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A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events
Abstract
40Ar/39Ar dating of specimens of moldavite, the formation of which is linked to the Ries impact in southern Germany, with a latest-generation ARGUS VI multi-collector mass spectrometer yielded three fully concordant plateau ages with a weighted mean age of 14.808 ± 0.021 Ma (± 0.038 Ma including all external uncertainties; 2σ; MSWD = 0.40, P = 0.67). This new best-estimate age for the Nördlinger Ries is in general agreement with previous 40Ar/39Ar results for moldavites, but constitutes a significantly improved precision with respect to the formation age of the distal Ries-produced tektites. Separates of impact glass from proximal Ries ejecta (suevite glass from three different surface outcrops) and partially melted feldspar particles from impact melt rock of the SUBO 18 Enkingen drill core failed to produce meaningful ages. These glasses show evidence for excess 40Ar introduction, which may have been incurred during interaction with hydrothermal fluids. Only partially reset 40Ar/39Ar ages could be determined for the feldspathic melt separates from the Enkingen core. The new 40Ar/39Ar results for the Ries impact structure constrain the duration of crater cooling, during the prevailing hydrothermal activity, to locally at least ~60 kyr. With respect to the dating of terrestrial impact events, this paper briefly discusses a number of potential issues and effects that may be the cause for seemingly precise, but on a kyr-scale inaccurate, impact ages.
... More recently, modeling of 40 Ar/ 39 Ar ages from authigenic feldspar in the Chicxulub impact melt rock suggests that the impact-generated hydrothermal system was active for as long as 6 Myr . Whereas, Ries melt rock has been constrained to cool to hydrothermal temperatures (~250-300 • C) within 60-700 kyr (e.g., Arp et al., 2013;Buchner et al., 2010;Reimold et al., 2013Reimold et al., , 2012aReimold et al., , 2012bSchmieder et al., 2018). ...
... A subset of zircon grains from Ries gave (U-Th)/He weighted mean age within the uncertainty of previously published ages from other chronometers ( Fig. 9; Buchner et al., 2010;Schwarz et al., 2020;Schmieder et al., 2018). Within the impact melt rock and the suevite, 25 Early Miocene grains were recovered (Fig. 9) as well as three granular to partially granular grains that recorded ages <10 Ma and two moderately shocked grains that recorded ages >20 Ma from the impact melt rock (not shown; see Supplemental Material). ...
... Weighted mean ages and associated 2σ uncertainties are also denoted. The black line represents 40 Ar/ 39 Ar impact melt age: 14.81 ± 0.02 (Schmieder et al., 2018). Grey bar represents the maximum hydrothermal system estimation from Reimold et al. (2012aReimold et al. ( , 2012bReimold et al. ( , 2013. ...
... One of the most well-known tektite varieties is moldavite (Figure 1), which is named after the main occurrence along the Vltava (or Moldau in German) River in the Czech Republic. This tektitic glass originated during a meteorite impact around 14.8 million years ago in the region of today's Nördlinger Ries in Germany (Schmieder et al. 2018). The impact created temperatures that melted the rocks on the surface and blasted the glassy droplets eastward over 400 km to the upper reaches of the Vltava River and into Moravia in today's Czech Republic. ...
... The impact created temperatures that melted the rocks on the surface and blasted the glassy droplets eastward over 400 km to the upper reaches of the Vltava River and into Moravia in today's Czech Republic. Occasionally, moldavites have also been found in Austria, Germany and Poland (Brachaniec et al. 2014;Schmieder et al. 2018). ...
... Moldavite most likely originated during the meteorite impact responsible for the Nördlinger Ries crater in Bavaria, Germany. This crater formed approximately 14.8 million years ago, during the Miocene (Bouška & Konta 1999;Böhme et al. 2002;Di Vincenzo & Skála 2009;Schmieder et al. 2018;Schwarz et al. 2020). Gentner (1971) performed age determination using the K/Ar method, and showed that moldavite and the Ries event both have the same age, thus linking their origins. ...
... As a contraindication for dating purposes, it should be in principle mentioned the CaO contents, ranging from some to various percentage units (e.g., Trnka and Houzar, 2002;Skála et al., 2009), which makes interference corrections from neutron-produced Ar isotopes from Ca isotopes during irradiation, far from being negligible (see e.g., Turner, 1971). Moldavites have been subjected to several geochronological studies during the last decades (see for more details about previous geochronological data, Di Vincenzo and Skála, 2009;Buchner et al., 2013;Schwarz and Lippolt, 2014;Schmieder et al., 2018a), since the early K-Ar data of the 60s (Gentner et al., 1963(Gentner et al., , 1967Zähringer, 1963). First 40 Ar/ 39 Ar results on moldavites date back to the 80s (Staudacher et al., 1982;Lange et al., 1995) and were followed by several other studies (Schwarz and Lippolt, 2002;Laurenzi et al., 2003;Di Vincenzo and Skála, 2009;Schmieder et al., 2018a). ...
... Moldavites have been subjected to several geochronological studies during the last decades (see for more details about previous geochronological data, Di Vincenzo and Skála, 2009;Buchner et al., 2013;Schwarz and Lippolt, 2014;Schmieder et al., 2018a), since the early K-Ar data of the 60s (Gentner et al., 1963(Gentner et al., , 1967Zähringer, 1963). First 40 Ar/ 39 Ar results on moldavites date back to the 80s (Staudacher et al., 1982;Lange et al., 1995) and were followed by several other studies (Schwarz and Lippolt, 2002;Laurenzi et al., 2003;Di Vincenzo and Skála, 2009;Schmieder et al., 2018a). However, most of these studies were completed using old generation of single-collector noble gas mass spectrometers and without using astronomically calibrated reference minerals (such as biotite GA-1550, FCs or ACs). ...
... However, most of these studies were completed using old generation of single-collector noble gas mass spectrometers and without using astronomically calibrated reference minerals (such as biotite GA-1550, FCs or ACs). Exceptions include the works by Di Vincenzo and Skála (2009), who used the FCs reference mineral, and that by Schmieder et al. (2018a), who used the GA-1550 reference mineral and a multicollector noble gas mass spectrometer. Both these studies proved a high intrasample and intersample reproducibility of 40 Ar/ 39 Ar age data and the absence of extraneous Ar (either excess or inherited Ar, see Kelley, 2002), which represents a recurrent potential drawback in the 40 Ar/ 39 Ar dating of impact glasses (e.g., Pickersgill et al., 2020;Di Vincenzo et al., 2021). ...
Moldavites (Central European tektites) are genetically related to the meteorite impact event that produced the 24-km diameter Ries crater (Germany) during the Langhian, and representing one of the youngest large impact structures on Earth. Despite the numerous geochronological studies over the last decades and the potential implications for stratigraphic, paleontological and paleoclimatic studies, the age of the Ries impact is still debated. In this study, I investigate in detail moldavite samples by multicollector ⁴⁰Ar/³⁹Ar laser dating in order to address the age of the Ries impact. Data were obtained relative to the key Fish Canyon sanidine (FCs) and Alder Creek sanidine (ACs) reference materials, over a period of nearly two years, in four irradiations of different duration. Results, completed through the step-heating and the total fusion techniques, demonstrate an excellent intrasample and intersample reproducibility of moldavites and prove that analytical performances of moldavites in terms of uncertainties on the ⁴⁰Ar*/³⁹ArK ratios are in line with those achievable by the most widely used FCs and ACs reference minerals. Results from total fusion analyses yield a mean RMoldaviteACs = 12.4952 ± 0.0038 (±2σ) and a mean RMoldaviteFCs = 0.52106 ± 0.00014 (±2σ). Data also allow to define a direct mean RACsFCs = 0.041703 ± 0.000032 (±2σ) and an indirect (using moldavites as intermediary), and more precise, RACsFCs = 0.041700 ± 0.000019 (±2σ). Using the internally consistent astronomically-calibrated ages for the ACs (1.1848 ± 0.0012 Ma) and the FCs (28.201 ± 0.046 Ma) reference minerals from the literature, the R-values yield indistinguishable ages for moldavites of 14.7495 ± 0.0045 (± 0.016 Ma, including all known source of errors) and 14.7486 ± 0.0039 Ma (± 0.025 Ma), respectively. The most recent astronomically calibrated age for FCs (28.176 ± 0.023 Ma) yields a slightly younger, but more precise, age of 14.7355 ± 0.0039 Ma (± 0.013 Ma). Results place definitively the age of the Ries impact in the reverse polarity chron C5ADr of the ATNTS2022, and suggest that the Ries impact preceded the switch from reversed to normal chron by at least 127 ± 15 ka.
... For instance, the sensitivity of 18 O w to regional, global, and topographic variations in paleotemperature, environmental conditions of an air mass prior to orographic ascent, evapotranspiration, water vapor recycling, and changes in vapor source has been shown to introduce uncertainties in stable-isotopebased elevation reconstructions (e.g., Mulch, 2016;Botsyun et al., 2020, Botsyun andEhlers 2021). In particular, isotopic changes over continental Europe could be related to a variety of factors such as declining pCO 2 levels (Pagani et al., 1999), variable ocean circulation and sea surface temperatures (Flower and Kennett, 1994;Wright et al., 1992), sea-level fluctuations (Foster and Rohling, 2013), paleogeographic changes (Herold et al., 2008;Poblete et al., 2021), and other processes affecting 18 O w (Botsyun et al., 2019;Poulsen et al., 2007;Risi et al., 2008;Roe et al., 2016;Sewall and Fricke, 2013;Sturm et al., 2010). We thus compare our newly refined near-sea-level 18 O estimate with paleoclimate simulations from the isotope-enabled ECHAM5-wiso atmospheric general circulation model (iGCM), which predicts changes in 18 O of precipitation. ...
... Our study of Miocene paleosol carbonate takes advantage of numerous pedogenic soil horizons embedded in a detailed geochronologic framework determined from previous studies of paleomagnetostratigraphy (Kälin and Kempf, 2009;Kempf et al., 1997;Schlunegger et al., 1996), radiometric age data on volcanic ash layers (Gubler et al., 1992;Schmieder et al., 2018), and mammal biostratigraphy (e.g., Bolliger, 1992;Kälin, 1997). The local paleomagnetic sections were correlated with the astronomically tuned Neogene timescale (ATNTS2012) of Hilgen et al. (2012). ...
... Pedogenic horizons are up to 50 cm thick and occasionally show mottling in grey, purple, and yellow. Age constraints of the Fontannen section are given by deposits associated with the Ries meteorite impact (14.81 ± 0.02 Ma; Schmieder et al., 2018) and by four mammal faunal sites in three zones (MN 4, MN 5, and MN 6;Kälin, 1997). We inferred pedogenic carbonate ages based on the local magnetostratigraphy (Schlunegger et al., 1996;Kempf et al., 1997;Figs. ...
Reconstructing Oligocene–Miocene paleoelevation
contributes to our understanding of the evolutionary history of the European
Alps and sheds light on geodynamic and Earth surface processes involved in
the development of Alpine topography. Despite being one of the most
intensively explored mountain ranges worldwide, constraints on the elevation
history of the European Alps remain scarce. Here we present stable and
clumped isotope measurements to provide a new paleoelevation estimate for
the mid-Miocene (∼14.5 Ma) European Central Alps. We apply
stable isotope δ–δ paleoaltimetry to near-sea-level
pedogenic carbonate oxygen isotope (δ18O) records from the
Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine
phyllosilicate hydrogen isotope (δD) records from the Simplon Fault
Zone (Swiss Alps). We further explore Miocene paleoclimate and
paleoenvironmental conditions in the Swiss Molasse Basin through carbonate
stable (δ18O, δ13C) and clumped (Δ47)
isotope data from three foreland basin sections in different alluvial
megafan settings (proximal, mid-fan, and distal). Combined pedogenic
carbonate δ18O values and Δ47 temperatures
(30±5 ∘C) yield a near-sea-level precipitation
δ18Ow value of -5.8±1.2 ‰ and, in
conjunction with the high-Alpine phyllosilicate δD value of -14.6±0.3 ‰, suggest that the region surrounding the
Simplon Fault Zone attained surface elevations of >4000 m no
later than the mid-Miocene. Our near-sea-level δ18Ow
estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for
the Northern Alpine Foreland Basin.
... The 26 km diameter 13 Ries impact structure (southern Germany; Fig. 1) is one of the best-preserved complex impact structures on Earth, e.g., Ref. 13 . The structure formed at 14.808 ± 0.038 Ma 17 . The nearby ~ 3.8 km in diameter Steinheim impact structure 19 is considered to have formed simultaneously, although this has been challenged in a recent publication 20 . ...
... Our age estimate of 14.74-14.91 Ma for the Blockhorizont impact ejecta layer is fully consistent with the 14.808 ± 0.038 Ma age of the Ries impact structure 17 . All results taken together, i.e., our palynological and paleontological investigations that reveal that mudstone and limestone clasts recovered from the Blockhorizont have ages that are consistent with equivalent lithologies in the Ries target stratigraphy, as well as the reported geochemical analysis, are consistent with clasts being derived from the Ries area, and, along with the age estimate of the horizon, support the hypothesis that the Blockhorizont is in fact indeed derived from the Ries impact. ...
... Ref.17 , and small, inset map in top left corner modified from Ref.15 .While the correlation between bentonites occurring in eastern Switzerland (Winterthur, Mollen-Waldkirch, Rengishalden-Bischofszell, and Schosstobel) was uncertain30 , recent zircon ages obtained by the British Geological Survey have now shown that all these horizons correspond to the same horizon28 , which is also supported by lithostratigraphy29 . The weighted mean age, as calculated by us, of the four localities is 14.34 ± 0.03 Ma. ...
Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.
... Millionen Jahren im Gebiet des heutigen Nördlinger Ries zurückzuführen ist (SCHMIEDER et al., 2018). Durch den Impakt des Meteoriten entstanden Temperaturen, die das an der Oberfläche anstehende Gestein verdampften und in Form von Schmelztropfen in östliche Richtung über 400 km bis an den Oberlauf der Moldau und nach Mähren im heutigen Tschechien verfrachteten. ...
... Die Entstehung von Moldaviten ist sehr wahrscheinlich auf den Meteoritenimpakt zurückzu-führen, durch welchen das Nördlinger Ries in Bayern entstanden ist. Dieser Impaktkrater wurde vor 14,808 (+/-0,038) Millionen Jahren im Miozän gebildet (SCHMIEDER et al., 2018, SCHWARZ et al., 2020. Durch den Einschlag wurde das anstehende Gestein stark verdichtet und hohen Temperaturen ausgesetzt, die zu einer Verdampfung führten und Schmelztropfen mit bis zu 25facher Schallgeschwindigkeit in östliche Richtung verfrachteten. ...
Eines der bekanntesten und populärsten natürlichen Gläser ist Moldavit, welches sich durch einen Meteoritenimpakt vor ca. 14,8 Millionen Jahren gebildet hat und heute insbesondere in Tschechien gefunden und abgebaut wird. Im Frühjahr 2022 besuchten die Autoren die größte derzeit im Abbau befindliche Mine, eine Sand und Kiesgrube nahe der kleinen Ortschaft Chlum im Süden Tschechiens, in der Moldavite als Beiproduktion gefunden werden. Der vorliegende Beitrag gibt einen Überblick zur derzeitigen Abbauaktivität im Allgemeinen und stellt zum anderen die gemmologischen Charakteristika der während der Exkursion gesammelten Proben vor.
... The Ries impact structure is an ideal site to study calcite deformation during impact cratering, because the impactites are well preserved with no influence from subsequent tectonic deformation since the impact about 14.8 Ma ago Schmieder et al., 2018aSchmieder et al., , 2018bSchwarz et al., 2020). In this study, we use the microfabrics of twinned calcite in calcite-bearing metagranite cataclasites within megablocks close to the peak ring of the Nördlinger Ries structure to identify the relevant deformation mechanisms (Figure 1). ...
