New Ordovician–Silurian drill cores from the Siljan impact structure in central

Sweden: an integral part of the Swedish Deep Drilling Program

OLIVER LEHNERT1, GUIDO MEINHOLD2, STIG M. BERGSTROM3, MIKAEL CALNER4,

JAN OVE R. EBBESTAD5, SVEN EGENHOFF6, ASA M. FRISK7, JUDITH L. HANNAH6,8,

ANETTE E.S. HOGSTROM9, WARREN D. HUFF10, CHRISTOPHER JUHLIN11, JORG MALETZ12,

HOLLY J. STEIN6,8, ERIK STURKELL13 and THIJS R.A. VANDENBROUCKE14

Lehnert, O., Meinhold, G., Bergstrom, S.M., Calner, M., Ebbestad, J.O.R., Egenhoff, S., Frisk, A.M., Hannah, J.L.,

Hogstrom, A.E.S., Huff, W.D., Juhlin, C., Maletz, J., Stein, H.J., Sturkell, E. & Vandenbroucke, T.R.A., 2012: New

Ordovician–Silurian drill cores from the Siljan impact structure in central Sweden: an integral part of the Swedish

Deep Drilling Program. GFF, Vol. 134 (Pt. 2, June), pp. 87–98. q Geologiska Foreningen. doi:

10.1080/11035897.2012.692707.

Abstract: New drill cores from the largest known impact structure in Europe, the relict of the Siljan

meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and

ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia.

The impact crater is an important target of the project “Concentric Impact Structures in the Palaeozoic”

within the framework of the “Swedish Deep Drilling Program”. Two core sections, Mora 001 and

Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late

Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the

classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different

facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus

occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in

the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be

related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus,

this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the

Caledonian collision between Baltica and Laurentia.

Keywords: Sweden; Siljan district; Ordovician; Silurian; impact structure; stratigraphy.

1GeoZentrum Nordbayern, Universitat Erlangen, Schloßgarten 5, D-91054 Erlangen, Germany;

lehnert@geol.uni-erlangen.de2Geowissenschaftliches Zentrum der Universitat Gottingen, Abteilung Sedimentologie/Umweltgeologie,Goldschmidtstraße 3, D-37077 Gottingen, Germany3School of Earth Sciences, The Ohio State University, 155 S. Oval Mall, Columbus, OH 43210, USA4Department of Geology, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden5Museum of Evolution, Uppsala University, Norbyvagen 16, SE-752 36 Uppsala, Sweden6Department of GeoSciences, Colorado State University, Fort Collins, CO 80523, USA7Palaontologisches Institut und Museum, Universitat Zurich, Karl Schmid-Strasse 4, 8006 Zurich,Switzerland8Geological Survey of Norway (NGU), 7491 Trondheim, Norway9Tromsø University Museum, Natural Sciences, N-9037 Tromsø, Norway10Department of Geology, University of Cincinnati, 500 Geology/Physics Building, Cincinnati, OH 45221-0013, USA11Department of Earth Sciences – Geophysics, Uppsala University, Villavagen 16, SE-752 36 Uppsala,Sweden12Institut fur Geologische Wissenschaften, Freie Universitat Berlin, Malteser Strasse 74-100, D-12249 Berlin,Germany13Department of Earth Sciences, University of Gothenburg, PO Box 460, SE-405 30 Goteborg, Sweden14UMR 8217 du CNRS: Geosystemes, Universite Lille 1, Avenue Paul Langevin, Batiment SN5, 59655Villeneuve d’Ascq, France

Manuscript received 27 March 2012. Revised manuscript accepted 8 May 2012.

GFFvolume 134 (2012), pp. 87–98.A Scandinavian Journal of Earth Sciences Article

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IntroductionResearch on impact structures in Scandinavia started in the1960s (e.g. Svensson & Wickman 1965) and was partlyconducted by the geoscientist Maurits Lindstrom since the 1980s(Bergstrom et al. 2007). Recently, the project “ConcentricImpact Structures in the Palaeozoic” (CISP; Hogstrom et al.2010) has become an integrated part of the “SwedishDeep Drilling Program” (SDDP; Lorenz 2010). CISP was oneof two projects within the SDDP that in 2009 was funded by theSwedish Research Council for impact studies in central Sweden.This work is part of the CISP project and focuses on the Siljancrater in Dalarna, central Sweden (Fig. 1), which is the largestimpact structure in Europe (Table 1) and among the largest inthe world (Table 2).

