MUDSTONE DEPOSITION IN AN INTERIOR BASIN DURING THE LATE DEVONIAN – THE LOWER BAKKEN SHALE MEMBER IN THE WILLISTON BASIN, NORTH DAKOTA, USA

The Upper Devonian lower shale member of the Bakken Formation in North Dakota consists mostly of organic-rich (up to 20 wt % total organic carbon (TOC)), black siliciclastic mudstones. Eleven fine-grained facies are identified in the succession and grouped into four distinct facies associations (FAs): siliceous black mudstones (FA 1), laminated mudstones (FA 2), bioturbated mudstones with macrofossil debris (FA 3), and silt-rich mudstones (FA 4). Each of the four FAs is interpreted to represent a distinct position along a proximal to distal offshore transect in this mudstone system. FA 4 rocks were deposited in the most proximal setting and are characterized by high-energy indicators such as phosphate and fossil clasts, and laminae with erosional bases. Basinwards, the mudstones of FA 3 and FA 2 are slightly finer-grained than the FA 4 deposits and reflect a decrease in depositional energy. The very-fine grained massive mudstones and local radiolarites of FA 1 record deposition in the most distal and tranquil regions of the basin.

Bedload transport processes were active across all FAs and are indicated by high-energy lenticular siltstone laminae likely caused by events, lag deposits, and clay clasts. Suspension settling processes are only responsible for depositing radiolarites and sub-mm laminae of clay-rich mudstones.

Stratigraphically, the lower Bakken member is subdivided into four intervals that re laterally traceable through the basin. The lower half of the succession, Intervals 1 and 2, are interpreted to represent deposits of a transgressive systems tract (TST), whereas the overlying interval 3 likely represents deposits of an early, and interval 4 of a late highstand systems tract (HST). The presence of Phycosiphon incertum fecal strings in all FAs, along with a small number of ichnofossils and shell fragments throughout the succession, indicates that the lower Bakken shale member was likely deposited in dysoxic conditions rather than anoxic conditions as previously suggested.