The Depositional Environment of the Lower Bakken Shale Member in the Williston Basin, Upper Devonian, North Dakota, U.S.A.

Abstract

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, ten siliciclastic and one carbonate, 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 are 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 rather than anoxic conditions as previously suggested.

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