Improved chronostratigraphy and fine-tuned timing for Late Triassic palaeoenvironmental changes in SW Britain using coupled magnetic polarity and carbon isotope stratigraphy

Abstract

Understanding the synchronicity of global climatic, environmental, and biotic events around the Norian-Rhaetian boundary (NRB) is problematic because of major international differences in biochronology. We instead use magnetostratigraphic and global carbon isotopic changes to produce more precise global correlation. This work focusses on the base and top of the Rhaetian, with principal age control from a new late Norian to latest Rhaetian magnetostratigraphy from Lavernock (southern Wales) which can be directly correlated to the proposed NRB sections at Pignola Abriola (Italy) and Steinbergkogel (Austria). A disconformity exists in the Lavernock section in its late Norian part (Branscombe Mudstone Formation), but the NRB interval is largely complete. The magnetostratigraphy and a composite δ¹³C_org stratigraphy from three British sections, demonstrate synchronous changes in both terrestrial and marine records. This analysis indicates the older proposed definition of the NRB from Steinbergkogel is in the upper few metres of the Branscombe Mudstone Formation, while the younger NRB definition from Pignola Abriola is in the upper parts of the Blue Anchor Formation. The latest Rhaetian magnetostratigraphy from Lavernock records reverse magnetochrons UT26r and UT28r which closely pre-date and post-date the widely recognised Marshi and Spelae carbon isotope excursions, respectively. Magnetochrons UT28r and UT27r were previously recognised at St Audrie's Bay (SW England), with relationships to the Newark Supergroup which tightly constrain the first phase of CAMP eruptions to overlap the Spelae excursion. The carbon isotope excursions present in the Blue Anchor Formation lacustrine successions, demonstrate the likely atmospheric, and global spread of these perturbations.