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Structural Cross Section Through the Blue Ridge Province in Central Virginia
(Many of the names on the drawing click to descriptions or other links for more details)
The modern Blue Ridge is an overturned anticline. That is, the rocks have been arched up into a fold, and then shoved over toward the west (left) so that the rocks on the western flank are now no longer right side up (follow the red dashed line).
Notice the Blue Ridge thrust fault at the base; the Blue Ridge province has been moved westward from its site of origin, perhaps as far east as Richmond. Below the Blue Ridge fault Cambrian and Ordovician sediments ("Lower Paleozoic Sedimentary Rocks") of the Valley and Ridge extend eastward under the Blue Ridge and piedmont provinces (cross section).
Descriptions of Rock Units
(in stratigraphic order beginning with the oldest on the bottom).
Lower Paleozoic Sedimentary Rocks. These are relatively unmodified, dominately carbonate rocks deposited in tidal and shallow shelf environments during the Cambrian and lower Ordovician. The are the same rocks exposed in the Valley and Ridge province (Page and Shenandoan valleys).
Formations include Shady, Rome, Elbrook, Conococheague, Stonehenge, Beekmantown, New Market, and Lincolnshire.
These rocks were thrust over by the Blue Ridge rocks during the Alleghenian orogeny. A larger view of the relationships of these rocks to the Blue Rige and be found in this cross section.
Evington. Currently upper greenschist and amphibolite facies meta-sedimentary rocks. They include the Alligator Back formation in North Carolina and Tennessee. Small mafic/ultramafic igneous bodies are common along the whole length. The entire sequence has undergone intense metamorphism and structural deformation directed from the east toward the west, probably during the Alleghenian orogeny. This formation is technically in the piedmont province as the Inner Piedmont belt.
These are interpreted as mid- to distal shelf deposits transitioning into slope and rise deposits of the Proto-Atlantic divergent continental margin (blurdgnesw). They would have been deposited above and to the east of the Proto-Atlantic axial rift. The mafic/ultramafic units are probably remnants of oceanic lithosphere (ophilolite suite).
Chilhowee Group: Weverton, Harpers and Antietam formations. Clastic sediments (conglomerates, sandstones, and shales) mostly deposited in river and submarine fan environments in an axial rift.
The Antietam is a quartz sandstone (metamorphosed to a quartzite) with the trace fossil Scolithus, indicative of a beach deposit (blurdgnesw).
Catoctin. About 2000 feet of basaltic lava flows (now metamorphosed to greenstone). On the northwest flank of the Blue Ridge these are subareal (i.e. flowed out on land), but on the southeast flank there are pillows preserved, indicating some of this was subaqueous (flowed out below water.)
These are interpreted to be flows accompanying the opening of the Proto-Atlantic ocean basin.
Lynchburg Group. Clastic sedimentary rocks (conglomerates, coarse sandstone and shales) deposited in a rift graben valley on the southeast flank of the Blue Ridge province. They are interpreted as submarine fan and alluvial fan deposits.
The Lynchburg is only one part of a series of related clastic sedimentary deposits found all up and down the Blue Ridge province. It includes the Mechum River graben in the center of the drawing above (but recent work indicates it may not be a graben at all).
In addition to the Lynchburg Group is included the Ocoee, Grandfather Mtn., and Mt. Rogers groups (NW flank, SE flank), sometimes accompanied by Crossnore volcanic pyroclastics and lava flows. Stratigraphic thicknesses range from about 3000 meters to 7000 meters. In virtually all these cases the graben borders are not preserved and their structure must be inferred from stratigraphic and other evidence.
The Crossnore is an igneous event dating from about 800 mya to 600 mya. It includes the remains of two rhyolitic volcanic piles (only one in Virginia), and an intrusive alkali granite batholith. The remains of one volcano is found in southcentral Virginia in the Mt.Rogers area. The second volcano is at South Mountain in southeast Pennsylvania.
The Robertson River batholith lies geographically between these two volcanic piles in the northern most part of the Blue Ridge province. It is intruded into Grenville metaplutons. All of these igneous rocks are closely associated with rift grabens and aulacogens.
Swift Run. Sandstones and shales (now metamorphosed to quartzites and phyllites) found on the northwest flank of the Blue Ridge in the northern part of the province. Probably equivalent to the Fauquier formation (NW flank) in northern Virginia. These are a regolith (weathered rubble) over the Grenville basement, although some outcrops show evidence of transportation and deposition in river systems.
UNCONFORMITY - an approximately 600 million year break in the rock record during which time the Grenville mountains were eroded down to a peneplain.
Grenville. The oldest rocks in Virginia (1.8-1.1 billion years) running in a NE-SW trend down the backbone of the Blue Ridge physiographic/geoglogic province (province map). They are dominantly deep forming igneous rocks (granite, granodiorites, etc.) having undergone various degrees of metamorphism to produce gneisses. Many are exposed along Skyline Drive in the Blue Ridge mountains, and in a strip of land to the east running from Galax north through Charlottesville and Culpepper. Related rocks are found all up and down the east coast from Newfoundland to North Carolina. In addition, the basement underlies most of eastern North America, including the Valley and Ridge and Allegheny plateau provinces in Virginia.
A large fault system (the Haysville-Fries-Rockfish Valley fault; HFRV) cuts down the middle dividing the Grenville rocks into the Pedlar massif to the northwest, and the Lovingston massif to the southeast. The fault probably represents a Grenville suture, but was reactivated in the Taconic.
Pedlar Massif. Western half of the Grenville basement exposed in the Blue Ridge. Typical plutons are the Nellysford, Flint Hill, and Lady Slipper (with a volcaniclastic protolith).
Lovingston Massif. Typical rock bodies in the Lovingston massif are the Stage Road Gneiss (with a sedimentary protolith) and various Grenville intrusives, including diorites, charnokites, and anorthosites.
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