Source: http://folk.uio.no/ftsikala/mjolnir/
Timestamp: 2019-04-20 04:29:53+00:00

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Welcome to the Mjølnir impact crater Homepage at the Geophysics Research Group, Department of Geology, University of Oslo.
Tsikalas, F., 2005, Mjølnir Crater as a result of oblique impact: asymmetry evidence constrains impact direction and angle. In: Henkel, H. and Koeberl, C. (eds) Impact Tectonism (Impact Studies), Springer, Berlin-Heidelberg (in press, March 2005).
Dypvik, H., Mørk, A., Smelror, M., Sandbakken, P.T., Tsikalas, F., Vigran, J.O., Bremer, M., Nagy, J., Gabrielsen, R.H., Faleide, J.I., Bahiru, G.M., and Weiss, H.M., 2004, The Ragnarok Formation and Sindre Bed: impact and ejecta material from the Mjølnir Crater in the Barents Sea. Norwegian Journal of Geology, v. 84, p. 143-167.
Tsikalas, F. and Faleide, J.I., 2004, Near-field erosional features at the Mjølnir impact crater: the role of marine sedimentary target. In: Dypvik, H. et al. (eds) Cratering in Marine Environments and on Ice (Impact Studies), Springer, Berlin-Heidelberg, p. 39-55 (abstract).
Shuvalov, V., Dypvik, H., and Tsikalas, F., 2002, Numerical simulations of the Mjølnir marine impact crater, Journal of Geophysical Research, v. 107 (E7), p. 1/1-13 (doi: 10.1029/2001JE001698).
Tsikalas, F., Faleide, J.I., Eldholm, O., and Dypvik, H., 2002, Seismic correlation of the Mjølnir marine impact crater to shallow boreholes. In: Plado, J. & Pesonen, L.J. (eds.), Impact Studies (Impacts in Precambrian Shields), Springer, Berlin-Heidelberg, p. 307-321 (abstract).
Tsikalas, F., Gudlaugsson, S.T., Faleide, J.I., and Eldholm, O., 2002, Porosity anomaly at the Mjølnir marine impact crater, Barents Sea. Deep-Sea Research Part II (Topical Studies in Oceanography), v. 49, p. 1103-1120 (full-text)(abstract).
Smelror, M., Kelly, S.R.A., Dypvik, H., Mørk, A., Nagy, J., and Tsikalas, F., 2001, Mjølnir (Barents Sea) meteorite impact ejecta offers a Volgian-Ryazanian boundary marker: Newsletters on Stratigraphy, v. 38, p. 129-140.
Tsikalas, F., Gudlaugsson, S.T., Faleide, J.I., Eldholm, O., 1999, The Mjølnir Structure, Barents Sea: a marine impact crater laboratory: In Dressler, B.O. and Sharpton, V.L. (eds), Large Meteorite Impacts and Planetary Evolution II, Geological Society of America Special paper 339, p. 193-204 (abstract).
Tsikalas, F., Gudlaugsson, S.T., Faleide, J.I., 1998, The anatomy of a buried complex impact structure: the Mjølnir Structure, Barents Sea: Journal of Geophysical Research, v. 103, p. 30,469- 30,484 (abstract).
Tsikalas, F., Gudlaugsson, S.T., Eldholm, O., and Faleide, J.I., 1998, Integrated geophysical analysis supporting the impact origin of the Mjølnir Structure, Barents Sea: Tectonophysics, v. 289, p. 257-280 (full-text)(abstract).
Tsikalas, F., Gudlaugsson, S.T., and Faleide, J.I., 1998, Collapse, infilling, and post-impact deformation at the Mjølnir impact structure, Barents Sea: Geological Society of America Bulletin, v. 110, p. 537-552 (abstract).
Dypvik, H., Gudlaugsson, S.T., Tsikalas, F., Attrep, M. Jr., Ferrell, R.E. Jr., Krinsley, D.H., Mørk, A., Faleide, J.I., and Nagy, J., 1996, Mjølnir Structure: An impact crater in the Barents Sea: Geology, v. 24, p. 779-882 (abstract).
Gudlaugsson, S.T., 1993, Large impact crater in the Barents Sea: Geology, v. 21, p. 291-294.
Tsikalas, F. and Faleide, J.I., 2005, Post-impact deformation of impact craters: towards a better understanding through the study of Mjølnir crater: Proceedings 36th Lunar & Planetary Science Conference (LPSC XXXVI), abstract No. 1022 (CD-ROM), 2 pp.
Dypvik, H., Mørk, A., Smelror, M., Tsikalas, F., Sandbakken, P.T., Bremer, M., Nagy, J. and Faleide, J.I., 2004, The Mjølnir impact crater: Norwegian Geological Society Abstracts & Proceedings, No 2, p. 37-40.
Tsikalas, F., Dypvik, H., Faleide, J.I., and Smelror, M., 2004, Towards a better understanding of marine impact processes: what a potential drilling of the Mjølnir Crater (Barents Sea) has to offer: Proceedings 35th Lunar & Planetary Science Conference (LPSC XXXV), abstract No. 1570 (CD-ROM), 2 pp.
Tsikalas, F. and Faleide, J.I., 2003, Mjølnir marine crater resulting from oblique impact: compelling evidence: Proceedings 3rd International Conference on Large Meteorite Impacts, Lunar & Planetary Institute, abstract No. 4005 (CD-ROM), 2 pp.
Tsikalas, F. and Faleide, J.I., 2003, Oblique Mjølnir marine impact: structural and geophysical diagnostic constraints: Proceedings 34rd Lunar & Planetary Science Conference (LPSC XXXIV), abstract No. 1015 (CD-ROM), 2 pp.
