Date of Award

1991

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

J.R. Reid

Abstract

Shoreline erosion at Lake Sakakawea has exceeded originally predicted rates. This thesis is a continuation of a project, begun in 1983, to study erosion rates, causes, and prediction; the purpose has been to describe variations in fracture patterns in shoreline banks and assess their affects on erosion rates.

During the first phase of this project (1983-1986), average bank recession was rapid (l.Sm/yr) and factors related to wave action, including fetch, bank orientation, and beach composition, were most important. Since 1986, low lake levels have persisted and wave action has not been a factor; however banks continue to recede, but at a slower rate (0.2m/yr), and bank properties, including fracture patterns, height, slope, and composi~ion have become mora important. Banks are not yet stabilizing, and factors related to lake levels, wave action, and bank properties must all be considered in predicting future bank recession rates.

Fracture patterns were described at each erosion station. The fractures result from regional stresses related to crustal uplift and NE SW plate motion, stress release associated with vertical and lateral unloading, subglacial deformation, and/or desiccation. Differences in average fracture size and abundance correspond to changes in lithology. Vertical fractures are smaller and more closely spaced where strong horizontal bedding or fracturing exists. Size and abundance are also affected by grain size, consolidation, weathering.

Consistently oriented N-S, E-W, NE-SW, and NW-SE orthogonal sets of straight, vertical fractures with matte surfaces are dominant in the Paleocene bedrock~ Horizontal fracturing also occurs where·bed4ing is xi well-developed; it is especially intense in and near lignite layers. The upper Medicine Hill till contains: sets of short, straight, vertical fractures, separated by near-horizontal fractures with straight or curved surfaces; most are sealed with mineral coatings. A columnar jointing pattern in the Upper Snow School and Upper Horseshoe Valley tills results from dominance of orthogonal sets of open vertical fractures with rough surfaces and the absence of horizontal structures.

The most important failure mechanisms affecting these cohesive bank sediments are toppling and high-angle sliding along large vertical bank parallel · tension fractures. Sediments with well-developed horizontal structures are more resistant to this type of failure because the fractures, formed by stress release along the exposed bank, cannot extend as deeply. Thus, Upper Medicine Hill till banks are receding 50% slower than other till banks. Bedrock banks with hard interbedded limestone lenses and strong lithological variations have also receded slower.

Vertical fracture orientations and abundance also affect erosion mechanisms and rates. However, because of the numerous other erosional factors, the multiplicity of fracture sets in the bedrock, and the high dispersion of fracture orientations in some of the tills, it is difficult to correlate these factors directly to variations in erosion rates.

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