DEPOSITIONAL PROCESSES ON THE LATE PLEISTOCENE COASTAL PLAIN OF SOUTHEASTERN USA: EVIDENCE FROM LONG RIDGE BAY, JONES COUNTY, NORTH CAROLINA

Carolina Bays are NW-SE oriented ovoid depressions found across the United States mid-Atlantic coastal plain. Recent work using high-resolution airborne LiDAR, OSL depositional ages, and modeling of glacial isostatic adjustment, together suggest that the Carolina Bays record one of several surficial processes that formed the periglacial landscape south of the Laurentide Ice Sheet. However, detailed studies of Carolina Bays and possible coeval depositional processes are few.

The Long Ridge Carolina Bay (LRB) field area is in eastern NC ~50 km inland from the modern coastline. The depression occurs in unconsolidated sandy sediment, has a ~850 m long axis, and is partly defined by a rim around its SE margin (<2 m total topographic relief).

Ground penetrating radar surveys (GPR; 250 MHz) crossing the LRB area reveal the stratigraphic structure of the upper 5 m of sediment. Below 4 m is a dome-shaped package with internal reflection geometries resembling meter-scale crossbedding. This deepest package may represent a dune or sand sheet. Overlying this feature at ~2-4 m is a laterally extensive package characterized by diffractions, which continues up to 0.5 km beyond the rim. Exposures in roadway ditches and river cut banks in the field area suggest this is a sandy deposit containing well-rounded pebbles. These two lower packages have no apparent relationship with the LRB geometry. The upper 2 m are partly obscured by ground surface multiples, root diffractions, and graded road-bed material, but distinct reflectors ~1 m under the NE rim gently dip towards and flatten within the LRB depression.

Sediment samples from the upper 1 m are predominantly fine to medium sand. Most also contain minor fine pebble percentages. Some contain substantial silt. LRB rim samples are consistently better sorted and sandier than those from the depression interior. Pebble percentages are highest in samples outside the depression (3-6%).

In summary, the LRB is a shallow feature (<2 m) unassociated with an inherited stratigraphic architecture. It may be the result of thermokarst thawing within a fluvial deposit, which was remobilized and sorted by aquatic and/or aeolian sediment transport.