New evidence for the age of the Quantico Formation of Virginia

A new fossil locality within the Quantico Formation of the Virginia Piedmont in Dale City, Virginia, about 40 km south of Washington, D.C., has yielded crinoids and a cephalopod that re-establishes an age younger than Cambrian for this formation. On the basis of regional considerations and the work of investigators in the early 1900s, an Ordovician age for the Quantico seems likely. Also, the Dale City pluton is not intrusive into the Quantico but is unconformable beneath it. Hence, the 560-m.y. zircon age for the Dale City pluton is no longer inconsistent with the temporal and stratigraphic relationships established for the stratigraphic units in the area. © 1980 Geological Society of America.