NEW CROSS SECTIONS DETAILING THE SUBSURFACE STRATIGRAPHY OF EARLY, MIDDLE, AND LATE PLEISTOCENE TERRACE DEPOSITS ON THE NORTH CAROLINA COASTAL PLAIN, FALKLAND 7.5-MINUTE QUADRANGLE

Abstract

Traditional geologic mapping methods (boots on the ground) are insufficient to characterize the geology and stratigraphy of the Atlantic Coastal Plain of eastern North Carolina. Due to low relief, sparse outcrops, recurring facies, and extensive wetlands cover, a non-traditional mapping method is needed. Three-dimensional (3D) subsurface mapping combines geomorphic landscape analysis with focused subsurface analysis that includes cores collected along specific profiles transecting landforms. Data are collected from a four-quadrangle study area that includes the Early Pleistocene Surry Paleoshoreline of eastern North Carolina and the incised Tar River valley. Mapping occurs at 1:24,000 and 1:8,000 scale in quarter-quad sections. The most recent data represent the northeast (NE) and northwest (NW) quadrants of Falkland quadrangle. New cross sections show a complex subsurface stratigraphy. A N-S-trending section in the NW quadrant shows the Quaternary stratigraphy of the ~27 m terrace. Here, the Cretaceous(?) basement consists of hard, brown, fine-grained micaceous muddy sand (Cape Fear?). Above its sequence boundary, the Quaternary includes a succession of facies. A basal, very coarse-grained olive gray muddy, sandy gravel (mixed siliciclastic and bioclastic) with mollusk hash and quartz and phosphate pebbles is overlain by a unit with alternating layers of olive gray, fine-grained muddy sand and sandy mud. Above this is very coarse feldspathic sandy gravel that locally includes finer intervals with heavy mineral-rich sands. Surficial facies consist of bioturbated, slightly gravelly muddy sand. Cross sections in the NE quarter-quad crosscut two sets of terraces in the incised valley of the Tar River: the 11-12 m and 6-8 m terraces are interpreted respectively as Middle (?) and Late (?) Pleistocene in age. Stratigraphy beneath both sets of fluvial terraces is similar. A Cretaceous (?) basement of olive gray sandy mud, hardpacked micaceous muddy sand, or “buckshot” clay is overlain by thin (2-4 m) Pleistocene, upward-fining sequences that consist of: 1) a basal lag with quartz, feldspar and peat/lignitic gravels; 2) pale orange, coarse-grained feldspathic gravelly sands, with heavy minerals concentrated in finer-grained intervals; and 3) a surficial yellow/orange slightly gravelly muddy sand.