TACONIC OROGENY

a mountain-building event that affected the Appalachian Geosyncline along the eastern coast of the United States. Evidence for the orogeny is most pronounced in the northern Appalachians, but its effects can be noted in the Ridge and Valley Province as far south as Tennessee and in the Piedmont Province as far south as Georgia. Originally thought to have occurred at approximately the Ordovician-Silurian boundary (about 438 million years ago), the Taconic orogeny is now considered by most investigators to have consisted of several pulses that extended from the mid-Ordovician Period up into the early part of the Silurian Period. Events ascribed to this orogeny include the development of angular unconformities in strata of the northern Appalachians, the Taconic allochthone (overthrust and displaced block) in New York, the uplift of the Green Mountains in Vermont, the development of clastic wedges of sediment (Queenston Delta in New York and the Blount Group in Tennessee), and igneous intrusion and regional metamorphism in both the northern and southern Appalachians. The Taconic orogeny may be considered an early phase of the Caledonian orogeny in western Europe. Thrust faulting occurred in a vast zone extending from the western coast of Newfoundland, southward along the St. Lawrence valley and New York, and into Pennsylvania. This zone of thrust faulting is known as Logan's Line, after Sir William Logan, the Canadian geologist who first recognized its significance. Strata to the west of Logan's Line have been little disturbed by Paleozoic deformation, whereas east of the line, Ordovician and older strata have been extensively deformed. The Ordovician shale facies is badly crushed, faulted, and overthrust westward in masses known as klippen, one of which forms the present Taconic Range.