REINTERPRETATION OF STRUCTURES IN THE HANGING WALL OF THE CHATTANOOGA FAULT, NEAR CHATTANOOGA, TENNESSEE

Recent excavations provide two new exposures of middle Paleozoic rocks near Chattanooga, Tennessee. These outcrops, coupled with detailed (1:12,000-scale) mapping, demonstrate that the geologic structure of the area is significantly different than was previously interpreted. They also reveal small-scale structures suggestive of displacement within the Chattanooga Shale (MDc) and at its lower contact. The 4-km2 study area, centered at 654,890 mE, 3,884,000 mN (Chattanooga 7.5-minute quadrangle), contains a map-scale anticline. Field relationships and structural data indicate that the fold is nearly horizontal with a northeast-southwest trend (approximately 045°). Second-order folds and undulations contribute to its noncylindrical geometry. Meter-scale folds in interbedded sandstone, siltstone and shale of the Rockwood Formation (Sr) are similarly oriented, as are centimeter- to meter-scale kink folds in MDc. Repetition of Silurian through Mississippian strata (Rockwood Formation through Fort Payne Chert) above the fold indicates at least one additional thrust not previously described. Cambrian Knox Group rocks are carried in an overlying thrust. These structures occur in the hanging wall of the Chattanooga fault. Distinctive greenish-gray shale that normally occurs at the top of Sr, directly below MDc, is absent at some locations in the study area and is interpreted to have been tectonically removed. The lower contact of MDc is also complicated by "clastic veins" and centimeter- to meter-scale intercalations of sandstone from the underlying Rockwood Formation. Locally, within MDc, an anastomosing pattern of shale with polished surfaces separates centimeter-scale ellipsoidal masses of Rockwood-derived ferruginous sandstone. Slickensides on polished surfaces trend northwest-southeast (295° to 320°). These structures are interpreted to indicate thrust-related displacement at the MDc/Sr contact and within MDc.