Site 1118

Hole 1118A (RCB):
9°35.110´S, 151°34.421´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 located 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. 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 pre-rift forearc basin sequence. The site is about 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.

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

The diabase conglomerate is overlain to 857 mbsf by a sequence of early 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 diabase-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 terrigeneous source of organic carbon. The whole sequence records turbiditic and hemipelagic sedimentation. It comprises mixed volcaniclastic sandstones, siltstones, and minor claystones, 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 high sedimentation rates observed.

Volcanic ash and volcaniclastic sands are ubiquitous throughout the Pliocene sedimentary section, but especially so between 3.0–3.6 Ma when a predominance of rhyo-dacitic glass reflects explosive silicic 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 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. 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 (~63°C/km), 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.


To 180 Site Summary for Site 1108

To 180 Table of Contents