Tsikalas, F. and Faleide, J.I., 2002, Near-field erosional features at the Mjølnir impact crater: the role of marine sedimentary target: Proceedings 33rd Lunar & Planetary Science Conference (LPSC XXXIII), abstract No. 1296 (CD-ROM), 2 pp.
Shuvalov, V., Dypvik, H., and Tsikalas, F., 2002, Numerical modeling of the Mjølnir marine impact event: Proceedings 33rd Lunar & Planetary Science Conference (LPSC XXXIII), abstract No. 1038 (CD-ROM), 2 pp.
Tsikalas, F., Faleide, J.I., Gudlaugsson, S.T., and Eldholm, O., 2001, From Mjølnir "Structure" to Mjølnir "marine impact crater": review of a decade of geophysical research: In Smelror, M., Dypvik, H. and Tsikalas, F. (eds) "Submarine Craters & Ejecta-Crater Correlation [7th Workshop, European Science Foundation (ESF)-IMPACT Programme], Norwegian Geological Society Abstracts & Proceedings, No 1, p. 84-86.
Tsikalas, F., Faleide, J.I., Eldholm, O., and Dypvik, H., 2001, Seismic correlation of the Mjølnir marine impact crater to shallow boreholes: In Smelror, M., Dypvik, H. and Tsikalas, F. (eds) "Submarine Craters & Ejecta-Crater Correlation [7th Workshop, European Science Foundation (ESF)-IMPACT Programme], Norwegian Geological Society Abstracts & Proceedings, No 1, p. 82-83.
Shuvalov, V., Dypvik, H., and Tsikalas, F., 2001, Numerical simulations of the Mjølnir marine impact: In Smelror, M., Dypvik, H. and Tsikalas, F. (eds) "Submarine Craters & Ejecta-Crater Correlation [7th Workshop, European Science Foundation (ESF)-IMPACT Programme], Norwegian Geological Society Abstracts & Proceedings, No 1, p. 73-75.
Tsikalas, F., Faleide, J.I., Eldholm, O., and Gudlaugsson, S.T., 1999, Porosity variation, seismic-amplitude anomalies and hydrocarbon potential of the Mjølnir impact crater: In Gersonde, R. and Deutsch, A. (eds) "Oceanic Impacts - Mechanisms and Environmental Perturbations [2nd Workshop, European Science Foundation (ESF)-IMPACT Programme], Reports on Polar Research (Berichte zur Polarforschung), v. 343 (1999), p. 97-100.
Tsikalas, F., Gudlaugsson, S.T., and Faleide, J.I., 1997, Seismic analysis of the Mjølnir impact structure, Barents Sea. In "Conference on Large Meteorite Impacts and Planetary Evolution (Sudbury, Canada, 1997)", p. 59, LPI Contribution No. 922, Lunar and Planetary Institute, Houston.
Gudlaugsson, S.T., Tsikalas, F., Eldholm, O., and Faleide, J.I., 1997, Geophysical modelling of the Mjølnir impact structure, Barents Sea. In "Conference on Large Meteorite Impacts and Planetary Evolution (Sudbury, Canada, 1997)", p. 19, LPI Contribution No. 922, Lunar and Planetary Institute, Houston.
Dypvik, H., Smelror, M., Mørk, A., Gabrielsen, R.H., Sandbakken, P.T., Nagy, J., Bremer, M., Tsikalas, F., and Faleide, J.I., 2003, Mjølnirkrateret I Barentshavet en hilsen fra verdensrommet, Gråsteinen (Geological Survey of Norway), v. 8, 68 pp.
Tsikalas, F., Dypvik, H., Faleide, J.I., Gudlaugsson, S.T., and Eldholm, O., 2001, Mjølnirkrateret, Høydepunkter Universitetets naturhistoriske museer og botanisk hage, Universitetet i Oslo, p. 31-33 (in norwegian).
Tsikalas, F. and Dypvik, H., 2001, Nedslagskratere, Høydepunkter Universitetets naturhistoriske museer og botanisk hage, Universitetet i Oslo, p. 22-24 (in norwegian).
Tsikalas, F., Dypvik, H., Faleide, J.I., Gudlaugsson, S.T., and Eldholm, O., 2000, Mjølnir: an impact crater offshore Northern Norway: Highlights Natural History Museums and Botanical Garden, University of Oslo, p. 31-33.
Tsikalas, F. and Dypvik, H., 2000, Impact craters: Highlights Natural History Museums and Botanical Garden, University of Oslo, p. 22-24.
Dypvik, H., Faleide, J.I., Mørk, A., and Smelror, M., 1999, Endeling har Mjølnirkrateret blitt boret: Geo, 1-99, p. 18-19. (in norwegian).
Tsikalas, F., 1997, A geophysical study of Mjølnir: a proposed impact structure in the Barents Sea: Dr. Scient. (Ph.D.) dissertation, Department of Geology, University of Oslo, 102 pp.
Dypvik, H., Tsikalas, F., Faleide, J.I. and Smelror, M., 2003, Drilling the Mjølnir Crater in the Barents Sea: Marine Impact Processes Environmental Consequences: Revised preliminary drilling proposal for the Integrated Ocean Drilling Program (IODP), 18 pp (Proposal 628-Pre2).
Tsikalas, F., 2003, Seismic records of impact structures: Inst. Geol., Univ. Oslo, Report #75, 85 pp.
