SITE SUMMARIES

Site 1115

Hole 1115A (APC [advanced hydraulic piston corer]):
9º11.389'S, 151º34.450'E; 1149.6 mbsl (meters below sea level)
0-4.40 mbsf cored; 4.43 m recovered (101%)

Hole 1115B (APC/XCB [extended core barrel]):
9º11.382'S, 151º34.437'E; 1148.8 mbsl
0-293.10 mbsf cored; 286.84 m recovered (98%)

Hole 1115C (RCB [rotary core barrel]):
9º11.383'S, 151º34.422'E; 1148.7 mbsl
0-283.5 mbsf drilled without coring; 283.2-802.5 mbsf cored; 291.63 m recovered (56%)

The objectives of our study at Site 1115 were to determine the sedimentology, biostratigraphy, and vertical motion history of the Woodlark Rise (the northern, upper plate margin to the Moresby detachment fault), including the prerift history of the Trobriand forearc basin sequence. The site was located ~35 km to the north of Site 1109 (Figs. F5, F7) to (1) better characterize the slope sediments and provide widely spaced data from shallower water depths for flexural subsidence models, (2) avoid the thick dolerite that prevented sampling of the prerift forearc sequence at Site 1109, and (3) sample the upper ~150 m of section that has been eroded by a submarine channel further south.

From bottom to top, the sedimentary succession cored at Site 1115 (Figs. F9, F10) shows (1) a shoaling and coarsening upward, middle Miocene forearc sequence, unconformably below (2) a late Miocene nonmarine coastal (fluvial?) and lagoonal succession, and (3) a shallow marine, then progressively deepening and fining upward, latest Miocene (5.54 Ma) to Pleistocene sequence related to the subsidence of the margin during the rifting of the Woodlark Basin. The synrift sequence is undeformed, with bedding dips <10º throughout, whereas the forearc sequence below exhibits a few normal, reverse, and strike-slip faults.

We cored the upper ~230 m of the >2-km-thick Trobriand forearc basin sequence. This section was deposited at >135 m/m.y. and is older than 12.1 Ma and younger than 15.1 Ma. The sequence comprises turbiditic sands, silts, and clays derived from calc-alkaline arc sources including distinctive clinopyroxene-phyric basic extrusives. The turbidites below 615 mbsf were deposited in upper bathyal depths (150-500 m). The turbidites above 659 mbsf were joined by redeposited neritic carbonates and deposits marked by sediment instability, possibly related to local channeling and/or regional tectonism. Benthic foraminifers indicate a change to neritic deposition (50-150 m) above 615 mbsf and by 13.6 Ma. The sediments record substantial input of shallow-water carbonate. The upward shallowing of the forearc sequence may be attributed to both filling of the basin and tectonic uplift.

An unconformity and hiatus at 574 mbsf resulted from the emergence of the forearc sequence. The unconformity is seismically imaged throughout the Trobriand forearc basin and is younger than 9.63 Ma at the Nubiam-1 well ~100 km to the northwest. At Site 1115, the unconformity is older than 5.54 Ma, by which time sediments younger than 8.6 Ma had accumulated to 513 mbsf at rates >13 m/m.y. The basal sediments above the unconformity are nonmarine (fluvial?) conglomerate (to 566 mbsf), topped by an organic-rich silty claystone (inner lagoonal), and capped by a siltstone with common to abundant shell fragments (open-marine lagoonal).

Margin subsidence is recorded by inner neritic (<50 m) sandy siltstone (to 475 mbsf) passing upward to silty sandstone (to 417 mbsf) deposited on an open shelf (50-150 m) influenced by traction currents. Sedimentation rates from 4.0-5.5 Ma averaged 45 m/m.y.

From 4.0 to 3.0 Ma, the average sediment accumulation dramatically increased to ~284 m/m.y. This resulted in undercompaction and anomalously high measured porosities between ~420 and ~300 mbsf (Fig. F11). Above 417 mbsf, turbidites were deposited in deeper water, upper bathyal (150-500 m), and they fine upward from sandy silty claystone to silty claystone. Volcaniclastic sand and silt horizons, originating from a calc-alkaline arc source, remain little changed throughout the Pliocene section. The pelagic carbonate component increases above 300 mbsf with CaCO3 concentrations increasing from ~20 to ~75 wt% at the seafloor.

Since 3.0 Ma the sedimentation rate markedly slowed, initially to ~79 m/m.y. (to 2.0 Ma) and then ~59 m/m.y. (to 0.5 Ma) and ~34 m/m.y. thereafter. The marked change in the sediment supply to the area corresponds in part to a decrease in the volcaniclastic sand deposited by turbidity currents. Above 169 mbsf (2.58 Ma), the input of fine metamorphic detritus marked by the presence of illite to 520 mbsf also ends. By this time and since then, the sedimentation rate was much slower than the margin subsidence and the surface deepened to upper middle bathyal (500-1150 m) depths. No record of the last 120 k.y. is preserved, and this may be responsible for the low porosities (65%-70%) near the surface. Pleistocene sedimentation was dominated by nannofossil ooze with volcanic ash. The Pleistocene volcanic ash fallout layers and middle-late Pliocene volcanogenic turbidites record a marked increase in explosive Trobriand Arc volcanism since 3.7 Ma.

