Mineralogy and chemistry of clay-rich sediments in the contact zone of the Bullion Creek and Sentinel Butte Formations (Paleocene), Billings County, North Dakota

The Bullion Creek and Sentinel Butte Formations (Paleocene) form over 50 percent of the surface and near surface sediments in the North Dakota part of the Williston Basin. These rocks are composed of sandstones, siltstones, claystones, and numerous lignite beds. A distinct surface color change from light colors in the Bullion Creek Formation to dark colors in the Sentinel Butte Formation serves as a boundary marker and is widely exposed in the Little Missouri River badlands. The color boundary is a product of weathering and is not readily apparent in the subsurface. The use of clay mineralogy was investigated as a possible criterion to distinguish between the two formations and provide an explanation of the coloring. Argillaceous units of both formations were sampled at three widely spaced sites along the Little Missouri River in Billings County, North Dakota. Samples were collected 32 km north of Medora, near Medora, and 25 km south of Medora. X-ray diffraction analysis of thirty-five samples established sodium montmorillonite with lesser amounts of mica-illite and iron chlorite as the major clay minerals. Minor amounts of kaolinite and the non-clay minerals quartz, calcite, dolomite, and feldspar are present in the clay-size fraction, the relative amounts and types of clay minerals are the same in both formations. Mica-illite and chlorite occur in about equal amounts and vary inversely with the amount of montmorillonite, the variation in clay mineral relationships within either formation is greater than the variation between the formations. Any difference in clay mineralogy across the formational contact is localized, chemical analyses of the clay-rich sediments showed a greater amount of iron and sodium in the Sentinel Butte Formation. The increase in iron, probably in disseminated form, i& suggested as the cause of the darker colors observed in outcrop. There appear to be two mechanisms of origin of the clay minerals in the Bullion Creek and Sentinel Butte Formations. Most of the clays are detrital material from highly montmorillonitic Cretaceous shales to the west and northwest. Some of the montmorillonite may be diagenetic and derived from the breakdown of eolian volcanic ash from the west