ABSTRACT

Geologic units and structure of the Williston Basin, Cedar Creek anticline, and Poplar dome affect the availability, movement, and quality of ground water in east-central Montana. Aquifers are above the Pierre Shale within the Fox Hills, Hell Creek, and Fort Union Formations and surficial deposits.

The Fox Hills Sandstone contains regionally continuous shoreface and estuarine sandstones. Scour of lower Hell Creek channels into the Fox Hills resulted in locally thick and thin areas of the Fox Hills-lower Hell Creek aquifer. Ground-water flow in the Fox Hills-lower Hell Creek toward the Williston Basin axis is diverted toward the Yellowstone River and toward a saddle between the Cedar Creek anticline and Poplar dome. Fox Hills-lower Hell Creek water is a sodium-bicarbonate type resulting from cation exchange and sulfate reduction.

The Hell Creek and Fort Union Formations contain discontinuous fluvial sandstones, flood plain mudstones, and coals; discontinuity makes aquifer prediction difficult. Most water wells are completed within 200 ft (60 m) of land surface. The water-table elevation mimics topography because intervening mudstones do not confine shallow aquifers. Ground-water flow in these units includes both local and regional systems. Generally poor and highly variable water quality results from the great range in lithologies and flow-path distances.

Surficial sediments 20 ft (6 m) thick within modern valleys provide sufficient quantities of ground water to be aquifers. Sediment on upland terraces typically does not have sufficient saturated thickness to form aquifers. Alluvial aquifers are hydraulically connected to underlying aquifers in the Fort Union or Hell Creek Formations.