The thermal gradient determined from five temperature measurements between 26 and 227 mbsf is 28ºC·km-1, yielding a heat flow of 28 mW·m-2 given the average thermal conductivity measured on cores from this interval of 1 W·m-1·ºC-1.

The magnetostratigraphic record of the upper 400 m is very good. The Brunhes/Matuyama/Gauss/Gilbert polarity transitions, and the Jaramillo, Olduvai, and Kaena Subchrons are identified, as are the Cobb Mountain and Reunion events. Very low magnetic susceptibilities characterize the intervals 210-410 mbsf and 480-550 mbsf, without apparent correlation to grain size, lithology, or sedimentation rate. The variation of susceptibility with depth observed at Site 1115 between 120 and 550 mbsf is similar to that observed at Site 1109 between 80 and 705 mbsf, and the susceptibility boundaries at both sites are time correlative (Figs. F12, F13).

Hole 1115C was successfully logged above 784 mbsf with triple combo geophysics and FMS-sonic tool strings. The well seismic tool was used to record check shots near the base of the hole, allowing depth correlation with seismic reflection lines.

The longest profile to date of the deep subseafloor biosphere was made at this site (Fig. F14). Bacteria were present in the deepest sample analyzed (801 mbsf) and both dividing and divided cells were present to 775 mbsf. The persistence of apparently living microbial life into indurated sedimentary rock ~15 m.y. old and to 801 mbsf extends the limit of the biosphere whose base remains undefined. Methane is present at levels above 1000 ppm from 250 to 450 mbsf and above 20,000 ppm from 572 to 802 mbsf (Fig. F14). The C1/C2 ratios generally exceed 1500 and 3000 over the same intervals, and broad maxima in ammonia are found within 200-450 mbsf and at the base of the hole. These observations are consistent with a biogenic origin for the volatile hydrocarbons and the presence of a significant amount of bacteria at depth.

High-resolution pore-water sampling (63 whole rounds) comprehensively documents the interstitial water chemical variations (Fig. F15). In the upper 300 mbsf of the cored section, pore-water variations primarily reflect the oxidation of organic matter and the concomitant early diagenesis of biogenic carbonate (including aragonite) leading to precipitation of dolomite, as well as alteration of detrital, mostly volcanic, matter. Further downhole, most of the pore-water variations are controlled largely by the alteration of volcanic minerals and the formation of clays and zeolites. Silicification appears to be a dominant process below 500 mbsf. The formation of calcite cements is significant in sediments of the forearc sequence.

Site 1109

Hole 1109A (APC):
9º30.390'S, 151º34.388'E; 2210.9 mbsl
0-9.50 mbsf cored; 9.96 m recovered (105%)

Hole 1109B (APC):
9º30.396'S, 151º34.391'E; 2211.1 mbsl
0-14.80 mbsf cored; 15.14 m recovered (102%)

Hole 1109C (APC/XCB):
9º30.392'S, 151º34.390'E; 2211.0 mbsl
0-375.70 mbsf cored; 323.11 m recovered (86%)

Hole 1109D (RCB):
9º30.380'S, 151º34.355'E; 2211.0 mbsl
0-352.80 mbsf drilled without coring; 352.80-802.00 m cored; 299.87 m recovered (67%)

Site 1109 is located on the Woodlark Rise, 11 km north of a major south-dipping normal fault system that is antithetic to the low-angle fault dipping north from Moresby Seamount (Figs. F5, F7). The site was positioned to cross a sequence boundary and an angular unconformity at about 350 and 770 mbsf, respectively, beneath which a lower stratified sequence, interpreted to be prerift forearc basin sediments, dips northward at ~10º.

Four holes were drilled at Site 1109: two short APC holes (1109A and 1109B), one APC/XCB (1109C), and one RCB (1109D). These allowed coring to a total depth of 802 mbsf, complete logging above 786 mbsf with the triple combo geophysical tool, logging two intervals (112-351 and 376-786 mbsf) with the FMS-sonic tools, and conducting a well seismic tool-vertical seismic profile (WST-VSP) with nine receiver locations from 378 to 460 mbsf. The following description uses unit boundaries derived from integrated core-log data interpretation.

Data from Site 1109 show a record of progressive subsidence (from subaerial to lagoonal, then shallow marine and deep water) over a period from latest Miocene to late Pleistocene (Fig. F10). These data provide the information to fulfill one of our primary objectives: to determine the sedimentology, biostratigraphy, and vertical motion history of the synrift sediments on the hanging wall margin to the Moresby low-angle normal fault.

A second objective, to determine the nature of the forearc basin sequence beneath the rift-onset angular unconformity, was thwarted by the unexpected presence of a massive dolerite with ophitic texture from 773 to 802 mbsf, overlain to 730 mbsf by a conglomerate of dolerite and some basalt cobbles in an altered clayey silty matrix with nonrecovered interbeds likely similar to the overlying unit (Fig. F9). The dolerite has seismic velocities of 5-6 km·s-1. Culminations observed on reflection seismic sections to be developed locally on the erosional unconformity--previously interpreted as reefs--may be volcanic constructions.

