SITE 1046

Site 1046 was drilled through the décollement zone of the northern Barbados accretionary prism 1.9 km west of the frontal thrust and about 900 m east of Site 1045 (Figs. 2, 3, 4). Site 1046 is a re occupation of Site 949, a site that was partially cored and instrumented with a Circulation Obviation Retrofit Kit (CORK). The negative polarity reflection that characterizes the décollement beneath Site 1045 diminishes in strength during the lateral transition to Site 1046. At Site 1046, LWD acquired spectral gamma-ray, resistivity, density, caliper, photoelectric effect, and neutron porosity logs from the surface, through the décollement, and to the oceanic basement at 832 mbsf (Fig. 7). All logs are of excellent quality except neutron porosity.

Traditional visual and multivariate statistical analyses define eight log units. These log units subdivide into two sedimentary packages corresponding to the sections above and below the décollement zone. Sediments above the décollement are carbonate- and clay-rich calcareous claystone and noncalcareous claystone. Log Units 1-3a are dominated by the calcareous claystone. Falling resistivity in log Units 3a and 3b suggest a transition to a more clay-rich lithology above the décollement zone. A low-density interval in log Unit 4 and 5a correlates with the décollement zone. High gamma-ray and potassium values indicate significant terrigenous input in log Units 5b through 6b. The spiky resistivity response in log Unit 6 indicates cyclical sedimentation, probably turbidite deposition. The low resistivity and low gamma-ray values in log Unit 7 suggest pelagic sediment accumulation. Higher gamma-ray and resistivity values in log Unit 8 suggest correlation to the calcareous ferruginous unit cored at the base of Site 543.

Deformational features from cores and seismic data correlate well with log-inferred structural features. Inversions in the gamma-ray and density curves indicate thrust faults at 162, 225, 280, and 350 mbsf. Results from adjacent Site 949 confirm thrusting at 260, ~280, and 350 mbsf, but not at 162 mbsf because of discontinuous coring at that depth. Seismic data indicate a thrust fault at 282 mbsf that apparently correlates with the log-inferred thrust at 280 mbsf. The structurally defined décollement zone from the cores lies between 370 and 437 mbsf. A sharp drop in the density curve suggests that the top of the décollement zone is in log Unit 4 at 380 mbsf. The lower contact is indistinct but probably below 437 mbsf in a gradient of increasing density. Broad geochemical anomalies and a lack of thermal anomalies around the faults and décollement at Site 949 indicate fluid flow is not currently active along these structures. The synthetic seismogram reproduces the reflection at the thrust fault at ~280 m, but only weakly identifies the décollement.

The density log agrees well with all available core densities at Site 949, except those from the carbonate-rich zone at 300-322 mbsf. Comparisons of mean densities through the underthrust turbidite sequence at Site 1046 and an equivalent sequence at the reference Site 1044 suggest selective consolidation of this interval, assuming equivalent starting densities. Apparently fluids are being drained seaward through the high permeability turbidites. This hydrogeologic phenomenon also explains the high thermal gradient observed here at Site 949 and the reference Site 672.

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