Date of Award

1986

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geology

First Advisor

A.E. Kehew

Abstract

In the past few years, a new interpretation of the formation of the Souris and Des Lacs glacial-lake spillways has been proposed. Now, based on geomorphological evidence, it is concluded that the Souris and Des Lacs Valleys were formed by massive glacial-lake outburst floods at the end of the late Wisconsinan deglaciation. Five phases of drainage have been recognized with the first forming the Des Lacs Valley and the other four associated with drainage in the Souris Valley from different proglacial lakes in Canada. The last of these floods down the Souris Valley was the most erosive.

This study is concerned with Holocene sediments that have filled the Souris and Des Lacs Valleys in Ward and Renville Counties. Most of the discussion is focused on the Souris Valley. Twelve cross sections were constructed from data gathered in the field, from the U.S. Army Corps of Engineers, and the North Dakota State Water Commission. These data include: texture, structure, fossils, and amount of organic material. Study of these data allows reconstruction of the depositional environment of the Holocene sediments and of the history of the valleys.

While the Souris Valley was being eroded during the last glacial-lake outburst, a headward-migrating knickpoint developed in the Souris Valley. This was caused by downcutting of the valley bottom while the flood was discharging into Glacial Lake Souris. After the flood subsided, an aggrading, meandering Souris River occupied the valley. Initially, the sides of both the Souris and Des Lacs Valleys were steep, and slumping occurred. Numerous intermittent streams entered the valleys from the sides and deposited coarse material close to the valley walls. The centers of the valleys contain silt with sand and clay lenses typical of fluvial systems dominated by meandering streams. The center of the Souris Valley, between its confluence with the Des Lacs River and the position of the Soo Line R.R. to the north, is characterized by a coffipletely different stratigraphic sequence that includes a central clay plug. Differences in the types of organic materials and fossil assemblages in this section suggest a swampy to shallow-pond environment. This pond may have been dammed by an alluvial fan deposited by the higher-gradient Des Lacs River.

Three organic horizons were recognized and are interpretee to represent cool, moist, climatic conditions that enabled vegetation to stabilize the steep valley sides. The lower horizon has been correlated with the Aggie Brown Member of the Oahe Formation of early Holocene ase by C-14 dating. The other two horizons are not dated but may be correlative with the late Holocene Riverdale Member of the Oahe Formation. A warm and drier climate during the raiddle Holocene was characterized by greater slope erosion and more deposition in the valleys.

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