Paper No. 214-1
Presentation Time: 9:00 AM-6:30 PM
LIDAR ELEVATION DATA PROVIDES NEW INSIGHT INTO THE BON AIR GRAVEL AND ATTENDANT CAROLINA BAYS
The Bon Air Gravel paves ~50 km^2 of a relatively undissected upland terrace which unconformably overlies Piedmont province bedrock in the vicinity of Midlothian, Virginia. Previous workers have considered these upland gravels to be fluvial deposits of Middle Miocene or older, and may represent a remnant of a formerly more expansive terrace. The Midlothian surface grades gradually from ~120 to ~100 meters above mean sea level (mamsl), west to east. Although referred to as a gravel, this unit is commonly subdivided into a lower gravel member and an upper quartzose loam member, and is correlated with the similarly subdivided gravel/loam Upland Deposits (formerly Brandywine) located at lower elevations to the east. As a curiosity, numerous aligned elliptical depressions have formed on this surface and have been considered “Carolina bay” landforms by many workers over the past 50 years. Here, we interrogate these bays' shape and orientation utilizing new digital elevation maps created from LiDAR data acquired in 2014. The new LiDAR additionally elucidates numerous Carolina bays established to the east on the adjacent gravel/loam sequence. Our findings suggest that these landforms adhere robustly to the “bayCarolina” archetypical ovoid planform found extensively (>22,000) in our Carolina Bay Survey, and have major axis orientations at 132º ± 2º rotation from north. When compared to bays we have documented to the north and south, this orientation is consistent with the systematic-by-latitude rotation noted in studies over the last 70 years. Studies by others indicate that the Midlothian bay rims are built solely within the massive loam member, whereas the loam member is absent within the basin proper and the gravel member remains intact throughout. The existence of bays on this surface is unexpected, since classic Carolina bay geomorphology considers them to be hosted above thick antecedent units of un-consolidated siliciclastic sediments. Furthermore, the uniformity of the gravel member beneath the bays adds to observations elsewhere that Carolina bays are created as voids in a unit of quartzose loam without disturbing the underlying surface. Their presence here is consistent with our Survey’s discovery of dense clusters of Carolina bays on isolated terrace remnants located west of the Coastal Plain up to 200 mamsl.