PHYSICAL PROPERTIES

Physical property measurements at Site 1259 were conducted on whole cores, split cores, and discrete samples. Whole-core measurements conducted with the MST included GRA bulk density (2.5-cm spacing), magnetic susceptibility (2.5-cm spacing), NCR (5-cm spacing), and NGR (15-cm spacing). 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 between 310 and 480 mbsf in Hole 1259B.

Moisture and density (MAD) properties were determined on discrete samples at a frequency of one per section from all three holes at Site 1259. Sampling for MAD was reduced across critical intervals, important transitions, and throughout the Cretaceous shale sequence. A full description of the various measurement techniques is in "Physical Properties" in the "Explanatory Notes" chapter.

Density and Porosity

MAD properties determined at Site 1259 include bulk density, porosity, grain density, water content, and void ratio (Table T20). Bulk density was determined on whole-core sections using the MST (GRA density) and on discrete samples. GRA-derived density tends 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 densities for Hole 1259A was 0.155 g/cm3. In the absence of logging data, it is important to calibrate the MST-derived GRA density with the MAD measurements from the laboratory. Average differences in the MAD- and GRA-derived densities between lithostratigraphic units has been calculated for samples taken from all three holes of Site 1259 (Table T21). 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.

There is no discernible increase in the bulk density of the upper 80 mcd of sediment at Site 1259, suggesting that this interval is apparently underconsolidated (Fig. F28). Large variations in both porosity and grain density occur through the interval. A significant increase in the bulk density and a corresponding drop in porosity and grain density occurs between 80 and 100 mcd and coincide with a general coarsening of the sediments (see "Lithostratigraphy") as well as a hiatus in sediment accumulation rate (see "Sedimentation Rates"). Below this perturbation, density increases and porosity decreases with depth from 110 to 370 mcd. Through this interval, variations in density, grain density, and porosity appear to track changes in the relative proportions of carbonate and biogenic silica (see "Lithostratigraphy"). An offset in MAD data at the lithostratigraphic Subunit IIB/IIC boundary (~285 mcd) occurs through an interval of sparse recovery that marks a shift from radiolarian ooze to nannofossil chalk.

Below 400 mcd, bulk density increases significantly across the K/T boundary, which separates the clayey nannofossil chalk of Subunit IIIA from the nannofossil chalk calcareous siltstone of Subunit IIIB. Density, porosity, and velocity measurements all increase significantly in variability between 410 and 475 mcd and then gradually decrease in both magnitude and variability below 475 mcd, down to the top of Unit IV. This variability is associated with cyclic changes in the composition and degree of lithification of the sediments. A finer-scale cyclic signal in this interval is best seen in the GRA data from the MST (Fig. F29).

Unit IV is characterized by low and variable bulk densities, decreasing grain density with depth, and a peak in porosity in the middle of the unit. Two anomalously low porosity values can be matched with peaks in the bulk density and are from limestone layers commonly found interbedded through the Unit IV shale sequences. A distinct increase in porosity through the middle part of Unit IV is unexplained. The frequency of MAD sampling is not sufficient to fully characterize the variability of Unit IV, and a better representation is given by the MST data described below. Limited sampling in Unit V reveals an abrupt increase in bulk density and grain density and a drop in porosity.

Acoustic Velocity

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 1259B (Table T22).

The general depth trend of acoustic velocity correlates directly with bulk density and inversely with porosity (Fig. F28). However, variability in the velocity is more pronounced through Subunit IIC and into the top of Subunit IIIA. This variability coincides with alternations between bands of light greenish gray sediment with high clay content (low velocity) and intervals rich in zeolite and calcite (high velocity) (see "Lithostratigraphy").

Beginning in the middle of Subunit IIIA and throughout Subunit IIIB, velocities increase to values between 1800 and 3000 m/s. This large range in velocities reflects the cyclic alternations between dark clay-rich intervals and light-colored calcite intervals. Just below the K/T boundary, velocity values reach ~3200 m/s before gradually decreasing to ~1900 m/s at the transition into Unit IV. In Unit IV (calcareous claystone with organic matter), velocity of the laminated organic-rich claystones averages 1800 m/s, whereas sandstone/limestone velocities vary between 2000 and >4000 m/s (Figs. F28, F29).

Anisotropy of velocity measurements were performed on cubes taken from Hole 1259B between 310 and 480 mbsf (Fig. F29). This interval coincides with the greatest variability in the velocity, density, and porosity measurements. Anisotropy increases downhole. At 300 mcd, velocities are ~1% faster along the longitudinal z-axis, but by 350 mcd, velocities are 2%–3% higher in the transverse direction. Transverse velocity is generally 1%–6% higher in Unit III, reflecting changes in sediment composition, varying rebound effects through clayey intervals, or a combination of both (Fig. F29).

