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Impact-induced carbonate-psilomelane vein in the Azuara structure of northeastern Spain

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... While this does not entirely rule out a low temperature alpine grass fire, the extreme desiccation observed on grain coatings (Fig. 6a) supports a hypothesized hot coating material at time zero with cracking occurring upon cooling. In addition, glassy spherules associated with the denser 'black mat' material, as well as the amorphous C (similar to that of impact origin shown by Schüssler et al. (2002)) and the presence of infrequent identification of platinum based metals-Rh and Ru, require further evaluation and explanation. It should be noted that the identification of platinum group metals with the SEM may be a chance encounter given both the limitations of the equipment and that the grains were scanned only under high voltage (20 keV) with electron beam penetration to~1 µm. ...
... The amorphous C shown in Fig. 18 of Schüssler et al. (2002) is similar in kind to what is shown in Fig. 4b herein. The origin of the C-rich encrustations discussed here might have resulted from impact in carbonate-rich rocks which melt at~3800°K based on the work of Schüssler et al. (2002). ...
... The amorphous C shown in Fig. 18 of Schüssler et al. (2002) is similar in kind to what is shown in Fig. 4b herein. The origin of the C-rich encrustations discussed here might have resulted from impact in carbonate-rich rocks which melt at~3800°K based on the work of Schüssler et al. (2002). ...
Article
A carbon-rich black layer encrusted on a sandy pebbly bed of outwash in the northern Venezuelan Andes, previously considered the result of an alpine grass fire, is now recognized as a ‘black mat’ candidate correlative with ClovisAge sites inNorth America, falling within the range of ‘blackmat’ dated sites (~12.9 ka cal BP). As such, the bed at site MUM7B, which dates to <11.8 ka 14C years BP (raw dates) and appears to be contemporaneous with the Younger Dryas (YD) cooling event, marks a possibly much more extensive occurrence than previously identified. No fossils (megafauna) or tool assemblages were observed at this newly identified candidate site (3800 a.m.s.l.), as in the case of the North American sites. Here, evidence is presented for an extraterrestrial impact event at ~12.9 ka. The impact-related Andean bed, located ~20 cmabove 13.7–13.3 ka cal BP alluvial and glaciolacustrine deposits, falls within the sediment characteristics and age range of ‘black mat’ dated sites (~12.9 ka cal BP) in North America. Site sediment characteristics include: carbon, glassy spherules, magnetic microspherules, carbon mat ‘welded’ onto coarse granular material, occasional presence of platinum group metals (Rh and Ru), planar deformation features (pdfs) in fine silt-size fragmental grains of quartz, as well as orthoclase, and monazite (with an abundance of Rare Earth Elements—REEs). If the candidate site is ‘black mat’, correlative with the ‘black mat’ sites of North America, such an extensive occurrence may support the hypothesized airburst/impact over the Laurentide Glacier, which led to a reversal of Allerød warming and the onset of YD cooling and readvance of glaciers. While this finding does not confirm such, it merits further investigation,which includes the reconnaissance for additional sites in South America. Furthermore, if confirmed, such an extensive occurrence may corroborate an impact origin.
... 4B and 5B, the N was presumably completely taken up in the gas cycle. Manganese oxidation by bacteria (Nealson, 1983; Bougerd and De Vrind, 1987) probably explains much of the Mn found as coating on grains in the Black Mat bed, the remainder coming from redox fluctuations in the aquifers (placons; Tilsley, 1977) resident in the section, or possibly from the impact itself (Schüssler et al., 2002). Crenulated or scalloped quartz shown in Fig. 6A is partly free of C coating but contains some Al and Cl. ...
... 4B and 5B, the N was presumably completely taken up in the gas cycle. Manganese oxidation by bacteria (Nealson, 1983; Bougerd and De Vrind, 1987) probably explains much of the Mn found as coating on grains in the Black Mat bed, the remainder coming from redox fluctuations in the aquifers (placons; Tilsley, 1977) resident in the section, or possibly from the impact itself (Schüssler et al., 2002). Crenulated or scalloped quartz shown inFig. ...
Article
Fired glaciofluvial beds in outwash considered to date from the onset of the Younger Dryas Event (~12.9 ka) in the northwestern Venezuelan Andes are considered equivalent to the Black Mat deposits described in other areas of North and South America and Europe. It may be equivalent to sediment recovered from other sites containing beds with spikes of cosmic nuclides and charcoal indicating the presence of widespread fire, one of the signatures of the Black Mat conflagration that followed the proposed breakup of Comet Encke or an unknown asteroid over the Laurentide Icesheet at 12.9 ka. In the northern Andes at Site MUM7B, sediment considered coeval with the Black Mat contains glassy carbon spherules, tri-coatings of C welded onto quartz and feldspar covered with Fe and Mn. Monazite with excessive concentrations of REEs, platinum metals including Ru and Rh, possible pdf’s, and disrupted/brecciated and microfractured quartz and feldspar from impacting ejecta and excessive heating summarize the data obtained so far. The purpose of this paper is to document the physical character, mineralogy and biotic composition of the Black Mat.
