Southeastern Section - 60th Annual Meeting (23–25 March 2011)

Paper No. 7
Presentation Time: 10:20 AM


BRADLEY, Philip J., North Carolina Geological Survey, Raleigh, NC 27699-1620 and MILLER, Brent V., Dept. of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115,

Detailed mapping and recent geochronology, partially funded by STATEMAP, in Orange County, North Carolina indicates that the majority of the County is underlain by weakly metamorphosed Late Proterozoic (~ 633 to 579 Ma) primary pyroclastic to epiclastic and intrusive rocks of the Carolina terrane. In southern Orange County, primary pyroclastic rocks and associated lavas form mappable units that are stratigraphically capped by epiclastic units consisting of mature conglomerates with subrounded to subangular clasts and thinly bedded siltstones. This stratigraphy suggests a local termination of volcanism followed by a period dominated by erosion and deposition. In northern Orange County, texturally identical primary pyroclastics and lavas are locally interlayered with thinly bedded siltstones and immature conglomerates with subangular to angular clasts of lava suggesting concomitant active volcanism and erosion of an older volcanic center. In southern Orange County, the East Farrington pluton (579 ± 6 Ma) truncates epiclastic units.

New U-Pb (zircon) crystallization ages for the top of the pyroclastic sequence in southern Orange County from a dacitic tuff (630 ± 1 Ma) from Morgan Creek and a dacite (628.5 ± 1 Ma) from the American Stone Quarry, coupled with previously published dates, indicate southern Orange County is dominated by ~ 630 Ma primary pyroclastics, associated lavas and intrusives. In northern Orange County, a new U-Pb age (613 ± 1 Ma) from a felsic tuff interlayer in a dominantly epiclastic unit, coupled with previously published dates from adjacent northern Durham County, indicate that the area is dominated by ~ 615 Ma lithologies.

Past workers designated the study area as part of the Hyco and Aaron formations of the Virgilina sequence, in which epiclastic rocks were assigned to the regionally extensive Aaron formation. Previously reported youngest detrital zircon ages of ~ 578 and ~ 588 Ma from two separate locations in the Aaron formation prevent direct correlation of texturally similar rocks in the Orange County area. It is speculated that each volcanic center had its own volcaniclastic apron that likely interfingered with volcaniclastic aprons of adjacent volcanic centers. Based on the new data, we have tentatively identified Upper (~ 615 Ma) and Lower (~ 630 Ma) members (informal) of the Hyco Arc.