Reworked crinoidal cherts and screwstones (Mississippian, Tournaisian/Visean) in the bedload of the River Maas, south-east Netherlands

Abstract

Rare Palaeozoic chert clasts collected from gravels in the bedload of the River Maas in the province of Limburg, south-east Netherlands, are rich in crinoid debris. These were transported during the Late Pliocene–Pleistocene by fluvial action from the southern Ardennes Massif (Namur-Dinant area, southern Belgium) and are of Early Carboniferous (Mississippian) age. Clasts are either completely chertified or may preserve macrofossils as natural moulds in a chert matrix; the latter are screwstones, a well-known lithology from rocks of this age. One screwstone preserves its fossiliferous debris, mainly crinoid columnals and pluricolumnals, in exquisite detail, including an archetypal ‘screw’, the mould of the axial canal. But the delicate ‘screw’ is not always preserved after fluvial transport. Further, the best-preserved specimen retains a mould of a pit in the latus, Oichnus simplex Bromley, probably a domicile excavated by an unmineralized invertebrate after the death of the crinoid.