PRELIMINARY RESULTS FROM THE 2009 AND 2010 CORE DRILLINGS OF THE MALINGEN STRUCTURE, A PROBABLE DOUBLET TO THE LOCKNE MARINE-TARGET IMPACT CRATER, CENTRAL SWEDEN

Målingen is a 1 km wide, circular structure located 16 km down-range of the 7.5 km wide, Upper Ordovician Lockne crater. The circular shape in the crystalline basement and exposed sedimentary breccias resembling the resurge deposits at Lockne have led to suggestions for an impact origin in relation to Lockne, but it has been generally accepted that the breccias are Caledonian allochthonous deposits of distal Lockne resurge materials and that the circular feature is merely a coincidence.

In 2009 we performed a 45 m core drilling (Målingen-1A) at the apparent center of the structure in order to understand the stratigraphic relationships of the sediments as well as the depth of the structure. As the core ended in sedimentary infill without reaching the crystalline basement the drilling was resumed in the same hole during 2010. The new drilling (Målingen-1B) reached the depth of 149 m. The two cores together reveal a sequence expected for a marine-target impact crater (i.e. Lockne): Post-impact deposits (3 m) of the same unit as at Lockne coarsen gradually downwards into calcareous siltstone, arenite, and sedimentary breccia (3 m). The breccia is mainly made up of fragments from the 50 m thick orthoceratite limestone of the upper target sequence. It rests with sharp contact on dark mudstone (97 m) originating from the 30 m thick Cambrian alum shale of the lower sedimentary target. At 105 m depth the dark mud passes into a polymict, matrix-supported breccia (3 m). It contains dm-size blocks of basement rocks, and some smaller clasts of limestone and alum shale, but gets increasingly clast supported and monomictic crystalline downwards. In the interval 105-119 m the rock is partially brecciated granite, which passes downwards into basement rocks of the regular appearance of the area.

The shape and depth of the Målingen structure together with the cored sequence support an impact origin. The stratigraphic and geographic relationship to Lockne indicates that it most likely occurred in conjunction with the Lockne impact. The thick infill of dark mud from the lower target and relatively much thinner deposits from the limestone target, as well as the near absent crystalline ejecta in the resurge are likely consequences of the relatively deep target water (500-700 m) causing most of the basement crater to develop due to displacement.