Geology, Petrography, Shock Petrography, and Geochemistry of Impactites and Target Rocks from the Kärdla Crater, Estonia

Meteoritics & Planetary Science 39, Nr 3, 425–451 (2004) Abstract available online at http://meteoritics.org Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Kärdla crater, Estonia V. PUURA,1* H. HUBER, 2† J. KIRS,1 A. KÄRKI,3 K. SUUROJA, 4 K. KIRSIMÄE, 1 J. KIVISILLA, 4 A. KLEESMENT, 5 M. KONSA,5 U. PREEDEN, 1 S. SUUROJA, 5 and C. KOEBERL 2 1Institute of Geology, University of Tartu, Vanemuise strasse 46, 51014 Tartu, Estonia 2Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 3Institute of Geosciences, University of Oulu, Box 3000, FIN-90401 Oulu, Finland 4Geological Survey of Estonia, Kadaka tee 80/82, 12618 Tallinn, Estonia 5Institute of Geology, Tallinn Technical University, Estonia pst. 7, 10143 Tallinn, Estonia †Present address: Institute of Geophysics and Planetary Physics, University of California Los Angeles, 595 Charles Young Drive, Los Angeles, California 90095, USA *Corresponding author. E-mail: [email protected] (Received 20 December 2002; revision accepted 7 January 2004) Abstract–The Kärdla crater is a 4 km-wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma-old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub-crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shock- metamorphosed allochthonous granitoid crystalline-derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20–35 GPa. An apparent K- enrichment and Ca-Na-depletion of feldspar- and hornblende-bearing rocks in the allochthonous breccia units and sub-crater basement is interpreted to be the result of early stage alteration in an impact-induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMX- mixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite-equivalent was found. INTRODUCTION these 15 impact craters (see Fig. 1), six structures of Middle to Late Ordovician (450–470 Ma) age form a prominent group, In the Baltic Sea region, 15 meteorite impact structures of i.e., Granby, Hummeln, Karikkoselkä, Lockne, Tvären, and Paleozoic age have been identified to date (Abels et al. 2002). Kärdla. The majority of them formed in a marine environment During the Paleozoic era, the Baltic area developed as a large (Puura et al. 1994; Abels et al. 2002). Kärdla is the best- and shallow epi-continental sea or flat sedimentary lowland. preserved crater among the Paleozoic marine impact At the end of the Paleozoic era, a thin (mostly <500 m, structures in this region. The Kärdla crater formed during the maximum ~3-km-thick) sequence of unconsolidated early Late Ordovician (see IUGS 2000) and has been dated at sediments covered the Precambrian basement. From the late 455 Ma (Puura et al. 1989; Puura and Suuroja 1992) based on Paleozoic era and during the Meso-Cenozoic era, the present the stratigraphic position of the Kärdla distal ejecta in the Fennoscandian Shield was re-exposed due to uplift and sequence of the Caradoc Series. erosion. Consequently, in the Paleozoic era, asteroids or Kärdla is also a good target for the study of unmelted comets hit a two-layered target either in marine or continental suites of impactites produced in marine environments and environments, depending on high or low sea levels. Among combined sedimentary-crystalline targets. Impactites from 425 © Meteoritical Society, 2004. Printed in USA. 426 V. Puura et al. Fig. 1. Geographic map of the 15 known Paleozoic impact structures in the Baltic Sea region. Among others, the Kärdla crater in Estonia belongs to the Middle to Late Ordovician age group, the most common in this region. The location of the Kärdla crater (lat. 58°59´ N, long. 22°46´ E) on Hiiumaa Island is shown in the close-up. The rectangle in the insert indicates the part shown in the geological map (Fig. 2). the Kärdla crater, both allochthonous crater-fill and ejecta layer outside the crater proper. Core K1 was drilled in autochthonous sub-crater rocks, are of silicate-dominated 1989–1990 in impactites with diameters of 60 and 40 mm composition. down to 815.2 m, and core K18 was drilled in 1984 with a During impact-induced processes, these rocks were diameter of 60 mm to a depth of 431.4 m. Both cores are enriched in potassium and depleted in sodium and calcium stored at the Arbavere field station of the Geological Survey (Puura and Suuroja 1992; Puura et al. 2000a, b). The of Estonia. Short descriptions of these two cores, including dominant type of alteration is metasomatic replacement of the core logs, were given by Suuroja (1996) and in unpublished rock-forming