The mid-Carboniferous Billefjorden Trough, typical of the narrow, asymmetric, apparently en echelon, Bashkirian rift basins on Spitsbergen and Bjørnøya, is filled with red-bed clastics, evaporites and carbonates. The synrift succession has an internal organization dominated by an unconformity-bounded, transgressive-to-regressive stratigraphic megasequence, some 700 m thick. This megasequence is constructed from repeated, smaller scale (up to 40 m thick) sequences which have boundaries that can be traced across the basin despite rapid lateral facies changes from basin margin to basin axis. Individual sequences probably betray short-term (c. l Ma) episodes of asymmetric subsidence, with initially rapid then slower rates of base level (relative sea level) rise during extension. Offset-stacking of sequences, initially back-stepping then forward-(basinward)-stepping, created a 20 Ma megasequence which records the long-term subsidence pattern related to Mid-Carboniferous rifting on Svalbard.

E. P. Johannessen, PO Box 300, 4001 Stavanger, Norway; R. J. Steet, Geo/ogica/ lnstitute, University of Bergen, 5007 Bergen, Norway.