Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK

Michael E Brookfield,Stephanie Suarez,Darren F Mark,C Giles Miller,Anne Kelly,Vincent Gallagher,Elizabeth J Catlos,Axel K Schmitt

doi:10.1144/jgs2020-107

Abstract

The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO4) grains from the Ludlow (n = 2) and stratigraphically higher Downton (n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238U–206Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma (n = 5 each, 238U–206Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert.Supplementary material: An Excel file containing detailed information on the SIMS analyses, a figure showing calibration curves for AS3 standards sputtered over sessions 1 and 2, and a figure showing CA-ID-TIMS U–Pb age data (concordia and weighted mean plots) are available at https://doi.org/10.6084/m9.figshare.c.5087031

Highlights

The late Silurian was a time of change from a marine to a nonmarine environment in the area of the Welsh Borderland, marking the final stages of the closure of the Iapetus Ocean, and an interesting interval for the evolution of life in general (e.g. Allen 1985; Calner 2008; Munnecke et al 2010; Blain et al 2016)
Using only four ages to characterize the depositional history of a unit is impossible, and we report these ages only because there are no radioisotopic results from the Ludlow Bone Bed sensu stricto
More zircon grains were found in the other samples, and their secondary ion mass spectrometry (SIMS) ages range from 471.6 ± 20.7 to 375.7 ± 14.6 Ma in sample 17b/4 and from 464.1 ± 25.2 to 396.2 ± 7.9 Ma in sample 14c (238U–206Pb ages, ±1σ; Figs 3 and 4)

Summary

Introduction

The late Silurian was a time of change from a marine to a nonmarine environment in the area of the Welsh Borderland, marking the final stages of the closure of the Iapetus Ocean, and an interesting interval for the evolution of life in general (e.g. Allen 1985; Calner 2008; Munnecke et al 2010; Blain et al 2016). The late Silurian was a time of change from a marine to a nonmarine environment in the area of the Welsh Borderland, marking the final stages of the closure of the Iapetus Ocean, and an interesting interval for the evolution of life in general The Ludlow Bone Bed itself is considered the type locality of several species of late Silurian fishes and was long thought to mark the first appearance of fossil fishes in the rock record Some of the earliest vascular land plants have been recovered from Wenlock age sediments and stratigraphic horizons just below the Ludlow Bone Bed in the Welsh Borderland (Edwards and Kenrick 2015)

Methods

Results

Conclusion