Concentric Impact Structures in the Palaeozoic - the Lockne and Siljan craters (CISP)

Principal Investigators:
     Erik Sturkell, Gothenburg University, Sweden.
     Anette E.S. Högström, Tromsø University Museum, Norway.
     Jan Ove R. Ebbestad, Uppsala University, Sweden.

Large impact structures are a key to understanding a diverse set of scientific problems since they record essentials about the geological history where they occur. The ambition of CISP is to characterize the large Lockne and Siljan crater structures, with decisive features in common. The Siljan structure, the largest impact structure in Western Europe, and Lockne, which is the best accessible and studied impact formed in relatively deep sea, occurred at the supposedly Himalayan-type Caledonian mountain front in Sweden. Their development therefore expresses interaction between a gigantic over-thrust plate and Baltica through about 80 Ma. The most important apparent interaction was depressing Baltica’s west flank to form the trough where both craters formed. Another was tectonic stress that influenced shape and mineralization in the craters. It is all the more important to elucidate the less understood complications that can help to understand other craters. Major issues are the importance of basement lithology and structure, depth of water or sediment cover at the time of impact.
In addition to impact tectonics, the effect on life from the impact event can be studied in the Lockne area where records of post-impact re-colonization are found. The Siljan crater, with its renowned carbonate mud mounds, also contains unique sedimentological and palaeontological records preserved solely within the crater structure.

In 2009 CISP was one of two projects within the Swedish Deep Drilling Program (SDDP) that was granted funds for site investigations in the Swedish Research Council’s (VR) annual call for scientific projects.  The project received about 1/2 of the applied funding, for the period 2010 – 2012, although the funds are available until the end of 2013.
Dr. Erik Sturkell was the main applicant, with drs Jens Ormö (Centro de Astrobiología, Madrid, Spain), Christopher Juhlin (Uppsala University, Sweden), Reinhard O. Greiling (Karlsruhe Institute of Technology, Germany) and Anette Högström listed as co-workers. Administration of the project is handled by the principal investigators. The project was presented in the SDDP Science & Technology Plan in May 2010 (http://www.sddp.se/sciencetechnology), as well as in Högström et al. (2010).

Fig. 1 – Anette Högström sampling at the Nittsjö section 2010. The ditch was re-filled when sampling was done.

Objectives and progression

The initial objective of the VR-proposal in 2009 for the project CISP was to establish reliable geological parameters for the deeper drilling of the Lockne and Siljan impact structures under the SDDP umbrella (Högström et al., 2010a).
Planned analytical approaches towards reaching this objective were:

• Shallow geophysics (resistivity and gravity in Lockne)

• High resolution deep reflection seismic in Siljan
• Shallow drilling
• Study of fluid inclusions to determine composition of the fluids and constrain temperature and pressure conditions in shear zones related to the cratering
• Field reconnaissance for drill sites
• AMS studies (anisotropy of magnetic susceptibility) on basement rock

The Science and Technology Plan of SDDP lists 16 participants in CISP, which per 2011 has almost doubled.

2010

In the initial year 2010, a general paper was published in where the project was presented (Högström et al., 2010a).
During the autumn, field work was undertaken in the Siljan area near Nittsjö. Here a new section was measured of the Lower Palaeozoic sediments with a basal contact to the precambrian granites (Högström et al., 2010b). The field work was financed from sources outside CISP. The principal investigators are Oliver Lehnert (Friedrich-Alexander University Erlangen-Nürnberg, Germany), Ebbestad, Högström and Åsa Frisk (Fig. 1). 
Field work was also undertaken in Siljan by Holly Stein and Judith Hannah from the AIRIE Program (http://www.airieprogram.org/) at Colorado State University in partnership with the Geological Survey of Norway. This work is in collaboration with Jan Ove R. Ebbestad. The aim was to sample hydrocarbons and shales to better understand Re-Os fractionation between oil and shale on maturation. The field work was financed through grant support to the AIRIE Program. Previous preliminary studies from AIRIE include Stein et al. (2009a, b, c) and Zimmerman et al. (2009). 
A drilling down to 147 m was made in the Målingen structure in Lockne 2009 and 2010. This is a possible satellite crater to Lockne. The drilling was undertaken with sources outside CISP. Other field work consisted of geophysical surveys (magnetometry and gravimetry; Fig. 2). Results related to this work and other studies in Lockne were presented at meetings and in publications (Ormö et al., 2010a-d).

