The Homerian (Silurian) stratigraphy of the Usk Inlier of Wales, UK: integration of carbon isotope data and sequence stratigraphy

 

Authors: Ray DC, Oostra L, Jarochowska E

Published in: Bulletin of Geosciences, volume 101, issue 3; pages: 365 - 387; Received 5 February 2026; Accepted in revised form 3 June 2026; Online 10 October 2026

Keywords: Silurian, Wenlock, carbon isotope, stratigraphy, Usk Inlier, Midland Platform, Avalonia,

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Abstract

The globally recognised Homerian (Silurian) carbon isotope excursion (CIE) is reported for the first time from the Usk Inlier of the southern part of the Midland Platform (Avalonia), Wales. There the first and second peaks of the Homerian CIE are documented from the limestone lithofacies of the Ton Siltstone (Trostrey Limestone Member) and Usk Limestone formations. The magnitudes of the CIE peaks are strongly influenced by the lithofacies with the relatively offshore nodular limestones of the Trostrey Limestone Member containing a diminutive first peak (0.6‰), when compared with the second peak (2.3‰) that is developed in the nearshore Usk Limestone Formation. A sequence stratigraphic interpretation has been facilitated by the occurrence of competing siliciclastic and carbonate sedimentary systems, which result from the proximity of the Usk Inlier to the landmass of Pretannia. Notably, the onset of the Homerian CIE is well expressed, and is associated with regressive wave-rippled sandy siltstones within the Ton Siltstone Formation, with the overlying first peak of the CIE within nodular limestones (Trostrey Limestone Member) that are representative of the early transgressive systems tract. Based upon the integration of carbon isotope data and sequence stratigraphy the Usk Inlier succession can be correlated with the Much Wenlock Limestone Formation of the wider Midland Platform, as well as key successions upon Baltica.