DETERMINING THE AGE AND DEPOSITIONAL ENVIRONMENT OF COASTAL PLAIN SEDIMENTS AT THE JACK QUARRY, VA USING PALYNOLOGY

This collaborative study between USGS and the VA Department of Mines, Minerals and Energy (DMME) uses palynologic analysis to establish the age and depositional environment of an undisturbed sedimentary cover deposit above granite bedrock at the Vulcan Materials Jack Quarry near Petersburg, VA. Recent mapping by Occhi et al. (2018) assigns interbedded gravel, sand, and silty clay at the quarry to the Cold Harbor formation (CHf), a shallow marine to estuarine unit of Pliocene age in the Coastal Plain of central VA. Field relations indicate that the CHf unconformably overlies the Pliocene Yorktown Formation (YF) in the Richmond, VA Fall Zone.

Sediments in the 16.5 meters of section ranged from a clast supported, sub-angular to rounded gravel/cobble of predominantly vein quartz composition with interstitial silty clay to an organic-rich clayey silt. While no marine sedimentary structures were observed during field work, previous investigations at a now-removed face at the quarry displayed shallow marine sedimentary structures including a clay-line burrow and an in-life position mollusk mold. These lithologies are interpreted to be nearshore material associated with a post-YF transgression.

A sample from a 0.5 m-thick bed of organic-rich clayey silt from near the middle of the exposed stratigraphic section was collected for pollen analysis. In descending order, the dominant taxa within this sample include Poaceae, Polygonaceae (Rumex), Asteraceae, Quercus, and Pinus. Taxa such as Amaranthaceae, Cyperaceae, Carya, Ambrosia, Myriophyllum, Sagittaria, Lycopodium and Pteracarya were also present, but in lower abundances. This pollen assemblage confirms the unit as Pliocene in age and suggests that the clayey silt was likely deposited in a grass-dominated swamp or marsh. Coarser sediments that bound the clayey silt may also be fluvial.

Understanding the depositional environments of regional Pliocene stratigraphic units can help locate and target heavy minerals deposits, which may include valuable Rare Earth Elements. Heavy mineral deposits near McKenney, VA, ~40 km to the south of Jack Quarry, occur in sand and gravel of Pliocene age. Relationships between the CHf, YF and the Pliocene Chowan River Formation in the southern Virginia Fall Zone are unclear and further investigations are necessary.