Fig. 2 – Fieldwork in the Lockne crater and in the possible satellite crater at Målingen located 12 km outside the Lockne structure. A) Gravity measurements performed in the autumn 2011. B) The crystalline impact breccia (Tandsbyn Breccia). C) Drilling in the centre of Målingen structure in the summer 2010. D) The drill-core recovered in Målingen, the hole finally reached almost 150 m.

2011
The main work performed during 2011 was the completion of two seismic lines on the west side of the Siljan structure. The principal investigator was Christopher Juhlin (Uppsala), and the work financed by CISP. The first line was 9 km and placed in an east-west direction just north of Mora. The second line was 12 km and placed in a north-south direction north of Orsa (Fig. 3). Each profile penetrated to about 2 km, and a number of good reflectors are evident. The seismic profile is being interpreted, and this will be the most important tool in determining the position of a deep drill hole.
During field work at Nittsjö 2010, Högström and Ebbestad came in contact with personnel from the company Igrene (http://www.igrene.se/). In June 2011 Igrene extended an invitation to study three drill cores they had taken up in connection with their prospecting. Each core was about 600 m, penetrating the Palaeozoic succession and the underlying basement. The cores were taken at Solberga, Stumsnäs and Mora (Fig. 4). Oliver Lehnert, Stig Bergström (Ohio State University, Columbus, USA) and Jan Ove R. Ebbestad made initial investigations of the cores, which turned out to be extremely interesting (see Igrene’s newsletter linked here).

Fig. 3 – Map of the Siljan structure with inset showing the position of the seismic lines in A) Mora area and B) Orsa area.

During October 2011 Lehnert, together with Guido Meinhold (University of Göttingen, Germany), continued a detailed logging of the core, sampling in detail for isotope- and biostratigraphy, geochemistry, thermal maturity, sediment provenance studies, and sedimentology (including detailed facies/microfacies studies). It seems that a palaeogeographical high existed to the west in Siljan, with a hiatus above the Holen Limestone. Preliminary graptolite identifications point to Silurian deposits following the Darriwilian Holen Limestone. The principal investigator is Lehnert. This work has been partly financed by CISP (Fig. 5). 

Fig. 4 – Map of the Siljan structure indicating sedimentological distribution and the position of the Ordovician carbonate mounds. The three Igrene drill sites are indicated. Transpression ridges superimposed following the suggestion by Kenkman & von Dalwigk (2000). Map modified from Ebbestad & Högström (2007).

Holly Stein and Judith Hannah from the AIRIE Program at Colorado State University sampled the cores for Re-Os analyses (Fig. 5). The intent was to sample both hydrocarbon and the Fjäcka and Kallholn shales. No hydrocarbon of quantity suitable for sampling was found. Several sections of Fjäcka were sampled from Solberga #1 and Stumsnäs #1. The former contains a reddish brown shale, whereas the Stumsnäs is a black shale and is more promising for Re-Os geochemistry. A starter industry-based grant to look at Re-Os fractionation in oil and shale has been secured by Stein and Hannah. A second proposal pending with NSF will augment the industry-based study by building a picture of stratigraphic ages in the Siljan region for linking with the greater CISP geoscience community (Ebbestad is the primary external collaborator). The first Re-Os results on drill core demonstrate success. A supplementary contribution of CISP funds for the Re-Os study would strengthen efforts by Stein and Hannah to raise funds external to Sweden. 

