The Lockne Crater: Revision and Reassessment of Structure and Impact Stratigraphy

Abstract.

The Lockne impact crater in central Sweden has features characterizing a relatively deep marine environment. Recently, improved outcrop (road construction, lumbering, etc.) has favoured examination of important features, resulting in greater precision and understanding. Under a water depth of at least 500 m, the target seabed consisted of 80 m Cambrian and Ordovician sediments. About one-half of this succession was lithified limestone; the other half was soft claystone and semi-lithified limestone. This succession rested on a peneplain cut into crystalline basement. The crater in the basement is just over 7 km wide. The sub-Cambrian peneplain can be followed all the way up to this crater, which thereby is shown to lack any significantly raised rim. The peneplain next to the crater was stripped of most sediment by the expanding water crater. Instead, there is a roughly 50 m thick brim of ejected crystalline rock, resting on the peneplain and on some remaining sediment. The brim is 2.5 km wide in the west, but less than 1 km in the east. Crystalline ejecta bodies, each with a volume of thousands of cubic meters, are scattered towards westerly directions as far as 9 km from the crater centre. An oblique impact from an easterly direction is indicated (Shuvalov et al., this volume). Radial furrows through the brim (“resurge gullies”) might have originated during ejection but were utilized by resurging seawater. Target carbonates in the wider surroundings were brecciated during the ejection and excavation stage. During the resurge stage, another breccia was deposited including both limestone and redeposited clasts of crystalline ejecta.