First Documentation of the Lacian–Alaunian (Early–Middle Norian) Transition in the Southern Hemisphere: A Multi-Proxy Norian Record from Palaeo-Antarctic Tasmania

The Lacian–Alaunian boundary (~221.5 Ma), marking the Early–Middle Norian transition within the Late Triassic, remains one of the least constrained sub-stage boundaries in Triassic stratigraphy, with documented sections confined almost exclusively to the Northern Hemisphere. Here we present a multi-proxy stratigraphic framework for drill cores from the Mount Nicholas Coal Measures (MNCM) of eastern Tasmania, Australia — a palaeo-Antarctic succession deposited at high southern latitudes — that provides what is, to our knowledge, the first record of the Lacian–Alaunian transition from the Southern Hemisphere. The chronostratigraphic backbone of the study rests on high-precision U–Pb TIMS zircon dating of felsic ash beds intercalated within the coal measures, constraining the BIC 3A core succession to 222.5–217.8 Ma, with a key horizon at 218.09 ± 0.09 Ma (Calver et al., 2021). A Bayesian age-depth model integrating these radiometric anchors constrains the 60–160 m interval of BIC 3A to 223–218 Ma, placing the Lacian–Alaunian transition (221.51 Ma; Jin et al., 2022) at approximately 113 m depth. This boundary position is independently supported by lithostratigraphic correlation between BIC 3A and the adjacent GY27 drillhole, where a dated tuff at a comparable stratigraphic level provides a convergent age constraint. Average sedimentation rates of ~2 cm/kyr, consistent with peat-forming low-accommodation floodplain and fluvio-lacustrine systems, validate the internal coherence of the age model over this interval. The organic-rich, fine-grained lithofacies of the MNCM — comprising carbonaceous mudstones, coal beds, and interbedded tuffs of alkali-olivine basalt and calc-alkaline affinity — record a sustained peat-swamp to floodplain depositional system with episodic volcanic input, providing a long-term conformable archive through the Lacian–Alaunian transition. Taken together, this integrated dataset — combining U–Pb TIMS geochronology, Bayesian age modelling, lithostratigraphy, and organic carbon isotope chemostratigraphy — establishes a robust multi-proxy framework for the Early–Middle Norian transition in a palaeo-Antarctic setting. The documentation of the Lacian–Alaunian boundary from a high-latitude Gondwanan succession opens new avenues for testing the global synchrony of this transition and for linking marine and terrestrial Triassic records across hemispheres.