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

This research focuses on Carboniferous rocks in North America that are commonly cyclical which result from sea level changes caused by changing climate conditions in an “Ice House” world. The Tyler Formation (Upper Carboniferious: Morrowan) occurs in the Williston Basin of Western North Dakota. The Tyler Formation contains meter-scale cycles of repeating lithofacies which can be used to create a sequence stratigraphic framework. This analysis will improve our understanding of the temporal and spatial resolution of this interval, as well as provide an understanding of the environmental and climatic significance of the nature of the cycles.

Examining core samples and well logs of the Tyler Formation along multiple transects from the basin center to the edges has shown cyclical lithofacies patterns repeating throughout the formation. While there is confidence these cycles repeat throughout the formation there is some difficulty correlating cycles from the basin depocenter to the edges because the sandstone bodies near the edge accumulated in possible incised valleys. A typical cycle within the Tyler contains calcareous mudstones overlying a brecciated carbonate, then show carbonaceous shales and interbedded mudstones and limestone, which are capped with a brecciated carbonate interval. The lower cycles suggest a more restricted or shallow marine conditions which is indicated by the presence of inarticulated brachiopods and bivalves. The middle and upper cycles indicate more open-marine conditions which is evidenced by the presence of crinoids, brachiopods, and bryozoans fossils. These changing cycles are related to sea level changes. Specifically, 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.

Continuing work on the stratigraphic framework will further aid in future studies of fossil distribution patterns of the Tyler Formation, and will continue to provide more insight into the climatic conditions of the Carboniferous in the Williston Basin which will improve the economic development of this formation.