From 713 to 730 mbsf, a very altered, clay-rich siltstone and fine-grained sandstone were recovered, including goethite concretions. A nonmarine, swampy setting is inferred. These sediments contain scattered basalt/dolerite clasts that logging data indicate were derived from discrete conglomerate intercalations. The oogonia of charophyte algae present in Sample 180-1109D-39R-CC indicate a locally freshwater environment at the top of this unit.

We envision deposition in a lagoonal setting of the silty claystone and clayey siltstone encountered between 672 and 713 mbsf. Shell, plant, and wood fragments are common to abundant. The lagoon was alternately brackish and, as indicated by the presence of dolomite, hypersaline. This unit has high natural gamma ray and porosity (45%-50%), low magnetic susceptibility, and velocities ~2 km·s-1.

The first (lower Pliocene: NN13 and N19/20) sediments deposited in a shallow-marine (<150 m water depth) environment occur at 588-664 mbsf and are mixed carbonate-siliciclastic rocks. From 599 to 672 mbsf the lithology consists dominantly of well-cemented, 30%-40% porosity, sandy bioclastic packstone/grainstone. A lower limestone section (643-672 mbsf) with high resistivity (3 m), has low natural gamma ray, 40-80 wt% calcium carbonate, and average velocities of 2.5-3.0 km·s-1. An upper sandstone section with lower resistivity (<2 m), has high natural gamma ray, 20-30 wt% calcium carbonate, and average velocities of 2.0-2.5 km·s-1. Above this is an interval (570-599 mbsf) of calcareous (bioclastic) sandstone with 40-50 wt% calcium carbonate and 45%-50% porosity. The entire sequence reflects relatively shallowwater sediments derived from both volcanic-related and neritic carbonate source materials that accumulated before 4 Ma at >70 m/m.y.

A succession of clay-rich siltstones and silty claystones, interlayered with thin (1-10 cm) medium- to fine-grained sandstones, at 390-570 mbsf was rapidly deposited (312 m/m.y.) in upper bathyal (150-500 m) water depths until ~3.35 Ma. Porosity increases (to 60%) as calcium carbonate decreases (to 25 wt%) upsection to ~480 mbsf, whereas velocities steadily decrease from 2.0 to 1.8 km·s-1 up the unit, and magnetic susceptibilities are constantly low above 540 mbsf. These are hemipelagic carbonate muds with turbiditic sand interbeds from a dominantly unaltered basalt-andesite volcanic source with minor neritic carbonates, deposited on a well-oxygenated and extensively bioturbated slope.

Above 380 mbsf, significantly greater magnetic susceptibilities that continue to 83 mbsf correspond to the influx of clays and silts from an additional source terrane, characterized by altered calc-alkaline volcaniclastic material and metamorphic detritus with mixed-layer, and probably smectite, clays. Between 330 and 390 mbsf (~3.07-3.35 Ma) the silty claystone is nearly devoid of sandy interbeds. The lower portion (up to 353 mbsf) has a high frequency of layers with unaltered volcaniclastics. The section from 295 to 330 mbsf was deposited at 69 m/m.y. between 2.57 and 3.07 Ma. Seismic reflection data show that this compressed section is part of a conformable slope sequence that substantially thickens downslope toward the rift basin to the south (i.e., there is a regional "onlap" relationship of flat basin turbidites laterally continuous with conformable slope deposits).

The margin continued to subside with an accumulation rate of 66 m/m.y. between 3.07 and 1.95 Ma (295-255 mbsf), and above 285 mbsf was at middle bathyal (500-2000 m) water depths. Distal silt-clay bioturbated turbidites with volcanic, terrigenous, and biogenic components rapidly onlapped the margin (225 m/m.y.) from 1.95 Ma until 1.0 Ma (~42 mbsf). These include a significant component of reworked slope sediments, as evidenced by dominantly upper Pliocene biota in the younger section. Volcaniclastic sands are most frequent from ~100 to 170 mbsf. Silt to coarse-grained sand interbeds are common from 170 to 247 mbsf, including a poorly recovered sand from 218 to 233 mbsf. These sands are remarkable for their high thorium and potassium contents, producing high natural gamma-ray counts, and lower porosities (40%-50%) and higher velocities (2 km·s-1) than adjacent intervals (60%-70% and 1.7 km·s-1, respectively).

Between 1.0 and 0.46 Ma the site was relatively sediment starved and/or intermittently eroded (the site is located in a submarine valley), with net accumulation rates of 21 m/m.y. of calcareous clayey silt and silty clay with some thin volcanic ash layers. Since 0.46 Ma, nannofossil-rich, calcareous sand, silt, and clay with volcaniclastic sand and volcanic ash, were deposited at 67 m/m.y. Only the surface sediment has a lower bathyal (>2 km) benthic fauna.

