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Cross Section G
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The Knox Unconformity, and
The Arvonia, Carolina Slate and Other Volcanic Arcs and Terranes
Early Ordovician; 500 - 450 mya
THERE ARE CARBONATES, . . . . AND THERE ARE CARBONATES
Great events in the history of the earth are not all geologically dramatic. For tens of millions of years, throughout most of the Cambrian and early Ordovician, the entire eastern seaboard, and all of North America, has been geologically quiet. Tens of thousands of feet of carbonate rock have accumulated (rift 4 x-section), and to quickly look at the lower Ordovician rocks would reveal . . . more carbonates.
In the early Ordovician all of Virginia is under water, the Sauk sea. Indeed, during the Ordovician most of North America will be under water. Fly westward from the Proto-Atlantic ocean, across the continental margin of Virginia and across the continent, nothing is visible below but water until the west coast, except perhaps for a few low islands of the transcontinental arch west of the modern Mississippi river. Carbonate deposition dominates the entire continent.
But, look at the rocks more carefully; there is a boundary where a great change takes place. Cambrian rocks have few fossils, and they are not very diversified. It is true that tidal environments (Stage E) support less life than the sea, but the scarcity of Cambrian fossils is still notable. In the Ordovician, however, the sea is alive with brachiopods, bryozoans, corals, cephalopods, crinoids, trilobites, and a host of other life, and their skeletons populate the rock in astronomical numbers. Small patch reefs are scattered everywhere. It is like a travel poster where the sea is aquamarine in color, crystal clear, and the bottom so close it seems like you can reach down and touch it. This is some of the best fossil collecting anywhere at any time.
Something has changed in the life on Earth. We can see the change in the fossil record, but explaining it is more difficult, and not really our task here. But, we can make a few observations. Most notably, the change in the fossil record occurs across the Knox Unconformity, an important boundary in the early Ordovician in the rock record. An unconformity is a gap in the rock record, when either no deposition occurred, or there was erosion. The Knox unconformity is the result of a major lowering of sea level (a regression), and as the sea drained off the continent the Cambrian sedimentary rocks were exposed, and began to erode. It marks the end of the Sauk sea, and the end of tidal domination in the seas.
In Virginia the extent of the Knox unconformity is not the same everywhere. There are places where the unconformity barely exists, and these were likely low areas, basins, that remained underwater even during the lowest part of the regression. But the unconformity is prominent in south central Virginia and indicates that the "southwest Virginia high" (map) stood higher than surrounding areas at the time and was thus more exposed and eroded during the regression.
Thus we see that not only was the rifted margin of eastern North America very ragged, it also varied a lot in elevation. That is, ancient continental edges had many of the same subtle features we find along modern continental edges. The discovery of such ancient and modern similarities gives us confidence we can use the present, which we know much more about, to model and fill in details of the past.
But at the end of the regression sea level rapidly rose again, eventually even higher than before, and all of North America was under the Tippecanoe sea. And along with the Tippecanoe came the great change in life from the Cambrian fauna to the Paleozoic fauna and the great expansion of the fossil record. The causes of this great change are still strongly debated, but the fact is the change did occur, and it shows up in the rock record. At first it takes a trained eye to notice the change in the rocks, but once pointed out it is obvious to almost anyone. The rock record will never be the same again.
THE ARVONIA, CAROLINA SLATE AND OTHER VOLCANIC ARC AND TERRANES
Beginning about 450 mya the tectonically quiet conditions of shallow carbonate deposition that had dominated Virginia for so long changed quickly and dramatically with the Taconic orogeny. The effects of the orogeny are wide spread in Virginia, including the Piedmont, Blue Ridge, Valley and Ridge, and the craton (the stable continent under the Allegheny plateau) to the west (these physiographic divisions only begin to take shape with the Alleghenian orogeny, Stage K). The effects of the orogeny extend along the entire eastern sea board from the Carolinas in the south through New England and the maritime provinces of Canada in the north.
Simplistically, the orogeny occurs when a terrane or series of terranes collides with eastern North America A terrane is any small floating block of crust; it includes volcanic arcs and/or microcontinents. A terrane by implication has been transported in from somewhere other than its present location. When first discovered they were called "suspect terranes" because geologists suspected that they had come from somewhere else, usually because their fossils or rocks were out of place for their present location.
The Taconic terranes are preserved today in the piedmont in a series of belts representing volcanic arcs and microcontinents. They include the Charlotte/ Chopawamsic belt and the Carolina and Eastern slate belts (click on map). Other terranes by other names in other places probably also contributed to the orogeny. It is not really clear whether they all came in together, or came in piece by piece, but some idea of the possible complexity can be gathered from the history of the Arvonia/Chopawansic arcs discussed in Stage F. The cross section at the top of this page suggests that the Arvonia, Chopawamsic, and a microcontinent may have come in together.
In this cross section the terranes are moving toward North America, but have not yet collided, because an east dipping subduction zone is eating oceanic crust. The remnant ocean basin (ROB in the cross section at top) between the terrane(s) and Virginia becomes narrower and narrower until it completely disappears down the subduction zone under the terrane, and collision occurs. It is the first stage of closing in the Proto-Atlantic Wilson cycle.
The Taconic is probably the most complicated of the orogenies to influence Virginia, and we explore some of its details in Stage H.