Abstract: Depositional Environments of Eagle Sandstone, North-Central Montana--Aid for Hydrocarbon Exploration

Dudley D. Rice

Depositional environments of the Upper Cretaceous Eagle Sandstone were determined at outcrops along the Missouri River and its tributaries from the town of Virgelle to the mouth of the Judith River. In this area, the Eagle, which overlies the Telegraph Creek Formation, is divided into three members. The Telegraph Creek accumulated in an offshore-transitional environment and grades upward into the shoreface, foreshore, and eolian sandstones of the Virgelle Sandstone Member at the base of the Eagle. The Virgelle was deposited along an eastward-prograding shoreline and ranges in thickness from 80 to 130 ft (24 to 40 m). The middle member of the Eagle represents coastal-plain deposition. In the eastern part of the area, the upper part of this member contains a massive, sheet- ike, delta-front sandstone capped by a thin delta-plain unit, which indicates a second progradation during Eagle deposition. The upper member is a transgressive, delta-destructional sandstone, which marks the beginning of the extensive Claggett transgression. In the eastern part of the area, this sandstone has the attributes of a shoreface deposit. In the western part, the member was deposited on a broad tidal flat. Chert pebbles in this upper member and in the basal part of the overlying Claggett Shale are a lag deposit derived from erosion of channels on the delta plain during the transgressions. The combined thickness of the middle and upper members of the Eagle is as much as 180 ft (55 m).

These depositional units and associated environments of the Eagle Sandstone can be identified in nearby wells. This information on depositional environments, combined with the determination of sandstone geometry from well logs, permits the prediction of shoreline trends and the location of potential stratigraphic traps.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana