Piedmont


 
Piedmont
 
The Piedmont is now relatively flat and topographically featureless due to erosion, however, it contains some of the most geologically significant rocks in Virginia. There are two distinct divisions within the Piedmont rocks: 1) a set of Late Proterozoic and Paleozoic igneous and metamorphic rocks (tan color on the map), and 2) lower Mesozoic (Triassic) sedimentary rocks (red on the map) deposited in graben basins faulted into the igneous and metamorphic rocks.
     
The Late Proterozoic and Paleozoic igneous and metamorphic rocks include three main components. First, the roots of several volcanic island chains such as in the Charlotte/Chopawamsic belt, and Carolina slate and Eastern slate belts (Map); second; several small continental fragments that are possibly Grenville in age (1.1-1.0 bya) (Sauratown Mountain in the south and Raleigh/Goochland belt running west of Richmond (Map); and third, the Inner piedmont belt running just east of the Blue Ridge Province (Map, or Blue Ridge cross section).
     
The volcanic arcs were comparable to volcanic islands seen in Japan, Borneo and Sumatra, and the Aleutian Islands. Many, more likely all, of these formed somewhere else and were brought to Virginia by later events. They are said to be allochthonous. Rocks still in the same place where they formed are termed autochthonous
    
The Grenville age (?) rocks may be microcontinental fragments torn loose and left behind during the Proto-Atlantic rifting, or continental fragments brought in from elsewhere. They contain high grade metamorphic rocks and igneous intrusions.
     
The Inner Piedmont belt contains rocks on the SE Flank of the Blue Ridge anticline. They are sediments (metamorphosed to greenschist and amphibolite) represented by the Evington/Alligator Back Formations (Cross Section), and mafic-ultramafic igneous bodies scattered along the whole length that represent old oceanic lithosphere (ophiolite suite). This is most likely a fragment of the Proto-Atlantic divergent continental margin
  
In addition, numerous late Paleozoic granite intrusions cut through the region, mostly in the eastern half. These were generated in the Taconic orogeny (e.g. Petersburg Granite dated at 320 mya; Map), and the Alleghenian orogeny.
     
Because these rocks have been deformed and metamorphosed, often several times, they are very complex. They also contain many economically important mineral deposits, including gold, talckyanite, and feldspar.
 
Contributed by Lynn Fichter 
Tuesday, July 29, 2014
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