The Mesoproterozoic Svinsaga Formation (SF) in central Telemark, South Norway, was unconformably deposited after ca. 1.347 Ga on the quartz arenite of the upper member of the Brattefjell Formation (UB), which is part of the Vindeggen Group. The unconformity is marked by a fracture zone a few meters thick developed in the UB quartz arenite. The lower part of the fracture zone, which contains sparse and closed fractures, and microfractures, grades upwards into a system of wider and mostly random oriented fractures and fracture patterns. The fractures are filled with mudstone. The fracture zone is overlain by several meters of in situ SF breccia, in which the fragments consist solely of the UB quartz arenite. The in situ breccia gradually grades into a basal breccia and clast- or matrix-supported conglomerates. The SF conglomerate beds contain solitary quartz arenite fragments and blocks up to 9.0 m3 that were derived from the UB. The fractures, fracture patterns, and microfractures in the UB quartz arenite and shattered quartz arenite fragments in the in situ SF breccia are ascribed to in situ fracturing, brecciation, and the accumulation of slightly moved blocks in a cold climate. The characteristics of the fractures and fracture patterns indicate rapid freezing and volumetric expansion. Because the paleotopography is not known, the origin of the breccia zone cannot be reliably established, but it most likely represents an ancient debris-mantled slope or blockslope accumulation. Some of the fragments in the breccia zone are slightly rotated and tilted, probably due to periglacial frost-heaving and mass-wasting processes. Chemical solution features around quartz arenite clasts are attributed to cold-climate chemical weathering. Observations suggest that the contact zone between the UB and the SF was affected by cryogenic weathering in a periglacial environment during the Mesoproterozoic. The block accumulation affected sedimentation patterns of the overlying SF, which was deposited by later alluvial processes. This study supports the idea that the unconformities and associated deposits record valuable information about the paleoclimate and paleosedimentology of the time.