Recent argon laser dating of the melt breccia gives a lateDevonian (Frasnian, 377 ^ 2 Ma) age for the Siljan impact(Reimold et al. 2005), and estimates of the pre-erosional craterdiameter range up to 90 km (Holm et al. 2011), although thepreserved present-day diameter is about 52 km (Tables 1 and 2).Long discussions about the origin of the Siljan ring structurewere settled when the discovery of planar deformation featuresin quartz grains and shatter cones provided clear evidence for animpact-formation mechanism (Svensson 1971, 1973; Holm et al.2011). In addition, the impact relationship of the ring is revealedby seismic data (Juhlin & Pedersen 1987, 1993) and the presenceof radial transpression ridges (Kenkmann & von Dalwigk 2000).

The ring-like depression encircling the approximately 30-km-wide central plateau (Fig. 2) of the exhumed impact structure isfilled with megablocks of Precambrian basement rocks andPalaeozoic sediments. The fortuitous preservation of Ordovicianand Silurian strata in the ring structure provides the onlypossibility to study sedimentary facies and ancient palaeoenvir-onments in a region where otherwise only Precambrianbasement rocks are exposed. Especially, the classic limestonesuccession in the eastern part of this structure showing theformation of two large and palaeogeographically interestingcarbonate mound complexes which had formed in deeper shelfenvironments (Kullsberg and Boda mounds; Fig. 2) was subjectof various palaeontological and sedimentological studies(references in Ebbestad & Hogstrom 2007).

Research on the Ordovician and Silurian succession in theSiljan district has a long tradition. This research history issummarized in Ebbestad & Hogstrom (2007). It started alreadyin 1734 with the report of crude oil in the Skarbacka quarry atOsmundsberget by Carl von Linne. His paper with observationsabout the geology of Dalarna was published posthumously(Linne 1788). Therefore, the first scientific publication from thisregion is the more detailed description of the oil and gasoccurrence in a paper by Tilas (1740) which was translated anddiscussed by Hedberg (1988). The lithologic terminology usedfor the units in the field area has changed over time, and we usethe nomenclature as summarized in the recent compilation byEbbestad & Hogstrom (2007) in this contribution.

The Palaeozoic succession in the Siljan district starts with theTremadocian Obolus beds, while the youngest pre-Caledonianstrata are Middle Silurian shales of the Nederberga Formation(Grahn 1998). Exposures in the area are, however, limited withonly a few long continous sections, and the succession is stillincompletely investigated. New drill cores provided by theSwedish private company IGRENE, therefore, fill an importantgap in the research on the stratigraphy of the Siljan district.

This paper documents the sedimentology and preliminarybiostratigraphy of the core sections as well as rock exposed in atrench dig at Nittsjo (Fig. 2). Together with future analysis of thecarbon isotope chemostratigraphy of these rocks, our data willallow for intra- and intercontinental correlations and dating ofCaledonian movements in the Siljan district. Furthermore, thedata will allow testing of current palaeogeographic reconstruc-tions (e.g. Cocks & Torsvik 2005), especially with respect to theclosure of the Iapetus Ocean and, hence, the collision betweenBaltica and Laurentia (e.g. Gee et al. 2010).

The IGRENE coresIn 2011, the Swedish company IGRENE drilled three boreholesin the Siljan region for natural gas exploration. Two of thesecores were sampled in detail by OL and GM in July and October2011 for different analytical purposes (Figs. 3, 4). The dip ofbedding is mostly horizontal except in the “lower shale member”of the Mora 001 core. In both cores, the classic lithostratigraphyof the Siljan district is apparent. It should be noted, however,that this terminology now is in need for revision and requirescalibration with the new international chronostratigraphy for theOrdovician, although this is out of the scope of this report.

Core descriptionsMora 001 core. The Mora 001 core was recovered at a sitesouthwest of the town of Mora (Fig. 2; geographic coordinates:WGS 84 – N 60 58.855, E 14 31.870; Swedish grid system –RT90 X676328, Y143096). The Lower to Middle Ordoviciansuccession from the contact with the Precambrian basementrocks to the top of the Holen Formation is 21.57 m in thickness(Fig. 5). At the contact with the underlying Precambrianporphyry, the sedimentary succession starts with a thinglauconitic limestone bed (0.16 m) followed by theLatorp (3.24 m), Lanna (5.64 m) and Holen limestones(12.52 m in total: with a basal Taljsten interval of 2.26 m).About 1.20 m above the base of the Latorp Limestone is a tongueof greenish Tøyen Formation (0.73 m grey limestone/green shaleintercalations, 0.08 m greenish nodular limestone, 0.50 mgreen/red shale intercalations). The upper 0.50 m of the TøyenFormation shows some gradual transition into the lower part ofthe Latorp Formation.

