Dalton is located in the Ridge and Valley Province of Georgia, about 100 miles northwest of Atlanta. The Ridge and Valley is the one area in Georgia that preserves sedimentary rocks that were deposited on the platform of the North American craton during the early Paleozoic. The Paleozoic rocks have been folded and faulted to produce long ridges that trend northeast to southwest. The faults bounding the province are thrust faults where sheets of sandstone, shale, limestone, and dolostone have been pushed northwestward on top of each other. As we will see, however, normal faults also occur in certain rocks of certain time periods. We will examine outcrops at three stops, defining a semicircle beginning on the northeast side of Dalton and ending on the Conasauga River at Tibbs Bridge.
As you can see from the state geological map, the outcrops we will visit lie between the Great Smoky (Cartersville) fault to the east and the Rome fault to the west. Find these two faults either on the above figure or, preferably, on an original copy of the state map which has the map key. Answer the following:
1. Which map units are found east and west of the Cartersville fault in the vicinity of Carters Dam? _______________________________________________________________ 2. Which map units are found east and west of the Rome fault? _______________________________________________________________
The field trip begins at the bedrock outcrop at the corner of the North Dalton bypass (US 76) and Chattanooga Avenue. To reach the outcrop, you take exit 336 from I-75. This will put you on US 76, which you take east for approximately 2 miles to the intersection with Chattanooga Avenue. The above view is looking west along Chattanooga Avenue from the outcrop. Note the long ridge in the background which trends northeast, parallel to the regional strike of the Paleozoic sedimentary rocks. According to the state geological map, the outcrop at STOP 1 is middle Ordovician, Bays formation or Mocassin limestone (Omb). Siltstone, shale, and limestone all occur. Changes in color are quite dramatic but partly due to chemical changes that have occurred since the sediments were deposited. Several large faults cut across the entire outcrop. The rocks have been smoothed and polished, and there is quartz material along the fault planes that formed during the faulting.
3. If the bed labeled A is the same on either side of the fault, would this be a reverse or a normal fault? ______________________________________________________ How can you tell? ______________________________________________________ In spite of the faulting, the rock retains grain size differences and other features that indicate the environment in which the sediment was deposited. Sedimentary structures such as those in the images below indicate a setting between high and low tide with a moderate degree of bioturbation or disturbance by burrowing organisms.
4. Image A shows mudcracks. What do these indicate about the depositional environment? ______________________________________________________ 5. Image B shows symmetrical ripple marks. What do these indicate? Is this the top or the bottom of the bed? a. ______________________________________ b. _____________________________________ 6. Image C shows a filled-in burrow, probably from a worm. Does this mark the top or the bottom of the bed? ______________________________________________________
The beds in the upper two thirds of the outcrop were thrust over the more laminar beds at the base. I measured a strike of N 40 deg E (see my instructions on measuring strike and dip). The beds dip at 78 degrees below the horizontal. Our main reason for visiting this outcrop was to observe the unique trace fossils ("ichnofossils") which Ken Leonard has found on bedding surfaces in this rock.
The small circular depressions in the rock immediately above my rock hammer are ichnofossils which Ken thinks were made by members of the phylum Cnidaria. 7. Look in your textbook and find the phylum Cnidaria. What are some modern organisms which are members of this phylum? _________________ _________________ _________________ _________________ 8. Describe one of the samples of the Conasauga "shale" that we collected from the outcrop at STOP 2: Color(s) ________________________________________ Hardness _______________ Texture __________________________________________________________________ Bedding/Foliation __________________________________________________________
Tibbs Bridge on the Conasauga River is reached by taking Tibbs Bridge Road from Spring Place on S.R. 225. The river channel makes abrupt right angle bends here so that the channel is either aligned with the bedrock strike or running perpendicular to it. Both limestone and shale are included in the unit shown on the state geological map. The outcrop that we will be looking at is under the bridge on the east bank. Shale erodes easily, so that the bank is covered with crumbled pieces of shale. If you break off a fresh slab and closely examine the top of each laminar bedding plane, you will be rewarded by the discovery of a fossil trilobite.
9. Describe one of the samples of the Conasauga shale that we collected from the outcrop at STOP 3: Color(s) ________________________________________ Hardness _______________ Texture __________________________________________________________________ Bedding/Foliation __________________________________________________________ 10. Now compare your descriptions and the samples from STOP 2 and STOP 3 with each other and with samples of shale, slate, and phyllite provided by your instructor. What rock name would you assign to: STOP 2 outcrop ___________________________________ STOP 3 outcrop ___________________________________
Chowns, T. M., 1972, Sedimentary environments in the Paleozoic rocks of northwest Georgia. Georgia Geological Survey, Atlanta.
Cressler, C. W., 1964, Geology and Ground-Water Resources of the Paleozoic Rock Area, Chattooga County, Georgia. Georgia Geological Survey Information Circular 27, Atlanta.
Pickering, S. M., and Murray, J. B., 1976, Geologic Map of Georgia. Georgia Geological Survey, Atlanta.
Munyan, Arthur Claude, 1951, Geology and mineral resources of the Dalton quadrangle, Georgia-Tennessee. Georgia Geological Survey, Atlanta.
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Copyright Donald M. Thieme
Valdosta State University, Valdosta, GA
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