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Feature dataset GeologicMap, part of Geologic Map of Northeasternmost Tennessee, and Adjacent Parts of Virginia and North Carolina

January 6, 2023

The geology of an area of 660 square miles mostly in the northeastern corner of Tennessee and small adjacent areas in Virginia and North Carolina is the subject of this report. The region lies principally in the Unaka province, with extensions northwestward into the Appalachian Valley and southwestward into the Blue Ridge province. The report combines results of surveys made between 1941 and 1953 by the U. S. Geological Survey, the Tennessee Division of Geology, and the Tennessee Valley Authority, and is published in cooperation with the Tennessee Division of Geology. Northeasternmost Tennessee is a region of widespread mineralization and was formerly important for mineral production. Iron, manganese, and bauxite have been mined, and the region has been prospected for phosphate, tripoli, zinc, pyrite, and barite. However, mineral deposits are dealt with only incidentally in this report. Chief attention is given to the rock formations, their structure, and their land forms, all of which are basic to an interpretation and evaluation of the mineral deposits. The consolidated rocks of northeasternmost Tennessee are largely of sedimentary origin and of early Paleozoic age, but they lie on a basement of plutonic and metamorphic rocks of Precambrian age. The basement rocks are principally exposed in the Blue Ridge province along the southeastern edge of the region of this report, but they also crop out in smaller areas farther northwest, in diverse structural situations. The basement rocks include some granitic intrusions that were probably injected as sheets at relatively shallow depth late in Precambrian time. But most of the basement rocks are evidently older and have had a much more complex history; their fabrics reflect structures superposed during successive epochs of plutonism, metamorphism, and deformation. During the earlier episodes, in Precambrian time, a terrane whose initial character is unknown was converted by plutonic metamorphism into gneiss, migmatite, and granitic rocks. During a subsequent episode, perhaps in early Paleozoic time, the basement rocks on the southeast were extensively sheared and mylonitized. In later Paleozoic time, when all the rocks of the region were deformed and broken into large-scale thrust blocks, the basement rocks were further sheared along relatively narrow zones of movement. In the northern part of the region the Mount Rogers volcanic group wedges in between the basement rocks and rocks of definite Paleozoic age. The group is a sequence of silicic flows and tuffs and clastic sedimentary rocks many thousands of feet thick, which were probably laid down during latest Precambrian time. The early Paleozoic sedimentary rocks include rocks of the Lower, Middle, and Upper Cambrian series, and of the Lower and Middle Ordovician series. Sedimentary rocks below the Middle Ordovician are 12,000 to 18,000 feet thick, of which the lower 6,000 to 10,000 feet belongs to the Lower Cambrian series. The Middle Ordovician series may exceed 5,000 feet in thickness in places. Because the Lower Cambrian series is very thick, and has been duplicated structurally, it occupies by far the widest area of outcrop in the region. In general, the older sedimentary rocks lie to the southeast, nearest the Precambrian basement, and the younger rocks lie to the northwest, in and near the Appalachian Valley, but in detail the sequence has been disordered by great low-angle thrusts, and lesser folds and faults. The initial Paleozoic deposit, the Chilhowee Group, is a mass of clastic rocks conglomerate, arkose, shale, and quartzite, with some thin beds of basaltic lava in the lowest formation. Diagnostic Lower Cambrian formations are known only near the top, although worm tubes (Scolithus) occur through the upper half. The Chilhowee Group forms the high ridges of the Unaka Mountains. The Chilhowee Group is overlain by a great carbonate sequence, which has been worn down into valleys and lowlands between the mountains. The lower two units of the sequence, the Shady dolomite and Rome Formation, belong to the Lower Cambrian series; succeeding them are the Conasauga Group (Middle and Upper Cambrian) and the Knox Group (Upper Cambrian and Lower Ordovician), a mass of dolomite and limestone with the thin Nolichucky shale present in places at the top of the Conasauga. The carbonate sequence is succeeded by a thick body of shale and sandstone of Middle Ordovician age, the youngest Paleozoic rocks still preserved in the region. Conglomerate interbedded in the Middle Ordovician rocks records an important orogenic episode, earlier than the late Paleozoic orogeny which produced most of the visible structures. The structure of northeasternmost Tennessee is representative of that of the southern Appalachians which were formed during later Paleozoic time and were characterized by great low-angle thrust faults that have been considerably warped. The traces of three major low-angle faults the Holston Mountain, Iron Mountain, and Stone Mountain faults divide the region into four structural units. Northwest of the Holston Mountain fault are the deformed Paleozoic rocks of the Appalachian Valley; between the Holston Mountain and Iron Mountain faults is the Shady Valley thrust sheet, which has been warped down into the Stony Creek syncline; between the Iron Mountain and Stone Mountain faults is the Mountain City window; southeast of the Stone Mountain fault are the plutonic and metamorphic basement rocks of the Blue Ridge province. The rocks of the Appalachian Valley and the Mountain City window are part of the same structural block, and have been overridden 18 miles or more by the rocks of the Shady Valley thrust sheet; this thrust sheet is, in turn, a lower slice of the great overriding mass of the Blue Ridge province than has moved along the Stone Mountain fault. The Shady Valley thrust sheet overrode previously deformed rocks; but the rocks of the thrust sheet lie in the relatively open Stony Creek syncline. Latest structures in the region are a series of right-lateral transcurrent faults, perhaps produced by continuation of thrusting movements southwest of the region of this report. Either during the deformation or shortly after, hydrothermal minerals were introduced locally in the consolidated rocks, producing small deposits of sphalerite, pyrite, specular hematite, and barite. During the Cenozoic era, degradation lowered parts of the land surface from levels near the present mountain summits to levels near the present streams. Degradation proceeded unequally; the limestone and dolomites especially were worn down to lowlands, with the quartzite and other clastic rocks remaining as high mountain ridges. Degradation also proceeded intermittently, with times of stillstand when the weaker rocks were extensively leveled and times of accelerated downcutting. There is little evidence of any former high-level erosion surfaces, except perhaps on the summits of Holston and Iron Mountains, but a very extensive former surface was cut lower down at the level of valley floors that stand several hundred feet above the modern streams. The time of cutting of the valley floor surface was one of deep and prolonged weathering, during which the carbonate rocks (especially the Shady dolomite) were thickly blanketed by residuum, and were in turn covered by quartzite wash from the adjoining mountains. It was also a time of mineralization, when widely distributed deposits of iron and manganese oxides were formed in the residuum, and local deposits of bauxite accumulated in depressions on the valley floor surface. Since the valley-floor surface was formed, the streams have cut down to their present levels, and talus and rock streams have accumulated on the mountain slopes, probably chiefly during the more rigorous climatic conditions of Pleistocene time.

Publication Year 2023
Title Feature dataset GeologicMap, part of Geologic Map of Northeasternmost Tennessee, and Adjacent Parts of Virginia and North Carolina
DOI 10.5066/P9JW4EPK
Authors Philip B King, Herman W Ferguson, Warren B Hamilton, Benjamin R Weinmann
Product Type Data Release
Record Source USGS Digital Object Identifier Catalog
USGS Organization Florence Bascom Geoscience Center