Steven K. Boss

Assistant Professor

Department of Geosciences

        A preliminary survey to assess the availability and location of sand resources offshore of the Outer Banks was commissioned by the Outer Banks Task Force, beginning in 1994. The primary intent of this survey was to collect reconnaissance data (single-channel, high-resolution seismic reflection and side-scan sonar profiles) to be used to develop baseline information regarding the availability of sand for beach nourishment and the shallow (<100 m depth) stratigraphy and sea-floor characteristics of the continental shelf waters within the jurisdiction of the State of North Carolina (3 nautical miles (nm)). The resulting survey obtained 1,164 km (628 nm) of profile data between 0.5 and 3 nm from shore. In 1995, geophysical data were supplemented with vibracores collected aboard the United States Army Vessel D/B Snell. One hundred twenty-one of these cores were analyzed and presented as part of the Task Force sand resource assessment.

        During 1999, geophysical and core data were analyzed and synthesized by the North Carolina Geological Survey working with Dr. Stephen K. Boss of the University of Arkansas to generate a working database documenting the general stratigraphy and sand resource potential (including sand volume estimates) offshore of four areas of critically eroding shoreline along the Outer Banks (Pea Island, Buxton, Diamond Shoals, and Frisco-Ocracoke). Each of these individual study areas (see map) was covered by an interim report and the overall project results were delivered to the Task Force in a final comprehensive report. These reports may be found in the Technical Information section of the OBTF website (www.OBTF.org).

The Database

        The interactive database consists of maps, images of the vibracores, images of grain size histograms, and tables of data. These files are hyperlinked to allow the user to navigate through the dataset and view the various data items. A typical path would be to locate a core of interest on a map, click its location to see the core image, click within a selected subsample interval to see the grain size histogram and then either go back to the core or to the table of subsample analytical data for that core. The dataset generally follows the structure of the four project study areas, however the hyperlinks permit easy "jumping" to another study area.

        North Carolina Geological Survey Open-File Report 2000-01 on CD-ROM contains the interactive dataset along with versions of the data files in native format (eg. Microsoft Excel spreadsheets) as well as in text format. For information on ordering, visit the NCGS website (http://www.geology.enr.state.nc.us/) and look under Publications.

      

Deploying the vibracoring rig ==>

       Resistant layers such as shells or well-packed sediment may impede penetration. Vibracores from this study ranged in length from 0.62 to 6.14 m and averaged 3.73 m. Core may also be lost. Despite a mechanism designed to prevent it, the sample sometimes falls out of the tube while the rig is being lifted from the seafloor.

       The vibracores provide physical samples of the geologic units exposed within the NC-12 study area. These are examined and described and related to the subseabottom information developed from the project's seismic data set and well as the seafloor texture data represented by the side-scan sonar data set.

      The picture on the left shows the core liner being pulled out of the steel tube onboard the coring vessel (D/B Snell). About two meters of sediment was recovered on this core. Note the clear, water-filled upper part of the tube. The supply of empty liners is visible in the bottom of the image. Onboard ship, the cores are marked, cut and capped. After offloading at the dock, cores are cut to 1-meter lengths and transported to the location where they get opened and processed.

Core Processing

       The NC-12 vibracores were opened, described, and sampled at the North Carolina Geological Survey Coastal Plain Office in Raleigh.  Here, the approximately 1-meter long segments were cut lengthwise.  One half of the core was sealed in polyethylene tubing and remains archived at this facility for reference or future research.  The other half was sampled at regular intervals and at zones of geological significance.  These "subsamples" were then taken to the DOT Soils Lab where they were washed, dried, and processed through stacked sieves of various screen sizes to determine the grain size distribution of the subsamples.  The subsampled intervals are shown on the core images of this interactive database.  The analytical results are represented in the grain size histograms that are linked to the intervals on the core images.  Tables provide grain size statistics for each subsample and for the entire core.

       Each histogram shows the weight percent of the sample within a given grain size class--for example, such as fine sand or gravel.  The subsample results can then be compiled  mathematically to provide textural characterization of entire cores or geological units.

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