Previous   |    Next

doi:10.2204/iodp.proc.310.203.2009

Conclusions

Measurements of the petrophysical properties, quantitative mineralogic composition, observations on the texture, and subsequent subdivision into petrophysical groups of 79 rock specimens of young Pleistocene and Holocene sediments have important implications for the understanding of the acoustic behavior of mixed carbonate-volcaniclastic rocks.

Primary control on the acoustic properties is exerted by porosity. The remaining variation in acoustic properties is explained by the mineralogical and textural parameters: the presence of volcaniclastic material and diagenetic alteration. No clear linear thresholds are defined; however, a general trend is that insoluble residue and fringing cement have opposite and overlapping effects on acoustic properties. Where clay minerals and organic material have a negative effect on acoustic velocity, introduction of magmatic minerals in the matrix results in positive deviation in velocity from general trends. Within carbonate-lithoclastic sand the amount of fringing cement controls acoustic velocity.

Common velocity-porosity transforms by Wyllie et al. (1956) and Raymer et al. (1980) only imperfectly explain the observed relationship between velocity and porosity in the data set. Clean carbonate samples generally fall onto Wyllie’s curve; magmatic minerals cause positive velocity deviations. Gardner’s experimental curve underestimates the velocity-density relation. Poisson’s ratio is introduced as a tool to identify maturation overprint of various sample sets from acoustic data alone. High Poisson’s ratios (~0.32) correspond to immature data sets, whereas data groups which experienced burial diagenesis display lower Poisson’s ratios (~0.25).

These findings suggest a more complicated relationship for mixed carbonate-volcaniclastics than that earlier documented for pure siliciclastics. Reasons for this may be the mineral mixing and the property of carbonate minerals to form more different intercrystalline boundaries under diagenetic alteration. The results of this quantitative analysis may have important implications for the prediction of porosity from acoustic velocities in similar mixtures of sediment. In addition, they may provide a direct link between acoustic properties and the primary depositional system and sequence stratigraphic history of the studied interval thereby connecting geology and seismic.

Top of page   |    Previous   |    Next