Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.277p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 277
Physics
Scientific paper
Research drill holes drilled in the 23 km diameter Lappajarvi Late Cretaceous meteorite impact crater give new information as to the structure, stratigraphy, and geochemistry of the impact formation in the crater (Pipping & Lehtinen, 1992). In one drill hole the sheet of glassy melt rock is 145 m thick and is underlain by 30 m of suevitic rock, followed by loose breccias down to 209 m where the hole ends. In a nearby hole 65 m of suevitic breccia was penetrated, underlain by a fine-grained allochthonous breccia to 160 m, followed by an autochthonous coarse breccia cut by suevitic dikes down to 275 m where the hole ends. A third hole was bored close to the crater rim. Topmost is 75 m of Quaternary glacial tills. After an erosional boundary follows 20 m of Mesoproterozoic (ca. 1200 Ma) sandy-silty sediments. These are underlain--with a tectonic boundary--by an autochthonous saprolite crust of about 40 m thickness. The hole ended in fresh mica schist--no signs of shock metamorphism--at 165 m. The structure typical for large impact craters was thus confirmed, i.e. an annular trough inside the rim of the crater, filled with rocks older than the impact. The stratigraphy of the crater fill is: lowermost autochthonous breccias followed by allochthonous breccias and suevitic breccias, with a melt rock cap covering a part of the crater fill. The general stratigraphy is: Palaeoproterozoic mica schists (1900 Ma), Mesoproterozoic saprolite (1400 Ma) in peneplanated schists, Neoproterozoic sediments (1200 Ma), Late Cretaceous crater (77 Ma, Jessberger & Reimold,1980) with a fill of impact rocks, Quaternary glaciation and sedimentation, and last, recent lake sedimentation. Impact rock geochemistry gives conclusive proof of an origin by meteorite impact. Conspicuous is the threefold Ni-content of the melt rock compared with the target rock, and the two-to-tenfold content of the Pt-metals in the melt rocks and suevites, compared with the target schists. Increased levels of K and Se are noted. References Jessberger, E.K. & Reimold, W.U. (1480) A Late Cretaceous 40 Ar - 39 Ar Age for the Lappajarvi Impact Crater, Finland. J. Geophys. 48, 57-59. Pipping, F. & Lehtinen, H. (1992) Geology, stratigraphy, and structure of the Lappajarvi meteorite crater, western Finland: preliminary results of deep drilling. Tectonophysics, in press.
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