CALCISILTITE COMPACTION IN THE DEEP OCEAN

The porosity of sediments is of importance to the study of fluid flow (both ground water and hydrocarbons) as well as in quantitative basin analysis. While silts are abundant in the clastic rock record they are consistently mixed with clay, sand, or both. This is not the case, however in the deep ocean, where silt-sized carbonate discs (coccoliths) often dominate the record. We compiled porosity results from two Ocean Drilling Project (ODP) legs which sampled sediments above the Walvis Ridge (ODP Leg 208, South Atlantic) and the Shatsky Rise (ODP Leg 198, Northwest Pacific), thus sedimentation remained above the CCD (Calcium Carbonate Compensation Depth).

We tested the impact of dilution by other grain sizes by fitting an exponential to samples that contained 90-100%, 80-90%, and 70-80% calcisiltite In both cases the samples containing 90-100% calcisiltite yielded a virtually identical result to those containing 80-90%, however, when the calcisiltite was diluted by 20-30% the results showed both an increased scatter and a shift to higher (SR) or lower (WR) porosities. This suggests that porosity is lithology dependent and that the effects of dilution are dependent on the material which is mixed in (mainly clay and/or calcareous sediments at WR and SR). Within each region, multiple boreholes were also analyzed separately. We found that the compaction of calcisiltite was quite consistent in each region. In some wells insufficient data was present to constrain the shallow portion of the curve, while in other cases accumulation rates seem to have a strong controlling influence.

The overall porosity vs. depth curves for samples with 80-100% calcisiltite in the two regions (WR and SR) were quite similar. However, porosities from WR were slightly higher than those from SR. The differences may be a result of differences in sedimentation rates or, the species of Coccolithophorid that live in the Central Atlantic as compared to the NW Pacific. For ODP Leg 208, the exponential curve is: f = 59.9exp (-z/1,722m), with a correlation coefficient of R=0.67 using 400 points. For ODP Leg 198, the exponential curve is: f =65.5exp (-z/1,754m), with a correlation coefficient of R=0.59 using 261 points.