Pine Mountain Thrust Fault

ES 767 Global Tectonics, Spring 2010

Wesley C. Smith

Geographic Setting

Pine Mountain, located in the eastern Kentucky Coalfields is a near linear ridge approximately 200 km in length that extends from near Jellico, Tennessee to Elkhorn City, Kentucky (Fig. 1). The crest of Pine Mountain rises gradually from the southwest to the northeast with elevations ranging from less than 670 m to more than 975 m (McGrain, 1983). These heights are possible due to the erosion resistant, quartz rich lower Pennsylvanian sandstones present at the crest. (Chesnut et al., 1998).

Figure 1. Physiographic map of Kentucky (McGrain, 1983)

Geologic Setting

Pine Mountain and the Pine Mountain thrust sheet are part of the four sided, trough shaped, Cumberland Overthrust Block, (McGrain, 1983) which is approximetly 200 km long and 40 km wide (Mitra, 1988). The block was pushed laterally by mountain building forces approximetly six miles from the southeast (McGrain, 1983). The Pine Mountain overthrust sheet is bounded to the west by the Pine Mountain Fault; which is bounded to southwest by the Jacksboro fault and to the northeast by the Russell Fork fault (Stearns, 1955). The Pine Mountain Fault is a bedding plane fault throughout most of its aerial extent (Miller, 1973) and dips to the southeast with a strike of about N60E (Kilsdonk et al., 1988). The fault represents the westernmost major thrust in the Appalachian thrust belt system separating the Valley and Ridge and Appalachian Plateau provinces (Mitra, 1988).

Figure 2. Generalized geologic map of Kentucky showing location of Pine Mountain Fault (McGrain, 1983))

The structural style of the Appalachian thrust belt system consists of a sequence of east-northeast trending major thrusts that climb from a detachment in the Cambrian Rome formation through successively younger units that flatten into a detachment in the Devonian Chattanooga Formation or climb to the surface (Mitra, 1988). Dips range from gentle to moderate in the southeastern direction (Mitra, 1988). (Fig. 3)

Figure 3. Generalized stratigraphic column of Appalachian Thrust System showing positions of major detachments (Mitra, 1988)

As shown in Fig. 3 and as described in Mitra, 1988 the column can be divided into 6 different lithotectonic units:

1) Precambrian crystalline basement and the Precambrian and Cambrian clastic sequence

2) Lower Cambrian Rome Formation and Middle to Upper Cambrian Conasauga Formation (mostly incompetent shales and some carbonates)

3) Cambrian Maynardville Formation and the Cambro-Ordovician Knox Group (consisting of interlayered dolomites and limestones which act as competent units and deform by fracturing and imbrication)

4) Middle Ordovician Chickamauga Group (mostly limestones which deform by minor folding, imbrication, fracturing and pressure solution)

5) Upper Ordovician, Silurian and Devonian units (consisting of an inhomogeneous sequence of thin-bedded shales alternating with limestones and sandstones)

6) Mississippian and Pennsylvanian units (consisting of compenent limestones and sandstones)

(Mitra, 1988)

Tectonic Setting

The Cumberland Thrust Block and Pine Mountain Thrust sheet are the result of deformation and thrust faulting from the continental-continental collision between Gondwana (West Africa) and Laurentia (North America) during the late Paleozoic (Chesnut et al., 1998). The mechanism for this collision began in the middle Ordovician when the neighboring oceanic plate (Iapetus) began subducting beneath the historically passive North American margin (USGS). At the end of the Permian the collision between Gondwana and Laurentia was complete and the Appalachians were raised to Himalayan like heights (USGS). This mountain building event, the Allegheny Orogeny, was the last of the Appalachian mountain building events and along with other collisions formed the most recent supercontinent, Pangea.

US 23 Highway Roadcut

Although the Pine Mountain Fault was already known as a classic example for studying fold and thrust belts (Mitra, 1988), additional studying was initiated when construction of a roadcut for US23 at Pound Gap (Fig. 4) uncovered a nearly complete sequence of Late Devonian, Mississippian and Early Pennsylvanian strata on the Kentucky side of Pine Mountain (Chesnut et al., 1998).

Figure 4. US 23 Roadcut Jenkins, KY (Photo by Wesley Smith)

Thrust faulting brought these rocks which usually lay approximately 610 m below ground in southeastern Kentucky to the surface (Chesnut et al., 1998). Below is a map, cross section and key which illustrates the stratigraphy of the Pine Mountain Fault (Fig. 5-7).

Figure 5. Location of G'-G'' cross section - southeast Kentucky (modified from McDowell et al., 1981)

Figure 6. NW-SE Cross Section through Pine Mountain Fault (see figure above for location) (modified from McDowell et al., 1981)

Figure 7. Geologic Key (modified from McDowell et al., 1981)

The outcrop has allowed stratigraphers, petroleum geologists and other scientists to examine rocks that would normally be at depth and has initiated additional detailed study of the fault. Due to its importance to the scientific community the KSPG designated it as the first “distinguished geological site” (Fig. 8) in the state of Kentucky (Chesnut et al., 1998). Fossils, small folds (Fig. 9) and other geological features can be readily seen throughout the exposure.

Figure 8. Distinguished Geological Site (Photo by Wesley Smith)

Figure 9. Small Fold - US 23 Roadcut near the KY-VA border (Photo by Wesley Smith)

Conclusion

The Pine Moutain Thrust Fault, located in southeastern Kentucky is responsible for the formation of Pine Moutain and a result of the collision between Gondwana and Laurentia. The US 23 roadcut through the fault has allowed geologists to study rock that would normally be buried deep within the crust. These studies have resulted in a better understaning of thrust faulting, tectonic deformation, depositional environments and oil and gas formation. As a result, the site was named the first “distinguished geological site" in Kentucky.

References

Chesnut, D.R., Jr., Eble, C.F., Greb, S.F. and Dever, G.R., Jr., 1998, Geology of the Pound Gap Roadcut, Letcher County, Kentucky: Kentucky Society of Professional Geologists, 169p.

Kilsdonk, B., Wiltschko, D.V., 1988, Deformation mechanisms in the southeastern ramp region of the Pine Mountain Block, Tennessee: Geological Society of American Bulletin, Vol.100, pp. 653-664

McGrain, P,, 1983, The Geologic Story of Kentucky: Kentucky Geological Survey, Special Publication 8, Series XI, 74p.

Miller, R.L., 1973, Where and why of Pine Mountain and other major fault planes, Virginia, Kentucky, and Tennessee: American Journal of Science, Cooper v.273A, pp.353-371

Mitra, S., 1988, Three dimensional geometry and kinematic evolution of the Pine Mountain thrust system, southern Appalachians: Geologic Society of America Bulletin, v.100, pp.72-95

McDowell, R.C., Grabowski, G.J., Moore, S.L., 1981, Publication Number 5859, Geologic Map of Kentucky 1:250000

Stearns, R.G., 1955, Low angle overthrusting in the central cumberland plateau, Tennessee: Bulletin of the Geological Society of America, Vol.66, pp.615-628

USGS, Geologic Provinces of the United States: Appalachian Highlands Province, http://geomaps.wr.usgs.gov/parks/province/appalach.html See source.

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