Integrated stratigraphical study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession in the Rheidol Gorge, Wales: a proposed Global Stratotype Section and Point for the base of the Aeronian Stage

Abstract

The Rheidol Gorge section, approximately 17 km east of Aberystwyth, mid Wales, exposes a ca. 20 m-thick succession of Llandovery (Silurian) strata from the upper Rhuddanian Pernerograptus revolutus Biozone through the lower Aeronian Demirastrites triangulatus Biozone and basal Neodiplograptus magnus Biozone. The section records deposition under a range of bottom-water oxygenation states. The Rhuddanian-Aeronian boundary is located 0.8 m above an abrupt lithological change from predominantly organic-poor, bioturbated 'oxic' mudrocks to an interval of black, richly graptolitic 'anoxic' shales. The graptolite fauna through the boundary interval, including the local lowest occurrence of D. triangulatus, allows precise correlation with other parts of the world. Graptolite assemblages indicative of separate divisions in the underlying revolutus Biozone and of the lower and upper parts of the triangulatus Biozone are also present. Chitinozoans are relatively well preserved in the section and indicate the Spinachitina maennili Biozone throughout the boundary interval, as is widely the case. The results of carbon isotope analyses from organic matter indistinctly show the weak interval of positive shift in δ 13 C org values through the Rhuddanian-Aeronian boundary interval, as observed globally, though local or regional processes appear largely to overprint the global signal. Overall, the excellent biostratigraphical record, well-documented local and regional stratigraphical context, historical significance, as well as easy access and assured longterm preservation, mean that the Rheidol Gorge section can be proposed as a strong candidate for a new Global Stratotype Section and Point for the base of the Aeronian Stage.

Locomotory and morphological evolution of the earliest Silurian graptolite Demirastrites selected by hydrodynamics

“…2018; Melchin et al . 2023), was selected as the original species for the reconstruction of a 3D digital model of Demirastrites (Fig. 1A).…”

“…The clear lineage from D. triangulatus to D. major in the Aeronian ) thus provides a very good lineage for modelling. In this study, D. triangulatus, which has been widely used as a marker for the base of the Aeronian Stage ( Storch 1994;Melchin et al 2023), was selected as the original species for the reconstruction of a 3D digital model of Demirastrites (Fig. 1A).…”

Graptolite biostratigraphy and biodiversity dynamics in the Silurian System of the Prague Synform (Barrandian area, Czech Republic)

“…Originally recognized as the Monograptus fimbriatus Biozone by Kettner & Bouček (1936) and Přibyl (1940a), the biozone was named after the senior synonym Demirastrites pectinatus by Přibyl (1948) and Bouček (1953). The graptolite assemblage of the lower pectinatus Biozone (Štorch et al 2018) equates with that of the British middle and upper triangulatus Biozone recognized by Zalasiewicz et al (2009) and Melchin et al (2018). The upper part of the Czech pectinatus Biozone should correspond with the British Neodiplograptus magnus Biozone but the latter index species has its first occurrence much lower in the Czech succession, as early as in the middle triangulatus Biozone of the Czech biozonal scheme.…”