AGE AND PROVENANCE OF CLASTS IN THE EARLY CRETACEOUS POTOMAC GROUP AT PUDDLEDOCK, VIRGINIA: IMPLICATIONS FOR PASSIVE MARGIN DEVELOPMENT IN EASTERN NORTH AMERICA

The Early Cretaceous Potomac Formation forms the basal unit of the mid-Atlantic Coastal Plain sedimentary sequence and is well-exposed in the Puddledock quarry near Hopewell, Virginia. At this locality, the Potomac Formation consists of 20 to 30 m of feldspathic sand and polylithic gravel. Sedimentary structures are consistent with fluvial deposition from a western source in a moderate- to high-energy environment. Cobbles and pebbles are sub-rounded to well-rounded, and are primarily vein quartz and well-cemented quartz sandstone with lesser amounts of felsic volcanic, granitic, and gneissic clasts.

Felsic volcanic clasts are a distinctive rhyolitic welded ash-flow tuff (70-83% SiO₂) with phenocrysts of K-feldspar, quartz, and relict fiamme in a fine-grained groundmass. The volcanic rocks were variably mylonitized under lower greenschist facies conditions and experienced pervasive K-metasomatism.

A population of 30 zircon grains from clasts of weakly deformed rhyolite yield a U-Pb SIMS age of 540±10 Ma. We interpret the clasts to be sourced from low-grade felsic volcanic rocks in the Roanoke Rapids or Carolina terranes, exposed 20 - 50 km to the west in the Piedmont. However, Puddledock volcanics are 10-20 million years younger than known ages from these terranes, and may represent a younger volcanic sequence that was eroded away during or since the Cretaceous.

Detrital zircons from feldspathic sand have peak age populations at 1100-1000 Ma and ~330 Ma with other broader populations between 460-410 Ma and 575-540 Ma. The population of Grenvillian zircons may have been sourced from the Blue Ridge, derived from recycled zircons in western Piedmont terranes with a Laurentian margin affinity, or sourced from the southern Goochland Terrane. Based on these data, the Early Cretaceous fluvial network was well-established and extended 50 to 150 km into the Appalachian hinterland.