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A progress report on geophysical and geological investigations in the 35–40 km diameter Azuara probable impact structure (northeast Spain) is given. The target is a 10 km thick sequence of Palaeozoic, Mesozoic, and Cenozoic sediments. Gravity measurements establish a negative residual anomaly of about 100μm/s2 and a mass deficiency of 1.24×1014 kg. They suggest the existence of a buried inner ring with half the size of the outer ring. The result of preliminary model calculations is compatible with the assumption of a flat structure only a few kilometers deep. Measurements of the total intensity of the Earth's magnetic field do not indicate causative bodies related to the structure. Geological mapping reveals a tectonic style which is characterized by folds and faults with radial and tangential elements and a strong horizontal component. Ejecta, abundant monomict and polymict breccias, dislocated megablocks, inverted stratigraphy, and a megabreccia up to 80 m thick in the outer ring are evidence for intense and violent deformation. Networks of typical breccia dikes well-known from many impact structure cut through nearly all stratigraphical units. Various types and generations of breccia dikes occur. In addition to previously described shock deformation in quartz and mica, we present further evidence of high-pressure and high-temperature signatures in rocks. The age of the impact is estimated to be Upper Eocene or Oligocene. We conclude that the formation of the Azuara structure is difficult to explain by mechanisms other than impact, and we discuss our observations within the scope of the contact/compression, excavation, and modification stages of impact cratering with special attention directed to the peculiarities of the Azuara
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The Rubielos de la Cérida impact structure forms a companion crater to the Late Eocene-Oligo-cene Azuara impact structure. Both are located more or less at the margin of the Iberian chains and the Ebro basin south of Zaragoza. Within the Rubielos structure, silicate melt rocks, carbonate-phosphate melts with small-scaled immiscibility features, very fine mixtures of silicate melt and carbonate forming clasts in suevite, as well as glassy particles of amorphous carbon were found. These melt rocks clearly reflect the chemical composition of various parts of the thick sedimentary pile in the target area and show the shock-induced high-temperature influence on these rocks.
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We report on the Azuara impact structure and its Rubielos de la Cérida compa- nion crater, which establish the largest terrestrial doublet impact structure presently known. Both structures have diameters of roughly 35 - 40 km and they have been formed in a purely sedimentary target. From stratigraphic considerations and palaeontologic dating, an Upper Eocene or Oligocene age is very probable Geological mapping has established abundant geologic impact evidence in the form of monomictic and polymictic breccias and breccia dikes, megabreccias, dislocated megablocks, remarkable structural features, extensive impact ejecta and impact sig- natures even in distant autochthonous deposits. The most striking impact evidence for both structures is given by strong shock metamorphism, including melt and dia- plectic glass, planar deformation features (PDFs), different kinds of impact melt rocks (from former silicate melt, carbonate melt, carbonate-phosphate melt) and suevite breccias. Glassy amorphous carbon particles in a solid C-O compound may be related with fullerenes and may originate from a quenched melt of extremely shocked coal or from extremely shocked limestones. It is assumed that the impact had considerable influence on the Mid-Tertiary regional geology of the Iberian System, and we suggest that respective geologic models which have so far not con- sidered this peculiar and far-reaching event, need considerable revision.
The Pelarda Formation is an isolated continuous deposit which is up to 200 m thick and extends over an area of about 12 X 2.5 km2. It was originally described as a Tertiary fluvial boulder conglomerate. From the occurrence of striated and plastically deformed boulders and pebbles which partly show moderate shock effects, we conclude that the Pelarda Formation is the remnant of an originally extended ejecta blanket around the large Azuara impact structure. This interpretation is substantiated by statistical analyses of the striae azimuth and the normals to locally developed bedding planes. The admixture of lo- cal substrate with the Pelarda Formation indicates secondary cratering as a consequence of ballistic transport and ballistic sedimentation of the primary ejecta. — Models of palaeogeography and morphogenesis based on the fluvial-deposit interpretation must be re-examined.
Article
Abstract— The 24 km diameter Ries impact crater in southern Germany is one of the most studied impact structures on Earth. The Ries impactor struck a Triassic to Upper Jurassic sedimentary sequence overlying Hercynian crystalline basement. At the time of impact (14.87 × 0.36 Ma; Storzer et al., 1995), the 350 m thick Malm limestone was present only to the south and east of the impact site. To the north and west, the Malm had been eroded away, exposing the underlying Dogger and Lias. The largest proportion of shocked target material is in the impact-melt-bearing breccia suevite. The suevite had been believed to be derived entirely from the crystalline basement. Calcite in the suevite has been interpreted as a postimpact hydrothermal deposit. From optical inspection of 540 thin sections of suevite from 32 sites, I find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt. Textural evidence of liquid immiscibility between silicate and carbonate melt in the Ries suevite includes carbonate globules within silicate glass, silicate globules embedded in carbonate, deformable and coalescing carbonate spheres within silicate glass, sharp menisci or cusps and budding between silicate and carbonate melt, fluidal textures and gas vesicles in carbonate schlieren, a quench crystallization sequence of the carbonate, spinifex textured quenched carbonate, separate carbonate spherules in the suevite mineral-fragment matrix, and inclusions of mineral fragments suspended in carbonate blebs. Given this evidence of liquid immiscibility, the carbonate in the suevite therefore has—like the silicate melt—a primary origin by impact-shock melting. Evidence of carbonate-silicate liquid immiscibility is abundant in the suevites from the southwest to east of the Ries crater. The rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts. This correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the Malm limestones are the source rocks of the carbonate impact melt. This correspondence shows that the suevites preserve a compositional memory of their source rocks. From the regional distribution of suevites with or without immiscible carbonate melts, it is inferred that the Ries impactor hit the steep Albtrauf escarpment at its toe, in an oblique impact from the north.
Memoria y Mapa, Hoja n° 40 (Daroca) del Mapa Geológico
ITGE (1991): Memoria y Mapa, Hoja n° 40 (Daroca) del Mapa Geológico de España. E. 1:200.000, Instituto Tecnológico GeoMinero de E
  • K Ernstson
  • J Fiebag
K. Ernstson, J. Fiebag (1992): The Azuara impact structure (Spain): new insights from geophysical and geological investigations. Geol. Rundschau, 81, 403-427.