LATE TRIASSIC FLUVIAL SEDIMENTARY ROCKS OF THE DURHAM SUB-BASIN, NORTH CAROLINA: AN EXCELLENT TEACHING OUTCROP FOR SEDIMENTARY STRUCTURES AND STRATIGRAPHIC RELATIONSHIPS

Late Triassic fluvial sedimentary rocks occur along the shoreline of Jordan Lake in Chatham County, NC at a site known locally to geologists as the “Martha’s Chapel outcrop.” The rocks are Late Triassic “red bed” sediments related to the Mesozoic continental rifting of Pangea and were deposited in the Durham sub-basin, the northernmost section of the Deep River rift basin. Along Jordan Lake, wave action has sculpted the shoreline into a continuous cliff exposure of fresh-surfaced rock. The cliffs expose nearly 100 meters of continuous section of sandstone, siltstone, and mudstone, allowing an excellent view into the architecture of the rocks.

The site is typified by consistent, rhythmic, fining upward sequences (1-6 m thick) in which coarse to very coarse-grained pebbly sandstones overlie scoured surfaces. The sandstones grade upward into mottled siltstones and mudstones. A striking color contrast exists between the light colored sandstones and the darker-hued finer grained mudstones and siltstones. The site is interpreted to represent lateral point bar aggradation and crevasse splays within an axial fluvial system surrounded by a vegetated floodplain.

The sandstones (arkoses and lithic arkoses) contain abundant coarse-grained muscovite and pink feldspar and are dominantly light in color: variously yellow, white, pink, or pinkish gray. Low angle, high amplitude cross bedding is characteristic of the sandstones at this outcrop. Sharp, scoured contacts are found at the base of the sandstones beds. Rip-up clasts and lithic pebbles can be observed scattered throughout the sandstone beds but are concentrated at the base above the scoured contact. Trains of imbricated pebbles can be observed in several sandstone beds. Many sandstones are bioturbated, a phenomena that intensifies at the overlying contact with the fine-grained sediments.

The siltstones and mudstones range in hue from dark red, to purplish red, to light red and are invariably mottled light greenish gray. The mottles are elliptical and generally extend from four to ten centimeters in length. The mottling is interpreted to arise from rooting of plants and the burrowing of organisms. The finer grained sediments are very poorly sorted and in many cases original fabric has been obliterated by bioturbation.