Slump Structures in the Middle Bakken Member, Williston Basin, North Dakota – In Search of Active Synsedimentary Fault Systems

Abstract

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 different stratigraphic levels of the middle Bakken succession. They display a distinct suite of deformed, extensional and compressional structures, as well as entirely disintegrated units. The goal of the present study is to assess the location 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. A preliminary data set shows that the slumps do follow known fault systems in the basin. Nevertheless, the thickness of slumps as well as the facies allow for locating the structures more precisely as it is assumed that slumps are thickest close to the fault in a down-dip direction. In order to assess fault activity, sequence stratigraphic surfaces such as parasequences boundaries have been 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, often obliterating parasequences contacts. Other faults, in contrast, exhibit only minor activity and are characterized by only a few minor slumps throughout the succession.

With normal faults, the succession is thinner on the footwall than on the hangingwall. 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.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018