Fig. 5 – Study and sampling of the Igrene cores. A) Stig Bergström and Oliver Lehnert studying core sections, June 2011. B) Oliver Lehnert and Guido Meinhold sampling the Mora 1 core. October 2011. C) Judith Hannah, Mats Budh, Holly Stein and Paul Storm discussing geology. October 2011. D) The Igrene core storage, June 2011. All photos by Ebbestad.

During 2011 geophysical investigations have continued in the Lockne area, and field reconnaissance have been made to determine the location for two shallow drillings (50-60 m) to investigate the structural composition around the so called Tandsbyn Gully. A number of reports and papers related to the Lockne structure has been published (Asensio et al., 2011; Broman et al., 2011; Melero Asensio et al., 2011a-d, Ormö et al., 2011a, b). 

2012
One of the main objectives of the application to the Swedish Research Council in 2012 was to acquire reflection seismics in the Siljan area, and this has been achieved (Juhlin et al. 2012). Other analytical approaches like shallow geophysics (resistivity and gravity in Lockne) and AMS studies (anisotropy of magnetic susceptibility) on basement rocks (Melero Asensio et al. 2011a-e; Ormö et al. 2010a-d; Ormö et al. 2011a, b; Agarwal et al. 2012), study of fluid inclusions to determine composition of the fluids and constrain temperature and pressure conditions in shear zones relate to the cratering (Broman et al. 2011), and shallow drilling (Ormö et al. 2010c; Lehnert et al. 2012a-e) have also been fulfilled, analyses are still in progress on the wealth of data gathered. A new application for additional work was submitted in spring 2012. Despite a favourable review, this project was unfortunately not funded. Thus, major work on CISP will be suspended at the end of the year until further funding can be secured. We will try to keep continuity in the scientific work at a low level with remaining funds from the 2011 grant.

Below is a summary of this year's activities:

The Siljan area

• A paper on the preliminary description of the Igrene drill cores in Siljan is now on line in GFF (Lehnert et al. 2012).
• A paper on the interpretation of two reflection seismic lines in the western part of the Siljan structure is now available online (Juhlin et al. 2012). This paper is open access.
• During the summer and autumn, Oliver Lehnert (Erlangen), Guido Meinhold and Arzu Arslan (both Göttingen), and lately also graptolite specialist Jörg Maletz (Berlin) worked on the three Igrene AB drill cores in Mora. The Stumsnäs core was successfully sampled and further fieldwork conducted in July. This included sampling for conodont stratigraphy in the Upper Ordovician (Hirnantian) strata of Osmundsberget with assistance of Jan Ove Ebbestad (Uppsala). Structural investigations were carried out on all cores.
• Analyses of Corg isotope measurements from the three Igrene cores and the Nittsjö trench (excavated by Ebbestad, Högström and Frisk 2009) are finished and need to be interpreted. Samples for TOC/Rock Eval are currently being analysed and whole-rock geochemical analyses (major and trace elements) will follow soon. Bentonite samples from the core are analysed in collaboration with Warren Huff (Cincinnati) who expects to have a full set of data in the beginning of next year. In addition a oil/bitumen study has been initiated with colleagues from CSIRO in Sydney (Ahmed et al. 2012).
• A status report for Igrene AB is finished.

The Lockne area

Fig. 6 – Field work in the Lockne area 2012. A) Inspection of the railroad profile just west of the Ynntjärnen Lake, this locality displays three different breccias formed during the impact and directly after it. Large clasts of ejected crystalline impact breccia (Tandsbyn Breccia), the impact breccia in the sedimentary sequence (Ynntjärn Breccia), the coarse resurge breccia (Lockne Breccia) and the fine grained (finale) resurge deposits (Loftarstone). B) Drilling near Tandsbyn in the beginning of November 2012. C) The graduate student Irene with the gravimeter on a small island in the Näckten Lake. All photos by Sturkell.

