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Impact Origin for the Hummeln Structure (Sweden) and Its Link to the Ordovician Disruption of the L Chondrite Parent Body

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Impact Origin for the Hummeln Structure (Sweden) and Its Link to the Ordovician Disruption of the L Chondrite Parent Body Downloaded from geology.gsapubs.org on March 27, 2015 Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body C. Alwmark1*, L. Ferrière2, S. Holm-Alwmark1, J. Ormö3, H. Leroux4, and E. Sturkell5 1Department of Geology, Lund University, Sölvegatan 12, 22362 Lund, Sweden 2Natural History Museum, Burgring 7, A-1010 Vienna, Austria 3Centro de Astrobiología (CSIC-INTA), Instituto Nacional de Técnica Aeroespacial, Carretera de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain 4Unité Matériaux et Transformations, UMR 8207, Université Lille 1 and CNRS, F-59655 Villeneuve d’Ascq, France 5Department of Earth Sciences, University of Gothenburg, PO Box 460, SE-405 30 Gothenburg, Sweden ABSTRACT Thus, despite the numerous investigations over Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 a period of almost 200 years, the origin of the Ma in our solar system’s asteroid belt. As a consequence, a large number of meteorite impacts Hummeln structure was still unknown. The occurred on Earth during the following few million years. The finding and characterization, drill core, recovered from the eastern part of the for the first time, of planar deformation features in quartz grains from rocks collected at the structure, confirmed that the depression is par- Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the tially filled with slumped Paleozoic sedimentary structure. The unambiguous shock features allow us to close an ~200-yr-old discussion about rocks, mainly Cambrian muddy sandstone and its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician limestone. Unfortunately the drill- Ordovician, adding an impact crater to the increasing number confirmed and properly dated ing was terminated at ~160 m depth, before from this period. Despite its relatively small size (~1.2 km in diameter), similar to the young reaching the bottom of the depression, due to Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well economic reasons. The top part of the sediment preserved, contradicting the general assumption that small craters are not preserved on Earth infill was shown to be glacially eroded, gener- for more than a few tens of thousands to a couple of million years. ating the conspicuous trail of Paleozoic rocks south of the lake. Furthermore, the core showed INTRODUCTION geophysical investigations of the structure, gave that the Paleozoic rocks in the structure occurred The Hummeln structure is located in the Små- further information in support of the impact as a sedimentary breccia and in places even with land province (57°22′N, 16°15′E) in southern hypothesis (Lindström et al., 1999; Ormö et al., overturned sequence. Detailed lithostratigraphic Sweden (Fig. 1). It consists of a >160-m-deep 1999), no conclusive evidence for an impact ori- comparisons with preserved coeval sequences and 1.2-km-wide depression in the Precambrian gin (such as shock deformation in minerals or at the nearby Baltic island of Öland, as well as crystalline basement, within a lake with the same traces of extraterrestrial matter) was presented. acritarch, chitinizoan, and conodont biostratig- name (Lake Hummeln; Lindström et al., 1999). The origin of the circular depression has puz- N57˚24´00˝ zled the geological community for almost 200 E16˚12´00˝ E16˚14´00˝ E16˚16´00˝ years. The structure was first mentioned by His- inger (1826), who noted a large concentration of what he interpreted to be glacially transported Paleozoic sedimentary rocks along a trail south Lockne/ of Lake Hummeln. Knowing that all Paleozoic Målingen sedimentary rocks have been eroded away in NORWAY this part of Sweden, this was an indication that a N57˚23´00˝ subterranean structure, where these rocks were SWEDEN FINLAND STOCKHOLM preserved, must exist nearby. This assumption Tvären Kärdla was confirmed by Linnarsson (1878) and Sved- Granby Diorite mark (1904), who concluded that the sedimen- Map location tary rocks occur in situ under the lake. Through Granitic rock (coarse-grained) Lake Hummeln Öland detailed lead soundings (Nordenskjöld, 1937) Granitic rock (ne-grained) 200 km it was shown that a sub-circular depression in Brecciated mudstone the crystalline basement was present under the southern part of Lake Hummeln. The structure Mudstone was at that time considered to be the result of Limestone N57˚22´00˝ H1 either explosive volcanism or tectonics (Nor- Trail of Paleozoic glacial erratics denskjöld, 1937, 1944; Asklund, in Norden- Outcrop of brecciated basement skjöld, 1944). Not until the 1960s, after detailed mapping of the lake topography, was an impact Limit of basement depression origin proposed for the structure (Fredriks- Core drilling H2 0 1 km son and Wickman, 1963; Svensson, 1966). Sample localities E16˚14´00˝ E16˚16´00˝ E16˚18´00˝ Although additional investigations, including the completion of a core drilling campaign and Figure 1. Simplified geological map of the Hummeln impact structure, Sweden (modified from Ormö et al., 1999; Lindström et al., 1999; completed during our 2012 field campaign). Other *E-mail: [email protected] verified Ordovician impact structures in Baltoscandia are marked with stars in the inset. GEOLOGY, April 2015; v. 43; no. 4; p. 279–282 | doi:10.1130/G36429.1 | Published online 18 February 2015 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 4 For| www.gsapubs.org permission to copy, contact [email protected]. 