarbon dating giving age <1 ka BP for 7 of 9 samples taken from the bottom layer of organic sediments from three Morasko craters. Same issue may exist with small charcoal pieces with age <2 ka BP (and several dated to ~700 BP) discovered deep in sinter-weathering crust of meteorites. Study of charcoal particles excluded their origin during post-sedimentary processes (like forest fire) unless meteorite fragments were laying directly on the surface for period between impact and wildfire (surviving whole time inhospitable climate conditions). There are two other possible explanations of charcoals. Either they were present at location as a result of past forest fire or they were produced during impact. However, similar small charcoal pieces were discovered around many craters (Kaali, Ilumetsa, Campo del Cielo, Whitecourt) and they were successfully used to estimate maximum age of those structures. During second stage of 14C dating the age obtained from three samples (taken few centimeters above the mineral bottom) was estimated ~5 ka BP. Looking for answer why these results are so different from previous once there may be mentioned at least two options. Lake sediments is difficult matter for radiocarbon dating and results may be hundreds or even thousands years older than real age. Second explanation may be that older matter (remnant of trees, paleosoil etc.), distributed around craters during impact, could be displaced by wind, rain, erosion and trapped in the bottom of cavities. The argument, which may be also against hypothesis of impact ~5 ka BP is meteorite shrapnel stuck in the roots of old tree. Result of expertise showed, that there exists mechanical damage in the wood and the only possible explanation is that meteorite has hit living tree. Maximum age, that this type of wood may preserve (inside building) is 1,8 ka while in natural environment not more than 500 years. Anyway some further examinations should be performed. The age obtained during AMS 14C dating of 2 samples from thin layer of paleosoil, discovered under overturned flap around largest Morasko crater, was ~5 ka BP. Observation that preserved layer of paleosoil is approximately 3 times thinner than thickness of modern soil may lead to conclusion that during impact top (younger) layer of organic sediments was removed and only older part “survived” in few locations close to the rim. The conclusion of the research was that dating provides maximum age of the impact (which does not exclude much younger impact <1 ka BP) so could be considered as right explanation of past discrepancies.

5

Kratery Morasko w świetle wiedzy na temat ukośnych impaktów

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Walesiak T.

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EN

It is considered that The Morasko meteorite shower happened about 5400 years BP with a trajectory from NE to SW (Bronikowska et al. 2015). Recent studies not only prove that this fall could be observed in Februrary 14, 1271, but also that location of most findings in the area to the east side of the craters, should not be taken as the main indicator of impact direction. The proper answer can be given base on knowledge related to oblique impacts, supported by understanding of complicated fragmentation process. The largest crater in Morasko Meteorite Restricted Area has main ejecta plume located in SE, side walls in SW and NE, and free of ejecta Zone of Avoidance (ZoA) in NW. This clearly shows that impact trajectory was approximatelly from NW to SE. There were also discovered other structures, 2 km North from Restricted Area, having common features. Their ejecta plumes as well as few raised ducts (possible traces of underground penetration) are directed to SE, pointing the largest structure – Umultowskie Lake, located approx. 2 km ESE from known main Morasko crater. Shapes of cavities, their walls and ejecta asymmetry allow estimating trajectories and defining new Morasko strewnfield, which matches not only location of impact strucures and discovered meteorite fragments, but also explains smaller (3–4 kg) findings in the neighborhood of Oborniki Wielkopolskie.

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Dalsze badanie struktur w okolicach Wyszkowa – wyniki prac terenowych

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2019

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vol. 10

168-192

EN

There are specified criteria used to verify if some geological structure was developed during hypervelocity impact. Some of them are related to the passage of a shock wave through rocks, so are considered as definitive. Porządzie, Jaszczulty & Ochudno are structures discovered in 2014 (based on LiDAR data). They have many features suggesting possible impact origin. In shape of Porządzie there can be even recognized possible traces of shock waves, similar to those present during hypersonic flow (similar to observed e.g. in an airburst shape of extraordinary bright meteors). Unfortunately, this kind of observations (morphometry) on Earth is still considered as not enough to confirm the crater. The aim of recent study was to search for additional traces, which may help to prove that mentioned structures were created during common oblique impact event. Initially there were chosen 3 locations for outcrops, but finally samples were collected from 6 smaller pits. In Porzadzie structure there were discovered cemented rocks with various shape and size (concretions or lithic, parautochtonous breccias) and 2 rocks with possibly shocked quartz, having basal sets of thin and straight lamellae. On once surface there were recognized hierarchal branched (including spoon-like) structures resembling shatter cones. In uprange, at depth of about 50–70 cm, soil was rich in small charcoal pieces (like in other relatively small craters: e.g. Campo del Cielo, Whitecourt, Kaali, Ilumetsa, Morasko). All these observations, taken together with previous results, are about to confirm impact origin of Porzadzie crater.

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Kratery Morasko – obrona hipotezy ukośnego impaktu w kontekście dostępnej wiedzy i wyników badań

100%

Walesiak T.