• The fieldwork in Jämtland took place at two localities and on two occasions during 2012. The main fieldwork was performed during July at both the Lockne and Målingen structures, mainly as geological mapping, levelling, and gravity measurements. Two drill sites were selected and all the landowner contacts and all other preparations for the November drilling campaign were made. During this stay, Reinhard Greiling, Agnes Kontny and a German student (Philip) from Karlsruhe joined the fieldwork for about a week (Fig. 6). In early November 2012 we made a six days visit to Lockne to supervise the core drilling in the western part of the structure.
• The geophysical fieldwork in Lockne was concentrated just north of the impact structure. An almost 5 km long profile was levelled (±1 mm accuracy) to produce heights for gravity corrections. The levelling started at the fix point “Sörviken” and followed the Marieby road for 500 m, at this point the levelling left the main road and turned into a small road. Following this small road gravity was measured at 124 sites along a 4.5 km long profile in the southwest direction. The purpose with the profile is to investigate the continuation of a shear zone that cuts the Lockne crater, and, as anticipated, the survey revealed a gravity low. During next field season a parallel profile will be made to confirm the direction of the shear zone.
• Reinhard Greiling, Agnes Kontny and a student (all Karlsruhe) sampled impact rocks for a study of anisotropy of magnetic susceptibility (AMS). The PhD student Amar Agarwal is analyzing the collected material (Agarwell et al. 2013). The Karlsruhe group was taken on an informative excursion in the Lockne area. One of the localities was the Sandbäcken creek that exposes down faulted and folded orthoceratitic limestone.
• The power plant at Högforsen was visited during the July field campaign. At Högforsen (former Högbroforsen), Häggenås, 40 km north of the Lockne impact centre, Maurits observed Loftarstone in the Håkan riverbed. This outcrop was flooded in 1985 due to the new power plant (Fig. 6). In the coming years the power plant will be overhauled after thirty years in service which will lower the water level and, hopefully, expose the Loftarstone.
• During the first four days in November a drill rig (Fig. 6B) arrived to Tandsbyn to make two short core holes. The holes (LOC10 and 11) were set on each side of the Tandsbyn gully to explore the overturned flap. Total depth was 53 and 63 m, respectively.
• In Målingen the geophysical survey continued with gravity. Målingen is situated about 15 km southwest of the 7.5 km wide Lockne crater and is a < 1 km wide structure formed almost simultaneously (within seconds) with Lockne. A 1.7 km long profile to the south of the structure was completed.
• In addition, 9 sites on islands in the Näckten Lake were included. This (hopefully) completed the gravity survey in the Målingen structure, which is now covered by 325 gravity sites. The height determination for the sites on land was done with precision levelling (±1 mm), as the expected anomaly is rather small (<0.5 mgal). For the survey on the islands (Fig. 6C), the level of the lake was determined on a calm day and during the same conditions as the height determinations of gravity sites on the island were made (using the lake level as reference surface). The accuracy for the height determination using the lake level is about ±5 mm. The survey on the islands is necessary for the determination of the regional field.
• During fieldwork, some gaps in the geological map could be filled and know outcrops were revisited. At 29 localities, extensive susceptibility measurements were performed (total 2131). These concluded the acquistion phase and evaluation, analyses and modelling is now taking over, continuing during the autumn and winter 2012/13.
• This summer Jan Ove Ebbestad (Uppsala) and Åsa Frisk (Zürich) collected nautiloid cephalopods in the Lockne structure.They will collaborate with Björn Kröger (Berlin) on analysing the ability of the shells to withstand imploding, thereby hoping to establish a depth parameter for the ocean at Lockne after the impact.
• The book (31 pages) “Locknekratern” written by Maurits Lindström (foreword by Sturkell and Ormö) tells the story about the research that led to the discovery of the marine-target Lockne impact crater. The printing costs (18 000 SEK) were shared by the Lockne crater museum and the CISP project. It is now for sale at the museum which will get the eventual profit.

