Avike Bay a km Diameter Possible Impact Structure at the Bothnian Sea Coast of Central Sweden

Herbert Henkel1, Vaino Puura2, Tom Floden3, Juho Kirs2, Mare Konsa4, Ulla Preeden2, Robert Lilljequist5 and Joanne Fernlund1

1Royal Institute of Technology, Department of Land and Water Resources Engineering, S-10044 Stockholm, Sweden ([email protected])

2Institute of Geology, University of Tartu, Vanemuise 46, EE-51014 Tartu, Estonia ([email protected])

^Department of Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden ([email protected])

4Institute of Geology, Tallinn Technical University, Estonia pst.7, EE-10143 Tallinn, Estonia ([email protected])

5Ecominas, Calle Horno 9, Estepuma, Spain ([email protected])

Abstract. Avike Bay is a 270° degree wide near-circular, 114 m deep bay on the Swedish coast of the Bothnian Sea, northeast of Sundsvall. The structure has a diameter of about 10 km. It was classified as a probable impact structure because of its extraordinary circular topography in the overwiew of impact structures in Fennoscandia. Recent studies lend further support to this interpretation. The structure has a submarine central mound, which is elevated some 40 m above the adjacent sea floor. It has a very distinct tangential and radial on-shore fracture pattern as seen in the topographic map. Along the southwestern shore of the Bay, an enigmatic quartzite breccia of unknown age occurs as part of a larger outcrop of polymict breccia with clasts of crystalline rocks and quartzite of unknown age. In thin section, planar fractures can be observed in quartz and feldspar grains. A detailed investigation showed that in a few cases the quartz grains contained microdeformation features closely resembling PDFs.

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