- Pierre Shale,
- Northern Great Plains,
- South Dakota
The Pierre Shale of the Northern Great Plains area of the United State is an Upper Cretaceous shale which was deposited in a marine environment. Geotechnically, the shale has been classified as an overconsolidated clay-shale. In South Dakota the Pierre Shale crops out extensively in the central and western portions of the state.
In this study an investigation was conducted for the purpose of clarifying and examining the existence of a discrepancy between geotechnically and geologically derived estimates of maximum past overburden for the Pierre Shale at Hayes, South Dakota.
The geological determination involved examining the topographic high points of Central South Dakota for the purpose of locating the remnants of any Tertiary rock that may have existed in the area. Based on this field evidence, a geologic profile showing estimated thickness of former strata in the area was constructed and the maximum thickness of past overburden was estimated for the Hayes study site.
The geotechnical determination was made by conducting laboratory high-pressure consolidation tests on samples from cores from Hayes. Preconsolidation pressures were estimated from the consolidation curves and the values were converted to thickness of overburden.
The geological determination of the maximum thickness of past overburden indicates that 600 to 1100 feet (183 to 3 3 5 m) of Tertiary rock and Cretaceous Pierre Shale has been removed by erosion at the Hayes site. Geotechnically, the preconsolidation pressures indicate that 3600 to 5000 feet (1100 to 1525 m) of material has been removed. The discrepancy that exists between the geological and geotechnical determinations is considered significant.
Because it was believed the geotechnical estimate was too high instead of the geological too low, the search for the cause of the discrepancy centered on those factors known to affect the determination of the preconsolidation pressure, Pc.
Laboratory factors were ruled out because their affect is to reduce the estimated Pc, thus reducing the discrepancy. Of the geological factors that are known to increase Pc, only two warranted assessment in this study --cementation and delayed consolidation. By default, these two mechanisms appear to be the possible causes for the discrepancy.
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