Dypvik, H., Tsikalas, F., Faleide, J.I. and Smelror, M., 2003, Marine Impact Processes: Drilling the Mjølnir Crater in the Barents Sea: Preliminary drilling proposal for the Integrated Ocean Drilling Program (IODP), 18 pp (Proposal 628-Pre).
Dypvik, H., Tsikalas, F., Faleide, J.I. and Smelror, M., 2003, Marine Impact Processes: Drilling the Mjølnir Crater in the Barents Sea: Preliminary drilling proposal for the International Continental Scientific Drilling Program (ICDP), 10 pp.
Tsikalas, F., 1997, The new role of impact cratering in earth sciences: environmental and economical aspects: Inst. Geol., Univ. Oslo, Report #71, 28 pp.
Gudlaugsson, S.T., Tsikalas, F., and the Mjølnir Research Group, 1995, Mjølnir - a probable impact structure in the Barents Sea: scientific potential, exploration significance and the need for drilling: Submitted to Norwegian Petroleum Directorate, IKU Petroleum Research, and eighteen oil companies, 19 pp., 39 illustrations.
Tsikalas, F. and Faleide, J.I., 2005, Post-impact deformation of impact craters: towards a better understanding through the study of Mjølnir crater: 36th Lunar & Planetary Science Conference (LPSC), March 14-18, 2005, Houston, USA.
Dypvik, H., Mørk, A., Smelror, M., Tsikalas, F., Sandbakken, P.T., Bremer, M., Nagy, J. and Faleide, J.I., 2004, The Mjølnir Impact Crater: "Arctic Geology, Hydrocarbon Resources and Environmental Challenges" Conference, May 24-26, 2004, Tromsø, Norway.
Tsikalas, F., Dypvik, H., Faleide, J.I., and Smelror, M., 2004, Towards a better understanding of marine impact processes: what a potential drilling of the Mjølnir Crater (Barents Sea) has to offer: 35th Lunar & Planetary Science Conference (LPSC), March 15-19, 2004, Houston, USA.
Tsikalas, F. and Faleide, J.I., 2003, Mjølnir marine crater resulting from oblique impact: compelling evidence: 3rd International Conference on Large Meteorite Impacts (organized by Lunar & Planetary Institute), Aug. 5-7, 2003, Nördlingen, Germany.
Tsikalas, F., Faleide, J.I ., and Eldholm, O., 2003, Mjølnir marine impact crater: summary of geophysical research: Norwegian Geological Society 18th Congress, January 6-8, 2003, Oslo, Norway.
Faleide, J.I., Tsikalas, F., and Eldholm, O., 2002, Mjølnir marine impact crater: summary of geophysical research: American Geophysical Union (AGU) Fall Meeting, December 6-10, 2002, San Francisco, California, USA.
Tsikalas, F. and Faleide, J.I., 2002, Resurge erosional features at the Mjølnir marine impact crater: European Science Foundation (ESF)-IMPACT Programme, 8th Workshop, May 31-June 3, 2002, Mora, Sweden.
Tsikalas, F. and Faleide, J.I., 2002, Near-field erosional features at the Mjølnir impact crater: the role of marine sedimentary target: 33rd Lunar & Planetary Science Conference (LPSC), March 11-15, 2002, Houston, USA.
Shuvalov, V., Dypvik, H., and Tsikalas, F., 2002, Numerical modeling of the Mjølnir marine impact event: 33rd Lunar & Planetary Science Conference (LPSC), March 11-15, 2002, Houston, USA.
Tsikalas, F., Faleide, J.I., Eldholm, O., and Dypvik, H., 2002, Seismic correlation of the Mjølnir marine impact crater to shallow boreholes: 25th Nordic Geological Winter Meeting, January 6-9, 2002, Reykjavik, Iceland.
Tsikalas, F., Faleide, J.I., Gudlaugsson, S.T., and Eldholm, O., 2001, From Mjølnir "Structure" to Mjølnir "marine impact crater": review of a decade of geophysical research: European Science Foundation (ESF) - IMPACT Programme, 7th Workshop, Submarine Craters & Ejecta-Crater Correlation, Aug. 29-Sept 3, 2001, Svalbard, Norway.
Tsikalas, F., Faleide, J.I., Eldholm, O., and Dypvik, H., 2001, Seismic correlation of the Mjølnir marine impact crater to shallow boreholes: European Science Foundation (ESF) - IMPACT Programme, 7th Workshop, Submarine Craters & Ejecta-Crater Correlation, Aug. 29-Sept 3, 2001, Svalbard, Norway.
Shuvalov, V., Dypvik, H., and Tsikalas, F., 2001, Numerical simulations of the Mjølnir marine impact: European Science Foundation (ESF) - IMPACT Programme, 7th Workshop, Submarine Craters & Ejecta-Crater Correlation, Aug. 29-Sept 3, 2001, Svalbard, Norway.
Tsikalas, F., Faleide, J.I., Gudlaugsson, S.T., Eldholm, O., and Dypvik, H., 2000, The Late Jurassic Mjølnir marine impact crater: 31st International Geological Congress (IGC), August 6-17, 2000, Rio de Janeiro, Brazil.
Tsikalas, F., Faleide, J.I., Eldholm, O., and Gudlaugsson, S.T., 1999, Porosity variation, seismic-amplitude anomalies and hydrocarbon potential of the Mjølnir impact crater: Oceanic Impacts - Mechanisms and Environmental Perturbations, European Science Foundation (ESF) IMPACT 2nd Workshop, April 15-17, 1999, Bremerhaven, Germany.
