METACHERT VS. MYLONITE: CONSEQUENCES OF MISINTERPETATION, TALLADEGA BELT, SOUTHERN APPALACHIANS

The Jemison Chert (JC) is a key fossiliferous late Early to early Late Devonian unit at the SW end of the Appalachian orogenic belt immediately N of the Coastal Plain. For >80 years geologists have included the JC (composed of metachert, subordinate siliceous argillite, carbonaceous phyllite, and metsasandstone) as a constituent unit within the Talladega slate belt (TSB) (SW extension of the W. Blue Ridge allochthon-WBR), and mapped it to the NE along much of the belt’s length, indicating that it is one of the youngest units in the metamorphic Appalachians. The presence of the JC and underlying fossiliferous Butting Ram Quartzite (BRQ) are important components of the decades-old argument that the Talladega belt (WBR allochthon) contains an anomalously young stratigraphy. Recently, however, it has been suggested (Higgins and Crawford, 2008) that the JC and BRQ are not constituent TSB units, and terminate against an unmapped ~NS fault beneath Interstate 65, questioning the existence of post-Cambrian rocks in the TSB and asserting that the rocks previously mapped as JC and BRQ are instead an unrelated belt of “Permian-Ordovician” mylonites which occur adjacent to and directly along strike of the JC. A number of relationships indicate, however, that the rocks mapped as “mylonites” are in fact, simply a sequence of metachert (JC) and metsasandstone/metaconglomerate (BRQ) that continue to the NE of I-65 as constituent units of the TSB for ~175 km. They exhibit stable greenschist facies metamorphic microstructures and slaty cleavage, but do not have the petrographic characteristics of mylonites. Some rocks mapped as mylonites even contain weakly deformed sponge spicules, brachiopods, and detrital silt grains. Sandstone/conglomerate grains are well sutured but do not show large distortions. SEM studies of the metachert both W and E of I-65 show pervasive, annealing-type, polygonal triple-point quartz microtextures that contrast strongly with non-metamorphic chert (King and Keller, 1992). Mean apparent crystal diameters progressively and gradationally increase from NW to SE across I-65, accompanied by a progressive development of slaty cleavage seen both in SEM and thin section in rocks both W and E of I-65.