MIDDLE COASTAL PLAIN GEOMORPHOLOGY AND STRATIGRAPHY, JOHNSTON COUNTY, NORTH CAROLINA

Soils, landforms and sediments were investigated in the Upper Middle Coastal Plains near Wilson’s Mills, North Carolina. The investigations are part of a study that includes Carolina bay landforms in the area. The Coastal Plain sediments belong to the Brandywine MSU (Pliocene) as described by Daniels et.al. This section of the Brandywine has an assemblage of landforms distinctively different from other Coastal Plain MSU in the county. The sediments in this unit occur in complex patterns with saprolites formed from meta-volcanic bedrocks. The landforms consist of isolated knolls and ridges that are are surrounded by lower lying poorly drained gently sloping to level rounded to irregular depressions . The lower lying sediments have zones of smetitic clays indicative of marine environments. The ridge tops and knolls are dominated by soils that are deep and typical of the Middle Coastal plain and are intermingled with soils formed over saprolites with gravels commonly associated . Ephemeral shallow stream channels have formed in the lowest landscapes. The landforms (erosional and depositional elements ) of this overall landscape do not appear to be related to the present drainage system. An auger traverse across one of the circular depressions revealed evidence of regressive-transgressive-regressive events as evidenced by stream dissection and soil formation (paleosols) followed by deposition of finer textured sediments containing smetitic clays. This deposional-erosional history may have part contributed to the complex nature of the overall landscape system. The Carolina Bays (features 1 & 2) are clearly younger features than the associated landscapes as indicated by their ovid forms (LIDAR imagery) and the fact that they truncate adjoining Brandywine age sediment units. These features appear to differ in age. Feature 2 is somewhat enigmatic. It does not contain a distinctive silty soil, but has a mature clayey soil sediment in the lowest bay depression, which appears to be related to the more recent Brandywine depositional elements as identified by the traverse. Samples from sandy rim sediments of these bays have been submitted for OSL dating to resolve questions concerning their relative ages.