IMPACT-RELATED CLASTIC INJECTIONS IN THE MARINE ORDOVICIAN LOCKNE IMPACT STRUCTURE, CENTRAL SWEDEN

Clastic injections generated in connection with the formation of impac t craters show many similarities to injections created by other geolog ical processes. However, circumstances such as their position relative to the impact structure and the evidence of forceful processes indica te an impact origin. The Ordovician Lockne impact structure was formed in a marine environment with both sedimentary (Cambrian and Ordovicia n) and underlying crystalline (Proterozoic) target rocks. Sea water pl ayed a substantial part in the cratering process, especially in the mo dification of the newly formed crater as the water surged back into th e structure. In the Lockne area elastic dykes and sills have long been known and have earlier been interpreted as neptunian dykes and conglo merates. So far seven cases of dykes and sills are known in the area. In this work these are interpreted as elastic injections formed in con nection with the Lockne impact. The elastic injections occur in the cr ystalline basement and the sedimentary sequence. The material in the i njections comes from all local lithologies (both sedimentary and cryst alline) but the sedimentary sequence dominates as a source. The dykes and sills were injected simultaneously with the fracturing and dilatio n of the host rock in the cratering process, and occur at different st ratigraphic levels. In some dykes, clasts from the host rock wall can be fitted back to their original position; the clasts are slightly rot ated and surrounded by exotic material. Quartz grains with planar defo rmation features were observed in the injected material. Most of the s ills within the bedded Ordovician limestone are restricted to marry be ds, except for the feeder dykes which cut through the overlying beds. This circumstance demonstrates how the decompression has opened the st rata along weaker layers and that the underpressure created subsequent ly sucked the material down. Laminar flow is a conspicuous internal st ructure in the dykes and sills and indicates viscous flow of injected material. The lamination in the injected material is parallel to the w alls in each case. The material was lithified prior to the event and w as crushed, mobilized in a water/sediment slurry and injected as dykes and sills.