PLEISTOCENE GEOLOGY OF NORTHWESTERN HORRY AND EASTERN MARION COUNTIES, SOUTH CAROLINA

Detailed 1:24,000 geologic mapping has defined the areal and three-dimensional distribution of Lower to Middle Pleistocene alloformations and fluvial terraces across four 7.5-minute quadrangles in Northwestern Horry and Eastern Marion Counties, South Carolina (Bayboro, Duford, Galivants Ferry, and Nichols). Mapping was based on observation of cuttings from power-auger and hand-auger holes, examination of surface exposures, geomorphic expression, elevation, and soil types. These factors were used to subdivide depositional sequences into marine, barrier-island complex, estuarine, and fluvial lithofacies.

Two Lower Pleistocene marine alloformations are preserved and overlie Upper Cretaceous and Pliocene marine deposits. The inland limit of the older alloformation (Wicomico) toes at +32–31.4 m (105–103 ft) and the younger alloformation (Penholoway) toes at +23.8–23.4 m (78–77 ft). Additionally, a middle Pleistocene alloformation (Ladson) is preserved as estuarine deposits in incised creek channels with a toe elevation of +17.4 m (57 ft). Four fluvial terraces are preserved within the Little Pee Dee River valley and its tributaries, and they overlie Upper Cretaceous and Pliocene marine deposits. Terrace IV is preserved as fluvial deposits between elevations of +27.4–23.8 m (90–78 ft). Terrace III is preserved as fluvial deposits between the elevations of +23.8–18.6 m (78–61 ft). Terrace II is preserved as terrace deposits between elevations of +18.6–15.2 m (61–50 ft) in. Terrace I is preserved as fluvial levees and terrace deposits between elevations of +15.2–10.9 m (50–36 ft). The surface expression of these Pleistocene units is modified by river morphology and Quaternary deposits such as Carolina bays and their associated sand rims, eolian sands, and anthropogenic modification.

The relationship between the allofomations and the fluvial terraces is not well understood. Correlation studies using geochronology could provide clarity to these relationships.