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. Due 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 paleosols and calcite veins crosscutting clasts in a conglomeratic red bed unit, to constrain their respective ages. Our data indicate that one of the studied units was deposited ca. 260 to 250 Ma (Ma – millions of years ago) 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 ca. 180 to 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: https://doi.org/10.6084/m9.figshare.c.6850741

U-Pb geochronology reveals that hydrothermal dolomitization was coeval to the deposition of the Burgess Shale lagerstätte

“…It is not precisely known how significant this bias is, but it is typically assumed to be less than a few percent, decreasing with larger spot sizes 84 . Thus, the use of a calcite reference material in dolomite U-Pb geochronology, each of which crystallizes in the trigonal crystal system, is commonplace in the geological community and the matrix effects between these minerals are generally considered to be negligible [92][93][94][95][96] .…”