Ries Crater Research Articles

Modeling the Ries‐Steinheim impact event and the formation of the moldavite strewn field

Abstract— Using detailed geological, petrographic, geochemical, and geographical constraints we have performed numerical modeling studies that relate the Steinheim crater (apparent diameter Da = 3.8 km), the Ries crater (Da = 24 km) in southern Germany, and the moldavite (tektite) strewn field in Bohemia and Moravia (Czech Republic), Lusatia (East Germany), and Lower Austria. The moldavite strewn field extends from ∼200 to 450 km from the center of the Ries to the east‐northeast forming a fan with an angle of ∼57°. An oblique impact of a binary asteroid from a west‐southwest direction appears to explain the locations of the craters and the formation and distribution of the moldavites. The impactor must have been a binary asteroid with two widely separated components (some 1.5 and 0.15 km in diameter, respectively). We carried out a series of three‐dimensional hydrocode simulations of a Ries‐type impact. The results confirm previous results suggesting that impacts around 30–50° (from the horizontal) are the most favorable angles for near‐surface melting, and, consequently for the formation of tektites. Finally, modeling of the motion of impact‐produced tektite particles through the atmosphere produces, in the downrange direction, a narrow‐angle distribution of the moldavites tektites in a fan like field with an angle of ∼75°. An additional result of modeling the motion of melt inside and outside the crater is the preferred flow of melt from the main melt zone of the crystalline basement downrange towards the east‐northeast rim. This explains perfectly the occurrence of coherent impact melt bodies (some tens of meters in size) in a restricted zone of the downrange rim of the Ries crater. The origin of these melt bodies, which represent chemically a mixture of crystalline basement rocks similar to the main melt mass contained (as melt particles <0.5 m in size) in the suevite, do not occur at any other portion of the Ries crater rim and remained enigmatic until now. Although the calculated distribution of moldavites still deviates to some degree from the known distribution, our results represent an important step toward a better understanding of the origin and distribution of the high‐velocity surface melts and the low‐velocity, deep‐seated melt resulting from an oblique impact on a stratified target.

40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Cònero Riviera, Ancona, Italy

A nearly complete and undisturbed Miocene carbonate sequence is present in the easternmost part of the Umbria-Marche basin, Italy, which is ideal for detailed and integrated stratigraphic investigations of the Miocene Epoch. In this study, we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 Ma Ries impact structure in southern Germany, located about 600 km to the north–northwest of the Umbria-Marche basin. The first step is to find coeval strata in the Umbria-Marche sequence. At the La Vedova section, Cònero Riviera, we dated a volcaniclastic biotite-rich clay layer, the Aldo Level, which is situated within planktonic foraminiferal Zone N8, at 14.9±0.2 Ma, using the 40Ar/39Ar method. Together with detailed geologic and stratigraphic information about the Aldo Level, the resulting age can be used confidentially to calibrate the Langhian stage. Besides providing new constraints on Miocene geochronology, this age can now be used for impact stratigraphic studies. To directly correlate the biotite ages of the La Vedova section with rocks from the Ries impact event, Ries impact glass was also analyzed and found to be coeval. Although unrelated to this impact event, the biotite-rich clay layer should help in the search for evidence of distal ejecta related to the Ries crater.

A natural shock-induced dense polymorph of rutile with α-PbO 2 structure in the suevite from the Ries crater in Germany

