Sedimentological significance of strain and sonic velocity anisotropy in fine-grained turbiditic and hemipelagic deep-sea sediments — An example from the Mississippi Fan

Abstract

In visually homogeneous and compositionally similar argillaceous sediment of the Mississippi Fan, depositional processes (turbiditic versus hemipelagic sedimentation) are reflected in the anisotropy of strain as well as in sonic velocity. The correspondence between the mode of deposition and anisotropy was observed in single beds as well as in entire depositional sequences. In single beds, the turbiditic part (as opposed to the pelagic portion) is characterized by an anisotropy within bedding and a comparatively low anisotropy between vertical and horizontal directions; as current influence decreases, anisotropy within bedding disappears, while anisotropy in vertical versus horizontal directions increases. Because of rapid sedimentation, the sediments are not influenced by bioturbation that would distort the primary fabric. In depositional sequences, the anisotropy pattern depends on the type of sequence; in fining-upward sequences, anisotropy in vertical versus horizontal directions increases upsection and anisotropy within bedding is less frequent. This pattern is explained by the decreasing influence of turbidity currents on sediment fabric and an increasing proportion of quietly accumulated, hemipelagic deposits. In coarsening-upward sequences the inverse trends are found. The general correspondence between trends in lithology and anisotropy only documents relative changes in the predominant depositional process within a specific fan sub-environment. On an absolute scale, values of both types of anisotropy differ from site to site; a clear pattern within the fan is found. Thus, anisotropy determinations provide useful supplementary information for interpreting fan environments. Compaction generally leads to an increasing anisotropy, but the relative differences in anisotropy between turbiditic and hemipelagic sediments are not significantly affected.