G - Knox Unconformity


 
The Knox Unconformity, and the Arvonia, Carolina Slate and Other Volcanic Arcs and Terranes 
 
Early Ordovician; 500 - 450 mya


~The Domination of Carbonate Deposition~
 
For tens of millions of years, throughout most of the Cambrian and early Ordovician, the entire eastern seaboard, and all of North America, was geologically quiet. Tens of thousands of feet of carbonate rock accumulated (Rift 4 X-Section), and to quickly look at the lower Ordovician rocks would reveal more carbonates.
     
Throughout most of the Ordocician, all of Virginia was under a body of water known as the Sauk sea. From the Proto-Atlantic ocean, across the continental margin of Virginia and across the continent, only few low islands of the transcontinental arch west of the modern Mississippi river are visible above water. Carbonate deposition dominated the entire continent. 
     
There is, however, a detectable boundary where a great change takes place. Cambrian rocks have few fossils and these are not very diversified. It is true that tidal environments (Stage E) support less life than offshore environments, 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 forms, and their skeletons populate the rock in astronomical numbers. Small patch reefs are scattered everywhere.
     
The change is obvious in the fossil record.  Most notably, the change 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 continuous. There are places where the unconformity barely exists, and these were likely low areas, or 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.  It is obvious, then, that the rifted margin of eastern North America very rugged and varied 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 analogues, which we know much more about, to model and fill in details of the past.  
 
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 cause of this great change is still strongly debated. 

~The Arvonia, Carolina Slate and Other Volcanic Arc and Terranes~
 
Beginning at about 450 mya, the tectonically quiet conditions of shallow Carbonate deposition that had dominated Virginia for so long changed quickly and dramatically with the onset of theTaconic orogeny. The effects of the orogeny were wide spread in Virginia and include 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 extended along the entire eastern sea board from the Carolinas through New England and the maritime provinces of Canada in the north.
     
Simplistically, the orogeny occurred when a terrane or series of terranes collided with eastern North America. A terrane is any small floating block of crust; it includes volcanic arcs and/or microcontinents. A terrane is 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 not synchronous with their current 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 were added at the same time, 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
     
In the cross section above, the terranes are moving toward North America, but have not yet collided, because an east dipping subduction zone is consuming 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 into the subduction zone under the terrane, and collision occurs. It is the first stage of closing in the Proto-Atlantic Wilson Cycle.
     
The Taconic Orogeny is probably the most complicated of the orogenies to influence Virginia, and we explore some of its details in Stage H

Contributed by Lynn Fichter 

Saturday, August 02, 2014
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