Image: Quartz vein in sandstone (Thunderhead Sandstone, Neoproterozoic; Clingmans Dome, Great Smoky Mountains, North Carolina, USA) 2 (36619574200)

Description: Quartz vein in sandstone in the Precambrian of North Carolina, USA. This is an outcrop of Thunderhead Sandstone in the Great Smoky Mountains. It's one of many stratigraphic units in the ~50,000 feet thick Ocoee Supergroup. The Ocoee has considerable deposits of sandstone and shale originally deposited as sands and muds in an ancient ocean trench. Trenches form along subduction zones, where a tectonic plate composed of thin, heavy oceanic lithosphere dives down into the mantle below another tectonic plate of either oceanic lithosphere or thick, lightweight continental lithosphere. The Thunderhead Sandstone in the park consists of coarse-grained sandstone with granules. It has quartz and potassium feldspar - it's an arkosic sandstone. Subangular grains appear to dominate. Some phyllite interbeds are present - these were originally shales, but have been metamorphosed. The prominent linear feature is a vein - a fracture that have since filled with one or more minerals (in this case, quartz). Note: Outcrops in the Clingmans Dome area have also been referred to as the Copperhill Formation (e.g., see Davis, 2019). But Southworth et al. (2012) notes: "The Thunderhead Sandstone and Copperhill Formation are probably the same unit". Stratigraphy: Thunderhead Sandstone, Great Smoky Group, Ocoee Supergroup, Neoproterozoic Locality: outcrop at Clingmans Dome (trailside & parking lot area), Great Smoky Mountains National Park, far-northern Swain County, far-western North Carolina, USA (vicinity of 35° 33’ 25.08” North latitude, 83° 29’ 46.12” West longitude)

References cited: Southworth et al. (2012) - Geologic map of the Great Smoky Mountains National Park region, Tennessee and North Carolina. Pamphlet to accompany United States Geological Survey Scientific Investigations Map 2997. 54 pp.

Davis (2019) - Roadside Geology of Tennessee. Mountain Press Publishing Company. Missoula, Montana. 385 pp.
Author: James St. John
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