Graptolite-rich shale in the Valley and Ridge province of Alabama, Georgia, Tennessee, and Virginia records subsidence and migration of the Sevier foreland basin, which formed in response to arc-continent collision during the Middle Ordovician Blountian phase of the Taconic orogeny. Rapid, tectonically induced subsidence of what had been a shallow carbonate platform led to pelagic deposition of graptolite shale in a deep basinal setting. The contact of the shale with the underlying carbonate rocks is a distinct record of initiation of subsidence.
Graptolites were collected extensively and systematically from the shale at 50 localities in the southern Appalachians. Correlation with zonal biostratigraphy demonstrated that the shale sequence is diachronous. In localities to the southeast, in Alabama and Georgia, it correlates with the Didymograptus murchisoni Zone. To the northwest, across strike in Alabama, Tennessee, and Virginia, the shale becomes progressively younger and correlates sequentially with the Glyptograptus teretiusculus, Nemagraptus gracilis, and Climacograptus bicornis Zones. This diachronism records the northwestward, cratonward migration of the foreland basin, which was driven by postcollisional plate convergence.
Biostratigraphic data from 22 localities were amenable to analysis by graphic correlation. At these localities, graptolites were collected from measured sections, and ranges of all species in the 22 sections were compiled into a CSRS (composite standard reference section) that correlates with the biostratigraphic interval from the G. teretiusculus Zone to the C. bicornis Zone. On the basis of the most reliable radiometric ages correlated into the graptolite zonation, the duration of the composite standard reference section is estimated to be 5 m.y. It is divisible into 89 CSUs (composite standard units), each with an approximate duration of 56 000 yr. Recorrelation of each of the 22 measured sections with the composite standard reference section allowed the stratigraphic base of the shale in each section to be expressed in terms of composite standard units, and such correlations permitted the diachronism of the basal shale contact to be measured with much greater precision than with zonal biostratigraphy. In addition, age differences of the basal shale contact between sections can be expressed in terms of time.
With the temporal information provided by the composite standard reference section and original distances between sections determined from a palinspastic reconstruction, the rate of migration of the foreland basin as recorded in the diachronous basal shale contact was determined. Between sections in northeastern Tennessee and adjacent Virginia, calculated rates averaged 13 mm/yr and decreased northwestward from 40 to 9 mm/yr. This cratonward decrease in rate may represent deceleration as foreland basin migration, and the arc-continent collision that drove it, slowed to a halt. The rate of 40 mm/yr may be considered as the maximum rate of plate convergence following arc-continent collision. Precollisional plate convergence rates may have been significantly greater. The rate of 40 mm/yr is, however, well within the range of modern rates of plate convergence. In addition, the deceleration recorded in the Sevier foreland basin is similar to the deceleration of postcollisional plate convergence at the Timor Trough.