Date of Award
Doctor of Philosophy (PhD)
Sargent County, in southeastern North Dakota, was glaciated during the Pleistocene Epoch. Objectives of a study of Sargent County were to (1) describe the surface sediment and interpret its origin, (2) correlate, where possible, this sediment with that of adjacent areas, and (3) recon struct the general geologic history of the county for the Quaternary Period.
A geologic map of the county was prepared using field data, topo graphic maps, aerial stereopairs, and soil maps. A map showing surface sediment origin was subsequently made using the available field and laboratory information.
Most of Sargent County is typical rolling prairie underlain by Quaternary sediment resting unconformably on Mesozoic rocks. The Quaternary sediment is primarily nonindurated sediment belonging to the Coleharbor and Walsh Formations. The Mesozoic rocks are mostly well-indurated sediment belonging to the Pierre, Niobrara, Carlile, Greenhorn, Belle Fourche, Howry, Newcastle, and Skull Creek Formations.
The Coleharbor Formation comprises about 95 percent of the county surface area and is primarily glacial sediment. The Formation includes five facies: diamicton (glacial sediment); sand and gravel (fluvial channel sediment); silt (lacustrine sediment); silty clay (lacustrine sediment); and silt, sand, silty sand, and clay (turbidity-current sediment).
The Walsh Formation comprises the remaining 5 percent of the county and includes four facies: clay (lacustrine sediment), sandy silt (slope wash sediment), clayey silt (fluvial overbank sediment), and silty sand (eolian sediment).
Evidence of previous glacier advances includes: (1) the occurrence of two different drift units exposed in a roadcut on the. Prairie Coteau, (2) the morphology and structure of the Whitestone Hills, and (3) the morphology and stratigraphy of the Lake Oakes Hills.
Two glacial diamicton beds are exposed in a Prairie Coteau roadcut; the upper bed, Drift A, is noticeably rich in shale fragments whereas the lower bed, Drift B, is shale-poor. Drift A correlates with the upper member of the Red Lake Falls Formation on the upper Red River Valley and the New Ulm Till of southwestern Minnesota. The shale fragments in Drift A were eroded by a glacier that moved down the valleys on the Red and Minnesota Rivers. The small percentage of shale fragments in Drift B indicates that it was deposited by a glacier that moved southward along the axis of the Red Lakes lowland, where there is little shale bedrock to erode. Drift B correlates with the lower member of the Red Lake Falls Formation of the upper Red River Valley and the Granite Falls Till of southwestern Minnesota.
The Whitestone Hills are streamlined and probably represent an over ridden landfonn from a previous glacier advance, as evidenced by oxidized diamicton occurring beneath unoxidized diamicton, separated by fluvial sand and gravel.
The Lake Oakes Hills in western Sargent County were found to be primarily composed of turbidity-current sediment that was deposited in an ice-marginal lake called Lake Oakes. Lake Oakes predates the last advance of ice because the hills are veneered with glacial diamicton deposited during overriding of the hills by the last glacier advance. The Lake Oakes sediment overlies another glacial diamicton evidently de posited by an earlier glacial advance.
The geologic events of the last glacial advance and ensuring de glaciation can be subdivided into three phases. During Phase 1, ice moved southward along the lowland of the Red and Minnesota Rivers. The Prairie Coteau caused the glacier to split into the James and Des Moines Lobes. Climatic warming caused the glacier to thin and retreat.
The presence of stagnant ice in Sargent County CTarks the beginning of Phase 2. Melt water from the James River in Dickey County and other streams in Sargent County and adjacent areas drained southward into South Dakota where it was eventually ponded by an end-moraine complex, result ing in the formation of glacial Lake Dakota. The lake eventually reached to several miles north of Cogswell in Sargent County.
Glacier ice was no longer active in Sargent County by the: beginning of Phase 3. Stiperglacial debris covered stagnant ice over most of the county, however, insulating it from rapid melting. Melt water discharged into glacial Lake Agassiz in the Red River Valley during this phase. Lake silt was deposited on stagnant ice above the Herman Beach level in Sargent County during the Milnor subphase. The lake dropped to the Herman level · (1,060 feet) during the Herman subphase and overflow from the Sheyenne River cut across northeastern Sargent County during the subsequent Milnor Channel subphase.
All glacier ice in Sargent County was gone by 10,000 B.P. The climate continued to warm resulting in a vegetative succession from forest to prairie. The increased dryness resulted in increased stream and wind erosion. Between 7,000 and 8,000 years B.P. maximum dryness was reached; resulting eolian activity produced dunes in some sandy and silty parts. of the county. From 7,000 until 4,000 years B.P. the climate gradually cooled. Since 4,000 years B.P. the climate of Sargent County has been relatively uniform.
Neilsen, Dennis N., "Quaternary geology of Sargent County, North Dakota" (1973). Theses and Dissertations. 208.