A dense post-rutile titanium dioxide (TiO 2) phase was discovered in shocked garnet gneisses from the Ries crater by reflected-light microscopy, laser microRaman spectroscopy and micro-beam X-ray diffraction. The Raman spectrum consists of nine bands at wave numbers: 152, 175, 285, 315, 340, 358, 428, 532 and 575 cm −1. These bands are identical to those of the α-PbO 2-structured polymorph of TiO 2 synthesized in a dynamic laboratory experiment. The diffraction pattern of the natural mineral revealed an orthorhombic lattice, similar to the α-PbO 2 polymorph with the cell parameters a=4.535(2) Å; b=5.499(2) Å; c=4.900(2) Å (space group Pbcn; columbite-type structure), density ρ=4.34 g/cm 2, where the numbers in parentheses are standard deviations in the last significant digits. This new polymorph is 2% denser than rutile. The rutile/α-PbO 2 polymorph phase transformation occurs exclusively at the grain boundaries between rutile and the shock-compacted host biotite and advances inwards in rutile. This textural relation establishes phase boundaries as the preferable fabric settings for dynamic-induced high-pressure phase transitions. Ab initio calculations negates the formation of the α-PbO 2 polymorph by inversion of a parental fluorite-structured polymorph during decompression. Heating of the experimentally produced α-PbO 2 polymorph above 500°C shows that it inverts back in a laboratory time scale to rutile. Hence, the survival of the α-PbO 2-structured polymorph in naturally shocked rocks constrains the post-shock temperature of the TiO 2-bearing assemblage at an upper bound of 500°C. The presence of this dense phase is expected in the Earth’s upper mantle below 123 km. The rutile/α-PbO 2-structured polymorph phase transition in subducted crustal limbs in the upper mantle should then be accompanied by considerable changes in the partitioning and fractionation of Nb and Ta between this TiO 2 polymorph and the coexisting dense silicates.

An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater.

We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.

Identification of shocked quartz by scanning cathodoluminescence imaging

Abstract— Quartz grains subjected to high‐strain‐rate shock waves owing to meteorite or cometary impact on Earth's surface commonly display shock lamellae. These lamellae appear as remarkably straight, thin, planar features (microstructures) in sets within which lamellae are essentially parallel to each other and spaced ≤ 20 μm apart. Two or more intersecting sets are typically present. Shock lamellae are commonly recognized and identified by optical methods, by use of the transmission electron microscope (TEM), and by etching polished sections and subsequent examination with a scanning electron microscope (SEM) operated in the secondary electron mode. We present here a method for observing planar microstructures in shocked quartz by using a cathodoluminescence (CL) detector attached to a SEM. The method relies on the fact that planar microstructures in quartz arising as a result of shock display no CL whatever; thus, they show up as distinct, thin, black lines on otherwise luminescent quartz grains. We used scanning CL imaging to study shocked quartz from the Ries Crater, Germany, a well‐known impact crater of Miocene age. We demonstrate that shock‐produced planar microstructures are clearly displayed in SEM‐CL images and can be distinguished from microfractures generated by tectonism, and subsequently filled with quartz, and other similar features not related to impact events. The SEM‐CL method provides a powerful supplement to other methods of identifying shocked quartz. It commonly provides better spatial resolution than does standard optical methods, and does not require etching of quartz grains. Further, it is easier and faster to use than are TEM methods, although it is not capable of the fine‐scale defect analysis possible with TEM.

In situ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries Crater, Germany

Reflected-light microscopy and fine-scale lasts microRaman spectroscopy of shocked garnet-cordierite-sillimanite gneisses in suevites of the Ries meteorite impact crater, Germany, led to the discovery of impact diamonds in their pristine graphite-diamond assemblages. Graphite-diamond textural relations permit a clear determination of the solid-state nature of the formation of diamond from graphite, which is estimated to have occurred at a peak-shock pressure between 30 and 40 GPa. Shock-induced transformations were promoted only in unkinked and undeformed graphite booklets at the graphite-garnet, graphite-sillimanite, or graphite-rutile interfaces, where the difference in shock impedance is very high. Reverberations of shock waves with short wavelengths similar to the grain sizes at the phase boundaries are probably important constraints for dynamic graphite-diamond phase transformation. Raman spectroscopic investigations of hard transparent carbon platelets intercalated between fine-grained diamond and deformed graphite revealed the platelets to be Raman inactive. The platelets are either dense amorphous carbon or an unknown dense crystalline carbon phase that is Raman inactive.

Carbonate‐silicate liquid immiscibility upon impact melting: Ries Crater, Germany

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.

