ABSTRACT

More than 9000 wireline logs were examined from North and South Dakota and Montana, over an area from the international border to latitude 45°, and from the eastern edge of the Dakotas to longitude 105°.

In the western Dakotas, the greatest thicknesses of the Newcastle Formation, 100-160 ft (30-49 m), occur in a zone 20-30 mi (30-45 km) wide from the Canadian border to the southern limit of the study area. Cores show sandstones, mudstones and coals, and represent fluvial channel and alluvial plain deposition. Elsewhere, the formation is 10-40 ft (3-12 m) thick, and thins to zero in central North Dakota. In the eastern Dakotas, the thickest Newcastle is 250 ft (75 m) thick, and consists of marginal marine sandstones and shales.

In eastern Montana, the Newcastle is generally 40-80 ft (12-24 m) thick, although some greater thicknesses occur. Except for fluvial deposits which extend across the border from North Dakota, there are few localized thicker trends. The Newcastle consists largely of shallow marine or marginal marine deposits.

The Newcastle Formation in North Dakota and immediately surrounding areas formed as a result of the following events: After Skull Creek deposition, sea level fell and exposed a large area. A major drainage system was incised into the Skull Creek shales, and drained into the marine basin in central Montana; the incised valleys were filled by fluvial sediments. After a brief lowstand, sea level rose and redistributed a large volume of sand as 10-40 ft (3-12 m) of shallow marine and marginal marine deposits across the study area. After further sea level rise, offshore marine conditions prevailed over much of the study area, and the thick Newcastle in the eastern Dakotas was deposited. Eventually, sea level rose and coarse clastic deposition ceased; Mowry shales were deposited over the entire area.