... The Ries is a complex impact structure with a diameter of about 26 km and was formed about 14.8 Ma ago in the middle Miocene Schmieder et al., 2018a;Schwarz et al., 2020). The impact affected the sedimentary rocks of the Swabian-Franconian Alb as well as the underlying Variscan basement, consisting mostly of metagranites, paragneisses, and amphibolites (e.g., Graup, 1978;Hüttner & Schmidt-Kaler, 1999;Von Engelhardt & Graup, 1984). ...
Shock-related calcite twins are characterized in calcite-bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre-impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (<7 GPa). Calcite, however, shows a high density (>1/μm) of twins with widths <100 nm. Different types of twins (e-, f-, and r-twins) crosscutting each other can occur in one grain. Interaction of r-and f-twins results in a-type domains characterized by a misorientation relative to the host with a misorientation angle of 35°-40°and a misorientation axis parallel to an a-axis. Such a-type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer.
... After extensive discussion of older age determinations, Schmieder et al. (2018) date the Ries event to 14.808 AE 0.038 Ma, based on moldavites. However, the closest reverse interval (C5ADr) ranges from 14.609 to 14.775 Ma (Raffi et al., 2020), which would require the Ries event to be slightly younger than 14.775 Ma and this correlates poorly with the magnetostratigraphic framework . ...
... A similar result is obtained by the most recent dating of Di Vincenzo (2022) with 14.7355 AE 0.013 Ma. If the ages of Schmieder et al. (2018) and Di Vincenzo (2022) are appropriate and the Ries impact occurred in the lower part of the reverse interval C5ADr, then there is a time gap of at least 100,000 years until the onset of fine-grained normalmagnetized lake sedimentation. This time interval is represented by coarse-grained fluvial sediments that do not allow polarity determination. ...
The Ries impact is the most important cosmic event in the younger geological history of Europe. Its effects reach far beyond the area considered so far and are documented in manifold evidence. In this paper, the widely scattered reports in the literature are compiled and supported with investigations by the authors. Besides well-known ejecta features like the Brockhorizont, Reuter's blocks, and moldavites, little known or forgotten indications, like a lechatelierite and β-cristobalite occurrence in Bavaria and unusual sedimentation phenomena in northern Germany, are presented. The paleogeographic reconstruction shows that the Ries impact occurred on the southern side of the Neogene Central European mainland. Large parts of this erosional area were devastated by the impact. Pressure waves and thermal radiation had a lasting effect on the landscape within hundreds of kilometers around the impact site. Destruction of the vegetation cover by impact-induced storms, wildfires, and heavy rainfall generated intense erosion. The adjacent sedimentation area to the north (Paleo-North Sea) experienced an increased and short-term supply of terrestrial debris to the marine environment. The stratigraphic coincidence of these exceptional sediments with the Ries event leads us to conclude that the distal effects of the impact are present here, which have so far received little or no attention in this context. The paper considers the different indications and sets them in a large-scale context.
... Accordingly, we recently suggested the two impact structures are likely of different age, supported by biostratigraphic, sedimentologic, and event stratigraphic evidence 3 . While the basal crater lake sediments within the ~ 14.81 Ma Ries crater 5,6 correspond to the transition from the European land mammal zones (ELMZ) MN 5 to MN 6 (Langhian), the oldest crater lake deposits at Steinheim fall in the transition zone of higher MN 6 to MN 7 (latest Langhian and Serravallian 3,12,34,35 ). ...
... The Ries seismite, i.e., the ~ 10-15 m-thick lower seismite unit and overlying distal Ries ejecta 3,14,18 , forms a unique continental seismite-ejecta couplet within a distance of 180 km from the crater 3 . The structural inventory within the Ries seismite, characterized by distinct soft-sediment deformation and dewatering structures described recently (such as sand spikes 3,27 ) indicates far-reaching seismic effects of the 14.81 Ma Ries impact 5,6 . The seismite, which is exposed at several localities within the NAFB 3,16,27 features metre-sized slumps, usually with NW-SE-striking slump axes, convolute bedding, and ball-and-pillow and flame structures 3 . ...
For decades, the Nördlinger Ries and Steinheim Basin in southern Germany have been regarded as a textbook example of a terrestrial impact crater doublet, although the oldest crater lake deposits in both craters suggest a biostratigraphic age difference of ~ 0.5 to 1 Myr. We previously presented stratigraphic arguments that challenged the double impact scenario and favoured a model of two temporally independent impact events in the Mid-Miocene. We here present, for the first time, four localities within a distance of ~ 50–100 km from the Ries and ~ 50–70 km from the Steinheim crater that expose two independent seismite horizons, together unique within the Upper Freshwater Molasse of the North Alpine Foreland Basin, each one featuring impressive water escape structures. The seismite horizons are separated by ~ 10 to 15 m of undisturbed Molasse deposits and, biostratigraphically, by an entire European Land Mammal Zone, thus providing evidence for two independent major seismic events within a time span of ~ 0.5–1 Myr. Both the lower and the upper seismite horizons can be correlated litho- and biostratigraphically with the basal crater lake sediments at the Ries and Steinheim craters, respectively, deposited immediately after the impacts. From a biostratigraphic point of view, the impact event that formed the Steinheim Basin probably occured around 14 Ma, some 0.8 Myr after the ~ 14.81 Ma Ries impact event.
... Considering the reverse magnetisation of the Suevite (in C5Bn.1r), the formation of the Ries crater must have occurred between 14.884 ±0.010 Ma and 14.870 ±0.003 Ma (Rocholl et al. 2018). A 40 Ar-39 Ar high-precision analysis of tektites yielded 14.808 ±0.038 Ma (Schmieder et al. 2018). In the deeper parts of the Ries impact structure the pre-Mesozoic crystalline basement was severely affected by impact metamorphism. ...
... Numerous geochronological studies in the Nördlinger Ries on various shocked crystalline target lithologies yielded Palaeozoic and especially Carboniferous ages which suggest that major portions of the local target rock were not or not fully chronometrically reset by the impact event (Staudacher et al. 1982;Horn et al. 1985;Schmieder et al. 2018;Schwarz et al. 2020;Tartèse et al. 2022). The Ries meteorite impact occurred in a two-layer target. ...
Monazite in lithoclasts of suevite impact breccia in the Nördlinger Ries (Bavaria, Germany) and its Th-U-Pb dating by electron probe microanalysis
Bernhard Schulz, Jan-Michael Lange, Joachim Krause, Dana Czygan
Abstract
In the Lehenberg (Lehberg or Limberg North) quarry in the NW part of the Megablock Zone of the Nördlinger Ries impact crater, granite and micaschist lithoclasts occur in a polymict suevite impact breccia. The lithoclasts display the petrographic characteristics of the shock metamorphism scale, as cavities filled with diaplectic glass, decorated planar elements in quartz grains, and severly kinked mica. In petrographic thin sections, monazite grains were detected by scanning electron microscope based automated mineralogy methods of spectral mapping. In backscattered electron imaging (BSE), monazite revealed the typical crystal shapes, and internal Th zoning and distribution pattern as known from igneous and metamorphic crystallization. Intragrain signs of shock metamorphism in a minority of monazite grains are strictly straight and parallel crack pattern resembling lamellae structures. Monazite mineral chemistry and bulk chemical Th-U-Pb ages were investigated by electron probe microanalyser (EPMA). The igneous and metamorphic monazites display contrasting and typical mineral-chemical properties. Metamorphic monazites follow strictly the cheralite substitution trend in Th + U vs Ca coordinates. Igneous monazite in an alkalifeldspar granite has the highest Y2O3 contents (~2 wt%) among all studied samples. In ThO2* vs PbO coordinates the monazite data define isochrones. Micaschist lithoclasts yielded 328 ± 3 Ma, 326 ± 6 Ma and 324 ± 5 Ma, interpreted to represent the thermal peak and post-peak age of metamorphic monazite crystallization. The 328 ± 5 Ma age of igneous monazite in the alkalifeldspar granite in contact to micaschist is interpreted to date the crystallisation of a synmetamorphic anatectic melt. This contrasts the 313 ± 3 Ma monazite crystallization age in a post-tectonic monzogranite. No indications of bulk Pb loss in monazite by shock metamorphism have been observed. The EPMA Th-U-Pb monazite ages from the lithoclasts match data from granites and meta-psammopelites in the outcropping pre-Mesozoic basement in the Western Bohemian Massif and the Black Forest. They confirm that the bottom of the Nördlinger Ries impact crater is situated in crystalline basement rocks belonging to the Moldanubian Zone.
Kurzfassung
Im Steinbruch am Lehenberg (auch Lehberg oder Limberg-Nord) im NW-Teil der Megablock-Zone des Nördlinger Rieskraters kommen Granit- und Glimmerschiefer-Lithoklasten in einer polymikten Suevit-Impaktbrekzie vor. Die Lithoklasten zeigen die petrographischen Merkmale der Stoßwellenmetamorphose-Skala, wie Einschlüsse mit diaplektischem Glas, Quarzkörner mit dekorierten planaren Elementen und stark geknickte Glimmer. In petrographischen Dünnschliffen wurde Monazit durch Spektralkartierung mit einem automatisierten Rasterelektronenmikroskop detektiert. Im Rückstreuelektronenbild (BSE) zeigt Monazit die für magmatische und metamorphe Kristallisation typischen Kristallformen sowie die internen Zonierungen und Verteilungsmuster von Th. Nur wenige Monazitkörner zeigen die für Stoßwellenmetamorphose typischen Internstrukturen wie scharf parallel angeordnete Risse die Lamellen bilden. Mineralchemie und Th-U-Pb-Alter der Monazite wurden mit der Elektronenstrahlmikrosonde bestimmt. Magmatische und metamorphe Monazite zeigen unterschiedliche Zusammensetzungen. In Th + U vs Ca Koordinaten folgen die metamorphen Monazite dem Cheralith-Substitutions-Trend. Magmatischer Monazit im Alkalifeldspat-Granit hat die höchsten Y2O3-Gehalte (~2 wt%) der untersuchten Proben. In ThO2* vs PbO Koordinaten definieren die Monazitanalysen Isochronen. In den Glimmerschiefer-Lithoklasten liegen die Th-U-Pb-Monazitalter bei 328 ± 3 Ma, 326 ± 6 Ma und 324 ± 5 Ma und werden als Alter der Metamorphose-Maximaltemperatur und nachfolgender Abkühlung interpretiert. Die Monazit-Isochrone von 328 ± 5 Ma im Alkalifeldspat-Granit im Kontakt zum Glimmerschiefer wird als Kristallisationsalter einer synmetamorphen anatektischen Schmelze interpretiert. Sie unterscheidet sich deutlich vom jüngeren Kristallisationsalter des posttektonischen Monzogranits mit seinen 313 ± 3 Ma alten Monaziten. Es ergeben sich keine Anzeichen für Verlust von Gesamt-Pb in Monazit durch die Stosswellenmetamorphose. Die Elektronenstrahlmikrosonden-Th-U-Pb-Alter der Monazite in den Lithoklasten gleichen denen von Graniten und Metapsammopeliten im prä-mesozoischen Basement der westlichen Böhmischen Masse und des Schwarzwalds. Dies belegt dass im Untergrund des Rieskraters die kristallinen Basementgesteine der Moldanubischen Zone anzutreffen sind.
Keywords: suevite impact breccia, micaschist, granite, monazite, microstructures, Th-U-Pb age dating, Moldanubian Zone
Schlüsselwörter: Suevit-Impaktbrekzie, Glimmerschiefer, Granit, Monazit, Mikrostrukturen, Th-U-Pb-Altersbestimmung, Moldanubisches Basement
... The glass fission-track method used to determine the age of several tephra samples from the Kanguk Formation was the zeta calibration method, which is based on analysis of age standards (Hurford and Green 1983;Wagner and Van den Haute 1992). The age standard used was the Moldavite tektite glass, which has a 40 Ar/ 39 Ar age of 14.808 6 0.021 Ma (2r) (Schmieder et al. 2018). The population-subtraction variant was used (Westgate 2015), and correction for partial track fading was done using the diameter correction fission-track (DCFT) method (Sandhu and Westgate 1995) or the isothermal plateau fission-track (ITPFT) technique (Westgate 1989). ...
... Â 10À10 year À1 . Zeta value was determined to be 311 6 4 based on seven irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, Canada, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass with an 40 Ar/ 39 Ar age of 14.808 6 0.021 Ma (2r) (Schmieder et al. 2018). Number of tracks counted is given in parentheses; number of tracks measured is given in square brackets. ...
More than 50 conspicuous tephra beds occur in the Kanguk Formation on the southwestern coast of Banks Island. Their glass shards are remarkably well preserved and permit comprehensive characterization, offering the potential for reliable, precise correlation of Upper Cretaceous sedimentary rocks across the three major depocentres of the Arctic Archipelago and adjacent northern continental margin: Sverdrup, Banks, and the Beaufort–Mackenzie basins. Twenty-one tephra beds were analyzed; all have a high-K, peraluminous, rhyolitic composition, with quartz, plagioclase, ilmenite, biotite, and zircon as the dominant minerals. Trace-element concentrations, especially low Nb and Ta, show that the parental magmas formed in a continental-margin subduction environment. Glass fission-track ages range from 100 Ma to younger than 60 Ma, and indicate a very low sedimentation rate giving a very condensed sedimentary sequence on southwestern Banks Island — a sequence that may well contain the K–Pg transition. Source calderas are unknown but most likely are situated in east-central Alaska and the central and northern Kuskokwim volcanic belt, some 1000 to 1500 km distant from southwestern Banks Island. It is also possible that some of the very thin tephra beds come from the Okhotsk–Chukotka volcanogenic belt in northeastern Russia.
... The mid-Miocene Ries impact in southern Germany is Central Europe's biggest cosmic disaster in Phanerozoic geologic history. While earlier theories posited the Ries impact was contemporaneous with the smaller nearby Steinheim event in a double-impact scenario, it was recently shown that the Ries, with a Langhian age of 14.81 Ma (Schmieder et al. 2018), predates Steinheim (Serravallian) by a few hundred thousand years . In addition to the distribution of impact ejecta, the Ries impact triggered a powerful (M W ~ 8.5) earthquake with far-reaching environmental effects . ...
... The estimated stratigraphic age of the Chöpfi host sandstone of ~ 15 million years, corresponding to early MN 6 (Bolliger 1998), postdates major seismotectonic activity within the North Alpine Foreland Basin (Keller 2012), but lines up well with the age for the Ries impact (Schmieder et al. 2018). At ~ 14.81 Ma, the local OSM deposits would have been unconsolidated and close enough to the land surface to experience strong seismic soft-sediment deformation, as observed at a number of localities across the basin Sach et al. 2020; Fig. 1B). ...
... In fact, as of 2020, 200 impact structures are listed in the compilation of terrestrial crater ages, and only 36 of them have ages with a precision better than ±2% at the two-sigma level (Schmieder and Kring, 2020; see also earlier work by Jourdan, 2012;Jourdan et al., 2009Jourdan et al., , 2012. Thanks to the advance of modern mass spectrometry, precision at the ±several ‰ level on isotopic ages is now achievable and even expected for well-behaved samples, with for example a precision of ±0.14 % (all uncertainties are given at 2r throughout the text) for the moldavite (Central European) tektites (14.808 ± 0.021 [0.038, including decay constant uncertainties] Ma; Schmieder et al., 2018), ± 0.36 % for the Australasian tektites (788.1 ± 2.8 [3.0] ka; Jourdan et al., 2019) and even as precise as ±0.033 % when a very large number of concordant analyses are pooled together, as for the Chicxulub impact (66.043 ± 0.022 [0.086] Ma; Renne et al., 2013). ...
