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U.S. Geological Survey Open-File Report 99-423

Geologic Map of the Green Bay 7.5-Minute Quadrangle, Prince Edward County, Virginia

The Green Bay 7.5 minute quadrangle includes an area of about 150 square kilometers in south-central Virginia, about 6 km southeast of Farmville, Virginia. Previous geologic mapping suggested that this quadrangle lies across the northern termination of the Carolina slate belt in Virginia.

A slightly westwardly overturned syncline, the Dryburg syncline, is mapped through the center of the Green Bay quadrangle. This syncline, mapped in detail since 1993 by personnel of the U.S. Geological Survey northward from the Roanoke and Dan River area, is a northern extension of the Virgilina synclinorium. It folds the Virgilina sequence of Carolina slate belt units, which here include higher metamorphic-grade equivalents of Aaron Formation slate and informal units of metamorphosed volcaniclastic rock of the underlying Hyco Formation. These units are traced progressively northward from the almandine zone, across a Barrovian series of staurolite and sillimanite metamorphic isograds into a higher metamorphic-grade terrain. This mapping shows that the Carolina slate belt units do not: 1) end against through-going faults in this area, or 2) rest on a higher metamorphic-grade "basement" to the north; as previously mapped (Laney, 1917; Jonas, 1932; Horton and others, 1991; Virginia Division of Mineral Resources, 1993). Instead, the same stratigraphic units increase in metamorphic grade northward extending into the central Piedmont of Virginia.

Metamorphic index minerals stabilities, and granite minimum melting temperatures, suggest that slate belt rocks in the Green Bay area were metamorphosed at depths of about 9 to 15 km, and temperatures of about 420° to 620°C. The area probably has been tilted southeastward about 30 degrees since a pre- or syn- Mississippian Redoak Granite period of metamorphism.

Late Alleghanian(?) arching and warping of foliation and bedding may be the result of transpressive deformation. The late Alleghanian(?) arches fold foliation and bedding on the west limb of the Dryburg syncline in the Keysville and Green Bay quadrangles.

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U.S. Department of the Interior | U.S. Geological Survey URL: https://pubsdata.usgs.gov/pubs/of/1999/0423/index.html Page Contact Information: Eastern Publications Group Page Last Modified: 17:41:20 Wed 07 Dec 2016