SEQUENCE STRATIGRAPHY AND FACIES VARIABILITY OF THE TRIDENT MEMBER OF THE LATE DEVONIAN THREE FORKS FORMATION IN SOUTHWESTERN MONTANA

Recent studies of Late Devonian Three Forks Formation outcrops and core provide new insights into the sequence stratigraphic relationships of the poorly understood Trident Member. The Three Forks Formation occurs in the Williston Basin of the US and Canada and extends across the Montana Shelf and into the Antler foredeep. Outcrops of the Three Forks on the tectonically active shelf have more pronounced sequences than in the Williston Basin, making this the perfect location for outcrop analysis. In western Montana, the Three Forks Formation includes the basal restricted Logan Gulch Member and the upper marine Trident Member. An interval of limestone, informally referred to as the Knoll Limestone, separates the two members. The Trident Member is progressively truncated to the east by a regionally recognized unconformity that separates the Three Forks Formation from the overlying Sappington/Bakken Formation. It has a zero edge along the Central Montana Uplift to the east and the Beartooth Shelf to the southeast.

The Trident Member is herein subdivided into 3 distinct lithofacies recognized from 7 measured sections. The shallow marine to lagoonal lower Trident Dolomite (lithofacies A) is a tan silty dolomite and was deposited during a regional lowstand. The Trident Shale (lithofacies B) is comprised of dolomitic silty shale and corresponds with deposition during a regional trangression. The Upper Trident Limestone (lithofacies C) is a fossiliferous limestone and was deposited during a period of renewed regression. Stratigraphic correlations made by previous investigators show interfingering of Lithofacies A and B along a NE-SW trend in the center of the study area. Further to the east and southeast, the Trident Member is almost entirely composed of Lithofacies A, suggesting the occurrence of an undulating coastal margin located along the SW-NE extent of the basin. Unlike the lower members of the Three Forks Formation and overlying Sappington Formation, Trident deposition was driven by a larger-magnitude relative sea level rise that lessened the restrictive effects created by the active basin and allowed formation of a defined basin edge.