GEOLOGY OF THE UPPER PART OF THE FORT UNION GROUP (PALEOCENE), WILLISTON BASIN, WITH REFERENCE TO URANIUM

摘要

Lignite-bearing sandy, silly, and clayey formations make up the Fort Union Group in the Williston basin, the upper part of which consists of the Tongue River Formation conformably overlain by the Sentinel Butte Formation. Two types of sandstone beds arc present in the Tongue River Formation, one of which is elongate in plan view, trough shaped, and as much as about 300 m wide and 20 in thick. This type, which is most common, was deposited in low-sinuosity, non-braided channels typical of the seaward part of a high-constructive delta. The second type of sandstone bed in the Tongue River is elongate in map view (probably), tabular in most cases about3 m thick, and as much as several hundred meters wide. Tin's type was deposited as lateral-accretion, point-bar deposits in high-sinuosity streams. Siltstone, claystone. lignite, and a small amount of limestone were deposited on natural levees, crevasse splays, and in flood basins between the distributaries.