Abstract In NW Spitsbergen, the infill of a large Old Red Sandstone (ORS) basin was affected by the Svalbardian deformation shortly after the sedimentation of the uppermost ORS units. In the Billefjorden area, along the eastern margin of the basin, folded and thrust-faulted Devonian deposits are unconformably overlain by undeformed Carboniferous clastic sediments and platform carbonate deposits. To re-examine the age of the Svalbardian deformation, samples for palynological investigations were taken from the youngest deformed ORS strata and the oldest post-Svalbardian sediments. The results of palynological investigations show that the folded and thrust-faulted uppermost ORS unit, the Plantekløfta member of the Mimerdalen Formation (Andrée Land Group), is Late Famennian in age. The lowermost undeformed and unconformably overlying post-ORS unit, the Triungen member of the Hørbyebreen Formation (Billefjorden Group), is ?Late Tournaisian to Viséan but not Famennian in age. Thus, the compressional west-directed folding and thrusting of the Svalbardian deformation took place after Late Famennian and before Late Tournaisian time.

Abstract In NW Spitsbergen, the Late Silurian to Late Devonian infill of an Old Red Sandstone (ORS) basin was affected by west-vergent folding and west-directed thrusting during the Early Carboniferous (Tournaisian) Svalbardian deformation. The brittle, predominantly compressional structures of the Svalbardian Fold-and-Thrust Belt are concentrated along at least five narrow, more or less north–south-trending zones. Three zones are exposed in the Devonian infill of the ORS basin. The involvement of the post-Caledonian ?Late Silurian to Earliest Devonian Viggobreen weathering zone and deposits Early Devonian in two thrust zones within the basement of the western basin margin indicates that the Svalbardian deformation also affected the basement areas along the west coast of NW Spitsbergen. Structures of the Svalbardian Fold-and-Thrust Belt are exposed within an area at least 100 km wide between the Billefjorden Fault Zone in the east and the west coast of NW Spitsbergen. Therefore, the Svalbardian deformation represents a much more important fold belt than previously recognized. On the basis of the timing, the large extent and the orientation of the fold-and-thrust zones, the Svalbardian Fold-and-Thrust Belt appears to represent the eastern continuation of the Ellesmerian Fold Belt in North Greenland and the Canadian Arctic Archipelago.