Events in the Geologic History


 
 
The Parts and Events in the Geologic History of Virginia
 
Geological divisions do not remain constant. Continents form, and then rift apart. Oceans open, and then close again, ceasing to exist. Mountains build up, and then erode to the sea. And through time, Virginia, and North America, drifted across the globe from south of the equator 600 mya (million years ago) to its present northern temperate location. 
     
In many places in Virginia are rocks that are now in contact with rocks that are of completely different in origin. Instances include igneous rocks that formed at great depths situated beside sedimentary rocks that formed on the surface. These rocks form by mutually exclusive processes so they must have formed at different times and/or in different places and then later been brought together. To understand the geologic history of Virginia we must be able to reconstruct these past events. The tables below list most of the essential pieces and events that make up Virginia's geologic history.
 
  • Supercontinents: In the geologic history of Virginia there were at least three supercontinents.
SUPERCONTINENTS IN VIRGINIA'S HISTORY
CONTINENT AGE OF EXISTENCE
RODINIA: North America + "Africa" (plus other continents joined to "Africa") Latest Precambrian and earliest Cambrian
LARUSSIA (Old Red Sandstone Continent): North America + Baltica (parts of Europe, Scandinavia and Siberia) Devonian. This supercontinent created the Caledonian mountains. Its effects did not directly influence Virginia, but the Acadian mountains joined with the Caledonian to form a long, continuous mountain chain for a short time.
PANGAEA: North America + Africa (plus South America, Antarctica, India, and Australia) Late Paleozoic and earliest Mesozoic

  • Ocean Basins: Supercontinents rifted at different times forming several ocean basins. It is likely that in the Proterozoic other rifting Events occurred with remnants preserved in Virginia (Grenville) geology. 
OCEANS IN VIRGINIA'S HISTORY
OCEAN NAME AGE OF EXISTENCE
Protoatlantic (Iapetus) Ocean Earliest Cambrian to Late Paleozoic
Rheic Ocean Between the Acadian and Alleghenian orogenies (perhaps longer). Devonian to Pennsylvanian
Atlantic Ocean Early Mesozoic to the present

  • Terranes: There were at least four island arc terranes, although it is possible some of these are the same with different names in different places. 
ISLAND AC (+MICROCONTINENT?) TERRANES IN THE HISTORY
VOLCANIC ARC NAME AGE OF EXISTENCE
Virgilina Active in the Cambrian
Chopawamsic Active in the Cambrian, although it probably went through its entire history out in the Protoatlantic before docking with Virginia
Arvonia + Charlotte Active in the Ordovician
Carolina Slate Belt (+Eastern slate belt?) Active in the Ordovician

  • Microcontinents: There were at least three microcontinent terranes, although it is possible some of these are the same with different names in different places. 
MICROCONTINENT TERRANES IN VIRGINIA'S HISTORY
MICROCONTINENT NAME AGE OF EXISTENCE
Gander (=Raleigh/Goochland?) Attached to Virginia in the Ordovician and still present in the Piedmont
Sauratown Mountain Attached mostly to North Carolina, but also to southern Virginia in the Ordovician and still present in the Piedmont
Armorica/Avalon Attached to Virginia in the Devonian as Armorica. Partially rifted when the present Atlantic ocean opened; the part remaining with North America known as the Avalon terrane.

  • Detached Orogenies: There were orogenies that took place elsewhere on the earth, but their rocks have been moved to Virginia and preserved. 
DETACHED OROGENIES IN THE HISTORY
OROGENY AGE OF EXISTENCE
Chopawamsic Middle Cambrian of the central and northern Piedmont. Attached to Virginia in the Ordovician. Arrived with the Arvonia volcanic arc and possibly the Raleigh/Goochland microcontinent.
Virgilina Cambrian of southwest Virginia and North Carolina. Probably came in in the Ordovician

  • Non-American Orogenic Events: There were orogenies that took place in other parts of the world at about the same time as those in Virginia but have different names. Often they are the same orogeny as in North America, caused by the same processes, just preserved in different places. For example, a collision orogeny is recorded on both colliding pieces, even though they are now separated. 
NON-NORTH AMERICAN OROGENIES IN THE HISTORY
OROGENY LOCATION AGE OF EXISTENCE
Caledonian Caused by collision between maritime provinces of Canada and Great Britain. Same Time as Acadian.
Ouachita Caused by collision between South America (as part of Gondwana) and the Gulf coast of North America. Remains exposed in Oklahoma and Texas. Other remains buried under the Coastal Plain. Same Time as Alleghenian.
Hercynian (Variscan) Southern Europe Same Time as Alleghenian.
Mauritanide Northwest Africa (Gondwana). This was the collsion effects opposite North America. Same Time as Alleghenian.

  • Virginia Orogenies: There were four orogenies that took place in Virginia, and up and down the eastern seaboard.
OROGENIES IN THE HISTORY
OROGENY AGE OF EXISTENCE
Grenville Late Proterozoic; between 9125-1130 mya, but may consist of more than one mountain building event. Several Wilson Cycles may be preserved.
Taconic Mid to Late Ordovician, dated 435-350 mya
Acadian Mid to Late Devonian, dated 350-370 mya
Alleghenian Probably began in the Mississippian, peaked in the Pennsylvanian, and continued into the Permian. Some consider this orogeny continuous with the Acadian.
 
Contributed by Lynn Fichter 
 
Friday, July 18, 2014
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