The stratigraphy of the Siktefjellet and Red Bay groups of northwestern Spitsbergen is revised in the light of new fieldwork. An early Early Devonian (Lochkovian) age is indicated by the rich fossil vertebrate faunas of the Red Bay Group. The underlying Siktefjellet Group is likely to be Late Silurian in age, although there is no direct fossil evidence of this. The most abundant sediment type in the two groups is fine-grained fluvial sandstone, although conglomerates and lacustrine siltstones also occur. The conglomerates that predominate in the lowest parts of both groups imply the presence of fault scarps at the surface. There has been much debate about the importance of strike-slip faulting in the area, particularly in relation to the major north–south trending Raudfjorden and Breibogen faults. Between these faults, the Biskayerhuken–Holtedahlfonna terrane contains a zone for which the deliberately genetic name ‘Siktefjellet Strike-slip Zone’ is proposed. This zone is characterized by evidence for a succession of events that, taken together, suggest a history of continuing, repeated strike-slip. These events were: (1) the tectonism and metamorphism of the basement (Krossfjorden Group) and the emplacement of the high-grade Richarddalen Complex, (2) development of crush zones in the basement, (3) deposition of the Siktefjellet Group in two distinct basins (perhaps of pull-apart origin), (4) deposition of the Red Bay Group (first as discrete scarp-related conglomerates, and then as part of a more stable fluvial basin), (5) localized compressional deformation of the Siktefjellet and Red Bay groups.