PROXIMAL BAKKEN FACIES – FLOODING THE WILLISTON BASIN IN THE LATE DEVONIAN-EARLY MISSISSIPPIAN, NORTH DAKOTA, USA

The Bakken Formation in the Williston Basin of North Dakota typically shows a four-fold subdivision: a mixed siliciclastic-carbonate middle member is sandwiched between two black shale units, the upper and lower Bakken members, and underlain by a discontinuous sandy to muddy unit, the Pronghorn member. Mostly siliciclastic, the Bakken locally shows carbonates. However, cores from the southern margin of the Bakken subcrop are different from classical Bakken cores: (1) they often lack a lower Bakken shale but may show shale clasts at the Three Forks-Bakken transition; (2) the Pronghorn member, if present, thickens and becomes sandy towards the margin; (3) the lower half of the middle Bakken contains mud-rich carbonates; and (4) the mud-rich carbonates exhibit pink and green colors, typical for the Three Forks Formation, but its textures and ichnofacies clearly separate it from this underlying unit.

The shale clasts are interpreted as reworked lower Bakken shale which was largely removed by the transgression that deposited the middle Bakken member. The scattered presence of the Pronghorn probably reflects local controls, likely pre-existing relief during the initial Bakken transgression. The carbonates in the basal middle Bakken probably show transgressive conditions and may therefore be time-equivalent to a thin carbonate lag at the base of the middle member which is present further distally. Only the overlying siliciclastic units correspond to the regression mirrored by the bulk of classical middle Bakken. The pink and green colors of much of the marginal middle Bakken could result from partly reworking Three Forks sediments, and/or sedimentary conditions similar to Three Forks times. The trace fossils, however, reflect hospitable living conditions unlike during Three Forks deposition.

The facies distribution shows that the Williston basin was flooded in steps, and flooding occurred on an uneven surface explaining the patches of transgressive facies. Towards the margin of this intracratonic basin, carbonates become prominent and in places substitute most of the classical middle Bakken facies. Erosion and non-deposition of siliciclastics play a major role in shaping the marginal Bakken facies and refute the idea of a simple layer-cake architecture of this major hydrocarbon-rich system.