THE ALEXANDER CITY FAULT: GEOLOGIC MAPPING OF KEY LITHOSTRATIGRAPHIC UNITS IN THE SOUTHERN APPALACHIAN EASTERN BLUE RIDGE, ALABAMA

In the Southern Appalachian eastern Blue Ridge, the southeast boundary of the Elkahatchee Quartz Diorite Gneiss is the Alexander City fault, a late shear zone, separating the Elkahatchee from the Wedowee and Emuckfaw Groups. As this fault is traced to the northeast, it has been previously proposed to mark the boundary between the Wedowee and Emuckfaw Groups, two key eastern Blue Ridge lithotectonic units. Early mapping suggested that the fault was a thrust, but more recently it has been interpreted as a normal fault, suggesting that it is a major, crust-penetrating structure that cuts the Appalachian thrust stack, the basal Alleghanian detachment, and pre-thrusting autochthonous basement. Detailed field mapping along the Alexander City Fault’s northeast projection has suggested it may not extend as far northeast as it has been previously mapped, and that the boundary between the Wedowee and Emuckfaw Groups may be a gradational stratigraphic contact. Near the apparently gradational stratigraphic contact, both groups contain graphitic garnet muscovite schist interbedded with metagreywacke, which has made them locally difficult to differentiate, evidenced by the fact that various geologists have placed the contact in significantly different places. In quads northeast of Alex City, field data does not support a fault along the contact between the Wedowee and Emuckfaw Groups. The projection of the Alexander City fault into this area was previously based upon tracing discontinuous magnetic anomalies from the southwest. However these anomalies are within the Emuckfaw and do not follow the contact as it is currently mapped. Away from the contact, both units can be more easily differentiated. The Emuckfaw Group is generally coarser grained, contains less graphite, has more abundant and thicker (up to 30 centimeters) metagreywacke layers, and contains distinctive amphibolite units. We intend to refine these field studies by conducting magnetometer transects across the Wedowee-Emuckfaw contact and across the magnetic anomalies in an attempt to determine their sources. We also plan to refine the geochemical studies of the important Emuckfaw (Beaver Dam) amphibolites in an effort to determine tectonic setting.