Saturday 16 September 2017

Understanding the deposition of suevites in the Miocene Ries Impact Crater in western Bavaria.

Impact craters are among the most common geological features in the Solar System, and understanding them is considered crucial to understanding the history of the planets. However such structures are rare on Earth, due to our active biosphere, oxidising atmosphere and tectonic plate system. One key structure associated with impact craters are suevites, breccias (sedimentary rocks made up of jumbled clasts of different sizes) found in and around impact craters, which show signs of shock metamorphism. A number of different origins have been suggested for these deposits, including the collapse of plume structures, deposition by density flows (similar to avalanches or pyroclastic flows) or deposition from fluid material released by the impact (similar to a lahar or mudflow). These theories are difficult to compare, however, as most known suevites are on other planets. Nevertheless, some suevites are found on Earth, including the first such deposit described, which is associated with the roughly 15 million year old Ries Impact Crater in western Bavaria, Germany.

In a paper published in the journal Geology on 26 July 2017, Susann Siegert of the Leibniz Institute for Evolution and Biodiversity Science at the Museum für Naturkunde, and the Institute of Geological Sciences at the Freie Universität Berlin, Michael Branney, of the Department of Geology at the University of Leicester, and Lutz Hecht, also of the Leibniz Institute for Evolution and Biodiversity Science at the Museum für Naturkunde, and the Institute of Geological Sciences at the Freie Universität Berlin, present a detailed study of the Ries Impact Crater, using both sedimentary analysis and trace element distribution to attempt to understand the origin of the suevite deposits there.

The Ries Impact Crater, or Nördlinger Ries, is a circular depression in western Bavaria roughly 26 km across and between 100 and 200 m lower than the surrounding country. This has been calculated to have been caused by an object with a diameter of about 1.5 km impacting the ground at a speed of about 20 kilometres per second, resulting in a blast 1.8 million times a large as the Hiroshima bomb.

The crater has a basement of crystalline rock overlain by a sedimentary sequence up to 600 m thick. This sedimentary sequence begins with an impact breccia, with a vitrophyric (glassy porphyritic igneous rock) matrix and large fluidal-shaped clasts of what appears to be ignimbrite (consolidated pumice material deposited by pyroclastic flows), overlain by a suevite deposit which varies between 10 and 400 m thick within the crater, and from 2 to 20 m thick outside the crater rim. The suevite comprises a mixture of angular clasts of varying sizes within a fine grained matrix. This is poorly sorted throughout most of the structure, with grading (ordering of clasts, with larger clasts in one direction and smaller in the opposite), where it exists, showing no overall pattern, though the lowest 4 cm shows some lamination, and the upper parts contains elutriation pipes (tubular structures formed by gas venting).

(A) Simplified geological map of the Ries crater, Germany (48°53′N, 10°37′E) displaying outer suevite exposures (in brown). Sample locations: 1–5—outer suevite in the western part; 6–8 outer suevite in the eastern part; 9—impact melt breccia; 10 and 11—crater suevite drill cores. (B) The very poorly sorted, matrix-supported massive nature of the Ries suevite is strongly indicative of deposition from a granular fluid–based density current. It closely resembles the deposit shown in (C). (C) Typical ignimbrite deposited from pyroclastic density currents at volcanoes. (D) Subvertical elutriation pipes of the Ries suevite; they are similar to the pipes shown in (E). (E) Subvertical elutriation pipes common in ignimbrite deposited by fluid-escape dominated deposition from pyroclastic density currents. F: The sharp base of the Ries suevite with inverse grading and diffuse low-angle cross-stratification with splayand- fade lamination (arrow) records deposition from lateral density currents. It is very similar to the deposit shown in (G). (G) Low-angle splay-and-fade stratified ash (arrow) that coarsens upward to massive lapilli tuff, seen widely in ignimbrites deposited from pyroclastic density currents. Siegert et al. (2017).

Most trace elements within rocks are prone to relocation by hydrothermal activity, which is an issue in the Ries Imapct Crater area, so in order to use the distribution of these elements to understand the formation of the suevite, Siegert et al. chose two elements that are not effected in this way; zirconium and cerium. The distribution of these elements proved to be somewhat surprising, with low levels in the west, where the concentration of the elements increases in shallower deposits, and higher levels in the east, where the concentration of the elements increase at greater depth.

Siegert et al. observe that the distribution of the suevite is uneven, with the deposit divided into wide areas of similar depth, rather than being thickest at a central point and thinning outwards. This is incompatible with a collapsing plume structure, but can be produced by density flows or fluid deposition. The presence of cross lamination in the bottom part of the suevite supports this finding, as a collapsing plume structure could not produce such bedding, as does the poor sorting of the clasts in the deposits, as plume deposition tends to sort clasts by size. Furthermore the presence of elutriation pipes is also at odds with a plume deposit interpretation, as such structure do not form in such deposits, as are the presence of clasts showing brittle breakage, which does not usually occur in plume deposits, where the material initially supporting the clasts is dust. Finally, computer modelling of the formation of the Ries Impact Crater deposits suggests that there is far much material in the suevite to have come from a plume.

Having ruled out deposition from a plume structure, Siegert et al. were left with two possibilities, deposition from a dry density flow or a wet fluid. Of these they conclude that a dry density flow is more likely, due to organisation of the clasts (poorly sorted, with local areas of grading in random directions), which is known to be associated with pyroclastic deposits, but not usually with deposition from a fluid, where clasts are almost always sorted so that they show vertical grading, with larger clasts at the bottom and smaller above. The presence of elutriation pipes and a cross-laminated basal layer is also more typical of density flows.

The distribution of the trace elements zirconium and cerium in the suevite can also be explained by a density flow. Siegert et al. hypothesise that these elements were present at a high density in the rocks beneath the eastern part of the crater. When these rocks where vapourised by the impact of the meteorite, they entered the density flow and spread through it, but as density flows are very short lived, did not spread evenly through the structure. Thus the concentration of these trace elements is densest in the deeper part of the eastern crater, then grow thinner moving upwards, then across the surface, then thinner still downwards on the western part of the crater.

Model of origin and emplacement of Ries suevite (t₁) Initial radial outflow of suevite material at the excavation to early modification stage (supported by central peak uplift), preserving chemistry of western (blue) and eastern (red) target rock heterogeneities. (t₂) Mixing within the crater (purple) during crater modification stage (central peak collapse). (t₃) Schematic cross section of Ries impact crater after the impact with positions of drill holes Nördlingen and Enkingen (dark gray is crystalline basement, light gray is sedimentary rock, brown is the Bunte Breccia, a polymict impact breccia). The outer suevite deposits and the lower part of crater suevite keep their original target lithology heterogeneities. The top of crater suevite (purple) represents the best mixed suevite (t₂) with a chemical composition close to average Ries suevite. Siegert et al. (2017).

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