Direct dating of deposition and rift-related alteration of fossil-barren red bed units in the North Sea

Abstract

Many fossil-barren or poorly fossiliferous red bed units are found in the North, Norwegian and Barents seas. Owing to the near-absence of biostratigraphic markers in these units, and the possibility of unconformities at their upper and lower boundaries, determining their depositional ages and stratigraphic assignment can be challenging. We apply in situ U–Pb carbonate dating on North Sea samples, including plant-root related carbonates (rhizoliths) from palaeosols and calcite veins cross-cutting clasts in a conglomeratic red bed unit, to constrain their respective ages. Our data indicate that one of the studied units was deposited c. 260–250 Ma and may represent an early development of the Skagerrak Formation. The second unit yielded an oldest age of 180.2 ± 9.7 Ma, interpreted to represent a late diagenetic event rather than deposition. High Mn/Sr and low δ 13 C values in many cements dated between c. 180 and 100 Ma are interpreted to represent precipitation of diagenetic carbonates and reprecipitation of root carbonates, potentially related to Jurassic–Cretaceous volcanism and uplift during rifting of the North Sea. These new data point to U–Pb carbonate geochronology as an effective tool for directly dating red bed units and subsequent diagenetic events. Supplementary material: Detailed methods for all geochemical analyses are available at https://doi.org/10.6084/m9.figshare.c.6850741