GENETIC STRATIGRAPHY AND GEOCHRONOLOGY OF SOUTH CAROLINA SHORELINES: IMPLICATIONS FOR MIS 5A SEA LEVEL

Prior to the Holocene, the most recent major warm period was marine isotope stage 5 (MIS 5, ca. 130-75 ka.). Global ice volumes during substage 5e (ca. 130-116 ka.) were slightly less, and sea-level positions were 3-6 m higher, than those of today. However, substages 5c (ca. 105 ka.) and 5a (ca. 82 ka.) represent secondary ice volume minima and sea-level maxima, the magnitudes of which are the subject of debate. For example, theoretical and empirical estimates for sea-level elevations during substage 5a range from -19 m to something approximating present day values. These contrasting views of substage 5a are not trivial, and correspond to differences in global ice volume that exceed present-day Greenland plus the West Antarctic Ice Sheet.

Most widely cited MIS 5 sea-level records are derived from uplifting oceanic islands and associated coral reefs. Clastic shorelines along passive continental margins have, by comparison, been underappreciated. This paper presents initial results of research along the South Carolina coast, between Charleston and the Santee River, where previous workers have raised a number of questions regarding the age and significance of shoreline successions. However, few issues have been resolved due to a lack of geochronological data.

Our investigations use detailed mapping, ground-penetrating radar, cores, and optical dating to address shoreline stratigraphy and geochronology. These data show that shoreline successions can be differentiated into discrete units with prograding barrier / shoreface lithosomes from one highstand truncated seaward by a scarp and underlying ravinement surface, which is in turn overlain by lagoonal facies and/or the next younger barrier / shoreface succession. Optical dates on shoreface sands indicates these shorelines represent MIS 7, MIS 5e, and substage 5a. Most importantly, MIS 5e sea-level indicators occur at elevations of +3-5 m, whereas substage 5a sea-level indicators occur at elevations up to +2-3 m. From these data we conclude that no significant net uplift or subsidence has taken place since MIS 5e in this area. It follows that the present elevation of the substage 5a shoreline cannot be explained by uplift or subsidence, and that relative sea-level positions for substage 5a along the South Carolina coast were approximately the same as MIS 5e or the present day.