Concentric impact structures in the Palaeozoic of Sweden – the Lockne and Siljan craters

Abstract

The Ordovician age Lockne and Devonian Siljan craters are two of the largest impact structures in northern Europe. Both formed in targets with a thick, low-strength upper layer. This target configuration is known to generate concentric crater structures with an outer, shallow crater in the low-strength layer, surrounding a central, deeper crater in the more resistant substrate. The concentric craters of Lockne and Siljan are excellent models for studies of similar concentric craters on Earth and elsewhere in the Solar system. Several structural issues remain, and drilling through the craters within the Swedish Deep Drilling Program intends to address the following: the extent of the craters with respect to the time of impact; the effects of cratering on the basement; and the role of basement structure for the crater formation. A problem for the Lockne crater is the relation to the Caledonian orogeny and the lateral extension of the ejecta blanket -the rim is interrupted by a radial depression that has been interpreted both as primary and secondary, tectonically induced. A second feature to study is the deeper and older (1.82-1.80 Ga) NNW-SSE shear zones that cut the basement. In the Siljan area the development of mega block associations comprising the infilling of the graben is disputed. The blocks may either be formed by sagging of peripheral parts of the fault blocks or alternatively by major radial movement involving kilometre long transport.

Relationships among magnetic fabrics, microfractures and shock pressures at an impact crater: A case study from Lockne crater, Sweden

“…The crystalline basement rocks were overlain by few kilometres of Caledonian nappe and several tens of metres of Silurian sediments (Högström et al, 2010;Kenkmann et al, 2007). The fractures, caused by pressure release from exhumation (Bahat, 1986(Bahat, , 1991Leith et al, 2014a), may be confused with those of an impact origin.…”

A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 1. Stratigraphical review with regional comparisons

“…For biostratigraphic comparisons, we use the same conodont zones and subzones as recognized in the Borenshult drillcore, which were first formally defined in the Siljan district (Bergström 1971(Bergström , 1983. Because it is in all probability based on a misidentification of the zone index (Bergström et al 2009b;Chen et al 2011), we do not use the Estonian Amorphognathus ventilatus Zone that was recently introduced without comment into the Swedish conodont zone scheme by Högström et al (2010).…”

“…In Dalarna, recent studies of these strata have been carried out by, among others, Marshall & Middleton (1990), Marshall et al (1997), Brenchley et al (1997), Maletz & Reich (1997), Grahn (1998), Suzuki & Bergström (1999), Loydell & Maletz (2002), Cocks (2005), Bergström (2007), , Schmitz & Bergström (2007), Suzuki et al (2009), Högström et al (2010, and Rasmussen et al (2010). For additional references, see .…”

A rock magnetic profile through the ejecta flap of the Lockne impact crater (central Sweden) and implications for the impact excavation process

“…Based on the standard relation between ballistic ejecta at the crater rim and the distance from where it would have travelled within the crater (e.g. Melosh, 1989) we can estimate that the mafic ejecta would originate from somewhere near the pre-impact shear zone that has been suggested to pass the center of the crater (Högström et al, 2010).…”