Acta Societatis Metheoriticae Polonorum

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2021

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vol. 12

108-128

EN

Over 2 years ago there was published an article (Bronikowska 2018a) challenging some results of research related to Morasko meteorite fall. Those results are huge problem for scientists who are not able to explain them using simplified models and maybe do not want to admit that some assumptions should be verified and redefined. One of hypothesis discussed in the paper was article manifesting that Morasko craters were created during oblique impact (Walesiak 2017) and that initial trajectory can be estimated by elongation of almost all cavities, their bilateral symmetries according to longer axis and observed asymmetries of rims. By comparison to Campo del Cielo craters Walesiak suggested that impact angle could be very shallow (10–20°), as ellipticity of all smaller Morasko structures is approx. 1,3. Further analysis of topography in neighborhood of Morasko revealed that there may exist more impact craters around this area, which may explain discovery of two iron meteorites near Oborniki village (17 km NW from Meteorite Morasko Restricted Area), unfortunately lost during World War II. In fact, Bronikowska confused definitions “oblique impact” and “elliptical craters”, what can be supported by given references (Elbeshausen et al. 2009) confirming Walesiak hypothesis. Unfortunately, this misunderstanding touches also another article of this author (Bronikowska et al. 2017), where obliquity was neglected. However, estimated during that modeling pre-impact angle (30–43°), according to known definitions written in many publications, should be classified as oblique impact. All elongated craters, as well as morphology of the largest structure in Meteorite Morasko Restricted Area suggest impact from NW while during reconstruction of meteoroid parameters Bronikowska assumed impactor coming from NE (based on existing strewn field). Considering not clear relation between meteorites and craters (e.g., lack of findings in cavities, relatively poor number of shrapnel fragments around structures and possibility that craters may be much older than fall of meteorites), it may be not enough reliable justification. Even if relation exists, example of Whitecourt (with distribution of meteorites downrange), compared with abundance of Morasko shrapnel fragments collected hundreds of meters NE from Restricted Area, undermines parameter used in modelling. Also, single fragmentation is doubtful, concluded after unreasonable omitting known craters outside Restricted Area (e.g., crater no 8 described by Pokrzywnicki (1964) and structure no 9 mentioned by Hodge (1994)). Finally, use of iSALE-2D shock physics code (valid for vertical impacts only) for modelling of elliptical craters formed during highly oblique impact (angle lower than 12°), even considering vertical component (which approximation is only applicable for impact into materials with a friction coefficient of about f=0,7 with no or very little cohesion) (Elbeshausen et al. 2009), also should not be considered as proper applied method leading to get valuable results. In this article there is proposed new strewn field definition for Morasko meteorite, based on elongation of all known and unconfirmed (yet) craters. Surprisingly, estimated (redefined) pre-impact trajectory gives convincing explanation for bilaterally symmetrical distribution of documented findings.

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‘Aegean’ – kolisty obszar w południowej Europie – jego geologia a największe kratery na Ziemi

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Walesiak T.

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EN

Analysis of online available references suggests that circular characteristics observed around Aegean Sea in southern Europe may represent protobasin class of impact craters, which small central peak and surrounded peak ring appear to be formed by the Cyclades. Thinner crust, upper mantle deformation (uplift), recognition of basin in upper mantle re-filled with rocks having lower density than surrounding mantle, central Aegean Sea area free of sediments and almost perfect circular anomaly of density with diameter over 500 km are only few arguments which may indicate its real origin. The strongest fact standing for hypothesis of impact is presence of deep, lithospheric fracture zones with both circular and radial character, detected up to the distance of ~516 km from common geometric point of all rings (providing estimated diameter of the largest circle over 1000 km). Existence of pseudotachylite veins and breccias in central Cyclades are contributory evidence. Mélange covered by Fe-Ni rich ores in Jurassic/Cretaceous boundary, if related to impact, may give clue to explanation of localized in Europe Tithonian mass extinction event 145 Ma. However, pseudotachylites and breccias can be produced by endogenic processes (e.g., plate tectonics). A further research is required to find evidence of passage of shock wave through target rocks.

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8 Walesiak T.

Jeszcze jedna dyskusja na temat daty spadku meteorytu Morasko

Wykorzystanie danych Numerycznego Modelu Terenu do poszukiwania i wstępnej identyfikacji struktur impaktowych

Analiza cech impaktu ukośnego na przykładzie struktur Porządzie, Jaszczułty i Ochudno

Przegląd najważniejszych prac dotyczących próby ustalenia daty spadku meteorytów oraz wieku kraterów Morasko

Kratery Morasko w świetle wiedzy na temat ukośnych impaktów

Dalsze badanie struktur w okolicach Wyszkowa – wyniki prac terenowych

Kratery Morasko – obrona hipotezy ukośnego impaktu w kontekście dostępnej wiedzy i wyników badań

‘Aegean’ – kolisty obszar w południowej Europie – jego geologia a największe kratery na Ziemi