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Post-impact deposits in Tvären, a marine Middle Ordovician crater south of Stockholm, Sweden

Published online by Cambridge University Press:  01 May 2009

M. Lindström ,
T. Flodén ,
Y. Grahn  and
B. Kathol
M. Lindström
Affiliation:
Department of Geology and Geochemistry, University of Stockholm, S-106 91 Stockholm, Sweden
T. Flodén
Affiliation:
Department of Geology and Geochemistry, University of Stockholm, S-106 91 Stockholm, Sweden
Y. Grahn
Affiliation:
Department of Geology and Geochemistry, University of Stockholm, S-106 91 Stockholm, Sweden
B. Kathol
Affiliation:
Department of Geology and Geochemistry, University of Stockholm, S-106 91 Stockholm, Sweden
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Abstract

The well-preserved Tvären crater is noteworthy for being one of a small number of Early and Middle Ordovician impact structures formed in a marine environment. It is demonstrated to be an impact structure by the presence of a breccia lens, consisting of crystalline basement rocks, and shocked quartz. The breccia lens formed under dry-hot conditions after expulsion of sea-water by the impact. Resurging sea-water thereupon deposited a positively graded, 60 m thick turbidite-like unit. This graded resurge deposit is a previously unknown feature, to be expected in open-sea impacts. Breccia in the lower part of this graded deposit contains fragments of a remarkably complete orthoceratite limestone succession that existed at the site of impact, resting on non-lithified sand of probably Early to earliest Middle Cambrian age. A sedimentary succession was deposited inside the crater at depths decreasing from more than 200 m in the initial stages to some 100 m at the time of deposition of the youngest preserved beds. The environment within the crater thus favoured deposition of an essentially complete stratigraphic succession with depth-controlled palaeoecology for a significant time interval after the impact. Whereas planktonic members, like graptolites and chitinozoa, are present throughout the post-impact succession, and asaphids, almost as persistent, became established at an early date, burrowers were somewhat reluctant to enter and remopleuridids and small strophomenids came in at a late stage. We suggest as a result of this study that structures formed by impact may offer unique information about the palaeogeology and palaeoenvironment of the region hit by the impact.

Type
Articles
Information
Geological Magazine , Volume 131 , Issue 1 , January 1994 , pp. 91 - 103
Copyright
Copyright © Cambridge University Press 1994

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