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doi:10.2204/iodp.proc.323.106.2011

Physical properties

Site U1342 was spudded at a water depth of ~830 m just south of the summit area of Ulm Plateau, a wave-beveled platform cresting the western end of Bowers Ridge. Seismic reflection data reveal a thin blanket of sediment overlying the ridge's basement framework, which is known from a few dredge hauls and from coring to be constructed of arc igneous rock older than middle or early late Miocene. Recovered cores from Holes U1342A–U1342D were sectioned and placed on the Special Task Multisensor Logger (STMSL) "fast track" to record whole-core magnetic susceptibility and GRA bulk density values. Core sections were allowed to warm to laboratory temperature and were then scanned on the Whole-Round Multisensor Logger (WRMSL) for GRA, magnetic susceptibility, and P-wave measurements. Only sediment samples from Hole U1342C were collected to measure moisture and density (MAD) properties with shipboard MAD procedures.

The sediment blanket at Ulm Plateau was sampled in Hole U1342C to ~45 mbsf. At this subsurface depth, basement of dominantly volcaniclastic rock was encountered. In Hole U1342D, basement was cored to a subsediment depth of ~83 m (44–127 mbsf). The top ~18.6 m of basement was drilled without coring, below which 13 XCB cores recovered ~41 m of dominantly volcaniclastic rock.

The sedimentary column was divided into two lithologic units (see "Lithostratigraphy"): Unit I (silt and laminated diatom ooze; surface to ~36 mbsf) and Unit II (sponge spicule– and diatom-bearing sand; ~36–45 mbsf).

Magnetic susceptibility

Figure F29 shows a downhole trend of decreasing magnetic susceptibility, from a near-surface value of ~100 SI units to about half this value at ~35 mbsf, the base of lithologic Unit I. Below this depth, magnetic susceptibility readings in Unit II increase in steps to as high as 350 SI units at 42 mbsf. The depth profile of magnetic susceptibility readings varies somewhat rhythmically between high and low excursions ranging from ~10 to 200 SI units in Unit I and from 50 to 350 SI units in Unit II (Fig. F29).

The cyclic fluctuations in Unit I readings presumably record alternating beds of dominantly biogenic and siliciclastic components. The downhole decreasing strength of magnetic susceptibility possibly records accumulating diagenetic degradation of magnetic properties. The higher values of Unit II likely track the downhole-increasing abundance of sandy material derived from the underlying volcanic basement, which a few recovered microfossils suggest was wave-base leveled sometime in the early late Miocene (see "Biostratigraphy" and "Lithostratigraphy"). In Hole U1342D, at a within-basement depth of ~73–74 mbsf (29–30 m below the sediment/basement contact), magnetic susceptibility values are much higher and average ~1500 SI units. At ~116–117 mbsf (~72–73 m into basement), magnetic susceptibility readings average ~750 SI units.

GRA wet bulk density

The WRMSL GRA data and MAD discrete sample measurements in Figure F30 show a steady increase in density with depth. The overall increase in density is from 1.35 g/cm3 near the seafloor to 1.65 g/cm3 at ~36 mbsf, which is the base of the silt and diatom ooze of Unit I. Below this depth, bulk density increases at a steeper gradient to ~1.90 g/cm3 just above basement at ~45 mbsf. The steeper gradient is within the sandy sediment of Unit II, which is presumably granular material derived during the final wave-beveling stage of destruction of a stratavolcano that formerly rose above Ulm Plateau. Core inspection implies that the increased density at the base of Unit II is a consequence of higher contents of siliciclastic and rock volcanic debris compared to biogenic components, in particular diatom frustules.

P-wave velocity

P-wave velocity increases downhole from near-seafloor readings of just over 1.45 km/s to ~1.55 km/s in Unit II near the contact with basement rock at ~45 mbsf (Fig. F31). In the sandy sediment of Unit II in Hole U1342D, P-wave velocity increased to ~1.60 km/s and somewhat higher. Velocity recorded in this unit is at a minimum because of serious drilling disturbance and the in-mixing of water.

Natural gamma radiation

The natural gamma radiation (NGR) readings displayed in Figure F32 increase irregularly downhole from a near-seafloor average of ~18 counts/s to ~24 counts/s at the base of Unit I. The gradient of increasing values steepens within the sandy beds of Unit II to reach ~40 counts/s just above basement contact at ~44 mbsf. The elevated NGR readings presumably record a downsection increase in clay minerals that may be derived within Unit II from the underlying bedrock of arc lava and volcaniclastic deposits. Rock clasts recovered in core sections exhibit evidence of oxidation. Clay minerals that account for the higher NGR readings in the basal sediment of Unit II presumably reflect subaerial weathering of the stratavolcano that towered above Ulm Plateau prior to its presumed early late Miocene destruction. At ~72–73 m into basement (~116–117 mbsf) in Hole U1342D, NGR readings are quite low, ranging from ~1 to 6 counts/s, but the cored volcaniclastic sandstone and breccia are commonly oxidized and red in color.

MAD (discrete sample) wet bulk density

MAD physical properties of wet and dry bulk density, moisture content, porosity, and grain density were measured on sediment samples (~10 cm3) taken from the working halves of split core sections. The samples were routinely collected from Sections 2, 4, and 6 of Hole U1342C cores (Table T14). The measured values of wet bulk density are shown in Figure F30 as open circles plotted with WRMSL GRA bulk density. In general, MAD values track those of GRA but with much greater measurement spacing. The contrast between Unit I and denser Unit II sediments is nevertheless clearly shown by the MAD data.

MAD porosity and water content

As shown in Figure F33, sediment porosity and water or moisture content track in parallel, decreasing downhole. The near-surface porosity is ~80%, but porosity decreases to ~55% in the sandy sediment of Unit II and just above basement at 44 mbsf. The downhole trend of overall decreasing porosity and water content presumably reflects compaction of the section, with notable excursions to higher values in the basal 10–12 m of Unit I that may identify higher relative content of siliceous microfossils.

Grain density

Grain density decreases downsection in Unit I from ~2.82 to 2.53 g/cm3 and possibly lower (Fig. F34). Grain density in underlying Unit II (siliceous microfossil–bearing sand) rapidly increases downhole to ~2.70 g/cm3 as bedrock is approached. The elevated grain density in Unit I, as compared to the upper beds of Site U1341 on the deeper flank of Bowers Ridge, can be attributed to the availability of nearby bedrock sources for Site U1342 deposits. Seafloor exposures of probably pre-Miocene volcaniclastic and lava units or their occurrence in the shallow subcrop most likely occur on the summit area of Ulm Plateau, located ~10 km northwest of Site U1342. The rapid excursion near the base of Unit I to a low density of just over 2.2 g/cm3 is probably a spurious measurement.