Graptolite Biostratigraphy and Depositional History of Middle Ordovician Shales, Southern Appalachians
Stanley C. Finney
Graptolite-bearing Middle Ordovician shales extend the length of the southern Appalachians in eastern thrust belts of the Valley and Ridge province. They directly overlie platform carbonates deposited on the continental shelf, and their initial deposition records subsidence of a foreland basin in response to the Taconic orogeny.
Recent studies of graptolite biostratigraphy based on extensive collecting from numerous outcrops demonstrate that the base of the shales and, thus, initiation of tectonically induced subsidence are progressively older to the southeast. Oldest ages are recorded in the Coosa belt of Alabama and east-central Georgia, where the Athens Shale and Rockmart Slate extend down into the tentaculatus Zone. Because of similar age and lithology, the position of these outcrops defines depositional strike. To the northeast, this trend projects under the Cartersville thrust, suggesting that similar strata extend beneath the allochthonous Blue Ridge and Piedmont. To the northwest, basal shales from the Helena thrust belt of Alabama and from immediately west of the Blue Ridge thrust in northeast Tennessee and southwest Virginia correlate with the teretiusculus Zone. The outcrops are also interpreted to define depositional strike, which is overridden by the Blue Ridge thrust between Georgia and northeast Tennessee and in central Virginia. Farther northwest across depositional strike, basal graptolite shales correlate with the gracilis Zone.
The oldest, easternmost shale outcrops correlate closely with shales in western Newfoundland that also record development of a foreland basin on the cratonic margin. Therefore, implications can be made about the pattern of initiation and progression of the Taconic orogeny.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.