Ramså basin, northern Norway: an integrated study

Abstract

The Ramså Basin in the northeast of the island of Andøya combines both a Mesozoic sedimentary succession and a subjacent deeply weathered basement. The basin must be considered as the only easily accessible analogue for the setting and development of the offshore Mesozoic basins. The aim of this study was first of all to study deep weathering and gain additional knowledge about weathering processes and the settings of weathering products underneath sedimentary strata potentially underlying the sedimentary basins on the Norwegian shelf and to develop a methodology to image these deep weathering remains with geophysics. Secondly, the delineation of structural geometries, depth-to-basement and fracture and fault systems of the Ramså Basin and the adjacent Andfjorden can provide important information about the tectonic evolution of the region and the development of the margin as an important link between the south-western Barents Sea and the Norwegian Sea. For this purpose, airborne and ground geophysical data were acquired for a comprehensive onshore-offshore study. Airborne gravity, magnetic and EM data were used to delineate the Ramså Basin extension and identified different basement domains and tectonic elements onshore and for the adjacent Andfjorden as well as additional sub-basins or a basin extension south of the Ramså Basin. Onshore and offshore seismic data secured structural information, which were interpreted jointly with the airborne data to derive a tectonic evolution model for the area. Bedrock mapping onshore Andøya, as part of the MINN project, and four new core drillings contributed to an updated bedrock map for northern Andøya. Geochemical and mineralogical analyses of samples from new and existing core material and from the surface revealed the weathering to be much less extensive than expected and disclosed a hardly observable weathering type, which to a large extent resemble the petrophysical parameters of compacted sediments or even basement.