279 Downloaded from geology.gsapubs.org on March 27, 2015 raphy, gave an age of formation of the sedimen- such small craters, usually expected to be quickly and the poles perpendicular to PDFs were deter- tary breccia to be roughly coeval, within a few eroded, can, under the right circumstances, sur- mined following the technique described in Fer- million years, with that of the Granby impact vive for hundreds of millions of years. rière et al. (2009a). SEM work was conducted structure, located in south-central Sweden (i.e., using a JEOL JSM 6610-LV at the Natural His- ca. 467 Ma; Grahn et al., 1996; Lindström et al., MATERIAL AND METHODS tory Museum (Vienna, Austria). The operating 1999; Ormö and Lindström, 2000). A diameter We prepared 21 polished thin sections from conditions were 15 kV accelerating voltage and of 1.8 km for the original pre-erosion crater 2 samples of the crystalline breccias, one from 0.1 nA beam current. The focused ion beam has also been suggested based on a reconstruc- a boulder along the southern border of the lake (FIB) technique was used for the preparation tion of the original target stratigraphy of ~130 shore (H1: 57°21′45.42″N, 16°15′30.53″E) and of a TEM foil, using a FEI Quanta 3D FEG m of Paleozoic sedimentary rocks and ~30 m one from an erratic boulder farther south (H2: DualBeam instrument at the Department of of seawater covering a peneplained Precam- 57°21′7.68″N, 16°14′46.56″E; Fig. 1). Both sam- Lithospheric Research, University of Vienna. A brian basement (Ormö et al., 1999). Brecciated ples have similar compositions, exhibiting the foil of 13 × 6 µm extent and ~100 nm thickness granite and dolerite are found in situ along the typical features of the matrix-supported crystal- was prepared and then investigated using TEM, southern border of the lake shore and as erratic line breccia, with angular fragments of primar- which was done with an FEI Tecnai G2–20 boulders along the shore and in the glacial trail ily granitic composition embedded in a finely TWIN instrument (LaB6, 200 kV) at the Univer- (Lindström et al., 1999). The crystalline brec- crushed cemented matrix. At macroscopic scale, sity of Lille (France). Conventional bright-field cia occurs as clast-supported monomict breccia the clasts vary in size from ~0.5 to 20 cm (smaller TEM imaging and electron diffraction were with a calcitic matrix and fragments as large as clasts are visible under the optical microscope) used to characterize shock effects in quartz. ~20 cm, and as a matrix-supported polymict and are mainly composed of quartz, microcline, breccia with clasts ranging in size from gravel and minor amounts of plagioclase and mica; the RESULTS to boulders in a finely crushed cemented sandy latter three are altered to various degrees. Our optical microscope survey of the thin sec- matrix (Ormö et al., 1999). The thin sections were searched for shock tions from the Hummeln structure revealed that From studies of meteorites, we know that a metamorphic features, first using the polarizing most of the quartz grains do not exhibit shock large disruption of an asteroid occurred ca. 470 microscope, and then using scanning and trans- features, even though many of them show undu- Ma in the main asteroid belt (Anders, 1964; mission electron microscopy (SEM and TEM). lose extinctions. However, during careful inves- Keil et al., 1994; Bogard, 1995; Haack et al., All quartz grains displaying planar deformation tigations, we were able to detect some quartz 1996; Korochantseva et al., 2007). This disrup- features (PDFs) were further investigated using grains displaying shock metamorphic features tion spawned large amounts of asteroid debris the universal stage (U-stage; cf. Reinhard, 1931; in the form of PDFs (Fig. 2). These grains are and dust, some of which entered Earth-crossing Emmons, 1943); the orientations of optic axes either part of larger clasts or free grains in the orbits, resulting in an increase of two orders of magnitude in the influx of smaller fragments (meteorites and micrometeorites) to Earth; the influx lasted for a couple of million years (e.g., A B Schmitz et al., 2001, 2003). It is assumed that B the disruption also resulted in the delivery of larger asteroid fragments to Earth that were responsible for the relatively high number of Ordovician craters (e.g., Schmitz et al., 2001; Ormö et al., 2014).
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