Velocity and Density of Sediments of Eirik Ridge, Labrador Sea: Control by Porosity and Mineralogy

A 767-m section of late Neogene (0-8 Ma) terrigenous sediments was cored at Ocean Drilling Program (ODP) Site 646. Continuous downhole geophysical logs, 161 laboratory measurements of core porosity and density, and 63 laboratory measurements of core velocity are used to analyze in detail the effects of porosity and mineralogy on the acoustic properties at this site. Porosity (determined from a resistivity log) agrees well with rebound-corrected laboratory measurements. Mineralogical variations (potassium feldspar, quartz plus plagioclase, calcite plus opal, and clay minerals) for the interval 206-737 mbsf were determined by matrix inversion of three logs: bound water, potassium, and uranium/ thorium ratio. These calculated mineralogical variations are similar in major features to mineral abundances from smear slides, but the wide depth spacing of smear slides and their subjective, semiquantitative mineral abundances preclude a detailed comparison. Calculated grain densities from mineralogy are consistent with laboratory measurements. A pseudodensity log from porosity and grain density is similar in character to the rebound-corrected, bulk-density measurements on cores, but about 0.1 g/cm lower than core measurements in the interval 340-737 mbsf. We found from our analyses that a strong synergy exists between downhole geophysical logs and core measurements of porosity and density: (1) core recovery is best at shallow depths, and logs are more reliable at greater depths; and (2) agreement between laboratory and log measurements corroborates the different assumptions made when analyzing the two data types. At Site 646, this synergy does not extend to laboratory measurements of velocity; laboratory velocities are lower than in-situ velocities, but higher than expected when rebound is considered. Observed trends of laboratory and log porosity, density, and velocity as a function of depth at Site 646 are in reasonable agreement with empirical trends. In contrast, empirical relationships of velocity to porosity do not agree well with our data. Application of Hookean elastic equations to our data is hampered by the lack of shear wave velocities and the sensitivity of the technique to small errors in porosities. Nevertheless, this theoretical approach yields a pseudovelocity log that agrees remarkably well with observed in-situ log velocities. INTRODUCTION The terrigenous sediments of the northeast Labrador Sea provide a high-resolution record of Neogene changes in bottomwater activity. During Leg 105, two holes were drilled at Site 646 (58° 12.56'N, 48°22.15'W) on the Eirik Ridge, in a water depth of 3451 m. Hole 646A was hydraulic-piston cored to a depth of 103.5 mbsf, and Hole 646B was cored with both the hydraulicpiston corer and extended-core barrel to a total depth of 766.7 mbsf. Both holes were continuously cored and had an overall average recovery of 55.7%. Here, we focus on the continuous downhole geophysical logs and discrete physical-properties measurements at Site 646. Our goals are to evaluate (1) the reliability of both types of measurements, (2) the extent to which velocity and density of these sediments are controlled by mineralogy and porosity, and (3) the consistency of these data with published empirical and theoretical models of porosity, velocity, and density variations as a function of depth and each other. In a companion paper (Busch, this volume), the porosity and compaction trend at Site 646 is analyzed in the context of trends at Sites 645 and 647 and in other terrigenous sequences. In a second companion paper (Jarrard and Arthur, this volume), the paleoceanographic implications of porosity and velocity variations at Site 646 are considered. 1 Srivastava, S. P., Arthur, M. , Clement, B. , et al., 1989. Proc. ODP, Sci. Results, 105: College Station, TX (Ocean Drilling Program). 2 Lamont-Doherty Geological Observatory of Columbia Univ., Palisades, NY 10964. 3 Graduate School of Oceanography, Univ. of Rhode Island, Narragansett, RI 02882. 4 Department of Geology and Geophysics, Univ. of New Orleans, New Orleans, LA 70148. Shipboard measurements of physical properties of core samples at Site 646 include 161 measurements of bulk density, grain density, and porosity, and 63 measurements of compressional wave velocity. The techniques used for density/porosity measurements (pycnometer) and velocity measurements (Hamilton frame velocimeter) are described in Srivastava, Arthur, et al., (1987a). With pipe set at 205 mbsf, the openhole interval from 205-742 mbsf at Hole 646B was logged with Schlumberger longspaced sonic, spherically focused, dual-induction, gamma ray, and caliper tools. Nearly the entire drilled interval from the seafloor to 743 mbsf was logged with spectral gamma ray, neutron porosity, and gamma spectroscopy tools (Srivastava, Arthur, et al., 1987b). In general, the quality and depth distributions of these physical-property measurements and geophysical logs complement each other. Because of decreasing core recovery downhole, 49% of the porosity and density measurements are from the upper 206 m of the site. In contrast, geophysical logs were of higher quality and number in the openhole interval below 206 mbsf; the through-pipe logs were not used in our analyses. Shipboard interpretation of both logs and physical-properties measurements are discussed in Srivastava, Arthur, et al., (1987b). The sedimentary sequence penetrated at Site 646 consists almost entirely of silty clays and clayey silts, deposited at a relatively uniform sedimentation rate of 90 m/m.y. during the last 8.5 m.y. (Srivastava, Arthur, et al., 1987b). The sequence is divided into two lithologic units on the basis of visual core descriptions and smear slides. Lithologic Unit I (0-236.4 mbsf, Holocene to late Pliocene) is divided into two subunits. Subunit IA (0-188.2 mbsf) consists of silty clays and clayey silts containing up to 40% carbonate and frequent ice-rafted dropstones. Subunit IB (188.2-236.4 mbsf) consists of poorly sorted muddy sands and silty muds, with frequent dropstones and an average of 10% carbonate. Glacial/interglacial cycles apparent in Sub-