... We illustrate the critical role of the 40 Ar/ 39 Ar method for determining ages of impact events. The method provides even more precise ages since the recent technological leap in multi-collection noble gas mass spectrometers (see e.g., Schmieder et al., 2018;Jourdan et al., 2019). Using the present data combined with numerical simulations of 40 Ar* diffusion and mixing, we show that some of the drawbacks of the method (Jourdan, 2012), e.g., sensitivity of the system to sample alteration, the presence of partly degassed clasts in the sample, or the presence of inherited 40 Ar*, can be overcome by careful sample preparation and methodology (see also e.g., Jourdan et al., 2009Jourdan et al., , 2012. ...
The possibility of a “death from above” cause for biotic crises and extinction events is intriguing, to say the least, but such claims must be supported by reliable and reproducible data, not only impact diagnostic criteria, but also accurate and precise radioisotopic ages of the impact structures/events. To date, only one example of such an impact related global extinction event is confirmed, at the end of the Cretaceous period.
Here we present and discuss results of newly obtained ⁴⁰Ar/³⁹Ar data from step heating analysis of impact melt rock samples from the 40 km-in-diameter Puchezh-Katunki impact structure, Russia, which allow us to precisely and accurately date its formation at 195.9 ± 1.0 Ma (2σ; P = 0.10). Based on these new data, we challenge the proposed temporal correlation with as many as five different extinction events (including the end-Triassic mass extinction) that were based on previous age estimations ranging from ∼164 to 203 Ma. Our new age for the formation of the Puchezh-Katunki impact structure allows us to exclude a relationship between this impact event and a known extinction event.
We also show that careful sample preparation and methodology can overcome problems with inherited and trapped ⁴⁰Ar, issues that are common when dating impact melt rocks. This is supported by ⁴⁰Ar* diffusion and mixing numerical models showing that the most prominent negative effects in the case of the Puchezh-Katunki impact melt rock samples are caused by hydrothermal alteration and undegassed melt rock domains present in an otherwise homogenized melt rock. Numerical modeling also shows that the ⁴⁰Ar* from high-Ca inherited crystals or clasts is decoupled from the melt rock during step heating experiments allowing to safely recover a plateau age.
Finally, our results highlight the importance of improving the database of ages of impact structures and show that caution should be practiced when suggesting connections between specific impact events and extinction events, especially in the case of poorly dated impact structures.
... We followed methods outlined in Wolfe et al. (2017) in all respects, with one exception: glass shards were etched with HF acid for an additional 15 s, thereby increasing total acid etch time from 180 s to 195 s. The glass fission-track age was calculated using the zeta-approach, based on the Moldavite tektite and its most current 40 Ar/ 39 Ar age of 14.808 ± 0.021 Ma (2σ; Schmieder et al., 2018). The Huckleberry Ridge tephra, with a 40 Ar/ 39 Ar sanidine age of 2.003 ± 0.014 Ma (Gansecki et al., 1998), was used as a secondary reference material for assessing the accuracy of the glass fission-track ages. ...
The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66−56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ∼50 m record of lacustrine silt and indicates an age ∼10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.
... The 24-km-diameter Ries impact structure, Germany (Fig. 1D), has been dated at 14.808 ± 0.021 Ma (Schmieder et al., 2018). The pre-impact stratigraphy from which the BB is derived comprises ∼600 m of sedimentary rocks, including ∼25 m of unconsolidated middle Miocene sandstones, mudstones, and marls, 190 m of Upper Jurassic limestone, 65 m of Middle and Lower Jurassic claystones and sandstones, and 300 m of Triassic sandstones and claystones (Engelhardt et al., 1995). ...
The presence of extensive clay minerals in the ancient Noachian terrains of Mars is often used to invoke past climatic conditions that were warmer and supported surface-stable liquid water. These clay-rich regions are also heavily cratered, leading to the possibility of a causal relationship. The aim of this study is to better understand the impact excavation and generation of clays and whether there are any mineralogical or geochemical indicators that could differentiate between these two origins, both on Earth and, by analogy, Mars. Here, we present a detailed field and laboratory investigation of the composition, texture, and setting of clay minerals in impactites at the well-preserved Ries impact structure, Germany. Authigenic impactite (syn- and post-impact) clay minerals in impact melt-bearing breccia deposits are compared with sedimentary-derived clay mineral-bearing units preserved from the time of the impact event. Our findings indicate: (1) impact-generated deposits comprise compositionally diverse, Al-dominant smectitic clay minerals that could have formed without appreciable exogenous volatiles through a combination of autometamorphism, hydrothermal alteration, and devitrification; and (2) the pre-impact sedimentary clay mineral assemblages were similar in composition to those in the impact-generated deposits such that only detailed, successive laboratory treatments and analyses could discern the two sample types. NASA’s Perseverance Mars rover mission is presently investigating its first science campaign and has identified secondary alteration products, including possible clay minerals. Our study suggests that the rover may explore impact-generated clay minerals in situ, though their provenance might only be determined from analysis of the returned samples in Earth laboratories.
... These differ significantly from after-organic diamonds which have a lower micro-Raman shift (1318-1324 cm À1 ) and may be more akin to the diamonds seen at Ries (Figure 6). Importantly, a lower volume of pre-impact organic life during the Mesoproterozoic (Brasier et al., 2019;Wellman & Strother, 2015) could mean the volume of carbon-rich impact products may differ significantly between the Stac Fada Member and the much more recent Ries and Kara impact craters, dated at~14.8 and 70.3 Ma, respectively, when a more modern biosphere existed (Schmieder et al., 2018;Schwarz et al., 2020;Trieloff et al., 1998). As noted in Shumilova et al. (2020), natural diamonds called "karite" are present at the Kara astrobleme, suggesting the conversion of organic life at the time of impact to diamond is possible. ...
The Stac Fada Member (Stoer Group) is a ~1.2 Ga melt‐rich impact breccia preserved and intermittently exposed along the NW coast of Scotland. Using a combination of x‐ray diffraction and micro‐Raman spectroscopy, we identify potential coesite that is spatially associated with micron‐sized diamonds, as well as disordered carbon phases. Comparing the graphite G‐band of disordered carbon phases in the impact breccia to samples from underlying units indicates that most of the carbon in the Stoer Group was ultimately derived from the underlying Lewisian basement. Disordered carbon phases within the Stac Fada Member have been modified by mild heating within a hot ejecta blanket rather than shock pressure. We also report the first evidence for impact diamonds discovered within the Stac Fada Member. These diamonds have an average Raman shift of 1328.5 cm ⁻¹ and are present within both the impact breccia and the shocked gneiss clasts that are present in sandstones directly underlying the Stac Fada Member contact, and within sandstone rafts entrapped in the unit. These findings have implications for the timing of deposition of the Stac Fada Member, which must have occurred after ballistic ejection of Lewisian basement clasts during the impact event.
... Die partienweise pflanzenführenden, bis zu ein Meter mächtigen Mergel-Schichten stehen im Josefstobel als Stillwasser-Einschaltungen innerhalb der sonst fluviatil geschütteten und teilweise aufgearbeiteten Sand-Fazies der Oberen Süßwassermolasse an. Die Pflanzenfundstelle befindet sich lithostratigraphisch nur ~ 1-2 Meter oberhalb des im Josefstobel vorkommenden ("unte-ren") Seismit-Horizonts, welcher erst jüngst von einem der beiden Autoren (V.J.S.) entdeckt wurde (BUCHNER et al. 2022) und durch das Starkbeben des Ries-Impakts vor ~ 14,8 Millionen Jahren (SCHMIEDER et al. 2018) verursacht wurde. Der pflanzenführende Mergel-Horizont deckelt diesen Ries-Seismit-Horizont in ungestörter Lagerung, so dass er und die in ihm enthaltene OSM-Flora zwar post-riesisch (nach dem Meteo-riteneinschlag) abgelagert wurde, aber sehr wahrscheinlich ebenfalls ein Alter von ~ 14,8 Millionen Jahre haben. ...
Im folgenden Beitrag soll eine - zumindest für den mitteleuropäischen Raum - als klassisch zu bezeichnende Fossilfundstelle neogener (jungtertiärer) Pflanzen, der sogenannte „Josefstobel“ nahe der oberschwäbischen Kreisstadt Biberach an der Riß, näher beschrieben werden. Der Josefstobel und seine Fundstelle befinden sich am sogenannten Hochgeländ bei Awengen, zwischen den beiden Ortschaften Ummendorf-Fischbach und Eberhardzell (Baden-Württemberg, SW-Deutschland). Die Pflanzenfundstelle befindet sich lithostratigraphisch nur etwa 1-2 Meter oberhalb des im Josefstobel vorkommenden „Unteren Seismit-Horizonts“, welcher durch das Starkbeben des Ries-Impakts während des Mittel-Miozäns verursacht wurde. Der – hier vorgestellte – pflanzenführende Mergel-Horizont deckelt diesen Ries-Seismit-Horizont in ungestörter Lagerung, so dass er und die in ihm enthaltene OSM-Flora zwar post-riesisch abgelagert wurde, aber zeitlich dennoch gut mit dem Ries-Alter von etwa 14,8 Millionen übereinstimmt. Bei den Fundobjekten handelt es sich insgesamt zu über 95 Prozent um Blattabdrücke von Daphnogene polymorpha, meist die schmalblättrige Form D. polymorpha bilinica. Selten sind schmälere bzw. breitere Reste anderer Daphnogene-Variationen oder -Arten belegt. Außerdem fanden sich einige Blattabdrücke und Samenkapseln verschiedener Pappel-Arten (Populus) und unbestimmter Bäume sowie Stängelreste von Schilf- und Schachtelhalm-Gewächsen (Phragmites, Equisetum). Auch einige Fossilreste der Fauna des ehemaligen Josefstobel-Gewässers liegen vor. So konnten bei den Sondierungen mehrere Gehäuse von Süßwasserschnecken (Planorbarius cornu, cf. Galba sp.) sowie teilweise zweiklappige Schalen von Süßwassermuscheln (Anodonta sp.) geborgen werden. Beim Schlämmen von Proben des pflanzenführenden Mergels fanden sich außerdem einige Fossilreste (Einzelzähne, Wirbel und Gräten) von Knochenfischen, darunter Schlundzähne von Weißfischen der Gattung Palaeoleuciscus.
... The sediment succession is situated within the Upper Freshwater Molasse (OSM) between two marker horizons: the so called Brock-Horizon (Ries crater material) below, at the base of the sand pit, and 14.6 Myr old bentonites above, which were dated by Rocholl et al. (2018b) and crop out in the nearby pit of Unterwohlbach. For the Brock Horizon currently differing magneto-stratigraphic results indicate ranges ages of 14,8 and14.9 million years, respectively (Rocholl et al., 2018a(Rocholl et al., , 2018bSchmieder et al., 2018aSchmieder et al., , 2018b. However, based on small mammal stratigraphy, a cyclical subdivision of the Middle Miocene Upper Freshwater Molasse prepared by Fiest (1986; and later published by Heissig (2006), allows to narrow down the age of the Entrischenbrunn deposits to an interval between 14.9 to 14.7 million years (Fiest, pers. ...
The 1 m thick intercalated marly sediments of Entrischenbrunn represent a floodplain pond within a braided fluvial system of late Langhian age. The organic rich upper part of the marls that are characterized by the presence of numerous plant macrofossils, are investigated for palynomorphs with SEM. The results reveal that many typical azonal broad leaved forest elements were present (e.g., Fraxinus and Liquidambar) and some of the common macrofossils, such as Hemitrapa, Podocarpium, Platanus sp., Quercus spp. etc. However, because of non-resistant pollen walls, the two dominant components of the macroflora, Daphnogene and Populus are not represented in the palynoflora. In contrast, plants not known from macrofossils or rare for this locality are present: several aquatic plants (e.g., Callitriche sp., Ludwigia sp., Decodon sp.), and a large portion of more subhumid and sclerophyllous plants such as Celtis “tournefortii-type”, two Ephedra spp., two Erica spp. affiliable with Mediterranean species, Quercus sp. section Ilex, a Sapoteae, Sideroxylon sp., Rehderodendron sp., Ulmus sp. section Foliaceae Zelkova “abelicea/serrata-type”, and Ziziphus “mucronata/mauritania/jujube-type”, whereas Diospyros sp. and cf. Royena sp. can be interpreted to present humid to subhumid conditions. The reasons for this mixture might be threefold: 1. localized well-drained substrates and water loss on particularly pebbly, gravelly and sandy deposits of the braided river system or 2. presence of a geographically controlled localized lower humidity in that area, or 3. because of a generally drier period within the Middle Miocene.
... This region is mainly composed of limestone formations hosting numerous caves and rock shelters, some of which were occupied by humans during the Pleistocene and Holocene. The main geological break in the region is represented by the N€ ordlinger Ries (NR in Fig. 1a), which is an impact crater of Miocene age (Schmieder et al., 2018). The portions of the Jura located to the West and East from the NR are known as Swabian and Franconian Jura, respectively. ...
After the Last Glacial Maximum, the Swabian and Franconian Jura (in SW and SE Germany, respectively) were repopulated by Magdalenian hunter-gatherers within the same communication network. However, while the Magdalenian settlement of the Swabian Jura dates to 17–14 ka cal BP, permanent Magdalenian occupations in the Franconian Jura date to 15–14 ka cal BP. In comparison with its western counterpart, the Franconian Jura was mostly excavated in the early days of archaeological research. Does this different chronology reflect the different history of research? Why did Magdalenian foragers establish permanent occupation in the Franconian Jura nearly 2 millennia after settling in Swabia, despite the fact these regions are only 150 km apart? To address these questions, we reinvestigated two sites in the Altmühl Valley with micromorphology and luminescence dating, namely Felsenhäusl-Kellerhöhle and Klausennische. Our data show that both sites have intact Pleistocene deposits. Among these, we identified sediments dating between 17 and 15 ka that show only rare lithic artifacts and microfeatures indicative of cold and arid conditions. Our work and published data suggest that the steady settlement of Magdalenian foragers in the Altmühl Valley starting 15 ka cal BP coincides with the end of this harsh period and the onset of cool and wetter environments. Data from the Swabian Jura demonstrated that in the Lone Valley, similar environments and Magdalenian occupations commenced earlier, starting 17 ka cal BP. Therefore, we propose that regional environments acted as a barrier against the dispersal of foragers in the Franconian Jura and determined its later Magdalenian occupation. Our research highlighted that different environments, taphonomic processes, and site uses probably coexisted across the German Jura. Therefore, it remains fundamental to expand the multisite data set proposed in this article to further test hypotheses about human/environment interaction in this region.
... The rather young age of this structure (c. 14.8 Buchner et al. 2013;Schwarz and Lippolt 2014;Schmieder et al. 2018;Schwarz et al. 2020) favored the preservation of a thick central fallback deposit and a far-reaching ejecta blanket outside the crater. An overview on the geology of the target rocks as well as the crater-fill and ejecta deposits along with some geophysical data is given by Pohl et al. (1977) and Stöffler et al. (2013). ...
