Sedimentology, facies architecture and sequence stratigraphy of the lower Bakken shale member in the Williston Basin, North Dakota, United States of America
Date
2014
Authors
Albert, Stacie, author
Egenhoff, Sven O., advisor
Wohl, Ellen E., committee member
Ҫavdar, Gamze, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Upper Devonian lower shale member of the Bakken Formation in the North Dakota portion of the Williston Basin consists predominantly of organic-rich (up to 20 wt % total organic carbon (TOC)), black siliciclastic mudstones and displays a total gamma ray (GR) log profile that commonly ranges between 200 and 900 API units. Eleven fine-grained facies, ten siliciclastic and one carbonate, are identified in the succession based on sedimentological characteristics and grouped into four distinct facies associations (FAs). FA 1 is characterized by the presence of pure radiolarites and also contains massive mudstones and abundant radiolaria-bearing mudstones. Laminated mudstones of FA 2 consist predominantly of partially- to well-laminated silt-bearing mudstones with foresets of alternating siltstone and mud-rich laminae in places. Deposits of FA 3, bioturbated mudstones with macrofossil debris, are dominated by phosphate- and fossil-debris bearing mudstones. The silt-rich mudstones of FA 4 are composed primarily of continuous and discontinuous siltstone laminae. The lower Bakken shale member is subdivided into four stratigraphic units termed Interval 1-4. Interval 1 is located at the bottom of the succession and is dominated by abundant bioturbation, fossil debris and lag deposits. The overlying Interval 2 is characterized by intercalated radiolaria-bearing mudstones with silt-bearing massive to faintly- and well-laminated mudstones. Stratigraphic Interval 3 is located just above Interval 2 and is comprised mainly of massive mudstones and abundant radiolaria-bearing mudstones that commonly coarsen-upward into siltstone laminae and thin beds near the base of the overlying Interval 4. The uppermost Interval 4 consists of siltstone laminae or lag deposits at its base that generally fine-upward into massive mudstones. Each of the four FAs recognized in the lower Bakken shale member are interpreted to represent a distinct position along a proximal to distal offshore ramp transect in this mudstone system. FA 4 rocks were deposited in the most proximal setting, as they are characterized by indicators of high-energy conditions in mudstones, such as phosphate and fossil clasts, as well as erosional bases. Basinwards of FA 4, mudstones of FA 3 and FA 2 are slightly finer-grained than the most proximal deposits and reflect an overall 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 interpreted as caused by events, lag deposits and clay clasts. Suspension settling processes are responsible for depositing some of the lower Bakken shale member facies, such as the radiolarites (facies 1) and sub-mm laminae of clay-rich mudstones (facies 3). Medium-scale parasequences in the lower Bakken shale member, represented by stratigraphic Intervals 1-4 in this study, can be traced laterally through the basin supporting a sequence stratigraphic interpretation. The lower half of the succession, Intervals 1 and 2, are interpreted to represent deposits of a transgressive systems tract (TST), characterized by massive and radiolaria-mudstones overlying silt-rich mudstones and heavily bioturbated, fossil-bearing mudstones. Interval 3 likely represents deposits of an early highstand systems tract (HST), as evidenced by rare high-energy event laminae and abundant massive and radiolaria-bearing mudstone laminae. Interval 4 possibly records a late highstand and in places progrades, but, in general, it displays a fining-upward trend that may represent a short-term transgressive pulse. 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.