EAST TENNESSEE GEOLOGICAL SOCIETY

The purpose of this trip is to observe structures in the field that we have discussed in class, along with some of the concepts and ideas that we have discussed in class this term. We will be looking at the transition from the external parts of the Appalachians into the internal parts. Similar-but not identical-transitions exist in most other mountain chains in the world regardless of whether or not they are the products of continent-continent collision, like the Urals, Alps, and Himalayas, or if they are part of the Cordilleran chain that extends from Alaska to southern Chile.

For us to accomplish these goals, we will be looking at rocks and structures ranging from an exposure in the Valley and Ridge to exposures in the westernmost parts of the western Blue Ridge. The rocks here range from Ordovician shales and limestones of the Appalachian Valley and Ridge to Neoproterozoic to Cambrian sandstone, shale, and other rock types in the western Blue Ridge that have a totally different appearance and history from the Valley and Ridge rocks (Fig. 1). The principal difference in terms of their origins is the Valley and Ridge rocks were deposited well up onto the ancient North American continental shelf, whereas the older sedimentary rocks were deposited at water depths from very shallow to very deep along the ancient margin in a rifting environment following the breakup of supercontinent Rodinia beginning -750 Ma or even earlier. The Ordovician shale and sandstone at the first stop on the field trip were also deposited in deep water, but in a basin that formed in front of the Taconic mountains, which formed by volcanic arc and related crust being shoved onto the outer continental margin, causing the region to the west (in front) of the overthrust arc material to subside and form a basin to the west. These clastic sedimentary rocks undergo a facies change to shallow-water limestone to the northwest, confirming that the source of the clastic material was to the southeast.

Once we cross the Great Smoky fault and enter Blue Ridge topography we will still be in sandstone, shale, and carbonate rocks that have not been deformed any more extensively than rocks in the Valley and Ridge, despite the fact that these rocks and those to the southeast have been transported some 400 km from the southeast to where they reside today (Fig. 2). We will stop to look at some of the carbonate rocks in the first block of Blue Ridge topography. These rocks are dolomite [CaMg(C03)21-limestone is composed mostly of calcite, CaC03-and are very similar to dolomite that we find in the Valley and Ridge, except that this dolomite unit (called Shady Dolomite) is older than the oldest rock unit of any kind exposed across the Tennessee Valley and Ridge. This rock unit comprises the first carbonate deposited in the Appalachians (from Alabama to Newfoundland) along the North American rifted margin following breakup of the supercontinent, indicating that the margin faced open ocean in the Early Cambrian.

From the vantage point of these carbonate rocks, we can look across a small creek located just down the hill and see that there is a somewhat different topography than that in the valley where we are standing. The creek follows an ancient fault, southeast of which are rocks that are markedly different from any of those to the west. The fault block to the northwest should really be considered a piece of the Valley and Ridge that happens to be located in Blue Ridge topography, so the Great Smoky fault simply brings up a suite of older rocks that do not occur in the Valley and Ridge. The mechanics of formation of this part of the Great Smoky fault, however, is identical to the mechanics of formation of Valley and Ridge faults: the fault propagated along a weak rock unit, e.g., shale, in the sedimentary sequence just below the Chilhowee Group rocks that underlie Chilhowee Mountain and the less erosionally resistant, overlying Shady Dolomite that underlies the linear valley making up Miller Cove. The fault then refracted across strong layers in the Chilhowee Group and Shady Dolomite (e.g., sandstone, limestone, or dolomite) to a higher weakness zone: probably the Rome Formation that overlies the Shady Dolomite and rests directly on the Precambrian basement rocks beneath the Valley and Ridge. The Miller Cove fault, located along and southeast of the creek, however, brought up rocks that contain structures and metamorphism that were produced -460 Ma, so what we are seeing here is an earlier crust-that we could call basement-that has been through an earlier tectonic event and was transported northwestward by a much younger fault, the Great Smoky fault. The expansive but thin mass of crust that comprises the Great Smoky (-Miller Cove-Blue Ridge) Blue Ridge-Piedmont megathrust sheet pushed the Valley and Ridge rocks in front of it like a snowplow: as it moved forward, thrusts propagated northwestward in front of it as snow piles up in front of the snowplow.

We will make several stops in these earlier deformed and metamorphosed rocks, and then arrive in an open, elliptical valley surrounded by high mountains. This valley is underlain by Valley and Ridge limestone and shale of the kinds exposed just west of the Great Smoky fault. This elliptical valley was produced by erosion through the great slab of the Blue Ridge (Great Smoky) thrust sheet to expose the rocks in Tuckaleechee Cove window and in the adjacent smaller but identical windows, Wear Cove (NE) and Cades Cove (SW). Measurement on the map from the southeast margin of Tuckaleechee Cove directly northwest to the trace of the Great Smoky fault where we first saw it provides a minimum displacement of -1 0 km, but seismic reflection and other geologic data permit us to conclude that the Great Smoky fault has a minimum displacement of >400 km (Hatcher et aI., 2007). For a window or series of windows to form, as we have here, the rocks of the hanging wall of the thrust sheet are arched into an antiform (upfold having the shape of an anticline) that was probably intensely fractured along its crest and permitted erosion to breach the thrust sheet and expose the rocks beneath the sheet. We will make three stops at exposures of the Great Smoky fault (counting the one at the boundary between the Valley and Ridge and Blue Ridge) and a fourth on the southeast side of Tuckaleechee Cove where we can see some structures beneath the sheet that enable us to formulate a model to explain the structure beneath Tuckaleechee Cove. We will then return to the frontal block of the Blue Ridge and turn southwestward onto the Foothills Parkway. Once we are on the Parkway we will make several stops to look southeastward into the Blue Ridge and northwestward across the Valley and Ridge. If the weather is clear, we will be able to see the high mountains along the crest of the Blue Ridge on the Tennessee-North Carolina line to the southeast and the Cumberland Plateau to the northwest of Oak Ridge. Our lunch stop will be at a convenient and scenic place along the Parkway, then we will continue southwestward to its southwest end on US 129 at Chilhowee Lake on the Little Tennessee River. We will then drive along US 129 to look at several interesting exposures of rifted margin sedimentary rocks, which were metamorphosed during the early Paleozoic.

Stop Location Map

Tuckaleechee Cove Area Stops (Guidebook Figure 11) Overlaid on LiDAR Topographic Hillshade (download PDF and flip between pages)

Tuckaleechee Cove Area Stops (Guidebook Figure 11) Overlaid on LiDAR Topographic Hillshade

3D Visualizations of Great Smoky Fault Structural Contours, Topography and Geology (PDF download)

Meet Up in the Morning

Stop 1

Stop 2

Stop 3

Stop 4

Stop 5

Bonus Stop

Stop 6

Stop 7

Stop 8

Lunch

Stop 9

Stop 10

Stop 11

ETGS extends special thanks to Dr. Robert Hatcher who graciously contributed assembly and production of the detailed guide book as well as his time and knowledge of the Blue Ridge Foothills Area geology.

Page updated December 10, 2022