SITE 1044

Site 1044 lies on the northern flank of the Tiburon Rise 6 km east of the Barbados accretionary prism frontal thrust at ODP Site 672 (Figs. 2, 3, 4). Site 672 was drilled as an undeformed reference locality to gauge changes in the sedimentary section caused by accretion or underthrusting observed at other DSDP and ODP sites to the west. Surprisingly, the structural and geochemical anomalies at the projected stratigraphic level of the décollement showed that the section at Site 672 is disturbed by the encroaching accretionary prism. LWD at Site 1044 and data from Site 672 provide valuable baseline information on the nature of the incoming sedimentary section and set new limits on possible extent of proto-décollement deformation.

Site 672 provides a wealth of information for correlation to the Site 1044 LWD data. Cores at DSDP Site 543, located 18 km north on the oceanic plate, constrain the nature of the lower Eocene to Cretaceous part of the section not penetrated at Site 672. The sedimentary section is about 650 m thick beneath Site 672 and overlies oceanic crust of probable Cretaceous age. The uppermost Pleistocene to lower Miocene hemipelagic unit extends to about 200 mbsf and is equivalent to the section incorporated into the accretionary prism. This upper 200 m section correlates to seismic Unit 1, which displays moderately continuous reflectors and is underlain by the proto-décollement. The proto-décollement shows no deformation at the resolution of the seismic data; sediments below this are underthrust. The underthrust upper Oligocene to middle Eocene units consist of alternating muddy and calcareous lithologies with terrigenous sandstone and siltstone interbeds. Lower middle Eocene to Cretaceous rocks are siliceous, calcareous, and clay-rich pelagic deposits overlying pillow basalt.

The principal questions addressed at Site 1044 are: (1) What is the log signature of the proto décollement? Does this deformation zone include any indicators of hydrofractures, such as observed at Site 948? (2) What are the overall physical properties of the incoming section, especially porosity? Equivalents of this sequence are the sources of fluids being expulsed from the accretionary prism and underthrust sequence. (3) How do the observed physical properties determined from LWD account for the seismic reflection signature of this well-imaged incoming sedimentary section?

Site 1044 penetrated through 685 m of sediment to the basaltic basement of the North American Plate 6 km east of the frontal thrust of the northern Barbados accretionary prism. LWD acquired spectral gamma-ray, resistivity, density, caliper, photoelectric effect, and neutron porosity logs in this hole (Fig. 5). All logs are of good quality except for neutron porosity. The high quality log data results from the in-gauge hole immediately behind the bit that the LWD tools sense. Ninety nine percent of the hole had differential caliper measurement of less than 1 in and 94% had less than 0.5 in. The density log mimics both the character and values of density measurements made from cores, further indicating log reliability.

Both traditional visual and multivariate statistical analyses of the logs define six log units, which account for the majority of the lithologic variations observed in the cores. This profile of log properties specifies the nature of the incoming sedimentary section and correlates well with Site 672 and 3-D seismic survey data. This core-log-seismic data suite provides an unparalleled reference for analysis of accretionary prism evolution to the west.

A decrease in density, resistivity, and gamma-ray logs from 169 to 189 mbsf defines log Unit 2, which correlates well with a structurally defined proto-décollement. Both logs and incompletely recovered cores recognize this low density unit, but logs better define its discrete boundaries and explicitly correlate it to a radiolarian mudstone interval at Site 672. A stratigraphically correlative mudstone interval always characterizes the décollement zone beneath the accretionary prism. Adjacent consolidation tests suggest high fluid pressures in this low density zone that would favor deformation there. The logs show no evidence in the proto-décollement for extreme fluid pressures, which might be seen as hydrofractures, and the geochemical evidence for fluid flow is equivocal here. The synthetic seismogram based on log results shows that this low density interval would have a reflection similar to that seen beneath much of the accretionary prism. Thus, incipient structures, physical properties, stratigraphic correlation, and seismic character all confirm that log Unit 2 is the proto-décollement.

Significant spikes in caliper, bulk density, resistivity and photoelectric effect plus a decrease in gamma-ray and an increase in resistivity values define log Unit 4 from 330 to 470 mbsf. This unit correlates with cyclically bedded sediments with high concentrations of sand and carbonate at Site 672. Anomalies in pore-fluid geochemistry suggest fluid flow in this unit. Lithologic properties derived from the logs indicate sand layers that could act as fluid conduits. A synthetic seismogram generated from the logs correlated through the 3-D seismic survey shows that this unit extends beneath the accretionary prism, where the fluids could be sourced.

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