Extensional deformation is very weak throughout the section, except for minor normal faults at about 260 and 360 mbsf and normal shear zones at 678-685 mbsf and in the dolerite. A folded region at 36-55 mbsf is interpreted as a slump.

Velocities linearly increase with depth from 1.5 km·s-1 at the surface to 1.9 km·s-1 at 520 mbsf, and then more rapidly to 2.2 km·s-1 at 590 mbsf with the increasing calcium carbonate. Velocities increase to 3-4 km·s-1 and are more variable in the bioclastic sandstones and limestones between 590 and 672 mbsf, and then return to 2 km·s-1 in the lagoonal sequence below that. Porosity decreases downward, but the usual negative exponential decay is interrupted at two levels (160-280 and 350-540 mbsf) where the higher porosity reflects undercompaction correlated with periods of high sedimentation rates (225-312 m/m.y.) (Fig. F11). The thermal conductivity generally mirrors the porosity-depth profile and ranges from 0.78 to 1.5 W·m-1·ºC-1, except for the dolerite, which shows values up to 2 W·m-1·ºC-1. Six temperature measurements down to 170 mbsf define a linear thermal gradient of 31ºC·km-1 and, when combined with an average thermal conductivity of 0.9 W·m-1·ºC-1 over this interval, a heat flow of 28 mW·m-2.

Although their number rapidly decreases with depth, bacteria are present in all samples obtained down to 746 mbsf. Total populations and numbers of dividing and divided cells show obvious relationships to the sediment geochemistry (Fig. F14). Pore-water constituent profiles show that carbonate diagenesis occurs at shallow depth (above 100 mbsf), aided by the bacterial decomposition of organic matter. Diagenetic transformation of pre-existing detrital clay minerals occurs to 300 mbsf. Bacterial activity further downhole is evidenced by deep-seated (430-550 mbsf) ammonia and alkalinity submaxima (the latter likely because of increased CO2 production). Of note is a generally increasing pH downhole, from 7.8 to greater than 8.6. Pore-water composition also reflects the alteration of volcanic components, formation of authigenic clay minerals, silica diagenesis, and diffusion of elements above the shell-rich and freshwater units below 672 mbsf. A crossover of Ca and Mg profiles, as often observed above igneous sills, occurs at 661 mbsf.

Headspace gas analyses show a typical methane profile, with concentrations increasing rapidly at 100 mbsf from ~5 to ~6000 ppm, then remaining between 1000 and 10,000 ppm down to 600 mbsf. Methane content begins to decrease below 600 mbsf, reaching 5 ppm by 720 mbsf. The only other hydrocarbon detected was C2, but it remained below 3.2 ppm throughout the entire cored section. The C1/C2 ratios did not drop below 1000, and organic carbon was generally <1% throughout the core.

Site 1118

Hole 1118A (RCB):
9º35.110'S, 151º34.421'E; 2303.6 mbsl
0-205.0 mbsf drilled without coring; 205.0-926.6 mbsf cored; 466.21 m recovered (65%)

Site 1118 is 1.8 km north of a major south-dipping normal fault system that is antithetic to, and bounds the rift basin above, the low-angle fault dipping north from Moresby Seamount (Figs. F5, F7). The location was selected in order to drill through the thick synrift section onlapping the northern margin and to penetrate an angular unconformity into north-dipping reflectors deep in the inferred prerift forearc basin sequence. The site is ~9 km due south of Site 1109 and has similar objectives in common to it and Site 1115, namely to determine (1) the sedimentology, biostratigraphy, and vertical-motion history of the northern margin, and (2) the nature of the forearc basin and basement sequence.

Results from Site 1118 record the progressive subsidence of a lower Pliocene, subaerially eroded and tropically weathered, landmass (Figs. F9, F10). A conglomerate of dolerite with minor basalt was recovered (Fig. F16) and imaged with FMS below 873 mbsf. Iron oxides and well-rounded clasts reveal that the dolerite, similar to that encountered at Site 1109 (although locally more pegmatitic), was exposed to subaerial alteration. Shearing and veining fragmented and partially brecciated the dolerite, which was deposited as a poorly sorted, probably fluvial conglomerate mixed with various clasts and sediment, including paleosols.

The dolerite conglomerate is overlain to 857 mbsf by a sequence of lower Pliocene limestones, calcareous paraconglomerates, and a volcaniclastic sandstone that was deposited in a marine lagoon with abundant calcareous algae. This sequence is well marked in the geophysical logs and by highs in the CaCO3 profile of >80 wt%. A VSP shows that the dolerite-limestone section corresponds to a strong reflector at the base of the sedimentary sequence that mantles underlying northward-dipping reflectors, which were not penetrated.

