Large Meteorite Impacts and Planetary Evolution 6002.pdf

THE MIDDLE ORDOVICIAN LOCKNE CRATER AS A SMALL SCALE MODEL FOR IMPACTS AT SEA. J. Ormo, M. Lindstrom, E. Sturkell and R. Tornberg. Dept. of Geology and Geochemistry, Stockholm university, S-10691 Stockholm, Sweden. [email protected].

The Lockne crater is situated in the county of Because water and wet sediments more easily form a Jamtland, central Sweden. It is one of 6 known melt than crystalline rocks, more energy is available craters from Baltoscandia and North America for the excavation. This results in craters with 20- formed at different water depths in the Middle 50% larger diameters than what would have been the Ordovician sea. The structure was soon covered by case with a single crystalline target (Kieffer & Caledonian overthrusting and has been protected Simonds 1980). Allthough sediments are more easily from erosion until the Cenozoic. The morphology melted, melt sheets are seldom found in craters with and lithology of the crater is therefore well sedimentary targets. The water and carbonates form preserved. volatiles that disperse the melt (Kieffer & Simonds The water depth at the time of impact was at least 1980). In the Lockne structure no melt sheet nor 200 m (Lindstrom et al. 1996). The target rocks suevite layer has been found though extensive search consisted of approximately 80 m of sedimentary both with drillings and with geophysics (Sturkell & rocks covering the crystalline basement. The water Ormo, in prep.). and the sediments served as a weak upper layer. The structure is phenomenologically a nested crater. The References: 7.5 km wide inner crater is developed in the Lindström, M., Sturkell, E.F.F., Törnberg, R. & Ormö, J. crystalline basement and surrounded by a shallow, 3 1996. The marine impact crater at Lockne, central km wide, outer crater developed in the sedimentary Sweden. GFF 118, 193-206. strata and the topmost part of the crystalline rocks Kieffer, S.U. & Simonds, C.H. 1980: The role of volatiles (Lindstrom et al. 1996). No clear rim has been and lithology in the impact cratering process. observed. This was most likely partly developed in Reviews of Geophysics and Space Physics 18, 143- the watermass and collapsed moments after the 181. impact. The resurging water was heavely loaded Sturkell, E. & Ormo, J.: Magnetometry of the marine, with debris and the resulting high density flows Ordovician Lockne impact structure, Jämtland, eroded deep gullies through the outer crater. The Sweden. Manuscript. resurge material filled most of the inner crater with a mixed breccia.

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