PALYNOLOGY AND SEDIMENTOLOGY/STRATIGRAPHY OF THE BAKKEN FORMATION, NORTH DAKOTA

Palynological and sedimentological analysis of the Bakken Formation in North Dakota has shed new light on the chronostratigraphic relationships and depositional history of the unit. A detailed palynological analysis of core samples examined palynomacerals and palynomorphs (algal cysts, acritarchs, and spores) and focused on all three members of the Bakken Formation (Lower, Middle, and Upper). Broadly, the lower and upper members are dominated by algal cysts (probably prasinophyte green alga) with a low diversity suite of acritarchs. Palynomacerals consist of structureless organic matter (SOM), most likely of marine origin, although some are terrestrial and highly degraded. The Middle Bakken samples yielded a mixed assemblage of spores and acritarchs with minor occurrences of algal cysts. Land plant derived palynomacerals dominate the Middle Bakken Member.

Age diagnostic taxa are rare but the key Late Devonian spore, Retispora lepidophyta, appears consistently in the Middle Bakken Member and is used to define the Devonian-Mississippian boundary, a significant sequence boundary within the Middle Bakken (lower and upper Middle Bakken contact). Distinct distribution patterns in acritarchs may prove to be of chronostratigraphic value. There is broad similarity between this study and other palynological studies undertaken in North America but examination of more wells will be required to clarify these relationships.

Three major sequence boundaries were identified in the cores and at each there are changes in the palynology. The contact of the Lower Bakken with the Middle Bakken is marked by an intensely burrowed sandy siltstone that contains abundant acritarchs. Much of the lower Middle Bakken was deposited in a shoreface setting and contains marine acritarchs throughout. This section shoals up to a sequence boundary at the lower Middle Bakken/upper Middle Bakken contact with a decrease up-section in acritarchs and the consistent presence of spores. The overlying upper Middle Bakken accumulated in tidally-influenced to storm-dominated environments, containing only black phytoclasts indicative of a high energy environment. The contact of the upper Middle Bakken with the Upper Bakken is marked by a scoured surface capped by a phosphatic sand lag and contains abundant SOM and increased numbers of algal cysts.