Seismic stratigraphy and tectonics offshore Scania, south-western Baltic Sea

Abstract: Geophysical data and descriptions of deep wells, produced during hydrocarbon explorations in the south-western part of the Baltic Sea offshore Scania, have revealed new information on the geological development of the area. This investigation is concentrated to the offshore triangular segment between the northwest-southeast Permian Sorgenfrei-Tornquist Zone through Scania and the east-west Caledonian Front north of Rügen.The investigated area is traversed by complex tectonic structures of different genesis. In the southern part of the Swedish offshore area, the westnorthwest-eastsoutheast directed low angle thrust fault of the Caledonian Deformation Front, is expressed along the Rømø-Møn Fault Zone. Listric and wrench faults are present in the Swedish and Danish areas, which are caused by alternating extensional and compressional tectonic regimes during different periods.The Caledonian, Variscan and Kimmerian events are presented in three-dimensional maps, based on stratigraphically determined seismic marker layers. A total of 9 marker layers in the sedimentary bedrock are interpreted from multichannel seismic data. They are as far as possible in this structurally complex area, assigned to their chronostratigraphical levels on the basis of well logs. The basal Unit-1 on top of the Precambrian crystalline basement, includes a thin but rather complete Lower and Middle Cambrian series, starting with the Hardeberga quartzitic sandstone. Unit-2 includes the Upper Cambrian with its black Alum shale. Unit-3 is a thin pyritic Lower Ordovician unit with a seismically undifferentiated lithology. Unit-4 consists of Lower Silurian dark shales. The upper parts of the Silurian and the entire Devonian are eroded throughout the investigated area, as verified in the deep wells. Unit-5 consists of Lower Carboniferous which increases in thickness towards the eastern part of the investigated area. It is present north-west of the Fakse Bay - Møn Swell but is not found in the Falsterborev-1 well offshore Scania. Unit-6, the Permian, is found only within the Danish area with a reduced thickness northwards from the Fakse Bay-Møn-Swell. A thin, but presumably rather complete, Triassic series is present in both the Swedish and Danish areas, but only Keuper which forms Unit-7 could be fully evaluated from the present seismic profiles. This unit is followed by uniform almost horizontal Jurassic deposits which form Unit-8. The uppermost Unit-9 consists of a thin sandstone deposit from the Cenomanian of the Upper Cretaceous. Although it forms an excellent reservoir rock, a suitable cap-rock is not present.A deep seismic profile in the Danish offshore area demonstrates the Caledonian Deformation Front as a low angle Ordovician-Silurian transpressional thrust fault, causing different depositional events between the southern part of the Fennoscandian Border Zone and the northern part of the North German-Danish zone. The Lower Palaeozoic reveals the steep south to south-west dipping top of the Precambrian crystalline basement along the northeast-southwest directed Agricola-Svedala fault. The changing tectonic styles of the investigated area, its shear zones, compressional and tensional regimes, and fault movements have been observed in the Mesozoic as well as in the Palaeozoic sequences.The northwest-southeast trend of the Carlsberg fault has been confirmed in the offshore area. Seismic interpretation in the Danish area shows the southward dipping pre-Permian Ringkøbing-Fyn High, which in the Møn area is covered on Lower Carboniferous deposits. Syn-rifting during the Early Carboniferous Visean age created an unconformity in the Swedish area forming two large scale grabens, whereas depression in the Danish area led to the deposition of Mesozoic deposits starting with a thin Triassic Keuper formation in a number of grabens.