The Lower Paleozoic Divergent Continental Margin and the Chopawamsic Volcanic Arc
Middle Cambrian and Lower Ordovician; 570 - 490 mya
Divergent Continental Margin
Following the late Proterozoic/early Cambrian rifting, the region became tectonically quiet for the next 120 million years along the east coast of North America. During this time the continental edge, subsiding quickly at first and then ever more slowly, continuously accumulated enough sediments to keep the water shallow. The resulted in a thick wedge of sediments, thinning toward the Craton and becoming many kilometers thick toward the Protoatlantic ocean (see above, and Rift Cross Section Four).
During this time, global sea level rose more or less continuously, creating what is known as the Sauk sea in North America. Virtually all of the United States was under water. The only exposed parts were central Canada, and a series of low islands running diagonally from northeast to southwest across the center of the continent known collectively as the transcontinental arch.
Throughout this time eastern North America lay 20-30(deg) south of the equator. Warm tropical waters, plus an absence of source lands to supply sandstones and shales, led to carbonate deposition (limestones and dolomites) many thousands of feet thick. These rocks were deposited in vast tidal flats. The figure to the right (click for larger version) is a model of tidal deposition in the Sauk sea. Tidal environments are divided into three zones: supertidal (above high tide), intertidal (between high and low tide), and subtidal (below lowest low tide). Characteristic processes and deposits occur in each tidal zone, and we have learned to recognize them by studying modern carbonate tidal flats. More about these deposits is explored in the further details link at the bottom.
The vast extent and thickness of the tidal deposits across eastern North America indicates evidence of large, powerful tides. These existed in part because the Cambrian moon was closer to the earth than it is now, and tidal attractions were stronger. It was likely that a large tidal bore, perhaps several feet high, rushed rapidly across western Virginia each day, and then drained off again. These Cambrian and Ordovician Carbonate rocks are now exposed throughout the Shenandoah and Page valleys in the Valley And Ridge Province, extend north into Maryland's Great Valley and into Pennsylvania, and contribute to the presences of good farmland in these regions.
The Lower Paleozoic divergent continental margin remained tectonically quiet over the next few million years (Cross Sections E, F, and G
Chopawamsic Volcanic Arc - Fixed and Mobile Worlds
Out in the Proto-Atlantic ocean, the Chopawamsic volcanic arc began to form (see cross section above). This arc of Middle Cambrian age is now firmly attached to Virginia (located in a narrow strip in the Fredricksburg area in northern Virginia; Piedmont Belt Map), but most of its history took place elsewhere. As described by the Plate Tectonic theory, volcanic arcs do not have to remain where they form and, in fact, it is highly likely they will migrate, at least relative to their origin location. For example, look at cross sections D and E for the simple Wilson Cycle. The continent on the left and the volcanic arc on the right are moving toward each another as the remnant ocean subducts. With plate tectonics no place is a fixed reference; all is relative.
In summary, the Chopawansic arc began its history somewhere else, and was transported to Virginia later. There is additional evidence to support this, explored in Stage F.
Contributed by Lynn Fichter