Rocks of Paleocene age are represented in the Cave Hills of northwestern South Dakota by the Ludlow, Cannonball, and Tongue River members of the Fort Union Formation. The Cave Hills are situated within the southern margin of the Williston basin, 80 mi (130 km) north of the Black Hills, South Dakota.

Numerous fine-grained, fining-upward sedimentary sequences comprise the Ludlow Member and are attributed to meandering streams occupying a low-gradient lower alluvial to upper deltaic plain. Major channel sandstones measuring up to 40 ft (12 m) in thickness, crop out and trend markedly to the northeast. Thinner sandstones adjacent to the large channel sandstones vary considerably in geometry and paleocurrent direction and are commonly associated with alternating siltstone, mudstone, claystone, and lignite deposits of levee, overbank, swamp, and possibly lacustrine origin.

The Cannonball Member is 130 ft (40 m) thick in the North Cave Hills and is represented by two fine-grained, coarsening-upward sandstone-mudstone sequences. A distinct vertical succession of sedimentary facies occur within each sequence representing offshore/lower shoreface through upper shoreface/foreshore depositional environments. A north to northeast depositional strike for the Cannonball shoreline is inferred from ripple crest and cross-bed orientations.

Numerous tree stumps in growth position are preserved along the upper surface of the Cannonball Member in the North and South Riley Pass mining districts. These stumps probably represent remnants of a cypress (Metasequoia) forest or swamp that stabilized the uppermost sands of the Cannonball shoreline.

Figure

The basal part of the Tongue River consists of approximately 40 to 50 ft (12 to 15 m) of lenticular sandstone, siltstone, mudstone, thin-bedded lignite, and kaolinite beds representing thin broad channels, point-bar, levee, overbank, and nearshore swamp depositional environments. Massive fluvial channel sandstones measuring several tens of ft in thickness overlie the fine-grained basal Tongue River lithologies. These channel sandstones represent the continued progradation of continental/fluvial/coastal plain depositional environments eastward over the marine sandstones of the Cannonball Member.

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