The Gauss/Gilbert Chron boundary (3.58 Ma), occurring at 846-850 mbsf, dates an upward-fining sequence disconformably above the limestones as all middle Pliocene and younger, in agreement with paleontological data (Biozones N20-N21 and NN16A-NN19A through the top of the cored sequence at 205 mbsf). The lower sedimentary section records a significant terrestrial input, including wood fragments, confirmed by the C/N ratio, which indicates a mixed-marine and terrigenous source of organic carbon. The whole sequence records turbiditic and hemipelagic sedimentation. It comprises mixed volcaniclastic sandstones, siltstones, and minor claystones, and then mostly siltstones and claystones interbedded with turbiditic sandstones and siltstones that decrease in proportion upward. The orientations of the subhorizontal maximum axes of the ellipsoids of magnetic susceptibility (corrected for bedding dip and core orientation) between 490 and 680 mbsf suggest an east-southeast-west-northwest-directed paleocurrent during sedimentation, almost perpendicular to the present-day slope.

The sedimentation rate from 3.58 to 2.58 Ma (387.5 mbsf) was 479 m/m.y., the highest encountered during Leg 180, with benthic foraminifers revealing an upper bathyal (150-500 m) paleowater depth. Between 2.58 and 1.95 Ma (288 mbsf), the sedimentation rate decreased to 155 m/m.y., and the paleodepth was middle bathyal (500-2000 m) to at least 205 mbsf. Apparently, rapid subsidence since 3.6 Ma was accompanied by sufficient sediment supply to limit deepening of the seafloor until 2.6 Ma. High porosities slowly decreasing from 50%-60% at 205 mbsf to 40%-50% at 800 mbsf likely reflect underconsolidation related to the observed high sedimentation rates.

Volcanic ash and volcaniclastic sands are ubiquitous throughout the Pliocene sedimentary section, but especially so in the portion dated to between 3.0 and 3.6 Ma (Fig. F17) in which a predominance of rhyo-dacitic glass reflects explosive acidic volcanism probably associated with rifting of the continental arc.

Most of the sedimentary section is undeformed with nearly horizontal beds and shows compaction-related minor faults as well as common slump folds. The abundance of synsedimentary features on such a nearly level seafloor suggests an unstable area periodically shaken by earthquakes and affected by mass movement.

As seen in other northern sites drilled during Leg 180, the variations in interstitial water constituents reflect the oxidation of organic matter mediated by microbial activity and the concomitant early diagenesis of biogenic carbonates (Figs. F14, F15). Volcanic alteration and authigenesis are important processes, particularly in the lower part of the hole. The abundance of volcaniclastic sands and the higher porosities in the lower part of the hole, when combined with the high temperature gradient (~63ºC·km-1), greatly influence the pore-water chemistry. In particular, the dissolved silica, lithium, and strontium show higher concentrations than might otherwise be expected. In addition, temperature measurements in the open hole during a logging run suggest migration of warm fluids at 700-800 mbsf.

Both methane and ethane, whose ratio is between ~5000 and ~1600, are present down to ~700 mbsf, below which ethane is not detected. The highest concentrations in these two volatile hydrocarbons occur where sulfate disappears from the interstitial water, attesting to a biogenic origin.

Bacteria population numbers and dividing and divided cells decrease rapidly with increasing depth and conform to the general model for their distribution in marine sediments. In extending their known distribution to 842 mbsf at this site, the deepest samples so far obtained, there is an indication that numbers are decreasing more rapidly than the model predicts, resulting in a sigmoidal depth distribution in these sediments (Fig. F14).

Site 1108

Hole 1108A (jet-in test only):
9º44.708'S, 151º37.514'E; 3162.7 mbsl
0-16.3 mbsf drilled without coring

Hole 1108B (RCB):
9º44.724'S, 151º37.543'E; 3177.2 mbsl
0-485.2 mbsf cored; 148.58 m recovered (30%)

Site 1108 is 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 (Fig. F7). 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 25º-30º beneath a 3.2-km-deep, asymmetric rift basin with more than 2 km of sediment fill (Fig. F4). 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 before 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 upper 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 and included dark siliciclastic sandstone and siltstone, volcanic breccia, microgranite, granodiorite, epidosite, greenschist mylonite, and biotite gneiss. Some glassy basalt fragments from submarine eruptions were incorporated in the same interval. 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 (Fig. F9). Ages increase from 1.67 to 1.75 Ma at 82.8 mbsf to <3.35 Ma at the base. Sedimentation rates increase downsection, from 324 m/m.y. at 1.7-2.0 Ma to 400 m/m.y. at 2.60-3.2 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 (Fig. F10).

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 200.2 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 of up to 35º, low-angle shearing, brecciation, and ubiquitous slickensides. The faults dip at moderate angles (~45º), 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.

Porosities measured in the laboratory 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 at least 400 m of sediments has been removed (Fig. F11). Half this amount may be associated with throw on the fault near 165 mbsf, the rest with erosion between the unconsolidated sands at 63 mbsf and the consolidated sandstones at 72 mbsf.

Temperature measurements suggest an average thermal gradient of 100ºC·km-1 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 94ºC·km-1 above 160 mbsf and 65ºC·km-1 below 200 mbsf, with a 10ºC offset formed in the last thousand years. Thermal conductivities in the upper several meters are 0.8-0.9 W·m-1·ºC-1 and are 1.0-1.7 W·m-1·ºC-1 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 wt%. 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) (Fig. F14). Starting at 391 mbsf, there is an increasing presence of higher chain volatile hydrocarbons indicative of thermogenically derived gas.

