PLIOCENE AND EARLY PLEISTOCENE STRATIGRAPHIC UNITS IN THE CLARKE CORE OF VIRGINIA: TO SUPPORT INTERSTATE CORRELATION OF FALL ZONE PLACER DEPOSITS

The North Carolina Geological Survey (NCGS), the Virginia Dept. of Energy and federal scientists are collaborating to evaluate existing stratigraphic nomenclature and upgrade descriptions for surficial Pliocene and Pleistocene deposits that include heavy mineral placers in the Fall Zone of Virginia (VA) and North Carolina (NC). This effort is funded through USGS STATEMAP and EARTH MRI programs. NCGS drilled new core at 127 sites in Drake and Red Oak 7.5-minute quadrangles in NC. In VA, mappers use traditional methods and an auger rig to map and define units. With STATEMAP funding for interstate correlation activities, NCGS drilled 367 ft of new core in VA in Dinwiddie and Cherry Hill 7.5-minute quadrangles at 13 sites along-strike.

To compare stratigraphy, core descriptions were standardized in graphic log format. Landscape positions were defined, and the logs with thick Coastal Plain units were chosen. In NC, the Daughtridge cores penetrated the Coharie terrace (150-246 ft; 45-75 m) between the Kenly and Wilson Mills paleoshorelines. In VA, the Clarke core cluster was situated on the Richmond Plain (205-240 ft; 62-73 m) between the Thornburg and Broadrock scarps in the Old Hickory heavy mineral deposit. All cores consist of 40 ft (12 m) of Coastal Plain section that overlies bedrock saprolite and occur at elevations of 200-217 ft (61-66 m). In VA, Clarke-03 was evaluated with a multi-sensor core logger system and sampled for cosmogenic nuclides. The units targeted may be the Cold Harbor (informal) and Bacons Castle Formations.

On display here is the Clarke core from VA which includes three sedimentary facies. The basal unit (7 ft: 2 m) is a med-coarse gravelly sand that coarsens upward into sandy gravel. Unit 2 (14 ft: 4 m) has a basal reworked gravel (2 ft: 0.6 m) that is sharply overlain by heavy mineral sands punctuated by quartz pebble layers; the unit may coarsen upward. Unit 3 (20 ft: 6 m) includes sands and gravels that become muddier upsection. These units are coarser-grained and more gravelly than the stratigraphy of the NC Daughtridge core, and represent a succession of more proximal shoreline environments (shoreface to foreshore, overlain by supratidal) than those in the NC.