Browsing by Author "Schmieder, Martin"

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    New aspects of the Middle-Late Triassic terrestrial impact cratering record : 40Ar/39Ar dating of the Rochechouart (France), Manicouagan (Canada), and Paasselkä (Finland) impact structures, paleoenvironmental considerations, and a critical look at the Late Triassic multiple impact theory
    (2010) Schmieder, Martin; Buchner, Elmar (PD Dr.)
    This thesis critically addresses the question whether Earth experienced a giant multiple cosmic strike in the Late Triassic ~214 million years (Ma) ago, as postulated by Spray et al. in 1998. According to this theory, the large ~100 km Manicouagan impact structure (Québec, Canada), the ~40 km Lake St. Martin impact structure (Manitoba, Canada), the ~25 km Rochechouart impact structure (France), the ~20 km Obolon impact structure (Ukraine), and the ~9 km Red Wing Creek impact structure (North Dakota, USA) are potential members of a ~4,500 km long impact crater chain, compatible with coeval impact ages available in 1998. However, no geologic evidence for the multiple Late Triassic impact scenario has been presented since that time. In this study, I present new 40Ar/39Ar data for the Rochechouart and Manicouagan impact structures, along with new constraints for the timing of the Obolon impact structure based on paleoenvironmental considerations. The new results are set in the context of recent chronological data available for the Lake Saint Martin impact structure, geochemical data, and the physical background of impact crater chain formation. Furthermore, paleoenvironmental implications are discussed. 40Ar/39Ar dating of sanidine and hydrothermally grown adularia separated from impact-metamorphosed gneiss from the Rochechouart impact structure yielded a Latest Triassic (Rhaetian) combined age of 201 ± 2 Ma (2s), indistinguishable within uncertainty from the age of the Triassic/Jurassic boundary. This new age is within the range of earlier dating results but distinctly younger than the previously obtained 214 ± 8 Ma (Karnian-Norian) age for Rochechouart and, therefore, is in conflict with the Late Triassic multiple impact theory. In agreement with the paleogeographic conditions in the western Tethys domain, 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 Islands and possible related deposits in other parts of Europe. Apparently shocked quartz reported from uppermost Triassic sediments of the Northern Apennines (Italy), as well as an enigmatic spherule layer recently detected in the upper Rhaetian Fatra Formation of the High Tatra Mountains (Slovakia) are discussed as possible distal Rochechouart ejecta. The comparatively small crater size of ~50 km, however, suggests no direct link between the Rochechouart impact and the global mass extinction at the Triassic/Jurassic boundary. 40Ar/39Ar dating of a fine-grained crystalline impact melt rock from the Manicouagan impact structure yielded a Late Triassic (Norian) age of 211 ± 2 Ma (2s) close to the widely cited ages of ~215 Ma. However, a set of precise and well-established U/Pb ages have previously been reported for the Manicouagan impact, and this dating attempt could not improve the impact age estimates. Recent (U-Th)/He ages obtained for the Lake Saint Martin impact structure suggest a Middle to Late Triassic (Ladinian to Karnian) impact age of ~235-230 Ma. No isotopic ages are available for the Red Wing Creek impact structure, stratigraphically dated at ~220-200 Ma. In addition, palaeogeographic studies have shown that the Obolon impact age is notably younger than previously stated, roughly ~185-170 Ma, placing the impact in the Early-Middle Jurassic. The new dating results for Rochechouart and Manicouagan, together with the recent age estimations for the Lake Saint Martin and Obolon impacts, rule out a ~214 Ma Late Triassic multiple impact event on Earth, and it is suggested that the five impact structures discussed were formed by spatially and temporally individual impact events in Triassic to Jurassic time. In addition, I present a first detailed petrographic and geochemical description of impact melt rocks from the Paasselkä impact structure, Finland. The degree of shock metamorphism suggests initial whole-rock melting at peak shock pressures of ~35 GPa and post-shock temperatures of up to ~1,500 °C; the geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield. 40Ar/39Ar dating of recrystallized feldspar glass particles separated from the impact melt rocks yielded a Middle to Late Triassic (Ladinian-Karnian) age of 229 ± 3 Ma (2s). This new age makes Paasselkä the first known Triassic impact structure dated by isotopic methods on the Baltic Shield. Due to the small crater size, a link between the Paasselkä impact and a postulated Middle/Late Triassic extinction event is unlikely. The new Paasselkä impact age is, within uncertainty, coeval with isotopic ages recently obtained for the Lake Saint Martin impact structure in Canada, indicating a new Middle to Late Triassic impact crater population on Earth.