The mixed siliciclastic-carbonate sabkhas of the Late Triassic Blue Anchor Formation (Prees 2 borehole, Cheshire Basin, UK): the prelude to the End-Triassic Mass Extinction

The Blue Anchor Formation represents a time period immediately prior to the earliest phase of the End-Triassic Mass Extinction (ETME). It therefore offers a unique opportunity to yield more information on the timings and causes of the ETME, which still pose many unanswered questions. The formation records a step transition from terrestrial ephemeral-lake sediments in the underlying Branscombe Mudstone Formation to deep marine shales in the succeeding Westbury Formation (e.g. Hesselbo et al., 2023). Millimetre-scale sedimentological analysis of the Prees 2C core, UK, alongside X-ray fluorescence analysis has allowed lithofacies and facies associations to be defined and the depositional history of the formation to be determined. Seven major lithofacies are recognised: 1) massive calcareous siltstone, 2) laminated calcareous siltstone, 3) silty sandstone, 4) microbial limestone (previously regarded as dolostone), 5) brecciated calcareous siltstone, 6) small cracked siltstone, and 7) large cracked siltstone. A sabkha setting is interpreted in an overall transgressive regime; at the base of the formation a continental sabkha system surrounding perennial lakes gradually transitions to a coastal sabkha system with lagoons. Stacked sequences record repeated cycles of subtidal-lacustrine to supratidal environments throughout the formation. Facies 1 and 2 represent low energy subtidal-lacustrine and lagoonal environments respectively, passing upward into facies 3 and 4, representing intertidal to near-shore supratidal environments including algal mats. The sabkhas themselves are not preserved but represented through facies 5, which indicates collapse breccia after dissolution of evaporites. The syneresis and desiccation cracks contained in facies 6 and 7 respectively represent hiatuses in both subaqueous and subaerial conditions. Appearances of facies 6 and 7, alongside teepee structures and nodular evaporites indicate increasing evaporative and hypersaline conditions upwards through the formation. Milankovitch forcing is suggested as the controlling factor in sabkha cyclicity, based on consistency and uniformity of parasequences. This will provide a framework for a cyclostratigraphic age model in further research, with palaeoclimate implications for the ETME.