The rifting event, once begun, proceeds rapidly, for by the early Jurassic ocean crust is beginning to form, and the Atlantic ocean open. The supercontinent Pangaea, built during the Alleghenian and Ouachita orogenies, is beginning to rift apart. A world at once familiar, and strange.įor as sparce as the geologic record is during the unconformity, by the late Triassic there is a rich record on both sides of the Atlantic, from northern Greenland to the tips of South America and Africa recording the initiation of renewed geologic activity. And the animals scurrying around under the canopy would have been early ruling reptiles (on their way to dinosaurs) and synapsids (mammal-like reptiles), but not a mammal in sight, not quite yet but they are evolving fast. Instead cycads, and ginkos, and conifer trees. There would be differences, of course the vegetation in the forests would look very strange to our eyes no hard wood trees, or flowering plants. So, as best we can sketch on the available evidence, on the eve of the Atlantic rifting the Mid-Atlantic probably looked very similar topographically to the world we have today. And, of course, the Triassic rift basins also have yet to form. Fomation of the coastal plain is in the future. One major difference, however, is that where the coastal plain is today lay Africa. It is harder to guess what the Blue Ridge and Piedmont were like, but also perhaps not dramatically different from what we see today. Cumberland image).Īnd the Alleheny Plateau would also have possessed dendritic drainage, much as it does today. Still, for example, the Valley and Ridge structure of imbricated ramp and flat thrust faults would have produced a trellis drainage pattern similar to today with long resistent ridges alternating with long softer valleys (cf. The Valley and Ridge, Blue Ridge, and Piedmont were all in their modern locations, with their modern structures, although not eroded as deeply as they are now. Physiographically, as erosion began, the landforms may have developed and looked similar to those today since most of the structural geology that underlies and helps create today's topography was in place by this time. Some of these long, east to west flowing river systems likely had their headwaters in Virginia, and flowed for thousands of miles before finally entering the ocean through southwest Texas and Mexico. We also know from other evidence that most of North America to the west was land area, built up by rivers carrying the sediment eroded from the Allgehanian mountains into the Absaroka sea. The erosion cut deep, however, all the way down to the Bluestone formation, something over 2000 meters of section. We base this on the deep erosion (last unconformity in the stratigraphic column) into the Pennsylvanian sediments found in southwest Virginia, although we have no way of knowing exactly when this erosion took place, except that it took place after the Harlan formation was deposited. On the other hand, rivers were either beginning or were actively incising deep canyons into the plateau. Likely though it was a high, broad, flat plateau since the great thickness of coal containing sediments that had accumulated on the western side of the Alleghanian mountains would have prevented erosion much below that level. ![]() We are reasonably sure that during the Permian the last parts of the Alleghanian mountains were eroding because a few layers of sediment eroded from them remain in West Virginia and southwest Pennsylvania (called the Dunkard).īy the lower Triassic, though, Virginia had little or no geological activity, and little topographic relief. We do know that Virginia lay deep within the supercontinent Pangaea, and that Virginia was firmly connected to northwest Africa. The best we can do is speculate based on conditions before and after, and reasoning from geologic principles and similar situations today. The second gap is after the Alleghanian orogeny.įrom the last sediments deposited in southwest Virginia during the Pennsylvanian ( Harlan formation) through the Permian, and until the late Triassic there is no geologic record in Virginia or the rest of the Mid-Atlantic to tell us what what going on. ![]() One of these record gaps (unconformities) is after the Grenville event at 1.1 billion ( Stage B) until the beginning of the Crossnore igneous activity at about 800 million ( Stage C). There are few times in Virginia and the Mid-Atlantic's history when there is just no rock record at all, nothing to tell us what was happening.
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