This section of the analysis of the Appalachian Mountains discusses the sedimentary deposits we observed on our trip. We inspected rocks at Thorn Hill, Pine Mountain, Hazard, the Kentucky River Fault Zone, Natural Bridge Park, and the surrounding area. We also looked at rocks at Bays Mountain and Ocoee Gorge, although not in as much detail.
1. Rome Formation
The Rome Formation is the bottom of the exposed section at Thorn Hill. The Rome Formation is early Cambrian in age. The Copper Creek thrust fault cuts through the Rome Formation. The rocks have a reddish color, and also, in some places, it shows mud cracks. Under the microscope, the calcite is very fine grained and well sorted.
This series of rocks could have been deposited in a continental shelf environment. The Rome Formation must have been in shallow water where the sediment could be exposed to air long enough to form mud cracks and for the cement to be oxidized to produce the reddish color we see under the microscope.
2. Conasauga Group
Both the Maryville Limestone and the Rogerville Shale are in the Conasauga Group. The limestone and the shale are interbedded. The limestone is massive, sometimes ten meters thick. There are no mud cracks like in the Rome Formation. There are shell fragments as well as evidence of bioturbation.
The Conasauga Group, would have been farther out in the water than the Rome. Since there were no mud cracks, these rocks were not exposed to air, but if they were, it was not for very long. The very thick limestones and shales and the thickness of the limestones indicate that there were no tides strong enough to disturb the sediments. The depth of the water would have been deeper than for any in the Rome Formation.
3. Chickamauga Group
While the Knox and Conasauga Groups are early Ordovician, the Chickamauga Group is middle Ordovician. There are sand-sized grains within the limestone. There are ash layers in this rock. Also, in the ash beds there are rip up clasts, nice lamination, but some soft sediment deformation. There is very little siliclastic input in the limestone.
The Chickamauga Group contains ash beds with nice lamination which indicates that these beds were relatively undisturbed.
4. Moccasin Formation
Brachiopods, corals, bryozoans, and criniods are very prolific in the beds of the Moccasin Formation. The limestone fossil beds alternate with limestone beds that contain no fossils. These beds are several tens of centimeters thick. In upper layers, mud cracks are visible. The beds are nicely laminated.
The Moccasin Formation has beds that are meters thick, some with fossils and some without. This would have to have been a changing subaqueous environment. This could be the time where water begins to recede from the beach.
5. Martinsburg Formation
This formation contains interbedded shales and limestones; there are no fossils in the shales. The limestone is mud rich and micritic. There is definite graded bedding. Ripple laminations are visible. Under a microscope, the limestone is poorly sorted. There are beds that contain shell hash that is graded.
The Martinsburg Formation shows ripples, so it must have been under water but far enough away from the shore that the waves still affect the sediment but are not strong enough to destroy them.
6. Juniata Formation
In this formation, mud cracks, plant roots, burrows, and a reddish color dominate. Under the microscope, we can see small quartz crystals in a muddy matrix which appears red. This rock is a limey mudstone to limestone.
The Juniata Formation shows a reddish color which usually means that the rock has been oxidized, and we can see (under the microscope) that it has an oxidized matrix. Also, there are small quartz grains which could be indicative of the next step in moving toward the upper delta plain.
7. Clinch Formation
This sandstone shows planar asymmetric cross stratification from which paleocurrent data could be collected (see figure data-1). There were fossils that were either roots or traces of burrows. The sands are large grains that are well sorted and subangular. This sandstone is very pure, there is almost no cement.
The Clinch Formation is a very pure sandstone whose grains are well rounded, which usually means that they have traveled a long distance (to be so well weathered). The sand would have been deposited first, the grains being the largest and would have dominated the beach above the tides.
At Pine Mountain, the most striking feature is the red shale. Also, there are no coals. In the Devonian sand and shale sequence, paleocurrent data was collected (see figure data-2).
The red color comes from oxidation. Some of these shales originated at a soil horizon where dead plants could oxidize which also indicates that they were not quickly buried. Paleosols are difficult to preserve. (Figure photo-1 is a photograph of Pine Mountain).
The Lexington Limestone has no fossils. There was minor bioturbation in some layers of this formation. These layers alternate between a thick, grayish layer and a thin, yellow-tanish layer in a general fining upward sequence. In almost every layer, we see mud cracks. Closer examination reveals very fine laminations and very definite grading. The sequence repeats itself for several meters (figure photo-2 is a photograph of the outcrop).
This carbonate platform shows characteristics of a supratidal environment. It has mud cracks and laminations. The fossils could have been brought up during an especially high tide since they are shallow marine life. Also, in the Lexington Limestone, there are mud intraclasts wish are also a diagnostic sedimentary structure.
Natural Bridge Park and Area
In the Natural Bridge Park, the Breathitt is in contact with the Newman and there is an unconformity between them. There are large planar foresets and uni-directional paleocurrents. We were able to collect paleocurrent data (see figure data-3). We also observed some larger grains imbedded in the fine grained rocks. (Figure photo-3 is a photograph of the Natural Bridge.)
In the general area around Natural Bridge, we can see channel like geometry with clear evidence of erosion; there is an obvious uncomformity. There are low angle foresets which are perpendicular to the channel and ripples are visible. The paleocurrent changes directions (see figure data-4). We also see coal seams which are overlain by sand that lacks evidence of scouring.
These rocks were most likely deposited by some kind of fluvial system. The obvious erosion that we see was probably due to a high velocity fluvial system. The planar cross bedding at the park indicates a fluvial system as well. We also see flaser bedding which could indicate an inconstant flow velocity. This could have been a meandering river system, the scouring taking place at the inside turns of the river. The coal beds could have been deposited after the river had moved away.
The Haddix Formation contained many terrestrial fossils, mostly plants (figure data-5 is a measured section). It also contained coals. In this formation, the contacts are fairly comformable. The sandstone at Hazard is much less clean that the sandstone at Natural Bridge. Fire clays are seen near the coals. In some places, it was possible to collect paleocurrent data (see figure data-6).
These rocks could have been deposited very close to the ocean, or in a place where the water table would have been high enough to support plant life, but not high enough to kill it, and also high enough to preserve the dead plants or at least to keep them from decomposing. The fire clays show that oxidation did occur. This could have been part of a large river system, like a meandering system, such as at Natural Bridge.
The rocks that formed Appalachian Mountains were obviously deposited in a marine setting. The rock sequence shows that these rocks had to be deposited on the edge of the continent. Some of them have been oxidized and others show evidence of very shallow marine life (bioturbation),while still more show no evidence of marine life. Some fossils show plant life and coal beds near each other which also implies that it was near a large quantity of water. As sediments, these rocks could have traveled through meandering rivers to the edge of the continent and be carried out to sea to settle once they reached a beach or a delta.
General Stratigraphic Column for the Appalachian Mountains
|Knox Conasauga||1000m||L. Ord.|