香川県高松クレーター産ガラスの岩石学的研究

Takamatsu crater has been identified and named by Kono et al., (1994), based on the presence of a concentric gravity low in the southern part of Takamatsu Plain, northeast Shikoku, Japan. The crater is 1∼2 km deep and ca. 4 km across, and buried under the Miocene volcanic formation and alluvium deposit. Both impact and volcanic origins have been proposed for the crater, but remained unresolved. We searched for the evidence of impact origin of the Takamatsu crater, such as shock metamorphic features of minerals, variable and extraordinarily high contents of SiO2, FeO, and NiO, together with low Na2O content of fresh glass in the Miocene deposit. The glass fragments in the pyroclastic flow deposit in the southern part of the Takamatsu crater are characterized by xenocrysts of quartz and feldspar and lack of idiomorphic phenocrysts. We did not find evidence for shock metamorphism in the minerals. The glass shows heterogeneous appearence with variable red tints and high vesicularity under the microscope, somewhat similar to the suevite of Ries crater. The glass of the Takamatsu crater, however, shows uniform and rhyolitic chemical composition, which is common for the calc-alkalic volcanics of the Miocene Setouchi volcanic suite. Furthermore, chemical composition of the glass is almost identical to those of welded tuff blocks and bulk matrix of the pyroclastic flow deposit, indicating that the glass fragments are of volcanic origin. The glass and associated rhyolite are dated by K-Ar method at around 14 Ma, which coincides with the age of the Setouchi volcanic activity. Although deep drilling has not been carried out for determining the subsurface structure, present analytical results argued for cauldron origin of the Takamatsu crater.

Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts

Abstract— Clasts of deep‐seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty‐three clasts, representing these lithologies, were analyzed for major element bulk composition.The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stöffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock‐fusion of a similarly composed basement.The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nördlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies.The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep‐burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.

Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada

The Slate Islands impact structure is the eroded remnant of a ∼30–32 km-diameter complex impact structure located in northern Lake Superior, Ontario, Canada. Target rocks are Archean supracrustal and igneous rocks and Proterozoic metavolcanics, metasediments, and diabase. A wide variety of breccias occurs on the islands, many of which contain fragments exhibiting shock metamorphic features. Aphanitic, narrow and inclusion-poor pseudotachylite veins, commonly with more or less parallel boundaries and apophyses branching off them, represent the earlist breccias formed during the compression stage of the impact process. Coarse-grained, polymictic clastic matrix breccias form small to very large, inclusion-rich dikes and irregularly shaped bodies that may contain altered glass fragments. These breccias have sharp contacts with their host rocks and include a wide range of fragment types some of which were transported over minimum distances of ∼2 km away from the center of the structure. They cut across pseudotachylite veins and contain inclusions of them. Field and petrographic evidence indicate that these polymictic breccias formed predominantly during the excavation and central uplift stages of the impact process. Monomictic breccias, characterized by angular fragments and transitional contacts with their host rocks, occur in parautochthonous target rocks, mainly on the outlying islands of the Slate Islands archipelago. A few contain fragmented and disrupted, coarse-grained, polymictic clastic matrix breccia dikes. This is an indication that at least some of these monomictic breccias formed late in the impact process and that they are probably related to a late crater modification stage. A small number of relatively large occurrences of glass-poor, suevitic breccias occur at the flanks of the central uplift and along the inner flank of the outer ring of the Slate Islands complex crater. A coarse, glass-free, allogenic breccia, containing shatter-coned fragments derived from Proterozoic target rocks (upper target strata), observed at two locations may be analogous to the ‘Bunte Breccia’ of the Ries crater in Germany. At one of these locations, this breccia lies close to a crater suevite deposit. At the other, it overlies parautochthonous, monomictic breccia. The Slate Islands impact breccias are superbly exposed, much better than breccias in most other terrestrial impact structures. Observations, including those indicative of multiple and sequential processes, provide insight on how impact breccias form and how they relate to the various phases of the impact process. Eventually they will lead to an improved understanding of planetary impact processes.

Diamonds from the iridium-rich K-T boundary layer at Arroyo el Mimbral, Tamaulipas, Mexico

