The Lilesville Gravels: A Neogene Strath Terrace Deposit from the Piedmont/Coastal Plain Boundary of North Carolina, USA

Abstract:

The Lilesville “gravels” are discontinuous Neogene cobble, sand and silt deposits exposed in gravel quarries in an area covering ~100 km² where the Pee Dee River crosses the Fall Line of North Carolina. These deposits unconformably overlie the western edge of the Inner Coastal Plain, and parts of the saprolitized Paleozoic Lilesville pluton, and are interpreted as strath terrace deposits.

In the Bonsal quarry, the Lilesville “gravels” comprise channel fills, channel bars, pebble lags, trough cross bedding, mud clasts, and mud drapes. These features define several facies including: 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. Paleoflow indicators suggest southerly transport, parallel to the modern Pee Dee River. GPR profiles and 3D models document facies boundaries on the quarry face and behind the high-wall. The tops of the Lilesville “gravels” are commonly marked by a pebble lag (deflation) horizon disconformably overlain by a medium- to coarse-grained, well-sorted quartz arenite interpreted as an aeolian deposit (Pinehurst Fm?). C14 dates from charcoal in the aeolian sand are 1638 +/- 46 calendar years BP (or 312 AD +/- 46).

The age of the Lilesville “gravels” remains uncertain. However, by using regional curves for ages of terrace deposits relative to their heights above river level (Mills 2000), it is suggested here that the Lilesville gravels are 7 to 12 Ma (Upper Miocene).

It is likely that the Lilesville “gravels” were deposited by the ancestral Pee Dee River when it was a braided stream flowing in a southerly direction. This suggests that a combination of regional uplift and a wetter paleoclimate in the Late Miocene may have been responsible for a larger discharge and coarser bedload than currently exist.