The Early Palaeozoic platform sediments in the foreland of the the Scandinavian Caledonides have generally been removed by erosion during postorogenic uplift of Fennoscandia during the formation of the Baltic or Fennoscandian Shield. An exception to the general picture is found in the Oslo Graben, where the Lower Palaeozoic sediments are preserved in a downfaulted position associated with the Permian graben tectonics. The Cambro Silurian of the Oslo region belongs to the Osen Røa nappe situated at the base of the lower allochton of the Caledonian nappe complexes (Bockelie & Nystuen 1985; Morley 1986; Bruton et al. 2010).
The basal decollement detachment surface forms a flat in the underlying autochton and parautochton of the Middle Cambrian Alum Shale deposited above the Precambrian crystalline basement. The Caledonian foreland deformation can be traced through the platform sediments to the southern Oslo Graben, where the orogenic deformation dies out
in the Skien-Langesund area.
The thrusted and folded, lithostratigraphical succession of the Oslo Region consists of Lower Palaeozoic epicontinental platform sediments deposited on the Baltoscandian margin of Baltica. The platform sediments were deposited on the sub-Cambrian peneplain that developed subsequent to the Sveconorwegian orogeny at 1100-900 Ma and intrusion of postorogenic granites at around 830 Ma (Graversen 1984; Graversen & Pedersen 1999; Berthelsen et al. 1996).
Thrusting and folding of the Lower Palaeozoic succession culminated during the late Caledonian, Scandian orogeny in Late Silurian time. During the evolution of the Oslo Graben, the folded, Lower Palaeozoic foreland succession was covered by thick plateau lavas and a thin cover of sedimentary rocks, and intruded by central volcanoes and batholiths during Late Carboniferous and Permian time. The Palaeozoic sedimentary rocks and the Permian lavas were stripped off from the Precambrian crystalline basement outcropping to the west and east of the graben during a later period of uplift and erosion.