The Homerian (silurian) Stratigraphy of the Usk Inlier of Wales, Uk: Integration of Carbon Isotope Data and Sequence Stratigraphy

Carbon isotope excursions (CIEs) are powerful tools for global stratigraphic correlation, yet their expression can vary dramatically with depositional setting. The Midland Platform (Avalonia) offers the opportunity to laterally assess the effect of depositional architecture on CIE records thanks to the geographic extent of outcrops spanning key stratigraphic intervals. Here we report the first occurrence of the globally recognised Homerian (Silurian) CIE from the Usk Inlier of the southern Midland Platform, Wales, and use it to explore how facies architecture shapes isotopic signals. Both the first and second peaks of the Homerian CIE are preserved within the limestone lithofacies of the Ton Siltstone (Trostrey Limestone Member) and Usk Limestone formations, but differ from other sections in that the magnitudes of both peaks are reversed: the relatively offshore nodular limestones of the Trostrey Limestone Member yield a diminutive first peak (0.6‰), whereas the nearshore Usk Limestone Formation records a more pronounced second peak (2.3‰). This lithofacies dependence underscores the importance of integrating chemostratigraphy with sedimentological context. The proximity of the Usk Inlier to the Pretannia landmass created competing siliciclastic and carbonate systems that, in turn, provide an unusually clear sequence stratigraphic framework. Notably, the onset of the Homerian CIE is associated with regressive wave-rippled sandy siltstones, with the overlying first CIE peak within nodular limestones interpreted as the early transgressive systems tract: a well-constrained tie between geochemical and systems-tract signals. This integrated approach enables robust correlation of the Usk Inlier succession with the broader Midland Platform and with key sections on Baltica, demonstrating the value of facies-aware chemostratigraphy for Silurian global correlation.