PRELIMINARY COMPILED BEDROCK GEOLOGIC MAP OF THE CHAPEL HILL 100K, NORTH CAROLINA

The N.C. Geological Survey is currently compiling a 1:100,000 scale bedrock geologic map of the Chapel Hill 100K Quadrangle in North Carolina’s Piedmont. The project is partially funded by the U.S. Geological Survey, National Cooperative Geologic Mapping Program under STATEMAP. Line work from multiple legacy sources is being utilized and modified according to new field observations and edge-matched with adjacent quadrangles. Legacy geologic data sources include NCGS data from recent STATEMAP mapping and older projects, USGS published maps, and master’s theses.

New and legacy data indicates the cryptic contact between the Charlotte (CLT) and Carolina (CT) terranes lies in the northwest corner of the map. Gneissic xenoliths within a batholithic-scale granodiorite have been found northwest of the terrane contact (CLT) with greenschist facies xenoliths present southeast of the contact (CT). A majority of the quadrangle is underlain by the greenschist facies CT, which includes the Hyco and Albemarle volcanic island arcs. The Durham and Sanford sub-basins of the Deep River Triassic basin contain sedimentary rocks from conglomerates to mudstones. The southeastern corner of the map area is underlain by metamorphosed crystalline rocks of the Cary sequence that is designated as the Easternmost CT and is interpreted to be part of the CT but separated from the rest of the terrane by the Triassic basin.

The Hyco Formation consists of dacitic to basaltic lavas, tuffs and volcaniclastic sedimentary rocks. The Aaron Formation is composed of distinct conglomerates to mudstones with sparse to abundant quartz grains. The Uwharrie Formation consists of a package of felsic to mafic volcanics and volcaniclastic sedimentary rocks. In the map area, the Albemarle group contains the Tillery and Cid Formations.

Field work is ongoing to determine the relationship of units previously mapped as Aaron Formation by past mappers which have detrital zircon dates that more closely associate the unit to the Albemarle arc. Additionally, the contact between the Uwharrie and the Hyco Formations north of Asheboro is not clear. Multiple structural models are being considered (i.e. folded unconformity vs fault bounded associated with intra arc rifting) and may require detailed mapping.