Tectonic and hydrothermal controls on environmental change and oceanic anoxia in the Miaolingian–Tremadocian Alum Oceanic Anoxic Episode of Baltoscandia

Oceanic anoxic events (OAEs) are commonly linked to global drivers, but the role of basin configuration and tectonic instability in sustaining long-lived anoxic conditions remains insufficiently understood. In Baltoscandia, the Miaolingian–Tremadocian Alum Shale Formation (~ 513–484 Ma) records the regionally restricted Alum Oceanic Anoxic Episode (AOAE), which persisted for ~30 Myr and led to the accumulation of kerogenous black shales. This updated review integrates stratigraphic, palaeontological, sedimentological, and geochemical evidence to assess the environmental and tectonic controls on basin circulation and redox conditions during the AOAE. Deposition occurred in a semi-enclosed, flat-floored epicontinental basin developed largely on the Transscandinavian Igneous basement. Restricted circulation, high productivity, and persistent water-column stratification promoted dysoxic to anoxic bottom-water conditions, favouring exceptional preservation of organic matter and enrichment in pyrite and redox-sensitive elements. The succession is punctuated by breccias, conglomerates, shelly limestones, karstic surfaces, synsedimentary faults and Neptunian dykes, recording recurrent tectonic pulses involving local uplift and tilting. These events intermittently modified basin geometry and circulation patterns, facilitating short-term oxygenation episodes, carbonate and phosphogenic production, and localized hydrothermal activity associated with polymetallic mineralization. The AOAE started following renewed transgression after the ‘Hawke Bay unconformity’, and ended with a major uplift and regression recorded by the Bjørkåsholmen Formation, both likely related to Caledonian tectonic activity. The Alum Shale Formation provides an exceptional example of how persistent anoxia can be sustained in a semi-restricted epicontinental basin through the interplay of palaeogeographic confinement, episodic tectonic, hydrothermal fluxes, and water-column stratification, largely independent of globally synchronous OAEs.