The marine impact crater at Lockne, central Sweden

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Abstract
The Lockne crater (63°00′20″N, 14°49′30″E), dated as Middle Ordovician, Caradoc, chitinozoan Zone of Lagenochitinal dalbyen‐sis, conodont Subzone of Baltoniodus gerdae, consists of a 7.5 km wide inner crater that is surrounded by a 3 km wide outer crater. The inner crater has been identified by drilling on both sides of Lake Lockne. Its floor is strongly crushed crystalline basement with a variable thickness of monomictic breccia. It is filled with more than 200 m of resurge deposits and nearly 100 m of secular, post‐impact Dalby Limestone, parts of which are covered by a thin Caledonian nappe outlier. The outer crater formed at the top surface of the crystalline basement that it exposes in a state of strong crushing and with a patchy cover of monomictic breccia. The presence and structure of the outer crater suggest that Lockne may be a nested crater with the basement acting as a strong, lower layer, and the sediment cover and the water acting together as a weak upper layer, the thickness of which can be estimated. The diameter of the large structure is 13.5 km. Resurge of the sea after impact eroded gullies radially through the outer crater. A halo of ejecta and resurge products can be traced at least 40 km outward from the impact centre. The reconstruction of this halo poses constraints on nappe movements in the cover rock. Because the crater itself is not much deformed one can now conclude that the basement in this area was relatively little affected by Caledonian and later deformation. Lindström, M., Sturkell, E.F.F., Törnberg, R. & Ormö, J., 1996: The marine impact crater at Lockne, central Sweden. GFF, Vol. 118 (Pt. 4, December), pp. 193–206. Stockholm. ISSN 1103–5897.