Sites 1110 through 1113

Site 1110

Hole 1110A (APC):
9º43.599'S, 151º34.511'E; 3246.4 mbsl
0-9.5 mbsf cored; 9.5 m recovered (100%)

Hole 1110B (APC/XCB):
9º43.609'S, 151º34.509'E; 3246.3 mbsl
0-22.3 mbsf cored; 5.37 m recovered (24%)

Hole 1110C (RCB):
9º43.599'S, 151º34.498'E; 3245.8 mbsl
0-15.0 mbsf drilled; no cores taken

Hole 1110D (RCB):
9º43.588'S, 151º34.526'E; 3245.8 mbsl
0-22.7 mbsf drilled; 22.7-28.7 mbsf cored;
0.10 m recovered (2%)

Site 1111

Hole 1111A (RCB):
9º43.059'S, 151º34.533'E; 3200.7 mbsl
0-173.7 mbsf cored; 15.19 m recovered (9%)

Site 1112

Hole 1112A (RCB):
9º44.749'S, 151º36.721'E; 3046.7 mbsl
0-122.4 mbsf cored; 5.85 m recovered (5%)

Hole 1112B (RCB):
9º44.746'S, 151º36.714'E; 3046.6 mbsl
0-126.1 mbsf drilled without coring; 126.1-164.6 mbsf cored; 1.19 m recovered (3%)

Site 1113

Hole 1113A (RCB):
9º45.449'S, 151º36.737'E; 2915.6 mbsl
0-25.2 mbsf cored; 0.44 m recovered (2%)

Sites 1110 through 1113 were drilled in various locations near the foot of Moresby Seamount (Fig. F8) in an attempt to find a viable alternate location to our primary Site 1108. With the hydrocarbon safety restriction of 485 mbsf, the depth reached at Site 1108, we sought to intercept the Moresby low-angle normal fault in other locations or at shallower depths. We first tried two locations ~6 km west-northwest of Site 1108, near the rift basin depocenter located due north of Moresby Seamount. We then tried two other locations updip of Site 1108, all without success.

At Site 1110, where the fault may be 400-450 mbsf, multiple holes were unable to penetrate below ~29 mbsf because of talus beneath ~9 m of surficial calcareous clay, late Pleistocene in age (<0.22 Ma). The pebbles and cobbles of the talus include micaschists, amphibolites, and rare granite porphyry.

Moving 1 km north, at Site 1111, we cored a single hole through ~154 m of Pleistocene deposits (<1.02 Ma): talus pebbles and cobbles in calcareous ooze, clay, and silty clay with lesser nannofossil-rich silt, sand, and gravel. The talus includes metasediments (micaschist and gneiss) and variably metamorphosed igneous rocks, both basic (dolerite, metadolerite, and lamprophyre) and acidic (granite porphyry). Coring was stopped at 174 mbsf in large metamorphic cobbles. Based on temperatures measured at the mudline and at 136 mbsf, the thermal gradient is 95ºC·km-1, similar to that encountered at Site 1108, and the heat flow is 86 mW·m-2.

Site 1112 is 1.5 km west of, and updip from, Site 1108, where the depth to the fault is ~450 mbsf. Another thick pile of talus deposits in Pleistocene (<1.75 Ma) sediments, including silty clay with occasional ash, required two RCB holes to reach 165 mbsf. The recovery consisted of pebbles only, mostly of metadolerite and epidosite, but also minor andesite, granite porphyry, micaschist, and sandstone.

After offsetting 1.3 km south, we attempted a "bare rock" spud at the base of the slope of Moresby Seamount where the fault crops out (Site 1113), but there was enough talus to make the hole unstable and repeatedly refill, and it was abandoned at 25 mbsf in micaschist and epidosite pebbles and cobbles.

Most of the metamorphic rocks recovered from the talus of Moresby Seamount have igneous protoliths. In addition, micaschists and gneisses record an early tectono-metamorphic stage characterized by the development of a foliation under epidote-amphibolite to amphibolite facies conditions. A later retrograde metamorphism in the greenschist facies affected all protoliths. This later stage was coeval to brittle extensional deformation and extensive hydrothermal alteration, probably during the normal faulting of Moresby Seamount and the subsequent unroofing of its basement.

Site 1117

Hole 1117A (RCB):
9º46.526'S, 151º32.945'E; 1663.2 mbsl
0-111.1 mbsf cored; 6.42 m recovered (6%)

Hole 1117B (RCB):
9º46.527'S, 151º32.951'E; 1663.2 mbsl
0-9.5 mbsf cored; 0.05 m recovered (0.5%)

Hole 1117C (RCB):
9º46.520'S, 151º32.943'E; 1663.2 mbsl
0-9.5 mbsf cored; 1.05 m recovered (11.1%)

Site 1117 is on the upper slope of the northern flank of Moresby Seamount, 3.5 km to the northwest of Site 1114 (Figs. F6, F8). It was a successful attempt to bare-rock spud into, and drill through, the main detachment fault where it crops out.

