SEDIMENTOLOGICAL CONTROLS ON FAUNAL CHANGES IN THE SILURIAN ROCKWOOD FORMATION, SOUTHEASTERN TN

Faunal changes in exposures of the Silurian Rockwood Formation near Chattanooga, Tennessee, show an upward transition from brachiopod-dominated to solitary rugose coral-dominated communities in a 12.5-meter section. A possible control on the faunal change is the high concentration of suspended sediments that interfere with filter-feeding organisms.

The Rockwood in this area is composed of medium- to thin-bedded intercalated limestones and shales interpreted as shallow-marine storm and suspension deposits. Current ripples, sand grains, single-valve (transported?) brachiopods and other death-assemblage fossil debris in the limestones suggest that these deposits were modified by storm energy. The shale units contain some carbonate material and most are fossiliferous with predominantly life assemblages.

XRD analyses of bulk shale from 1.3, 2.9, 9.6, 11.1 and 12.5 meters in the section all indicate quartz, clay and calcite. Oriented mounts of the clay fraction from 9.6 meters in the section reveal broad (>0.35 deg. FWHM) peaks that correspond to d-spacings of 14.11, 9.97, 7.10, 4.97, 4.70, 3.53, and 3.32 Å. These are interpreted as due to a combination of kaolinite, illite and chlorite-group clays. Heating at 550 deg. C for 2 hours destroyed kaolinite, as indicated by elimination of the 7.1 and 3.5 Å peaks. After heating, the 14.1 Å peak increased slightly in intensity and shifted to 13.62 Å, due to alteration of the brucite layer in chlorite. Further, the 14.1 Å d-spacing did not increase when glycolated, which would have indicated a smectite-group clay. The 10, 5 and 3.3 Å peaks were unaffected by either heat or glycolation, indicating illite.