SITE 1048

Site 1048 was drilled 2.1 km east of the deformation front into the sedimentary section coming into the northern Barbados accretionary prism and is 3.8 km west-southwest of co-located Sites 1044 and 672 (Figs 2, 3, 4). At Site 1048, LWD acquired spectral gamma-ray, resistivity, density, caliper, photoelectric effect, and neutron porosity logs in a tectonically undisturbed oceanic section from the seafloor, through the proto-décollement, to a total depth of 324 mbsf (Fig. 9). All logs are of excellent quality except neutron porosity.

Traditional visual and multivariate statistical analyses define four log units whose lithology can be interpreted by correlation to Site 672. Log Unit 1 (0-105 mbsf) is characterized by increasing density, resistivity, and photoelectric effect with depth, and a normal compaction trend. These log signatures are consistent with a carbonate-rich lithologic unit that occurs from 0 to 123 mbsf at Site 672. Log Unit 2 (105-187 mbsf) shows a general decrease in resistivity, density, photoelectric effect, and gamma-ray values. Comparisons to Site 672 suggest that these log responses probably signify a downhole transition to an increasingly underconsolidated clay-rich unit. An interval of very low density, low resistivity, low photoelectric effect, and low gamma-ray values defines log Unit 3 (187-207 mbsf). This interval probably correlates to a lower Miocene radiolarian mudstone interpreted as the proto-décollement at Site 672. Log Unit 4 (207-324 mbsf) is characterized by increases in the density, gamma-ray, and photoelectric effect values. The highly variable (spikey) nature of the curves suggests interbedding of contrasting lithologies, and correlates with the muddy turbiditic unit beneath the décollement at Site 672.

Comparisons of the logs from the more seaward reference Site 1044 to Site 1048 show good correlation but a slightly thicker section at 1048. A synthetic seismogram generated from the density log reproduces well the shape and amplitude of the negative polarity reflection from the proto-décollement. The location of the proto-décollement occurs slightly higher in the upper reflective seismic unit than at Site 1044; thus, the basal surface of this seismic stratigraphic unit appears to be time transgressive. The most important result of Site 1048 is confirmation of the extremely low-density nature of the proto-décollement and the associated implications for localization of the detachment as the accretionary prism forms to the west.

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