Abstract Until very recently, most geologists were conditioned to seek the effects of three major events–the Taconian, the Acadian, and the Alleghanian–within the Appalachian orogen. Things are not that simple, however, as the importance of older deformations is increasingly being recognized. Although this chapter is concerned primarily with the Taconic orogen (sensu stricto), two older deformational events are considered herein. These events are the Blountian and Penobscottian orogenies. The Penobscottian event has been recognized for some time (Neuman, 1967; Hall, 1969, 1970), but its importance in Appalachian geology has only recently become apparent by work in northern Maine (Osberg, 1983; Boone and others, 1984) and the Potomac Valley of Virginia and Maryland (Drake and Lyttle, 1981; Drake, 1987). In Maine, the Penobscottian can only be dated as pre-late Ibexian (pre-Arenigian), whereas in the Potomac Valley it is thought to be of late Middle Cambrian to early Late Cambrian (Dresbachian) age. Neither syn- nor post-orogenic sediments are recognized that could have resulted from the Penobscottian deformation. On the contrary, the Blountian event is recognized because of its syn-and post-orogenic sediment wedge, but deformational features related to the event have not as yet been recognized in the Blountian hinterland, although isotopic dating within the Blue Ridge is permissive of deformation at this time. The Blountian orogeny has been recognized for many years (Kay, 1942), and Rodgers (1953) has termed it the Blountian phase of the Taconicorogeny. In my opinion, it was a separate tectonic event that was completed prior to the Taconic (sensu stricto), as its uppermost molasse is overlain by distal Taconian syn- and post-orogenic deposits. It goes without saying that the effects of the Penobscottian and Blountian orogenies are difficult to recognize and separate from those of the Taconic orogeny. For this reason, the effects of the earlier events will be discussed with those of the Taconic where they are believed to be present.

Abstract: Mudstones from the Taconic and Blountian foreland basins were analyzed for whole-rock chemical composition and clay mineral composition. These foreland basins formed during the middle and late Ordovician when exotic terranes collided with Laurentia. The purpose was to determine if the mudstones record first-order trends in provenance that are related to tectonic history. In both basins, the ratio of chlorite to illite and the ratio of the concentration of three "mafic" elements (Ti, Cr, Ni) to Nb (a "felsic" element) increases with time. However, mudstones from the Taconic foreland basin have a higher proportion of Ti, Cr, and Ni than those from the Blountian foreland basin. Results from the Blountian foreland basin showed the greater amount of scatter. The outboard terranes that collided with Laurentia were the most important sources of siliciclastic sediment because carbonate platforms fringed the continental margin of each basin. Thus, compositional trends in each basin reflect an increase in the proportion of sediment eroded from mafic source rocks within the colliding terrane. In presently accepted tectonic models for the Taconic foreland basin, the colliding terrane is an arc system. The provenance signature in the mudstones suggests the sediment source shifts from a non-magmatic outer arc to the inner volcanic arc during the collision. The comparatively lower concentration of mafic elements in the Blountian foreland basin mudstones may indicate that the colliding terrane was composite or that its angle of convergence was more oblique.

The southern Appalachian crystalline core is composed of lithotectonic assemblages that are largely sedimentary in origin. Sixteen paragneiss samples from the Blue Ridge and Inner Piedmont of North Carolina and Georgia, and one sample of Middle Ordovician rocks from the Sevier-Blountian clastic wedge in the Tennessee Valley and Ridge were sampled for sensitive high-resolution ion microprobe (SHRIMP) U-Pb detrital zircon geochronology, whole-rock geochemistry, and zircon trace-element analyses. Detrital zircon ages range from Archean (~2.7 Ga) to Middle Paleozoic (~430 Ma), with a notable abundance of Mesoproterozoic zircons (1.3–0.9 Ga). Many samples also contain moderate populations of slightly older Mesoproterozoic zircons (1.5–1.3 Ga). Minor populations of Paleoproterozoic (2.3–1.5 Ga) and Neoproterozoic (754–717 and 629–614 Ma) ages occur in several samples; however, Paleozoic detrital zircons (478–435 Ma) are restricted to samples from the Cat Square terrane. Depositional periods of the metasedimentary terranes are bracketed by detrital zircon, metamorphic, and magmatic ages, and include: (1) Mesoproterozoic, (2) Neoproterozoic to early Paleozoic, and (3) middle Paleozoic. A xenolith from the ~1.15 Ga Wiley Gneiss suggests a post–~1.2 Ga period of sedimentation prior to the ~1.15 Ga Grenvillian magmatic event. Detrital zircon populations of Neoproterozoic to Middle Ordovician suggest a mixed Laurentian provenance with Amazonian and peri-Gondwanan sources deposited in divergent and convergent plate settings. Blue Ridge and Inner Piedmont detrital zircon ages, whole-rock geochemistry, lithologic assemblages, and field relationships are compatible with deposition of immature clastic material in a rift and passive-margin setting from the Neoproterozoic to early Paleozoic. Occurrence of 1.3–0.9 Ga, 1.5–1.3 Ga, and 754–717 Ma detrital zircon ages indicate a dominantly Laurentian provenance for the Cartoogechaye, Cowrock, Dahlonega gold belt, Smith River allochthon, and Tugaloo terranes. Minor Paleoproterozoic populations in these terranes suggest input from distal terranes of the Laurentian midcontinent or the Amazonian craton. Transition to a convergent plate margin in the Middle Ordovician resulted in collision of central Blue Ridge and Tugaloo terranes and recycling of material from these terranes into the Mineral Bluff Formation and Sevier Shale. Ordovician and 629–614 Ma detrital zircons from the Cat Square terrane document the first occurrence of peri-Gondwanan material, which was deposited in a convergent setting between the Laurentian margin and the accreting Carolina superterrane during the Late Silurian to Devonian.

Abstract Black shales are integral parts of most foreland-basin deposits and, because they typically reflect maximum basin subsidence, their distributions serve as proxies for the extent of foreland-basin development. In the United States Appalachian area, the distribution of Middle–Upper Ordovician black shales suggests that the Taconian Orogeny proceeded from south to north along the eastern Laurentian margin and that Taconian tectophases were mediated by convergence at continental promontories. In the Late Ordovician Taconic tectophase, changes in the distribution of the Martinsburg and Utica black shales support a reversal of subduction polarity that effected the reactivation of basement structures and basin migration sufficient to yoke the Appalachian foreland basin with adjacent intracratonic basins. Shale distribution suggests that early Chatfieldian (late Sandbian–early Katian), east-verging subduction early in the tectophase generated a cratonic extensional regime with a narrow foreland basin that developed along reactivated Iapetan basement structures. Abruptly, in late Chatfieldian–early Edenian (early Katian) time, westwards migration of basinal Utica black shales and an underlying unconformity suggests change to a compressional regime and westwards subduction vergence. The coincidence of changes in basin shape and migration with the shifts in subduction polarity suggests a causal relationship.