feldspars, hornblende, and biotite with reports of the Geological Survey of Estonia. A detailed secondary minerals. The occurrence and mechanisms of description of core K1 is given by Suuroja et al. (2002b). large-scale K-metasomatism in impact structures remain Puura and Suuroja (1992) published preliminary data on poorly studied. At many craters, fissures, vugs, and pores in chemical and mineralogical changes of Precambrian impactites are filled with Ca- and Na-rich minerals, as basement silicate target rocks. described for the Manson crater in the USA (McCarville and The aim of the present paper is to present results of Crossey 1996) or the Popigai, Kara, and Puchezh-Katunki systematic studies of impact lithologies and target rocks of the craters in Russia (Naumov 1999, 2002; Masaitis et al. 2004). Kärdla crater. The research focused on the geochemical- These secondary minerals include zeolites, clay minerals, and mineralogical composition and metasomatic alteration, on the carbonates. However, at Kärdla, veins and vugs filled with the spatial distribution of shocked minerals, and on the mixing of Ca-Mg-rich minerals calcite, dolomite, and chlorite are only a target rocks and the admixture of a meteoritic component. considerably small admixture in K-enriched impactites. Drilling of the Kärdla crater was performed by the GEOLOGICAL SETTING OF THE CRATER AND Geological Survey of Estonia at 30 different sites with IMPACT LITHOLOGIES maximum depths of 800 m, but most cores penetrated to an average depth of 300 m (Suuroja 1996; Puura and Suuroja The Multilayer Target 1992). The studied samples are from 13 of these drill cores (for locations, see Figs. 2 and 3) that penetrated between 79.7 The buried crater structure is located in the northwestern and 815.2 m into the crater-fill deposits and sub-crater marginal part of the Russian platform, within the exposure of basement in the crater moat, central uplift, crater rim, and the Upper Ordovician and Lower Silurian. The crater site is Impactites and target rocks from the Kärdla crater 427 Fig. 2. Subsurface geological map of the Kärdla crater. Post-impact formations are removed, and contour lines show the subsurface topography of target rocks and impact breccias that underlie the post-impact cover-rocks. The Paleozoic rocks in the rim area are mainly Cambrian sandstones and claystones. Cross sections AB and CD are shown in Fig. 3. Fig. 3. Generalized cross sections of the Kärdla crater. Top: SW-NE section across the uplifted crystalline basement in the rim (drill cores at Paluküla and Tubala). Bottom: NW-SE section through the gullies in the rim. The locations of the cross sections are shown in Fig. 2 (AB, CD). Post-impact cover sediments are shown in gray. Logs of the allochthonous breccias of boreholes K1 and K18 are given in Figs. 4a–4c. For the subsurface geology below the cover, see Fig. 2. 428 V. Puura et al. on Hiiumaa Island, Estonia, in a flat, low, near-shore area. allochthonous breccias within the crater (0.5) (including: Only low hills with up to 23 m elevation mark the most sediment-dominated [0.3] and crystalline [0.2]). For the elevated buried fragments of the crystalline rim (e.g., the calculations, the vertical parabolic cross section of the crater Paluküla and Tubala hills; see Fig. 2). A flat wetland was divided into truncated cones considering the thickness of corresponds to the crater proper. target layers in the volume of the transient cavity and impact- At present, the composite lithologies of the Kärdla area stratigraphic units in the present crater. The morphology of consist of 170 m-thick uppermost Middle Ordovician to breccia units and the proportions of sedimentary to crystalline Cambrian and uppermost Vendian (Neoproterozoic III, after rocks in the impact breccia units were estimated from logs of IUGS 2000) sedimentary rocks underlain by Paleoproterozoic cores K1, K18, and K12. The results of the calculations were (Svecofennian, 1.9–1.8 Ga) crystalline basement (see Fig. 3). rounded to the nearest 0.1 km 3. The sea at the impact site was a shallow-water, near-shore The crater rim wall consists of four horseshoe-shaped zone of the Paleo-Baltic basin, which opened to the Iapetus elevated parts, including Paluküla High in the east-northeast Ocean far in the southwest. The lower Paleozoic sedimentary and Tubala in the west-southwest. The crest of the wall cover and Paleo-Proterozoic crystalline basement were both segments was eroded deeply enough to expose the crystalline affected by the impact.

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