Uranium-lead dates from Livian (Middle Viséan) bentonites of the Namur-Dinant Basin, Belgium

Three new high-precision U–Pb zircon dates are presented from diagenetically altered volcanic ash layers (bentonites) interbedded with cyclically-bedded Livian (middle Viséan) limestones of the Namur-Dinant Basin, southern Belgium. These bentonites were sampled from the Livian boundary stratotype at the cliffs of Lives-sur-Meuse and have precise biostratigraphic control. The U–Pb dates provide important isotopic age tie points for improved calibration of the Mississippian timescale. The Banc d’Or de Bachant (bentonite L1), the base of which defines the base of the Livian Substage, yielded a U–Pb zircon chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) date of 341.2 Ma. This is substantially younger than the base of the Livian Substage inferred in most published Carboniferous timescale calibrations and, together with field observations, is interpreted to indicate a substantial hiatus across the Moliniacian-Livian boundary interval in the Livian boundary stratotype. In light of this, the Livian boundary should be redefined and the stratotype relocated to an area characterised by more continuous sedimentation. The Livian Substage is estimated to be 5.1 Myrs long. This duration is comparable to most Phanerozoic stages and supports that the Viséan Stage should be formally subdivided to attain better stratigraphic precision. Livian shallowing-upwards carbonate parasequences have average cycle durations of between 61 and 130 kyrs/cycle. This range overlaps with the Milankovitch c. 100 kyrs short eccentricity cycle, which is the dominant beat in younger Warnantian and Asbian parasequences that are widespread in Western Europe and are widely interpreted to be glacio-eustatic in origin. The Livian parasequences may also be glacio-eustatic and record an early expression of late Palaeozoic glaciation.