Middle Pennsylvanian megabreccia adjacent to the Odellfjellet Fault in Billefjorden, central Spitsbergen

The Billefjorden Trough, a well studied onshore analogue to Carboniferous rift basins in the Barents Sea, is bounded to the west by the Billefjorden Fault Zone. Structural field analysis of a megabreccia and adjacent strata of the Minkinfjellet Formation in the hanging wall of the Odellfjellet Fault segment of the Billefjorden Fault Zone suggests that slope failure occurred along the basin-bounding fault in the Middle Pennsylvanian during the “through-going fault zones” phase of extension, i.e., during decreasing tectonic activity. Structural field data acquired in the megabreccia adjacent to the Odellfjellet Fault using the scanline method show no relationship between fracture density and the presence and/or proximity to a major fault. In the early Cenozoic, mild reworking of Middle Pennsylvanian deposits, including plastic deformation within the megabreccia, reflects Eurekan strain partitioning and decoupling during reactivation of the Billefjorden Fault Zone. The study also reveals the occurrence of abundant WNW–ESE-striking faults in Middle Pennsylvanian sedimentary deposits, which probably formed parallel to the preexisting structural grain trending highly oblique to Grenvillian and Caledonian grains.