STRUCTURE CONTOUR MAP OF THE CRETACEOUS PEEDEE FORMATION, HORRY COUNTY, SOUTH CAROLINA WITH PALEO-VALLEYS AND PALEO-ESCARPMENTS

Borehole logs generated during detailed 1:24,000-scale geologic mapping in Horry County, South Carolina, were used to define the areal and three-dimensional distribution of Pliocene to Holocene depositional systems. A derivative product of this work was the development of a structure contour map of the upper surface of the Cretaceous Peedee Formation. When combining this work with existing structure contour maps along the northeast coast of South Carolina (Barnhardt et al., 2009), multiple post-Cretaceous paleochannels can be traced inland. Some surficial expression of the paleochannels exists in places, such as along portions of the Waccamaw River, but most of the paleochannels are now covered by Pleistocene and Holocene sediments. Associating these channels to their paleo-river systems is tenuous since no river currently flows parallel to the paleochannels, but the Great Pee Dee, Little Pee Dee, and Lumber Rivers are the most likely candidates. With the formation of the Horry barrier system, the drainage has migrated to the southwest to its current position as postulated by Wykel and Doar (2019). Of note are two apparent paleo-escarpments: one +6 m (+20 ft) MSL and the second +13.7 m (+45 ft) MSL. While the paleochannels trend NW-SE, normal to the escarpments, it appears that these escarpments may be indicators of erosion from prior sea-level highstands, not the river systems. The possible age constraints are not yet known, but future work may reveal additional detail with regards to the timing related to the escarpments’ formation.