SEDIMENTARY FACIES AND BOUNDING SURFACES ASSOCIATED WITH THE EARLY PLEISTOCENE SURRY PALEOSHORELINE COMPLEX, COASTAL PLAIN, NORTH CAROLINA
Three-dimensional (3D) subsurface mapping techniques were used to characterize facies, establish a sequence stratigraphic framework, and define surficial landforms for this Early Pleistocene relict landscape. The method combines comprehensive landscape analysis with targeted subsurface coring along key transects that crosscut landforms, i.e., the Surry Scarp. To interpret landform elements, position cross sections, and define depositional systems, high-resolution LiDAR (2006 bare earth – vertically accurate to 25 cm per 1m X 1m tile) data was acquired (www.ncfloodmap.com), and processed to produce hillshade, slope and contours (0.25 and 0.5 m). This was used with orthoimagery. Geoprobe cores were collected along key transects using the continuous, discrete sampling method. Maximum penetration depth was 72 ft; recovery was >80%. Graphic logs were prepared to display and interpret stratigraphic data using a standardized method of textural classification that is independent from composition.
A NW-SE cross section that includes quarry outcrops and cores shows bounding surfaces, and a series of fossiliferous units, heterolithic strata (HS) with detrital plant debris, and feldspathic siliciclastic facies. An unconformity occurs at the base of the Plio-Pleistocene section. This is overlain by marine facies that infill paleotopographic lows. The infill includes mixed (bioclastic + siliciclastic) coarse debris (fossils, quartz and phosphate) in the quarry, or an upward coarsening sandy siliciclastic parasequence topped by a clastic shell bed, to the east. A second widespread surface truncates the unit in the topographic lows. The overlying transgressive systems tract (TST) consists of a basal bed of reworked clasts that locally exhibits carbonate dissolution and concentrated phosphate. Above this, the high-stand systems tract (HST) coarsens upward from marine mud, into heterolithic strata (HS), and ultimately shoreface sands. Identification of additional bounding surfaces is pending.