Amphibolite clasts in the suevite of the Ries impact crater contain shock-induced melt veins (SMVs) with high-pressure phases such as majoritic garnet, jadeitic clinopyroxene and others. In addition, heat conduction from hot SMVs into adjacent rock portions locally produced further high P–T melt pools. These melts were preferentially generated in rock domains, where the SMVs cross older (‘pre-Ries’) veinlets with analcime or prehnite and larger grains of sericitized plagioclase. Melting of such chemically different local bulk systems (Na-, Ca-, Ca-Na- and K-Na-rich) was facilitated by low solidus temperatures of the original secondary OH-bearing phases. From the resulting shock-induced melts, liebermannite, kokchetavite, jadeite, nonstoichiometric and albitic jadeite, grossular, vuagnatite, lawsonite + coesite, and clinozoisite crystallized during pressure release. Vuagnatite is now proven to be a genuine high-pressure phase. Its ubiquitous distance of 20–35 μm from the hot shock veins suggests a temperature sensitivity typical for an OH-bearing phase. In local Na-rich melts albitic jadeite appears instead of the assemblage jadeite + SiO2. Liebermannite, a dense polymorph of K-feldspar was identified by Raman spectroscopy. After stishovite, liebermannite constitutes the second known high-pressure phase in the Ries that contains silicon exclusively in six-fold coordination. The KAlSi3O8-polymorph kokchetavite was formed in alkali-rich melt glasses. Pressure and temperature values in the range of about 8–11 GPa and ~ 800–1100 °C were estimated from the chemical compositions of locally occurring majoritic garnets (Si = 3.21–3.32 and 3.06–3.10 apfu), respectively, and the presence of fine-grained aggregates of lawsonite and coesite. Generally, the neighboring areas of the veins are characterized by a sequence of variable high-pressure phases documenting strongly falling P–T conditions with increasing distance from the vein. These novel features enlighten the dynamic event during passage of a shock wave.
... Der Rieskrater ist ein komplexer Impaktkrater mit einem Durchmesser von etwa 26 km und entstand vor etwa 14,8 Ma im mittleren Miozän (Rocholl et al. 2018a(Rocholl et al. , 2018bSchmieder et al. 2018aSchmieder et al. , 2018bSchwarz et al. 2020). Vom Impakt betroffen waren einerseits die Sedimentgesteine des Deckgebirges der Schwäbisch-Fränkischen Alb und andererseits das unterlagernde Grundgebirge, bestehend aus Gneisen, Metagraniten, Amphiboliten und Ultrabasiten. ...
... The oldest strewn field known is the North American (NA) one of 35.3 Ma age (± 0.1) associated with the 85 km diameter Chesapeake Bay (USA) impact structure and includes Bediasites, Georgianites, Barbados and Cuba tektites. The Central European (CE) or moldavite strewn field of 14.8 Ma age (± 0.2; Schmieder et al. 2018) is associated with the Nördlinger Ries impact structure of about 24 km in diameter (Nördlinger Ries, Bavaria, Germany). There is another impact crater, the Steinheim crater, ~3.8 km in diameter, located about 42 kilometers west-southwest from the center of Nördlinger Ries. ...
OVERVIEW
On Earth, natural glasses are typically produced by rapid cooling of melts, and as in the case of minerals and rocks, natural glasses can provide key information on the evolution of the Earth. However, natural glasses are products not solely terrestrial, and different formation mechanisms give rise to a variety of natural amorphous materials. In this chapter, we provide an overview of the different natural glasses of non-magmatic origin and on their formation mechanisms. We focus on natural glasses formed by mechanisms other than magmatic activity and included are metamorphic glasses and glasses produced from highly energetic events (shock metamorphism). The study of these materials has strong repercussions on planetary surface processes, paleogeography/paleoecology, and even on the origin of life.
... Moldavites (named after the Moldau/Vltava River), or Central European tektites, known for their attractive deep-green colour were formed from the uppermost sedimentary rocks layer during the Ries impact event in Miocene (Skála et al., 2016;Buchner et al., 2020). The best estimates of the moldavite age as well as that of the Ries crater (Bavaria, Germany), corrected for most recently suggested 40 K decay constants, (Jourdan et al., 2012in Skála et al., 2016 currently vary between 14.74 ± 0.20 Ma and 14.83 ± 0.15 Ma (Di Vincenzo and Skála, 2009;Buchner et al., 2010;Schwarz and Lippolt, 2014;Schmieder et al., 2018). The moldavite tektites were formed by an early ejection of surficial impact melt or from condensates of vaporized surficial sediments, Oligocene and Miocene fluviatile-lacustrine sediments (mainly sands and limestones of up to 50 m total thickness; see Stöffler et al., 2013 and references therein). ...
The area of Southern Bohemia between Novohradské hory Mountains and the Třeboň and Budějovice Basins – the Novohradské Foothills (in the south of the Czech Republic) is known for the occurrence of moldavites. These tektites are abundant in fluvial Koroseky sands and gravels. This study focuses on reconstructing the paleostreams, which deposited the moldavite-bearing sediments, and their relationship to present streams. Based on a review of the deposits' occurrences, a connection between moldavite-bearing sediments and deposits of the present watercourses, Vltava, Malše, Stropnice and Svinenský, is proposed. The link between moldavite-bearing deposits and the current streams were further supported by analyses of river geometry (longitudinal profiles, stream gradient) and terrain morphology (valley floor ratio, basin asymmetry). It is suggested that the moldavite-bearing deposits were sedimented during the Pliocene or Pleistocene by predecessors of the present streams. The occurrences of particular fluvial deposits and changes in terrain morphology and river geometry can be traces of changes in the river network and the dynamic evolution of local relief induced by tectonic activity during the Pliocene and Pleistocene.
... It is important to note that the timing of sand spike formation in the NAFB is not only stratigraphically consistent with the age of the Ries impact in the Mid-Miocene (Langhian) 20,21 , but also at odds with the timeline of active tectonism and seismicity associated with the Alpine orogeny 64 No major seismic events are evident in the NAFB after 17.5 Ma 64 and, accordingly, the UFM is generally unaffected by Alpine folding [33][34][35] . The Mid-Miocene decline in Alpine tectonic activity is coupled with decreasing subsidence in the NAFB, marked by the so-called "pre-Riesian hiatus" around 16 Ma 33 . ...
Sand spikes, pin-shaped, carbonate-cemented sandstone bodies of variable size widely
interpreted as sedimentary concretions, have been enigmatic for nearly two centuries. We
here present a high-energy mechanism for their formation. Two classic sand spike occurrences are found in the North Alpine Foreland Basin of Central Europe and at Mount Signal in southern California, USA. A distinct seismite horizon in Mid-Miocene Molasse sediments of southern Germany, genetically linked with the Ries impact event, exhibits dewatering structures and contains numerous sand spikes with tails systematically orientated away from the Ries crater. Sand spikes at Mount Signal, strikingly similar in shape to those found in Germany, have tails that point away from the nearby San Andreas Fault. Based on their structural and stratigraphic context, we interpret sand spikes as a new type of seismite and a promising tool to identify strong impact-induced or tectonic palaeo-earthquakes and their source regions in the geologic record.
... In this study, it is assumed that the meteorite impact occurred at ∼14.9 Ma (Langhian, Miocene) in line with most ages published in the last decade: 14.92 ± 0.02 Ma (Rocholl et al., 2018), 14.808 ± 0.038 Ma (Schmieder et al., 2018), 14.89 ± 0.34 and 14.75 ± 0.22 Ma (Schwarz et al., 2020) (see details in Data repository A). Shortly after the impact, a fluvial system was established within the crater (Jankowski, 1981;Buchner and Schmieder, 2009;Stöffler et al., 2013). ...
The Nördlinger Ries Crater lacustrine basin (South-West Germany), formed by a meteorite impact in the Miocene (Langhian; ∼14.9 Ma), offers a well-established geological framework to understand the strengths and limitations of U-Pb LA-ICPMS (in situ Laser Ablation-Inductively Coupled Plasma Mass Spectrometry) geochronology as chronostratigraphic tool for lacustrine (and more broadly continental) carbonates. The post-impact deposits include siliciclastic basinal facies at the lake centre and carbonate facies at the lake margins, coevally deposited in a time window of >1.2 and <2 Ma. Depositional and diagenetic carbonate phases (micrites and calcite cements) were investigated from three marginal carbonate facies (Hainsfarth bioherm, Adlersberg bioherm and Wallerstein mound). Petrography combined with C and O stable isotope analyses indicate that most depositional and early diagenetic carbonates preserved pristine geochemical compositions and thus the U-Pb system should reflect the timing of original precipitation. In total, 22 U-Pb ages were obtained on 10 different carbonate phases from five samples. The reproducibility and accuracy of the U-Pb (LA-ICPMS) method were estimated to be down to 1.5% based on repeated analyses of a secondary standard (speleothem calcite ASH-15d) and propagated to the obtained ages. Micrites from the Hainsfarth, Adlersberg and Wallerstein facies yielded ages of 13.90 ± 0.25, 14.14 ± 0.20 and 14.33 ± 0.27 Ma, respectively, which overlap within uncertainties, and are consistent with the weighted average age of 14.30 ± 0.20 Ma obtained from all the preserved depositional and early diagenetic phases. Data indicate that sedimentation started shortly after the impact and persisted for >1.2 and <2 Ma, in agreement with previous constraints from literature, therefore validating the accuracy of the applied method. Later calcite cements were dated at 13.2 ± 1.1 (nw=2), 10.2 ± 2.7 and 9.51 ± 0.77 Ma, implying multiple post-depositional fluid events. This study demonstrates the great potential of the U-Pb method for chronostratigraphy in continental systems, where correlations between time-equivalent lateral facies are often out of reach. In Miocene deposits the method yields a time resolution within the 3rd order depositional sequences (0.5–5 Ma).
... If tektite production was on average characterizing less than 10% of large impacts, the probability to obtain a sequence of 3 out of 4 events is less than 3 per mil. The next-youngest well-dated large craters include El'gygytgyn, at 3. 6 Ma, and the tektite producing Ries crater at 14.81 Ma 10,44 . Therefore, there is still a majority (4 out of 6) of large tektiteproducing craters since the Middle Miocene. ...
Tektites are terrestrial impact-generated glasses that are ejected long distance (up to 11,000 km), share unique characteristics and have a poorly understood formation process. Only four tektite strewn-fields are known, and three of them are sourced from known impact craters. Here we show that the recently discovered Pantasma impact crater (14 km diameter) in Nicaragua is the source of an impact glass strewn-field documented in Belize 530 km away. Their cogenesis is documented by coincidental ages, at 804 ± 9 ka, as well as consistent elemental compositions and isotopic ratios. The Belize impact glass share many characteristics with known tektites but also present several peculiar features. We propose that these glasses represent a previously unrecognized tektite strewn-field. These discoveries shed new light on the tektite formation process, which may be more common than previously claimed, as most known Pleistocene >10 km diameter cratering events have generated tektites.
... The Ries impact structure (Germany) is an ~24-km-diameter impact structure (Pohl et al., 1977) with an age of 14.808 ± 0.021 Ma determined by 40 Ar-39 Ar (Schmieder et al., 2018;Schmieder and Kring, 2020). The target at the Ries structure is composed of a Hercynian crystalline basement covered by Mesozoic sedimentary rocks (Schmidt-Kaler, 1978). ...
Quantitative insights into the geochemistry and petrology of proximal impactites are fundamental to understand the complex processes that affected target lithologies during and after hypervelocity impact events. Traditional analytical techniques used to obtain major- and trace-element data sets focus predominantly on either destructive whole-rock analysis or laboratory-intensive phase-specific micro-analysis. Here, we present micro–X-ray fluorescence (μXRF) as a state-of-the-art, time-efficient, and nondestructive alternative for major- and trace-element analysis for both small and large samples (up to 20 cm wide) of proximal impactites. We applied μXRF element mapping on 44 samples from the Chicxulub, Popigai, and Ries impact structures, including impact breccias, impact melt rocks, and shocked target lithologies. The μXRF mapping required limited to no sample preparation and rapidly generated high-resolution major- and trace-element maps (~1 h for 8 cm2, with a spatial resolution of 25 μm). These chemical distribution maps can be used as qualitative multi-element maps, as semiquantitative single-element heat maps, and as a basis for a novel image analysis workflow quantifying the modal abundance, size, shape, and degree of sorting of segmented components. The standardless fundamental parameters method was used to quantify the μXRF maps, and the results were compared with bulk powder techniques. Concentrations of most major elements (Na2O–CaO) were found to be accurate within 10% for thick sections. Overall, we demonstrate that μXRF is more than only a screening tool for heterogeneous impactites, because it rapidly produces bulk and phase-specific geochemical data sets that are suitable for various applications within the earth sciences.
... In general, the ejecta deposits of most terrestrial impact structures are either eroded or covered by post-impact sediment. A notable exception in this regard is the Ries crater, Germany, which despite its age of 14.81 Ma (Schmieder et al., 2018) has a rather well preserved two-layer ejecta blanket, whereby a layer of suevite (an impact breccia with clastic as well as melt clasts, for example, Stöffler & Grieve, 2007;Stöffler et al., 2013Stöffler et al., , 2018) overlies a variegated, polymict lithic breccia layer known as Bunte Breccia (Sturm et al., 2013, and references cited therein). Ries suevite forms the upper unit of the proximal ejecta blanket. ...
Geomorphological study of some of the just more than 200 known terrestrial impact structures has demonstrated that despite extensive degradation, important geomorphological keys, such as drainage pattern, topographic signatures, erosional landforms, and depositional features, can still be assessed. They can provide possible indicators to assist in the recognition of further impact structures, especially on Precambrian shields and cratonic landmasses. This study documents the surface features and landforms of the Paleoproterozoic, about 11 km diameter Dhala impact structure in India. The Dhala structure has an estimated age that is constrained stratigraphically between 1.7 and 2.5 Ga. This structure is deeply eroded, and barely has a morphological resemblance to other known terrestrial or extraterrestrial impact structures. We have analyzed the operative surface‐forming processes for the Dhala area.
We demand to continue the in‐depth study of all terrestrial impact structures, especially the pre‐Paleozoic ones, so that geomorphological criteria can be rigorously constrained and applied in conjunction with a priori remote sensing and field data to support the identification of new structures prior to their ultimate confirmation using diagnostic evidence of shock metamorphism.
... The 24 km diameter (Pohl et al. 1977) Ries impact structure, Germany (Fig. 2a), has been dated at 14.808 AE 0.021 Ma (Schmieder et al. 2018). The target is comprised of a 500-800 m thick, predominately Mesozoic sedimentary sequence overlying a crystalline basement having a variety of gneisses, amphibolites, and granites (Graup 1978). ...
The impact melt‐bearing breccias at the Ries impact structure, Germany, host degassing pipes: vertical structures that are inferred to represent conduits along which gases and fluids escaped to the surface, consistent with hydrothermal activity that occurs soon after an impact event. Although the presence of degassing pipes has been recognized within the well‐preserved and long‐studied ejecta deposits at the Ries, a detailed mineralogical study of their alteration mineralogy, as an avenue to elucidate their origins, has not been conducted to date. Through the application of high‐resolution in situ reflectance imaging spectroscopy and X‐ray diffraction, this study shows for the first time that the degassing pipe interiors and associated alteration are comprised of hydrated and hydroxylated silicates (i.e., Fe/Mg smectitic clay minerals with chloritic or other hydroxy‐interlayered material) as secondary hydrothermal mineral phases. This study spatially extends the known effects of impact hydrothermal activity into the ejecta deposits, beyond the crater rim. It has been suggested that the degassing pipes at the Ries are analogous to crater‐related pit clusters observed in impact melt‐bearing deposits on Mars, Ceres, and Vesta. The results of this work may inform on the presence of crustal volatiles and their interaction during the impact process on rocky bodies throughout the solar system. The Mars 2020 Perseverance rover may have the opportunity to investigate impact‐related features in situ; if so, this work suggests that such investigations may provide key information on the origin and formation of clay minerals on Mars as well as hold exciting implications for future Mars exploration.
