Article

Cratered cobbles in Triassic Buntsandstein conglomerates in NE Spain: Shock deformation of in-situ deposits in the vicinity of large impacts

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Abstract

Quartzite cobbles in Lower Triassic Buntsandstein conglomerates from northeastern Spain display unusual millimeter- to centimeter-sized circular craters, commonly having central mounds and surrounded by radial fractures. The conglomerates are also marked by intense fracturing down to microscopic scale. These features have traditionally been attributed to tectonic compression and pressure dissolution at cobble contacts. Sections through the cratered cobbles reveal pervasive internal fracturing, segments detached along concave spall fractures, and zones marked by quartz grains with planar deformation features. Comparison with results of impact experiments on artificial conglomerates suggests that these features were produced by internal accelerations, grain collisions, and spallation related to shock-wave propagation through inhomogeneous deposits. The proximity of the outcrops to the Azuara and proposed Rubielos de la Cérida impact structures suggests that shock deformation of conglomerates can provide an easily recognizable regional impact signature.

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... Also omitted on the EID map are larger-scale cataclastic lithofacies assemblages. These are exemplified by the well-studied, although controversial Azura/Rubielos de la Cérida structures in northeast Spain (Ernstson et al. 1985(Ernstson et al. , 2001Diaz-Martinez et al. 2002), and large subsurface complexes such as that in South Australia described by Glikson et al. (2013) to contain multiple quartz microdeformation styles of currently uncertain origin. The global locations and general details of suspected impact sites (including possible and probable among other technical confidence categories) was provided by Rajmon (2010). ...
... The latter author drew comparisons to the currently unconfirmed (EID) northeast Spanish Azura and Rubielos de la Cérida impact structures (Palaeogene). These are characterized by a similar, though more intense variety of (Early Triassic Bundsandstein) cobble deformation fracture styles (clast spallation, radial fracturing, quartz planar features/PDFs) considered to be indicative of Hertzian-style dynamic shock mechanisms at impact magnitudes (Ernstson et al. 2001;Ernstson and Hiltl 2002; see also www.impact-structures.com). ...
... Here are found grains displaying open fracture ('spallation') features (Figs. 5.9d-f) similar to those considered by Ernstson et al. (2001) to represent dynamic contact concussion. However, controls and timing of the observed granular microdeformation features, as well the potential co-occurrence of diagnostic distal ejecta material (shocked quartz, spherules and/or heavy minerals) remain uncertain. ...
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The Late Triassic (Carnian to Rhaetian Stages: ca. 237–201 Ma) has a long history of geological research, although controversy remains over the precise definition of key sub-unit boundaries, including those defining the three constituent stages. Within this context, at least five terrestrial bolide impact structures ranging from 9 to 85 km in diameter have been identified at present-day northern latitudes, the proximal remnant crater aspects of which have been studied in increasing detail over the last few decades. The more elusive distal sedimentary expressions of these multi-sized hypervelocity events remain largely unknown, although if preserved, identified and interpreted correctly, may (as precisely dateable event horizons) help to address certain existing stratigraphic uncertainties, particularly pertaining to the (longest) Norian Stage. Detailed absolute age-dating using a range of radioisotopic methods (e.g. U-Pb and 40Ar/39Ar) currently indicates that at least three of the confirmed Late Triassic impact craters formed prior to commencement of the major Rhaetian Central Atlantic Magmatic Province (CAMP) volcanic episode by several million years. Impact research efforts to date have focused mainly on describing and process modeling the relatively well-preserved largest impact structure, Manicouagan (215.5 Ma; 85 km diameter) located in northeastern Quebec, Canada and, to a lesser extent, the Saint Martin (227.8 Ma; 40 km) and Rochechouart (ca. 207–201 Ma; ca. 23–50 km) structures in central Manitoba, Canada and west-central France respectively. The smaller, subsurface Red Wing structure (ca. 200 Ma; 9 km diameter, ca. 2.5 km burial depth) located in South Dakota, USA, also has attracted significant economic interest. Unlike the well-documented End Cretaceous Chicxulub impact (66 Ma; ca. 180 Km), attempts to establish a globally significant causal extinction connection between the larger impacts (e.g. Manicouagan and Rochechouart) and Late Triassic marine and terrestrial bioevents, culminating with the ‘End Triassic Extinction’ (ETE), have essentially proved unsuccessful.
... French and Christian Koeberl when mentioning the cobble deformations of the Spanish Buntsandstein conglomerates (French and Koeberl 2010) in connection with the publication of Ernstson et al. (2001) in the GEOLOGY journal show that they have obviously not understood spallation and the complex physical process related with shock propagation in spherically shaped cobbles. ...
... So far, so good. In 1992, Eidelman & Reches were of course not aware of the Spanish spallation features and the paper of Ernstson et al. (2001), by the way printed in the same journal GEOLOGY, otherwise they could perhaps have discussed also a dynamic formation of their tensile fractures by spallation. ...
