A new view of the Munk Marl bed in the Danish Central Graben (Upper Hauterivian to Lower Barremian)

The lower Barremian Munk Marl Bed (MMB) is a key marker bed in the Danish Central Graben, North Sea, typically defined by organic-rich laminated fine-grained sediments bounded by bioturbated organic-poor mudstones, marlstones or marly chalks. The Deep Adda-1 well in the Danish Central Graben, comprise an upper Hauterivian – lower Barremian core in the Tuxen Formation spanning the lower boundary of the laminated organic-rich Munk Marl Bed (MMB). The core provides a record of mid-Cretaceous sedimentation on the eastern flank of the intrabasinal Adda–Tyra inversion high. Multidisciplinary sedimentological–biostratigraphic–palaeoecological data document an abrupt environmental shift at the boundary accompanying the shift from chalk to marl sedimentation. The upper Hauterivian – lowermost Barremian chalk and marly chalk (nannozones BC 10 – lowermost BC 14) represents a well-ventilated, current-swept setting supporting a diverse nektonic and benthic fauna (bivalves, brachiopods, bryozoans, crinoids, sponges, belemnite and asterozoans), ‘typical’ chalk trace fossils 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 BC 14) 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 dominant trends in the nannofloral dataset reflect a temporal shift in upper water mass conditions between the Hauterivian and the Barremian that was initiated prior to the onset of deoxygenation of the sea floor.
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 late early Barremian oxygen depletion in concert with additional potential forcing factors such as coeval volcanism and watermass warming. The latter is indicated by the nannofossil assemblage. There is no nannofloral evidence that the stratification and oxygen depletion experienced at the sea floor imposed on the activities in the photic zone
The research leading to these results has received funding from the Danish Offshore Technology Centre (previously Danish Hydrocarbon Research and Technology Centre) under the Tight Reservoir Development program.