Aggregates of ilmenite with varying amounts of rutile, ferropseudobrookite, and pseudorutile in suevites from the Ries impact structure have been analyzed by light microscopy, analytical scanning electron microscopy, electron microprobe analysis, and Raman spectroscopy to constrain their formation conditions. The tens to hundreds of micrometer aggregates comprise isometric ilmenite grains up to 15 µm in diameter that form a foam structure (i.e., smoothly curved grain boundaries and 120° angles at triple junctions). Grains with foam structure show no internal misorientations, indicating a post-impact formation. In contrast, ilmenite grains with internal misorientation occurring in the core of the aggregates are interpreted as shocked remnant ilmenite originating from the target gneisses. They can contain twin lamellae that share a common {1120} plane with the host, and the c-axis is oriented at an angle of 109° to that of the host. Similarly, the new grains with foam structure display up to three orientation domains, sharing one common {1120} plane for each pair of domains and c-axes at angles of 109° and 99°, respectively. This systematic orientation relationship likely reflects a cubic supersymmetry resulting from the transformation of the initial ilmenite upon shock (>16 GPa) to a transient perovskite-type high-pressure phase (liuite), subsequent retrograde transformation to the polymorph wangdaodeite, and then back-transformation to ilmenite. Whereas, the new grains with foam structure formed from complete transformation, the twin domains in the shocked ilmenite are interpreted to represent only partial transformation. Ferropseudobrookite occurs mostly near the rim of the aggregates. An intergrowth of ferropseudobrookite, ilmenite, and rutile, as well as magnetite or rarely armalcolite occurs at contact with the (devitrified) matrix. The presence of ferropseudobrookite indicates high temperature (>1140 °C) and reducing conditions. The surrounding matrix provided Mg2+ to form the ferropseudobrookite-armalcolite solid solution. Rutile can occur within the aggregates and/or along the ilmenite boundaries; it is interpreted to have formed together with iron during the decomposition of ilmenite at lower temperatures (850–1050 °C). We suggest magnetite in the rims formed by electrochemical gradients driven by the presence of a reducing agent, where Fe2+ within ilmenite diffused toward the rim. Subsequent cooling under oxidizing conditions led to the formation of magnetite from the iron-enriched rim as well as pseudorutile around ilmenite grains.
Our study demonstrates that the specific crystallographic relationships of ilmenite grains with foam structure indicate a back-transformation from high (shock) pressures >16 GPa; moreover, the presence of associated Fe-Ti-oxides helps indicate local temperature and oxygen fugacity conditions.
Large‐scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic anomalies. Despite magnetic anomalies in craters being well‐documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post‐impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact‐melt bearing lithologies. We report new chemical, rock‐, and mineral‐magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO‐18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low‐cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post‐impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral‐to‐reducing post‐impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak‐ring.
The Ries impact structure (southern Germany) formed ca. 15 Ma and is 22−26 km in diameter, making it one of the youngest and best-preserved mid-size terrestrial impact craters, yet the subsurface has not been studied with modern geophysics. We present the first high-resolution seismic profiles of the Ries impact structure; the profiles show discontinuous intra-basement reflectors and a central crater floor without a significant central topographic high. The inner crystalline ring sits adjacent to, not on top of, the crater terrace zone. These morphologies indicate that during the crater modification stage, the rebounding central uplift at Ries rose and then collapsed without the continued outward motion required to form a fully developed peak ring. The Ries impact structure may be best considered a transitional complex crater form between a central-peak crater and a peak-ring crater as documented on the Moon and other rocky planets. A series of high-amplitude, discontinuous, topographically influenced reflectors overlying the basement implies that the suevite within the crater basin was emplaced via lateral transport.
Der Riesimpakt ist das bedeutendste kosmische Ereignis in der jüngeren Erdgeschichte Mitteleuropas. Seine Wirkungen reichen weit über Süddeutschland hinaus und dokumentieren sich in vielfältigen Zeugnissen. In dieser Arbeit werden die umfangreichen, aber sehr verstreuten Hinweise aus der Literatur zusammengetragen und mit eigenen Untersuchungen untersetzt. Neben den bekannten Ejekta, wie Brockhorizont, Reutersche Blöcke und Moldavite, werden hier vor allem die wenig bekannten oder vergessenen Indizien und ungewöhnliche Sedimen-tationserscheinungen wie der Spezialton Heide und die Seeser Geröllgemeinschaft in der Lausitz sowie der Helle Horizont in Norddeutschland behandelt. Der Artikel diskutiert die Plausibilität der Korrelation mit dem Riesereignis.
The Ibex Hollow Tuff, 12.08 ± 0.03 Ma (40Ar/39Ar), is a widespread tephra layer erupted from the Bruneau-Jarbidge volcanic field of southern Idaho. Tephra from this eruption was deposited across much of western and central North America and adjacent ocean areas. We identified the Ibex Hollow Tuff at Trapper Creek, Idaho, near its eruption site, and at 15 distal sites, from the Pacific Ocean to the Gulf of Mexico, by the chemical composition of its glass shards, using electron-microprobe analysis, instrumental neutron activation analysis, and laser-ablation–inductively coupled plasma–mass spectrometry. By these methods, we distinguished the Ibex Hollow Tuff from overlying and underlying tephra layers near its source and at distal sites. Fluvially reworked Ibex Hollow Tuff ash was transported by the ancestral Mississippi River drainage from the interior of the North American continent to the Gulf of Mexico, where it is present within an ~50-m-thick deposit in marine sediments in the subsurface. The minimum fallout area covered by the ash is ~2.7 million km2, with a minimum volume of ~800 km3, and potential dispersal farther to the north and northeast. The areal distribution for the Ibex Hollow Tuff is similar to that of the Lava Creek B (0.63 Ma) supereruption. The Ibex Hollow Tuff represents a unique chronostratigraphic marker allowing a synoptic view of paleoenvironments at a virtual moment in time across a large terrestrial and marine region. The Ibex Hollow Tuff is also an important marker bed for North American Land Mammal Ages, and it coincides with climatic cooling in the middle to late Miocene documented in marine cores.
This paper presents a brief synopsis of the Miocene Epoch, an important transitory chapter in the history of the Earth. It was during the Miocene that the major continents and oceans attained a “modern” configuration in terms of paleogeography and tectonics, oceanic ventilation and circulation, ocean chemistry, and faunal and floral assemblages. It also was during the Miocene that global climate fully transitioned into its current icehouse state, including marked growth of the Antarctic ice sheet and initiation of the Arctic ice cap. Long-term global cooling was controlled by a number of factors including tectonics, the large-scale changes in the distribution of flora, particularly the expansion of grasslands, and by fluctuating orbital parameters of the Earth. This global cooling trend was briefly interrupted by a short period of warming in the middle Miocene.
Miocene sea-level changes consisted of a number of glacio-eustatic third-order (1–5 million year [m.y.] duration) cycles superposed upon three longer-term, second-order (5–20 m.y. duration) supercycles. Development of large-scale tropical carbonate systems in the Miocene was relegated to three main geographic regions: the circum-Caribbean, Mediterranean, and Indo-Pacific. In addition, a pronounced cool-water platform system developed along the southern margin of Australia. Miocene reefal buildups were dominated by tropical to subtropical framework assemblages consisting primarily of large scleractinian corals, encrusting red algae, and rhodoliths (free-living coralline red algae) that grew on platform margins and interiors or on isolated atolls. Miocene carbonates were deposited in a variety of oceanic and structural settings and constitute important petroleum reservoirs, particularly in Southeast Asia. Deep-water terrigenous clastic sediments of Miocene age are also important petroleum reservoirs in some regions. In addition, the Miocene interval contains numerous prolific petroleum source rocks, most composed of Type III (gas-prone) kerogen.
The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum. The Miocene climatic optimum could have been triggered by coeval increase in magmatism, suggests global carbon cycle simulations constrained by seawater osmium isotope data from Pacific Ocean sediments.
For decades, the Old Crow tephra has been a prominent stratigraphic marker for the onset of Marine Isotope Stage (MIS) 5e, the last interglaciation, in subarctic northwest North America. However, new zircon U-Pb dates for the tephra suggest that the tephra was deposited ca. 207 ka during MIS 7, with wide-ranging implications for chronologies of glaciation, paleoclimate, relict permafrost, and phylogeography. We analyzed ∼1900 detrital glass shards from 28 samples collected at Integrated Ocean Drilling Program Site U1345 in the Bering Sea, which has a well-constrained age model from benthic foraminiferal δ18O. Except for one possibly contaminant shard dated at 165 ka, Old Crow tephra was absent from all samples spanning 220–160 ka. Old Crow tephra appeared abruptly at 157 ka, comprising >40% of detrital shards between 157 and 142 ka. This abrupt increase in the concentration of detrital Old Crow tephra, its absence in earlier intervals, and its presence at low concentrations in all samples between 134 and 15 ka collectively indicate that the tephra was deposited during the middle of MIS 6 with a likely age of 159 ± 8 ka. As a result, the late Quaternary chronostratigraphic framework for unglaciated northwest North America remains intact, and the timing of key events in the region (e.g., bison entry into North America; interglacial paleoclimate; permafrost history; the penultimate glaciation) does not require wholesale revision.
Lacustrine deposits are extensively investigated because they play a pivotal role as environmental recorders and host valuable economic resources. However, chronostratigraphic reconstructions in these settings are usually hampered by the scarcity of data required to establish the depositional age of the system. The prime objective of this PhD project was examining benefits and limitations of carbonate U-Pb (LA-ICPMS) dating (LAcarb) as new chronostratigraphic tool for lacustrine deposits. The second objective was to investigate the dating potential, in terms of dating success and time resolution, of various carbonate types and mineralogies with the aim to better organize future LAcarb based research. To achieve these goals, depositional (microbialites, ooids, oncoids) and early diagenetic carbonate phases of known absolute age were collected from two lacustrine settings: the Ries Crater basin (Miocene, SW Germany) and the Yacoraite formation (Fm.) from the Salta rift basin (Cretaceous-Paleogene, NW Argentina). A sedimentologic and diagenetic study based on petrography and Oxygen (O) and Carbon (C) stable isotope analysis allowed to select carbonate phases that most possibly preserved the pristine U-Pb geochemical composition and consequently inform on the timing of deposition. In the Ries Crater basin, LAcarb provided accurate ages that allowed chronostratigraphic correlations at a time resolution of the 3rd order stratigraphic sequence (0.5–5Ma). In the framework of the Yacoraite Fm., two depositional age depth model were obtained along a stratigraphic section. One derived from zircon (ash layer) geochronology (maximum depositional age depth model; MDA depth model) and the other from LAcarb (minimum depositional age depth model; MIDA depth model). The two models remarkably overlap and describe the same sedimentation rate dynamic with a time resolution between 0.9 and 2% (2σ). The revised depositional age of the Yacoraite Fm. was then integrated in a chronostratigraphic model merging biostratigraphic, chemostratigraphy and magnetostratigraphy data from literature. Accordingly, the KPg limit was interpreted to be located in the Yacoraite Fm. whereas the two Paleocene-Eocene hyperthermals (Paleocene-Eocene Thermal Maximum, PETM; early Eocene Climate Optimum, EECO) were identified in the two overlying formations. A statistical analysis was achieved based on the ages of 80 carbonate phases from the Yacoraite Fm. Microbialites provided the lowest dating success (41% ages consistent with the MDA depth model) and age precision (2σ < 10%) in contrast with lacustrine cements that yielded the highest dating success (64% ages consistent with MDA depth model) and age precision (2σ < 3%). Furthermore, calcitic phases usually provided more precise ages than dolomitic phases. The better comprehension of potentialities and limitations of LAcarb acquired in this PhD allowed to propose a workflow to build a robust depositional age depth model in lacustrine settings. Three possible perspective scenarios were introduced: 1) the study of the Ries Crater basin as analogue of paleolakes on Mars; 2) the use of LAcarb to select pristine carbonates for chronostratigraphic studies; and 3) basin scale chronostratigraphic correlations in the Yacoraite Fm.
Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for ⁴⁰K-⁴⁰Ar (or ⁴⁰Ar/³⁹Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (<1 Ma) tephra. Here, we demonstrate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2σ uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and ⁴⁰Ar/³⁹Ar ages: Biederman tephra (178 ± 17 ka), HP tephra (680 ± 47 ka), Gold Run tephra (688 ± 44 ka), Flat Creek tephra (708 ± 43 ka), PA tephra (1.92 ± 0.06 Ma), Quartz Creek tephra (2.62 ± 0.08 Ma), Lost Chicken tephra (3.14 ± 0.07 Ma), and GI tephra (542 ± 64 ka). We also present newly revised glass fission-track and ⁴⁰Ar/³⁹Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for ²³⁰Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other stratigraphically important tephra in the region.
Impact crater central peaks and peak ring complexes are important exploration targets for future missions to other planetary bodies, because they provide access to material uplifted from lower crustal levels. Material exposed there could also provide chronological constraints on crater formation events. Therefore, it is essential to understand if uplifted peak material preserves the chronological records of igneous and metamorphic protolith crustal rocks, or if such records are reset during impact events. To investigate this issue, we collected shocked gneiss and granite samples from uplifted crystalline basement megablocks in the 24 km diameter Ries impact crater in Germany, which is dated at ~14.8 Ma. Petrographic observations, electron beam imaging, and Raman spectroscopy suggest that these samples record the peak pressures of ~10–15 GPa. In situ U-Pb dating shows that monazite U-Pb systematics have not been affected by the Ries impact, as gneisses and granites yielded monazite U-Pb dates of ~370 and 330 Ma, consistent with known Variscan metamorphic and magmatic events. The U-Pb systematics of some zircon grains yielded U-Pb dates of approximately 5–10 Ma, which is younger than the age of the Ries impact event. These young dates correspond to U-rich metamict domains and may reflect recent Pb loss and/or U-gain during postimpact hydrothermal alteration or weathering. These observations indicate that dating uplifted crystalline material in impact craters on other bodies might provide useful petrological and chronological constraints on the underlying target rocks rather than directly dating impact events, for which sampling impact melt and impact melt-bearing lithologies should remain the primary target.
Wellsch Valley tephra, near Swift Current, southwestern Saskatchewan, and Galt Island tephra, near Medicine Hat, southeastern Alberta, have been referenced in the literature since the 1970s, but little is available on their physical and chemical attributes — necessary information if they are to be recognized elsewhere. This study seeks to remedy this situation. Both have a calc-alkaline rhyolitic composition with hornblende, biotite, plagioclase, pyroxene, and Fe–Ti oxides being dominant. They have a similar composition but are not the same. Wellsch Valley tephra has a glass fission-track age of 0.75 ± 0.05 Ma, a reversed magnetic polarity, and was deposited at the close of the Matuyama Chron. Galt Island tephra has an age of 0.49 ± 0.05 Ma, a normal magnetic polarity, and was deposited during the early Brunhes Chron. Rich fossil vertebrate faunas occur in sediments close to them. Major- and trace-element concentrations in their glass shards indicate a source in the Cascade Range of the Pacific Northwest, USA, but differences in trace-element ratios suggest they are not consanguineous.
Data returned by NASA’s Mars Science Laboratory Curiosity rover showed evidence for abundant secondary materials, including Fe‐oxides, phyllosilicates, and an amorphous component on and below Vera Rubin ridge in the Murray formation. We used equilibrium thermochemical modeling to test the hypothesis that altered sediments were deposited as detrital igneous grains and subsequently underwent diagenesis. Chemical compositions of the Murray formations’ altered components were calculated using data returned by the chemistry and mineralogy X‐ray diffraction instrument and the alpha particle X‐ray spectrometer on board Curiosity. Reaction of these alteration compositions with a CO2‐poor and oxidizing dilute aqueous solution was modeled at 25–100 °C, with 10–50% Fe3+/Fetot of the host rock. The modeled alteration assemblages included abundant phyllosilicates and Fe‐oxides at water‐to‐rock ratios >100. Modeled alteration abundances were directly comparable to observed abundances of hematite and clay minerals at a water‐to‐rock ratio of 10,000, for system temperatures of 50–100 °C with fluid pH ranging from 7.9 to 9.3. Modeling results suggest that the hematite–clay mineral assemblage is primarily the result of enhanced groundwater flow compared to the Sheepbed mudstone observed at Yellowknife Bay, and underwent further, localized alteration to produce the mineralogy observed by Curiosity.
