Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK
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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,238 U–206 Pb, ±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,238 U–206 Pb, ±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.
Catlos , E J , Mark , D F , Suarez , S , Brookfield , M E , Miller , C G , Schmitt , A K , Gallagher , V & Kelly , A 2020 , ' Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK ' , Journal of the Geological Society , vol. 178 , no. 1 , jgs2020-107 . https://doi.org/10.1144/jgs2020-107
Journal of the Geological Society
Copyright © 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London.
DescriptionSupport for obtaining the geochemical data in this paper was provided by funds to E.J.C. provided by the Jackson School of Geosciences at The University of Texas at Austin. This work was also funded by the Scottish Universities Environmental Research Centre. SIMS facilities at Heidelberg University acknowledge support through DFG Scientific Instrumentation and Information Technology programme.
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