The upper boundary of the Holen Limestone is a distinctunconformity (Fig. 4). The unconformity is overlain by aSilurian siliciclastic succession that encompasses upperLlandovery through Wenlock strata and comprises a minimumthickness of about 224 m.

We identify six members with informal status, and theseinclude a “lower shale member” (64.5 m; broadly coeval withthe more calcareous Kallholn Formation, its lateral equivalent tothe east), a “lower shale–mudstone member” (54.14 m), a“tricolour member” (7.66 m; thin layers of red and green shalesand grey silty shales), a “sandstone member” (27.23 m), an“upper shale member” (36.63 m) and an “upper shale–mudstonemember” (33.84 m).

The upper part of this clastic succession is equivalent to theNederberga Formation of Grahn (1998). He describedsandstones at the base of that formation, covered by shale ofearly Wenlock age (Grahn 1998). Hence, the NederbergaFormation correlates at least in part with the “sandstone

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Fig. 1. Impact structures on the Scandinavian Peninsula and adjacent countries (orange and red dots; the numbers are explained in Table 1). TheNASA landsat image was taken on 19 February 2003, showing true-colour MODIS (Moderate Resolution Imaging Spectroradiometer) by JacquesDescloitres, MODIS Rapid Response Team, NASA/GSFC (http://eoimages.gsfc.nasa.gov/images/imagerecords/3000/3239/scandinavia.TMO2003050_lrg.jpeg). The distribution of the shown impact craters is based on data accessible in the Earth Impact Database of the“Planetary and Space Science Centre”, New Brunswick, Canada (PASSC; www.passc.net).

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member” and with the overlaying “upper shale” and “uppershale–mudstone members” in the Mora 001 core. Accordingly,based on this new core section, a redefinition of the NederbergaFormation, including all the clastic members observed on top ofthe Middle Ordovician Holen Limestone (Fig. 5), is necessary.

Several thin K-bentonite beds occur within the lower part ofthe Silurian clastic succession; at 220.20–220.18 m, 213.015–213.01 m, 161.57–161.56 m, 161.53–161.50 m and 152.11–152.07 m (Fig. 6).Solberga 1 core. The Solberga 1 core was recovered ca. 700 m

north of Solberga quarry (Fig. 2; geographic coordinates: WGS

84 – N 60 59.296, E 15 12.735; Swedish grid system – RT 90X676393, Y146820). In contrast to the Mora section, the corerecovered from Solberga shows a classical Ordovician carbonatesuccession well known from the Siljan district and other parts ofSweden. A composite of the sedimentary succession based ondifferent thrust slices is shown in the measured profile in Fig. 6.

Sedimentation in the Solberga area starts with the lateTremadocian Obolus beds (1.15 m, often called Obolusconglomerate in the literature). The bulk of the material in thisunit consists of material reworked from crystalline basementrocks. Irregular limestone lenses with the eponymous brachio-

Table 1. List of impact structures in Europe based on data accessible in the Earth Impact Database of the “PSSC”, New Brunswick, Canada (PASSC; www.passc.net).

No. Crater name Country Location Diameter (km) Age (Ma)