Research Article| November 01, 1997 Diamonds from the iridium-rich K-T boundary layer at Arroyo el Mimbral, Tamaulipas, Mexico R. M. Hough; R. M. Hough 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar I. Gilmour; I. Gilmour 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar C. T. Pillinger; C. T. Pillinger 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar F. Langenhorst; F. Langenhorst 2Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany Search for other works by this author on: GSW Google Scholar A. Montanari A. Montanari 3Osservatorio Geologico di Coldigioco, I-62020, Frontale di Apiro (MC), Italy4École des Mines de Paris, Boulevard Saint-Michel, 75272, Paris Cedex 06, France Search for other works by this author on: GSW Google Scholar Author and Article Information R. M. Hough 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom I. Gilmour 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom C. T. Pillinger 1Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom F. Langenhorst 2Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany A. Montanari 3Osservatorio Geologico di Coldigioco, I-62020, Frontale di Apiro (MC), Italy4École des Mines de Paris, Boulevard Saint-Michel, 75272, Paris Cedex 06, France Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (11): 1019–1022. https://doi.org/10.1130/0091-7613(1997)025<1019:DFTIRK>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. M. Hough, I. Gilmour, C. T. Pillinger, F. Langenhorst, A. Montanari; Diamonds from the iridium-rich K-T boundary layer at Arroyo el Mimbral, Tamaulipas, Mexico. Geology 1997;; 25 (11): 1019–1022. doi: https://doi.org/10.1130/0091-7613(1997)025<1019:DFTIRK>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Diamonds, up to 30 µm in size, were found in the iridium-rich layer from the K-T boundary site at Arroyo El Mimbral and the spherule bed from Arroyo El Peñon, northeastern Mexico. Stepped heating experiments indicate two or more isotopically distinct diamond components with carbon isotopic compositions characteristic of a mixture of carbon sources. The diamonds' crystal form is cubic—not the hexagonal polymorph of diamond, lonsdaleite, which has been used previously to infer formation due to shock transformation of graphite. The size, crystallography, and mineralogic associations of K-T diamonds are similar to those of impact-produced diamonds from the Ries crater in Germany where both shock transformation of graphite and a mode of formation by condensation from a vapor plume have been inferred. The discovery of impact-produced diamonds in association with high Ir contents for these sediments supports their impact origin, K-T age, and the inference that their source was from the buried impact crater of Chicxulub on the Yucatan peninsula, Mexico. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Diamond and silicon carbide in impact melt rock from the Ries impact crater

SHOCK-PRODUCED diamond and lonsdaleite (the hexagonal polymorph) were first observed in experiments involving explosions1. Several classes of meteorites2,3 contain microcrystalline diamond aggregates that are thought to be produced by impacts with the Earth or in space. Diamonds have also been found in association with several Russian impact craters4 and in Cretaceous/Tertiary boundary impact ejecta5,6; these too have most often been interpreted as having formed by shock in the solid state4. Here we report the occurrence of diamond lonsdaleite plates and cubic diamond in association with silicon carbide, in impact melts from the Ries crater in southern Germany. We interpret these occurrences as evidence that these phases can be formed by chemical vapour deposition from the ejecta plume of an impact crater. It follows that cubic diamond and silicon carbide may be formed at any impact site from vaporized carbon-bearing rocks, and hence may be used as a reliable diagnostic tool for hypervelocity impact on Earth. This process may also explain the occurrence of diamonds found in sediments (carbonados7), which may result from the 'heavy bom-bardment' period of early Earth history, rather than from mantle-derived diatremes8.

Suevite breccia from the Ries crater, Germany: Origin, cooling history and devitrification of impact glasses

Abstract— The distribution and petrography of surficial suevite breccias of the Ries impact crater in Southern Germany are reviewed, and the morphology, petrography and chemical composition of impact glasses in suevite breccias and their postdepositional devitrification is synthesized. Origin and thermal history of suevite breccia and suevite glasses are inferred from these data and from recent results of cooling and crystallization experiments with suevite glass melts under controlled conditions. In a montmorillonitic groundmass, the suevite breccia contains pieces of glass, up to some decimeters in size, and crystalline rock clasts of all stages of shock metamorphism. The glass particles originated in impact melt of basement gneisses and cooled by adiabatic pressure release from ∼80 GPa to atmospheric pressure during ejection from the crater. They were deposited on the ground together with the other suevite components at a temperature of ∼750 °C. Fractured glass pieces in the breccia show that during deposition of the suevite the temperature was below the temperature at which undercooled melt transforms to rigid glass. The suevite cooled after deposition mainly by convection of heat by emanating gases and vapors. In chilled layers at the base and at the top of suevite deposits, the glasses are preserved in vitreous state. Between these zones, the glasses were devitrified, yet crystallization of pyroxene, plagioclase and magnetite took place below the glass‐transformation temperature. Annealing experiments show that this unusual devitrification below the transformation temperature can be explained by the impact origin of suevite glasses. Due to rapid adiabatic cooling on decompression, the glasses were oversaturated with water and internally strained. Under these conditions, devitrification, especially the formation of plagioclase, was possible at temperatures below the transformation range. The origin from adiabatically cooled impact melt of deep‐seated rocks distinguishes water‐bearing suevite glasses from the Ries‐derived, water‐free moldavite tektites, which are interpreted as condensates of vaporized, surficial sediments (Engelhardt et al., 1987).

