Journal of Conference Abstracts

Burial History Deduced from Loading Experiments and Microstylolites in Caribbean Mixed Sediments

Ida Lykke Fabricius (iggill@pop.dtu.dk)

Dept.Geol. Geotech., DTU, b.204, DK-2800 Lyngby, Denmark

Compaction curves for 11 samples from the mixed sediments and calcareous chalk with clay from the Caribbean (ODP Sites 999 and 1001) are discussed with reference to compaction curves for calcareous ooze and chalk of the Ontong Java Plateau (ODP Site 807) (Lind, 1993a). The compaction curves follow the classical pattern for constant rate of strain tests: following a stress interval with elastic deformation, i) each sample reaches in situ stress or maximal prior stress, ii) yields, and iii) enters the normal-consolidated interval, where it undergoes plastic pore collapse. If the test is continued to high strains, work hardening sets in.

A difference in diagenetic history of the three sites is immediately apparent by comparing the velocity-depth trends (velocity-data from Sigurdsson et al. 1997). 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. The burial history is discussed from preconsolidation data and present burial conditions and suggests a removal of 400 m sediment at a hiatus (Sigurdsson et al. 1997) at 166 mbsf at Site 1001. This interpretation predicts a previous burial to more than 500 mbsf for depth intervals containing microstylolites, which corresponds to observations at Sites 999 and 807. Thus three sites from two widely separate regions indicate that microstylolites in carbonates form at minimum burial depths deeper than 500 m. No direct link between formation of microstylolites and cementation was found, suggesting that dissolution and precipitation are not necessarily related. No macroscopic stylolites were found in the argillaceous calcareous sediments of Sites 999 Site 1001, whereas wispy lamination was commonly observed. In the pure chalk sediments of the Ontong Java Plateau macroscopic stylolites occur below a burial of 830 mbsf at Site 807 (Lind, 1993b). Porosity rebound during core retrieval could not be detected for soft sediments, whereas a porosity rebound of 2% is deduced for deeper, cemented intervals.

Comparing the compaction curves, two distinct rates of porosity loss are noted: (1) Fine-grained-carbonate-supported samples (with less than 45% insoluble residue) compact at the same rate irrespective of the content of non-supporting microfossils or pore-filling clay. The slopes of the compaction curves are similar to calcareous sediments from Site 807. Each compaction curve has a distinct shift along the porosity axis, reflecting the content of microfossils (adding to porosity) and clay (diminishing porosity). The similar compaction rate is interpreted to be due to microfossils playing only an insignificant passive role during compaction; and despite the generally high clay content of these Caribbean samples (8% - 43% insoluble residue), the compaction curves are controlled by the fine-grained calcite, so that the fine-grained clay fraction appear to be moving dispersed in the pore fluid between the calcite grains. (2) Samples dominated by clay (above 45% insoluble residue) compact at a higher rate than samples dominated by fine-grained carbonate. This is probably a consequence of the bimodal distribution of fine carbonate particles and clay. The compaction of these samples appears to be controlled mainly by the fine-grained clay fraction.

Lind IL, Proc. ODP, Scientific Results, 130, 673-686, (1993a).

Lind IL, Proc. ODP, Scientific Results, 130, 445-451, (1993b).

Sigurdsson H, Leckie RM, Acton, GD, et al, Proc. ODP, Init. Repts, 165, (1997).

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