Confirmation of the impact origin of the Late Ordovician Tvären impact structure (southeast Sweden) and emplacement of impactites in a marine setting

Abstract The Tvären structure in southeastern Sweden has been listed as a confirmed marine‐target impact structure for decades. However, to date, no measurements and/or indexed data of planar deformation features in quartz grains from the structure have been published or any other unequivocal evidence of impact. Here, we present an investigation aimed at searching for shocked quartz in the 224 m deep Tvären‐2 drill core. We confirm that the rocks of this core contain, on average, about 5% of shocked quartz, either displaying up to 10 sets of planar deformation features, planar fractures, or both. Petrographic investigation resulted in a division of the core into four main stratigraphic units: (i) lithic impact breccia; (ii) coarse melt‐bearing resurge deposit; (iii) finer melt‐bearing resurge deposit; and (iv) post‐impact deposit. Studying sedimentary facies and structures as well as using petrographic and textural characteristics of the material made it possible to recreate the process of crater formation. We describe a sequence of excavation stage‐generated crystalline breccia overlain by early modification stage polymictic breccia with crystalline and limestone clasts formed when the collapse of the crater rim and walls had begun. This breccia is, in turn, overlain by a resurge deposit comprised of material brought into the crater as the water returned. Hence, studying macro‐ and microstructures is yet another approach to better understand the mechanisms involved in the formation of small impact craters and their associated deposits in marine environments.