STRATIGRAPHY AND AGE OF CRETACEOUS ROCKS AT MOUNT EVERTS, YELLOWSTONE NATIONAL PARK, WYOMING AND MONTANA

The National Park Service, U.S. Geological Survey, and Montana Bureau of Mines and Geology are conducting a multidisciplinary study of the Cretaceous stratigraphy, sedimentology, and paleontology of Mount Everts, a broad faulted uplift at the north end of the Yellowstone plateau along the Gardner River near Mammoth Hot Springs in Yellowstone National Park.

The western face of Mount Everts is comprised almost entirely of 4,000 ft of Cretaceous rocks, including in ascending order, the Kootenai Formation, Thermopolis Shale, Muddy Sandstone, Mowry Shale, Frontier Formation, Cody Shale, Telegraph Creek Formation, Eagle Sandstone, Everts Formation, and Landslide Creek Formation.

Results of current research include: (1) The presence of Bear River Formation fauna in rocks identified as Muddy Sandstone at Mount Everts indicates that the Muddy Sandstone of the Mount Everts area is chronostratigraphically equivalent to the Bear River Formation of southwestern Wyoming. (2) The lower part of the Mowry Shale on Mount Everts consists of pastel-colored bentonitic mudstone that is lithostratigraphically equivalent to lower Mowry beds in south-central Montana and the Vaughn Member of the Blackleaf Formation in southwestern Montana. (3) The presence of middle Cenomanian palynomorphs in the lower part of the Frontier Formation and middle Turonian ammonites in the upper part supports the presence of an unconformity within the unit. (4) Samples of the Everts Formation collected on Mount Everts yielded fossil palynomorphs indicative of a nonmarine depositional environment. Based on the species present, the age of the formation can be no older than Coniacian. An ash bed near the top of the Everts Formation just north of the Park yielded an Ar40/Ar39 radiometric date of 77.81 ± 0.31 Ma, which indicates a Campanian age. (5) Fossil palynomorphs found in the Landslide Creek Formation are indicative of a Campanian or younger age, possibly as young as Maastrichtian.