OTTAR-BASIN, SW BARENTS-SEA - A MAJOR UPPER PALEOZOIC RIFT BASIN CONTAINING LARGE VOLUMES OF DEEPLY BURIED SALT

Seismic mapping and gravity modelling of the Ottar Basin - a little studied Upper Palaeozoic graben in the south-western Barents Sea - demonstrates the presence of a major rift basin with large accumulations of unmobilized salt. Buried beneath thick, flat-lying Mesozoic strata, the NE-trending fault-bounded basin is at least 170 km long, varies in width between 50 and 80 km and coincides with a negative gravity anomaly of more than -10 mgal. Seismic observations show that the south-western part is a half-graben tilted to the north-west whereas the north-eastern part appears to be more symmetric in shape. A large mass deficiency in the north-eastern part of the basin, indicated by a gravity anomaly of more than -30 mgal, makes it necessary to postulate large amounts of salt within the basin. The preferred gravity model shows a total basin depth of 9.5 km, basin relief of 4.2 km and a salt volume of 6800 km(3) corresponding to a 2.4-km-thick salt layer. Similar basin depths, but only 500-600 km(3) of salt, are indicated beneath the Samson Dome in the south-western part of the basin. Unlike salt bodies in other Barents Sea basins, the thick salt deposit in the north-eastern part of the Ottar Basin is relatively unaffected by halokinesis. Interfingering of different basin facies, lack of tectonic reactivation of the basin and a relatively late differential loading by prograding cover strata probably explain these differences in development. The large size and voluminous salt deposits establish the Ottar Basin as one of the major Barents Sea evaporite basins and an important structural component of the Upper Palaeozoic rift system.