ABSTRACT

The Late Devonian and Early Mississippian (Famennian to Middle Tournaisian) Bakken Formation is thin and spatially continuous throughout the Williston Basin. The thinly laminated, fine grained, organic-rich mudstone of the Lower and Upper Members accumulated in a distal marine setting affected by weekly circulating bottom water. An abundance of in situ and detrital pyrite inclusions and lack of evidence of benthic faunal activity suggests that toxic concentrations of dissolved hydrogen sulfide developed in pore waters and possibly in the water column. The gray mudstone to fine sandstone deposits of the Middle Bakken Member are sub-divided into three sub-units: 1) sub-unit A, massive, burrow mottled to horizontal plane-bedded offshore mudstone; 2) sub-unit B, lenticular-bedded, wavy-bedded and flaser-bedded fine quartz sandstones overlain by trough cross-bedded and tabular cross-bedded, fine to medium quartz sandstones; and 3) sub-unit C, a second interval of massive, burrow mottled to horizontal plane-bedded offshore mudstone. Sub-units A and B represent a continuum of depositional processes in a lower to upper shoreface environment. A transition from deeper offshore to shallower shoreface followed by a return to an offshore setting is reflected by a Nereites ichnofacies followed by a Skolithos grading to Cruziana ichnofacies and the return of a Nereites ichnofacies. The Lower and Upper Bakken Members are composed of variable concentrations of quartz, orthoclase feldspar, illite, dolomite, calcite, pyrite and high concentrations of marine organic matter (up to 30% TOC). During deposition, increases in the abundance of siliciclastic minerals and organic carbon in the Lower Bakken Member suggest increased burial efficiency in bottom sediments enhanced organic matter preservation. The uniform mineralogy of the Upper Bakken Member accompanied by significant changes in organic carbon content imply increased productivity enhanced organic matter preservation by increasing the amount of organic material deposited on the sea floor. While stagnant conditions, including anoxia, on the basin floor are important, conditions in the overlying water column contribute most to the highest concentrations of organic material in the Bakken Formation.