The origin of the marine Lockne impact structure, Jämtland

Abstract: The Lockne impact structure in Jämtland (63o00'20"N, 14o49'30"E), formed in the Middle Ordovician at approximately 455 Ma. The structure is a concentric crater with a total diameter of 13.5 km. The impact took place in a marine environment. The sea-water played an important role in the cratering process and in crater morphology and the amount of melt remaining in the structure. The sea-water rushed back into the crater in a resurge, eroding and redepositing the ejecta among the resurge deposit. Sea-water furthermore facilitated the hydrothermal system, which was driven by the residual heat in the structure.The Lockne structure hosts shocked quartz and an iridium anomaly. The rim wall round the crater collapsed in the modification stage of the crater and was annihilated by the resurge. The fractured basement and the impact breccia were initially rich in open cavities. These became partly filled with dominantly calcite. The filling contributed to a low density contrast, generating a negative gravity anomaly of 22 gu. The gravity model indicates a central uplift and a NW-directed tilt of the structure. This tilt is also seen in the magnetic models. The apparent absence of any impact melt is probably real and related to the environment of impact.