A comparison of settling and sieve techniques for the analysis of bioclastic sediments

This paper examines differences in the use of sieve and settling techniques for the determination of textural parameters of grain-size distributions for 30 bulk bioclastic (reefal) deposits and examines the interpretive use of the results. A comparison of results of the key textural parameters shows that sieve-size estimates deviate from settled-size determinations of mean grain-size and dispersion values by up to 2 and up to 1.5 phi units, respectively. These differences are explained by the different grain properties measured by the two techniques exacerbated by the heterogeneous character of bioclastic deposits. Changes in sample components which control grain shape, and grain bulk-sample density, also influence the magnitude of size and dispersion differences. Observed differences are discussed in relation to interpretation of the depositional environment and hydraulic regime, leading to the conclusion that sieve results can grossly distort depositional and energy processes operating within bioclastic environments. In contrast, settled-size estimates (an index of deposit hydraulic behaviour) show that bioclastic sediment transport systems operate within a much narrower energy regime than sieve-sizes suggest. Hydraulic-size estimates are considered to provide a better tool with which to interpret sedimentary processes in bioclastic environments.