DEFORMATION FEATURES IN CHATTANOOGA SHALE, NEAR CHATTANOOGA, TENNESSEE

Several exposures of deformed middle Paleozoic rocks in the westernmost Valley and Ridge, near Chattanooga, TN, reveal a variety of structures. These include cm-scale ellipsoidal to lens-shaped masses of sandstone in Mississippian to Devonian Chattanooga Shale (MDc). Fissility in the carbonaceous shale is intensified adjacent to these lenses and conforms to their biconvex shapes. Slickensides on rinds of highly polished shale trend NW-SE. The walnut- to fist-size masses are composed of fine- to medium-grained ferruginous quartz sandstone. Thin-section study reveals that the quartz grains retain detrital characteristics, but that many are broken and displaced; cataclastic deformation is particularly evident in microbreccia zones. XRD studies confirm the mineralogy determined petrographically, but proved to be of little value in establishing a relationship between the lenses and other sandstones.

The sandstone lenses occur low in MDc, near its contact with the underlying Silurian Rockwood Formation (Sr), which consists of alternating cm-scale layers of mudstone and ferruginous sandstone. Where sandstone lenses are found in MDc, Sr is complexly folded; hinge lines trend NE-SW. In a particularly spectacular exposure that contains numerous sandstone lenses, the uppermost sandstone layer in Sr is locally duplexed with m-scale horses. Elsewhere in this same outcrop, this sandstone layer displays boudinage necking or is entirely absent. The sandstone lenses have not been observed where MDc and adjacent units are not deformed.

Based on their shape, surface features, internal characteristics, structural and stratigraphic distribution, circumstances of their occurrence, and associated fabrics in surrounding shale, we interpret the sandstone lenses as tectonic intercalations derived from sandstone layers near the top of the underlying Sr. Boudinage of these layers may result in cm-scale ellipsoidal to lens-shaped masses of sandstone that become entrained in MDc and continue to deform during subsequent flow of the shale. Observations near Fort Payne, AL, are suggestive of an alternative mechanism, whereby joint-bounded blocks of the sandstone layers are entrained in MDc and are geometrically modified during subsequent flow. In either case, cataclastic flow has resulted in mesoscopic ductile behavior.