File:Concretions in sandstone (Thunderhead Sandstone, Neoproterozoic; Clingmans Dome, Great Smoky Mountains, North Carolina, USA) 3.jpg

English: Concretions 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 conspicuous rounded masses are concretions, which are post-depositional structures found in some sedimentary successions, particularly in shales. They vary in mineralogy, size, and shape, but often have slightly flattened spheroidal shapes. They typically form by post-depositional mineralization around some nucleus (a rock fragment or fossil fragment). The concretions at this locality are in sandstone and have a spherical to subspherical to slightly irregular shape. Note that in this photo, two concretions have merged to form a somewhat dumbbell-shaped mass.

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)