Site 1109

Hole 1109A (APC):
9°30.390´S, 151°34.388´E; 2210.9 mbsl
0—9.50 mbsf cored; 9.96 m recovered (105%)
Hole 1109B (APC):
9°30.396´S, 151°34.391´E; 2211.1 mbsl
0—14.80 mbsf cored; 15.14 m recovered (102%)
Hole 1109C (APC/XCB):
9°30.392´S, 151°34.390´E; 2211.0 mbsl
0—375.70 mbsf cored; 323.11 m recovered (86%)
Hole 1109D (RCB):
9°30.380´S, 151°34.355´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. 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 pre-rift forearc basin sediments, dips northward at ~10%.

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 the early Pliocene to late Pleistocene. 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. The dolerite has seismic velocities of 5—6 km/s. In hindsight, culminations observed on reflection seismic sections to be developed locally on the erosional unconformity– previously interpreted as reefs–are more likely to 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 Section 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 about 2 km/s.

The first (early Pliocene: NN13 and N19/20) shallow-marine (<150 m water depth) deposits 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 ohm m), has low natural gamma ray, 40—80 wt% carbonate, and average velocities of 2.5—3.0 km/s. An upper sandstone section with lower resistivity (<2 ohm m), has high natural gamma ray, 20—30 wt% carbonate, and average velocities of 2.0—2.5 km/s. Above this is an interval (570—599 mbsf) of calcareous (bioclastic) sandstone with 40—50 wt% carbonate and 45%—50% porosity. The entire sequence reflects relatively shallow-water 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/fine-grained sandstones, at 390—570 mbsf were rapidly deposited (312 m/m.y.) in upper bathyal (150—500 m) water depths until ~3.35 Ma. Porosity increases (to 60%) as carbonate decreases (to 25 wt%) upsection to ~480 mbsf, whereas velocities steadily decrease from 2.0 to 1.8 km/s 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, one that is 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 1.00 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 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) than adjacent intervals (60%—70% and 1.7 km/s, respectively).

Between 1.00 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. The site subsided to lower bathyal depths (>2 km) by NN21 (0.26 Ma).

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 36—55 mbsf is interpreted as a slump.

Velocities linearly increase with depth from 1.5 km/s at the surface to 1.9 km/s at 520 mbsf, and then more rapidly to 2.2 km/s at 590 mbsf with the increasing carbonate. Velocities increase up to 3—4 km/s and are more variable in the bioclastic sandstones and limestones between 590 and 672 mbsf, and then return to 2 km/s 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 (300—400 m/m.y.). The thermal conductivity generally mirrors the porosity-depth profile and ranges from 0.78 to 1.5 W/(m•K), except for the dolerite, which shows values up to 2 W/(m•K). Six temperature measurements down to 170 mbsf define a linear thermal gradient of 31°C/km and, when combined with an average thermal conductivity of 0.9 W/(m•K) over this interval, a heat flow of 28 mW/m .

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. 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 sub-maxima (the latter likely due to 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. Below 600 mbsf, methane content begins to decrease, 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.


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