Tsikalas, F., 1999, Mjølnir Structure, Barents Sea: a unique impact crater: Norwegian Geological Society Bergen, March 25, 1999, Bergen, Norway.
Tsikalas, F., Faleide, J.I., Gudlaugsson, S.T., and Solberg, C.E., 1994: Geophysical signature of the proposed Mjølnir impact structure, Barents Sea: Impact Cratering and Evolution of Planet Earth, 2nd International Workshop, The Identification and Characterization of Impacts, May 31 - June 5, 1994, Lockne, Sweden.
Gudlaugsson, S.T., Tsikalas, F., Faleide, J.I., and Dypvik, H., 1994: Impact origin of the Mjølnir impact structure, Barents Sea: Impact Cratering and Evolution of Planet Earth, 2nd International Workshop, The Identification and Characterization of Impacts, May 31 - June 5, 1994, Lockne, Sweden.
Tsikalas, F., Faleide, J.I., Gudlaugsson, S.T., & Eldholm, O.: "From Mjølnir 'Structure' to Mjølnir 'marine impact crater': review of a decade of geophysical research".
Tsikalas, F., Faleide, J.I., Eldholm, O., & Dypvik, H.: "Seismic correlation of the Mjølnir marine impact crater to shallow boreholes".
Tsikalas, F., Faleide, J.I., Eldholm, O., & Gudlaugsson, S.T.: "Porosity variation, seismic-amplitude anomalies and hydrocarbon potential of the Mjølnir impact crater".
Tsikalas F., Faleide J.I., Gudlaugsson S.T., & Eldholm, O.: "Mjølnir Structure, Barents Sea: a marine impact crater laboratory".
Gudlaugsson S.T., Tsikalas F., Eldholm O. & Faleide J.I.: "Geophysical modelling of the Mjølnir impact structure, Barents Sea".
Tsikalas F., Gudlaugsson S.T. & Faleide J.I.: "Seismic analysis of the Mjølnir impact structure, Barents Sea".
Tsikalas F., Faleide J.I., Gudlaugsson S.T. & Solberg C.E.: "Geophysical signature of the proposed Mjølnir impact structure, Barents Sea."
The 40-km-diameter Mjølnir Structure is located on the Bjarmeland Platform in the southwestern Barents Sea (Fig. 1). In 1993, Mjølnir was first interpreted as an impact structure by S.T. Gudlaugsson from its geophysical signature and overall geological setting. The impact hypothesis motivated the acquisition of high-resolution geophysical data by the Norwegian Defence Research Establishment that together with stratigraphic and sedimentological information from a nearby drillhole obtained from IKU Petroleum Research (Fig. 1) comprise an extensive and unique database. The great scientific potential offered by this database led to the initiation of a research project, "Mjølnir - a large impact crater in the Barents Sea", at the Department of Geology, University of Oslo. A central part of this project was the doctorate study - Tsikalas F., 1996, A geophysical study of Mjølnir: a proposed impact structure in the Barents Sea - that further refined and strengthened the impact interpretation for the structure, contributing to the confirmation of the Mjølnir Structure as an impact crater.
Figure 1. The 40-km-diameter Mjølnir Structure relative to the main Upper Jurassic-Lower Cretaceous structural elements in the SW Barents Sea.
Impact cratering has traditionally not been included among the common geological processes on Earth. Planetary exploration and extensive lunar research in the 1970s, however, demonstrated clearly the ubiquitous presence and the unremitting occurrence of impact phenomena in the solar system, establishing impact cratering as one of the most important processes that affects planetary surfaces. On the other hand, the significance of the bombardment of impactors for the geological evolution of the Earth is not easily recognized, as the structures and deposits formed during impacts are exposed to surface weathering processes that conceal, degrade and even erase them completely.
A turning-point in terrestrial impact studies occurred in the early 1980s, following the discovery of impact evidence at the Cretaceous-Tertiary (K-T) boundary, 65 Ma ago, and the suggestions that impact caused the mass extinction at the K-T boundary, extinguishing the dinosaurs among others. The impact hypothesis was initially based on the discovery at the boundary of a high concentration of the element iridium (Ir) that is almost absent from Earth compositions but abundant in extra-terrestrial bodies. Further research added additional impact indicators at the same boundary, the most important of which is shocked quartz grains exhibiting deformation features obtained under pressures higher than common geological processes can reach.
After more than one decade of intensive research, the "smoking-gun" of that enormous impact event was recently identified as the Chicxulub impact structure in Mexico, having a diameter on the order of 200-300 km and believed to have caused by a 10 km asteroid body. Impact cratering is gradually becoming more accepted, increasing the awareness in the earth science community of the potential importance of impact in geological history. Moreover, the spectacular impact of the comet P/Shoemaker-Levi 9 on Jupiter in 1994 provided a unique opportunity to study an actual impact event as it happened.
Approximately 150 terrestrial impact structures are currently known. The spatial distribution of the known craters is not random. Almost all are on land, whereas only a few underwater craters are known due to the difficulty in identifying buried and in-situ submarine craters. Nevertheless, the significance of marine versus land impacts is mainly ascribed to the preservation of marine craters by post-impact sediments. Marine geophysics is a major tool during the initial recognition and study of water-covered terrestrial impact craters. Nonetheless, geophysical studies alone do not confirm an impact origin, and the need for geological, mineralogical and/or geochemical evidence is indispensable.
