Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Reflection seismology surveys in Georgia and North Carolina have undoubtedly demonstrated the allochthonous nature of the western Piedmont. Adjoining parts of South Carolina are also most probably allochthonous. Beneath the allochton are mildly metamorphosed Cambrian and Ordovician sediments. Although no drill holes have pierced autochthonous sediments beneath the western Piedmont, these seismic data and geologic evidence in the Grandfather Mountain window confirm the presence of sediments. Most probable formations include equivalents of latest Precambrian Ocoee Supergroup, Cambrian Chilhowee Group, and possibly up to the Lower Ordovician Knox Group.

Strong reflections are best observed on the COCORP southern Appalachian survey, Lines 1 and 2. Reprocessing of these data on the University of Wyoming VAX 11/780 has yielded seismic sections of greater resolution than those first presented by Cook and others (1979, 1981). Their interpretation of the reflection data hypothesized the continuation of a master decollement and underlying metasediments from the western Piedmont into the eastern Piedmont, Charlotte belt, and Carolina slate belt. From an additional five seismic lines, Cook and others (1981) presented the idea “... that a major detachment extends eastward beneath the crystalline rocks of the Eastern Piedmont and Coastal Plain.”

An alternative interpretation of these seismic data is presented which terminates the metasediments beneath the allochthonous Piedmont approximately 60 km southeast of the Brevard zone. Seismic definition of the master decollement surface in the eastern Piedmont is less well defined. Optimum data processing has emphasized the important series of dipping reflections beneath Kings Mountain Belt in the eastern Piedmont. These reflections are observed on COCORP Lines 1, 4, and 5 to a depth of at least 15 km and possibly down to 20 km. Limited three dimensional data indicates dip around 30° east. These prominent dipping reflections are interpreted as a zone of intense deformation due to many hundreds of kilometers total displacement along a wide zone of thrust faults, the master decollement root zone into the lower ductile crust. This interpretation differs considerably from Cook and others (1979, 1981) where the prominent dipping reflections are interpreted as Cambrian continental-slope deposits beneath a regional detachment. My interpretation views these reflections as representative of the master decollement root zone which might exist along the entire Appalachian chain. Reflections beneath the eastern Piedmont are a result of complicated structure within an ancient island arc accreted to the coast of North America.