Site 1047
(Leg 110, Site 676)

Site 1047 was drilled 1 km west of the deformation front through the décollement zone and more than 300 m into the underthrust section of the northern Barbados accretionary prism (Figs. 2, 3, 4). Site 1047 is a re-occupation of Site 676, which was cored to 310 m and documented processes associated with initiation of sediment offscraping. At Site 1047, LWD tools acquired spectral gamma-ray, resistivity, density, caliper, photoelectric effect, and neutron porosity logs from the seafloor through the décollement and to ~300 m below it into the underthrust section (Fig. 8). All logs are of excellent quality except neutron porosity.

Traditional visual and multivariate statistical analyses define six log units (Fig. 8). Log Unit 1 (0 158 mbsf) is characterized by increasing density with depth, a normal compaction trend, high resistivity, and high photoelectric effect. These log signatures are consistent with a carbonate-rich lithologic unit that occurs over the same depth interval at Site 676. Log Units 2, 3a, and 3b (158 276 mbsf) show a general decrease in resistivity, density, photoelectric effect, and gamma-ray values. These log responses reflect a downhole transition to an increasingly underconsolidated clay-rich unit above the décollement zone. An interval of very low density, low resistivity, and low gamma-ray values defines log Unit 3c (276-300 mbsf). This interval correlates to a lower Miocene radiolarian mudstone that includes mud-filled veins, a thrust fault, and a methane anomaly. This interval was interpreted as the incipient décollement zone at Site 676. An increase in differential caliper here and increased torque on the drill bit suggest that the muddy section was extruding into the drill hole, perhaps because of overpressuring. Log Units 4 (300-493 mbsf) and 5 (493-562 mbsf) exhibit a highly variable photoelectric effect and gamma-ray response, suggesting interbedded lithologies. Resistivity data in Unit 5 indicate sand interbeds that are probably correlative to the turbidite sequence cored at Sites 671 and 672. Log Unit 6 (562-619 mbsf) is characterized by decreasing photoelectric effect and density, and increasing and variable gamma ray values. Log Unit 6 correlates with the early Eocene-aged noncalcareous claystones and siliceous claystones recovered at Sites 672 and 543.

Comparisons of the density curves from the reference Site 1044 and Site 1047 show selective consolidation in the décollement zone (log Unit 3) and the underthrust sandy turbidite section (log Unit 5). Presumably both intervals of localized consolidation are due to dewatering in response to localized deformation and loading due to underthrusting. A synthetic seismogram generated from the density log reproduces the negative polarity reflection from the décollement and the strong reflections from the underthrust turbidite sequence, but poorly matches reflections in the accretionary prism.

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