Recently, the carbonate belt of the northwestern side of the northern Appalachian Orogen has been interpreted as an early Paleozoic shelf within a continental margin province that evolved synchronously with the
This plate tectonic model incorporates results of various studies of the past century, commencing with the original Taconic controversy over the age of Taconic rocks. The last Taconic controversy revolved around the interpretation of Taconic sedimentary rocks that are surrounded by carbonate. Either they are overthrust, as originally suggested by Keith, or they are autochtonous lateral facies within a basin surrounded by the carbonates, as originaly proposed by Dale. The overthrust interpretation was subsequently modified by Ruede- manii to become the Taconic klippen hypothesis that, during the 1960's, has been virtually proven by many workers, but principally by Zen. Kay, as a result of his classic work on the Trenton Group of New York, proposed a miogeosyncline-eugeosyncline (shelf-island arc) couple model for the pre-Middle Ordovician, western Appalachian belt, a concept which was to become the key for future refinement of our understanding of regional stratigraphic-structural relations of the various facies and volcanic assemblages. It provided the basis of the collapsing continental rise model of Dietz and of Dietz and Holden that followed Drake, Ewing, and Sutton's comparison of Kay's model with the present North American Atlantic continental margin. Rodgers then proposed that the southeastward termination of the carbonate shelf was represented by breccia facies that can be observed in the field, that the carbonates and breccias were of a Bahama bank and bank-cdgelike environment, and that this termination was the termination of the Ordovician continental margin. More recently Bird and Dewey incorporated these models into the plate tectonics model of an evolving so-called Appalachian Atlantic Ocean, following Wilson's proposal of a driven-out Proto-Atlantic Ocean and Dewey and Kay's and
Commencing in the Ordovician, the Appalachian portion of the enormous sheet of Cambro-Ordovician carbonates of central and eastern North America was involved in the extensive diachronous deformation that formed the Appalachian Orogen. Therefore, reconstruction of the continental margin to its condition before deformation involves the sorting out of bulk stratigraphic-tectonic units, the comparison of their relative chronologies, and the recognition of various sedimentary and structural environments. Using present- day lithosphere plate relations and the analogy of an Atlantic type of continental shelf-rise-abyss facies relationship, a plate tectonics model can be constructed from these various stratigraphic-tectonic elements of the carbonate belt and associated marginal, overlying, and allochthonous rocks that integrates otherwise seemingly diverse and unrelated aspects and provides an actualistic model for the evolution of the North American Ap-
The key to reconstructing the pre-Taconian or pre-Middle Ordovician relations is in the stratigraphic- structural assemblages of the Taconian thrust belt. By paleontological considerations of the various structural units or klippen one by one, the chronostratigraphic relations between the carbonate shelf assemblage and the sediments of the thrust sheets can be determined. Essentially, the earliest emplaced thrust sheets contain sediments whose age range matches that of the underlying shelf (autochthon), which was as pointed out a number of years ago by Zen. In addition, the facies of these sediments fit very well the model of shelf- continental rise assemblages that accumulated in a starved environment. The bulk of the initially emplaced klippen, the Giddings Brook Slice, is composed of these offshelf and synchronous sediments. The lowest known fossils in these sediments are in Lower Cambrian orthoquartzite, which matches petrographically
The bulk of the overthrust sediments, however, are pre-Lower Cambrian shales and clastics, which are thousands of feet thick and which in the lowest portion are apparently nonmarine graben assemblages containing extrusive basaltic rocks. All these rocks are at least slightly metamorphosed in subgreenschist facies. Additionally, similar facies occur in the autochthon below the Lower Cambrian basal unconformity of the carbonate platform, which also locally contains extrusive basalt and rhyolites. These relationships were
Essentially then, with the eastward limit of the carbonate belt being taken as a shelf edge, reconstruction of the overlying structural assemblages indicates a history of late Precambrian continent separation beginning with early horst and graben tectonics and sedimentation, followed by establishment of an Atlantic-type continental margin through to Late Cambrian and Early Ordovician time. Then, ocean closing by subduction along the continental margin converted the margin basement and sediment assemblage to an Andean-like system, diachronously through to Middle to Late Devonian continent-to-continent colli ion. The carbonate belt then lay along the north side of a Himalayan-like mountain system. Its present geographic position is a consequence pf subsequent plate evolution commencing in the Late Triassic as indicated by the Newark Basin assemblage.