Site 1108

Hole 1108A (jet-in test only):
9°44.708´S, 151°37.514´E; 3162.7 mbsl
0–16.3 mbsf drilled without coring
Hole 1108B (RCB):
9°44.724´S, 151°37.543´E; 3177.2 mbsl
0–494.90 mbsf cored; 148.58 m recovered (30%)

Site 1108 is located in the seismically active region of incipient continental separation 1 nmi ahead of the neovolcanic zone of the Woodlark Basin spreading center, Papua New Guinea. Here, a continental fault block (Moresby Seamount: summit 120 mbsl) forms the footwall to a low-angle normal fault imaged to 9 km that dips 25%–30% beneath a 3.2-km-deep, asymmetric rift basin with more than 2 km of sediment fill. At Site 1108 we sought to drill through ~900 m of the rift basin sediments, the low-angle normal fault zone, and into the footwall metamorphics. The primary objectives at this site were to (1) characterize the composition and in situ properties (stress, permeability, temperature, pressure, physical properties, and fluid pressure) of the active low-angle normal fault zone to understand how such faults slip and (2) determine the vertical motion history of the hanging wall and the footwall as local ground truth for models of the timing and amount of continental extension prior to spreading initiation. Hole 1108A was a jet-in test in anticipation of reentry operations. Hole 1108B was rotary cored to 485 mbsf, with ~60 m of open-hole logging using the triple combo and temperature tools, before unstable hole conditions terminated operations. The site was not deepened because of pollution prevention and safety concerns, and hence the primary objectives were not met.

The first core contained late Pleistocene nannofossil-bearing hemipelagic sediment: calcareous clay with minor volcaniclastic silt and sand. Talus from Moresby Seamount was recovered as isolated clasts from 14.5 to 62.7 mbsf, including dark siliciclastic sandstone and siltstone, volcanic breccia, microgranite, granodiorite, epidosite, greenschist mylonite, and biotite gneiss. Some glassy basalt fragments were incorporated from submarine eruptions. Trace amounts of adhering sediments reveal biostratigraphic ages >1.25 Ma by 24.1 mbsf. Quartzo-feldspathic-lithic sand from 62.7 to 63.4 mbsf may relate to and herald the overlying talus. Gas observed bubbling out of the top of the core barrel for this interval may reflect penetration of a gas hydrate layer.

Terrigenous turbidites, now lithified to sandstones, siltstones, claystones, and minor conglomerates, constitute the remainder of the section 72.3–485.2 mbsf. Ages increase from 1.67 to 1.75 Ma at 82.8 mbsf to less than 3.35 Ma at the base. Sedimentation rates increase downsection, from 325 m/m.y. at 1.7–2.0 Ma to 425 m/m.y. at 3.0–3.2 Ma. Benthic foraminifers indicate deposition in deep water (lower bathyal: >2000 m), except for middle (500–2000 m) to lower bathyal conditions below 410 mbsf.

The majority of the turbidites comprise interbedded sandstones, siltstones, and claystones in which medium- to coarse-grained sediments dominate. The sandstones above ~330 mbsf contain a high proportion of metamorphic-derived lithoclasts, related mineral grains, and altered igneous-rock-derived grains (volcanic and ophiolitic), whereas those below contain large amounts of material derived from basic and acidic volcanic and minor plutonic rocks. Planktonic foraminifers and bioclasts of shallow-water origin are common to both sections.

A subunit from 139.4 to 202.2 mbsf comprises foraminifer-bearing clayey siltstone and silty claystone with occasional fine-grained sandstone. Bioturbation is common. Minor disseminated pyrite is suggestive of relatively low-oxygen bottom conditions at times. Subunits of thin conglomerate were recovered near 313, 380, and 437 mbsf. The clayey siltstone subunit shows abundant evidence of brittle deformation characterized by bedding dips up to 35%, low- angle shearing, brecciation, and ubiquitous slickensides. The faults dip at moderate angles (~45%) and most of the structures indicate normal senses of displacement in an extensional fault zone. The greatest frequency of fractures and faults is concentrated between 158 and 173 mbsf. Within this interval there is an age offset from 2.0 to 2.58 Ma between 159.6–164.8 and 172 mbsf. Based on the sedimentation rates above and below, ~200 m of section appears to be cut out by this normal fault near 165 mbsf.

Within the more competent lithologies below, the intensity of tectonic deformation falls off markedly and bedding is subhorizontal. However, below 350 mbsf the turbidites become finer grained (more claystones, siltstones, and fine sandstones), and the section is once more deformed with scaly fabrics, fractures, and evidence of shear along fault planes. Tectonic deformation appears to be concentrated in the finer grained units.

Lab-measured porosities show an expected exponential decay with depth below ~160 mbsf, but the misfit of these values when extrapolated to the surface with the measured surface porosities indicates that about 400 m of sediments has been removed. Half this amount may be associated with throw on the fault near 165 mbsf; the other half, with erosion between the unconsolidated sands at 63 mbsf and the consolidated sandstones at 72 mbsf.

Temperature measurements suggest an average thermal gradient of 100°C/km to 390 mbsf. Alternatively, the same data may be explained by advection of fluids along the ~165-mbsf fault, and/or by a thermal gradient of 94°C/km above 160 mbsf and 65°C/km below 200 mbsf, with a 10°C offset formed in the last thousand years. Thermal conductivities in the upper several meters are 0.8–0.9 W/(m•K) and are 1.0–1.7 W/(m•K) below 130 mbsf.

Three processes appear to control the pore-water geochemistry. Bacterially mediated oxidation of organic matter depletes sulfate 75% by 83 mbsf (and totally below 172 mbsf), the depth where methane concentrations become elevated and there is a salinity minimum. The downhole decrease in K+ and Mg2+ and the increase in salinity, Na+, Cl, Ca2+, Li+, and Ca/Mg (locally modulated by the formation of calcite cements) are consistent with diagenesis of volcanic matter to form clay minerals. Depth profiles of all these ions show offsets or local deviations associated with the fault at ~165 mbsf.

Organic carbon contents average 0.5%. The C/N ratios mainly between 8 and 20 suggest a mixed terrigenous and marine origin for the organic matter. Headspace gas data show a C1/C2 ratio decreasing from ~2000 at 335 mbsf to 138–195 in the deepest samples (467 and 476 mbsf). Starting at 391 mbsf, there is an increasing presence of higher chain volatile hydrocarbons indicative of thermogenically derived gas.


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