Settling behaviour related to sieve analysis of skeletal sands

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ABSTRACT

One thousand grains selected from sieved samples of a bioclastic sand have been individually measured, weighed and timed in free unhindered fall at terminal velocity in a 250 cm column of sea water.

Four fall regimes are represented: straight fall, spinning and spiral modes and erratic tumbling. As size increases grains pass through this series at rates dependent upon shape and effective density.

Computed best-fit curves for velocities/intermediate diameter and an equivalent sphere/intermediate diameter illustrate considerable divergencies in behaviour between the eleven grain types and five shape classes examined. Current methods of analysis of carbonate sediments, by sieving, by grain-counting of components, and by sedimentation balance, provide different kinds of information which it is impractical to consider as having any simple relationship to each other.

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