Geochemistry of macrofossil, bulk rock and secondary calcite in the Early Jurassic strata of the Llanbedr (Mochras Farm) drill core, Cardigan Bay Basin, Wales, UK

Abstract

The Llanbedr (Mochras Farm) core (Wales, UK) yielded a >1300 m long mudrock sequence that has excellent potential for establishing an integrated stratigraphic scheme for the entire Early Jurassic Epoch. Lithological variations in the core are predominantly driven by hierarchical changes in the carbonate content. These changes also dominate – or may impact upon – many geochemical and physical properties of the core. The bulk carbonate C isotope record displays systematic fluctuations, the largest of which correspond to previously identified phases of environmental perturbation. The magnitudes of negative C isotope excursions in carbonate are inflated compared with equivalents previously described elsewhere as a result of diagenesis and the concomitant loss of primary carbonate. The marine macrofossil record of Mochras reveals biological and isotopic patterns that are generally comparable with other UK basins. Potentially significant differences between the Cleveland and Cardigan Bay basins are observed in the Pliensbachian and Toarcian fossils. This different expression may be related to different habitat structures or palaeoceanographic and water depth differences between these basins. Minima in macrofossil δ18O values generally coincide with peaks in macrofossil wood abundance and sea-level lowstands inferred from sequence stratigraphic interpretation of other UK sections. This relationship suggests a possible relative sea-level control on the observed O isotope records and sediment provenance.