ALBERT

Notes: Clastic injections generated in connection with the formation of impact craters show many similarities to injections created by other geological processes. However, circumstances such as their position relative to the impact structure and the evidence of forceful processes indicate an impact origin. The Ordovician Lockne impact structure was formed in a marine environment with both sedimentary (Cambrian and Ordovician) and underlying crystalline (Proterozoic) target rocks. Sea water played a substantial part in the cratering process, especially in the modification of the newly formed crater as the water surged back into the structure.In the Lockne area clastic dykes and sills have long been known and have earlier been interpreted as neptunian dykes and conglomerates. So far seven cases of dykes and sills are known in the area. In this work these are interpreted as clastic injections formed in connection with the Lockne impact. The clastic injections occur in the crystalline basement and the sedimentary sequence. The material in the injections comes from all local lithologies (both sedimentary and crystalline) but the sedimentary sequence dominates as a source. The dykes and sills were injected simultaneously with the fracturing and dilation of the host rock in the cratering process, and occur at different stratigraphic levels. In some dykes, clasts from the host rock wall can be fitted back to their original position; the clasts are slightly rotated and surrounded by exotic material. Quartz grains with planar deformation features were observed in the injected material. Most of the sills within the bedded Ordovician limestone are restricted to marly beds, except for the feeder dykes which cut through the overlying beds. This circumstance demonstrates how the decompression has opened the strata along weaker layers and that the underpressure created subsequently sucked the material down. Laminar flow is a conspicuous internal structure in the dykes and sills and indicates viscous flow of injected material. The lamination in the injected material is parallel to the walls in each case. The material was lithified prior to the event and was crushed, mobilized in a water/sediment slurry and injected as dykes and sills.

Notes: Abstract The Lockne impact crater south of Östersund formed in the early Middle Cambrian with a diameter of 7 km. It is identified by its rim wall of crushed Precambrian basement granite, by fragments of impact melt, and by grains of shocked quartz. The exceptional preservation, in particular of the rim wall, is due to a complicated geological history, the first stage of which consisted of burial by marine sediments. This stage lasted until the Middle Ordovician, or over 50 million years. An early Caradoc lowering of the sea-level may have induced debris flows that stripped the rim wall of much of its sedimentary cover. Because normal marine sedimentation recommenced soon after this event, the structure was not seriously damaged, as it was buried again. The Caledonian orogeny emplaced an overthrust nappe as ultimate protection, which was removed by a recent erosion episode from all but the center of the structure. Structures formed by the impact of extraterrestrial bodies are very rare throughout much of Europe, because such structures are neither well preserved nor displayed in young mountain belts or sedimentary basins. However, northern Europe has several ascertained structures of this kind (Svensson &Wickman, 1965;Svensson, 1968;Bruun &Dahlman, 1982;Kala et al., 1984;Flodén et al., 1986;Wickman, 1988). Unfortunately, the hitherto known structures are either poorly preserved or hidden by younger deposits. We are reporting the discovery of a well-preserved exposed and accessible impact structure that has escaped the notice of geologists although important features of it have been described and puzzled over by generations of researchers (Wiman, 1900;Hadding, 1927;Thorslund, 1940;Lindström et al., 1983). The structure is located in the Lockne area to the south of Östersund in central Sweden. It has a diameter of 7 km and its center is near Tramsta on the northwest shore of Lake Locknesjön (Fig. 1). Its middle is covered by folded Lower to Middle Ordovician Orthoceratite Limestone with the sheared Lower Ordovician Töyen Shale at its base. This local expanse of deformed rock is the remainder after erosion of an extensive nappe of overthrust rocks emplaced during the Caledonian orogeny. The good preservation of the impact structure is due to the nappe cover, which had to be removed before erosion could attack the underlying structures. The rim of the impact crater is outlined by a wall of strongly shattered fragments of Proterozoic crystalline rocks (Fig. 2), which formed the local bedrock (Strömberg et al., 1984) at the time of impact. The rim wall is best preserved along the western part of the structure. It was referred to as »arkose-like breccia« byThorslund (1940), who interpreted it as the result of continental weathering, but the components rather show evidence of intense crushing than of weathering (Simon, 1987a). The »arkose-like breccia« does not contain components derived from the lower Palaeozoic deposits of the area.