MINERALOGY AND STRUCTURAL SETTING OF THE MIDDLE ORDOVICIAN T3 BENTONITE AT CHICKAMAUGA DAM, CHATTANOOGA, TENNESSEE

Middle Ordovician limestone of the Chickamauga Supergroup and the interbed of T3 bentonite are well exposed at the Chickamauga dam, Chattanooga, Tennessee. These rocks lie in the immediate footwall of the southeast-dipping Missionary Ridge fault and are complexly deformed. Structures include numerous NE-SW-trending meter-scale folds, some of which are overturned and have detached limbs. The bentonite and structural geology at the dam are of engineering interest related to the present construction of a new lock.

The T3 bentonite in these exposures is up to 1 meter thick and varies from a well-indurated light gray to medium green claystone, where fresh, to a very friable clay-rich soil, where deeply weathered. The clay is plastic when wet.

Powder X-ray diffraction (XRD) of the moderately weathered bentonite indicates an illite/smectite mixed-layer clay, quartz, sanidine, and calcite. Efforts to quantify XRD patterns using SIROQUANT (tm), software for full-pattern Rietveld refinement, were hampered by the structural complexity of the clay. As a work-around, clay peaks were excluded from refinements; clays were considered nondiffracting phases in SIROQUANT's calculation of amorphous content. ZnO (approximately 10 wt.%) was used as a spike. Little or no true amorphous material is indicated. Taking this approach to quantification of XRD patterns for several samples from a single outcrop, the bentonite was found to contain 76 to 85 wt.% clay ("amorphous content"), 7.5 to 8.5 wt.% quartz, 3.5 to 7.5 wt.% sanidine, and 2.5 to 9.0 wt.% calcite. Chi-squared values of 5.26 to 7.83 indicate relatively good fits for these refinements.

Calcite is interpreted as a secondary mineral in the bentonite derived from adjacent limestone. Sanidine and quartz are interpreted as primary igneous phases from the original volcanic ash.

To test and improve upon quantitative analysis of mineralogy, clay and nonclay fractions were concentrated using a hydraulic separator, in which the clays were carried upward, out the top of a vertical tube, by a very slow upward flow of water. Sieving and simple suspension, were also used to concentrate clays and nonclays. Calcite was removed by dissolution in HCl. Analyses of variously processed samples are generally consistent with those of the unprocessed bentonite.