Mn-rich (3.26–15.6 wt.% MnO) sedimentary layers (coticules, Strohschiefer) occur in Ordovician siliciclastic sedimentary rocks that were deposited on the Avalonian shelf and subsequently metamorphosed between 340 and 380 Ma. The Mn-rich layers formed during periods of restricted clastic input, when Mn concentrated by a redox-controlled dissolution–precipitation process near the sediment–water interface, comparable to recent Mn-crusts in the boreal settings of the Baltic Sea, the Barents Sea, and the Kara Sea. Enhanced clastic sedimentation brought the process to a halt and buried the Mn-rich layers. Formation of Mn-rich layers resumed upon renewed reduction in clastic input. Nd and Sr isotope data exclude a volcanic or hydrothermal derivation of Mn. Instead, Mn input occurred via rivers from the continental crust. The Nd isotopic composition and geochemical fingerprints of Mn-rich and hosting Mn-poor sedimentary rocks suggests that the clastic input was dominated by material derived from Gondwana rather than from Avalonia. The occurrence of coticules, however, seems to be restricted to the Avalonian shelf.