Based on seismic reflection profiles, the Mjølnir Structure is defined as a large, 850-1400 km3, volume of disturbed seismic reflectivity patterns that has the form of a deep, parabolic bowl at the centre and turns into a shallow broad brim at the periphery of the structure (Fig. 2). The disturbance is characterized by a systematic loss of reflection coherency from its deeper and outer parts to the upper and central parts caused by a progression of seismic facies from disrupted layering and diffractions to chaotic and reflection-free zones.
Figure 2. Schematic cross section showing the Mjølnir physical impact and the types of deformation associated with the structure. Reflectors UB and LB bound the time of impact. URU, Late Cenozoic upper regional unconformity; UB, lower Barremian; TD, the first continuous reflector above the seismic disturbance; LB, base Upper Jurassic.
Siliciclastic deposits dominate the post-Permian sedimentary succession of the southwestern Barents Sea. The seismic profiles indicate that the deformation, and consequently the Mjølnir impact, took place near the Jurassic/Cretaceous transition. The impact occurrence in a water-covered target gives rise to a dynamic water cavity (Fig. 2). Collapse of this cavity leads to processes that could account for several structural differences between land and marine impacts, and is also associated with distinct environmental effects. During impact, the passage of the shock-wave results in extensive in-situ fracturing and brecciation. Target material is excavated and ejected in ballistic trajectories upwards and outwards from the impact site. As excavation of the brecciated volume advances, a geometrical feature, the excavated crater, is formed (Fig. 2). It delimits the provenance of material expelled from the crater and provides the void space for subsequent infilling. As target rock material is displaced laterally and downward, the result is a parabolic-shaped cavity, referred to as the transient cavity, that is about three times deeper than the excavated crater (Fig. 2). However, the transient cavity is a short-lived feature as collapse of the crater walls under the force of gravity results in an increased crater diameter and the development of fault-blocks at the periphery, an elevated crater floor, and a centrally located high (Fig. 2).
The seismic correlation from the Mjølnir Structure to the nearby drillhole, located 30 km from the periphery of the structure (Fig. 1), indicated that the top of the seismic disturbance at Mjølnir intersects the drillhole at a stratigraphic level estimated as of Volgian-Berriasian age (141-149 Ma). The correlatable interval revealed unequivocal indicators of meteoritic impact, i.e. shocked quartz grains and a strong enrichment in iridium. In particular, the shocked quartz grains showed presence of planar fractures and recrystallized planar deformation features (Fig. 3), both common diagnostics for the high shock pressures caused by impact. An iridium peak of about 15 times the background value was found approximately at the same level, undoubtedly indicating presence of extraterrestrial material.
Figure 3. Cathodoluminescence-scanning electron microscopy of a shocked quartz grain (from Dypvik et al., 1996).
The top of the seismic deformation at the Mjølnir Structure is overlaid by a regionally prominent limestone bed of lower Barremian age (Fig. 4). Seismic mapping of this bed clearly shows that the major structural features of Mjølnir are typical of large complex impact structures. Excellent seismic images bring out a distinct radial zonation pattern (Fig. 5), comprising a 12 km wide complex outer zone, including a marginal fault zone and a modest peak ring, a 4 km wide annular depression, and an uplifted central high, 8 km in diameter. In addition, key features include the sharp rim faults with a cumulative throw of ~150 m that separate highly deformed strata within the crater from intact platform strata, and a 45-180 m thick seismic unit, formed during impact, characterized by disturbed and incoherent reflectivity patterns and confined by prominent fault-blocks and the post-impact strata (Fig. 4).
Figure 4. High-resolution single-channel seismic profiles, and interpretations, crossing the entire structure through the centre. M, marginal fault zone; P, peak ring.
Figure 5. Illuminated perspective image of residual two-way traveltime to reflector UB, lower Barremian. Two-way traveltime readings along 2100 km of seismic reflection profiles in Mjølnir area were cross-over corrected, a second-order polynomial surface was removed, and a spline surface was fitted to the residual values. The view is directly from above; light sources at azimuths 30o, 290o, and 340o. The grey area on top of the central high shows where reflector UB is truncated by erosion. Vertical exaggeration ~20x.
Following the impact, progressive sediment accumulation took place over the impact structure. The sedimentation gradually filled in the crater relief, resulting in considerable thickening and lateral facies changes of the early post-impact deposited strata influenced by the underlying structure. Continued deposition, subsequently created a substantial, ~2-2.5 km thick overburden that was greatly eroded during the Plio-Pleistocene Northern Hemisphere Glaciation. By removing the effects of the post-impact burial and the associated deformation from the present crater morphology (Fig. 4), we are able to reconstruct the original crater relief immediately after impact. Reconstruction brings out an initially subtle structure with only ~30-40 m average residual depth.
It has been qualitatively shown that impact-generated structural and physical property anomalies, in combination with the prograding sedimentary load resulted in radially varying differential subsidence and compaction faulting within the structure that took place in successive phases. The periphery of the structure and the surrounding platform compacted considerably more than the central crater which was underlain by a denser, less porous core. The central high developed as a prominent feature during the extensive post-impact burial, and the peak ring and the cumulative throw on the rim faults were further enhanced. Thus, the present very distinct expression of Mjølnir (Fig. 5) is largely a post-impact burial phenomenon.
In order to access the possible consequences of the Mjølnir impact, it is important to constrain the magnitude of the impact event (impactor size and mass, energy release) as accurately as possible. Based on various scaling-laws, revised estimates for these parameters indicate that the impactor was 1-3 km in diameter, had a mass of 1 billion tons, and the released energy during impact was 400 000 megatons TNT. The waiting time between impacts of Mjølnir's magnitude, approximately 0.7-1 Ma, is much shorter that the average length of a stage in the geological time scale (typically 5 Ma). This clearly shows that Mjølnir-sized impact events cannot be associated with significant mass extinctions. However, dissipation of the energy released during the Mjølnir impact is sufficient to have caused several short, near-field perturbations; a large magnitude earthquake, displacement of a considerable amount of material from the impact site, and high-amplitude tsunami waves.
