TECTONIC AND STRATIGRAPHIC IMPLICATIONS OF CARBONATE-RICH TURBIDITES IN THE MISSION MOUNTAIN FORMATION OF THE MINERAL BLUFF GROUP, GEORGIA WESTERN BLUE RIDGE

In the western Blue Ridge (WBR) of north Georgia the base of the Paleozoic Mineral Bluff Group (MBG) is in angular unconformable contact above older metasedimentary rocks of the Paleozoic Hiwassee River Group (HRG) and the Precambrian Great Smoky Group (GSG). The MBG is the youngest sequence preserved in this portion of the WBR and occurs in the core of the regional-scale isoclinal Murphy synclinorium (MS). The well documented pre-Mineral Bluff Group unconformity is indicated by truncating angular relationships with underlying units and significant lithologic changes across the contact. Paleontologic evidence also supports the interpretation of an unconformity at this contact. The Mission Mountain Formation (MMF), the basal formation of the MBG, contains diverse lithologies, including: siliciclastic metaturbidites, coarse polymictic metaconglomerates, thick metasandstones, and carbonate-rich metaturbidites. The carbonate-bearing units in the MMF were mapped by early workers as Murphy Marble (MM) of the HRG. Subsequent work by Fairley (1963) demonstrated that these units were distinct from and stratigraphically above the MM and within a unit that he mapped as the Andrews Schist calcareous facies (ASCF). Recent work has shown that most of Fairley's Andrews Schist is MBG. These carbonate units are important as marker units for structural analysis, and because of their diverse sedimentologic origin and tectonic implications. They have been informally called the “Keithsburg marble” or ASCF. Here they are referred to as the Keithsburg facies (KF) of the MMF. Folding associated with the MS results in at least two outcrop belts of the KF in north Georgia. Several important stratigraphic relationships distinguish the KF from the MM. First, unlike the MM, the KF is never in contact with the underlying Brasstown Formation. Second, the siliciclastic-rich KF is lithologically distinct from the relatively pure marble of the MM. Finally, the KF always occurs within the MMF several hundred meters above the basal MMF contact. These relationships show that the KF must have been formed from a diverse source, most likely exposed areas of eroding HRG (including the MM) and possibly GSG.