Seismic Reflection Profiling

Initial selection of proposed Site HE-4A, which became Site 1205, was based on data gathered at DSDP Site 432, where basement was reached at ~42 mbsf (Jackson, Koizumi, et al., 1980). We conducted a single-channel seismic profile, 3.5-kHz PDR, and magnetometer survey in the vicinity of Site 432 to ensure proper hole location and suitability for basement drilling. Analog seismic data (Glomar Challenger, 8 August 1977, 2230 hr Universal Time Coordinated [UTC], and S.P. Lee, 8-76-NP, 8 October 1976, 2230 hr UTC) were acquired in the vicinity of Site 432. Figure F40 shows the track line of the Leg 197 survey superimposed on ETOPO5 seafloor bathymetry with a contour interval of 100 m. Tick marks along our survey lines mark half-hour intervals. Line 1 is approximately in the same location and azimuth as the S.P. Lee 8-76-NP line.

Survey Lines 1 and 4 pass directly over Site 1205, which is 100 m southeast of DSDP Site 432. Approximately 3-km-long sections from Lines 1 and 4 are shown in Figures F41 and F42, respectively. The midpoint of the active portion of the streamer was ~218 m astern of the water gun source, which was ~4.5-6 m deep. The midpoint between the active section of the streamer and the water gun source was ~191 m astern of the ship's recorded GPS position. Firing of the water gun was set at every 4 s. Each shot record is 3 s in record length, beginning 100 ms before the water gun was triggered. This 100-ms delay, created by the trigger control panel, was removed in SIOSEIS record processing. Ship speed averaged 5.12 kt (2.77 m/s) during Line 1 and 4.57 kt (2.48 m/s) during Line 3. All three lines were processed with SIOSEIS seismic processing software (version 2001.3) (http://sioseis.ucsd.edu), using predictive deconvolution, bandpass filtering from 60-150 Hz, and Fk-migration with a velocity of 2000 m/s. Table T17 shows the SIOSEIS script processing parameters for each line.

At Site 1205, a thin sequence of sediment overlies the volcanic basement. A strongly coherent reflection at 1.81 s two-way traveltime (TWT) marks the contact between the ~40-m-thick sediment and the basement. This contact, dipping to the west, shows little relief except for a shallow trough seen on Line 4. The truncation of dipping lava flows beneath the sediment/basement contact suggests an erosional unconformity (Fig. F42). The lack of lateral coherence of the seafloor reflector is possibly due to very low velocity sediment producing a velocity gradient relative to the seawater velocity.

Two different types of basement are present in the vicinity of Site 1205, producing distinctly different seismic signatures. The upper sequence of dominantly alkalic basalt lava flow units interbedded with soil layers (see "Physical Volcanology and Igneous Petrology") produces laterally coherent, low-frequency, and west-dipping reflections beneath the sediment layer. This section was penetrated in Hole 1205A, at ~1.88 s TWT. Basement underlying this soil-lava sequence, which appears to be more massively bedded given its lack of laterally coherent internal reflectivity, rises gently upward to the southeast. The strongly reflective upper sequence of lava flows and soil interbeds continues to thin to the southeast until it is only ~0.02 s TWT thick at shotpoint 1431 on Line 1 (Fig. F41). The prominent rise in the top of the lower incoherently reflective basement at shotpoint 1351 (Fig. F41) does not appear to be fault controlled and is more likely an erosional or constructional surface. The change in acoustic character of the basement sequences might be related to soil horizons occurring far less commonly below Core 197-1205A-29R (~203 mbsf) than above this depth, which corresponds to ~1.88 s TWT.

The boundary in the two types of basement might mark the top of dominantly tholeiitic flow units, having higher eruption rates, and thus leaving less time for the formation of soil horizons and interbedded sediment necessary to produce the layered character of the upper basement section.