NEW GEOLOGIC MAP AND GEOCHRONOLOGY OF LOUDOUN COUNTY, VA

A cooperative investigation between the U.S. Geological Survey and Loudoun County produced a new 1:50,000-scale Geologic Investigations Series Map I-2553. Loudoun County is situated within the Blue Ridge and Piedmont provinces. Land use reflects the geologic framework of the crystalline rocks in the western part of the county and the Mesozoic basin fill strata in the eastern part of the county. Field mapping at 1:12,000 scale conducted from 1988 to 1993 emphasized surficial deposits and about 40 crystalline rock units of the Blue Ridge province. Geochemical analyses of igneous rocks, structural analyses, and newly acquired aeromagnetic image data compliment the map plate.

Geochronologic studies using U-Pb analyses of zircon and monazite, 40 Ar/39 Ar spectra of mica and hornblende, and fission tracks of zircon and apatite document a complex tectonic history spanning 1.1 billion years. Mesoproterozoic plutonism and tectonism occurred from about 1153 to 1055 Ma, with hornblende cooling ages of 1000 to 920 Ma, associated with the Grenvillian orogeny. Neoproterozoic plutonism at 722 Ma followed by sedimentation and bi-modal volcanism from 568 to 555 Ma was during Iapetan extension. Paleozoic metamorphism and deformation at lower greenschist facies occurred during the Devonian with further deformation during the Alleghanian orogeny in the Pennsylvanian. Westward tectonic transport and emplacement of the North Mountain thrust sheet at 305 Ma was during the late phase of the Alleghanian orogeny. Jurassic to present-day cooling was at average effective uplift and denudation rates of about 20m/m.y. The proto-Potomac River breached the Blue Ridge Mountains at the Harpers Ferry water gap in mid-Oligocene to early Miocene.