The Deep Adda-1 well in the Danish Central Graben, North Sea, provides a record of mid-Cretaceous sedimentation on the eastern flank of the intrabasinal Adda–Tyra inversion high. An upper Hauterivian – lower Barremian core in the Tuxen Formation spans the lower boundary of the laminated organic-rich Munk Marl Bed (MMB), a key marker bed in North Sea Cretaceous stratigraphy. Multidisciplinary sedimentological–biostratigraphic–palaeoecological data document the abrupt environmental shift at this boundary. The upper Hauterivian – lowermost Barremian lower Tuxen Formation (nannozones BC10 – lowermost BC14), beneath the MMB, represents a well-ventilated, current-swept setting supporting a diverse benthic fauna and characterized by a condensed succession with hardgrounds, at one level defining a biostratigraphic hiatus, and stacked, thin shallowing-upward parasequences. The succeeding lower Barremian MMB (nannozone BC14) attests to poorly oxygenated bottom waters and a total lack of epi- and infauna; the calm, inhospitable sea floor was intermittently disturbed by muddy turbidity currents and debris flows. The base-MMB surface is a complex fractured hardground indicative of relative sea-level fall and protracted winnowing of the cemented sea floor. The Deep Adda-1 core thus records a sea-level excursion that accompanied the onset of early Barremian oxygen depletion in concert with additional potential forcing factors such as coeval volcanism and watermass warming.