37. CORRELATION OF WELL LOGS, PHYSICAL PROPERTIES, AND SURFACE SEISMIC REFLECTION DATA, MIDDLE VALLEY, JUAN DE FUCA RIDGE1

Synthetic seismograms calculated from well logs and shipboard physical properties measurements from Holes 858F and 857C have been correlated with surface seismic reflection records. Hole 858F was drilled in an active vent site and Hole 857C was drilled into nearby sediments with high heat flow. Physical properties measurements in unlogged portions of the holes were corrected for temperature, pressure, and anisotropy and merged with the log data; this allowed synthetic seismograms to be calculated from the seafloor to the bottoms of the holes. Log velocities in Hole 858F between 129 and 250 mbsf are 150-250 m/s higher than velocities in Hole 857C, which are in turn 200 m/s higher than those expected for abyssal turbidites. These increases occur despite probable velocity reductions associated with high in-situ temperatures. Site 858 was not logged for density, but physical properties measurements indicate that densities are as much as 0.5 g/cm3 larger than normal abyssal turbidites. Site 857 has logged densities 0.2 g/cm3 higher than normal abyssal turbidites, indicating that it too has been altered, but not as much as Site 858. A strong increase in density and/or velocity occurs near the seafloor at Site 858; this increase occurs over a short distance relative to the wavelength of the seismic reflection data and reflects a large amplitude signal 30 ms beneath the seafloor. This event can be tentatively correlated to the change at 28 mbsf from lithologic Subunit HA to IID which is more altered. Low core recovery and no logs from this interval render the exact configuration and nature of this transition unknown. The reflection has only been observed at Site 858 which indicates that the rapid increase in alteration with depth at Site 858 is a local phenomenon. At Hole 857C the transition from lithologic Subunit IIA to IIB is a gradual increase in degree of alteration that does not result in a strong reflection. At both sites the transition from sediments to interlayered basalt and sediments is marked by high-amplitude reflections. Modeling of the interlayered basalts and sediments at Hole 857D shows that these closely spaced, high-contrast layers result in an interference pattern when imaged by a low-frequency seismic source. Reflections are high in amplitude, but do not correlate to individual sill units.