The pre‐impact Ordovician Stratigraphy of the tvären bay impact structure, se Sweden

Abstract

Abstract The Middle Ordovician impact crater in the Tvaren Bay south of Stockholm has a well preserved sequence of sedimentary rocks in an area of Sweden where the Lower Palaeozoic sedimentary rock cover has been eroded away. The crater was formed in the Late Kukrusean by a cosmic body that hit a marine environment The resurge breccia deposited shortly after the impact consists mostly of limestone fragments, but also of fragments from the Precambrian crystalline bedrock. Limestone clasts in the lower part of the breccia were studied by means of visual appearance and microfossil content, and by using conodonts for age determination correlations were made between the pre‐impact limestones of Tvaren and limestones in other parts of Sweden and Estonia. The conodonts range from the Volkhovian Microzarkodina parva Zone to the Upper Kukrusean Prioniodus variabilis Zone. The majority of the species are Llanvirnian‐Llandeilian. The findings show (1) that although the pre‐Volkhovian Lower Ordovician is poorly devel...