SGU Research Paper C 832 Research Paper C 832 Intra-cratonic dextral transtension and inversion of the southern Kattegat on the south- west margin of Baltica – Seismostratigraphy and structural development Mikael Erlström & Ulf Sivhed Intra-cratonic dextral transtension and inversion of the southern Kattegat... Research Papers C 832 Intra-cratonic dextral transtension and inversion of the southern Kattegat on the southwest margin of Baltica – Seismostratigraphy and structural development Mikael Erlström & Ulf Sivhed Sveriges Geologiska Undersökning 2001 ISSN 1103-3371 ISBN 91-7158-657-1 Cover: View of Kullen and Skälderviken Bay from Torekov. Photo M. Erlström. © Sveriges Geologiska Undersökning Geological Survey of Sweden Layout: Agneta Ek, SGU Print: Elanders Tofters, Östervåla 2001 CONTENTS Abstract ............................................................................................................................................................. 4 Introduction ...................................................................................................................................................... 4 Geological setting .............................................................................................................................................. 6 Used data set ..................................................................................................................................................... 8 Reflection seismic data ............................................................................................................................... 8 Well data .................................................................................................................................................... 8 Bouguer gravity and magnetic anomaly data ............................................................................................... 9 Onshore geology ........................................................................................................................................ 10 Lithostratigraphical representation .................................................................................................................... 11 Palaeozoic ................................................................................................................................................... 11 Mesozoic .................................................................................................................................................... 11 Seismic interpretation ........................................................................................................................................ 12 Time structure maps ................................................................................................................................... 12 Basement ............................................................................................................................................. 12 Top Carboniferous and Lower Palaeozoic ............................................................................................. 12 Top Permian ........................................................................................................................................ 12 Top Triassic .......................................................................................................................................... 13 Isopach maps .............................................................................................................................................. 15 Geoseismic profiles ..................................................................................................................................... 16 Bouguer gravity and magnetic anomaly maps .................................................................................................... 22 Model of bedrock geology ................................................................................................................................. 23 Structural development ..................................................................................................................................... 25 Pre-rift ........................................................................................................................................................ 25 Syn-rift ....................................................................................................................................................... 26 Post-rift ...................................................................................................................................................... 27 Upper Permian ..................................................................................................................................... 27 Triassic ................................................................................................................................................. 27 Jurassic – Early Cretaceous ................................................................................................................... 28 Late Cretaceous – Palaeogene ............................................................................................................... 29 Neogene uplift ............................................................................................................................................ 29 Discussion ......................................................................................................................................................... 30 Conclusions ...................................................................................................................................................... 31 Acknowledgement ............................................................................................................................................. 31 References ......................................................................................................................................................... 31 ABSTRACT The geological framework of the southern part of the preted as Late Palaeozoic extension faults. This indicates Kattegat includes the regionally important Sorgenfrei- that the structural style of syn-rift sedimentation in Scania Tornquist Zone, around which regional stresses in the was similar to that in the southern Kattegat. The Scanian crust have been released since the Late Palaeozoic. The deposits, however, were removed by erosion during Late zone evolved as an intra-cratonic fault zone as a result of Cretaceous–Palaeogene inversion and uplift. The Triassic– stresses induced on the south-western margin of Baltica Early Jurassic is characterised by post-rift subsidence and during the formation of Pangaea. Stratigraphic informa- the Kattegat evolved as a marginal part of the Norwegian- tion from deep wells and high-resolution seismic surveys Danish Basin, where a thick Triassic succession was de- have made it possible to reconstruct the structural evolu- posited. During the Late Triassic–Middle Jurassic the area tion and fault geometry of a key area of the south-western experienced fault-controlled differential subsidence inside part of Baltica. Faulting during the Late Palaeozoic Varis- the Sorgenfrei-Tornquist Zone, leading to great variabil- can orogeny significantly weakened this area. Thick Rot- ity in sediment thickness and stratigraphical detail. Most liegendes clastics were deposited in rapidly subsiding half- of the Quaternary subcrop in the study area is composed grabens as a result of rifting and transtension in a dextral of Jurassic strata. The well-constrained onshore geology strike-slip regime along the major faults of the Sorgenfrei- of Scania is correlated with offshore data for the southern Tornquist Zone. These deposits disconformably overlie Kattegat. The offshore continuations of faults and rock a 3.5–4.0 km thick sequence of Cambrian, Ordovician, units are displayed, as are the extensions of the Precam- Silurian, and Carboniferous strata. Lower Palaeozoic stra- brian Hallandsås and Kullen horsts. The Late Cretaceous ta underlying the syn-rift sequence provide valuable infor- to Palaeogene inversion is clearly verified by reverse reacti- mation on the original thickness of the pre-rift deposits on vation of the major faults along the Sorgenfrei-Tornquist the margins of Baltica. The syn-rift sequence has a thick- Zone. ness of up to 2.5 km in the southern Kattegat. Wells and geophysical data verify that the rifting was accompanied Key words: Kattegat, seismostratigraphy, structural evolu- by volcanism. Aeromagnetic and Bouguer gravity data in- tion, strike-slip, extension, rifting, inversion, Sorgenfrei- dicate highly magnetic and dense rock bodies which prob- Tornquist Zone. ably correspond to intrusive magmatic rocks in the crust and basaltic layers in the Carboniferous–Permian succes- Mikael Erlström, Ulf Sivhed; Geological Survey of Sweden, sion. In Scania, the aerogeophysical data indicate the ex- Kiliansgatan 10, 223 50 Lund, Sweden, istence of north–south directed zones which are inter- e-mail: [email protected], [email protected] INTRODUCTION Reconstruction of the Phanerozoic structural evolution western Scania, the southern Baltic Sea area, the waters of the south-western part of the Baltica plate is largely surrounding Bornholm, and Hanö Bay. based on subsurface data from deep wells and geophysical Interpretations of the seismic