Latest Cenozoic stratigraphy of Lake Sakakawea area, northeast Mercer County, North Dakota
The latest Cenozoic in northeast Mercer County, North Dakota, is represented by five formations. They are, from bottom to top: the Charging Eagle Formation, the Medicine Hill Formation, the Snow School Formation, the Coteau Formation, and the Oahe Formation. The Charging Eagle Formation is mainly fluvial sediment. Most of the sediment was derived from the west with only minor amounts from the northeast. It contains the first evidence of glaciation in the area. The lower Medicine Hill Formation is mainly sediment deposited by a northeast-flowing ancestor of the Missouri River and by melt-water streams. The upp.er Medicine Hill Formation is glacial sediment. The upper member contains the oldest preserved glacial sediment in the area. Many large inclusions of gravel,' sand, and clay characterize the upper member. The lower Snow School Formation is mainly fluvial sediment deposited by a northeast-flowing ancestor of the Missouri River and by melt-water streams. The upper Snow School Formation is glacial sediment. The upper member contains the youngest glacial sediment in the area. The lithologic homogeneity, columnar jointing, and a weathering profile characterize the upper member. The Coteau Formation is mainly alluvium and slopewash sediment. It is found at the base of steep hillslopes. Much of it was deposited during unstable (warm and dry) climatic conditions, probably contemporaneously with much of the Oahe Formation. The Oahe Formation (late Wisconsinan and Holocene) is mainly wind-blown silt. It is found on the hilltops and in gentle depressions. The characteristics of the four members (the Mallard Island, Aggie Brown, Pick City, and Riverdale Members) are controlled by changes in hillslope stability resulting from clilllatic changes.
Geology--North Dakota--Mercer County; Geology, Stratigraphic--Cenozoic, Geology
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