ABSTRACT

Recent biostratigraphic and lithostratigraphic work indicates placement of the Mississippian-Pennsylvanian boundary of central Montana at the contact between the Amsden and Alaska Bench Formations. Fergus Group is introduced to include related Upper Chesterian formations.

An east-west trending rifted basin dominated Mississippian seaways and sedimentation. The rifted basin formed a separate ecosystem that controlled restricted marine and brackish faunas of the central Montana trough. These faunas are unlike other North American Mississippian open marine faunas but do mimic the restricted marine Upper Mississippian faunal assemblage of the aulacogen generated Russian Moscow Basin.

The central Montana Basin extends westward from the Williston Basin near Glendive to the Montana overthrust belt. It is bounded on the north by the Cat Creek-Weldon fault zone and on the south by the Forsyth-Devils Basin fault zone. The basin is about 400 mi long by about 60 mi wide (665 by 100 km). During Late Mississippian this major depression contained alluvial systems during low sea level stands and estuarine to restricted marine systems during high stands. Sea level stands were eustatic in response to continental glaciation near Gondwanaland's south pole.

Central Montana Mississippian Chesterian formations include those of the Big Snowy Group (Kibbey, Otter and Heath), and the Fergus Group (Lower Tyler, Bear Gulch,Upper Tyler, Cameron Creek, and Alaska Bench). The faunas in these formations are sparse and restricted. Absent are open marine cephalopods, thick shelled brachiopods, crinoids, corals, foraminifera and sponges. Prevalent forms include an estuarine assemblage of thin shelled brachiopods, stromatolites, pelecypods, ostracods, shrimp, conodonts, gastrapods and fish. Faunal studies indicate the entire section to be Chesterian, the top of the Mississippian occurring at the top of the Alaska Bench Formation.