General

• The CISP proposal for an ICDP workshop 2013 was reviewed by the ICDP. In general, ICDP was positive to the topic but suggested further preparatory work by the CISP working group before re-applying for and arranging an international workshop.
• On April 30th CISP collaborators Oliver Lehnert (Erlangen), Judith Hannah (Colorado), Holly Stein (Colorado) and Jan Ove Ebbestad (Uppsala) featured in a Science Program on Swedish national televison, where the prospecting for hydrocarbons by Igrene AB in Siljan was the focus.
• In the late summer, Chris Juhlin and Jan Ove Ebbestad (Uppsala) met with Vladimir Kutcherov (Stockholm) and Mats Budh (Mora) where possible collaboration on future seismic lines and drill cores in Siljan was discussed.
• In August, Ebbestad presented the CISP project at the Cambridge Arctic Shelf Programme (CASP), after kind invitation of Olga Bogolepova (CASP).

The CISP working group

Prof. Erik Sturkell, Gothenburg University, Sweden.
Dr. Anette S.E. Högström, Tromsø University Museum, Norway.
Prof. Jan Bergström, Swedish Museum of Natural History.
Prof. Stig M. Bergström, The Ohio State University, USA.
Dr. Mikael Calner, Lund University, Sweden.
Dr. Jan Ove R. Ebbestad, Uppsala University, Sweden.
Dr. Oliver Lehnert, Friedrich-Alexander University Erlangen-Nürnberg, Germany.
Dr. Jens Ormö, Centro de Astrobiología, Madrid, Spain.
Prof. Christopher Juhlin, Uppsala University, Sweden.
Prof. Reinhard O Greiling, Karlsruhe Institute of Technology, Germany.
Dr. Gudio Meinhold, Göttingen University, Germany.
Prof. Holly Stein, Colorado State University, USA.
Prof. Judith Hannah, Colorado State University, USA.
Dr. Jörg Maletz, Frei University Berlin, Germany.
Prof. Lars Holmer, Uppsala University, Sweden.
Prof. Risto Kumpulainen, Stockholm University, Sweden
Dr. Paula Lindgren, Stockholm University, Sweden.
Prof. Warren Huff, University of Cincinnati, USA.
Prof. Sven Egenhoff, Colorado State University, USA.
Dr. Arne Thorshøj Nielsen, Natural History Museum of Denmark.
Dr. Yutaro Suzuki, Shizuoka University, Japan.
Dr. Olga Bogolepova, CASP, England.
Dr. Jaak Nolvak, Tallinn University, Estland.
Dr. Thijs Vandenbroucke, Gent University, Belgium.
Dr. Åsa. M. Frisk, Zurich University, Switzerland

References

Ahmed, M., Lehnert, O., Fuentes, D., Sestak, S., Meinhold, G. & Gong, S. 2012. Biomarker evidence for the origin of seep oil and solid bitumen from the Late Devonian Siljan impact structure, Sweden. 17th Australian Organic Geochemistry Conference: Biogeochemistry from Deep Time through Petroleum Resources to Modern Environments. Abstract. Macquarie University, Sydney, New South Wales, Australia, 2nd - 5th December 2012.

Agarwal, A., Lied, P., Srivastava, D., Kontny, A., & Greiling, R. 2013. Evolution of magnetic fabrics before and after the impact in the Lockne Crater, Sweden. Geophysical Research Abstracts 15, EGU General Assembly 7–12 April, Vienna.

Angerer, T. & Greiling, R. 2012. Fabric evolution at basement–cover interfaces in a fold-and-thrust belt and implications for de´collement tectonics (Autochthon, Lower Allochthon, central Scandinavian Caledonides). International Journal of Earth Sciences (Geolgische Rundschau) 101, 1763–1788.

