STRUCTURAL AND STRATIGRAPHIC CHARACTERISTICS OF THE COOSA SHEAR ZONE AS EXPOSED IN THE ROME SOUTH (GA), LIVINGSTON (GA), AND INDIAN MOUNTAIN (GA-AL) QUADRANGLES FROM ROME, GEORGIA, TO NORTH OF ROCK RUN, ALABAMA

The Coosa Fault was first mapped and named by Hayes (1894) for its occurrence along the southern edge of the Coosa Valley (Cressler, 1970). Generally, the Coosa Fault carries the Rome Formation in the hanging wall and juxtaposes this unit with various lithologies of the Conasauga Group. Hayes (1894) described fault-related rock in the footwall of the Coosa Fault, and subsequent workers redefined this fault-related rock as a stratigraphic unit of the Conasauga Group. Recently completed, detailed geologic mapping supports the fault-related rock described by Hayes, as indicated by the presence of a highly folded and mylonitic tectonostratigraphic unit; this unit is defined herein as the Coosa Shear Zone (CSZ). The structural upper boundary of the CSZ is the Coosa Fault and the lower structural boundary is the Web Creek Fault. Outcrop widths of the CSZ vary between 30 and 7,000 feet. Core drilling near Glen Hollow, Floyd County, indicates that CSZ is up to 120 feet thick. In the Livingston and Rome South quadrangles, outlying klippen of Rome Formation and fensters through the CSZ indicate a relatively thin sheet associated with a ramp to flat transition of the Coosa Fault, whereas on the Indian Mountain quad, the lack of Rome Formation klippen and fensters indicate the Coosa Fault has a higher-angle ramp geometry.

Fold axial planes are highly variable, ranging from nearly recumbent to steeply inclined, overturned, and northwest verging. Fold axes are doubly plunging, suggesting at least two, possibly three periods of movement within the CSZ. Major fold axes plunge to the southwest between 5° and 47° and northeast between 5° and 19°.

The CSZ is characterized by lithologies derived from the underlying Conasauga Group and overlying Rome Formation. Rocks within the CSZ are generally olive-green silty shale, greenish fine- to medium-grained sandstone, and siltstone which are intensely deformed and mylonitic. Shale and siltstone weather green to tan; shearing has produced a distinct waxy sheen, giving the rocks a phyllonitic texture with visible muscovite on the shear surfaces. All lithologies are highly contorted and locally contain abundant concordant and discordant vein quartz which increase toward the Coosa Fault. Locally, limestone and reddish sandstone of the Rome Formation occur as thin, discontinuous layers and pods in the CSZ.