By Chris Kozloski
The Southern Appalachin Mountains were formed in three orogenies. The first of these was the Taconic orogeny that occured 450-500 million years ago in the mid to late Ordivician. During this orogeny, the Proto-North American plate collided with the westward moving Piedmont continental fragment causing the accretion of the latter onto Proto-North America along the Blue Ridge-Piedmont boundary. This marks the first period of deformation as the sediments from the now closed basin that was between the two plates were thrusted upon Proto- North America. Following the Taconic, was the Acadian Orogeny that occured in the mid-Devonian around 350-400 million years ago. This event caused extensive deformation and thrusting as the Carolina Shale Belt Island Arc collided with the now accreted Piedmont plate after the closing of the Iapetus Ocean. Once again, miosynclinal sediments form the former ocean were overthrust upon land. (The Proto-North America/Piedmont Plate) Finally, a third colisision took place, this time between Proto-North America/Peidmont/Carolina Slate Belt and Africa. This was the Allegheny Orogeny and is charactorized by extensive overthrusting and igneous activity in the southern Appalachins. This occured 250-300 years ago which places the event in the late Paloezoic.
These three Orogenies, plus 250 million years of erosion, are the progenetors of the modern Appalachin landscape. Geologists have classified this landscape into several physiographical provences based on geological and structural features with similar histories. Traveling from west to east, these provences are: the Appalachin Plateau, the Valley and Ridge provence, the Blue Ridge provence, the Inner Piedmont provence, the Carolina Slate Belt, and finally, the Coastal Plain. Taken in order these provences are breifly charactorized as follows:
The Appalachin Plateau is a structural basin that marks the most westerly deformation associated with the Appalachin orogenies. Gently folded throughout most of its area the basin becomes more convoluted ad one travels east, finally ending with subsurface thrust faults and decollement allochthons. The Cumberland Plateau and Eastern Kentucky coal fields fall into this provence. East of the Appalachin Plateau is the Valley and Ridge provence. As the name implies the region is charactorizes by alternating topographical valleys and ridges that strikr NE to SW. These ridges are the upthrust ends a series of imbricate thrusts that ride along a sole fault on the crystaline basement. Walden and Misssionary ridges around Chattanoga Tennessee are examples of these thrusts.
Next, as we travel east and cross the Great Smokey Fault, there is the Blue Ridge provence. Charactorized by extensive deformation and metamorphisim from all three southern appalachin orogenies, the Blue Ridge marks the colision of Proto-North America and the Piedmont Fragment. Once thought to be exposed preCambrian basement, The Blue Ridge provence has been proven to overly undeformed Cambrian rocks. It is now concluded that The Blue Ridge provence is an allochthon overthrust during the mountain building processes that shaped the region.
After the blue ridge there is the Inner Piedmont provence. Charactorized by volcanic and plutonic rocks this region contains the remenants of the old seafloor that once separated the Proto-North America and Peidmont plates before the onset of the Acadian.
Next we enter a zone of metamorphaosed volcanic material called the Carolina Slate belt. Volcanics, ranging in age from 700 million (preCambrian) to 500 million (Late Cambrian) years old represent the material produced by subductrion ans the Carolina Slate Belt Island Arc moved towards Proto-North America / Peidmont. The Eastern portion of this arc is overlain by the Coastal Plain which finishes off the provences with its 200 million years of recent sediment.
Now for the field trip. The first stop we made was at the Cumberland Plateau Decollement zone near Dunlap Tennessee- the furthest deformation of the Alleganian orogeny. The purpose of this stop was to examine a major bedding plane thrust fault that was the plane of movement for the decollement. This fault was centered on a weak coal seem that was exposed in the upper part of the road cut. Deformational structures such as drag, Z, and S folds were apparant throughout the cut. hese folds indicated that the units above the coal seem, the fault, were relatively undeformed, while the units below, which bore the stress of the overthrusting allochthon, were strained.
