Bentonite beds and discontinuity surfaces as correlative tools in the Lower Ordovician carbonates of Baltoscandia

The Orthoceras Limestone on the East European Platform during the Early to Middle Ordovician is characterized by numerous discontinuity surfaces. The most prominent represent important sequence boundaries. The contemporaneous marginal and basinal deposits are interrupted by several bentonite beds that can be traced laterally onto the paltform, where the ash-beds disappear to be replaced by characteristic discontinuity surfaces. The ash-beds and the lateralli equivalent discontinuity surfaces are correlated using high-resolution biostratigraphy.