Ordovician sediments sandwiched between Proterozoic basement slivers: tectonic structures in the Stumsnas 1 drill core from the Siljan Ring, central Sweden

The Stumsnas 1 core, drilled in 2011 in the southern part of the Siljan Ring, represents a structurally complex section and provides new insights into the impact-induced local structure in central Sweden. The Siljan Ring was formed around the central uplift of a Late Devonian meteorite crater, the largest known impact structure in Europe. The Stumsnas 1 core section reveals that about 90m of Palaeozoic (Lower to Upper Ordovician) sedimentary rocks are sandwiched between Proterozoic igneous basement rocks. The sedimentary contact to the underlying approximate to 260m of Proterozoic basement is a prominent unconformity. The contact to the overlying approximate to 190-m-thick slab of Proterozoic basement is a few metres wide complex fault zone, comprising alternating thin slices of sedimentary and granitic rocks together with fault breccia and gouge. The tectonic emplacement of basement rocks over the Palaeozoic sedimentary succession is apparently impact-related and caused folding and faulting of the underlying sediments, some of which were overturned and cut out. Minor fault zones occur throughout the Stumsnas 1 core section and have large damage zones with intense fracture networks along which alteration and mineralisation took place, likely also impact-related. Small-scale faults and fractures are common and are critical for fluid migration and hence for ongoing exploration for natural gas and geothermal energy reservoirs in the Siljan impact structure.