DYNAMIC LANDSCAPE EVOLUTION AND SEQUENCE STRATIGRAPHY IN THE EARLY PLEISTOCENE – ATLANTIC COASTAL PLAIN, NORTH CAROLINA, USA

The Atlantic Coastal Plain of North Carolina features a well-preserved, relict Pleistocene terrane that demonstrates how geomorphic features evolved during Gelasian and Calabrian stages during a series of transgressive-regressive (T-R) events that followed Marine Isotope Stage (MIS) 103, which marks the base of the Quaternary (2.58 Ma). Landform analysis using high-resolution LiDAR was paired with detailed subsurface analysis along cross sections that transect key geomorphic features to develop a dynamic stratigraphic model for landscape evolution near the Surry Paleoshoreline. The Surry shoreline complex (toe at 30 m) is a regional-scale geomorphic feature that may mark a major change from a normal to a forced regression in the Calabrian (?). Features at elevations of ~22-36 m were examined in the context of the underlying stratigraphy, and falling stages associated with the forced regression.

The base of the Quaternary (~12-24 m thick) is marked by a thin (mm to 3 m) coarse-grained, mixed to bioclastic shell bed, dominated by cobble-sized mollusks, and phosphate and quartz pebbles. This bed is commonly clast-supported, the largest mollusk valves occurring at the top of the bed, akin to a pavement. Laterally and upsection, the bed may assume a dissolution fabric, that lacks carbonate and consists of the insoluble residue of quartz, concentrated phosphate and other grains. This transgressive lag defines the base of the Quaternary: five Sr isotope ages on Mercenaria from the bed range in age from 1.7 to 2.65 Ma. This time frame corresponds to North American Pre-Illinoian I, J, K stages. This lag is abruptly and conformably overlain by marine mud interbedded with thinner shell beds, that fines upward to a maximum flooding surface (MFS). These marine facies correlate with the Chowan River Formation. Above this, the high-stand systems tract (HST), consists of tidally bedded heterolithic strata that thin seaward; this is overlain by upward-fining feldspathic coarse sands, which might represent falling stage parasequences.

The terraces mapped in LiDAR demonstrate how the forced regression impacted dynamic landscape evolution: the Surry paleoshoreline was abandoned abruptly. As sea level dropped in a series of steps (1-2 m), incipient incised valleys formed along the borders of a series of downstepping marine shorelines.