Masters Theses

Date of Award

3-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Otto C. Kopp

Committee Members

Kenneth R. Walker, Richard W. Arnseth, Steven H. Stow

Abstract

The nature and properties of the residuum developed on top of the Copper Ridge and Chepultepec Formations of the Knox Group at West Chestnut Ridge, Oak Ridge, Tennessee were investigated. Physical, mineralogical, and geochemical analyses of both the residuum and selected Lower Knox Group bedrock samples were used to characterize the weathering behavior of siliceous and argillaceous dolostones.

Lower Knox Group lithologies at West Chestnut Ridge are dominated by cherty dolostones with fewer beds of limestone, shale, and sandstone. The bedrock has weathered to form an overlying mantle of clay- and chert-rich residuum and soils that thicken to 30 meters or more on ridge tops. Samples were available from seven cores at various depths over an interval of one to 30 meters. Minerals in the residuum include illite, quartz, kaolinite, hydroxy-interlayered vermiculite, mixed-layer clays, amorphous iron-, manganese-, and aluminum-oxides and oxyhydroxides, as well as gibbsite, hematite, goethite, feldspars, dolomite, and calcite.

Mineralogical and chemical analyses for the residuum suggest a discontinuity at a depth of 15 meters in the two most complete cores (9 and 16) overlying the Copper Ridge Formation. Above approximately 15 meters, the residuum is more extensively leached, while at greater depths the composition of the residuum more closely reflects the character of the original parent bedrock. Based on chemical analyses, two zones identified in the residuum can be shown to be independent populations using the t-test: (1) an upper, more thoroughly leached zone generally free of detrital feldspars and carbonates; and (2) a lower zone of accumulation in which traces of feldspars and carbonates are still present.

Chemical compositions of available bedrock samples were compared with chemical compositions of the residuum to obtain an approximation of the extent of the changes that took place during the weathering process. Upon examination of the data, several elements (such as K, Na, Ti, and Mn) appear to have been enriched during the weathering process; however, when the relative weight percentages are compared, it is apparent that these elements have been removed from the system. Only Fe, Al, and perhaps P have been concentrated in the system. These elements appear to have been leached from the upper zones and concentrated in the lower zone of accumulation. Ca is the most mobile element and was almost completely removed by groundwater. Mg is also mobile, but some Mg was retained in clay minerals. For purposes of comparison, Si02 is assumed to have been neither added to nor removed from the system. It was retained in the residuum in the form of relatively insoluble quartz (mostly in the sand and silt fractions), chert fragments (in the gravel, sand and silt fractions), and in clay minerals.

Ionic adsorption capacities (CEC's and AEC's) of clay minerals are dependent on many factors such as the clay mineral species present, the pH of the weathering environment (pH values indicate the soils and residuum at West Chestnut Ridge are acidic), the amount of clay surface areas (estimated from grain size analyses), and the amount of amorphous oxides, oxyhydroxides, and organics. Correlation coefficients indicate there are direct, dependent relationships between CEC's and pH (in distilled water), and CEC's and percentages of the clay-size fractions.

In the event radionuclides are buried at the West Chestnut Ridge site in the future, the soils and residuum could come in contact with the waste leachate. As a result, this study is important for the understanding of the weathering behavior of the residuum overlying the siliceous and argillaceous dolostones of the Lower Knox Group. This information will provide a basis for the interpretation of future hydrologic and chemical investigations of the site.

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