Asensio, I., Martín-Hernández, F., and Ormö, J., 2011. Evidence of ferrichromite of extraterrestrial origin by means of rock magnetic studies from the Loc-9 drill core (Lockne crater, Sweden). American Geophysical Union Fall Meeting, 5-9 December, San Francisco, USA.

Broman, C. Sturkell. E. & Fallick, A.E. 2011. Oxygen isotopes and implications for the cavitygrown quartz crystals in the Lockne impact structure, Sweden, GFF 133, 101-107.

Ebbestad, J.O.R  & Högström, A.E.S. 2007. Ordovician of the Siljan District, Sweden. In Ebbestad, J.O.R., Wickström, L.M. & Högström, A.E.S. (eds.) WOGOGOB 2007. 9th meeting of the Working Group on Ordovician Geology of Baltoscandia. Field guide and Abstracts. SGU Rapporter och meddelanden 128, 7-26.

Högström, A.E.S., Sturkell, E., Ebbestad, J.O.R., Lindström, M., and Ormö, J., 2010a. Concentric Impact Structures in the Palaeozoic of Sweden – the Lockne and Siljan craters. GFF 132, 65–70.

Högström, A.E.S., Ebbestad, J.O.R. & Frisk, Å.M. 2010b. Om ett dike i Siljansringen ur ett historiskt klimatperspektiv. Geologiskt Forum 68, 20–23.

Juhlin, C., Sturkell, E., Ebbestad, J.O.R., Lehnert, O., Högström, A. E. S. & Meinhold, G. 2012: A new interpretation of the sedimentary cover in the western Siljan Ring area, central Sweden, based on seismic data. Tectonophysics Onlinedoi>10.1016/j.tecto.2012.08.040

Kenkmann, T. & von Dalwigk, I., 2000: Radial transpression ridges: A new structural feature of complex impact craters. Meteoritics and Planetary Science 35, 1189–1202.

Lehnert, O., Meinhold, G. Bergström,  S.M., Calner, M. Ebbestad, J. O. R., Egenhoff, S. Frisk, Å.M., Högström, A.E.S. & Maletz. J., 2012a. The Siljan Meteorite Crater in central Sweden – an integral of the Swedish Deep Drilling Program (SDDP). ICDP (International Continental Scientific Drilling Program) DFG Schwerpunktkolloquium, March, 7 - 9. Abstracts: 102-103; Geomar (Kiel, Germany).

Lehnert, O., Meinhold, G. Bergström,  S.M., Calner, M. Ebbestad, J. O. R., Egenhoff, S. Frisk, Å.M., Högström, A.E.S. & Maletz. J,.  2012b. The Siljan Ring in central Sweden - a window into the Palaeozoic history of Baltoscandia. General Assembly 2012 of the European Geosciences Union (EGU), Vienna, Austria, 22 – 27 April 2012.

Lehnert, O., Meinhold, G. Bergström,  S.M., Calner, M. Ebbestad, J. O. R., Egenhoff, S. Frisk, Å.M., Högström, A.E.S. & Maletz. J.,  2012c. The Siljan Impact Structure - an important integral for reconstructing the early Palaeozoic history of Baltoscandia. GV and SEDIMENT Meeting September 23-28, 2012, Hamburg.

Lehnert, O., Meinhold, G., Bergström, S.M., Calner, M., Ebbestad, J.O.R., Egenhoff, S.,  Frisk, Å.M., Hannah, J.L., Högström, A. E. S., Huff, W.D., Juhlin, C., Maletz, J., Stein, H.J., Sturkell, E. & Vandenbroucke, T.R.A. 2012d: New Ordovician-Silurian drill cores from the Siljan impact structure in central Sweden an integral part of the Swedish Deep Drilling Program. GFF 134, 87–98.

Melero Asensio, I., Martín-Hernández, F., and Ormö, J., 2011a. Rock-Magnetic Properties of Drill Core LOC-9 from the Lockne Crater, Sweden. LPS XLII. Houston, USA. #1463

Melero Asensio, I., Martín-Hernández, F., and Ormö, J., 2011b. Rock magnetic properties of drill core LOC-9 from the Lockne crater, Sweden. European Geosciences Union General Assembly, 3-8 April, Vienna, Austria. #9131.

