Upper Triassic–Lower Cretaceous seismic sequence stratigraphy and basin tectonics at Bornholm, Denmark, Tornquist Zone, NW Europe

Abstract

An integrated analysis of seismic sequence stratigraphy and structural evolution of a dynamic intraplate fault basin was conducted along a segment of the Tornquist Zone at the boundary between the Northwest European Craton and the Baltic Shield-East European Platform. High-quality regional seismic-reflection lines document the Late Triassic through early Late Cretaceous tectonic history and depositional sequence architecture. The stratigraphic package was divided into three major transgressive/regressive cycles (1st-order sub-cycles). The upper Upper Triassic through Lower Cretaceous siliciclastic cycles I and II are divided into 2nd-order transgressive/regressive facies cycles composed of higher frequency 3rd-order sequences. The lowstand basin fill comprises stacked successions of lowstand systems tracts produced by relative sea-level rises and falls. During the 3rd-order regressive intervals highstand-dominated depositional conditions prevailed to the northwest while lowstand-dominated conditions occurred to the southeast. Cycle III records an Upper Cretaceous major siliciclastic–chalk cycle. The magnitude of the relative sea-level highs and lows increased up through the studied interval. A two-dimensional structural-evolution analysis is based on the variation of the sediment thickness across the fault blocks up through the sedimentary column. Basin subsidence can be divided into successive structural/tectonic regimes. Fault block rotation and structural inversion characterized the early subsidence of the basin floor. During the second interval, inversion ceased while fault-block rotation continued. In the third interval, the rotation also stopped and no deformation of the basin is recognized. The three major transgressive/regressive facies cycles and associated relative sea-level cycles are linked with the structural regimes. Contemporaneous deposition of 3rd-order highstand- and lowstand-dominated complexes related to differential subsidence and uplift is also evidence of basin tectonics as the controlling factor on relative sea-level changes. Comparison of the relative sea-level curve developed for the study area with eustatic sea-level curves shows that they follow different trends. The deviations between these curves, and the correlation of sequence boundaries with changes of the tectonic regime, support the conclusion that tectonics was a major control on relative sea-level fluctuations in the study area. The 2nd-order cycles linked with deformation episodes along the Tornqist Zone correlate with Lower-Middle Jurassic 2nd-order cycles in England. Deformation along the Tornquist Zone may thus have influenced the sequence stratigraphy across the entire Northwest European Craton.