Site 1261 differed significantly from the other four sites drilled during Leg 207. A 360-m Neogene unit of clayey nannofossil-rich material lays atop expanded highly lithified middle Eocene chalks and limestones. The bottom 60 m of the Neogene unit was a large debris flow. Below the expanded middle Eocene deposits, the lithology of Site 1261 begins to reflect that of the other sites (see "Lithostratigraphy").
Physical property measurements at Site 1261 were conducted on whole cores, split cores, and discrete samples. Whole-core measurements, conducted with the MST, included GRA bulk density (2.5-cm intervals), magnetic susceptibility (2.5-cm intervals), NCR (5-cm intervals), and NGR (15-cm intervals). Compressional (P)-wave velocity was measured in the transverse direction on split cores at ~50-cm intervals and along both transverse and longitudinal directions on cube samples taken at a frequency of two per core in Hole 1261B between ~380 and 560 mbsf.
Moisture and density (MAD) measurements were conducted on discrete samples at a frequency of one per section from both holes at Site 1261. Sampling for MAD was reduced across critical intervals, important transitions, and throughout lithostratigraphic Unit IV (Cretaceous black shale sequence). A full description of the various measurement techniques can be found in "Physical Properties" in the "Explanatory Notes" chapter.
MAD properties determined at Site 1261 include bulk density, dry bulk density, porosity, grain density, water content, and void ratio (Table T18). Bulk density was determined on whole-core sections using the MST (GRA density) and on discrete samples. GRA-derived densities tend to underestimate the true bulk density because RCB cores do not completely fill the inner diameter of the liner. The average difference between the GRA and MAD density for Hole 1261A was 0.199 g/cm3. Differences in the MAD- and GRA-derived density vary between lithostratigraphic units and have been calculated for samples taken from both holes at Site 1261 (Table T19). If the GRA data sets are to be used for quantitative purposes it is recommended that they should first be corrected to the MAD data.
The upper 300 m of Site 1261 appears normally consolidated, with bulk density increasing linearly with depth (Fig. F18). An abrupt drop in the grain density and porosity occurs at 300 mcd but is not reflected in the bulk density. Porosity and, to a greater degree, grain density are highly variable throughout a debris flow deposit between 310 and 370 mbsf (Subunit IC), reflecting the variable composition of clasts and matrix. In contrast, bulk density maintains a linear increase with depth through this interval, indicating postdepositional normal consolidation.
The calcareous chalk and limestone of Unit II has greater variability in all MAD properties (Fig. F18). This variability arises from cyclic alternations between light greenish gray and greenish gray calcareous chalks with the darker layers containing higher proportions of clay (see "Lithostratigraphy"). Porosity increases across the Unit II/Subunit IIIA boundary, coincident with the P/E boundary, and is matched by a decrease in both bulk density and grain density.
In Subunit IIIA, density increases with depth (porosity covarying), indicating normal consolidation. This is supported by the grain density, which does not change downhole. Throughout Subunit IIIB, porosity increases and bulk density and grain density decrease. Light–dark color bands, produced by changing abundances of carbonate and clay, are responsible for the variability in bulk density, grain density, and porosity in this subunit (Fig. F18).
Unit IV is characterized by low and variable bulk density, highly variable grain density, and a slight tendency toward higher porosity in the middle of the unit. Peaks in the density and grain density are associated with variably cemented limestone and sandstone intervals interbedded with the organic-rich, low-density laminated claystone.
No MAD samples were taken in Unit V.
P-wave velocity was measured on split cores using the modified Hamilton Frame apparatus. In addition, measurements of transverse (x- and y-direction) and longitudinal (z-direction) velocity were conducted on cube samples from Hole 1261B (Table T20).
The general depth trend of acoustic velocity correlates directly with bulk density and inversely with porosity (Figs. F18, F19). In Unit II, the variability in the MAD data is even more pronounced in the velocity data, reflecting the cyclical alterations of green clay-rich bands (low velocity) and light variably cemented calcite-rich (higher velocity) bands ("Lithostratigraphy"). A reduction in the variability of the velocity data and a shift toward decreasing velocity with depth characterizes the bottom 30 m of Unit II. This shift is controlled by increasing clay content downhole through the transition into Unit III.
Velocity increases to the base of Subunit IIIA, driven by the increase in density with depth, which reflects normal consolidation in this part of the section. Velocity then decreases through Subunit IIIB due to increasing clay content.
In Unit IV (black shale sequence), the average velocity of the laminated organic-rich claystones is 1700 m/s, whereas sandstone/limestone velocity varies between 2000 and 4000 m/s (Fig. F19). Unit V has high velocity (~4400 m/s) (Fig. F19), consistent with a well-indurated quartz sandstone lithology.
MST data through the clay-rich sequences of Subunits IA and IB have a high and constant NGR signal of 50 counts per second (cps) and an average magnetic susceptibility of 30 (magnetic susceptibility is reported here as dimensionless instrument units. See "Physical Properties" in the "Explanatory Notes" chapter for conversion of these data to SI units), matching the uniformity in sediment composition. NGR and magnetic susceptibility become more variable in Subunit IC, and a distinct and abrupt drop in both between 320 and 340 mcd corresponds to an expanded light green sequence in the debris flow of Subunit IIC through Cores 207-1261A-16R and 17R (Fig. F20).
A drop to background noise levels of both the magnetic susceptibility and NGR values marks the transition into the calcareous chalk and limestone of Unit II. At ~450 mcd, there is a rise in both the NGR and magnetic susceptibility signals that continues across the boundary with Subunit IIIA. GRA density is highly variable through Unit II, reflecting the changing sediment composition (alternating high–low clay content). This interpretation is supported by the comparison between GRA density and MAD bulk density (Table T19).
Crossing into Subunit IIIA, a spike in both NGR and magnetic susceptibility marks the P/E boundary. GRA density drops abruptly across this interval and then increases slowly downhole through Subunit IIIA. A distinct drop in magnetic susceptibility and NGR emissions occurs at ~530 mcd. From here, NGR emissions fluctuate across the hiatus separating the Paleocene and Cretaceous (Subunit IIIA/IIB boundary). NGR counts increase downhole through Subunit IIB with a concomitant decrease in GRA density (Fig. F20), reflecting increasing clay content.
Unit IV is characterized by highly variable GRA and NGR signals that correspond to the alternation of laminated organic-rich intervals and cemented limestone and sandstone (Fig. F20). NGR emissions are highest between ~560 and 585 mcd. Another distinct aspect of the NGR signal is its slow rise near the bottom of Unit IV, where values gradually climb back to levels seen at the top of the sequence. At the contact with Unit V, NGR emissions drop to instrument noise levels. Magnetic susceptibility remains relatively constant through Unit IV, with a narrow spike at ~ 605 mcd, representing a glauconite-rich calcareous claystone (see "Lithostratigraphy"). This spike is preceded by a ~10-m interval devoid of any measurable magnetic susceptibility signal (Fig. F20). The GRA profile through the shale sequence shows two minima between ~590 and 600 mcd and between 630 and 640 mcd; these broadly correlate to peaks in MAD-derived porosity (Fig. F18).
The boundary between Units IV and V is sharp and best defined by the increase in GRA density and drop in NGR emissions. There is no discernable change in magnetic susceptibility across this interval.