Preferred Orientation and Velocity Anisotropy in Marine Clay-Bearing Calcareous Sediments

In order to better understand the role of preferred orientation of calcite in the compressional velocity (V _p ) anisotropy of calcareous marine sediments, ultrasonic V _p and x-ray pole figure goniometry measurements were made on selected laminated calcareous claystones, laminated clay-bearing limestones, and nonlaminated limestones from various DSDP sites. Although all samples exhibit V _p anisotropy (up to 20%), none exhibit calcite-preferred orientation. Thus, V _p anisotropy in these calcareous sediments is not caused by calcite-preferred orientation, in agreement with findings of other researchers. Pole figures and thin section observations of the laminated carbonate samples indicate that poles to (001) of kaolinite and illite are strongly aligned normal to bedding. Clay-preferred orientation is probably responsible for some of the observed V _p anisotropy. The V _p anisotropy in calcareous claystones is found to be correlated to calcite content, in contrast to the relation found by others for pelagic chalks and limestones, suggesting a dependence upon lithology. Most of the anisotropy in laminated calcareous claystones appears to be controlled by flat pores oriented parallel to bedding, which could slow acoustic waves traveling perpendicular to bedding. Pelagic chalks and limestones tend to have irregularly shaped pores that do not affect anisotropy in the same way as in calcareous claystones.