Composition and deposition of Early Eocene ashes in northern Germany

North Atlantic rifting during the Palaeocene-Eocene was accompanied by explosive volcanic eruptions. These led to distribution of about 200 ash layers of mainly basaltic composition covering wide areas of NW and Central Europe, also reaching the Tethys realm (Obst et al. 2015).

The ash layers, which are often interbedded in clayish successions, are known from offshore and onshore drillings but also from surface exposures, e.g., cliff sections or clay pits. In part, the pyroclastic material is well preserved in eogenetically carbonate cemented concretions, which occur in northern Germany and Denmark in glacially dislocated rafts of Eocene sediments or as isolated glacial erratic boulders named cement stones (“Zementsteine”).

Petrographic and sedimentological investigations of numerous cemented ashes from several locations in northern Germany (Fehmarn, Klütz Höved, Groß Roge, Grimmen, Wobbanz/Rügen and Greifswalder Oie) allow to distinguish different types of preservation. Single and rarely double ash layers up to 15 cm in thickness may either be preserved undisturbed, intensively bioturbated or reworked. Especially in shallow marine environments, the ashes can partly be eroded by currents or waves, and the basaltic glass particles may be redistributed.

In detail, variations in thickness and grain size as well as varying glass composition and alteration can be used to characterize distinct layers and will help to correlate ashes of the same volcanic event between different occurrences. Furthermore, changes of the sedimentation environment are documented in a NW–SE transect reflecting still water conditions in the central part of the North Sea Basin and near-shore environments at the eastern basin margins.