The burial diagenesis of calcareous sediments has been widely studied (e.g., Schlanger and Douglas, 1974; Scholle, 1977; Maliva and Dickson, 1992; Borre and Fabricius, 1998) and linked to geotechnical properties (e.g., Hamilton, 1976; Lind, 1993a; Audet, 1995) or acoustic velocities and physical properties (e.g., Mayer, 1979; Hamilton et al., 1982; Kim et al., 1985; Urmos et al., 1993). The aim of the present paper is to compare compaction properties for the mixed sediments of the Caribbean to compaction curves obtained in a pure carbonate lithology, primarily from the Ontong Java Plateau, to interpret the role of textural differences and burial history. The role of rebound of core material after removal from in situ conditions is also discussed. Caribbean Leg 165 Sites 999 and 1001 were chosen because they both penetrated long sections of pelagic clayey mixed sediments. Leg 130 Site 807 from the calcareous sediments of the Ontong Java Plateau was selected as a reference.
The differences in diagenetic history of the three sites is immediately apparent by comparing the velocity-depth trends (Fig. 1). The sonic velocities indicate soft sediments down to a depth of ~570 meters below seafloor (mbsf) at Site 999, down to ~170 mbsf at Site 1001, and down to ~1100 mbsf at Site 807. Below these depths, the sonic velocity indicates indurated lithologies. Differences in burial history may be determined from preconsolidation measured by loading experiments and from the occurrence of stylolites.
As shown by Jacobsen (1972), preconsolidation is difficult to assess from consolidation tests. This difficulty is illustrated by data on the overconsolidation ratio (OCR) of Masters and Maghnani (1993) and Moran (1997). OCR is the ratio between the preconsolidation stress and the present effective overburden stress. OCR data for calcareous ooze of the Ontong Java Plateau range between 0.7 and 3.6 (Masters and Maghnani, 1993). OCRs of 0.3-0.5 for calcareous ooze of the Ceara Rise (Moran, 1997) suggest that the ooze is underconsolidated to a surprising degree.
An independent estimate of previous burial may be obtained from the occurrence of wispy lamination or flaser structures, which may be interpreted as solution seams (Garrison and Kennedy, 1977). They typically form as microstylolitic swarms in limestones containing >8%-10% clay, whereas in purer limestones, stylolites form (Bathurst, 1987). At the Ontong Java Plateau, wispy laminations were observed below 490 mbsf at Site 807 and below 630 mbsf at Site 806 (Kroenke, Berger, Janecek, et al., 1991; Lind, 1993b), and macroscopic stylolites below a burial of 830 mbsf at Site 807 (Lind, 1993b).
The physical properties measured in the laboratory will differ from in situ values if the core material expands as a result of the stress release during core retrieval. This rebound may be assessed from consolidation tests. Hamilton (1976) described a procedure to estimate rebound from the unloading part of compaction curves. For calcareous sediments he found a rebound (increase) in porosity ranging from zero near the seafloor to 5% at a depth of 500 mbsf, whereas for pelagic clay, he found a larger rebound (increase) in porosity ranging from zero near the seafloor to 7% at a depth of 300 mbsf. In general accordance with these results, Moran (1997) found a porosity rebound of 6% at a depth of 300 mbsf for the calcareous sediments of the Ceara Rise, and Masters and Maghnani (1993) found a rebound in porosity ranging from 1% to 4% in calcareous ooze of the Ontong Java Plateau (Leg 130). By contrast, using a compaction cell designed for high stresses, Lind (1993a) found a rebound of <0.1% for the calcareous ooze and chalk of the Ontong Java Plateau down to depths of 945 mbsf. This difference may be attributed to a difference in elasticity of the applied apparatus. If the compaction cell by itself has a significant bedding and deforms elastically, it becomes difficult to distinguish bedding of the apparatus from elastic deformation of the sample. Hamilton (1976), Masters and Maghnani (1993), and Moran (1997) used oedometers, which may imply significant bedding of the compaction cell (Jacobsen, 1972). This problem may have been overcome in the high-stress instrument used by Lind (1993a) and in the present study.