Melero Asensio, I., Ormö, J., and Sturkell, E., 2011c.Preliminary Geophysical Survey of the Målingen Structure, Sweden. LPS XLII. Houston, USA. #1542

Melero Asensio, I., Ormö, J., and Sturkell, E., 2011d. Preliminary geophysical survey of the Malingen structure, Sweden. European Geosciences Union General Assembly, 3-8 April, Vienna, Austria. #9148.

Melero Asensio, I., Ormö, J., and Sturkell, E., 2011e. Geophysical survey of the proposed Målingen marine-target crater, Sweden. Fragile Earth - Geological Processes from Global to Local Scales and Associated Hazards, 4-7 September, Munich, Germany.

Ormö, J., Hill, A., and Self-Trail, J.M. 2010a. A chemostratigraphic method to determine the end of impact related sedimentation at marine-target impact craters (Chesapeake Bay, Lockne, Tvären).  Meteoritics and Planetary Science 45, 1206–1224.

Ormö, J., Sturkell, E., and Lepinette, A., 2010b. Geological and numerical evidence for a crater bound sedimentary impact breccia lining the basement crater at the Lockne impact structure. LPS XLI. Houston, USA.

Ormö, J., Sturkell, E., Melero Asensio, I., and Frisk, Å., 2010c: Preliminary results from the 2009 and 2010 core drillings of the Målingen structure, a probable doublet to the Lockne marine-target impact crater, central Sweden. Geological Society of America Annual Meeting, Denver, USA.

Ormö, J., Lepinette, A., Sturkell, E., Lindström, M., Housen, K.R., and Holsapple, K.A. 2010d. Water resurge at marine-target impact craters analyzed with a combination of low-velocity impact experiments and numerical simulations” In: R.L., Gibson, and W.U., Reimold (Eds.) “Large Meteorite Impacts and Planetary Evolution IV”. Geological Society of America Special Paper 465, 81–101.

Ormö, J., King, D. T., Jr., Sturkell, E., and Lepinette, A., 2011a. Catastrophic water movements at marine impact events. Fragile Earth - Geological Processes from Global to Local Scales and Associated Hazards, 4-7 September, Munich, Germany.

Ormö, J., Sturkell, E., Melero Asensio, I., Frisk, Å., Lepinette, A., and Moro Martin, A., 2011b. The Målingen Structure: a Probable Doublet to the Lockne Marine-Target Impact Crater, Central Sweden. LPS XLII. Houston, USA. #1048.

Stein, H.J., Yang, G., Zimmerman, A., Hannah, J.L., Egenhoff, S., 2009a. Re‐Os fractionation on instantaneous maturation at the Siljan impact site, central Sweden: AAPG Annual Convention and Exhibition Abstracts Volume, v. 18, p. 203.

Stein, H., Yang, G., Hannah, J.L., Zimmerman, A., and Egenhoff, S., 2009b. Re‐Os fractionation on instantaneous maturation at the Siljan meteorite impact site, central Sweden: Geochimica et Cosmochimica Acta, v. 73, no. 13, p. A1268.

Stein, H., Zimmerman, A., Yang, G., Hannah, J., and Egenhoff, S. (2009) Hydrocarbon maturation and Os mixing on bolide impact at the Frasnian‐Famennian boundary: Geophysical Research Abstracts, v. 11, European Geosciences Union (EGU), EGU2009‐11085‐1 (electronic).

Zimmerman, A., Yang, G., Stein, H.J., Hannah, J.L., and Egenhoff, S. (2009) Out of this world crude oil – separating meteoritic and hydrocarbon Re-Os components:  Geochimica et Cosmochimica Acta, v. 73, no. 13, p. A1536.