The estimated energy release during the Mjølnir impact translates into a potential 8.3-magnitude earthquake with a magnitude range of 7.7-8.7, being sufficient to trigger slumping on nearby sedimentary slopes. The estimated volume of the displaced material during an impact of Mjølnir's magnitude is in the order of 140-180 km3. It was deposited in the vicinity of the crater and was probably associated with a major disturbance of the water column. The impact of the Mjølnir projectile into a shallow shelf environment with water depths of 300-500 m and the subsequent collapse of the impact-generated water cavity is expected to have given rise to large-amplitude tsunami waves. A paleogeographic reconstruction for Early Cretaceous time indicates that the shores of northeastern Greenland, northern Fennoscandia, and Novaya Zemlya may have experienced coastal erosion as they fall within the 400 km radius from the impact site and were affected by tsunami waves with amplitudes greater than 5-10 m.
The seismic reflection method is potentially the most powerful of the geophysical methods in providing an effective means of mapping the large-scale geometrical structure of an impact crater at depth with a high degree of horizontal and vertical resolution. It has been extensively applied to impact structures within crystalline targets. However, the innovative contribution of the Mjølnir Structure is that it demonstrated the effectivity of the method in sedimentary targets. The regular, pre-impact stratification of these targets provides a series of reference horizons against which the impact-induced structures can be identified and mapped in great detail.
With the possible exception of the Chesapeake Bay impact crater in USA, the phenomenon of post-impact deformation and structural modification induced by sedimentary loading, particularly in water-covered sedimentary targets, appears not to have been studied or quantified much at the impact sites. In the case of Mjølnir, we demonstrated that the extensive post-impact overburden combined with impact-induced variations in physical properties at depth is the major controlling factor of the present distinct crater relief.
Of the 24 known craters ³30 km in diameter, excluding the Mjølnir Structure, only eight have preserved proximal ejecta deposits and several are eroded beneath the crater floor. Well-preserved crater/ejecta pairs are therefore extremely rare in the terrestrial impact cratering record. As a result, many aspects of the impact process remain poorly constrained. The Mjølnir Structure is within the twenty largest impact structures discovered on Earth, ranking eighth among those presently not exposed at the surface. The Mjølnir impact occurrence in a marine environment and the subsequent deposition of post-impact sediments over the crater relief contributed to the preservation of the structure and associated deposits ejected from the impact site during the event. Both are among the best-preserved source-crater/ejecta-layer pairs in the terrestrial impact record. The structure also provides one of the best records of marine impacts available, establishing Mjølnir as a milestone in norwegian and international impact cratering geology.
The general exploration significance of an impact crater lies in the fact that an undisputed effect of a meteor impact into brittle target rocks is the formation of a large volume of fractured rocks with potentially high porosity and permeability that can be conductive to hydrocarbon accumulations. Considering that the total volume of the impact-deformed platform strata at Mjølnir is on the order of 850-1400 km3 it seems worthwhile to investigate the hydrocarbon potential of the structure further. However, impacts into marine sedimentary basins are less well understood and it is yet uncertain how much fracture volume will be generated or whether this volume can be maintained against porosity-reducing processes such as compaction and diagenesis.
Thorough examination of the high-resolution single-channel seismic profiles reveals a range of intra-sedimentary features considered as classical gas indicators, including acoustic blanketing, enhanced reflectors and smearing, and restricted columnar disturbances. All these features are indicative of possible shallow or deep hydrocarbon accumulation within the Mjølnir Structure. However, a probable negative factor is that the major faults are not efficiently sealed and several reach shallow depths, truncated by the Late Cenozoic glacially erosional unconformity (Fig. 4). Further evaluation of the hydrocarbon indicators by reprocessing of the digital single-channel seismic data will better quantify the interrelation and spatial distribution of gas and seismic-amplitude anomalies.
A prospective drilling of the structure will unambiguously prove the impact origin and will further clarify the many questions related to petroleum potential. Drilling should be preceded by a local 3D seismic survey in the Mjølnir area. The well preserved structure offers a great potential for a well confined, high-quality case-study. Furthermore, a substantial search should be initiated to locate the ejecta layer and associated impact-wave deposits, as well as other regional short-term perturbations in the appropriate sedimentary successions from outcrops and cores in locations surrounding the Barents Sea at the time of impact.
En gang i tidsrommet fra 149 til 141 millioner år siden, nær overgangen mellom jura og kritt periodene, passerte et himmellegeme gjennom jordens atmosfære og slo ned i det grunne Barentshavet. Den 1 milliard tonn store meteoritten med diameter på 1-3 km, fordampet store vannmasser og kastet havbunnsedimenter opp i luften. Dermed ble det dannet et karakteristisk sår i den myke havbunnen, et krater som senere ble dekket av yngre sedimenter. Globalt regnes det 40 km diameter store Mjølnirkrateret med omliggende nedslagsavset-ninger blant de best bevarte spor etter nedslag i marine områder.
