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      Age of the Sääksjärvi impact structure, Finland: reconciling the timing of small impacts in crystalline basement with regional basin development

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          Abstract

          We report a new age for the Sääksjärvi impact structure, Finland, a 6 km diameter feature that formed in crystalline rocks of the Precambrian Baltic Shield. Two previous studies reported 40Ar/ 39Ar data for Sääksjärvi and suggested conflicting formation ages of ≤330 Ma or c. 560 Ma. The former age represents a possible complication for models which indicate that the region was covered by sediments of the Caledonian foreland basin throughout much of the Phanerozoic. We conducted a study combining imaging, microstructural analysis and U–Pb dating of shocked zircon from Sääksjärvi. The U–Pb dataset indicates a c. 600 Ma impact into predominantly c. 1850 Ma target rocks. A concordia age of 608 ± 8 Ma (2 σ) confirms Sääksjärvi as the first known Ediacaran impact structure in the Baltic Shield and only the second worldwide. Our data indicate that the Sääksjärvi impact structure formed in exposed crystalline basement rocks of the Baltic Shield prior to the development of the Caledonian foreland basin. Given that most impact structures on Earth are relatively small features, radiometric dating of small impact structures in crystalline basement may place boundaries on the timing and spatial extent of palaeobasins that might otherwise be difficult to constrain.

          Supplementary material: Additional imaging and microstructural analysis of zircon, additional concordia plots, and U–Pb data tables are available at https://doi.org/10.6084/m9.figshare.c.5043462

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          Age significance of U–Th–Pb zircon data from early Archaean rocks of west Greenland—a reassessment based on combined ion-microprobe and imaging studies

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            Structures of ZrO2 polymorphs at room temperature by high-resolution neutron powder diffraction

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              Is Open Access

              Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits

              This article presents a current (as of September 2019) list of recommended ages for proven terrestrial impact structures (n = 200) and deposits (n = 46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth–Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magneto-stratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.
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                Author and article information

                Contributors
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                Journal
                Journal of the Geological Society
                Journal of the Geological Society
                Geological Society of London
                0016-7649
                2041-479X
                November 02 2020
                November 2020
                November 2020
                July 01 2020
                : 177
                : 6
                : 1231-1243
                Affiliations
                [1 ]Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
                [2 ]Radiation and Nuclear Safety AuthorityLaippatie 4, Box 14, FI-00811 Helsinki, Finland
                [3 ]Lunar and Planetary Institute – Universities Space Research Association, Houston, TX 77058, USA
                [4 ]Hochschule Neu-Ulm, Neu-Ulm University of Applied Sciences, D-89231 Neu-Ulm, Germany
                [5 ]School of Earth and Planetary Sciences, Curtin University, Perth, WA 6845, Australia
                Article
                10.1144/jgs2020-034
                a61145c6-c950-4412-a3d1-7fcbc7e3d3bc
                © 2020
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