SEQUENCE STRATIGRAPHY OF THE TYLER FORMATION (LOWER PENNSYLVANIAN/MORROWAN) IN THE WILLISTON BASIN, NORTH DAKOTA

This research is being done because Carboniferous rocks in North America are commonly cyclical (repeating lithofacies) because of changing climate conditions resulting in sea level changes during an “Icehouse” world. The Tyler Formation (Upper Carboniferous: Morrowan) occurs in the Williston Basin of Western North Dakota. It contains meter-scale cycles of repeating lithofacies. A sequence stratigraphic analysis of this formation would improve temporal and spatial resolution of this interval, as well as provide environmental and climatic significance to the nature of the cycles.

The examination of core samples and well logs of the Tyler Formation examined along multiple transects from the basin center to the edges has shown cyclical lithofacies patterns repeating throughout the formation. These cycles begin with calcareous mudstones overlying a brecciated carbonate, then show carbonaceous shales and interbedded mudstones and limestone, and are capped with a brecciated carbonate interval. Lower cycles contains fossils (inarticulate brachiopods, bivalves) that indicate more restricted or shallow marine conditions, whereas middle and upper part fossils (crinoids, brachiopods, bryozoans) indicate more open-marine conditions. The lower cycles indicate a transgressing sea, whereas the middle and upper cycles occur above a maximum flooding surface or horizon. It’s expected that the middle and upper Tyler cycles will show greater lateral basin extent than the lower cycles.

An enhanced stratigraphic framework is expected from this analysis, which will aid in future studies of fossil distribution patterns of the Tyler, may provide more insight into the climatic conditions of the Carboniferous in the Williston Basin, and improve the economic development of this formation.