Jurassic heritance of the geomorphology in Mid Norway

The age and formation of the Scandinavian mountains and the western coastal areas (the ‘strandflat’)  have long been the subject of debate. Some suggest that the present-day mountains are remains of the Caledonian orogen while others claim that the Caledonian nappes after denudation were covered by Mesozoic sediments and subsequently exhumed. We have tried to clarify these issues by studying remains of chemically weathered rocks (saprolites) along two profiles from the coast to the interior of central Norway. This multidisciplinary study includes the following data: digital topography, electrical resistivity tomography (ERT), XRD, XRF, palynological analyses and K–Ar dating of samples from outcrops, trenches and core drilling. The coastal areas are dominated by an outer ‘strandflat’ and an inner ‘joint-valley’ landscape, while the interior and mountainous areas are  characterised by smoother landscapes referred to as ‘palaeo-surfaces’. Remnants of pre–Tertiary weathering occur in the joint-valley landscape as well as on the palaeo-surfaces. The deep saprolites are found within fault- and fracture-zones and at depths exceeding 50 m in drillholes. It is suggested that the old saprolites were strongly eroded along the coast and in the fjords and valleys like Orkdalen and Sunndalen. K–Ar dating of clay from saprolites on the mainland commonly show Jurassic ages, seen along a profile that stretches from the coast to the Dovrefjell (approx. 1400 m a.s.l.).The age of the smectite- and kaolinite-containing saprolites seems to be almost contemporaneous along this profile, implying that the entire area was subject to weathering in a warm and humid climate, such as prevailed during the Late Triassic and Jurassic. Palynological remains in the clayey saprolites contain thermally altered pollen and spores from the Triassic and Jurassic, which supports the interpretation and dating of the saprolites.  It is therefore suggested that the Mesozoic landscape in central Norway was shaped by uplift and deep weathering in the Jurassic. However, saprolites occurring along a second profile south of the Trondheimsfjord show Carboniferous and Permian K-Ar ages, indicating that this area constitute a Permian or Triassic sediment basin that was eroded during the late Cenozoic. Thus, it is likely that the entire Trøndelag county was covered by Mesozoic sedimentary rocks, right up to the start of the Cenozoic erosion. Important processes that governed the shaping of the landscape were tectonic uplift and erosion throughout the Cenozoic, followed by extensive abrasion and erosion by glaciers and meltwater during Pleistocene. We therefore conclude that both the studied saprolites and the shape of the present-day landscape in central Norway are characterized by the landscape formed during the Jurassic. This includes the deep profiles of chemical weathering and a drainage pattern that changed in the Pleistocene.