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Neu-Ulm, Germany

Buchner E.,University of Stuttgart | Buchner E.,Neu-Ulm University | Schwarz W.H.,University of Heidelberg | Schmieder M.,University of Stuttgart | Trieloff M.,University of Heidelberg
Meteoritics and Planetary Science | Year: 2010

40Ar/ 39Ar dating of recrystallized K-feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km Nördlinger Ries crater (southern Germany) yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (2σ). This new age for the Nördlinger Ries is the first age obtained from (1) monomineralic melt (2) separated from an impact-metamorphosed target rock clast within (3) Ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure. The new age goes very well with the 40Ar/ 39Ar step-heating and laser probe dating results achieved from mixed-glass samples (suevite glass and tektites) and is slightly younger than the previously obtained fission track and K/Ar and ages of about 15 Ma, as well as the K/Ar and 40Ar/ 39Ar age data obtained in the early 1990s. Taking all the 40Ar/ 39Ar age data obtained from Ries impact melt lithologies into account (data from the literature and this study), we suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event. © 2010 The Meteoritical Society.

Schmieder M.,University of Western Australia | Schmieder M.,Curtin University Australia | Buchner E.,Neu-Ulm University
Zeitschrift der Deutschen Gesellschaft fur Geowissenschaften | Year: 2013

The ~3.8 km wide Steinheim Basin (Baden-Württemberg, SW Germany), is formed in a sequence of Triassic to Upper Jurassic sedimentary rocks that support the karstified plateau of the eastern Swabian Alb. It is a well-preserved complex impact structure with a prominent central uplift. Shatter cones from the Steinheim Basin count among the most typically developed shatter cones so far known from terrestrial impact structures and were first described in 1905. In addition to the widely known, well-developed shatter cones in Upper Jurassic limestones of the crater rim domain and the central uplift, shatter cones were also noted in the Middle Jurassic "Eisensandstein" sandstones at the flanks of the central uplift. Recently, we discovered shatter cones in concretionary claystone nodules of the underlying Middle Jurassic "Opalinuston" Formation that was temporarily accessible during water catchment works at the top of the central uplift (Steinhirt). The Steinheim shatter cones are highly variable in their lithologic and structural properties, with well-defined individual or nested cones running either in one main or opposite directions, as well as cones arranged in a "sun-like" pattern radiating outward around concretionary cores within the "Opalinuston" nodules. Our observations suggest that, at least at Steinheim, shock-wave scattering and shatter-cone formation were not dominated by the general impact geometry as commonly stated, but governed by local, micro- to meso-scale target rock effects (e.g. rock inhomogeneities or local impedance). In particular, the "Opalinuston" shatter cones indicate that even comparatively soft clayey lithologies may be conductive to shock waves. © 2013 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.

Buchner E.,Neu-Ulm University | Schmieder M.,University of Western Australia | Schmieder M.,Curtin University Australia | Schwarz W.H.,University of Heidelberg | Trieloff M.,University of Heidelberg
Zeitschrift der Deutschen Gesellschaft fur Geowissenschaften | Year: 2013

In the last years, various new 40Ar/39Ar ages have been obtained for the approximately 24 km Nördlinger Ries crater (southern Germany) by dating Ries tektites, suevite glass (mixed melt) and recrystallised K-feldspar melt particles separated from partially molten biotite granite in impact melt rocks. These 40Ar/39Ar ages, obtained by step heating and laser total fusion analyses, are slightly younger than the previous fission track, K/Ar and 40Ar/39Ar ages of ~15 Ma reported in the early 1990s. By the recalculation of the standards used and the revision of the 40K decay constant, Ries impact ages of 14.74 ± 0.20 Ma (this study) and 14.83 ± 0.15 Ma (2δ), respectively, were presented recently. The indirectly achieved new age of ~14.9 Ma (U/Pb method), as well as the recently obtained ages for the Ries impact event by the application of the (U-Th)/ He method, are also discussed in this study. © 2013 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.

Schmieder M.,University of Stuttgart | Buchner E.,University of Stuttgart | Buchner E.,Neu-Ulm University | Schwarz W.H.,University of Heidelberg | And 2 more authors.
Meteoritics and Planetary Science | Year: 2010

40Ar/39Ar dating of potassium feldspar (primary spherulitic-blocky and secondary idiomorphic K-feldspar) separated from impact-metamorphosed gneiss found near Videix in the western central part of the Rochechouart impact structure (NW Massif Central, France) yielded a Rhaetian combined age of 201 ± 2 Ma (2σ), indistinguishable within uncertainty from the age of the Triassic/Jurassic boundary. Ballen quartz intergrown with the primary K-feldspar indicates post-shock temperatures exceeding approximately 1000 °C that affected the precursor gneiss. Geochemically, both feldspar types represent essentially pure potassium end-members. Apart from the approximately 15 km diameter impact deposit area, the youngest crystallization age known for basement rocks in this part of the Massif Central is approximately 300 Ma. No endogenic magmatic-thermal events are known to have occurred later in this region. The K-feldspar recrystallized from local feldspar melts and superimposed post-shock hydrothermal crystallization, probably within some thousands of years after the impact. It is, therefore, suggested that the 40Ar/39Ar age for the Videix gneiss (as a potassic "impact metasomatite") dates the Rochechouart impact, in consistence with evidence for K-metasomatism in the Rochechouart impactites. The new age value is distinctly younger than the previously obtained Karnian-Norian age for Rochechouart and, thus, contradicts the Late Triassic multiple impact theory postulated some years ago. In agreement with the paleogeographic conditions in the western Tethys domain around the Triassic/Jurassic boundary, the near-coastal to shallow marine Rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest Triassic of the British Isles and possible related deposits in other parts of Europe. © The Meteoritical Society, 2010.

Buchner E.,Neu-Ulm University | Buchner E.,University of Stuttgart | Schmieder M.,Philamlife Village
Icarus | Year: 2015

The ~24. km Nördlinger Ries and the ~3.8. km Steinheim Basin in southern Germany are thought to represent a ~14.8. Ma old impact crater doublet. The complex craters of the Steinheim Basin with its crater fill deposits and the Nördlinger Ries and its voluminous impact ejecta blanket are still widely preserved. Although located in an environmental setting that presumably underwent the same erosional history as the Ries crater, field geologic studies suggest that no proximal or distal ejecta of the Steinheim impact event are presently preserved. Generally, the lack of the ejecta blanket around the crater could be explained either by intense erosion, the scarcity of outcrops, or it never formed. In contrast to the lack of ejecta, fluvial and lacustrine Middle Miocene sediments deposited prior to, synchronous with, and shortly after the impact are preserved in many places in the surroundings of to the Steinheim Basin.On low-density asteroids or planets with highly porous target rocks (≥30-40% effective porosity), impact structures can form without significant ejecta outside the craters due to the compaction of porosity and a concordant drastic reduction of the ejecta velocity. In the Steinheim area, the target rocks comprised loose, porous Miocene sands, Upper Jurassic limestones and Middle Jurassic porous sand- and claystones. The average porosity of the entire sedimentary target suite may have reached 20-30% or even higher values assuming the existence of open karst cavities in the Upper Jurassic carbonates. Compaction of the porous target rocks, resulting in the reduction of ejected material, in combination with erosion could explain the apparent lack of impact ejecta in the wider periphery of the Steinheim impact structure. The Steinheim Basin represents the first proposed terrestrial example of an impact crater characterized by porosity-related ejecta suppression, and it is suggested that other sediment-hosted impact structures on Earth might exhibit analogous excavation-process characteristics. © 2014 Elsevier Inc..

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