Soil development and its relation to the ages of morphostratigraphic units in Horry County, South Carolina

Soils of the lower Coastal Plain in Horry County, South Carolina, were sampled to determine whether their chemical, mineralogical, and morphological properties are related to soil age. These soils have developed on marine barrier and backbarrier deposits that paleontological data indicate are 200 ka (Conway and Jaluco barrier systems) and 2.5 to 3.0 Ma (Horry barrier system). These three barrier systems are the major morphostratigraphic units of barrier systems present on the southern limb of the Cape Fear Arch, which has affected deposition and deformation of Coastal Plain sediments at least since the Cretaceous. The late Cenozoic tectonic history of the arch is not known in detail, but previous studies have suggested regional uplift throughout the Neogene. The present topographic positions of barriers and back barriers are due to either eustatic changes in sea level, regional uplift, or a combination of the two. Barrier deposits consist predominantly of well-sorted, fine-grained and very fine grained quartzose sand; interbeds of sandy clay and clay range from less than 1 to 5 m thick. Barriers of the Horry system have crest heights of 30 to 40 m, the Conway barrier reaches a maximum altitude of 17 m, and the Jaluco barriers have heights up to 12 m. The preserved topography is similar on all barriers. The Horry barrier system is somewhat more dissected and is more deeply incised than the Conway or the Jaluco barrier systems. Carolina bays are ubiquitous on the sandy surfaces of the barrier systems. Humates (or spodic horizons) are present within many of the barrier sands and are almost always associated with Carolina bays. Field descriptions of Horry back-barrier soils indicate argil lie horizon thicknesses ranging from 60 to 90+ cm. Textural and microfabric data indicate that many of these soils have argillic horizons thicker than 150 cm. These soils are therefore considered to be Paleudults, whereas soils on the Conway and Jaluco back barriers are Hapludults. Well-drained barrier soils of the Horry, Conway, and Jaluco systems all clas'U.S. Geological Survey, Doraville, GA 30360. 2U.S. Geological Survey, Reston, VA 22092. 3U.S. Department of Agriculture, Soil Conservation Service, National Soil Survey Laboratory, Lincoln, NE 68508. 4U.S. Department of Agriculture, Soil Conservation Service, Columbia, SC 29201. sify as Hapludults. Soils developed on the 200-ka Conway and Jaluco systems are similar both physically and chemically to those developed on the 2.5to 3.0-Ma Horry barrier system. Data indicate that soils on the Horry barrier system have bisequal development; our samples were from the surface horizons only and do not represent the full profile. Two sample sites were near or on the rims of Carolina bays on terraces of the Little Pee Dee River. The differences in dominant grain size and sorting of the parent material preclude a direct comparison of soils developed on barriers with those developed on Carolina bays. The characteristics of Carolina bay soils, however, appear to be more like those of the Conway and Jaluco barrier soils than the Horry barrier soils. By inference, then, soils associated with the Carolina bays are apparently nearer 200 ka than 2.5 to 3.0 Ma. All the soils developed on the Horry, Conway, and Jaluco barrier systems and on Carolina bays have an accumulation of low-charge hydroxyinterlayer dioctahedral vermiculite in the upper parts of their profiles; kaolinite dominates the lower horizons and the parent material. Gibbsite and goethite as well as quartz are present in the clay-sized fractions. Halloysite is present in soils of each age that have significant Bt horizons. Although halloysite appears to be an indicator of in situ development of clay, its presence or absence seems to be associated more with drainage conditions than with the ages of sola. Only a few pedogenic parameters were found to be useful for estimating soil and surface ages. The position of Bh or spodic horizons within the profile may indicate differences in profile ages, but not enough is known about the origin of spodic horizons to allow their use for age determination. The thicknesses of sola and argillic horizons and the mineralogies of the <2-(jim fractions all show trends with increasing age, but they also are related to drainage, as the soils containing high percentages of medium-grained to very coarse grained sand illustrate. The lack of thick argillic horizons in the barrier sands may be the combined result of excessive drainage, little inherited clay, and a small percentage of weatherable minerals. Microfabric analysis of back-barrier profiles proved to be the most reliable indicator of the depth and degree of sand, silt, and clay translocation and may prove reliable for estimating soil age. Soils and Morphostratigraphy in Horry County, S.C. 1