SEDIMENTOLOGICAL ANALYSES RELATED TO FAUNAL CHANGES IN THE SILURIAN ROCKWOOD FORMATION, SOUTHEASTERN TN

The Silurian Rockwood Formation in the Chattanooga area contains shallow-marine sediments deposited as intercalated medium- to thin-bedded limestones, siltstones, fine-grained sandstones and shales. The sediments of more resistant beds appear to have been modified by storm energy. There is a faunal change from a brachiopod-dominated to a rugose- and brachiopod-dominated population in the upper section. There is a shift from grey to tan color (weathered and fresh) in the outcrop that is coincident with the increased coral population. Thin sections were made of the resistant limestones, sandstones and siltstones collected from 1.4m, 1.8m, 2.4m, 4.2m, 5.0m, 7.5m, 9.0m, 9.6m, 11.1m, and 12.5m. Small (< 0.06mm) euhedral dolomite rhombs are pervasive through most of the section. All thin-sections were stained with an Alizarin Red S and potassium ferricyanide mix dissolved in 5% HCl to identify calcite (pink stain), ferroan dolomite (blue stain) and non-ferroan dolomite (no stain). HCl enhanced the relative relief among the siliciclastic, calcium- and calcium/magnesium carbonate grains. All slides contain euhedral to subhedral ferroan dolomite as indicated by stains. Fine-grained sandstones and siltstones at 1.8m, upper 9.6m and 11.1m contain a low concentration of ferroan dolomite, which is disseminated among siliciclastic grains. The carbonate-rich units at 2.9m, 4.2m, 5.0m, 7.5m, 9.0m, basal 9.6m, and 12.5m all contain > 50% ferroan dolomite in the matrix. Many fossil fragments retain original calcitic structure. Bioturbation is pervasive through the carbonate-rich units. The 12.5m thin-section contains zoned euhedral dolomite crystals that are strongly iron-stained (natural stain). Future work will include XRF analysis of these samples. These data will be used to more accurately determine clay content variation in the section. We will infer clay content with the alumina fraction from results. Also, we will use XRF results to assess iron variation through the section.