SLUMP STRUCTURES AS A TOOL TO RECOGNIZE SYNSEDIMENTARY FAULT ACTIVITY IN THE MIDDLE MEMBER OF THE BAKKEN FORMATION, WILLISTON BASIN, NORTH DAKOTA

The middle member of the Bakken Formation in North Dakota is a carbonate-siliciclastic unit that is intercalated between two black shales, the lower and upper Bakken members. It consists of mostly siliciclastic sediments with local minor carbonate intercalations. Although never described in detail, slump structures are very characteristic for the middle Bakken member and occur in several stratigraphic levels of the middle Bakken succession. They display a distinct suite of deformed extensional and compressional structures, as well as entirely disintegrated units. This study shows the distribution of the slumps, their thickness, the vertical succession of structures present, as well as their relation to the thickness changes in the middle Bakken member.

The slumps reflect synsedimentary movements, e.g. along faults that were active during sedimentation. The study shows that the slumps follow known fault systems in the basin. Nevertheless, the net thickness of slumps allows for locating the structures more precisely as it is assumed that slumped units are thicker close to a fault. In order to assess fault activity, sequence stratigraphic surfaces such as parasequence boundaries are used to establish timelines within the middle Bakken member. Based on these timelines, the investigated fault systems show strongly varying amounts of synsedimentary activity: very active fault systems have slumps occurring throughout the succession. Other faults, in contrast, exhibit minor to no activity and are characterized by only a few slumps throughout the succession.

Areas accumulating slumps correspond to down-dip positions, or depositional troughs. The thickness of the middle Bakken member is, therefore, also a reflection of synsedimentary sediment movement, and the slump facies is limited to the downthrown side of the fault. Nevertheless, potential strike-slip faults do not show thickness variations mirroring uplifted and downthrown sides and may be exclusively detectable by such slumps. The presence of slumps shows that the middle Bakken member was characterized by high synsedimentary tectonic activity and therefore indicates that intracratonic basins are not as tectonically passive as previously thought.