The influence of microfossil content on the physical properties of calcareous sediments from the Ontong Java Plateau

Abstract

This work is based on results from Ocean Drilling Program Leg 130, which drilled the Ontong Java Plateau, a broad submarine plateau in the western equatorial Pacific. The Ontong Java Plateau has a unique combination of geographic location and bathymetry that makes it ideally suited for paleoceanographic studies. This study addresses several aspects of plateau sediment physical properties (e.g., porosity, velocity, acoustic impedance, compression and expansion indexes, and rebound) within the broad framework of establishing relationships between physical properties and microfossil content and preservation. Samples obtained in Hole 803D were analyzed to determine their microfossil constituents. The resulting data are compared to shipboard-measured physical properties data to assess the relationships between small-scale fluctuations in physical properties and microfossil content. Impedance was found to increase with increasing grain size and planktonic foraminifera content. Variations in the coarse fraction constituents appear to have a more significant effect on the physical properties than do variations in the fine fraction constituents, though the fine fraction make up greater than 85% of the samples by weight. Many of the seismic reflectors identified by shipboard scientists could be related to changes in the relative percentages of microfossil constituents. The consolidation behavior of Ontong Java Plateau sediments was observed to relate to sediment composition. In general, consolidation test parameters from this study are consistent with those of other researchers for sediments of similar carbonate contents. We found that both compression and expansion indices decrease with increasing carbonate content and with increasing foraminifer content. Rebound curves derived from consolidation tests on Ontong Java Plateau samples yield porosity rebounds of 1% to 4%for these sediments at equivalent depths of 200 to 1200 meters below seafloor (mbsf), The exception is a radiolarian-rich sample that has 6% rebound. We combined the rebound correction derived from the porosity rebound vs. depth data with the correction for pore-water expansion to correct the shipboard laboratory porosity data to in-situ values. The rebound-corrected laboratory data can be used as in situ data in place of missing or erroneous downhole logging data.