Geology of the 1:24,000 Roanoke East Quadrangle and investigations of the Long Island Creek Gneiss within the southernmost Appalachians, Alabama and Georgia

Abstract

The geology of the 1:24,000 Roanoke East, Alabama, Quadrangle has been determined to be of high mapping priority by the State Geologic Mapping Advisory Committee due to the rapid development along the I-85 corridor west of Atlanta, Georgia. Objectives for this study are: (1) to map and describe lithologies and their distributions; (2) to analyze structures and fabrics; (3) to produce a vector ArcGIS geologic map of the Roanoke East, Alabama, Quadrangle; (4) to characterize the Long Island Creek Gneiss through both geochronological and geochemical analyses, and; (5) to synthesize the geologic history. The key findings are seven-fold. (1) The lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle are not easily divided into separate, mappable units as a result of their lithological similarities and their inter-gradational nature. The current author divides the lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle into two main lithofacies types: a structurally lower section defined mostly by garnetiferous-graphitic-quartz-biotite schists and phyllites interlayered with micaceous quartzites; and an upper section of variably graphitic, garnetiferous-sericite-chlorite schists and phyllites, with no interlayered quartzites. The Long Island Creek Gneiss intrudes these units. (2) The lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle are also not easily distinguishable from lithologies of the immediately adjacent units (the Emuckfaw Group and schists within the Waresville Schist). Outcrops of cataclasites defining the Abanda and Katy Creek faults bounding the Jacksons Gap Group were not observed and, hence, placing the upper and lower boundaries of the package proved to be a difficult task. (3) The current author defines the upper limit of the Emuckfaw Group within the Roanoke East Quadrangle based on the presence/lack of metagraywackes, which occur together with rare, thin amphibolites. I also have defined the lower limit of the Waresville Schist by the occurrence of amphibolites, interpreting them to be part of the metavolcanics of the Dadeville Complex. This contrasts with the placement of the same contacts by workers in Georgia, who instead place these amphibolites within lithologies of the Brevard shear zone. (4) The Long Island Creek Gneiss that intrudes the Jacksons Gap Group does not appear to cross over the boundaries of the Brevard shear zone within the area of the Roanoke East Quadrangle. (5) Early D1 fabrics and lithologic contacts are locally truncated at the Katy Creek fault where syn- to late-D1 fabrics parallel it, implying the juxtaposition of the Dadeville Complex and Jacksons Gap Group during a syn- to late-D1/M1 event. An inverted metamorphic gradient along the Katy Creek fault may have been formed as a result of down-heating that occurred during the emplacement of the hot Dadeville Complex on top of the cooler Jacksons Gap Group. (6) The age of Rock Mills Granite Gneiss remains to be constrained geochronologically, but it is younger than the 466 Ma Waresville Schist it intrudes (VanDervoort et al., 2017). (7) Based on available evidence, the Long Island Creek Gneiss is a highly fractionated melt emplaced at 293.1 +/- 5.3 Ma during the end of the period of Alleghanian plutonism spanning between ~330 and 295 Ma (Lin, 2015), placing a maximum on the timing of right-slip movement along the Brevard shear zone.