GEOLOGIC MAPPING OF THE JACKSONS GAP GROUP, BREVARD FAULT ZONE, ALABAMA AND ITS IMPLICATIONS FOR SOUTHERN APPALACHIAN TECTONIC EVOLUTION

Miogeoclinal sequences are rarely preserved in the crystalline core of the southern Appalachians. The cover sequence deposited upon Grenville basement in the Pine Mountain window is one occurrence that has been lithologically correlated with units exposed in the Talladega slate belt of the foreland; the latter units are biostratigraphically correlated with Neoproterozoic to middle Paleozoic sequences deposited along the Laurentian margin. We report new lithostratigraphic and metamorphic observations from the Jacksons Gap Group (JGG), a 2,100 m thick package of siliciclastics lying between the slate belt and the Pine Mountain window in AL and GA that lithologically defines the Brevard zone. The JGG has a faulted basal contact (Abanda fault) with metasedimentary units (Emuckfaw Group) and Middle Ordovician granites (Kowaliga and Zana) of the eastern Blue Ridge. Different types of rocks marking the structural base of the JGG are chlorite-hornblende-biotite schist, orthoquartzite, garnetiferous phyllite, and a package of phyllitic quartzite containing interlayers of carbonate-bearing phyllite, sericite phyllite, and metasiltstone. Structurally above these basal units is a micaceous quartzite containing interlayers of metaconglomerate and orthoquartzite, the latter being locally cross stratified. The structural top of the JGG is marked by the Katy Creek fault that emplaced the metavolcanic/metaplutonic Dadeville Complex. Preservation of sedimentary structures requires a low degree of strain in rocks of the JGG. Conglomerates contain clasts ranging up to boulders with aspect ratios of 0.5 x 0.2 m. Conglomerate clasts are mainly orthoquartzite or phyllite, the latter being flattened and deformed around the former. Some units appear to be lower grade than terranes faulted against the JGG (chlorite zone [?] versus kyanite zone, respectively). Some JGG metasiltstones contain bedding/cleavage relations, further elucidating the lower degree of strain that allowed for the preservation of primary structures. We describe possible lithologic correlations of the JGG and how being in between higher grade metamorphic terranes requires a metamorphic inversion, both of which are significant for southern Appalachian tectonics.