PROVENANCE OF THE CRETACEOUS POTOMAC GROUP FROM DETRITAL ZIRCON ANALYSIS, CENTRAL-EASTERN VIRGINIA

A detrital zircon (DZ) provenance study of the Cretaceous Potomac Group (Kp) was used to determine the paleogeographic setting of structurally juxtaposed terranes and intrusive units that occur to the west of and beneath the Coastal Plain as well as the source of clasts of volcanic rocks that occur in the unit.

The Kp is an immature siliciclastic unit that forms the base of the Coastal Plain and crops out along the Fall Line of VA and MD. In central VA, The Kp overlies ~300 Ma Petersburg Granite (PG). To the W-NW are sedimentary and intrusive rocks of the Triassic Richmond-Taylorsville basin that are separated by the Hylas fault zone from Mesoproterozoic to Devonian medium to high-grade rocks of the Goochland terrane (GT). To the W-SW are low-grade metasedimentary and metaigneous rocks of the Roanoke Rapids terrane (RRT). West of the RRT and the Gaston Dam-Hollister faults are low to medium grade gneisses and schists of the Raleigh, Spring Hope and Triplet terranes.

The Vulcan Materials Puddledock Quarry near Hopewell, VA exposes the Kp beneath the Paleocene Aquia Formation. Kp in the quarry consists mostly of feldspathic sand and gravel. Pebble to boulder-size clasts within the gravel include relatively undeformed volcanic rocks, along with more common vein quartz and quartzite. DZ analyses of Kp matrix sand indicate, in descending abundance, populations at ~1065 Ma, 328 Ma, 455-409 Ma, and 574-540 Ma. Meso- and Neoproterozoic DZ ages compare well to published ages from the GT and RRT, respectively, but outcrops of undeformed volcanic rocks are sparse in the RRT and absent in the GT. Grenvillian DZ may have been sourced from the Blue Ridge, or recycled from other Piedmont terranes. Carboniferous DZs are ~30 My older than published ages for late Paleozoic plutons that crop out in the eastern Piedmont. Mid-Paleozoic plutons are only known from the western Piedmont in VA, but include the Concord-Salisbury plutonic suite and potentially rocks of the central Piedmont shear zone in NC. Exposures of undeformed volcanic rocks similar to those in the quarry are unknown from any of these terranes in VA.

DZ analyses suggest that either (1) the provenance of the Kp, including volcanic clasts, is more distant than expected based on immaturity of the sediment; or (2) the DZs and clasts may represent sources that were completely eroded during the Cretaceous.