Date of Award
Master of Arts (MA)
The lower Tullock Formation (Paleocene), which consists of a section 5 to 30 feet thick, was studied in Slope County, southwestern North Dakota to determine its depositional environments. The lithol ogies present are sand, silt, clay and lignite. A bed of lignite or yellow silt marks the base of the Tullock Formation throughout the study area.
The silt beds are interpreted as having been deposited as natural levees because they are wedge-shaped and contain climbing ripple cross-stratification, interbedded sand lenses and lignitized paleo-raot systems. Well-drained swamp deposits are difficult to distinguish, but are probably located in the silt beds.
There are two types of clay beds which were deposited in different envirorunents. The first type of clay bed is dark-colored, rich in organic matter, silty and shaly. It is interpreted as having been deposited in a poorly-drained swamp in a fluvial floodbasin because of its organic content and interbedded relationships with lignite beds. The other type of clay bed is light-colored and contains less organic material and silt than the first type. It is interpreted as having been deposited in small lakes within a floodbasin because of its low organic content, massively bedded clay and stratigraphic position above lignite beds.
Lignite beds are interpreted as having been deposited in poorly drained swamps in fluyial floodbasins because of their wood content, lateral persistence, thickness and interbedded relationships with dark colored clay beds.
The sand beds of the lower Tullock Formation are interpreted as having been deposited as crevasse-splays because their grain size (very fine- to medium-grained) is coarser than levee deposits, and they show lateral thinning, scoured bases and small-scale ripple cross stratification. The sand beds are underlain by lignite and overlain by dark-colored clay indicating the sand was rapidly dispersed into poorly-drained swamps.
Prodelta and near- and off-shore lacustrine environments of deposition are rejected because of incorrect vertical successions of beds. Commonly clay beds rich in organic matter are overlain by lignite beds, which, in turn, are overlain by silt beds. Lacustrine deposits would show clay beds overlain by silt and sand beds, in turn overlain by lignite beds. The succession present is what would be expected for a fluvial environment.
Hickey, William K., "Depositional environments of the lower Tullock Formation (Paleocene), Slope County, southwestern North Dakota" (1973). Theses and Dissertations. 136.