PRELIMINARY COMPILED GEOLOGIC MAP OF DURHAM COUNTY, NORTH CAROLINA

A preliminary detailed geologic map, partially supported with STATEMAP funds, has been completed for Durham County. The northwestern third of the county is underlain by greenschist facies crystalline lithologies of the Neoproterozoic to Cambrian Carolina terrane. Central and southern portions are underlain by Triassic sedimentary rocks and Jurassic diabase of the Durham sub-basin of the Deep River Mesozoic basin. The eastern edge of the County is underlain by the Neoproterozoic Easternmost Carolina terrane.

Within Durham County, the Carolina terrane is composed of two lithotectonic units: 1) the Hyco Formation of the Hyco Arc, which consists of layered meta-volcaniclastic and meta-plutonic rocks, and 2) the Aaron Formation of the redefined Virgilina sequence, which consists of meta-volcaniclastic rocks. Available age dates indicate the Hyco Formation in the area may be divided into lower (ca. 630 Ma) and upper (ca. 615 Ma) members (informal) with an apparent intervening hiatus in magmatism. The Aaron Formation outcrops in the northwestern corner of the County.

Hyco Formation units of meta-volcaniclastic rocks include various lithologies that are grouped together to represent interpreted environments of deposition (or lithofacies). The dacitic lavas and tuffs unit is interpreted to represent dacitic domes and proximal pyroclastics. The felsic tuffs unit is interpreted to be distally deposited air fall tuffs and associated volcanosedimentary rocks. The andesitic to basaltic lavas and tuffs unit is interpreted as lava flows and associated pyroclastics. The epiclastic/pyroclastic units are interpreted to be deposits of active volcanics and eroded volcanic highlands.

In Durham County, the Easternmost Carolina terrane is composed of upper greenschist facies meta-plutonic rocks of the Beaverdam Complex and meta-volcaniclastic rocks. The Jonesboro Fault separates the Easternmost Carolina terrane from the Durham sub-basin.

The Durham sub-basin consists of variously interlayered conglomerates, sandstones, siltstone and mudstones which are separated into lithofacies associations. Diabase dikes and sills intrude basin sediments. Diabase dikes also intrude the crystalline rocks of the County. Quaternary aged alluvium is present in most major drainages throughout the County.