BENTONITE-CONTROLLED DEFORMATION OF MIDDLE ORDOVICIAN LIMESTONE AT CHICKAMAUGA DAM, CHATTANOOGA, TENNESSEE

Middle Ordovician Carters and Tyrone Limestones, and the T3 bentonite, are well exposed along the north shore of Chickamauga Lake, east (upstream) of the dam. Complex deformation of these rocks, in the immediate footwall of the Missionary Ridge fault, presumably relates to Alleghanian overthrusting of Cambrian and younger strata.

One outcrop, in particular, provides excellent 3D exposure of a fault and related folds. The front face of the outcrop, a vertical E-W-trending surface that measures 7 m long and 4 m high, is dominated by 1.3 m of Carters Limestone that is repeated (2.6 m total thickness) by a thrust having a classic ramp-and-flat geometry, indicating top-to-southeast displacement, antithetic to regional thrusting; the bedding-parallel flat dips gently (10° to 15°) to the east; the ramp dips moderately (40°) to the west. Striations indicate that slip was only slightly oblique to this near-perfect cross section.

The repeated section of Carters Limestone underlies the T3 bentonite. The bentonite is approximately 1 m thick, where it is relatively undeformed, and is tectonically thickened above and adjacent to the fault. A layer of black chert, several cm's thick, occurs at the top of the repeated section, directly below the bentonite and directly below the hanging wall of the fault, along its flat. This layer provides distinctive hanging-wall and footwall cutoffs, which are separated by approximately 5 meters, measured along the trace of the fault.

At the leading edge of the hanging wall, the chert layer is downward facing in the overturned limb of a recumbent fold. Remarkably, 8 m of the NE-trending fold hinge is exposed behind the front face of the outcrop, directly above a pavement-like platform of the chert layer on the footwall. The hinge of the fold, as defined by the chert layer, is torn along much of its length, revealing internal aspects of the structure. To the north and above, bedrock is concealed by a south-facing embankment of bentonite.

Earthworx (Bodkin) conducted a high-definition survey (LiDAR laser scan) of the outcrop from multiple perspectives. The resultant georeferenced color-mapped point-cloud model of the exposed structure was enhanced by probing vertically into the bentonite embankment.

We attribute many aspects of this structure to the presence of the bentonite as an outcrop-scale mushwad.