GEOLOGIC MAPPING IN THE CAROLINA TERRANE IN ORANGE, DURHAM, AND CHATHAM COUNTIES, NORTH CAROLINA – A PROGRESS REPORT WITH IMPLICATIONS OF NEW AGE DATES

The North Carolina Geological Survey, through the 50:50 cost-share STATEMAP component of the National Cooperative Geologic Mapping Program, has been mapping in the Carolina terrane in Durham, Orange, and Chatham Counties since 2002. To date, the Carolina terrane portion of Orange County is complete at the 1:24,000-scale. Most of Durham County and approximately half of Chatham County have also been mapped at this scale. Rock types include various metamorphosed intrusive, extrusive, and volcanogenic sedimentary rocks of the Hyco Formation of the Hyco arc and the Aaron Formation of the Virgilina sequence. The primary volcanic and volcano-sedimentary rock types have been grouped into map units based on related chemical and/or interpreted genetic relationships. Primary volcanic lithologies were formed in subareal to subaqueous depositional environments with concomitant deposition of various types of clastic volcanogenic sediments.

Previous interpretations of the regional stratigraphy, based on a limited number of age dates, divided the Hyco Formation into lower (ca. 630 Ma) and upper (ca. 615 Ma) members (informal) associated with an apparent intervening hiatus of magmatism. New U-Pb zircon LA-ICPMS ages indicate that magmatism in the lower member of the Hyco Formation may be as old as ca. 650 Ma and that Aaron Formation rocks may be present in units previously interpreted as Hyco Formation.

The new age dates also indicate that older (ca. 650 Ma) granitoids may be intermingled with younger (ca. 579 Ma) granitoids. The accurate assigning of map units to the upper or lower members of the Hyco Formation may not be possible without abundant age dates.

Additionally, recent mapping in Chatham and Durham Counties has utilized Hillshade LiDAR data to identify multiple mile-long lineaments. Data collection within the lineaments indicate the local occurrence of diabase, evidence of brittle faulting, and abundant quartz that often resembles quartz cataclasite. The lineaments are interpreted as Mesozoic-aged faults.