Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

Abstract

The Clydach Valley Subgroup (Courceyan) records a dip section through an Early Carboniferous shallow marine, carbonate shelf and consists of three oolitic formations separated by paludal/peritidal units with abundant evidence of subaerial exposure in proximal areas. The lower part correlates with the Kinderhookian–Osagean Boundary Excursion, with allochem data indicating a minimum δ13C value for marine carbonate of +4.5‰, with associated δ18O of −4.6‰. Marine carbonate δ13C and δ18O values of +2‰ and −2‰, respectively, were estimated for the younger part. Allochem isotopic data yield a well‐defined mixing line consistent with stabilisation in meteoric water at varied water: rock ratios, the degree of stabilisation increasing up‐dip and up‐stratigraphy. Two distinct diagenetic styles closely correlate with evidence for the presence (Diagenetic Regime 1) or absence (Diagenetic Regime 2) of subaerial exposure, non‐ferroan and ferroan calcite cement dominating respectively. Five cement zones (Zones 2–6) defined by iron content occur in Diagenetic Regime 1, irrespective of formation, indicating a similar sequence of palaeohydrological changes affected repeated depositional cycles. Zones 3–5 are considered meteoric based on their isotopic composition (δ18O: −5.8 to −11.1‰ and δ13C: −3.7 to −6.2‰) and form distinct clusters dependent on zone, age and location. Pedogenic carbonates and meteoric cements record a long term increase in meteoric δ18O values: −7.8‰ during the Kinderhookian–Osagean Boundary Excursion, −6.9‰ during diagenesis of the upper part of the subgroup and −6.3‰ associated with initial deposition of the overlying Llanelly Formation. This is consistent with global sea water trends, but an element of climate change cannot be ruled out. Increases in cement δ18O values as meteoric systems become established (Zone 3–4) indicate repeated short term variation in rainwater composition probably driven by climate change, but also suggests a link between climate and the depositional cycle.

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

“…During Tony's term as a junior lecturer at what is now Cardiff University, he developed an interest in the early Mississippian oolites in South Wales, which lead to him supervising two PhD research projects on these units after moving to Nottingham University. The results of one of these studies are re‐evaluated by Raven (2023) who shows how the integration of palaeosol, meteoric cement and allochem isotopic data from a series of stacked oolitic sand bodies allows the recognition of global isotopic trends, regional climate effects and broad local hydrological changes. The Kinderhookian‐Osagean Boundary Excursion, which had not been recognised when the work was undertaken, is evident in allochem (grain) δ 13 C values.…”