Geologic Story of Cades Cove
Erosion by Abrams Creek and its tributaries has exposed Ordovician (450 million year old (my)) Jonesboro Limestone that contains various types of fossils beneath the Great Smoky fault. Within the Great Smoky Mountains National Park , the Great Smoky fault is a regional thrust fault that juxtaposed Precambrian (770-545 my) metamorphosed sandstone and siltstone of the Ocoee Supergroup onto the nonmetamorphosed limestone of the Knox Group with tens to hundreds of miles of displacement. From southwest to northeast in eastern Tennessee, the area near Calderwood, Cades Cove, Crib Gap, Tuckaleechee Cove, and Wear Cove, are erosional windows through the Great Smoky fault that expose carbonate rocks usually restricted to the Great Valley of the Valley and Ridge Province, west of Chilhowee Mountain. The bedrock reveals a complex geologic history.
Rocks above the Great Smoky fault were deposited "before the Cambrian", thus they have been called Precambrian, Late Proterozoic, and Neoproterozoic--all terms for a period of time between the Middle Proterozoic (Mesoproterozoic) and Cambrian. These Neoproterozoic rocks are called the Metcalf Phyllite (Zm), Elkmont Sandstone (Ze), Cades Sandstone (Zc), and Shields Formation (Zs), of the Ocoee Supergroup. They do not have fossils so their age is uncertain. The rocks are younger than the underlying 1 billion year old granitic gneiss exposed near Cherokee and Bryson City, NC, and they are older than the 545 my fossiliferous Cambrian rocks that overly them. They are clastic sedimentary rocks, composed of pieces and fragments (clasts) derived from the erosion of the granitic gneiss. These clastic rocks were deposited in a deep water basin that formed as the early continent of North America (called Laurentia), rifted, or split apart, resulting in the formation of an ancient ocean called Iapetus that existed 345 my before the Atlantic Ocean.
Rocks of the Ocoee Supergroup are subdivided into the Snowbird Group (Metcalf Phyllite), Great Smoky Group (Elkmont Sandstone, Cades Sandstone), and the Walden Creek Group (Shields Formation). These rocks were named after localities in and around the park where they are well exposed (King and others, 1958). Metcalf Phyllite was named for rocks typically exposed at Metcalf Bottoms, Elkmont Sandstone for rocks exposed near Elkmont, Cades Sandstone for rocks exposed on Cades Mountain, and Shields Formation for rocks exposed on Shields Mountain east of Pigeon Forge. These rocks were metamorphosed and structurally deformed at depth in the earths crust tens to hundreds of miles east of the park, about the same time that the Ordovician Jonesboro Limestone was being deposited in shallow water of the Iapetus Ocean. Rocks similar to the Ocoee Supergroup were buried deeply beneath the limestone as it was being deposited.
The contacts between all of the rock units are thrust faults of undetermined displacement. The Greenbriar fault places Elkmont Sandstone of the Great Smoky Group on top of Metcalf Phyllite of the Snowbird Group. The fault predates early Paleozoic regional metamorphism and it has been folded (Hadley and Goldsmith, 1963). Since the fault places younger rocks on top of older rocks, the Greenbriar fault is interpreted to be a normal fault that formed in the Neoproterozoic extensional rift basin (Robinson and others, 1992). In this area, all of the Ocoee Supergroup rocks and the Greenbriar fault were folded, faulted, and metamorphosed to biotite-chlorite grade of greenschist-facies in Ordovician and Devonian time (Connelly and Woodward, 1992; Connelly and Dallmeyer, 1993). Both the Metcalf Phyllite and Cades Sandstone are thrust on each other, and the Cades Sandstone was thrust onto the Shields Formation along the Rabbit Creek fault. The Rabbit Creek fault may be part of the Dunn Creek fault that places Snowbird and Great Smoky Group rocks onto rocks of the Walden Creek Group, east of Pigeon Forge.
Approximately 250 my ago the Iapetus Ocean closed as the continental tectonic plates of North America and Africa/Eurasia collided resulting in the Alleghanian orogeny. The folded and faulted metamorphosed rocks were uplifted and transported westward tens to hundreds of miles along the low-angle Great Smoky fault . Even later, the Gatlinburg fault cut the other faults and it still forms a straight prominent escarpment on the south side of the cove.
The primary foliation, or layering, in the Elkmont Sandstone, Cades Sandstone, and Jonesboro Limestone is bedding, which is mostly indicated by graded grainsize, and aligned rock fragments and fossils. Elkmont Sandstone and Cades Sandstone have a secondary foliation, called cleavage, that is related to folding. The primary foliation in the Metcalf Phyllite and Shields Formation is pervasive cleavage. The secondary foliation in the Metacalf Phyllite is spaced crenulation cleavage, and it is bedding in the Shields Formation.
Beds of Elkmont Sandstone, Cades Sandstone, and Shields Formation were folded, cleavage developed, and faults formed during early Paleozoic regional metamorphism and deformation. The lustrous Metcalf Phyllite was produced from shearing between faults.
Late Paleozoic folding and faulting related to emplacement of the Great Smoky thrust sheet produced upright folds with crenulation cleavage in the Metcalf Phyllite and broad folds in the Jonesboro Limestone.
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