Date of Award
Master of Science (MS)
The Sentinel Butte Formation (Paleocene) near Lost Bridge, Dunn County, west-central North Dakota, is 190 meters thick, and is charac terized by four main lithologies: sand, silt, clay, and lignite. The study interval is ISO meters thick, and lies between the basal sand of the formation and the Bullion Butte lignite bed, neither of which are exposed in the immediate study area. Sand, gray to yellow, makes up about 3S% of the interval studied. It occurs in tabular beds 2-18 meters thick. It is fine to very-fine, angular, and composed largely of quartz, feldspar, rock fragments, with minor amounts of biotite, chlorite, and organic materials. Sand beds become finer grained from bottom to top, are poorly to very poorly sorted, and have an erosional base. Large-scale cross strata are visible in the lower and middle parts of the sand beds, and small-scale cross-strata are usually abundant in upper parts of the beds. Sand beds where deposited by meandering rivers as indicated by their tabular and U shapes, sedimentary structures, and textural analysis. Paleocurrent data suggest a general southeasterly stream flow.
Silt, light yellow to grayish brown, makes up about 40% of the formation and occurs in beds 1-15 meters thick. Small-scale cross-strata and plant remains are common. Fossil gastropods and pelecypods indi cate a freshwater origin. Climbing ripple cross-strata, sand lenses, sillclfied stumps found in growth position, and iron-rich concretions with organic centers are evidence of a natural-levee environment for light-colored silt beds. A floodbasin environment is indicated for darker colored silt beds that are finer-grained, contain abundant plant debris, and are associated with clay and lignite beds.
Clay, gray to brown, makes up about 15% of the formation and occurs in beds up to 5 meters thick. Plant remains and carbonaceous materials are abundant. Montmorillonite, mica, kaolinite, and chlorite are the principal clay minerals. The dark color, organic matter, and association with ligntte beds indicate a lower floodbasin environment of deposition for the clay beds.
Lignite beds are black, hard, blocky, woody-textured, and are found associated with brown to black, soft, clayey, carbonaceous debris. Most lignite beds are 1 meter or less thick, are excellent aquifers and enhance vegetation growth. Because lignite beds have a woody texture and are associated with floodbasln deposits and fluvial sands, they probably have resulted from peat accumulation in the lowest areas on the floodbasin between major streams.
Nesemeier, Bradley D., "Stratigraphy and sedimentology of the Sentinel Butte formation (Paleocene) near Lost Bridge, Dunn County, west-central North Dakota" (1981). Theses and Dissertations. 209.