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Visit the full web article via http://www.impact-structures.com/impact-educational/meteorite-impact-spallation-from-mega-to-micro-scale/ ******* Abstract.- Spallation, a well-known process in fracture mechanics, plays a major role in meteorite impact events and is evident on all scales from megascopic spall plates and with larger structural features, over mesoscopic dynamic deformations in particular of conglomeratic target deposits, down to microscopic shock spallation in mineral grains. Here I report on experimental spallation, on various geological implications of impact spallation and on mineralogical spallation evidence, with a special focus on the Spanish Azuara and Rubielos de la Cérida impact structures and the Ries impact crater in Germany.
... within!hard!cobbles!has!been!discussed!already!earlier!for!cobbles!from!the!Tüttensee! crater!and!also!in!a!general!way! (Ernstson!et!al.!!2001(Ernstson!et!al.!! ,!2010,!and!it!has!been!explained! among!others!by!the!strongly!non6linear!process!of!shock!propagation.!In!the!present!case! ...
... (here:!Azuara!and!Rubielos!de!la!Cérida!in!Spain)! (Ernstson!et!al.!2001).!! ! ...
... Likewise, the widely open fractures in the otherwise coherent cobbles with smooth surface and without any shearing (Fig. 10, upper) cannot possibly have originated from tectonics. Instead, these so-called spallation features are the typical result of dynamic shock deformation well known from shock experiments in fracture mechanics (Fig. 10, lower right) and also observed in conglomerates near large impact structures[22][23], (Fig. 10, lower left). We emphasize that the examples shown in the figures do not represent scarce finds but regularly occur in and around the strewn field craters. ...
... Therefore, the special target conditions, that is hard and dense cobbles and boulders in an uncemented soft matrix, are discussed to have enabled a focusing of shock intensity as has earlier been considered for the Barringer crater Coconino sandstone[48]and for a shocked conglomerate[22]. ...
Article
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... The preservation of a complete chondrule within micrometeorite WF1202A-001 is unique as asteroids are subjected to a range of collisional events and alteration processes throughout their residence in the solar system. Based on previous studies regarding the behavior of shock waves in heterogeneous media (e.g., Melosh 1996;Ernstson et al. 2001), Genge et al. (2005) proposed that dust production predominantly affects chondrules due to their higher shock impedance with respect to the chondritic matrix. Since particle and shock wave velocities migrate faster through denser materials, chondrules tend to fragment more easily. ...
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... These so called spallation features are the typical result of dynamic shock deformation well known from observed conglomerates near large impact structures [17,18]. ...
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We use Schmieder and Kring's article to show how science still works within the so-called "impact community" and how scienti c data are manipulated and "rubber-stamped" by reviewers (here, e.g., C. Koeberl and G. Osinski). We accuse the authors of continuing to list the Azuara and Rubielos de la Cérida impact structures and one of the world's most prominent ejecta occurrences of the Pelarda Fm. in Spain 1 2 as non-existent in the compilation. The same applies to the spectacular Chiemgau impact in Germany, which has been proven by all impact criteria for several years. For the authors' dating list, we propose that the multiple impact of Azuara is included together with the crater chain of the Rubielos de la Cérida impact basin as a dated candidate for the third, so far undated impact markers in the Massignano outcrop in Italy.
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We use and variegate the title of this article published in Earth-Science Reviews to show how science may (mal)function, how scientific results are manipulated, and how a few exposed impact researchers (the authors of the Earth-Science Reviews article included) are counteracting exactly the ideas presented in that article.
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A nappe-like thrust of Cambrian over Tertiary, the Daroca thrust, in northeast Spain has puzzled gelogists since longtime. Because of a lacking root zone and a lacking relief it didn't match a reasonable geologic pattern. In the younger regional geologic literature the thrust is nevertheless incorporated in Alpine regional tectonics. An obviously first closer investigation of the involved Cambrian and Tertiary units, their facies and structural setting leads to a model that relates the Daroca thrust to the nearby roughly 40 km-diameter Azuara impact structure. The thrust is part of the excavation stage of impact cratering which may have affected both the Cambrian plate and the diamictic Tertiary below. The model is strongly substantiated by comparison with the Ries impact structure where similar thrusts and related features occur. The Daroca thrust is one more example reflecting the work of the regional geologists who pretend the giant Azuara impact event with the formation of the Azuara impact structure and the adjacent about 70 km Rubielos de la Cérida elongated impact basin never happened. Hence, all their regional geologic models still developed which completely ignore the impact and its radical influence on the Tertiary regional geology are without any scientific relevance.
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