GEOLOGIC MAP AND GEOCHRONOLOGY OF THE FREDERICK 30' X 60' QUADRANGLE, MARYLAND, VIRGINIA, AND WEST VIRGINIA

The new Frederick 1:100,000-scale Geologic Investigations Series Map I-2889 lies within the Potomac River watershed. Mapped at 1:24,000-scale, and compiled at 1:100,000 scale, the map covers the Great Valley of the Valley and Ridge, the Blue Ridge, the Piedmont, and a small part of the Coastal Plain provinces. The Piedmont province is subdivided into the Potomac and Westminster terranes, the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium, and the Culpeper and Gettysburg basin. U-Pb analyses of zircon and monazite and 40 Ar/39 Ar spectra of both white mica and hornblende document a complex tectonic history. Mesoproterozoic plutonism in the Blue Ridge occurred from 1170±10 Ma to 1055±2 Ma. Episodes of Grenvillian metamorphism occurred from 1130 to 1033 Ma, with hornblende cooling ages between 1000 and 920 Ma. Neoproterozoic extension during Iapetan rifting occurred during 722 Ma plutonism and bi-modal volcanism from 568 to 555 Ma. Neoproterozoic and early Paleozoic metasedimentary and metavolcanic rocks were deposited on the margin of the rifted continent and in deep trenches in the Iapetus Ocean.

In rocks of the Potomac terrane in the eastern Piedmont, Ordovician amphibolite-facies metamorphism and deformation occurred. Greenschist-facies metamorphism and deformation in the western Westminster terrane was in the early Silurian (~430 Ma). Greenschist-facies rocks of the central and eastern Piedmont and Blue Ridge were deformed from the Devonian to the Carboniferous (about 380 to 340 Ma). Final emplacement of the rocks was along the North Mountain thrust fault during the Permian Alleghanian orogeny. Deformation, including regional development of cleavage, generally progressed from east to west, but many out of sequence faults and shear zones are found. Across the study area there is a regional east to west progression of amphibolite- to greenschist-facies metamorphism. The Potomac foliation fan and the South Mountain cleavage fan are a composite feature that resulted from different tectonic events over a period of at least 180 m.y.