Abstract
The Lower Jurassic Toarcian Stage ( c. 183–174 Ma) is marked by one of the largest global exogenic carbon-cycle perturbations of the Phanerozoic, which is associated with the early Toarcian Oceanic Anoxic Event (T-OAE; c. 183 Ma). Climatic and environmental change at the T-OAE is reasonably well constrained in the marine realm, with marine anoxic or euxinic conditions developing locally across both hemispheres, at the same time as the T-OAE negative carbon-isotope excursion. However, high-resolution stratigraphic comparison between different palaeo-ocean basins and with the continental realm can be complicated. Palaeomagnetic reversals can provide a precise and accurate stratigraphic correlation tool between marine and continental sedimentary archives, and even between sedimentary and igneous successions. Here, we present a high-resolution magnetostratigraphic record for the Toarcian Stage in the biostratigraphically complete and expanded Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, Wales. This study provides the first geomagnetic polarity reversal scale that is integrated with high-resolution biostratigraphy and carbon-isotope stratigraphy for the entire Toarcian Stage. This stratigraphic framework also provides a new, precise correlation with the basalt lava sequence of the Karoo–Ferrar Large Igneous Province, linking the Pliensbachian–Toarcian boundary and T-OAE climatic and environmental perturbations directly to this episode of major volcanic activity. Supplementary material: Details of the palaeomagnetic data and dip direction are available at https://doi.org/10.6084/m9.figshare.c.4052720
Highlights
Core samples are not affected by drilling-induced formation of biscuits
Samples for magnetic-polarity analysis were selected at 2–5 m resolution, spanning the entire Toarcian interval in the Mochras Borehole, from 601.83 to 863.5 m below surface
During progressive alternating field (AF) demagnetization three classes of behaviour were identified: Class A, samples for which demagnetization trajectories tended towards the origin, and lines could be fitted to the demagnetization trajectories (65 samples); Class B, samples for which the polarity could be determined, but with very poor line-fits and maximum angular deviation of >30°, so that no inclination was computed (45 samples; natural remanent magnetization (NRM) intensities were generally very low); Class C, samples that demonstrated unstable behaviour during demagnetization, or where the NRM intensities were too low to provide meaningful demagnetization data (38 samples)
Summary
The Mochras slabbed core (in 1 m core-lengths) is stored in boxes at the British Geological Survey, Keyworth, Nottingham, UK, at normal atmospheric conditions. The Toarcian Stage in the Mochras borehole occurs over the depths of 863.50–601.83 m. The core was drilled vertically, with the bedding dipping gently (of the order of 10°) to the east (Tappin et al 1994; Hesselbo et al 2013; see the Supplementary Material). The core slabs of the Toarcian are reasonably well preserved Core samples are not affected by drilling-induced formation of biscuits (see Pearson & Thomas 2015). The surface of the Toarcian core slab is c. Samples for magnetic-polarity analysis were selected at 2–5 m resolution, spanning the entire Toarcian interval (except that the aalensis Zone is partially truncated owing to unconformity) in the Mochras Borehole, from 601.83 to 863.5 m below surface (mbs). 148 cubes (each 2.3 cm × 2.3 cm × 2.3 cm; ∼12 cm3) were prepared from the core sections, composed predominantly of mudstones with varying concentrations of calcium carbonate
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