Late Cretaceous Tectonic Evolution of the Sorgenfrei–Tornquist Zone (STZ): Deposition and Palaeoceanographic Implications Based on Reflection Seismic Data

We present a structural and depositional interpretation in the southeasternmost Sorgenfrei–Tornquist Zone (STZ), one of the most prominent Late Cretaceous compressional inversion structures in Northern Europe. Detailed stratigraphic analysis of seismic facies and well data shows that the spatial and temporal variability of gravitational deposits, contourite drifts, and moats in the marginal troughs are related to the polyphase inversion tectonic history and the associated palaeoceanographic changes. The Hanö Bay and Bornholm Basin contain a sand-rich mounded depositional feature proximal to the STZ. This unit is resolved with high vertical resolution in seismic data and represents a clear example of a siliciclastic-carbonate mixed depositional system, where deposition was controlled by the interplay between inversion events and eustatic sea-level changes. Following the progradational and aggradational deposition during an early inversion phase and tectonic quiescence, a notable back-stepping pattern is observed in the upper Santonian–lower Campanian. The increased accommodation space outpaced sediment infill during eustatic sea-level rise in the late Santonian. We interpret that the marginal trough subsided during multiple inversion pulses associated with elastic flexure in response to inversion tectonics. The comparison of sequence-stratigraphic indicators and the global (eustatic) sea-level curve allows for a refined reconstruction of the inversion history and points to a major uplift in the Santonian–Campanian. Further, we attribute a penecontemporaneous change in the depositional pattern, i.e., the erosional Campanian–Maastrichtian contourite moat system, to intensified bottom current activity related to significant global cooling, in conjunction with the palaeoceanographic modification induced by the inversion tectonics described in this study.