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Norwegian Journal of Geology





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Normal-sense movements along two major strands of the Billefjorden Fault Zone controlled sedimentation in the Carboniferous Billefjorden Trough. The Billefjorden Trough is a more than 2000 meters thick, west-dipping half-graben basin where accommodation was provided by combinations of extensional faulting and folding throughout the basin history. Whereas previous workers have interpreted several of these folds as due to Tertiary contraction, we argue that they developed during rifting, as extensional forced folds in the style described by other workers from rifts such as the Gulf of Suez. The present basin geometry indicates that accommodation was facilitated by a combination of fault relief and fault-related folds such as extensional fault-propagation monoclines, longitudinal rollover folds and a broad syncline that developed in the hanging wall of the master faults. Major fault-tip monoclines with sharp hinges and steep fold-limbs were allowed to develop in the basin as it became dominated by thick sabhka deposits. Some of the monoclines can be traced laterally into intra-basinal half-grabens where we document significant growth of the hanging wall stratigraphy. The basin shows a close link between structural style and the distribution of main facies associations, confirming a tectonic control on the main depositional systems. Three major alluvial fan complexes formed the main fairways for clastic supply into the basin; of which two were associated with relay ramps. The alluvial fan systems mostly kept up with subsidence or prograded far into the basin. This restricted the extent of marine incursions, and resulted in the development of isolated playas. Initial floodplain deposition in the gently subsiding rift-basin centre was followed by marine flooding events localized mainly to the hinges of longitudinal folds and to relay zones. The gently dipping fold flanks caused tidal flats to be developed in lenticular depocentres. As the main segments of the BFZ linked during strain localization, the western basin-margin monocline was likely breached, triggering rapid subsidence with marine flooding and a change to a westward thickening, wedge-shaped depocenter. Subsequently, a gentle relief was gradually re-established in the basin during renewed growth of fault-monoclines. Finally, the entire basin became submerged, with fault-monoclines being reactivated. Causes for this late subsidence include eustacy, thermal relaxation, differential compaction of the basin fill, or faulting driven by regional extension.

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