LITHOLOGIC, GEOPHYSICAL, AND PALEOENVIRONMENTAL FRAMEWORK OF RELICT INLET CHANNEL-FILL AND ADJACENT FACIES: NORTH CAROLINA OUTER BANKS

The geophysical, sedimentological and chronostratigraphic framework of the Outer Banks barrier island system (North Carolina, USA) is being defined. Previous researchers have reconstructed a complex evolutionary history of this barrier island system, including periods of barrier island partial collapse followed by barrier island transgression and extensive inlet formation. The focus of the present project is to define the ages and regional extent of the collapse and subsequent transgressive and channel-fill facies. Over 100 km of high-resolution ground penetrating radar (GPR) data were acquired between Kitty Hawk and Ocracoke Inlet. GPR facies were characterized using shore-parallel GPR transects and 3-D surveys at selected locations. GPR data were correlated to sediments collected in 36 vibracores to provide the regional shallow (<8 m) stratigraphic framework. Lithologic and microfossil data have been used to define specific facies and depositional environments. Organic and carbonate material have been dated using AMS 14C analyses and quartz sand units were dated using optically-stimulated luminescence (OSL) analysis.

GPR data reveal multiple, previously undocumented relict inlet channels constituting ~60% of the shallow (<8 m below ground surface) geologic framework. The data reveal different sequences of fill within inlet complexes. Channel-fill sands are characterized by prominent clinoform packages, sometimes bounded by erosional surfaces, indicating variable sediment transport directions from the NE and NW over the majority of each inlet complex and from the SE and SW near the southernmost edge. These data reveal shoal development and migration late in the closing of the inlets. Paleochannels vary in size and depth from those comparable to the recent (2003) Isabel Inlet (ca. 8 m deep and 1.5 km in width) to massive migrating inlet complexes that span distances of 2-5 km and that may have been active for centuries. OSL dates on channel-fill sands indicate a period of inlet-fill activity from ~0.5 to 0.3 ka (2-sigma range). Use of OSL on channel-fill sand appears to produce accurate ages, but does not provide the resolution needed to determine if all inlets were active simultaneously.