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.Supplementary material: Analytical data for the Mochras core are available at https://doi.org/10.6084/m9.figshare.c.5463508
Highlights
Lithification and diagenetic modification of the sediment did not induce strong depletions in Sr, 13C or 18O in the carbonate phases, with diagenetic stabilization occurring before deep burial
The magnitude of C isotope excursions in bulk carbonate was amplified by a reduction in the carbonate content during environmental perturbations, increasing the biasing effect of added diagenetic carbonate containing C sourced from oxidized organic matter
The rarity of macrofauna during large environmental perturbations illustrates the negative effects of such events on faunal assemblages
Summary
The drill core and related samples are stored at the National Geological Repository of the British Geological Survey, Keyworth, UK. About half of the originally nearly fully recovered core was broken up soon after recovery. For the Jurassic part of the core, only the archive half from 1282 to 601 m depth, corresponding to the uppermost Sinemurian to top Toarcian stages, is still present with the stratigraphic context preserved. 1.4 m of cored interval, are archived. Nearly 7800 discrete fossil specimens registered with a core depth of one-inch precision, and aggregate bags of c. 500 g of core fragments, each corresponding to c.
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