The base of the cored section consists of a noncumulate, quartz-magnetite gabbro (Fig. F18) that passes upward into brecciated and then mylonitized equivalents, with a fault gouge at the surface. The upward-increasing shearing and alteration confirm that the northern flank of the seamount is an outcropping fault surface. The first undeformed gabbro occurs at 86 mbsf, but brecciated gabbro was found deeper in the section, at 96 mbsf. Therefore, the minimum thickness of the shear zone preserved within the footwall is about 100 m.

In the surficial core we recovered 4 m of soft, light-colored, clayey material with a soapy feel, interpreted as a fault gouge (Fig. F19). This material contains talc, chlorite, calcite, ankerite, and serpentine, which is consistent with hydrothermal alteration of the underlying deformed gabbro. It has low porosity (~30%), bulk density of ~2.2 g·cm-3, unconfined compressive strength in the range of 65-90 kPa, thermal conductivities of 1.3-1.8 W·m-1·ºC-1, and transverse sonic velocities of ~2 km·s-1. These physical properties strongly contrast with those common to near-seafloor deposits and to the gabbro protolith below (porosity 3%, bulk density 2.76 g·cm-3, thermal conductivity up to 3.8 W·m-1·ºC-1, and sonic velocity 6.0-6.4 km·s-1). Although the fault gouge has been exposed at the seafloor, its characteristics still reflect its deformational origin. The shape parameter of the ellipsoid of magnetic susceptibility (T = 0.2-0.8) indicates an oblate magnetic fabric and the degree of anisotropy of the magnetic susceptibility is maximum in the fault gouge (Pj = 1.1-1.2), both indicative of flattening as a result of high shear strain.

The deformation textures in the gabbro range from brecciated to cataclastic to mylonitic, almost totally obscuring the initial subautomorphic texture. Mylonite clasts recovered down to 57 mbsf show a well-developed foliation with S-C structures (Fig. F20). Within the foliation, the association of epidote-rich and very fine grained layers of quartz, epidote, and chlorite reveals greenschist facies conditions during deformation. Asymmetrical fibrous quartz pressure shadows present around pyrite also attest to syntectonic metamorphism. The gabbro between 62 and 86 mbsf shows evidence of increasing brecciation upward, passing progressively to the mylonite. Quartz + epidote veins, reoriented parallel to the foliation in the mylonite, suggest that the brecciation was assisted by silica-rich fluids. Late veins of epidote and calcite cut the rock, attesting to more carbonate-rich fluids in the late stage of shearing. Late alteration, associated with fluid flow within the shear zone, has produced chlorite, talc, and fibrous amphibole replacing primary plagioclase and clinopyroxene.

The mineralogy and texture of the gabbro are similar to those of high-level gabbros occurring in ophiolites. These gabbros, together with the presence of dolerites reminiscent of a sheeted dolerite complex at Site 1114, suggest that Moresby Seamount may be part of an ophiolitic complex exhumed by extension along the northward-dipping low-angle normal fault bounding the seamount to the north.

Site 1114

Hole 1114A (RCB):
9º47.613'S, 151º34.504'E; 406.5 mbsl
0-352.80 mbsf cored; 43.78 m recovered (12%)

Site 1114 is just north of the crest of Moresby Seamount where seismic reflection data indicate that the basement beneath a south-southwest-dipping normal fault is shallowest (Figs. F5, F8). The primary objective was to determine the internal structure and composition of Moresby Seamount, particularly the nature of the basement (rock type, pressure-temperature history, structural fabric, and deformation history). A second objective was to determine the sedimentology, biostratigraphy, and vertical motion history of the ~300 m of local sedimentary cover, which may correspond to the uplifted and partly eroded synrift sequence.

In Hole 1114A, we drilled ~286 m of Pliocene-Pleistocene sediments separated by a 6-m-thick tectonic breccia from a metadolerite that forms the basement (Fig. F21). The dolerite was metamorphosed under low-grade greenschist facies conditions before its upper part, which contained a chilled margin, was reactivated by normal faulting, leading to its unroofing.

The sediments consist of rift-related, mostly volcaniclastic, turbidites deposited in middle bathyal water depths (500-2000 m). The benthic foraminiferal assemblage is characteristic of a suboxic environment of deposition, which may correspond either to a basinal situation with restricted circulation or to the oceanographic oxygen minimum situated at middle to upper bathyal water depths. From bottom to top, the deposition changed from mostly coarse-grained sandstones, ~100 m thick, to a finer grained intercalation of sandstones, siltstones, and claystones. The mineralogy of the sandstones suggests that most of the turbidites were derived from calc-alkaline extrusive rocks, but minerals and clasts from ultramafics and metamorphics (serpentinite and calc-schists) are also present, as well as ubiquitous bioclasts.

