EVIDENCE FOR AN ESTUARY: THE EKALAKA MEMBER OF THE FORT UNION FM, SE MONTANA

The Paleocene Fort Union Fm near Miles City, MT comprises the Tullock, Lebo and Tongue River Members. East of the Miles City coal field the Tullock and Lebo Members correlate to the Ludlow Member where all 3 members are interpreted as meandering stream and overbank deposits within an aggrading alluvial plain. A recently recognized unit near Ekalaka, Montana is equivalent to the Lebo and upper Ludlow Members. The Ekalaka Member comprises 3 facies associations: (A) inner to central estuary tide-dominated subaqueous sand bars, (B) sandy fluvial-dominated inner estuary point bar deposits, and (C) coastal plain channel, levee, crevasse splay and floodbasin deposits. The splays of facies association C contain plant and trace fossils indicating both fresh and brackish water conditions on an estuarine shoreline. All three facies associations contain Ophiomorpha, Diplocraterion, Thalassinoides, Planolites and Skolithos, including Skolithos linearis.

The Ekalaka Member is bounded by widespread unconformities. At its base, the Ekalaka Member is 64 Ma and rests unconformably on the Ludlow Member and Hell Creek Fm. The basal unconformity probably formed due to uplift of the Miles City arch. After uplift ceased, an estuary developed and received Ekalaka Member sediments. The estuary opened southeastward into the Cannonball Sea and at its mouth thick sand deposits formed in the Cave Hills area. Fractured facies association A deposits record earthquake-triggered slump events within the estuary.

The upper unconformity occurs where the Tongue River Member rests unconformably on the Lebo, Ludlow or Ekalaka Members. This unconformity extends throughout the western Williston Basin and is marked by well-developed paleosols and paleovalley fills. It likely is due to a drop in sea level. The Ekalaka Member beneath the unconformity is about 62.5 Ma. The Tongue River Member above the unconformity is about 61 Ma, and the hiatus is about 1.5 Ma.

Marine diatom (Coscinodiscus), foram (Trochammina, Miliammina and Eggerella) and ichnofossil-bearing horizons occur in the lower 100 meters of the fluvial-deltaic Tongue River Member from Miles City to the Little Missouri River. These body and trace fossils occur interbedded with freshwater deposits, suggesting deposition in interdistributary bays in a low gradient delta formed during a marine transgression.