Formation of the Central European tektites, known as moldavites, has been associated with a large meteorite impact in southern Germany 14.8 m.y. ago. The geochemical link between moldavites and their source materials, and the processes of their possible chemical differentiation still remain uncertain. Some differences in chemical composition between moldavites and sediments of corresponding age from the surroundings of the Ries crater could be explained by a hypothesis according to which biomass covering the pre-impact area contributed to the source materials. In a comparison of the geochemical compositions of a large representative set of moldavites and suitable Ries sediments, enrichment in elements K, Ca, Mg, and Mn and depletion of Na in moldavites, similar to redistribution of these elements during their transfer from soil to plants, could indicate the unconventional biogenic component in moldavite source materials. Simple mixing calculations of the most suitable Ries sediments and a model biogenic component represented by burned biomass residue are presented. The plausibility of the estimated biomass contribution considering reconstructions of the middle Miocene paleoenvironment in the pre-impact Ries area is discussed. No significant vapor fractionation is required to explain the observed variability of moldavite chemical composition.
A completely automated system for measuring in situ K–Ar ages using laser ablation for argon extraction and potassium measurement, and mass spectrometry for argon determination is described. The system consists of a laser system, an optical spectrometer, a vacuum line, a noble gas mass spectrometer, and control software. The automated prototype was designed with ease of use in mind, specifically planned for dating many samples at low cost and reasonable precision. This instrumentation can perform one hundred K–Ar analyses every 24 h with the current setup, including blanks, reference materials and unknowns. Measurement results for the reference materials HDB‐1 biotite, GL‐O glauconite, LP‐6 biotite, Mica‐Mg phlogopite, Alder Creek sanidine, and two tektites (moldavite and indochinite) are presented. It is shown that very different sample matrices require specific calibrations. Uncomplicated sample preparation, fast analytical times and uncertainties typically below 5% (1 standard deviation), depending on the age and potassium content, are the hallmarks of the system, which is suitable for many exploratory geology applications (e.g., geological mapping, mineral provenance, dating of drill cores).
Reconstructing Oligocene-Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth’s surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps, however, remain scarce. Here we present stable and clumped isotope geochemistry measurements to provide a new paleoelevation estimate for the mid-Miocene (~14.5 Ma) European Central Alps. We apply stable isotope δ-δ paleoaltimetry on near sea level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30 ± 5 °C) yield a near sea level precipitation δ18Ow value of −5.8 ± 0.2 ‰ and in conjunction with the high-Alpine phyllosilicate δD record suggest that the region surrounding the SFZ attained surface elevations of > 4000 m no later than the mid-Miocene. Our near sea level δ18Ow estimate is supported by paleoclimate (iGCM Echam5-wiso) modeled δ18O values, which vary between −4.2 and −7.6 ‰ for the Northern Alpine Foreland Basin.
The Nördlinger Ries Crater lacustrine basin (South-West Germany), formed by a meteorite impact in the Miocene (Langhian; ~14.9Ma), offers a well-established geological framework to understand the strengths and limitations of U-Pb LA-ICPMS (in situ Laser Ablation-Inductively Coupled Plasma Mass Spectrometry) geochronology as chronostratigraphic tool for lacustrine (and more broadly continental) carbonates. The post-impact deposits include siliciclastic basinal facies at the lake centre and carbonate facies at the lake margins, coevally deposited in a time window of >1.2 and <2Ma. Depositional and diagenetic carbonate phases (micrites and calcite cements) were investigated from three marginal carbonate facies (Hainsfarth bioherm, Adlersberg bioherm and Wallerstein mound). Petrography combined with C and O stable isotope analyses indicate that most depositional and early diagenetic carbonates preserved pristine geochemical compositions and thus the U-Pb system should reflect the timing of original precipitation. In total, 22 U-Pb ages were obtained on 10 different carbonate phases from five samples. The reproducibility and accuracy of the U-Pb (LA-ICPMS) method were estimated to be down to 1.5% based on repeated analyses of a secondary standard (speleothem calcite ASH-15d) and propagated to the obtained ages. Micrites from the Hainsfarth, Adlersberg and Wallerstein facies yielded ages of 13.90±0.25, 14.14±0.20 and 14.33±0.27Ma, respectively, which overlap within uncertainties, and are consistent with the weighted average age of 14.30±0.20Ma obtained from all the preserved depositional and early diagenetic phases. Data indicate that sedimentation started shortly after the impact and persisted for >1.2 and <2Ma, in agreement with previous constraints from literature, therefore validating the accuracy of the applied method. Later calcite cements were dated at 13.2±1.1 (nw=2), 10.2±2.7 and 9.51±0.77Ma, implying multiple post-depositional fluid events. This study demonstrates the great potential of the U-Pb method for chronostratigraphy in continental systems, where correlations between time-equivalent lateral facies are often out of reach. In Miocene deposits the method yields a time resolution within the 3rd order depositional sequences (0.5-5Ma).
The identification and distinction of fluvial from lacustrine deposits and the recognition of catchment changes are crucial for the reconstruction of climate changes in terrestrial environments. The investigated drill core succession shows a general evolution from red–brown claystones to white–grey marlstones and microcrystalline limestones, which all have previously been considered as relict deposits of an impact ejecta‐dammed lake, falling within the mid‐Miocene Climate Transition. However, recent mammal biostratigraphic dating, suggests a likely pre‐impact age. Indeed, no pebbles from impact ejecta have been detected, only local clasts of Mesozoic formations, in addition to rare Palaeozoic lydites, have been found outside of the study area. Lithofacies analysis demonstrates only the absence of lacustrine criteria, except for one charophyte‐bearing mudstone. Instead, the succession is characterized by less diagnostic floodplain fines with palaeosols, palustrine limestones with root voids and intercalated thin sandstone beds. Carbonate isotope signatures of the mottled marlstones, palustrine limestones and mud‐supported conglomerates substantiate the interpretation of a fluvial setting: Low, invariant δ¹⁸Ocarb reflects a short water residence time and highly variable δ¹³Ccarb indicates a variable degree of pedogenesis. Carbonate ⁸⁷Sr/⁸⁶Sr ratios of the entire succession show a unidirectional trend from 0.7103 to 0.7112, indicating a change of the source of solutes from Triassic to Jurassic rocks, identical to the provenance trend from extraclasts. The increase in carbonate along the succession is therefore independent from climate changes but reflects a base‐level rise from the level of the siliciclastic Upper Triassic to the carbonate‐bearing Lower to Middle Jurassic bedrocks. This study demonstrates that, when information on sedimentary architecture is limited, a combination of facies criteria (i.e. presence or absence of specific sedimentary structures and diagnostic organisms), component provenance, and stable and radiogenic isotopes is required to unequivocally distinguish between lacustrine and fluvial sediments, and to disentangle regional geological effects in the catchment and climate influences.
Since its recognition as an impact structure 60 years ago, no volcanics were anticipated in the circular depression of the 14.8 Ma old Nördlinger Ries. Here, we describe for the first time a volcanic ash‐derived clinoptilolite‐heulandite‐buddingtonite bed within the 330 m thick Miocene lacustrine crater fill. Zircon U‐Pb ages of 14.20 ± 0.08 Ma point to the source of the volcanic ash in the Pannonian Basin, 760 km east of the Ries. The diagenetically derived zeolite‐feldspar bed occurs in laminated claystones of the Ries soda‐lake stage and represents the first unequivocal stratigraphic marker bed in this basin, traceable from marginal surface outcrops to 218 m below surface in the crater center. These relationships demonstrate a deeply bowl‐shaped geometry of crater fill sediments, not explainable by sediment compaction and corresponding stratigraphic backstripping alone. Since most of the claystones formed at shallow water depths, the bowl‐shaped geometry must reflect 134 +23/−49 m of sagging of the crater floor. We attribute the sagging to compaction and closure of the dilatant macro‐porosity of the deeply fractured and brecciated crater floor during basin sedimentation and loading, a process that lasted for more than 0.6 Myr. As a result, the outcrop pattern of the lithostratigraphic crater‐fill units in its present erosional plane forms a concentric pattern. Recognition of this volcanic ash stratigraphic marker in the Ries crater provides insights into the temporal and stratigraphic relationships of crater formation and subsidence that have implications for impact‐hosted lakes on Earth and Mars.
The Carpathian obsidian samples from the Slovakian part of the Zemplín – Tokaj area have been studied by means of fission-track dating (FT) and geochemistry to better understand the provenance of the archaeological obsidians from the Central Europe realm. New FT obsidian ages obtained by the isothermal plateau method (ITPFT) are in a narrow time interval between 12.45 ± 0.45 and 11.62 ± 0.25 Ma, and indicate a short-time monogenic volcanic evolution rather than a long-lasting volcanism over the 16–10 Ma period, as was previously thought. Geochemically, these obsidians belong to the silica-rich, peraluminous, high-potassium, calc-alkaline rhyolite series volcanic rocks with a ferroan character which were derived by multi-stage magmatic processes from mixed mantle and crustal sources during subduction in a volcanic arc tectonic setting. Chemical composition of the Carpathian obsidians clearly exhibits a common similarity among all examined localities (Brehov, Cejkov, Hraň, and Viničky). A comprehensive provenance study, including physical properties of the obsidians, confirms a general congruence within the studied obsidians and the use of common provenance labelling, such as Carpathian-1 (C1) for the Slovakian – Zemplín area obsidians, is recommended.
Four newly discovered moldavites from the East and West Gozdnica pits, SW Poland, are characterized. All specimens, including other four, reported earlier, are from Upper Miocene fluvial sediments of the Gozdnica Formation. Their weight varies between 0.529 and 1.196 g. The moldavites are bottle green in colour and have bubbles and inclusions of lechatelierite. Low degree of corrosion suggests short river transport, apparently eastward from Lusatia.
The Ries crater represents one of the best investigated, large, complex
terrestrial impact structures with a well preserved 'ejecta blanket'.
The Ries crater is, therefore, used as an important reference point for
the discussion of impact cratering mechanics. A synopsis is presented of
field and laboratory data relevant to the mechanics of the
crater-forming process. The structure and composition of the target is
considered and a description is presented of the surface formations of
the crater, taking into account present morphology and surface geology,
classification and composition of impact formations, structure and
stratigraphy of outer impact formations, and shock metamorphism and
thermal history of various impact formations. The subsurface structure
of the crater is examined, giving attention to drillholes, geoelectric
measurements, and gravity measurements. A cratering model is also
presented.
A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the ~10 km Paasselkä impact structure in southeast Finland was dated via 40Ar/39Ar step heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) (MSWD = 0.32; p = 0.93) is equal to the previously published pseudo-plateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.
In the last years, various new Ar/Ar 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 Ar/Ar ages, obtained by step heating and laser total fusion analyses, are slightly younger than the previous fission track, K/Ar and Ar/Ar ages of ∼15 Ma reported in the early 1990s. By the recalculation of the standards used and the revision of the K 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.
Palaeoenvironmental considerations suggest that the continental Miocene Ries-Steinheim impact on the Swabian-Franconian Alb plateau, as a rare case on Earth, affected a region dominated by swamplands and lakes. This is supported by the contemporaneous phreatomagmatism of the nearby Urach Volcanic Field, the widespread occurrence of pre- and early post-impact lacustrine and palustrine sediments that overlie deeply karstified, water-saturated Upper Jurassic target limestones, as well as by a subtropical-humid palaeoclimate and a high groundwater level at the time of impact. The characteristics of the Ries ejecta (e.g. accretionary lapilli) display further evidence. Both impact craters became host to crater lakes soon upon impact. A water-saturated, lacustrine-palustrine phreatic impact scenario is also in agreement with the characteristics of proximal Ries and Steinheim impact ejecta, as well as recent numerical modelling results for the Ries impact.
A reevalution of the geologic setting and properties of suevite at the Ries Crater reveals a new hypothesis based on "phreato-magmatic"-like explosions of a clast-laden impact melt sheet induced by surficial water.
For impact craters with dimensions such as the Ries crater (corresponding to a 1 km meteorite) it has become a standard reference in textbooks on planetary science that under terrestrial conditions distal transfer of boulders may reach as far as 200 km. In order to test this assumption we simulated the impact-induced ballistic transfer of limestone boulders ejected out of the Ries crater and have come to the conclusion that “Reutersche Blöcke” and “Ries-Brockhorizonte,” found at distances of up to 130 km away, are distal Ries ejecta. Boulders alleged to be Ries components found in Northern Switzerland at distances of up to 200 km away can be related to the Ries event, if the parameters of our numerical simulation are stretched to its limits. Our simulation includes the following assumptions and variables: (1) boulders are ejected from the interference zone at a very early stage of impact; (2) starting conditions may range between velocities of 1 and 4 km/s and 35° to 65° for the flight path angle; (3) drag-free and transitional conditions at the impact site have been incorporated into the density model of the atmosphere; (4) a typical boulder is represented by an suitable aerodynamic drag model; (5) an aerothermal heat model was used to determine heat load.
Large impact events like the one that formed the Chicxulub crater
deliver significant amounts of heat that subsequently drive hydrothermal
activity. We report on numerical modeling of Chicxulub crater cooling
with and without the presence of water. The model inputs are constrained
by data from borehole samples and seismic, magnetic, and gravity
surveys. Model results indicate that initial hydrothermal activity was
concentrated beneath the annular trough as well as in the permeable
breccias overlying the melt. As the system evolved, the melt gradually
cooled and became permeable, shifting the bulk of the hydrothermal
activity to the center of the crater. The temperatures and fluxes of
fluid and vapor derived from the model are consistent with alteration
patterns observed in the available borehole samples. The lifetime of the
hydrothermal system ranges from 1.5 to 2.3 Myr depending on assumed
permeability. The long lifetimes are due to conduction being the
dominant mechanism of heat transport in most of the crater, and
significant amounts of heat being delivered to the near-surface by
hydrothermal upwellings. The long duration of the hydrothermal system at
Chicxulub should have provided ample time for colonization by
thermophiles and/or hyperthermophiles. Because habitable conditions
should have persisted for longer time in the central regions of the
crater than on the periphery, a search for prospective biomarkers is
most likely to be fruitful in samples from that region.
Impact-generated glasses from fallout suevite deposits at the Ries
impact structure have been investigated using analytical scanning
electron microscopy. Approximately 320 analyses of glass clasts were
obtained. Four glass types are distinguished on the basis of composition
and micro- textures. Type 1 glasses correspond to the aerodynamically
shaped glass bombs studied previously by many workers. Major oxide
concentrations indicate the involvement of granitic rocks, amphibolites,
and minor Al-rich gneisses during melting. Type 2 glasses are chemically
heterogeneous, even within individual clasts, with variations of several
wt% in most of the major oxides (e.g., 5770 wt% SiO2). This
suggests incomplete mixing of: 1) mineral-derived melts or 2) whole rock
melts from a wide range of lithologies. Aluminium-rich clinopyroxene and
Fe-Mg-rich plagioclase quench crystals are present in type 1 and 2
glasses, respectively. Type 3 glasses contain substantial amounts of H2O
(~12 17 wt%), low SiO2 (5053 wt%), high Al2O3 (1721 wt%),
and high CaO (57 wt%) contents. This suggests an origin due to
shock melting of part of the sedimentary cover. Type 4 glasses form a
ubiquitous component of the suevites. Based on their high SiO2 content
(~85100 wt%), the only possible protolith are sandstones in the
lowermost part of the sedimentary succession. Calcite forms globules
within type 1 glasses, with which it develops microtextures indicative
of liquid immiscibility. Unequivocal evidence also exists for liquid
immiscibility between what are now montmorillonite globules and type 1,
2, and 4 glasses, indicating that montmorillonite was originally an
impact melt glass. Clearly, the melt zone at the Ries must have
incorporated a substantial fraction of the sedimentary cover, as well as
the underlying crystalline basement rocks. Impact melts were derived
from different target lithologies and these separate disaggregated melts
did not substantially mix in most cases (type 2, 3, and 4 glasses and
carbonate melts).
