PHYSICAL PROPERTIES OF THE ICDP-USGS EYREVILLE CORE, CHESAPEAKE BAY IMPACT STRUCTURE

During 2005 the International Continental Scientific Drilling Program and the U.S. Geological Survey (USGS) cored the 35.8 million-year-old Chesapeake Bay impact structure (CBIS) near the southern end of the Delmarva Peninsula. The Eyreville A and B hole was cored from 125 to 1,766 m depth with about 90 percent recovery. Borehole logging equipment problems, and a collapsed coastal plain core-hole below 1,099 m, prevented all but temperature and natural-gamma data collection within the drill rods. The lack of physical property information prompted us to select sections of the CBIS core for scanning with the USGS Geotek multi-sensor core logger (MSCL) in Menlo Park, California. Data from the MSCL included density, P-wave velocity, magnetic susceptibility, resistivity, and fractional porosity. Cores were sampled at one-centimeter intervals using core photos to allow visual comparison of data and core. The Exmore resurge sediments (444 to 600 m) have P-wave velocities that average 1,538 m/s, density 1.99 g/cm³, magnetic susceptibility 5.6 (cgs), fractional porosity 0.45, and resistivity 5.7 ohm-m. Slumped Cretaceous sediments (~600 to 1,096 m) have an average p-wave velocity of 1,704 m/s, density 2.22 g/cm³, magnetic susceptibility 2.4, fractional porosity 0.31, and resistivity 10.4 ohm-m. Granite blocks (1,096 to 1,371 m) have an average P-wave velocity of 3,970 m/s, density 2.51 g/cm³, magnetic susceptibility 372.4, fractional porosity 0.15, and resistivity 18.6 ohm-m. Sand and lithic blocks beneath the granite (1,371 to 1,393 m) have an average P-wave velocity of 2,959 m/s, density 2.40 g/cm³, magnetic susceptibility 8.9, fractional porosity 0.26, and resistivity 412 ohm-m. Suevite and lithic breccia (1,393 to ~1,550 m) have an average P-wave velocity of 5,208 m/s, density 2.22 g/cm³, magnetic susceptibility 27.8, fractional porosity 0.32, and resistivity 374 ohm-m. Schist and pegmatite near the bottom of the corehole (1,393 to 1,756 m) have an average P-wave velocity of 4,722 m/s, density 2.50 g/cm³, magnetic susceptibility 21.2, fractional porosity 0.16, and resistivity 244 ohm-m. In detail CBIS physical property variations are complex. Density vs P-wave velocity plots within the granite, for example, show data clusters that suggest that at least five different granite types are present in the core.