Only the uppermost sediments are Pleistocene in age. The indurated sediments below 35 cm in Core 180-1114A-2R are all late and middle Pliocene (from >1.67 Ma to 3.09-3.25 Ma) and were deposited at rates of at least 176 m/m.y. (Fig. F10). The offset of the porosity vs. depth curve suggests that from 220 to >400 m of the Pleistocene section has been removed (Fig. F11), which we tentatively relate to the uplift of Moresby Seamount in the footwall of bounding normal fault systems.

The deformation in the recovered sedimentary rocks preferentially occurs in the fine-grained strata, where it is expressed as a scaly fabric with numerous striated surfaces. It increases in intensity approaching the tectonic breccia. The bedding dips in the upper ~110 m range from 0º to 30º, then increase to 25º-60º below that. The FMS data reveal (Fig. F22) that (1) bedding dip directions are dominantly northwest but range from north to west; (2) the basement/sediment faulted contact outlined by the breccia dips ~60º toward the southwest, slightly oblique to the main normal fault that offsets basement by >2 km; and (3) in contrast, faults within the overlying sedimentary rocks mostly dip to the north, but a few dip to the south just above the breccia. The sense of motion on faults within the sedimentary rocks is dominantly normal, but reverse and oblique slip faults also exist. These observations suggest that a component of left-lateral motion exists on the bounding, south-southwest-dipping normal fault, in agreement with regional evidence for north-south extension.

The brecciated fault contact is also marked by an abrupt increase in the degree of hydrothermal alteration. The alteration in the breccia and metadolerite is characterized by massive clay and calcite veins that crosscut numerous quartz and epidote veins. In the breccia, the latter veins are restricted to the clasts. The hydrothermal alteration tends to decrease downward in the metadolerite, suggesting that fluids were channeled into the tectonic breccia.

Site 1116

Hole 1116A (RCB):
9º51.934'S, 151º34.508'E; 1851.3 mbsl
0-158.90 mbsf cored; 32.61 m recovered (21%)

Site 1116 is on the southern flank of Moresby Seamount, 8 km south of Site 1114, within a tilted block bounded by two normal faults that each offset the basement by >1 km to the south-southwest (Figs. F5, F8). The objectives for this site were to characterize the early rift sediments and the seamount basement (which was not reached).

Very low surface porosity (~30%), and high bulk density (~2.15 g·cm-3), sonic velocity (2.2-2.8 km·s-1), and thermal conductivity (1.0-2.2 W·m-1·ºC-1) values, suggest that 700-1000 m of the section has been removed by faulting and/or erosion (Fig. F11). This is consistent with the lack of a Quaternary and uppermost Pliocene section, as well as the seismic stratigraphy.

The recovered section is dominated by Pliocene indurated sandstones alternating with siltstones/claystones (Fig. F21). The sandstones are fine to medium, and occasionally coarse grained, and display parallel, wavy, and convolute laminations, which indicate they originated from turbidity currents in a near-source slope setting. A paraconglomerate is present at 34-63 mbsf. The sandstones below this are subarkosic with a calcareous matrix. Measured CaCO3 contents are <5 wt%. Some thick-bedded, reverse-graded sandstones with occasional intraformational rip-up clasts were deposited from high-density turbidity currents. However, normal grading and lamination in some siltstones/sandstones also indicate deposition by low-concentration turbidity or bottom currents. The matrix-supported conglomerates, interbedded with sandstones and siltstones, comprise relatively unaltered, mainly angular clasts and are interpreted as debris-flow deposits.

Benthic foraminifers indicate middle bathyal paleowater depths (500-2000 m), with inner neritic (<50 m) benthic foraminifers redeposited within turbidite beds. The N20/N21 boundary (3.35 Ma) occurs between 104 and 128 mbsf, and the minimum age of 1.95 Ma at the surface yields a minimum sedimentation rate of 70 m/m.y. (Fig. F10). Burrows are either abundant or relatively depleted, suggesting alternating poorly and well-oxygenated subseafloor conditions with, in both cases, abundant detrital organic matter input and a mostly terrigenous source, as indicated by C/N ratios.

The orientations of the subhorizontal maximum axes of the ellipsoids of magnetic susceptibility (corrected for bedding dip and core orientation) below 100 mbsf indicate northeast-southwest or northwest-southeast-directed paleocurrents during sedimentation.

The sources of clastics are little-altered basic extrusive rocks of mainly calc-alkaline affinities, probably derived from the Miocene Trobriand Arc, but also include acidic extrusives, shallow-water bioclasts, metamorphics, and serpentinite. The serpentinite, rare chromite, and some of the gabbro and dolerite grains probably have an ophiolitic origin (Paleocene-Eocene Papuan Ultramafic Belt?).

The upper 100 m of the section shows abundant evidence of synsedimentary deformations, including folding and low-angle extensional faulting typical of gravity-driven processes. The bedding dips <10º, except for fold limbs where it is up to 50º. Common dewatering and fluidization features were possibly seismically triggered. Faults and scaly fabrics are within a narrow zone between 100 and 120 mbsf. They include steep strike-slip and 25º- to 55º-dipping pure normal faults. Below the fault zone beds dip ~15º consistent with the dip of seismic reflectors.

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