10. Site 1116¹

Shipboard Scientific Party²

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. 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. 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. 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. 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.