SEQUENCE STRATIGRAPHIC ANALYSIS OF THE UPPER TYLER FORMATION (MORROWAN; LOWER PENNSYLVANIAN) IN WESTERN NORTH DAKOTA

The Tyler Formation (Upper Carboniferous: Morrowan) occurs in the subsurface of the Williston Basin of Western North Dakota. Previous work by this group has focused on both the lower Tyler and entire unit as whole. This study focuses on the Upper Tyler and the transition zone between the upper and lower units. The increased understanding of the Tyler Formation in terms of temporal and spatial relationships has led to improved insights into climate change during ice house conditions.

The Tyler contains meter-scale cycles of repeating lithofacies, which can be used to create a sequence stratigraphic framework. Examining core samples and well logs of the Tyler along multiple transects from the basin center to the edges has shown cyclical lithofacies patterns repeating throughout the formation. The use of XRD analysis along with gamma ray markers solidifies the correlation of cycles in the Lower Tyler across much of the basin. A weakly incised valley fill has destroyed much of the lithofacies along the basin edge creating some uncertainty in correlating the Lower Tyler to the basin. Correlation of the Upper Tyler is currently only accomplished using gamma ray markers, although cores containing the Upper Tyler are less frequent, which adds uncertainty to the correlation across the entire basin. Cycles in the lower part of the Tyler are relatively thick (7 to 20 ft.) and begin end with fossiliferous marine shale overlying a paleosol (ancient soil). The cycles in the upper part are thin (3 to 6 feet) and have more limestone and evaporate minerals.

Clay analysis has shown increasing amounts of Kaolinite moving up section in the Lower Tyler. Currently, we interpret environments of the Lower Tyler to be changing from a relatively direr climate to relatively more humid climate up section. The thinner cycles along with red beds and evaporites in the Upper Tyler suggest a relatively drier climate than the Lower Tyler. It can be interpreted that long term climate change resulted in the cyclic lithofacies in this unit. Continued stratigraphic framework will provide refined insight into the climatic conditions of the Carboniferous in the Williston Basin.