Geology of the Milltown Alabama 7.5’ Quadrangle and 40Ar/39Ar geochronology of muscovite from select rocks of the east-central Alabama Piedmont

Abstract

The Milltown 7.5’ Quadrangle is located within east-central Alabama and contains features important to understanding the formation of the southern Appalachians. The Brevard fault zone passes through the Milltown Quadrangle and divides the Laurentian margin metasedimentary rocks of the eastern Blue Ridge from allochthonous metavolcanic units assigned to the Inner Piedmont terrane. In the study area, the eastern Blue Ridge is composed primarily of schists and phyllites separated from the Inner Piedmont by the Brevard fault zone, which is bounded by the Abanda and Katy Creek faults. Within the fault zone are lithologically distinct rocks of the Jacksons Gap Group which can be traced from the Upper Cretaceous onlap in Alabama all the way to Atlanta, Georgia. Within the study area and to the southwest the Jacksons Gap Group can be divided into several subunits that become less distinct to the northeast. The Inner Piedmont is composed of the Waresville Schist, a diverse group of metamorphosed volcanic and volcaniclastic rocks that has been intruded by the Rock Mills Granite Gneiss. Late-stage Alleghanian right-lateral shearing overprinted the Brevard zone mylonites and phyllites an extensive area of rocks in both the eastern Blue Ridge and Inner Piedmont and collectively the affected zone is referred to as the Brevard shear zone. Geological mapping of the Milltown Quadrangle and 40 Ar/ 39 Ar cooling dates for muscovite have produced five key findings. (1) Within the Milltown Quadrangle, Brevard zone lithologies (i.e. Jacksons Gap Group) are not easily separable into individual map units as has been recognized for other areas to the southwest. Units have gradational contacts and display only slight lithologic differences. The Jacksons Gap Group is subdivided into three units: a structurally lower section consisting predominantly of fine-grained garnetiferous, graphitic, quartz phyllites; a middle section of interlayered garnetiferous quartz schists and phyllites; and an upper section of sericitic quartz phyllites. Along-strike structural and/or stratigraphic variations have caused many units to pinch and swell or to be completely excised. (2) The Waresville Schist contains metamorphosed mafic and more felsic volcanics that are extensive enough to potentially be mapped as distinct lithologies. A previously unreported lithology, garnetite, was identified in the Waresville Schist that extends possibly 8 kilometers as a mapable unit, and likely extends farther to the east of the Milltown Quadrangle. The protolith is interpreted as a sedimentary deposit hosted within hydrothermal exhalatives. (3) Late D 2 deformation produced the large scale folding within the Dadeville Complex including the Penton synform that trends N70°E and plunges 28° with a half-wavelength of ~10 km and an amplitude of ~3 km. Notably, the Penton synform does not affect underlying units of the Jacksons Gap Group requiring its detachment from the lower plate along the Katy Creek fault. (4) 40 Ar/ 39 Ar Cooling ages for muscovite extracted from lithologies of the Inner Piedmont are ~319 Ma with a standard deviation of 0.47 Ma and those from the Jacksons Gap Group are ~315 Ma, consistent with dates reported for wholesale uplift and cooling of these terranes. And, (5) muscovite from en echelon quartz veins within the Hog Mountain tonalite are ~321 Ma, indicating that they were emplaced during early Alleghanian metamorphism at shallower depths than the Inner Piedmont, and not during later extension of the Southern Appalachians.