Abstract–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.
Abstract–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.
Melt-bearing clastic deposits (suevites) at impact craters have traditionally been regarded as plume fallout deposits. We present new field, textural, and chemical evidence that the subcircular blanket of suevite at the type locality, the Ries impact crater, Germany, was emplaced by a radial, granular fluid-based particulate density current, analogous to those that form ignimbrites of volcanic origin. Newly mapped chemical zoning patterns in the blanket record the response of the current to changing topography during the earliest modification stages of impact crater formation. The eastern sector of the suevite blanket has a different high field strength element composition than the western sector. The crater-fill facies also shows vertical gradational zoning that records changes in the composition of suevite deposited with time. The lateral zoning is best explained by radial outflow of the density currents, but changes in the crater topography caused the flow directions of the melt-bearing density current to change (return flow). The later convergence of flow paths allowed more thorough mixing in the crater, and is recorded by the more uniform composition of the later deposited upper parts of the crater-fill suevite. Emplacement by density currents is indicated by (1) topography-influenced (ponded) thickness variations of the suevite sheet, (2) very poor sorting, (3) matrix support, (4) massive nature, (5) subtle coarse-tail grading, (6) abundant elutriation pipes, (7) abundance of broken and whole matrix-supported concentric-laminated accretionary lapilli in uppermost parts, and (8) an inverse-graded basal layer with low-angle cross-stratification. These are classic features of deposits from granular fluid-based density currents, such as ignimbrites deposited by pyroclastic density currents at explosive caldera volcanoes, but differ markedly from fallout deposits worldwide.
The Middle Miocene Upper Freshwater Molasse sediments represent the last cycle of clastic sedimentation during the evolution of the North Alpine Foreland Basin. They are characterized by small-scale lateral and temporal facies changes that make intra-basin stratigraphic correlations at regional scale difficult. This study provides new U–Pb zircon ages as well as revised ⁴⁰Ar/³⁹Ar data of volcanic ash horizons in the Upper Freshwater Molasse sediments from southern Germany and Switzerland. In a first and preliminary attempt, we propose their possible correlation to other European tephra deposits. The U–Pb zircon data of one Swiss (Bischofszell) and seven southern German (Zahling, Hachelstuhl, Laimering, Unterneul, Krumbad, Ponholz) tuff horizons indicate eruption ages between roughly 13.0 and 15.5 Ma. The stratigraphic position of the Unterneul and Laimering tuffs, bracketing the ejecta of the Ries impact (Brockhorizon), suggests that the Ries impact occurred between 14.93 and 15.00 Ma, thus assigning the event to the reversed chron C5Bn1r (15.032–14.870 Ma) which is in accordance with paleomagnetic evidence. We combine our data with published ages of tuff horizons from Italy, Switzerland, Bavaria, Styria, Hungary, and Romania to derive a preliminary tephrochronological scheme for the Middle Miocene in Central Europe in the age window from 13.2 to 15.5 Ma. The scheme is based on the current state of knowledge that the Carpathian–Pannonian volcanic field was the only area in the region producing explosive calc-alkaline felsic volcanism. This preliminary scheme will require verification by more high-quality ages complemented by isotopic, geochemical and paleomagnetic data.
Ejecta surrounding the 26-km-diameter Ries Crater, Germany, may be helpful to the interpretation of the Cretaceous Tertiary (C T) Boundary Event. The Ries ejecta can be classified into three major facies: (1) moldavite tektites, (2) Bunte Breccia, and (3) suevite, each of which represents a temporally and spatially distinct ejection regime. The petrographic and geochemical characteristics of each facies are also distinct, reflecting an orderly stratigraphic succession of the Ries target. Moldavites represent early high-speed ejecta originating at or close to the projectile-target interface; Bunte Breccia reflects the major excavation and ejection phase and comprises >90% of all ejecta beyond the rim crest; suevite is deposited last and is derived from the most deep-seated target strata. Details of the tektite transportation mechanism) are still poorly understood, but it is virtually certain that drag forces in a rapidly ascending cloud of vaporized target and projectile materials must be invoked. In contrast, individual components ofthe Bunte Breccia are ejected in direct ballistic trajectories and at sufficient velocities to generate a secondary cratering action upon landing, which in turn leads to a highly turbulent, ground-hugging debris surge. Finally, transportation and deposition of suevite seems to require a turbulent, radially expanding gas phase composed of volatiles (H2O; CO2) liberated from the target rocks. It is estimated that this vapor cloud persisted for a minimum of 5 min and that the ambient atmosphere was severely perturbed for at least this long. Using various scaling laws that relate the bolide's kinetic energy to crater geometry or volume and assuming 25 km/s as impact velocity, a projectile diameter of 1 to 2 km results for a stony object; corresponding ratios of ejecta mass (Me ) and projectile mass (Mp) range from 6 ×lO1,) to 4 × I04 (max); Me/Mp of = 102 seems to be a reasonable estimate. These calculations contrast with the bolide mass as estimated by geochemical means. Geochemical studies reveal that projectile dissemination is heterogeneous and that maximum extraterrestrial contamination modeled as a C( chondrite is 4 × 10 3 wt %; moldavites-at most-contain 4 × 10 s wt % C chondrite. Thus, projectile masses based on cratering theory conflict by orders of magnitude with measured concentrations of meteoritic indicator elements. This discrepancy seems to imply that most of the bolide mass is lost to the atmosphere. Observations from the Ries and other terrestrial craters indicate that tektites and.
Moldavites from southern Bohemia, from western Moravia, from the Cheb Basin, from Lusatia (Germany), and from Waldviertel (Austria) are the only known European tektites. In the present paper, we briefly sum up the existing knowledge about their strewn fields and geology, about their properties, and their origin. The present survey should enable a detailed comparison with other groups of tektites and separation of primary differences from differences caused by earth history. The extent of moldavite occurrences is a result of intensive denudation and redeposition of the initial strewn field. All regions of moldavite occurrences are spatially associated with regional basins and depressions. The oldest moldavite-bearing sediments with very short-transported material are unsorted colluvio-fluvial gravelly sands and clays of Middle to Upper Miocene age. Fluvial transport of moldavites to more distant places determined their present distribution and led to a substantial lowering of their content in the sediments. Roughly 106 metric tons of moldavite matter (macrotektites) formed initially. Only about 1% of this mass has been preserved till the present. Most moldavites are splash-form moldavites. No ablation features were found on their surface. Muong Nong type moldavites occur sporadically but their amount could be much higher at the time of their formation. Micromoldavites were not found. Their preservation in the conditions of continental sediments over a time period of about 15 m.y. is not probable. It is, however, a question whether they were formed or not. Moldavites represent the most acid group of tektites with silica content of around 80 wt%. They are relatively rich in K2O, too. On the other hand, they are characterized by low average contents of Al2O3, TiO2, FeO and Na2O. These low contents of TiO2 and FeO lead to their higher translucency, similarly as in georgianites. In the same way as with other tektites, moldavites originated by fusion and ejection of porous target rocks during an oblique impact of a large meteorite. The impacting body - in the case of moldavites - was probably a chondrite 500-1000 m in diameter. Its impact also created the Ries crater at approximately 14.4-15.1 Ma.
We describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of 238U-230Th disequilibrium. Our results support independent inferences that zircon can remain suspended for long periods (> 100 ka) in the convecting portions of the magma from which it crystallizes. Because the crystallization ages date when the magma cooled to its zircon saturation temperature, even when the zircons have long magmatic residence times, 238U-230Th zircon dating can be used to constrain the thermochemical evolution of silicic magmas. 238U-230Th ages have been determined for individual zircons from rhyolites associated with the Long Valley magmatic system of eastern California. The samples are from Deer Mountain, an 115 ± 3 ka low-silica moat rhyolite, and from the coarsely porphyritic, low-silica rhyolite of South Deadman dome, one of the ∼ 0.6 ka Inyo domes. Previous investigations have suggested that the two lavas were derived from the same magma reservoir. A few of the zircon model ages, calculated with respect to the isotopic characteristics of the whole rocks, are within error of that for eruption of the Deer Mountain rhyolite. However, the majority of zircons from both lavas cluster around an age of ∼ 230 ka. This common interval of zircon nucleation and growth, for petrologically similar lavas, suggests that the younger Inyo dome lava may have tapped the same magma body from which the Deer Mountain rhyolite erupted more than 100 ka before. On the other hand, most of the zircon model ages are younger than previous episodes of silicic volcanism in the Long Valley Caldera, suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of Long Valley caldera. Zircon saturation temperatures for the rhyolites studied (795-810°C) are the same as those obtained from coexisting Fe-Ti oxides (809 ± 4°C), showing that the magma cooled to < 815°C more than 200 ka ago. The surprising consequence of these temperatures is the apparent longevity of the shallow magma reservoir from which relatively small ( < 1 km3) volume magmas erupted. The magma reservoir could have remained molten because of the regular influx and differentiation of mafic magma, resulting in accumulation of a much larger volume of magma than that erupted.
An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.
Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160 Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30 ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the oceanic crust. Indeed, many short-wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies.
More than 25 years after the first moldavite find in Lusatia (German: "Lausitz"), there are still no detailed studies of these glasses and their host sediments. This work contributes to the discussion on the origin of the Lusatian moldavites. Results suggest that their origin cannot be completely explained by fluvial transport from the South Bohemian sub-strewn field. Physicochemical characteristics of the Lusatian moldavites tagether with the paleogeographical and stratigraphical position of the moldavite-bearing sediments support this argument. Most Lusatian moldavites display features that are unknown in Bohemian moldavites and occur only in the Moravian moldavites. This suggests an independent Lusatian sub-strewn field within the moldavite strewn field.
Über 25 Jahre nach dem ersten Auffinden eines Moldaviten in der Lausitz fehlen immer noch eingehende Untersuchungen an diesen Gläsern. Diese Arbeit soll zur Diskussion über die Herkunft der Lausitzer Moldavite beitragen. Vorliegende Ergebnisse schließen eine ausschließliche Herleitung der Lausitzer Moldavite aus dem südb.hmischen Substreufeld aus. Neben Überlegungen zu Paläogeographie und Stratigraphie der Fundschichten sprechen dafür besonders die physikochemischen Eigenschaften der Lausitzer Moldavite. Die Mehrzahl weist Merkmale auf, die bei Moldaviten aus Böhmen unbekannt sind und nur in Mähren auftreten. Dies führt zur Annahme eines eigenständigen Lausitzer Substreufeldes innerhalb des Moldavitstreufeldes.
In this chapter we will focus primarily on the promise and limitations of the ocean crust as a recor- der of geomagnetic field variations, emphasizing the record of past geomagnetic field variations recorded in anomalies (and therefore in source magnetization) on timescales of 103 years (excursions) to 104–106 (reversals) and 107–108 (superchrons). We review the origin of the magnetization in the various crustal source layers responsible for lineated magnetic anomalies and conclude by mentioning some appli- cations to deciphering how oceanic crust formed and by speculating on future directions. The chapter is based mostly on published literature that appeared since the last major review of ocean crust magnetiza- tion by Smith (1990).
Seven impact melts from various places in the Nördlinger Ries were dated by 40Ar-39Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2–40% excess 40Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.
Suevite and melt breccia compositions in the boreholes Enkingen and
Polsingen are compared with compositions of suevites from other Ries
boreholes and surface locations and discussed in terms of implications
for impact breccia genesis. No significant differences in average
chemical compositions for the various drill cores or surface samples are
noted. Compositions of suevite and melt breccia from southern and
northeastern sectors of the Ries crater do not significantly differ.
This is in stark contrast to the published variations between
within-crater and out-of-crater suevites from northern and southern
sectors of the Bosumtwi impact structure, Ghana. Locally occurring
alteration overprint on drill cores—especially strong on the
carbonate-impregnated suevite specimens of the Enkingen
borehole—does affect the average compositions. Overall, the
composition of the analyzed impact breccias from Ries are characterized
by very little macroscopically or microscopically recognized
sediment-clast component; the clast populations of suevite and impact
melt breccia are dominated consistently by granitic and intermediate
granitoid components. The Polsingen breccia is significantly enriched in
a dioritic clast component. Overall, chemical compositions are of
intermediate composition as well, with dioritic-granodioritic silica
contents, and relatively small contributions from mafic target
components. Selected suevite samples from the Enkingen core have
elevated Ni, Co, Cr, and Ir contents compared with previously analyzed
suevites from the Ries crater, which suggest a small meteoritic
component. Platinum-group element (PGE) concentrations for some of the
enriched samples indicate somewhat elevated concentrations and
near-chondritic ratios of the most immobile PGE, consistent with an
extraterrestrial contribution of 0.1-0.2% chondrite-equivalent.
The extent of impact-generated hydrothermal activity in the 24 km sized Ries impact structure has been controversially discussed. To date, mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact-melt bearing breccias, specifically the crater-fill suevite. Here, we present new petrographic, geochemical, and isotopic data of postimpact carbonate deposits, which indicate a hydrothermal activity more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the inner crystalline ring of the crater, show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. In particular, the streamer carbonates, which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 °C, are characteristic of a hydrothermal origin. While much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis, a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive Eu anomaly, indicating a hydrothermal fluid convection in the crater basement. Finally, the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250,000 yr after the impact, which would be much longer than previously assumed.
A new drill core (SUBO 18) that intersects a continuous sequence of impact breccias has been obtained at the town of Enkingen at the southern edge of the Ries inner crater. Detailed petrographic analysis of this core provides important new information regarding the nature and origin of within-crater suevite, which is not only relevant for the understanding of the formation of the Ries impact structure but has strong implications for impact breccia deposition in general. In the Enkingen core, below 21 m of crater sediment, 90 m of impact breccia have been sampled. Most of this material is what in the past has been referred to as suevite defined as groundmass of elastic material entraining cogenetic melt fragments and target rock clasts. Only the bottommost 10 m of core are melt-dominated impact breccia. Any relatively thick melt rock intersection along this core is revealed to be composed of individual centimeter- to decimeter-sized melt bodies best described as "agglomerate" due to its close resemblance to volcaniclastics of that type. Detailed macro-, meso-, and microscopic petrographic analysis has established that the SUBO 18 impact breccias cannot be divided into distinct units. The groundmass is extensively altered to phyllosilicate (smectite or chlorite) or carbonate minerals. Results of a detailed scanning electron microscopic investigation of the groundmasses of samples from all parts of this sequence emphatically support the traditional definition of suevite, irrespective of scale. Modal analysis at macroscopic to submicroscopic scales has shown that the overall, average melt content of Enkingen suevite is on the order of 49 vol %. This exceeds by far previous estimates and brings the Ries crater melt volume in line with values traditionally suggested from crater and melt volume scaling for similar-sized impact structures in crystalline and mixed targets.
