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.

Latest CISP news: An ICDP workshop proposal was submitted on the 15th January 2012.

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. 2 – 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). 

CISP 2012 and beyond
The main goal of CISP as described in the VR proposal of 2009 has been met to a great extent, since the seismic line in Siljan has been made. With the reduced funding (50%) from VR most of the other aims are only partially met, and will in the extension of CISP be addressed through other means.
However, through opportune financial support from other sources and the generosity of Igrene it has been possible to study the Lower Palaeozoic succession in Siljan and the relationship with the underlying Precambrian basement. During the next year the success of these work demanding studies depends on a successful continuation of funding. A shallow seismic line would be appropriate to investigate the geometries of the 3 major facies belts in the Ordovician succession of the Siljan ring structure to show the transition from the Ordovician platform into a Silurian shale basin to the SW discovered during the studies of the Mora 001 core by Lehnert. The geometry of this basin is of major interest for the palaeogeographical reconstructions and the evolution of this margin of Baltica during Ordovician times. Furthermore, a seismic line is needed in the Lockne area to be able to decide the placement of a deep core hole. Below is a list of tasks for 2012 (nonspecific ordering).

• Analyses of the seismic profiles and decision for placement of a deep core (Principal investigators Sturkell and Juhlin).
• Analysis of the sedimentological succession in the Igrene cores and the Nittsjö profile (Principal investigator Lehnert).
• Analyses of shallow seismic profiles in relation to the sedimentary succession in the Igrene cores (Principal investigator Juhlin and Lehnert).
• Re-Os analyses of key shale intervals from Igrene drill holes to compare with Re-Os data from outcrops and to model with Re-Os data from oils; link with biostratigraphy (Principal Investigators, Stein and Hannah, with Ebbestad and others)
• Study of the deeper Precambrian part of the Igrene cores (Principal investigator Sturkell and Ormö).
• Sampling for magnetic studies from the Igrene drill cores (c. 20 oriented samples) (Principal investigator Reihard Greiling).
• Drilling of two shallow cores in the Tandsby gully, Jämtland (Principal investigators Sturkell and Ormö).
• Geophyscial investigations in the Lockne structure (Principal investigators Sturkell, Ormö).
• Field investigations (structural data, magnetic susceptibility) and sampling for magnetic studies (c. 20 oriented samples) along mafic dykes from the unaffected basement towards the Lockne impact crater (Principal investigator Reihard Greiling).
  Sampling for magnetic studies from drill cores (c. 20 oriented samples) (Principal investigator Reihard Greiling).
• Planning for an ICDP work shop application January 15th (Principal investigators Sturkell, Högström and Ebbestad).
• Planning for a VR application in April on the continuation of CISP (Principal investigator Sturkell, Högström and Ebbestad).
• 2012 it is planned to print a popular science book (about 25 pages) about the research history for the Lockne area and the discovery of the impact structure. The book is written and illustrated by Maurits Lindström and will be distributed through the Lockne impact museum (http://www.locknekratern.se/).

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