AGE, ORIGIN, AND TECTONIC SIGNIFICANCE OF THE ARVONIA AND BUFFARDS FORMATIONS IN VIRGINIA’S PIEDMONT

Rocks in Virginia’s Piedmont formed during multiple Paleozoic orogenies and the suturing of exotic terranes to Laurentia. The Arvonia and Buffards formations, in the Chopawamsic arc terrane, have long been considered Late Ordovician Taconian successor basin deposits. These units range from quartzite, schist, and commercial-grade slate of the Arvonia Formation, to conglomeratic and micaceous schist of the Buffards Formation. The Quantico Formation occurs to the northeast of this area and is correlative to the Arvonia Formation. Previous workers concluded that these formations are Late Ordovician, based on fossil assemblages that include crinoids, bryozoa, trilobites, and brachiopods. Additionally, the stratigraphic and structural relationship between the Arvonia and Buffards formations is contested – some workers hypothesize that the Arvonia Formation is older, while others suggest that the Buffards Formation is older. We use ICP-MS laser ablation geochronology on detrital zircons (from 5 samples) and new mapping to constrain the depositional age of the two formations and better understand their significance. Ongoing work includes geochemical and pseudosection analyses to estimate the P-T-t path, as the precise conditions of metamorphism in these formations is poorly constrained.

Mapping and observations of bedding-foliation relationships and facing indicators show that the Buffards Formation is stratigraphically above the Arvonia Formation. Our U/Pb detrital zircon age dates reveal detritus in these units is as young as 400 Ma (Devonian). Both formations yield Mesoproterozoic and Paleozoic populations, though the three Buffards samples contained a smaller Mesoproterozoic peak, and a larger percentage of post-Ordovician zircons than the two Arvonia samples. The difference in age populations indicates that there was a shift in provenance between deposition of the Arvonia Formation and the Buffards Formation. Furthermore, these age data suggest that the Arvonia-Buffards successor basin is Acadian (late Devonian), and likely related to the docking of the Goochland and Dinwiddie terranes to the east.