INTRODUCTION
EVENTS IN THE CLOSURE OF THE POTO-ATLANTIC
As discussed above, structural deformation, metamorphism, deposition of clastic wedges, volcanism and to some degree plutonism, are events that "cluster" around several time intervals throughout the Paleozoic in the Appalachians. We refer to each of these tectonic episodes as orogenies.
Taconic Orogeny
There are several geologic occurrences which suggest that a major tectonic event, the Taconic Orogeny, occurred during the Middle Ordovician (Rodgers, 1971). A major metamorphic event occurred in the Appalachian Piedmont and the New England Appalachians during the Ordovician (Rodgers, 1971; Hatcher, 1978).
A major lithospheric flexural event is recognizable in the Middle Ordovician strata of the Valley and Ridge. The flexural event created tow basins, an eastern basin (moat) in which the Edinburg and Martinsburg were deposited (Stops 2 and 3), and a western basin in which the Reedsville, Oswego, and Juniata were deposited (Stop 6). The two basins were separated by an arch (peripheral bulge) between them. The Virginia portion of the clastic wedge is described in detail by Diecchio (1985, 1986a, 1896b; and Fichter and Diecchio, 1986b).
Numerous Middle Ordovician bentonites are found in the Valley and Ridge (Haynes, 1992). There are several post-Middle Ordovician (Silurian and Devonian) plutons in the Piedmont of the Carolinas (Wright and others, 1975). All of these related geologic phenomena are the basis for our understanding of the Taconic orogeny.
It has been proposed that the Taconic event represents the collision between eastern North America and a volcanic arc, or a microcontinent (Hatcher, 1975, Diecchio, 1980), with subduction toward the east. In any case, this is one event that occurred during subduction associated with closure of the Proto-Atlantic Ocean.
The time of onset of subduction is controversial. Mussman and Read (1986) suggest that the sea-level drop at the end of the Early Ordovician (Know-Beekmantown unconformity) represents uplift of a peripheral bulge associated with the onset of subduction. Others (Vail and other, 1977) relate this to a global sea-level drop. Strata we will visit on this trip suggest that deepening, possibly associated with the development of a flexural moat (Martinsburg basin), occurred during the Middle Ordovician. The oldest bentonites are Middle Ordovician, suggesting the onset of volcanism.
Acadian Orogeny
An event, similar in nature to the Taconic, occurred during the Middle Devonian, and is referred to as the Acadian Orogeny. Devonian metamorphism is represented by a low grade event in the Central and Southern Appalachians (Hatcher, 1978). Lithospheric flexure of Devonian age is recognized in the Appalachian Basin (Ettensohn, 1987). We will visit one stratigraphic section (Stops 9-13) representative of the Devonian clastic wedge (Catskill "delta"), described in detail by Fichter (1986). A prominent Middle Devonian age volcanic ash deposit (Tioga bentonite) has been recognized throughout the Appalachian basin (Dennison and Textoris, 1970). There are a cluster of post-Devonian (Pennsylvanian-Permian) plutons in the Piedmont of the Southern Appalachians (Wright and others, 1975). These phenomena are interpreted as representing the Acadian Orogeny and represent another stage of closure of the Proto-Atlantic.
The Acadian event has been interpreted as plate convergence and collision between a continental block (Avalon) and North America. It is on the Avalon terrane that Richmond now stands. Subduction was to the east, so Avalon had a volcanic arc along its western edge. Avalon did not converge directly on North America but slid in obliquely, colliding and scraping at a number of points from the maritimes of Canada to the southern Appalachians. A major impact in the southeastern New York area created a large foreland basin into which poured the Catskill clastic wedge. Many of the formations of the clastic wedge in Virginia and West Virginia (Stops 9-12) are associated with this basin. The Pocono formation (Stop 13) was derived from the last impact occurring in the southwest Virginia area.
Alleghanian Orogeny
The major deformation of the Appalachian Valley and Ridge, and final uplift of the Blue Ridge occurred late in the Paleozoic and is called the Alleghanian Orogeny. This event is represented by the development of the fold and thrust belt that is so characteristic of the Valley and Ridge and Blue Ridge. This event overprinted many previously formed structures, and was responsible for the primary structures we see today. The Alleghanian event occurred during the Pennsylvanian and Permian.
The Alleghanian is not associated with a metamorphic event, nor a recognized bentonite. There is a late Paleozoic clastic wedge spreading westward. In short, this orogeny is very different from the earlier two. To many investigators, there is not a distinct boundary between the Acadian and Alleghanian orogenies. But, in any case, the Alleghanian represents the culmination of the Appalachian mountain-building associated with the collision between North America and Africa.