Site 1115

Hole 1115A (APC [advanced hydraulic piston corer]):
9°11.389´S, 151°34.450´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]):
9°11.382´S, 151°34.437´E; 1148.8 mbsl
0–293.10 mbsf cored; 286.84 m recovered (98%)
Hole 1115C (RCB [rotary core barrel]):
9°11.383´S, 151°34.422´E; 1148.7 mbsl
0–283.5 mbsf drilled without coring; 283.5–802.5 mbsf cored; 291.56 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 pre-rift history of the Trobriand forearc basin sequence. The site was located ~35 km to the north of Site 1109 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 pre-rift 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 shows (1) a shoaling and coarsening upward, middle Miocene forearc sequence, unconformably below (2) a late Miocene nonmarine (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 <10% throughout, whereas the forearc sequence below exhibits a few normal, reverse, and strike-slip faults.

We cored the upper ~230 m of the ~5-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). Above 659 mbsf they are 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 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 sandstones (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 to 3 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. 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. The pelagic carbonate component increases above 300 mbsf, with CaCO3 concentrations increasing from ~20 to ~75 wt% at the seafloor.

Since 3 Ma the sedimentation rate markedly slowed, initially to ~79 m/m.y. (to 2 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% to 70%) near the surface. Pleistocene sedimentation was dominated by nannofossil ooze with volcanic ash. The Pleistocene ash 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 28°C/km, yielding a heat flow of 28 mW/m2 given the average thermal conductivity measured on cores from this interval of 1 W/(m•K).

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.

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. 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 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. The C1/C2 ratios generally exceed 1500 and 3000 over the same intervals, and broad maxima in ammonia occur 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. 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.


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