1 Siljan Sweden N 618 20, E 148 520 52 376.8 6 1.72 Mjølnir Norway N 738 480, E 298 400 40 142.0 ^ 2.63 Keurusselka Finland N 628 80, E 248 360 30 ,18004 Boltysh Ukraine N 488 450, E 328 100 24 65.17 ^ 0.645 Ries Germany N 488 530, E 108 370 24 15.1 ^ 0.16 Lappajarvi Finland N 638 120, E 238 420 23 73.3 ^ 5.37 Rochechouart France N 458 500, E 08 560 23 214 ^ 88 Obolon Ukraine N 498 350, E 328 550 20 169 ^ 79 Dellen Sweden N 618 480, E 168 480 19 89.0 ^ 2.710 Logoisk Belarus N 548 120, E 278 480 15 42.3 ^ 1.111 Ternovka Ukraine N 488 080, E 338 310 11 280 ^ 1012 Paasselka Finland N 628 20, E 298 50 10 ,180013 Lumparn Finland N 608 90, E 208 60 9 ,100014 Mien Sweden N 568 250, E 148 520 9 121.0 ^ 2.315 Neugrund Estonia N 598 200, E 238 400 8 ,54016 Vepriai Lithuania N 558 50, E 248 350 8 .160 ^ 1017 Lockne Sweden N 638 00, E 148 490 7.5 , 45818 Kardla Estonia N 598 10, E 228 460 7 ,45819 Soderfjarden Finland N 638 20, E 218 350 6.6 ,60020 Saaksjarvi Finland N 618 240, E 228 240 6 ,56021 Gardnos Norway N 608 390, E 98 00 5 500 ^ 1022 Mizarai Lithuania N 548 10, E 238 540 5 500 ^ 2023 Dobele Latvia N 568 350, E 238 150 4.5 290 ^ 3524 Suvasvesi N Finland N 628 420, E 288 100 4 ,100025 Steinheim Germany N 488 410, E 108 40 3.8 15 ^ 126 Zeleny Gai Ukraine N 488 40, E 328 450 3.5 80 ^ 2027 Zapadnaya Ukraine N 498 440, E 298 00 3.2 165 ^ 528 Granby Sweden N 588 250, E 148 560 3 ,46629 Iso-Naakkima Finland N 628 110, E 278 90 3 .100030 Ritland Norway N 598 140, E 68 260 2.7 520 ^ 2031 Rotmistrovka Ukraine N 498 00, E 328 00 2.7 120 ^ 1032 Tvaren Sweden N 588 460, E 178 250 2 ,45833 Karikkoselka Finland N 628 130, E 258 150 1.5 ,23034 Saarijarvi Finland N 658 170, E 288 230 1.5 .60035 Kaalijarv Estonia N 588 240, E 228 400 0.11 0.004 ^ 0.00136 Morasko Poland N 528 290, E 168 540 0.1 ,0.0137 Ilumetsa Estonia N 578 580, E 278 250 0.08 ,0.0066

Notes: The ages of the Lockne, Kardla, Granby and Tvaren structures are according to Grahn et al. (1996); the age of the Neugrund structure is according to Suuroja & Suuroja (2000). Thepresent-day diameter of the Siljan impact structure is about 52 km (Grieve 1988), but estimations by Holm et al. (2011) suggest an original diameter of up to 90 km, based on the distributionof shock metamorphic features in the basement rocks. The focus of the CISP project, the Siljan and Lockne impact structures, is highlighted in bold.

Table 2. List of the 15 largest impact structures in the world reflecting the importance of the Siljan impact structure ranking together with Kara-Kul as the 12th largestworldwide.

Ranking Crater name Province, country Location Diameter (km) Age (Ma)

1 Vredefort Free State Province, South Africa S 278 00, E 278 300 300 2023 ^ 42 Sudbury Ontario, Canada N 468 360, W 818 110 250 1850 ^ 33 Chicxulub Yucatan, Mexico N 218 200, W 898 300 170 64.98 ^ 0.054 Manicouagan Quebec, Canada N 518 230, W 688 420 100 214 ^ 14 Popigai Krasnojarsk, Russia N 718 390, E 1118 110 100 35.7 ^ 0.25 Acraman South Australia, Australia S 328 10, E 1358 270 90 ,5905 Chesapeake Bay Virginia, USA N 378 170, W 768 10 90 35.5 ^ 0.36 Puchezh-Katunki Volga Federal District, Russia N 568 580, E 438 430 80 167 ^ 37 Morokweng North West Province, South Africa S 268 280, E 238 320 70 145.0 ^ 0.88 Kara Nenetsia, Russia N 698 60, E 648 90 65 70.3 ^ 2.29 Beaverhead Montana, USA N 448 360, W 1138 00 60 ,60010 Tookoonooka Queensland, Australia S 278 70, E 1428 500 55 128 ^ 511 Charlevoix Quebec, Canada N 478 320, W 708 180 54 342 ^ 1512 Siljan Dalarna, Sweden N 618 20, E 148 520 52 376.8 6 1.712 Kara-Kul Berg-Badachschan, Tajikistan N 398 10, E 738 270 52 ,5

Source: Earth Impact Database of the PASSC, New Brunswick, Canada; www.passc.net. The Siljan impact structure is highlighted in bold.

