Marine ichnogenera within Torrejonian facies (Paleocene) of the Fort Union Formation, southeastern Montana

Amherst College, Department of Geology, Amherst, MA, United States
University of North Carolina, United States
Franklin & Marshall College, United States

The trace fossils Ophiomorpha, Diplocraterion, Skolithos linearis, "Skolithos," Thalassinoides, and Planolites have been identified in Paleocene strata near Ekalaka, Montana. These ichnogenera are confined to specific lithofacies within a new stratigraphic unit, the Ekalaka Member of the Fort Union Formation. The Ekalaka Member is of early to mid-Torrejonian age, is bounded by unconformities in the vicinity of the Miles City Arch, and varies in thickness from 20 to 70 m, thickening to the east, where the lower unconformity disappears. This new member has been mapped from the vicinity of Ekalaka to the South Dakota border, a distance of 50 km. The Ekalaka Member probably correlates with the massive D-Bed and E-Bed sandstones in the Cave Hills of northwestern South Dakota; both lithostratigraphic units lie below the U ₃ unconformity. The six ichnogenera from the Ekalaka Member are all marine. However, the low numbers of individuals within each ichnogenus and the few numbers of individual ichnogenera suggest a brackish-water depositional environment. The most common ichnogenus, "Skolithos" (defined as a short, thin, irregular tube with a vertical to subvertical sand wall), has been found in the same bed as Ophiomorpha and Skolithos linearis. Diplocraterion, Planolites, and Thalassinoides are also found in Ekalaka strata. Ophiomorpha, Diplocraterion, and Skolithos linearis, although less common than "Skolithos," have been interpreted by previous workers to be euryhaline marine ichnogenera. Furthermore, the strata in which they occur show tidal reversed-ripple laminae on cross-beds and neap/spring tidal bundle laminae. Nevertheless, the attribution of marine influences on Ekalaka deposition can be more strongly argued on the basis of the same ichnogenera found in the Cave Hills sandstones, which at the same time contain marine sharks, skates, and rays.

This record provided courtesy of AGI/GeoRef.