CHARACTERISTICS AND SIGNIFICANCE OF REGIONAL UNCONFORMITIES IN SOUTHERN APPALACHIAN METAMORPHIC TERRANES

Unconformities are key features in tectonic analysis because they commonly signal a change in a region's tectonic setting. In metamorphic terranes, however, recognition is difficult because most recognition criteria used in unmetamorphosed sequences either cannot be, or are difficult to apply. It is thus no surprise that most published geologic maps of the metamorphic Appalachians show no unconformities. However, they must be much more common in crystalline terranes than currently thought, and their discoveries have the potential to provide important tectonic insight. Where such contacts exhibit mappable discordance, most unconformities have likely been misinterpreted as faults; in concordant sequences, most have been regarded as conformable. This is the case with the important contacts described here as unconformities. These have the following characteristics: A) Sedimentary structures in sequences above the contact always young away from the contact; B) The contact marks a significant change in lithofacies, and is sharp. Repetition by tight folding can make a contact appear gradational, but those between the differing lithofacies in the folded zone are sharp; C) Rare paleontologic evidence can support the presence of an unconformity; D) Regional discordance of units below the contact can be observed; and E) Detritus from older units occurs in units directly above the contact. In addition to nonconformities at the Grenville basement/cover rock contacts, four significant regional unconformities in the Blue Ridge/Talladega belt have been recognized. In each, the older units were only mildly deformed prior to formation of the unconformity. An unconformity between the Neoproterozoic Snowbird and Great Smoky Groups in Georgia separates sequences that formed during two stages of continental rifting. The sub-Lay Dam Formation unconformity in the Talladega belt separates a thick post-Ordovician clastic wedge from the underlying Cambrian-Lower Ordovician carbonate bank. The sub-Mineral Bluff Group unconformity in the Murphy belt also separates the shallow shelf drift facies sequence from a thick Ordovician or younger clastic wedge. On the overturned W. limb of the Ducktown anticline in Tennessee and Georgia evidence suggests an unconformity between the Nantahala/Brasstown and the younger Wilhite Fms.