Date Thesis Awarded

5-2020

Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Christopher M. Bailey

Committee Members

Brent E. Owens

Robert S. Leventhal

Abstract

In central Virginia, a heterogeneous sequence of layered low-grade metamorphic rocks crops out along the boundary between the eastern Blue Ridge and western Piedmont. Two competing models exist to explain this sequence of phyllite, quartzite, carbonate and greenstone. The first interpretation names this sequence the Evington Group and considers these rocks to be on the eastern limb of the Blue Ridge anticlinorium. In this model, the Evington Group is a lateral equivalent of the western Blue Ridge Chilhowee Group. The second model argues that the “Evington Group” is a false unit that is instead the older Alligator Back Formation repeated across the Bowens Creek Fault.

Our mapping across the Blue Ridge – Piedmont transition near Gladstone, Virginia indicates that there are several northeast-southwest trending belts of metamorphic sedimentary and volcanic rocks. The sequence is >3 km thick and includes (from bottom to top): phyllite, marble, quartzite, phyllite, greenstone, phyllite and marble. Some layers are repeated by folding. The greenstone is chemically similar to the underlying Catoctin Formation. Carbonate rocks include: a blue-gray micaceous to quartzose ± graphite ± tremolite marble and a milky white to pink, biotite-bearing marble. Bulk geochemical analysis of the marbles reveals a higher MgO in the white marbles (17%) than the blue-gray marbles (2%). The depositional environment was likely a distal muddy marine shelf below wave base with significant detrital input. U/Pb detrital zircon and apatite dating indicates that these rocks were deposited after 600 Ma and experienced metamorphism associated with the Taconian Orogeny (~440 Ma).

In central Virginia, the Evington Group is a major stratigraphic sequence in the eastern Blue Ridge. Evington Group strata were medial to distal Iapetan deposits on the Laurentian continental margin; these deposits received Grenvillian detritus with occasional periods of carbonate production and sea floor volcanism. The Bowens Creek Fault, as portrayed in southern Virginia, is not present in the Gladstone area, nor do characteristic Alligator Back Formation lithologies occur in this region. Evington Group rocks experienced at least two episodes of deformation, and regional metamorphism reached mid-greenschist conditions.

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