Published data interpret the oil-productive sandstone (Muddy Formation) at the Bell Creek and Ranch Creek fields (T8S, R54E, and T9S, R53E) as marine-deltaic and barrier-bar sandstones that are facies equivalent to, or younger than, freshwater channel deposits on the east and south. Core and log studies now show the Bell Creek sandstone (an informal member of the Muddy Formation) to be removed by postdepositional erosion of an incised valley between the fields. Thus, impermeable valley-fill shale, siltstone, and sandstone, with thin coals, are younger than the adjacent Bell Creek sandstone.
The stratigraphic relationships are as follows: (1) the Bell Creek sandstone was deposited as a widespread regressive sandstone genetically related to the underlying marine Skull Creek Shale, (2) the younger incised valley between the Bell Creek and Ranch Creek fields is approximately 1 mi (3 km) wide, contains up to 30 ft (9 m) of impermeable fill, trends north-south, and probably connects with a major southwest-trending valley, 8 mi (26 km) to the south (Rocky Point field area), and (3) valley fill forms the seal along the east side of Ranch Creek, and possibly east of Bell Creek as well.
Isopach thinning of the combined interval of Skull Creek Shale and Muddy Formation indicates a northeast-trending paleostructure that may have controlled facies distribution, drainage incisement patterns, and fluid migration.
The new age interpretations can be related to a sea level highstand for deposition of the Skull Creek and overlying Bell Creek sandstone; erosion of valleys related to a lowstand; fill of the valleys during a rising sea level, followed by deposition of the marine Mowry Shale under highstand conditions.