THE LILESVILLE GRAVELS: SEDIMENTOLOGY AND DEPOSITIONAL ENVIRONMENTS OF A TERTIARY FLUVIAL DEPOSIT OF THE INNER COASTAL PLAIN OF NORTH CAROLINA

The Lilesville “gravels” are discontinuous Tertiary gravel, sand and silt deposits covering 102 km²along the Pee Dee River in North and South Carolina. These deposits unconformably overlie the western edge of the Inner Coastal Plain and parts of the Paleozoic Lilesville pluton and country rock. The study site is a 500-meter long highwall in a gravel quarry.

The origin of the Lilesville gravels have been debated for >150 years. Proposed ages for the deposits range from Late Eocene to Pleistocene. Based on regional stratigraphy, Owens (1989) assigned the deposits to the Lower Pliocene. That age is younger than predicted by regional curves for ages of terrace deposits relative to their heights above river level (Mills, 2000). The Lilesville gravels range from 87 to 107 meters above the current Pee Dee River, suggesting they are 7 to 12 Ma (Upper Miocene).

Channel fills, channel bars, pebble lags, trough cross bedding, mud clasts, and mud drapes occur in the quarry. These features define several facies in the Lilesville gravels. In stratigraphic order (oldest to youngest) they are: 1) an imbricated clast-supported conglomerate with a medium- to coarse-grained lithic arenite matrix; 2) a lenticular silt- to medium-grained, lithic arenite partly interbedded with the conglomerate; 3) a pebbly cross-bedded medium- to coarse-grained lithic arenite; and 4) a mottled medium- to coarse-grained lithic arenite. Flow indicators suggest southwest transport. Multi-frequency GPR profiles have been acquired to project facies boundaries into the high-wall exposure and to locate crystalline basement beneath the quarry floor. The top of the Lilesville gravels is commonly marked by a pebble lag horizon overlain by a medium- to coarse-grained, well-sorted quartz arenite interpreted as an aeolian deposit (Pinehurst Fm?). C14 dates of charcoal samples from the aeolian sand are 1638 +/- 46 calendar years BP (or 312 AD +/- 46).

The Lilesville gravels were deposited by the ancestral Pee Dee River when it was a braided stream flowing to the southwest. This suggests that uplift associated with the Cape Fear arch may have influenced the regional river flow pattern, and that a wetter paleoclimate in the Late Miocene to Early Pliocene may have been responsible for a larger discharge and bed load than currently exist.