Geology of the Early Palaeozoic Lockne impact structure, Central Sweden

Abstract

The diameter of the crater at Lockne as seen on topographic maps is 7–8 km. The Tandsbyn Breccia, the principal evidence of impact, consists of strongly crushed local basement. Its distribution follows the crater margin. On the north margin, basement granite rests on an inverted sub-Cambrian erosion surface on the lowermost Middle Cambrian. Tangential faults occur at the periphery of the structure. After the impact occurred under the sea during the middle Ordovician, there was a resurgence of ejecta-loaded sea water which deposited a chaotic breccia (Lockne Breccia), which has the appearance of a debris flow. The Lockne Breccia, together with a less coarsely grained turbidite (“Loftarsten”), which lies immediately above it, predominantly consist of clasts of lower to middle Ordovician limestone and, mainly subordinate, inclusions of Tandsbyn Breccia and isolated clasts of crystalline basement. Fragments of impact melt are important components of the turbidite. A protective cover of sediments formed after the impact, is, in turn, overlain by an outlier of a Caledonian overthrust nappe, which occupies the central and topographically deepest part of the crater. Shocked quartz has been identified in the Loftarsten.