| 1. |
- Ormo, J., et al.
(author)
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First known Terrestrial Impact of a Binary Asteroid from a Main Belt Breakup Event
- 2014
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 4
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Journal article (peer-reviewed)abstract
- Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Malingen crater, formed by the impact of a binary, presumably 'rubble pile' asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
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| 2. |
- Ormo, J., et al.
(author)
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Oceanic resurge deposits at the Rochechouart impact structure (France) suggest a marine target environment
- 2023
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In: Geological Magazine. - : Cambridge University Press (CUP). - 0016-7568 .- 1469-5081. ; 160:4
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Journal article (peer-reviewed)abstract
- The Rochechouart impact structure, located in the western part of the Massif Central in France, has been suggested to be one of the largest impact structures in western Europe. Various age datings have placed the event in a span from the Late Triassic to the Early Jurassic, but the most recent works favour a Late Triassic age. Very little is known about the target environment at the time and location of the impact event. Seemingly coeval, potential tsunamites along palaeoshorelines of the sea that covered parts of continental Europe at the time have been suggested to be related to the impact event and may indicate a marine target setting. Here we apply the method of visual line-logging of the graded suevite in the Chassenon SC2 drill core. This method has previously been used to investigate the depositional environment of similar deposits in several marine target impact craters. It allowed us to compare the deposits at these craters with those at Rochechouart, and in this way not only confirm the marine target setting, but also estimate the target water depth to be similar to 200 m. Altogether, our results indicate a palaeogeographic target setting in a newly opened seaway connecting the Paris Basin with the Aquitaine Basin, which may indicate an age of impact at the younger end of the hitherto suggested age-span, i.e. in the late Rhaetian - Early Jurassic.
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| 3. |
- Ormo, J., et al.
(author)
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The geology of the Malingen structure: A probable doublet to the Lockne marine-target impact crater, central Sweden
- 2014
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In: Meteoritics & Planetary Science. - : Wiley. - 1086-9379. ; 49:3, s. 313-327
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Journal article (peer-reviewed)abstract
- The Malingen structure is an approximately 700m wide, rimmed, sediment-filled, circular depression in Precambrian crystalline basement approximately 16.2km from the concentric, marine-target Lockne crater (inner, basement crater diameter approximately 7.5km, total diameter in sedimentary strata approximately 13.5km). We present here results from geologic mapping, a 148.8m deep core drilling from the center of the structure, detailed biostratigraphic dating of the structure's formation and its age correlation with Lockne, chemostratigraphy of the sedimentary infill, and indication for shock metamorphism in quartz from breccias below the crater infill. The drill core reveals, from bottom to the top, approximately 33m of basement rocks with increased fracturing upward, approximately 10m of polymict crystalline breccia with shock features, approximately 97m of slumped Cambrian mudstone, approximately 4.7m of a normally graded, polymict sedimentary breccia that in its uppermost part grades into sandstone and siltstone (cf. resurge deposits), and approximately 1.6m of secular sediments. The combined data set shows that the Malingen structure formed in conjunction with the Lockne crater in the same marine setting. The shape and depth of the basement crater and the cored sequence of crystalline breccias with shocked quartz, slumped sediments, and resurge deposits support an impact origin. The stratigraphic and geographic relationship with Lockne suggests the Lockne and Malingen craters to be the first described doublet impact structure by a binary asteroid into a marine-target setting.
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| 4. |
- Alwmark, C., et al.
(author)
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Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body
- 2015
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In: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 43:4, s. 279-282
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Journal article (peer-reviewed)abstract
- Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar system's asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years.
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| 5. |
- Alwmark, Carl, et al.
(author)
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Shocked quartz grains from the Malingen structure, Sweden-Evidence for a twin crater of the Lockne impact structure
- 2014
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In: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 49:6, s. 1076-1082
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Journal article (peer-reviewed)abstract
- The Malingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Malingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Malingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne-Malingen pair).
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| 6. |
- Melero-Asensio, I., et al.
(author)
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Geophysical signature of Malingen, the minor crater of the Lockne-Malingen doublet impact structure
- 2018
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In: Meteoritics & Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 53:7, s. 1456-1475
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Journal article (peer-reviewed)abstract
- Malingen is the 0.7km wide minor crater associated to the 10 times larger Lockne crater in the unique Lockne-Malingen doublet. The craters formed at 458Ma by the impact of a binary asteroid related to the well-known 470Ma Main Belt breakup event responsible for a large number of Ordovician craters and fossil meteorites. The binary asteroid struck a target sequence including similar to 500m of sea water, similar to 80m of limestone, similar to 30m of dark mud, and a peneplainized Precambrian crystalline basement. Although the Lockne crater has been extensively studied by core drillings and geophysics, little is known about the subsurface morphology of Malingen. We performed magnetic susceptibility and remanence, as well as density, measurements combined with gravity, and magnetic field surveys over the crater and its close vicinity as a base for forward magnetic and gravity modeling. The interior of the crater shows a general magnetic low of 90-100nT broken by a clustered set of high-amplitude, short wavelength anomalies caused by bodies of mafic rock in the target below the crater and as allogenic blocks in the crater infill. The gravity shows a general -1.4mgal anomaly over the crater caused by low-density breccia infill and fractured crystalline rocks below the crater floor. The modeling also revealed a slightly asymmetrical shape of the crater that together with the irregular ejecta distribution supports an oblique impact from the east, which is consistent with the direction of impact suggested for the Lockne crater.
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| 7. |
- Sturkell, Erik, 1962, et al.
(author)
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Early modification stage (preresurge) sediment mobilization in the Lockne concentric, marine-target crater, Sweden
- 2013
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In: Meteoritics & Planetary Science. - : Wiley. - 1086-9379. ; 48:3, s. 321-338
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Journal article (peer-reviewed)abstract
- Lockne is a concentric impact structure due to a layered target where weak sediments and seawater covered a crystalline basement. A matrix-supported, sedimentary breccia is interlayered between the crystalline breccia lens and the resurge deposits in the crater infill. As the breccia is significantly different from the direct impact breccia and the resurge deposit, we propose a separate unit name, Tramsta Breccia, based on the type locality (i.e., the LOC02 drilling at Tramsta). We use granulometry and a novel matrix line-log method to characterize the sedimentology of the Tramsta Breccia. The obliquity of impact combined with the layered target caused an asymmetric, concentric transient crater, which upon its collapse controlled the deposition of the breccia. On the wide-brimmed downrange side of the crater where the sedimentary target succession was removed during crater excavation, wide, overturned basement crater ejecta flaps prevented any slumping of exterior sediments. Instead, the sediments most likely originated from the uprange side where the brim was narrow and the basement crater rim was poorly developed, sediment-rich, and relatively unstable. Here, the water cavity wall remained in closer proximity to the basement crater and, aided by the pressure of the collapsing water wall, unconsolidated black mud would flow back into the crater. The absence of interlayered resurge deposits in the Tramsta Breccia and the evidence for reworking at the contact between the overlying resurge deposits and the Tramsta Breccia indicate that the slumping was a rapid process (<75 s) terminating well before the resurge entered the crater.
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