THE 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 defined a complex evolutionary history of this barrier island system, including a period of barrier island collapse followed by barrier island transgression and extensive inlet formation. The focus of this project is to define the age and regional extent of collapse, transgressive and inlet fill facies. Greater than 100 km of high-resolution ground penetrating radar (GPR) data have been acquired between Oregon and Ocracoke inlets along the Outer Banks. GPR data are being correlated to sediments collected in 33 vibracores to provide the regional shallow (<8 m) stratigraphic framework. Analysis and description of sediments for grain-size and fossil assemblages (macro-fossils and foraminifera) will define specific facies and determine depositional environment. Samples are being dated using AMS 14C analyses (for organic and carbonate material), and optically-stimulated luminenscence (OSL) analyses (for quartz sands).

Geophysical data reveal multiple, previously undocumented relict inlet channels. Inlet-fill sediments constitute in excess of 60% of the shallow (<8 m below ground surface) geologic framework in this region. Paleochannels here vary in size and depth from those comparable to the recent 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 may have been active for centuries. OSL ages of these relict inlets are pending. GPR facies are being defined using shore-parallel GPR transects and 3-D surveys of selected paleochannels. Six 3-D GPR surveys have been acquired, including a 120 m x 83 m grid revealing 3 different sequences of fill within one inlet complex, characterized by prominent clinoform packages bounded by erosional surfaces, and indicating variable sediment transport directions occurring from the NE and NW. These data reveal shoal development and migration late in the closing of this inlet. Relict inlets are incised into adjacent facies which define the evolution of this system from open-marine (dated from ~3000 to approximately 1250 calibrated years B.P.), to estuarine, to transgressive back-barrier (overwash and marsh).