Magnetostratigraphy of the Toarcian Stage (Lower Jurassic) of the Llanbedr (Mochras Farm) Borehole, Wales: basis for a global standard and implications for volcanic forcing of palaeoenvironmental change
Mac Niocaill, C
Journal of the Geological Society
© 2018 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/).
The Lower Jurassic Toarcian Stage (~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; ~183 Ma). Climatic and environmental change at the T-OAE is reasonably well constrained in the marine realm, with marine anoxic/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 34 perturbations directly to this episode of major volcanic activity.
We acknowledge funding from Shell International Exploration & Production B.V., the International Continental Scientific Drilling Program (ICDP) and the Natural Environmental Research Council (NERC) (grant number NE/N018508/1). All authors thank the British Geological Survey (BGS), especially Tracey Gallagher and Scott Renshaw, for enabling access to the Mochras core. Clemens 470 V. Ullmann is acknowledged for the help with sampling and the general discussion. James B. Riding publishes with the approval of the Executive Director of the British Geological Survey (NERC). This manuscript is a contribution to IGCP 655 (IUGS-UNESCO): Toarcian Oceanic Anoxic Event: Impact on marine carbon cycle and ecosystems, and IGCP 632 (IUGS-UNESCO): Continental Crises of the Jurassic: Major Extinction events and Environmental Changes within Lacustrine Ecosystems.
This is the final version of the article. Available from the Geological Society via the DOI in this record.
Published online 17 April 2018