Petrofabric examination of the main suevite of the Otting Quarry, Nördlinger Ries, Germany

Abstract— Extensive textural studies have been carried out at the suevite in a quarry several kilometers east of the rim of the Nördlinger Ries crater. The composition, grain size and clast orientation of suevite on a 6 m high vertical section were quantified macroscopicaily, as well as microscopically. There exists a strong correlation between the texture of a clastic rock and the transport mechanism of its components. This correlation can be used to obtain information about the transport mechanism of the suevite components, which is fundamental to the understanding of the impact process. A consolidated main suevite enriched in “Flädle” and “Bomben” can be distinguished from a poorly consolidated base suevite, deprived of “Fädle” but relatively well sorted in clast grains. It has been proven that the glass clasts in the main suevite exhibit an inverse gradation, while the crystalline clasts in the lower half of the section show a normal gradation. Eighty one percent of the samples investigated possess orientated clasts ≥2 mm. From the results of this investigation, a predominantly horizontal transport of the main suevite is indicated for the area of investigation. This transport could occur in the form of a suevitie flow similar to that of a pyroclastic flow.

Gas chromatographic/mass spectrometric analysis of cembrenoid diterpenes in kerogen from a lacustrine sediment

AbstractAlkaline hydrolysis of kerogen from a Miocene lacustrine sediment (Nördlinger Ries, southern Germany) afforded a series of cembrenoid diterpenes characterized by a 14‐membered ring system together with a second, probably related, series of yet unidentified structure. This is the first report of the occurrence of these compounds in fossil organic matter. The interpretation of cembrenoids as molecular markers of arid or semiarid climate is consistent with other information about the depositional environment of the former Nördlinger Ries crater lake.

Probing impacts in Fennoscandia

A symposium on impact crater structures in Fennoscandia was held on the Otaniemi Campus of the Helsinki University of Technology (HUT), Espoo, Finland, May 29–30, 1990. The symposium was hosted by the faculty of Materials Science of HUT (led by M. Peltoniemi and B. Söderholm), and sponsored by the Geological Surveys of Finland and Sweden, the Nordic Council of Ministers, the Academy of Finland, the Swedish National Science Research Council, and the Finnexploration Company. Sixty scientists, mainly from the Nordic countries but a few from the United States, Canada, Austria, Germany, Switzerland, Estonia, Czechoslovakia, and the Soviet Union participated in the meeting. Although similar symposia on impact structures have been held recently, for example, the Vredefort Symposium in South Africa (see Tectonophysics special issue, 1990) and the Ries Crater Symposium in Germany (see Meteoritics special issue, 1989), this was the first of its kind held in Scandinavia.

Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany—a review

The lithologies and distribution of the various ejecta formations from the Ries crater, Germany, are described. They may be subdivided into: (1) low shock level (< 10 GPa), represented by the Bunte Breccia (sediments from the upper 600 m of the target area), megablocks of sedimentary rocks, and megablocks and monomict breccias derived from the crystalline basement, and (2) high shock level (> 10 GPa) represented by polymict crystalline breccias, fall-out suevites, crater suevite, and tektites (moldavites). The ejecta from the Mesozoic sedimentary column do not show effects of shock pressures exceeding 10 GPa, and only basement gneisses were shock melted to yield suevite glasses. It is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level, where a complex of gneisses underlies shallower granitic intrusions. The estimated depth of the transient crater was about 2700 m and its diameter was 12–13 km. Bunte Breccia, megablocks of sedimentary rocks, and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits, extending up to a radius of 22 km. Fall-out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta. In contrast, the crater suevite never left the crater cavity; it represents the main mass of suevite. Moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised Tertiary sands, covering the surface of the target area.

