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.