I lærebøkene betraktes ikke meteorittnedslag som en viktig geologisk prosess på jorden, men nyere forskning viser at de dominerer på planetenes overflater. Slike strukturer er vanskelige å påvise på jorden fordi de raskt, helt eller delvis, ødelegges av forvitring og erosjon. Imidlertid førte ideen om at et stort nedslag ved kritt-tertiær grensen hadde forårsaket masseutryddelser, blant annet av dinosaurene, til fornyet interesse for terrestriske nedslagshendelser tidlig på 1980-tallet. Denne hypotesen ble styrket ved at det ble funnet tynne, antatt samtidige, lag med høye konsentrasjoner av iridium (Ir). Dette elementet opptrer i liten grad i terrestriske bergarter, men er typisk for ekstra-terrestriske legemer. Det ble også påvist sjokkdeformerte kvartskorn som hadde vært utsatt for et større trykk og temperatur enn ved normale terrestriske prosesser, altså en annen nedslagsindikator. Den påfølgende oppdagelsen av Chicxulub krateret i Mexico ble tatt som bevis på et nedslag for 65 millioner år siden. Det 200-300 km diameter store krateret er antatt å skyldes en asteroide med diameter på 10 km.
Etterhvert har meteorittnedslag blitt mer akseptert som en viktig geologisk prosess, også på jorden. I verdensrommet ble prosessen nylig illustrert ved det praktfulle nedslaget av kometen P/Shoemaker-Levi 9 på Jupiter. Vi kjenner i dag omlag 150 terrestriske nedslagskratere. Et lokalt eksempel er Gardnoskrateret i Hallingdalen omtalt av J. Dons og J. Naterstad i Geonytt 3/92. Det er verd å merke at nesten alle terrestriske kratere er funnet på land, mens det er få sikre observasjoner fra de marine områder som utgjør ca. 2/3 deler av jordens overflate. Derfor var funnet av et meget godt bevart krater på Bjarmelandsplattformen i Barentshavet en svært velkommen nyhet (Fig. 1). Vi understreker at sikker påvisning av marine kratere krever geofysiske undersøkelser med høy dataoppløsning, supplert med geologiske, mineralogiske og geokjemiske studier av samtidige sedimentære lag.
Figur 1. Beliggenhet av Mjølnir med hovedstrukturer av øvre jura-nedre kritt alder.
Et meteorittnedslag i et grunnhav fører til at vannlaget lokalt fordamper og at det genereres en sjokkbølge som leder til omfattende oppsprekkning i den underliggende sedimentære berggrunn. Samtidig vil en del av de deformerte sedimentene kastes ut av selve nedslagsområdet. Figur 2 viser at denne prosessen danner et "utgravd krater" som senere fylles igjen. Hele området som påvirkes av nedslaget utgjør et parabolformet, "transient hulrom". Dette området er imidlertid ustabilt. Umiddelbart etter nedslaget kollapser kraterveggene og det dannes forkastninger i områdets perifere deler. Samtidig heves kratergulvet og det dannes en sentral høyde. Dermed øker størrelsen på området som direkte påvirkes av selve nedslaget. Disse primære strukturene har imidlertid også gjennomgått endringer knyttet til sedimentær overlagring i tiden etter nedslaget. Derfor avbilder strukturen slik vi ser den i dag to ulike deformasjonsprosesser.
Det var oppdagelsen av slike typiske sirkulære nedslagsstrukturer i seismiske refleksjonsprofiler fra Barentshavet som vakte mistanken om en kraterstruktur. På Bjarmelandsplattformen kan vi avgrense et markert, lokalt område med forstyrret seismisk mønster i en ellers uforstyrret post-permisk lagrekke dominert av sandsteiner og skifre (Fig. 2).
Figur 2. Skisse av nedslaget og kraterutviklingen. Reflektorene UB og LB avgrenser tiden for nedslaget. URU, sen-kenozoisk regional inkonformitetet; UB, nedre barrem; TD, den første kontinuerlige reflektor over den seismiske forstyrrelsen; LB, bunn øvre jura; TP, topp perm; RF, randforkastning.
Den skålformede strukturen har et volum på 850-1400 km3 og består av en dyp sentral del omgitt av et grunnere perifert område, en såkalt "omvendt sombrero" (Fig. 2). Det forstyrrete området er dekket av et regionalt kalksteinslag av nedre barrem alder. Detaljkartlegging av dette laget viser flere strukturelle og morfologiske trekk som er typiske for store, komplekse nedslagsstrukturer. Vi har påvist en klar radiær sonering (Fig. 3). Ytterst finnes en 12 km bred sone som omfatter et område med normalforkastninger (M) og en liten ringhøyde (P), dernest et 4 km bredt ringbasseng (A), og innerst en hevet sentralhøyde (S) med omlag 4 km radius (Fig. 4). Krateret er begrenset av markerte randforkastninger (RF) som skiller det deformerte området fra de intakte plattform lagene.
Figur 3. Relieff av nedre barrem reflektoren (UB). Det grå feltet på toppen av sentralhøyden viser hvor reflektoren er erodert. 20x vertikal forstørrelse.
Figur 4. En-kanals seismiske profiler med tolkninger. Profilene har høy oppløsning og går gjennom sentrum av strukturen. Reflektor navn er vist i figur 2 og de radielle strukturelementer er beskrevet i teksten.
(Dypvik et al., 1996). Samlet har de geofysiske og geologiske indikatorer ført til at strukturen nå er anerkjent som et terrestrisk nedslagskrater (Grieve et al., 1995).
Etter nedslaget ble først krateret fylt igjen og deretter dekket av stadig tykkere avsetninger. Vi antar at det var 2-2.5 km overlagring for 3-4 millioner år siden. Denne belastningen forårsaket en betydelig forsterkning av krater-relieffet. Storparten av de overliggende sedimentene ble imidlertid fjernet under de Plio-Pleistocene nedisninger. Dersom vi korrigerer for denne sekundære deformasjonen står vi tilbake med et opprinnelig krater som kun var 30-40 m dypt (Fig. 2).