The second stop we made was in Chattanooga at an overlook on Walden ridge, where we overlooked the valley and ridge provence. We observed the series of parallel ridges that charactorize the region. Walden Ridge, which we were standing on, its continuation across the Tennessee river, Signal Mountain, and Missionary Ridge are examples of some of these ridges. The origin of these features are a series of imbricate thrust faults that expose Pennsylvanian sandstone units. Since these units erode at slower rates then the surrounding limestones, the ridges are higher than the surrounding countryside. The pattern is formed by the parallel faults.
Stop number 3 was located at Ocoee dam #1 in Ocoee Gorge Tennessee. It is located at the transition from the Valley and Ridge provence to the Western Blue Ridge. The rocks in the road cut exhibit extensive brittle failure due top their proximity to the Great Smoky fault. This failure is the lowest of increasing metamorphic facies that we traveled through on the trip. With no metamorphisim and little ductility it portrayesn how rocks fail without the great heat and pressure to be seen at later stops.
Stop "B" at Maddens Branch was done from the seat of our vehicle due to space restrictions in the road cut and shoulder. The site presented the metamorphisim of the Murray shale unit of the Chilhowee group so it exhibited slaty cleavage. Also Several boudins were present. This was the next step up the ladder of metamorphisim from stop "A". (To save time and space later let me mention that all of the subsequent stops made in the trip are in increasing grade of metamorphisim as we change facies. I will reverse the order of stops 4 and 5 to adhere to this parameter.)
Stop "C" on the ocoee gorge trip marks the onset of prevalent deformation of the rock units. Both brittle faulting and ductile pinching of the layers are exhibited. Severe folding also occurs throughout the varying units, a large anticline and overturned syncline being promenent features. This extensive deformation, plus the onset of prominent ductility, represents the next step in the increase of the grade of metamorphisim.
The final stop (Stop "D") of the Ocoee Gorge section of the trip was the stop at Boyd Gap. The change of constituaants from shales and slate to phyllite and the clastics to prevalent quartzites and metagraywackies indicate increased pressure in the region. Also, large veins of pressure solutioned quartz start to become more prevalent. Slaty cleavage and folding round out the list of features for this stop.
Chunky Gal Mountain and Glade Gap should have been our fifth stop but we missed the outcrop on our first pass so it was our sixth. The purpose of the stop was to observe lithologies that charactorise the medium grade metamorphisim of the region-here derived from the Hayesville thrust. The Chunky Gal Mountain Fault, boudins, and gneisic lithologies are the highlights of this stop. I would like to elaborate on Glade Gap but since we never found the stop I don't know what to emphasize.
Finally, we reached the thermal peak of deformation with the last stop at Winding Stair Gap, the best exposure of granulitre facies in the southern Appalachins. Six high grade metamorphic facies are represented along the stop and we spent most of our time looking for them. These facies were formed in melts and recrystalization of an origonal sequence of sedimentary and volcanic rocks with a sliver or ultramafics. Extensive Boudinage indicates the origonal compositional layer. This stop represents the most advanced deformation in the southern appalachins. It was my favorite stop for this reason- and the neat rocks I collected.
Normally At this time I would go into a fancy closing paragraph where I try to summarize everything in the report but I am not about to thrust myself into the obligation. You cant fault me, its a normal reaction to stress. It is now time I inflict my revenge with puns for the convoluted bedding schedule of quick nappes I've been running to finish this report on time. So, in closing, I leave you with these Questions and statements:
I thought the moorman syncline was located in Utah, not Kentucky.
My first attempt at fault nomenclature was accompanied by a lot of heaving objects, pitching fits, and throwing tantrums but my understanding hinged on your translation which offset my problems and allowed me to separate the facts from the garbage.
I really enjoyed your letting us look for exposures at Potter Hall. I saw some impressive cleavage through the window.
Some of your test questions on material types were viscous
I am doing strain puns for the sheer joy of it.
Principle stress is a teachers strike
How's "practice test" for an oxymoron.
fast food - pun and oxymoron
and finally... Beware of falling breccia. It might gouge your eye out!