The Siljan Meteorite Crater in central Sweden – an integral of the Swedish Deep Drilling Program (SDDP)

New drill cores from the largest known impactstructure in Europe, the Siljan crater, provide superbpossibilities to reconstruct Early Palaeozoic marineenvironments and ecosystems, and to document changes insedimentary facies, sea-level and palaeoclimate. Themeteorite crater is a major target of the project ConcentricImpact Structures in the Palaeozoic (CISP) in theframework of the Swedish Deep Drilling Program.Studies of Ordovician and Silurian strata in the SiljanDistrict have a long tradition and many scientific papersdeal with the geology of the area. The Palaeozoicsuccession starts with the Tremadocian Obolusconglomerate, the youngest pre-Caledonian strata areMiddle Silurian shales of the Nederberga Formation.However, exposures are limited, there are few continoussections, and the Early Palaeozoic sedimentary rocksresting on the Precambrian basement are incompletelyinvestigated.Detailed sedimentological and biostratigraphicalstudies of the cores and the Nittsjö trench together withanalysis of the carbon isotope chemostratigraphy will allowintra- and intercontinental correlations and the dating ofCaledonian movements. Our preliminary studies show thatdifferent and yet undefined facies belts are preserved in theSiljan District. The recent findings of palaeokarst in thearea together with similar new findings in other parts ofBaltoscandia reflect times of subaerial exposure of thebasin regionally and challange the idea that theBaltoscandian basin was a deep and tranquil depositionalenvironment.Our preliminary data provide a first base forreinterpretations of this part of Sweden, previouslyregarded as representing a stable cratonic area unaffectedby the Caledonian collision between Baltica and Laurentia.The erosional unconformity and the substantial hiatusbetween Middle Ordovician limestones and late EarlySilurian shales in the western part of the crater suggests anextended period of uplift and erosion presumably related toflexural forebulge migration toward the east due to tectonicloading by the Caledonian nappes to the west. The Lowerto Middle Ordovician carbonate succession is only about21 m thick, with a sharp flooding surface on top of theMid-Ordovician Holen Formation. The overlyingsiliciclastic succession (Upper Llandovery, based ongraptolite data) comprises a minimum thickness of about224 m. The sudden deepening after the eastward migrationof the forebulge is indicated by rapid deposition of shalesand shale/mudstones displaying unstable conditionsexpressed by megaslumps, debris flows, turbidites andseveral synsedimentary tectonic features. The intercalationof a sandstone unit reflects a strong regression in this shalebasin followed by rapid transgression and deposition ofdark, organic-rich shale and mudstone.In contrast to this development, a classicalOrdovician/Silurian carbonate/shale succession, well103 IODP/ICDP Kolloquium Kiel, 07. – 09.03.2012Figure 6: Bathymetry map of Lake Ohrid. Proposed sites aremarked by the red dots.Figure 5: Multichannel seismic line crossing the DEEP-Site inthe central basin of Lake Ohrid.known from other parts of Sweden, formed in the northern(Skattungbyn-Kallholn), northeastern (Furudal), andsoutheastern part (Boda) of the Siljan District. Detailedsampling of the cores for stable isotopes, thermal maturity,geochemistry, sediment provenance, facies and microfaciesstudies in the autumn of 2011 now helps in solvingregional problems as well as stratigraphical andpalaeogeographical questions.CISP - Concentric Impact Structures of the Palaeozoi