An in-depth approach of 40Ar/39Ar dating of the ∼23 km Lappajärvi impact structure (Finland) was performed using carefully selected single-grain aliquots of optically fresh, clast-poor, impact melt rock and recrystallized K-feldspar melt particles separated from impact-metamorphosed granite pegmatite. Step-heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven K-feldspar samples. The melt rocks yielded fully concordant ages with a weighted mean of 76.37 ± 0.46 Ma. Dating of the K-feldspar melt particles resulted in a series of younger plateau ages ranging from 75.11 ± 0.36 to 76.11 ± 0.35 Ma. The melt rock results combined with the oldest syn-melt rock K-feldspar age and including all sources of uncertainties yielded a weighted mean age of 76.20 ± [0.29] Ma (2σ; MSWD = 1.02, P = 0.41), which is interpreted to represent the best-estimate age of the Lappajärvi impact. The age spread of at least 1.1 ± 0.5 Ma between the crystallization of the impact melt rocks and the K-feldspar melt particles warns against the use of single 40Ar/39Ar analysis to derive a meaningful impact age. In turn, the age offset monitors both rapid cooling of the Lappajärvi melt sheet versus slow cooling and impact-induced hydrothermal circulation within the crater basement, most likely driven by long-lasting heat flow from the hot central uplift of the impact structure. This interpretation is in line with the grain-size dependent argon diffusion parameters and apparent closure temperatures of ∼230–410 °C determined for various domain sizes of K-feldspars observed in this study. Our study shows that even in comparatively small impact craters post-impact hydrothermal activity can be estimated to last between ∼600 ka and ∼1.6 Ma and is technically resolved by means of the 40Ar/39Ar dating technique. Therefore, the longevity of hydrothermal systems in medium-sized impact craters might be an order of magnitude longer than previously estimated. Prolonged post-impact heat flow in medium-sized craters, in analogy to Lappajärvi, might have played an important role in the emergence of life on early Earth and possibly Mars.
A new 100 m drill hole in a local magnetic anomaly area located in the southern inner ring of the Ries crater yielded 80 m of suevite and, surprisingly, massive impact melt rocks. The petrology and magnetic properties of the core are discussed.
The Ries basin is a shallow, nearly circular depression about 17 miles in diameter that lies between the Swabian and Franconian plateaus of southern Germany. Great masses of breccia and a system of thrust sheets associated with the Ries have been studied by German geologists for about a century. E. Werner and Otto Stutzer suggested that the Ries was an impact crater, but the consensus of the principal investigators has been that it was formed by some sort of volcanic explosion. The only direct evidence of magmatic activity at the Ries is the presence of glass in scattered patches of a breccia called suevite. Some of the glass has long been recognized as sintered fragments of old crystalline rocks. We have found that glasses of various composition coexist in single specimens of suevite. In addition, coesite, a high-pressure polymorph of SiO2, and lechatelierite, SiO2 glass, occur in the sintered rocks in the suevite. The presence of the same phases in sintered rock fragments at Meteor Crater, Arizona, and the coexistence of glasses of different composition suggest that the glassy components of suevite are of impact rather than volcanic origin.
New U-Pb geochronologic data from the Sudbury structure directly link extensive hydrothermal activity in crater-fill breccias with the 1850 Ma impact event, and constrain complex impact-induced processes to < 4 m.y. Semiconformable alteration of breccias in the Onaping Formation includes silicification, albitization, chloritization, calcitization, and complex feldspathization, which directly underlies Zn-Cu-Pb ore deposits. Geochronological data demonstrate a middle to lower crustal source for the crater-fill deposit that was subsequently affected by the hydrothermal event at 1848.4 +3.8/-1.8 Ma. These and other data confirm that the crater-fill breccia, hydrothermal system, Sudbury igneous complex, and its sublayer Ni-ore-bearing units were emplaced as a result of impact. The ages of events associated with the Sudbury structure are not more closely constrained than in any other meteoric impact structure.
The 24-km diameter Ries crater, in Germany, is one of the best-preserved terrestrial complex impact structure and clay minerals in the groundmass of the Ries suevites have been recognized for several decades. It is generally accepted that these clays were formed by post-impact aqueous alteration of impact-generated glasses and/or finely comminuted crystalline basement material at temperatures above ambient conditions. The Ries impact structure provides a good opportunity to study the evolution of post-impact development of impact craters. Here we present a study of boron isotopic compositions (δ11B) and B concentrations of the secondary smectite clay fraction in fall-out and crater suevites from the Ries crater (Germany). These data were used to study the boron fractionation in clay mineral precipitation processes and to model, using B-isotope fractionation, the characteristics (pH, temperature, and salinity) of fluids during fluid–rock interaction at different positions within the crater.
Alteration of surficial suevites at Ries crater, Germany was studied by means of X-ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post-impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al-Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post-impact cooling, then the alteration was of limited character and is completely masked by later weathering.
There are 174 confirmed impact structures known on Earth (e.g., http://www.unb.ca/passc/ImpactDatabase/; late 2008) but a far smaller number of impact structures has yielded a well-constrained age. Precise and accurate age constraints are crucial for (1) correlating causes and effects on the bio- and geosphere of catastrophic processes, (2) better constraining the impactor flux through geological time and evaluation of potential impact periodicity, (3) calibrating the absolute chronostratigraphic time scale, (4) calibrating the age of within-crater continental sedimentary deposits (e.g., for regional paleo-climatic analysis), and (5) correlating impact events and distal impact ejecta occurrences.
We report results of an interdisciplinary project devoted to the 26 km-diameter Ries crater and to the genesis of suevite. Recent laboratory analyses of "crater suevite" occurring within the central crater basin and of "outer suevite" on top of the continuous ejecta blanket, as well as data accumulated during the past 50 years, are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations (Artemieva et al. 2013). The properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting. In a new approach, we reconstruct the minimum/maximum volumes of all allochthonous impact formations (108/116 km 3), of suevite (14/22 km 3), and the total volume of impact melt (4.9/8.0 km 3) produced by the Ries impact event prior to erosion. These volumes are reasonably compatible with corresponding values obtained by numerical modeling. Taking all data on modal composition, texture, chemistry, and shock metamorphism of suevite, and the results of modeling into account, we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement. Numerical modeling indicates that only a very small fraction of suevite can be derived from the "primary ejecta plume," which is possibly represented by the fine-grained basal layer of outer suevite. The main mass of suevite was deposited from a "secondary plume" induced by an explosive reaction ("fuel-coolant interaction") of impact melt with water and volatile-rich sedimentary rocks within a clast-laden temporary melt pool. Both melt pool and plume appear to be heterogeneous in space and time. Outer suevite appears to be derived from an early formed, melt-rich and clast-poor plume region rich in strongly shocked components (melt ≫ clasts) and originating from an upper, more marginal zone of the melt pool. Crater suevite is obviously deposited from later formed, clast-rich and melt-poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper, central zone of the melt pool. Genetically, we distinguish between "primary suevite" which includes dike suevite, the lower sublayer of crater suevite, and possibly a basal layer of outer suevite, and "secondary suevite" represented by the massive upper sublayer of crater suevite and the main mass of outer suevite.
The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12–16km for the Ries and 90–100km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous–Tertiary boundary mass extinction event.
Important sources of systematic error in 40Ar/39Ar dating arise from uncertainties in the 40K decay constants and K/Ar isotopic data for neutron fluence monitors (standards). The activity data underlying the decay constants used in geochronology since 1977 are more dispersed than acknowledged by previous geochronologically oriented summaries, and compilations of essentially the same data in nuclear physics and chemistry literature since 1973 have consistently produced lower estimates (and larger assigned uncertainties) of the constants for 40K → 40Ar and 40K → 40Ca decay. Considering also uncertainties in 40K/K, and the questionable existence of a γ-less electron capture 40K → 40Ar decay direct to ground state, the total 40K decay constant is known to no better than ±2% at the 2σ level.40Ar∗/40K ratios for individual standards are known to better than ±2% in some cases, but interlaboratory discrepancies of more than 2% in the 40Ar/39Ar ages of secondary standards like the Fish Canyon sanidine (FCs) suggest larger uncertainties.
Core materials from the Manson impact structure (MIS), Manson, Iowa, are examined in order to evaluate postimpact alteration processes. Interpretation of the high-temperature postimpact hydrothermal system is based on mineralogic investigation. MIS rocks from the M1, M7, M8, and M10 cores obtained by the continental scientific drilling project (CSDP) in 1991 and 1992 are used in this study. All lithologies, including the sedimentary clast breccias (SCB), crystalline clast breccias (CCB), and central peak crystalline peaks (CPC), have been described previously. Emphasis is placed on fluid conduits that cross-cut all these lithologies. Analytical techniques include petrography, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). The minerals are grouped according to their temperatures of occurrence in modern geothermal systems. The highest temperatures in the MIS are represented by a garnet and ferroactinolite assemblage (assemblage I). Assemblage II contains epidote, prehnite, and wollastonite, which represents slightly lower temperatures in the system. The existence of laumontite, quartz, and adularia defines a third assemblage. Assemblage IV is defined by calcite and clays, and represents the lowest alteration temperature at the MIS. These temperature-sensitive calc-silicates serve to constrain the fluid temperatures of the MIS hydrothermal system. Assemblage I suggests that the system reached over 300 C. Successively decreasing temperatures through time, approaching ambient temperatures, are suggested by the lower temperature assemblages II, III, and IV. A model for the cooling history of the MIS is reported elsewhere. The distribution of these high-temperature minerals points to the central uplift, not the melt sheet, as being the heat source for the system.
Mass extinctions manifest in Earth's geologic record were turning points in biotic evolution. We present 40Ar/39Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub
bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than
5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins.
Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The
Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.
An understanding of impact-generated hydrothermal systems is crucial for
deciphering the environment on early Earth and Mars, as well as
predicting promising locations for finding evidence of past life on
Mars. Impact events and early life may be tightly connected -- a sudden
increase in the number of impact events which occurred at ˜3.9 Ga
coincides remarkably well with the earliest isotopic evidence of life on
Earth at ˜3.85 Ga, and hydrothermal systems generated by impact
events can provide a habitable environment for thermophilic organisms.
Several hydrothermal systems associated with terrestrial impact craters
have been identified on the basis of mineralogical evidence. Examples of
known systems include the 35 km Manson crater, the 80 km
Puchezh-Katunki crater, and the 250 km Sudbury crater. In order to
better constrain the expected lifetimes of these systems and further
understand their mechanics, a finite-difference computer simulation is
used to evaluate the effects of convective cooling by circulating water
and steam. In this work we present modeling results of water and heat
transport shortly after the formation of the Sudbury impact crater in
present-day Ontario, Canada. Our model predicts that an impact-induced
hydrothermal system associated with a Sudbury-sized impact crater can
remain active for at least 105 - 106 years. While
the location and volume of the habitable zone within the crater changes
as the crater undergoes cooling, it is sufficiently long-lived for an
ecosystem to develop. The insight into the mechanics of these systems
gained from this model can help locate hydrothermal vents and
hydrothermally altered minerals at Martian impact craters.
Moldavites in Lower Austria are discussed, including the analysis of
tektites from Altenburg and Radessen. The samples show signs of surface
alterations by water and/or humic substances. The geographical
distribution, petrographic characteristics, and geological settings of
the moldavites are examined. Chemical analysis of the tektites show a
close similarity with other moldavites in the Bohemian or Moravian
group. The study confirm previous findings of Austrain moldavites,
showing that the moldavite tektite strewn field is not confined to
Czechoslovakia, but extends further south than previously assumed.
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).
Abstract— Surficial suevites from the Ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy. The groundmass of these suevites comprises calcite, clay minerals, impact melt glass, crystallites (plagioclase, garnet, and pyroxene), francolite, and Ba-phillipsite. The latter zeolite is a secondary phase. Abundant textures have been observed: intricate flow textures between the various groundmass phases, globules of each phase in the other phases, spheroids of pyrrhotite in calcite, the “budding-off” of clay globules into silicate glass and/or calcite, euhedral overgrowths of francolite on apatite clasts, and quench-textured crystallites in the groundmass. Groundmass-forming calcite displays higher FeO, MnO, and SiO2 contents than limestone target material. The composition of suevite “clay minerals” is highly variable and not always consistent with montmorillonite. Three types of glasses are distinguished in the groundmass. Type 1 glasses are SiO2-rich and are clearly derived from sandstones in the sedimentary cover, while the protoliths of the other two glass types remains unclear.Analytical data and micro-textures indicate that the calcite, silicate glass, francolite, and clay minerals of the groundmass of the Ries suevites represent a series of impact-generated melts that were molten at the time of, and after, deposition. On cooling, plagioclase, pyroxene, and garnet crystallized from the groundmass. These results are at variance with the current, traditional descriptive definition of suevite. Given that Ries is the original type occurrence of “suevite,” some modification to the traditional definition may be in order. As the results of this study are most consistent with the groundmass of Ries surficial suevites representing a mix of several types of impact-generated melts, we suggest that a possible origin for these suevites is as some form of impact melt flow(s) that emanated from different regions of the evolving crater.
Any hypervelocity impact generates a hydrothermal circulation system in resulting craters. Common characteristics of hydrothermal fluids mobilized within impact structures are considered, based on mineralogical and geochemical investigations, to date. There is similarity between the hydrothermal mineral associations in the majority of terrestrial craters; an assemblage of clay minerals–zeolites–calcite–pyrite is predominant. Combining mineralogical, geochemical, fluid inclusion, and stable isotope data, the distinctive characteristics of impact-generated hydrothermal fluids can be distinguished as follows: (i) superficial, meteoric and ground water and, possibly, products of dehydration and degassing of minerals under shock are the sources of hot water solutions; (ii) shocked target rocks are sources of the mineral components of the solutions; (iii) flow of fluids occurs mainly in the liquid state; (iv) high rates of flow are likely (10−4 to 10−3 m s−1); (v) fluids are predominantly aqueous and of low salinity; (vi) fluids are weakly alkaline to near-neutral (pH 6–8) and are supersaturated in silica during the entire hydrothermal process because of the strong predominance of shock-disordered aluminosilicates and fusion glasses in the host rocks; and (vii) variations in the properties of the circulating solutions, as well as the spatial distribution of secondary mineral assemblages are controlled by temperature gradients within the circulation cell and by a progressive cooling of the impact crater. Products of impact-generated hydrothermal processes are similar to the hydrothermal mineralization in volcanic areas, as well as in modern geothermal systems, but impacts are always characterized by a retrograde sequence of alteration minerals.
Combined field studies, optical and scanning electron microscopy, and electron microprobe studies of impactites from the Ries impact structure, Germany, have allowed a clearer picture of the hydrothermal system associated with the Ries impact event to be made. Hydrothermal alteration is concentrated within impact-generated suevites in the interior of the crater (crater suevites) and around the periphery (surficial suevites), with minor alteration in the overlying sedimentary crater-fill deposits. The major heat source for the Ries hydrothermal system was the suevite units themselves. Hydrothermal alteration of crater-fill suevites is pervasive in nature and comprises several distinct alteration phases that vary with depth. An early phase of K-metasomatism accompanied by minor albitization of crystalline basement clasts and minor chloritization, was followed by pervasive intermediate argillic alteration (predominantly montmorillonite, saponite, and illite) and zeolitization (predominantly analcite, erionite, and clinoptilolite). Hydrothermal fluids were typically weakly alkaline during the main stage of alteration. In contrast to the crater-fill suevites, alteration within surficial suevites was typically restricted to montmorillonite and phillipsite deposition within cavities and fractures. The pervasive nature of the alteration within the crater-fill suevites was likely due to the presence of an overlying crater lake; whereas alteration within surficial suevites typically occurred under undersaturated conditions with the main source of water being from precipitation. There are exceptional outcrops of more pervasively altered surficial suevites, which can be explained as locations where water pooled for longer periods of time. Hydrothermal fluids were likely a combination of meteoric waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks.