Continent after continent was thrust and sutured onto the North American craton as the Pangean supercontinent began to take shape. Mountain building continued periodically throughout the next 250 million years ( Caledonian, Acadian, Ouachita, Hercynian, and Alleghanian orogenies). This was just the first of a series of mountain-building plate collisions that contributed to the formation of the Appalachians. Streams carried rock debris downslope to be deposited in nearby lowlands. As mountains rose, erosion began to wear them down. Thrust faulting uplifted and warped older sedimentary rock laid down on the passive margin. Volcanoes grew along the continental margin, coincident with the initiation of subduction. With the creation of this new subduction zone, the early Appalachians were born. The once quiet Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic plate, the Iapetus, collided with and began sinking beneath the North American craton. ĭuring the Middle Ordovician Period (about 458-470 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event ( Taconic orogeny) in North America. When seas receded, terrestrial sedimentary deposits and erosion dominated. Thick layers of sediment and carbonate rock were deposited on the shallow sea bottom when the region was submerged. During this interval, the region was periodically submerged beneath shallow seas. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal Plain Province. ĭuring the earliest part of the Paleozoic Era, the continent that would later become North America straddled the equator. Paleogeographic reconstruction showing the Appalachian Basin area during the Middle Devonian period.
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