SITE 1045

Site 1045, located 2.7 km west of the frontal thrust, was drilled through the décollement zone of the northern Barbados accretionary prism (Figs. 2, 3, 4). This is the first penetration of a high amplitude negative polarity reflection of the Barbados décollement. This reflection might indicate high fluid pressures and perhaps physical dilation and hydrofractures. Drilling stopped 58 m below the top of the décollement zone because of hole instability but recorded a log profile through the bottom of this structure. At Site 1045, LWD acquired spectral gamma-ray, resistivity, density, caliper, photoelectric effect, and neutron porosity logs in this hole (Fig. 6). All logs are of excellent quality except neutron porosity. High quality is due to an in-gauge hole. Ninety-eight percent of the hole had differential caliper measurement of less than 1 in, below which density measurements are reliable.

Traditional visual and multivariate statistical analyses of the logs define eight log units. Because Site 1045 was never cored, we inferred lithology from log properties and seismic correlation. Carbonate-rich and clay-rich lithologies apparently dominate the accretionary prism, with a substantial clay-rich interval for about 100 m above the décollement zone. Correlation of the logs to the seismic reflection data identifies an interval of extremely low density between 425 and 438 mbsf as the décollement zone. Density and resistivity curves correlate positively, except below the décollement zone. These curves are inverted along two thrust faults that can be identified in the seismic reflection data and at several other depths in the logs where no structures are resolved seismically.

The density log places significant constraints on the hydrogeology of the accretionary prism, the décollement zone, and the underthrust sediments. A reversal of the consolidation trend in density occurs about 100 m above the top of the décollement zone. We believe that thrust imbrication of a low permeability clay-rich section has hindered consolidation. Extremely low densities down to 1.5 g/cm3 characterize the décollement zone. Sharp changes in density (~0.2 g/cm3) mark the top and bottom of the décollement zone. A comparison of the density profile through the proto décollement at Site 1044 and the décollement at Site 1045 shows that density lows at Site 1045 are higher, thinner, and more sharply defined than the broader low in the Site 1044 density data. Therefore the density distribution in the décollement can be explained by compaction of the proto décollement zone; dilation is not required. A synthetic seismogram based on the density log at Site 1045 reproduces the observed negative polarity reflection. Beneath the décollement zone, consolidation in the clay-rich upper part of the underthrust section is retarded, probably due to its low permeability and rapid loading by the overthrusting accretionary prism.

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