Stratigraphy and history of the Sakakawea Sequence, south-central North Dakota

The Sakakawea Sequence (Pleistocene) in North Dakota contains six formations that make up the Coleharbor Group (Pleistocene). The Braddock Formation (Wisconsinan) is mainly glacial sediment. The Emmons Formation (late Wisconsinan) is mainly glacial sediment. The Four Bears Formation (Wisconsinan) is mainly either fluvial or lacustrine sediment. The Coteau Formation (Holocene) is mostly fluvial, colluvial, or slough sediment. The Denbigh Formation (Holocene) is mostly eolian sediment. The Oahe Formation (late Wisconsinan and Holocene) is mostly eolian sediment (loess) The Oahe Formation is divided into three members: the Mallard Island (late Wisconsinan), Aggie Brown (latest Wisconsinan to earliest Holocene), and Riverdale (middle and late Holocene) Members. During preglacial time major rivers flowed toward Hudson Bay. With the initiation of glaciation, ice blocked the rivers and forced them to flow southward. During the Pleistocene at least four major glaciations occurred. The Dunn Glaciation was pre-Wisconsinan that Verone Glaciation was post Dunn and pre-Napoleon, the Napoleon Glaciation was early Wisconsinan (?), and the Lostwood Glaciation was late Wisconsinan. Three phases (Cattail Creekt Zeeland, and Long Lake) are recognized within the Lostwood Glaciation~ The Cattail Creek was an early advance of the Lostwood Glaciation. The Zeeland and Long Lake Phases were later advances; they may have been contemporaneous. Numerous tundra polygons indicate permafrost conditions existed at the beginning of the Lostwood Glaciation. Polygons occur on all but the youngest glacial surfaces (Zeeland and Long Lake). Lakes Standing Rock and McKenzie were formed when the Lostwood ice dammed the Missouri River. Lake McKenzie was formed when ice of\u27 the Cattail Creek Phase blocked the flow of the Missouri River in the Strasburg Channel. Lake Standing Rock probably formed when ice blocked the Missouri River in north-central South Dakota. In postglacial time, more detailed evidence allows reconstruction of the history of hillslope stability. Variations in hillslope stability are largely controlled by vegetation and runoff, which are controlled by precipitation and temperature. In cool, moist areas, like North Dakota, stable conditions occur when the climate is cooler and moister. Soil formation occurs during these times. The following series of stable and unstable episodes are interpreted to have occurred in south-central North Dakota during late Wisconsinan and Holocene time. The McCarny Unstable Episode began with the deposition of the yellow loess of the Mallard Island Member of the Oahe Formation and continued until after deglaciation when a spruce woodland migrated into the area. The Leonard Stable Episode began about 12,000 to 11,500 B. With the formation of a red brown forest soil (Aggie Brown Member of the Oahe Formation). This soil continued to form until the. Climate warmed slightly about 10,000 B.P. Spruce woodland was replaced by prairie grass, and a prairie soil formed on top of the forest soil. The Wolf Creek Unstable Episode began about 8500 B.P. when the climate warmed again. Tall-grass prairie was replaced by short-grass prairie and hillslope stability decreased. Deposition of the gray loess of the Riverdale Member of the Oahe Formation began and continued for about 4000 years. During this unstable time, barchans formed where there was an adequate sand supply. The Thompson Stable Episode began about 4500 B.P. when the climate cooled. The Thompson Paleosol formed during that time. The Garrison Unstable Episode began during the late Holocene when the climate warmed again. Deposition of the gray loess of the Riverdale Member of the Oahe Formation occurred again during this time. The Jules Stable Episode began when the climate cooled again. Deposition of the Jules Paleosol occurred during that time. The cool climatic conditions continued until about 1929 when the Dirty Thirties Unstable Episode began. It lasted for about 10 years, after which the Mandan Episode began