VALLEY FILL FACIES AND CHARACTERISTIC GAMMA-RAY LOG RESPONSE PATTERNS, LATE PLEISTOCENE TO HOLOCENE TRANSGRESSION, OUTER BANKS, NORTH CAROLINA, USA

A suite of nine rotasonic drill cores from the North Carolina Outer Banks records the Late Pleistocene (<14 cal. ka) to Holocene facies succession that infilled the paleo-Roanoke River valley, and its transition into the overlying modern barrier island complex. These data were collected as part of a cooperative research program including NCGS, USGS, ECU, University of Delaware and the Virginia Institute of Marine Science. Facies divisions and stratigraphic interpretations are based on integrated litho- and bio- stratigraphic analyses (foraminifera, diatoms), gamma logs, 14C and amino acid racemization (AAR) ages, and nearby seismic reflection profiles from Albemarle Sound and the adjacent inner continental shelf.

The paleo-Roanoke valley trends east-west and is up to 12 km wide and 36 m deep. Its cross section is asymmetrical, with the steeper side to the north. The base of the valley unconformably overlies a muddy open shelf deposit with an AAR age of 900-1200 ky. Above this, the valley fill includes a succession of four distinct facies (Units 1-4).

Unit 1 (0-6 m thick) is a basal gravelly sand that is barren of microfossils and fluvial in origin. Its gamma response is blocky and 5-20 cps. Unit 2, a complexly interstratified mud, sand, and intraclast gravel (<8 m thick), is problematic, with marine, marginal marine and fluvial-lacustrine indicators. Six 14C ages (10-14 cal. ka) indicate that Unit 2 includes the Younger Dryas chronozone. Foraminifera assemblages are inconclusive, but diatom data suggest that salinity increases upsection. The lower and upper boundaries of Unit 2 are flooding surfaces that are marked by abrupt shifts in lithology and gamma response. It has a sawtooth to spiky pattern (20–60 cps).

Units 3 and 4 contain open shelf foraminifera assemblages and are Holocene in age. Unit 3 (as thick as 12 m) is a gradational, shoaling (coarsening) upward sequence of burrowed mud and sand that infilled the paleo-Roanoke embayment. As grain size increases upsection, mud content and gamma response (i.e., cps) decrease (funnel-shaped log response). Unit 4, the Colington Shoals complex, is a 13 m thick interval of graded gravelly sands with a blocky gamma response (3-15 cps).

The valley fill and adjacent interfluves are overlain by Unit 5, a complex of barrier island sands as thick as 10 m, with a gamma response of 5-20 cps.