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pod (now named Ungula, see Ebbestad & Hogstrom 2007) areintercalated into the beds along with clasts derived from theNeoproterozoic granite and mixed with glauconitic sands. TheTremadocian Obolus beds are overlain by a late Tremadocianthrough middle Katian limestone succession including theLatorp (4.28 m), Lanna (4.06 m), Holen (19.10 m, with a basalTaljsten interval of 3.20 m), Segerstad limestones (4.30 m), theinterval of the Skarlov–Seby–Folkslunda limestones (4.20 m;undifferentiated), Dalby–Furudal limestones (54.50 m; undif-ferentiated), Freberga Formation (13.45 m), a transition from theFreberga Formation into the Slandrom Limestone (1.80 m) andan incomplete succession of the Slandrom Limestone (4.84 m)cut off by a major thrust fault (Fig. 6). Small-scale palaeokarst

morphologies have been found in several thrust slices at thetop and within the Slandrom Limestone.

Similar to the development of the Lower Ordovician successionin theMora 001 core,1.17 mabove thebase of the Latorp Limestoneis a tongue of Tøyen Formation (about 0.65 m of grey nodularlimestone in a green shaly matrix). The Tøyen Formation iscalcareous and much thinner than in the Mora 001 core, reflecting ashallowing from the Mora area (where parts of the formation aremuch more shaly) towards the east. The contact between the TøyenFormation and the Latorp Limestone is also transitional in theSolberga 1 core, showing a clear facies interfingering.

K-bentonites are concentrated in the upper part of the DalbyLimestone (Sandbian), and lower part of the Freberga

Fig. 2. Simplified geological map of the Siljan district with location of IGRENE drill sites (1, Mora 001; 2, Stumsnas; 3, Solberga), the Nittjo trenchand other locations mentioned in the text.

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Formation, occurring at 179.19–179.16 m, 178.92–178.89 m,178.625–178.59 m, 176.30–175.43 m (presumably the Kinne-kulle K-bentonite based on thickness), 171.39–171.37 m and171.00–170.52 m.

A complete succession of the late Katian Fjacka Shale(6.64 m) is only preserved in the middle part of the package ofthe thrust slices (depth in the Mora 001 core: 124.65–131.29 m;Fig. 6). Limestone belonging to the Jonstorp Formationis preserved only within different thrust slices and it is,therefore, not possible to establish the thickness of this unit.In the Solberga 1 core, typical greenish-grey (LowerJonstorp Formation) and reddish (Upper Jonstorp Formation)packstone with trilobites and brachiopods were recovered fromthe lower part of the package of thrust slices, in between the twomajor fault planes. In between these lower and upper members isan interval displaying some transition of these colours, whichpresumably correlates with the middle member of theJonstorp Formation, the Oglunda Limestone. The thickestinterval of Lower Jonstorp Formation with a sedimentarycontact at the base comprises 3.57 m of grey/greenishlimestones, providing a minimum thickness estimate of theunit. The thickest succession of the Upper Jonstorp Formationshows a tectonic contact at the base and the top and is 6.60 m inthickness. The uppermost approximately 4 m of the interval withthrust slices comprises several thrust slices with greenish marlsof the lower part of the Loka Formation, partly mixed with BodaLimestone. Above the upper main thrust, 4.48 m of Upper

Jonstorp Formation is overlain by the marly lower part of theLoka Formation (2.33 m) with sedimentary contact, followed bya “tongue” of Boda Limestone (3.79 m), and an upper portion ofgreenish, marly Loka Formation (7.46 m). The Boda Limestoneis fully developed in the Solberga quarry to the south of the drillsite. Sedimentary contacts between a marly lower part of theLoka Formation, the Boda Limestone and an upper more nodularLoka Formation reflects the interfingering of these units indeeper environments on the outer shelf.

The early Llandovery succession of the Motala Formationcompares well with the lithologies exposed in the type area inOstergotland (Bergstrom & Bergstrom 1996). There are threeinformal members, a marly lower member (6.90 m), a shalymiddle member with limestone nodules (16.63 m) and an uppernodular limestone member (4.13 m). The top of the EarlyPalaeozoic succession recovered from the Solberga 1 corecomprises the black shales of the Llandovery KallholnFormation (59.53 m). Within this shale unit, several thin K-bentonite beds have been observed, occurring at 59.86–59.82 m,58.22 – 58.19 m, 48.47 – 48.42 m, 45.76 – 45.70 m, 44.32 –44.30 m, 44.185–44.15 m, 33.19–33.05 m, 32.71–32.70 m,and 30.38–30.34 m.

