Taconic Orogeny

Mountain-building event that affected the Appalachian Geosyncline along the eastern coast of the U.S. Evidence for the orogeny is most pronounced in the northern Appalachian Mountains, but its effects can be noted as far away as Tennessee and Georgia.

Events ascribed to it include the development of New York's Taconic Range and Vermont's Green Mountains. Originally thought to have occurred at approximately the Ordovician-Silurian boundary (с 443 million years ago), it is now generally considered to have consisted of several pulses from the mid-Ordovician to the early Silurian Period.

* * *

▪ geological event

      first of three mountain-building events forming the Appalachian Mountains in eastern North America, the Acadian (Acadian orogeny) and Alleghenian (Alleghenian orogeny) orogenies being the second and third events, respectively. Originally viewed as a single event, the Taconic orogeny is now known to consist of at least three episodes. The first took place in the Early Ordovician (Ordovician Period) Epoch near Maine and Newfoundland. The second was centred on eastern Tennessee in the Middle Ordovician Epoch. The third occurred during the Late Ordovician Epoch, principally in eastern Virginia, Pennsylvania, and New York.

      The three episodes are thought to represent the oblique collision of a terrane (a crustal block or formation of related rocks) with the eastern edge of North America. In the Appalachians, the Taconic orogeny produced angular unconformities (interruptions in the deposition of sedimentary rock) in the Appalachian Basin and the Taconic Allochthon in New York, and it also caused igneous (igneous rock) intrusions and regional metamorphism (metamorphic rock) in the northern and southern Appalachians. Flexure of North America during the Taconic collisions produced deep sedimentary basins that accumulated up to 300 metres (about 1,000 feet) of sediment in some areas, such as the Queenston Delta in New York and the Blount Group in eastern Tennessee. More-distant effects included gentle uplift of the Nashville Dome and Cincinnati Arch. The third episode of the Taconic orogeny triggered a regional extinction of many marine invertebrates (invertebrate) in eastern North America.

* * *