200 YEARS OF GEOLOGIC MAPPING IN CHATHAM COUNTY, NORTH CAROLINA – LOCATION OF ONE OF THE FIRST DISTINCT GEOLOGIC ELEMENTS IDENTIFIED IN NC

In 2022, the North Carolina Geological Survey (NCGS) published a compiled geologic map of Chatham County. This map is a culmination of over 200 years of geologic work in the county. In 1820, Denison Olmsted, a professor at UNC Chapel Hill and creator of the first geological survey in the US, published a letter that described sandstones and coal deposits in Chatham County. In 1823, Olmsted published the nation’s “First State Geologic Map” identifying the Triassic area and the Great Slate Formation in Chatham County.

Ebenezer Emmons, the first State Geologist of NC, published the Map of the Deep River Coal Field in 1856. The map differentiated the Triassic rocks, identified diabase as “Trap” and identified the crystalline rocks as the Taconic Slates with their associated mineral resources of soapstone (pyrophyllite) and copper. The US Civil War brought the exploitation of coal and iron in Chatham and adjacent counties. After the Civil War to the 1920’s, geological investigations continued centered around coal and its extraction. Work by Campbell and Kimball, published in 1923, is the first modern study that laid the foundation for future work within the Triassic basin. The next major advancement in geologic mapping in Chatham County was by Reinemund (1955; USGS Professional Paper 246). His careful work resulted in a map showing the Triassic sedimentary rocks and coal deposits with such excellent accuracy and detail that his work is still relevant today.

With the help of the USGS STATEMAP Program, the geologic understanding of Chatham County catapulted forward with detailed geologic mapping in the 1990’s through 2022. Coupled with the work of other researchers in the region, we now know that Chatham County is underlain by: rock units belonging to two volcanic island arcs (the Hyco and Albemarle arcs of the Carolina terrane); a continental rift basin (Deep River Basin of the Newark Supergroup); Jurassic diabase; and remnants of Coastal Plain sedimentary cover. As NCGS celebrates its 200^th anniversary, new “state-of-the-art” detrital zircon data and their unexpected results has brought to light how much we still have to learn about the geology of Chatham County and the Piedmont of NC.