164 Preliminary Report


Figure 1. Location map of Leg 164 sites on the southeastern North American continental margin (modified after Dillon and Paull, 1983). Contours are in meters. Shaded region along the Blake Ridge and Carolina Rise indicates the areas where gas hydrate occurrence has been inferred on the basis of BSRs.

Figure 2. Bathymetry (3.5 kHz precision depth recorder) over the Cape Fear Diapir showing the location of Sites 991, 992, and 993.

Figure 3. Composite stratigraphic sections for Sites 991, 992, and 993 showing core recovery in all holes, a simplified summary of lithology, depth of lithologic-unit boundaries, and age.

Figure 4. Interstitial-water chloride profiles at the (A) Cape Fear Diapir (Sites 991, 992, and 993) and (B) Blake Ridge Diapir (Site 996).

Figure 5. Seismic reflection profile (CH-06-92 Line 31) for the transect of Blake Ridge Sites 994, 995, and 997. The profile crosses perpendicular to the topography of the Blake Ridge. Vertical scale is in seconds of two-way traveltime.

Figure 6. Composite stratigraphic section for Sites 994, 995, and 997 showing simplified lithostratigraphy, depth of lithologic-unit boundaries, biostratigraphic ages, and total carbonate contents (wt% CaCO3) of the sediments.

Figure 7. Age-depth relationships of biostratigraphic markers of calcareous nannofossils at Site 994 (dashed line), Site 995 (thin solid line), and Site 997 (thick solid line). Vertical error bars show sample interval uncertainties, and the box indicates combinations of sample interval uncertainty and age uncertainty. The horizontal bar at the bottom of the figure indicates the range of Amaurolithus amplificus.

Figure 8. Interstitial-water chloride concentration profiles for Sites 994, 995, and 997.

Figure 9. Preliminary results of in situ temperature measurements at Sites 994, 995, and 997. Triangles indicate measurements made with the Adara tool, squares are WSTP measurements, and circles denote data collected with a prototype probe designed by E. Davis and H. Villinger. The open squares correspond to anomalous temperature measurements obtained within the gas hydrate stability field at Site 994. Errors in temperature are estimated subjectively, based on the quality of the equilibration record and the magnitude of the correction applied to the raw data. Errors in the depths of the measurements have not yet been taken into account. The reported gradients were calculated from linear regressions weighted proportional to the inverse of the squared error. None of the thermal gradients are in equilibrium with the temperatures recorded at the mudline, and heat flow estimated using these gradients and laboratory thermal-conductivity measurements is more than 20% lower than determined during conventional surveys at the same Blake Ridge locations in 1992.

Figure 10. Acoustic velocity log for Sites 994, 995, and 997. Data are shown for both near (DTLN) and far (DTLF) measurements from the long spacing sonic tool (LSS).

Figure 11. Resistivity log for Sites 994, 995, and 997 showing deep-reading electrical resistivity data from the phasor dual induction tool (DIT).

Figure 12. Velocity profiles based on the results of the zero-offset VSP at Sites 994, 995, and 997. From the flank of the Blake Ridge to the crest, the profiles show similar, possibly slightly increasing, velocities above the hydrate stability zone (~450 mbsf) and a significant decrease in velocity toward the crest below this zone. These preliminary velocity-depth functions were produced by inverting the air gun first-arrival times using a weighted, damped, least-squares inversion that weights mean traveltimes by the inverse of their standard error. The velocity-depth curves were produced by assigning equal weight to fitting the traveltime data and to producing a smooth velocity-depth function.

Figure 13. Single-channel seismic line acquired over the Blake Ridge Diapir showing the location of Site 996. Data have undergone constant velocity migration (1500 m/s) and a mild three-trace mix. Note the highly reflective sediments in the center of the figure bounded above by a reversed polarity reflector interpreted as the base of hydrate stability. Vertical scale is in seconds of two-way traveltime.

Figure 14. Bathymetry (3.5 kHz precision depth recorder) over the Blake Ridge Diapir showing the location of Site 996. Detailed summary lithologic columns for Site 996 reflect the position of holes drilled into a small depression on the crest of the diapir. H = gas hydrate occurrences and C = carbonate nodules. Subunits are indicated with arrows.

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