Proposal for the lower boundary-stratotype of the Keila Regional Stage (Upper Ordovician)

According to the latest view on stratigraphy, chronostratigraphic units, stages inclusive, can be unambiguously defined only by their lower boundary-stratotypes. The Ordovician regional stages of Estonia are mostly distinguished based on their content rather than their boundaries, and in many cases lack appropriate lower boundary-stratotypes. In this study we propose the boundary-stratotype for the Keila Regional Stage. The lower boundary of the Keila Stage has commonly been understood as the base of the Kinnekulle K-bentonite. The suggested stratotype section and point is defined as the base of the Kinnekulle K-bentonite in a subsurface tunnel at Pääsküla Hillock, south of Tallinn, North Estonia. This K-bentonite bed can be widely traced in Baltoscandia, and the approximately coeval Millbrig K-bentonite is widespread in North America. Several microfossils could also be used to distinguish the lower boundary of the Keila Stage within Baltoscandia.