The Hedmark Basin was formed by rifting c. 750-590 Ma ago in an area about 130-230 km NW of the
present position of the basinal sequence in the Caledonian nappe region. A pre-rift sequence may be
represented by fine-grained ftuvial sandstones. The rift episode is divided into three thermo-mechanical
phases which individually are characterized by one or several spans of structural-sedimentary evolution.
A western graben with marine turbidite sandstones and black shales was formed during the phase of
initial crustal stretching (A). The main phase of crustal stretching (B) included basin widening by an
eastward moving fault system, renewed deep fracturing with basalt extrusion, block faulting causing sub-basins and local structural inversions, and possible strike-slip movements along the Imsdalen Fault (IMF ).
Sedimentary environments included alluvial fans and plains, are as with shallow-marine sand and carbonate mud, fan deltas, sub-marine fans and basin lows with organic-rich mud. The phase of thermal cooling (C) was characterized by waning fault activity, decreasing deposition in sub-basins and a long period of slow regional subsidence. The Varangerian glaciation (c. 650 Ma) took place during this phase and was succeeded mainly by fluvial and shallow-marine siliciclastic sedimentation. The Baltoscandian rift episode was terminated by the Early Cambrian transgression.