Upper Mississippian-Lower Pennsylvanian Stratigraphy of Central Montana and Williston Basin

Abstract

Tyler formation is the name of the rock sequence between the black shales of the Heath formation and the carbonates of the Amsden formation (restricted). The Tyler formation is divided into two members, and the division is made at the base of the A zone, which consists of the Bear Gulch limestone tongue and its sand equivalent. The upper member is called the Cameron Creek member. The Alaska Bench limestone is considered a member of the Amsden formation (restricted). The sand map of the Tyler formation shows what is believed to have been a normal marine to restricted beach-offshore bar type of depositional environment. This is true both in central Montana and Williston Basin, but the erosional unconformity at the base of the Tyler formation is evident only in central Mon ana. The sands in the lower member of the Tyler formation in central Montana are called the Sumatra sands. The upper sand in the Tyler-Heath of Williston Basin is termed the Fryburg sand, and the middle sand is the Fritz sand. The age of the Heath formation is believed to be Late Chester, whereas the overlying Tyler formation is Early Pennsylvanian (Morrow). The Amsden formation (restricted) is dated as Morrow-Atoka.