Lake Karikkoselka impact structure, central Finland:: New geophysical and petrographic results

The Lake Karikkoselka meteorite impact structure is located at 62 degrees 13.3'N, 25 degrees 14.7'E within the Early Proterozoic granitic terrane in central Finland, This well-preserved simple impact structure has a diameter of less than or equal to 2.4 km, The structure owes its discovery to the circular bathymetry of Lake Karikkoselka. The impact origin of the structure is indicated by extensive fracturing of the target rock, by shock metamorphic features in a breccia boulder, and by shatter cone-like features observed in lake shore outcrops of porphyritic granite. The microscopic shock metamorphic features include multiple sets of planar deformation features in quartz clasts, kink banding in biotite, mosaicism in quartz, and feldspars and incipient vesiculation in feldspars, Geophysical data show distinct anomalies related to the structure. In particular, the structure is associated with exceptionally strong airborne electromagnetic anomalies, The gravity data reveal a low of 3.8 mGal, consistent with the size of this impact structure. Integrated geophysical data have been used to construct a five-layer model of the structure: (1) lake water, with (2) a thin veneer of mud at its bottom, (3) a sediment and/or impact breccia layer, (4) fractured granite below and beyond the lake, and (5) unfractured target granite (basement), The gravity and the electromagnetic models are mutually consistent and suggest a depth of similar to 120 m for the bottom of the sediment and/or breccia layer, thus implying an anomalously shallow structure (depth/diameter, <0.1). Paleomagnetic data suggest an age of 230-260 or 530-560 Ma for the impact, but older age interpretations are possible.