DiscussionIn the Mora 001 core, the Lower to Middle Ordovician carbonatesuccession is only about 21 m thick, with a major unconformity atthe top of the Middle Ordovician Holen Limestone (Fig. 5). Theoverlying siliciclastic succession is upper Llandovery throughearly Wenlock (based on preliminary graptolite data) and is at least224 m thick. A substantial deepening of the depositionalenvironment in the early Silurian is associated with highdepositional rates of shale and mudstone with megaslumps, debrisflows and turbidites suggesting increasing angle of depositionalslopes. The presence of abundant synsedimentary tectonic features(e.g. slumping structures and localized faulting) in this part of thesuccession further implies active tectonism in the area. Weinterpret the hiatus and the subsequent deepening of depositionalenvironment as due to passage of a Caledonian peripheralforebulge from the west towards the east in Late Ordovicianfollowed by a movement back to the west due to increasedtopographic loading in the earliest Silurian. The intercalation of asandstone unit in the Silurian shales reflects a regression of theshoreline in Wenlock times, followed by rapid transgression anddeposition of dark, organic-rich shale and mudstone.

In contrast to this development, a classical Ordovician–Silurian carbonate-shale succession, well known also from otherparts of Sweden, formed in the north-western and northern part(Skattungbyn–Kallholn–Kargarde), north-eastern part (Furu-dal) and south-eastern part (Boda) of the Siljan district (Fig. 2).It is also recovered in the Solberga 1 core, although not totallyundisturbed. There is a 41.06-m-thick interval of multiple thrustslices between two major thrust planes at the depth of 105.25and 146.31 m (Fig. 6). Within this package, however, there arealso sedimentary contacts between the Slandrom Limestone andthe Fjacka Shale, as well as between the Fjacka Shale and theLower Jonstorp Formation. This allows a detailed study of theFjacka Shale succession, which is an important regional sourcerock for hydrocarbon generation. Regardless of the structuralcomplications, Solberga 1 gives the first nearly continous coresuccession through the classical Ordovician–Silurian (lateTremadocian through early Llandovery) succession in this

Fig. 3. A and B. Core description and sampling activity at the IGRENEcore store at Mora, Vattymyrsvagen. Core boxes laid out for loggingand sampling in the IGRENE core storage facility.

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region. Bergstrom et al. (2011) pointed out that conodont datafrom sections in three different quarries in the impact structure(Bergstrom 2007) had to be compiled for a comparison of thelate Darriwilian to early Llandovery strata recovered from theBorenshult core at Motala in Ostergotland (upper FurudalLimestone through basal Motala Formation) with the successionexposed in the Siljan area. In this large impact structure, nopreviously known core or outcrop contains the correspondingstratigraphic interval in the Borenshult core.

The palaeokarst found in several thrust slices at the top andwithin the Slandrom Limestone in the Solberga 1 core is coeval

with the palaeokarst reported from the south-eastern part of theSiljan ring structure (Calner et al. 2010a) and in other parts ofBaltoscandia, reflecting a time interval of subaerial exposure ofthe basin during a glacial event (Calner et al. 2010b).

Besides Mora 001 and Solberga 1, a third yet unlogged andunsampled core (Stumsnas 1, geographic coordinates: WGS 84 –N 60 52.689, E 14 57.222) has been recovered from a drill sitewest of Rattvik (Fig. 2). First observations show that the lowerTremadocian through Darriwilian part of the sedimentarysuccession in the Stumsnas 1 core is lithologically comparableto that in the Mora 001 and Solberga 1 cores. However, debris

Fig. 4. Examples of core boxes from the Mora 001 and Solberga 1 drill sites. A. Core box 35 of Mora 001 showing the upper part of the DarriwilianHolen Limestone overlain by Silurian shales. B. Core box 11 of Solberga 1 showing typical lithologies of the earliest Silurian Motala Formation.The core box with marks shows exemplarily the sampling for carbon isotopes.

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Fig. 5. Simplified log of the Mora 001 core. Note that 0 m in the illustrated section correlates with the 14 m depth mark in the IGRENE core and waschosen as the undisturbed top of the “upper shale–mudstone member” of the Silurian clastic succession.

