Abstract: Stratigraphy, Depositional Environments, and Lignite Resources of Fort Union Formation, West-Central North Dakota

Edward A. Rehbein

Total lignite resources of the Fort Union Formation (Paleocene) in North Dakota are estimated to be 1.3 trillion tons (1.2 Tt), or more than two times the latest published estimate of 531 billion tons (482 Gt). The strippable resource estimate is increased to between 37 and 47 billion tons (34.6 and 42.6 Gt).

The increased estimate of total resources results from the demonstrated lateral continuity of the lignite beds. Both the Harmon and Hansen lignite beds in the Tongue River Member of the Fort Union extend east-west and north-south for more than 100 mi (161 km) and cover an area exceeding 6,600 sq mi (17,087 sq km).

Three members of the Fort Union contain lignites. The lowest, Cannonball Member, is of marine origin and does not contain lignite. The Cannonball Member and the Ludlow Member are time-equivalent, intertonguing units. Two kinds of peat deposition typical of deltaic environments are present in the Ludlow Member. Some of the Ludlow lignites are extensive and probably formed in the plains of abandoned delta lobes; other are lenticular and local in extent and probably formed in interdistributary areas of actively prograding deltas.

The Tongue River Member contains the Harmon and Hansen beds which were formed in a fluvial depositional system. Sediments were transported from west to east through fluvial channels within stabilized meander belts. Peat was deposited in large, elongate swamps that roughly paralleled the east-west trending meander belts. Occasional deposition of overbank sediments in the flood-plain areas adjacent to meander belts interrupted peat deposition and formed partings. Thick peat deposits formed away from the meander belts.

Lignites of the overlying Sentinel Butte Member also were deposited between stable meander belts. Sediment transport during this time was primarily from northwest to southeast, and the geometric orientation of the lignites reflects the change of direction.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC