Doctoral Dissertations

Date of Award

8-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Abstract

The Oak Ridge Reservation is located approximately eight kilometers southeast of the Cumberland Plateau in the most foreland portion of the southern Appalachian fold-thrust belt in the east Tennessee Valley and Ridge province. On the reservation, a detailed understanding of the factors controlling ground water flow paths, flow rates, residence times, and storativity is needed when planning to restore and protect the environment from past and present disposal operations. Although numerous detailed site specific studies have been conducted across the reservation to characterize ground water flow, the Oak Ridge Reservation Hydrology and Geology Study is a reservation wide investigation of the geology and the ground water flow system. Since the only available geologic map of the entire reservation was made from reconnaissance mapping in the fifties, the first phase of the project is to remap the area in detail and record all mesoscopic structures. Although remapping of the entire reservation is not yet complete, my assignment was to map part of the reservation (Bethel Valley quadrangle and surrounding area) at a scale of 1 : 12,000. As a result of mapping, structural analysis, and compilation of regional data, knowledge of the geologic evolution of the area is improving which will aid in ground water flow characterization.

The Copper Creek, Whiteoak Mountain, and Kingston faults cross the map area and repeat the Paleozoic strata in three northeast striking and southeast dipping thrust sheets. The stratigraphy ranges in age from the Lower Cambrian into the Lower Mississippian and consists of 33 formations. The Early Cambrian through Early Ordovician stratigraphic units (Rome Formation; Conasauga Group, and Knox Group) have similar characteristics in each thrust sheet. Particular emphasis, however, was placed on dividing, naming, and correlating the stratigraphy of the Middle and Upper Ordovician Chickamauga Group in the Whiteoak Mountain and Kingston thrust sheets. Results indicate that the Chickamauga Group in the Whiteoak Mountain sheet can be correlated with units previously defined along strike in the Valley and Ridge of northeast Tennessee, but the Chickamauga in the Kingston sheet can be correlated with units in middle Tennessee (Stones River and Nashville Groups). Displacement on the Whiteoak Mountain fault is considered the cause for juxtaposition of such different stratigraphic sequences in the Chickamauga Group.

Structural analysis of the area indicates that the Copper Creek and Whiteoak Mountain faults have distinctly different structural geometries that represent different elements of a thrust system. The Copper Creek fault places a lower hanging-wall flat in the Rome Formation on a footwall flat in the Moccasin Formation and the Whiteoak Mountain fault places a lower hanging-wall flat in the Rome Formation on a footwall ramp. Both faults have a regional extent with displacements that may range from a minimum of 10 km, to as much as 70 km. The present southeast dip of the Copper Creek fault is due to the folding of the Copper Creek fault by displacement along the Whiteoak Mountain fault. Folding of thrust faults, such as the Copper Creek, clearly documents the hinterland-to-foreland sequence of thrusting in the area, however, minor imbricate faults formed in both a break-forward and break-back sequence. In addition, the Whiteoak Mountain fault may have reactivated out-of-sequence during continued expansion of the fold-thrust belt. A change in the mechanical characteristics of the stratigraphy cannot be used to explain the different structural geometries between the Copper Creek and Whiteoak Mountain faults. Other external variables, however, such as, strain rate and confining pressure may have influenced the deformational style. Finally, results suggests that the critical taper of the southern Appalachian thrust belt wedge may been have been maintained by a combination of out-of-sequence thrusting and toe versus underplating accretion mechanisms.

The low effective matrix porosity and permeability of the rock units has resulted in ground water flow paths that are controlled by the fracture system. The fracture system in the area consists of systematic and nonsystematic fractures. The systematic fracture sets were V studied in detail because their characteristics are- considered the most suitable for transmitting ground water long distances: The predominant fracture sets are oriented normal to bedding and are bedding strike parallel with northwest dips and bedding strike perpendicular with either northeast or southwest dips. The majority of the fractures appear to be Mode I extension fractures, although hybrid and shear fractures also occur. Besides strike-parallel and strike - perpendicular fracture sets, a number of additional fracture sets have developed, but are not as pervasive across the area. For example, some fracture sets only formed adjacent to the major thrust faults. The predominant fracture sets appear to have formed prior to regional folding and faulting because they are cut by bed-parallel fractures and veins, have commonly twinned calcite filling, and have orientations controlled by bedding geometry. The early formed fracture sets, however, may have been reactivated during the development of the fold thrust belt. In addition, regional compilation of fracture data indicates that similar fracture sets occur in the Plateau province suggesting that the fractures in the area did not form as a result of active folding and faulting. Analysis of the stress magnitudes produced during basin subsidence was coupled with fracture mechanics failure criteria to model the growth of the prethrusting Mode I extension fractures.

Lastly, a numerical model has been developed to assess the influence of rock fracture toughness, confining pressure, and strain rate on the growth of Mode I cracks located in the hinge region of a buckle fold. The model was developed to examine the relationship between buckle fold evolution and layer fracturing, but does not apply to any particular structure in the map area. The approach involves coupling fracture mechanics theory with finite element models of buckle folding to examine some of the variables that influence the development of folds in foreland fold-thrust belts. The propagation of cracks from preexisting flaws is one mechanism of strain softening a layer undergoing buckle folding and localizing the development of a throughgoing fault. Initial results indicate that particular combinations of confining pressure, strain rate and rock fracture toughness will prevent cracks from propagating and weakening the layer allowing the development of a tight fold. However, other combinations will result in little folding before cracks propagate from flaws and possibly cause failure of the layer. The results of the model are discussed in relation to the development of structural geometries in a thrust system where initial buckle folding is important.

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