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Fig. 6. Simplified log of the Solberga 1 core. Note that 0 m in the illustrated section correlates with the 14.80 m depth mark in the IGRENE core andwas chosen as the undisturbed top of the Kallholn Formation. The transitional interval displaying a greenish-grey to red colour change likelyrepresents strata coeval to the Oglunda Limestone which is the middle member of the Jonstorp Formation. The key is given in Fig. 5.

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Fig. 7. Preliminary lithostratigraphic correlation for the Mora 001 and Solberga 1 core sections. Note that only a few preliminary biostratigraphicdata (graptolites) exist from the cores. The nomenclature is according to Ebbestad & Hogstrom (2007).

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flows and olistoliths in the upper part of the core reflect a positionon a slope where due to instability and tectonic movementsmaterial was sliding as blocks or shed as debris flows towards abasin forming to the west (present-day coordinates).

ConclusionsThe late Devonian meteorite impact in the Siljan district createdspecial conditions for the preservation of Early Palaeozoicsuccessions from later erosion by protecting them as megablocksthat slid back into the crater shortly after the impact. This reportsummarizes the general Lower Palaeozoic stratigraphy based ontwo new core sections from the ring structure (Mora 001 andSolberga 1). The conclusions are as follows:

. The Mora 001 and Solberga 1 cores include strata rangingfrom the late Tremadocian to Wenlock in age.

. Ordovician K-bentonites in the Solberga 1 core are concentratedin the upper Dalby Limestone (including the Kinnekulle K-bentonite as a marker horizon) and in the Lower FrebergaFormation, which can be compared to the occurrences of theselayers in other parts of Baltoscandia (e.g. Bergstrom et al. 1997).Furthermore, possible correlation may exist between the few thinK-bentonite beds in the Silurian shale succession of the Mora 001core and the well-known K-bentonite horizons in shallowerenvironments related to major eruptions (e.g. Osmundsberget K-bentonite; Kiipli et al. 2006; Bergstrom et al. 2008).

. The erosional unconformity and hiatus between the MiddleOrdovician Holen Limestone and early Silurian shales in theMora area (Fig. 7, Mora 001 core), spanning some 30 Ma, isinterpreted as a result of passage of the Caledonian peripheralforebulge due to tectonic loading by thrust sheets to the west.The classic upper Middle and Upper Ordovician carbonatesuccession with large mound structures (Kullsberg and Bodamounds), characteristic for the eastern part of the basin (Figs.2, 7), is missing in the western part of the impact structure.

. A substantial regression in the Silurian clastic succession ofthe Mora 001 core is indicated by major influx of silt(“tricolour member”) and proximal delta sands (“sandstonemember”). This regression may reflect the global sea-leveldrop during the Sheinwoodian (early Wenlock) glaciation.This assumption is justified by preliminary graptolite datafrom the upper part of the “lower shale member” indicating alate Llandovery age (Oktavites spiralis Biozone, or slightlyyounger) for these strata. The graptolite data also show that the“lower shale member” correlates broadly with the morecalcareous Kallholn Formation, which is seen as a lateralequivalent of these shales.

Acknowledgements. – OL and GM are very grateful for support of the studies related to theCISP project of the “SDDP” in the frame of the “International Continental Scientific DrillingProgram” (ICDP) by the Deutsche Forschungsgemeinschaft (DFG project LE 867/8-1). MCacknowledges support from the Swedish Research Council. We are indebted to Mats Budh,Soren Hedberg and Andreas Gidlund of IGRENE in Mora. Comments from Sanna Holm andCarl Alwmark (Lund University) as well as by Mark Johnson (Univerity of Goteborg)improved the manuscript.We all thank Prof. Maurits Lindstrom, who led numerous researchprojects not only on Palaeozoic meteorite craters but also in different fields ofpalaeontology, sedimentology and regional geology in Europe. Before he passed away on14 November 2009, Prof. Lindstrom and fellow applicants (E. Sturkell, Gothenburg;J. Ormo, Madrid; C. Juhlin, Uppsala; R.O. Greiling, Karlsruhe; A.E.S. Hogstrom, Tromsø)initiated the project “CISP”. OL is indebted to Maurits Lindstrom for inviting him to work inthe CISP project on various geoscientific questions.This paper is a contribution to the IGCP591 project “The Early to Middle Paleozoic Revolution”.

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