When the mechanism of turbidity currents was first proposed, it was thought that ooze deposits, which alternated with coarse to fine sand, were deposited on abyssal plains by pelagic sedimentation between turbidity current pulses. Later it was thought that concentrations of calcareous bioclastic material in the upper part of the ooze pointed to turbidity current deposition of the sand and lower part of the ooze, with only the upper ooze a result of pelagic sedimentation. Because CaCO <sub>3</sub> tests dissolve in sinking, no calcareous tests of upper ocean origin are found below 5500 to 5600 m. However, a study of terrigenous turbidites from abyssal plain samples taken at depths exceeding 5900 m which follow the sand to ooze sequence with foraminiferal concentrations in the upper ooze demonstrate that the beds are turbidite alone. It is hypothesized that any pelagic deposits forming between turbidity currents would be eroded away at the beginning of each pulse and that the fossil concentrations in the upper ooze result from delayed settling of the gas or protoplasm filled tests.

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