The marine Lockne impact structure, Jamtland, Sweden: a review

The Lockne impact structure in Jamtland (63 degrees 00'20 " N, 14 degrees 4 9'30 " E) formed in the Middle Ordovician at approximately 455 Ma. The stru cture is a concentric crater with a total diameter of 13.5 km. The impact t ook place in a marine environment. Seawater played an important role in the cratering process and in crater morphology and the amount of melt remainin g in the structure. Seawater rushed back into the crater in a resurge, erod ing and redepositing the ejecta among the resurge deposit. Seawater further more facilitated the hydrothermal system, which was driven by the residual heat in the structure. The Lockne structure hosts shocked quartz and an iri dium anomaly. The rim wall round the crater collapsed in the modification s tage of the crater and was annihilated by the resurge. The fractured baseme nt and the impact breccia were initially rich in open cavities. These becam e partly filled with dominantly calcite. The filling contributed to a low-d ensity contrast, generating a negative gravity anomaly of 22 gu. The gravit y 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.