Loss of radiogenic argon from shocked granitic clasts in suevite deposits from the Ries Crater

Five granitic clasts from the Otting and Aumühle quarries in the suevite ejecta deposits beyond the rim of the Ries, Germany, impact crater have been characterized as to modal composition, degree of shock, and loss of radiogenic argon. The shock pressures experienced by this selected suite of samples range from < 15 to > 60 GPa, and the samples have been modestly shocked to heavily shocked, or even melted. Ages for these samples were determined by the 39Ar 40Ar technique. The least shocked specimen (10–15 GPa) shows very little loss of radiogenic Ar relative to the approximately 320 My age of the Variscan basement at the Ries. All other samples (~28 GPa, ~42 GPa, ~52 GPa, and an impact melt) suggest complete to nearly complete loss of radiogenic Ar at the time of the 15 My old impact event. Most of the radiogenic Ar in these granitic samples is contained in feldspar, with a lesser amount in biotite. Diffusive loss of Ar occurs relatively easily from these highly disordered feldspars. The essentially complete loss of radiogenic Ar from 4 of the 5 samples is, in large part, ascribed to thermal activation during the cooling history of the suevite rather than during initial passage of the shock wave. These data are consistent with the conclusion from previous paleomagnetic studies that the suevite was deposited at temperatures above 500°C. The fact that Staudacher et al. (1982) found almost no loss of radiogenic Ar in a wide variety of Ries samples is ascribed to two causes: a) most of their samples were shocked to < 15 GPa and were collected in drill cores from the modestly shocked crater bottom, which was not as hot as the suevite; and b) their observations were primarily made on hornblende and biotite separates that are more resistant to shock effects and to diffusive loss of Ar, compared to shocked feldspars. Loss of argon from our suevite samples likely occurred in a “hot”, post-impact ejecta layer. From data presented here we estimate that < 50% of the material composing the “hot” Ries ejecta should show at least partially reset K-Ar ages; such materials compose no more than 5% of the total displaced crater volume.

The Allochthonous Polymict Breccia Layer of the Haughton Impact Crater, Devon Island, Canada

Abstract— The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain‐size characteristics, and degree of shock metamorphism for the grain‐size ranges 10–∼ 50, 1–10, 0.03–1, and &lt;0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock‐melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine‐grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock‐melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (∼850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air‐borne mode of emplacement for the upper section of the breccia in analogy to the fall‐back suevite in the Ries crater. A calculation of the excavation (Z‐model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock‐melted crystalline basement rocks, if present at all, must be confined to the very center of the structure below the excavation cavity.

The Haughton Impact Structure: Summary and Synthesis of the Results of the HISS Project*

Abstract— Surface and subsurface structural studies undertaken under the Haughton impact structure study (HISS) project indicate that the 23 Ma‐old Haughton impact structure, (Devon Island, Canadian Arctic) consists of a central basin of uplifted strata, an inner zone of uplifted megablocks at 3.5–5.5 km radius, a complex, faulted annulus of megablocks at 5.5–7.0 km radius and an outer zone of downfaulted blocks. No evidence of a previously suggested structural multi‐ring form was found. The geophysical studies suggest an original diameter of 24 km, slightly larger than previous estimates and the seismic data indicate considerably more faulting in the western portion than has been mapped from surface exposures. Detailed studies of the allochthonous breccia deposits found no major radial variations in lithology and shock levels. The only anomaly is the concentration of highly shocked, cobble‐sized clasts in the central area coincident with the maximum gravity and magnetic anomalies. It is suggested that this local component is related to the highly shocked rocks of the central uplift and may have been shed from the uplift during late stage adjustments. There is no visible central topographic peak of uplifted bedrock at Haughton but studies of the post‐impact Haughton Formation suggest that the center of the structure subsided 300–350 m soon after formation. Breccia studies also indicate the occurrence of shock‐melted sediments, including shales, but no evidence of shock melted carbonates, the most common target lithology. This may be ascribed to the ease with which carbonates are volatilized by relatively moderate shock levels. The large amount of volatiles released on impact helped disperse the highly shocked products leading to the formation of a relatively cool clastic and polymict breccia deposit in the interior, as opposed to a coherent melt sheet. In this regard, the breccia deposit is somewhat analogous to the suevite deposits within the Ries crater. Sedimentological studies indicate that the Cretaceous‐age Eureka Sound Formation was present at the time of impact and that the Haughton area has undergone as much as 200 m of erosion since the time of impact.