A new age model for the Ordovician (Sandbian) K-bentonites in Oslo, Norway

Abstract

During the Late Ordovician, large explosive volcanic eruptions deposited worldwide K-bentonites, including the Millbrig and Deicke K-bentonites in North America and the Kinnekulle K-bentonite in Scandinavia. We have studied a classical locality in Oslo containing one of the most complete sections of K-bentonites in Europe. In a 53 m section of Sandbian age, we discovered 33 individual K-bentonite beds, the most notable beds being the Kinnekulle and the upper Grimstorp K-bentonite. Magnetic susceptibility (MS) measurements on two intervals show significant periodicity peaks interpreted as Milankovitch cycles and thus astronomically forced changes in sediment supply and composition. These cycles fit remarkably well with both the expected Milankovitch periodicities for the Ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of Milankovitch cycles from the lower Paleozoic so far. Five of the K-bentonites have been dated by high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon geochronology, where the Kinnekulle K-bentonite gives an age of 454.06 ± 0.43 Ma. We have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 K-bentonites. Using the age model, we further present a new age for the Sandbian-Katian stage boundary. The section in Oslo provides the highest resolution window into the Upper Ordovician K-bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the Paleozoic and the relationship with the global carbon cycle changes that followed.