Date of Award:

1988

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. McCalpin

Abstract

Pocatello Valley in southeastern Idaho and northern Utah is a structural and topographic basin bounded on all sides by mountains composed of Paleozoic platform carbonates and elastics. In the late Pleistocene it contained pluvial Lake Utaho, which, prior to 1981, was considered to have been an arm of Lake Bonneville. This study corroborates the finding of Currey (1981) that the two lakes were separate.

The Quaternary deposits examined in this study are divided into two broad groups: those that were deposited prior to the last pluvial lake cycle, and those that were deposited during and after the pluvial lake maximum, (since approximately 16 ka) when the area was occupied by Lake Utaho and Lake Bonneville. Pediment gravels, alluvial fans, piedmont colluvium, and talus comprise the older group: the younger deposits include stream channel deposits, lacustrine sediments, and loess.

Quantitative front sinuosity height ratios) geomorphological techniques (mountain ratios and valley floor width-valley indicate that the bounding ranges on the east and west margins of Pocatello Valley are slightly to moderately active tectonically. Precise surveying of the Lake Utaho highstand shoreline revealed significant deviations from the smooth, isostatically rebounded shoreline elevation curve of Crittenden (1963). The greatest deflections occur where the sinuous shoreline crosses the more linear inferred range front faults at the base of Samaria Mountain, on the east margin of the valley. The deflections (as much as 6.4 m over a horizontal distance of 900 m) suggest that movement has occurred along the range front faults since the shorelines were created approximately 16 ka, but no fault scarps were formed. A buried colluvium estimated to be 95 ka + 15 ka that was exposed draped toward in a over the trench at the range the inferred fault, valley. There are front is monoclinally and dips as much as 49° no fractures in the colluvium, which suggests that, although relative movement between the mountain and valley blocks has occurred, the displacement has only warped the colluvium. This further suggests that any earthquakes accompanying the movements must have been below the magnitude threshold (ML 6.2-6.3) necessary for surface rupture.

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