Nedslaget førte også til laterale endringer i de sedimentære bergarters fysiske egenskaper (tetthet, porøsitet, seismisk hastighet). Vi kan vise at den opprinnelige kraterstrukturen og de endrete bergartsegenskaper i nedslagsområdet, sammen med de sekundære lasteffekter, førte til radiær differentiell innsynkning og forkastningsaktivitet i tiden etter nedslaget. Særligt ble strukturens ytre deler kompaktert mye mer enn det sentrale krateret med sin tettere og mindre porøse kjerne. Derfor er randforkastningene og den typiske sentralhøyden (Fig. 3 og 4) strukturer som er forsterket av belastningseffekter etter selve nedslaget.
Studier av terrestriske kratere viser at nedslag av Mjølnirs størrelse opptrer hvert 0.7-1.0 Ma. Dermed er det lite sannsynlig at slike nedslag kan ha forårsaket masseutryddelse av livsformer, selv om de nok kan ha hatt store konsekvenser på lokal og regional skala. For eksempel, vil energien som ble frigjort under Mjølnirs nedslag være tilstrekkelig til å forårsake hendelser som jordskjelv, tsunamibølger, og vidtrekkende spredning av sedimentfragmenter fra nedslagsområdet.
Vi har beregnet at energien som ble frigjort under nedslaget tilsvarer 400 000 megatonn TNT, og at nedslaget kan ha utløst et jordskjelv av størrelse ~8.3 på Richters skala. Et slikt skjelv vil kunne føre til massebevegelse selv på svakt hellende sedimentære flater. Vi har beregnet volumet av sedimentene som ble kastet ut av krateret til 140-180 km3. Størstedelen av sedimentene ble trolig re-avsatt i og nær krateret. Tsunamibølgene oppstår fordi nedslag i et 300-500 m dypt grunnhav produserer et hulrom i vannlaget som deretter kollapser (Fig. 2). I tidlig kritt tid befant Nordøst-Grønland, det nordlige Fennoskandia og Novaya Zemlya seg innenfor en radius på 400 km fra nedslaget, hvor 5-10 m store tsunamibølger vil føre til betydelig erosjon i kystområdene.
Mjølnirkrateret er blant de 20 største kjente terrestriske nedslagsstrukturer. Av kratere med diameter større enn 30 km er det, i tillegg til Mjølnir, kun påvist avsetninger med typiske iridiumanomalier og sjokkdeformerte kvartskorn fra 8 av 24 kjente strukturer. Mange kratere er også erodert til under kratergulvet. Med andre ord, godt bevarte kratere med bevart nedslagsmateriale er fremdeles en sjeldenhet. Av denne grunn er flere forhold i samband med selve nedslagsprosessen og den sekundære kraterdeformasjonen enda ikke tilfredstillende undersøkt og forstått. Mjølnirs nedslag i et grunnhav og den påfølgende avleiring av yngre sedimenter har gjort at både selve krateret og nedslagsavsetningene er blant de best bevarte fra terrestriske nedslag. Faktisk representerer Mjølnir en milepel når det gjelder kartlegging av marine kraterstrukturer. I tillegg gjør beliggenheten det enkelt å foreta nye geologiske og geofysiske undersøkelser. Derfor vil Mjølnir kunne bli et sentralt laboratorium for framtidige studier av marine nedslag.
Vi peker spesielt på at Mjølnir gjør det mulig å studere de forandringer som inntreffer på grunn av sedimentbelastning etter nedslaget. Med unntak av Chesapeake Bay krateret i USA, er dette prosesser som i liten grad er tatt hensyn til. Mjølnir dokumenterer både en betydelig sekundær deformasjon, og at denne har kontrollert det nåværende kraters karakteristiske relieff.
Den store oppsprekkning ved et meteorittnedslag vil normalt øke bergartenes porøsitet og permeabilitet, altså kan nedslaget danne et hydrokarbonreservoar. Volumet av den deformerte Mjølnirstrukturen gjør det interessant å vurdere dens hydrokarbon potensiale. For eksempel, viser våre seismiske profiler attributter som kan tolkes som gass indikatorer. Vi presiserer likevel at det er store usikkerheter knyttet til hvordan det oppsprukne nedslagsområdet reagerer på kompaksjon og diagenese, prosesser som normalt reduserer porøsiteten.
Våre undersøkelser på Bjarmelandsplattformen viser at den seismiske refleksjonsmetoden er et effektivt redskap til å kartlegge primære og sekundære nedslagseffekter i sedimentbassenger. Sammenliknet med nedslag i krystalline bergarter, vil et sedimentbasseng gi opphav til klare laterale og vertikale seismiske kontraster mellom krateret og de omliggende, uforstyrrete lagpakker, samtidig som det kan foretas detaljerte stratigrafiske korrelasjoner. Videre undersøkelser bør i hovedsak baseres på utvidet seismisk kartlegging, fortrinnsvis ved en lokal 3-D undersøkelse med god vertikal og horisontal oppløsning. Denne vil også legge grunnlaget for et dedikert borhull med kontinuerlig prøvetakning. I tillegg vil det være nyttig å sette i gang et "leteprogram" i kjerneprøver og blottninger fra de omliggende områder for å finne flere spor av nedslagsproduktene og mulige andre hendelser som skyldes nedslaget.

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