Whole-Core Multisensor Track

Poor core recovery hampers the interpretation of MST data trends in Unit I and Subunit IIA. Unit II tends to have a somewhat higher magnetic susceptibility and gamma ray signal than Unit I, with a drop in the density, magnetic susceptibility, and NGR signals at 100 mcd (Fig. F30).

The transition into the siliceous foraminifer nannofossil chalk of Subunit IIB is marked by a decrease in both the magnetic susceptibility and NGR data to background noise levels. Near the bottom of Subunit IIB, beginning at ~250 mcd, a gradual increase in magnetic susceptibility and NGR can be discerned. This increase continues across the boundary into Subunit IIC and culminates at 310 mcd, where recovery in Holes 1259B and 1259C supplement the record and a distinct series of peaks in both magnetic susceptibility and NGR sensors is seen. NGR counts through the middle part of Subunit IIC, an interval of alternating clay and zeolite/calcite–rich layers reach 20 counts per second (cps), whereas magnetic susceptibility rises to >15 (magnetic susceptibility values are reported herein as raw instrument units; see "Physical Properties" in the "Explanatory Notes" chapter for conversion of these data to SI units).

NGR and magnetic susceptibility signals are cyclical through the lower 40 m of Subunit IIC, where the P/E boundary is recovered, and are represented by a short pronounced excursion in magnetic susceptibility values to 25 and NGR emissions of 30 cps. Just below the P/E boundary, there is a 30-m gap in recovery in the upper part of lithostratigraphic Subunit IIIA.

Following this gap in recovery, a continuous spliced sequence is made (see "Composite Depths") that extends from the middle of Subunit IIIA, across the K/T boundary, through the nannofossil chalk and calcareous silstone of Subunit IIIB, and down to the transition into Unit IV. Alternating dark clay-rich intervals and light-colored carbonate intervals bracket the K/T boundary, which is seen in all three holes as a peak in magnetic susceptibility (>30) and a sharp rise to just less than 30 cps in NGR emissions.

GRA density gradually decreases below the K/T boundary. NGR emissions through this same interval remain relatively constant until ~485 mcd, where a gradual rise correlates with the appearance of the glauconitic sands transitioning into the organic-rich interval of Unit IV.

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 intervals (Fig. F31). In general, the NGR readings are highest near the top of the unit, with emissions reaching 120 cps. NGR counts decrease until ~520 mcd, where a spike in the magnetic susceptibility and GRA records represents a distinct coarse-grained glauconite-rich calcareous claystone (see "Lithostratigraphy"). Below this spike, NGR emissions vary around an average of ~35 cps.

Excellent covariance is shown between the NGR and GRA signals in Unit IV, with organic-rich layers having lower density and higher radioactivity and limestone intervals with higher density but lower radioactivity. A density minima occurs through the interval from 490 to 550 mcd (Fig F31) that may reflect higher organic carbon content (higher porosity). The magnetic susceptibility signal through Unit IV shows a small but gradual increase beginning below the glauconitic calcareous claystone at ~520 mcd. This change may indicate a concomitant change in sediment composition that explains the observed trend in GRA density.

The boundary between Units IV and V is sharp and best defined by a large increase in GRA density. An increase in susceptibility across this boundary, however, is more gradual. NGR emissions drop upon entering Unit V but remain above 25 cps.

Synthetic Seismograms

A synthetic seismogram is generated from formation velocity and density. In the absence of logging data at Site 1259 and reliable MST data from RCB cores, density and velocity measurements were obtained from MAD properties and Hamilton Frame measurements, respectively. MAD samples taken within 2.5 cm of Hamilton Frame velocity measurements were used in the calculation of downhole impedance:

velocity (m/s) x density (Mg/m3).

Downhole impedance contrasts across successive layers yield the reflection coefficient series (Fig. F32). An Ormsby wavelet was convolved with the reflection coefficient series to generate a synthetic seismogram (Fig F33).

Hole 1259A was offset 300 m from the seismic line, and changes in the depth structure of the shallowest sediments are difficult to account for over this offset; however, by aligning Reflector C on the seismic section with the prominent reflection at the base of the synthetic seismogram there is excellent agreement between marked impedance contrasts and major reflectors. Once shifted, the synthetic seismogram matches accurately to the seismic data, which allowed regional Reflectors B, B, and C to be correctly identified in the time domain (Fig. F33). Reflector C represents the base of the black shales. Reflector B matches the impedance spike found at ~520 ms (two-way traveltime) (Fig. F32), the height of distinct cyclical alternations between dark clay-rich intervals and light-colored limestone intervals (~475 mbsf) (Fig. F32), whereas Reflector B is the density and velocity step across the K/T boundary denoting the transition from Subunits IIIA to IIIB (see "Lithostratigraphy").

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