Sediment Texture Analysis Techniques

If the sediment sample contained gravel, the entire sample was analyzed. If the sample was composed of only sand, silt, and clay, an approximately 50-gram, representative split was analyzed. The sample to be analyzed was placed in a pre-weighed 100-ml beaker, weighed, and dried in a convection oven set at 75°C. When dried, the samples were placed in a desiccator to cool and then weighed. The decrease in weight due to water loss was used to correct for salt. The weight of the sample and beaker less the weight of the beaker and the salt correction gave the sample weight.

The samples were disaggregated and then wet-sieved through a number 230, 62µ (4ø) sieve using distilled water to separate the coarse- and fine-fractions. The fine fraction was sealed in a Mason jar and reserved for analysis by Coulter Counter (Shideler, 1976). The coarse fraction was washed in tap water and reintroduced into the pre-weighed beaker. The coarse fraction was dried in the convection oven at 75°C and weighed. The weight of the coarse (greater than 62µ) fraction is equal to the weight of the sand plus gravel. The weight fines (silt and clay) can also be calculated by subtracting the coarse weight from the sample weight. The coarse fraction was dry-sieved through a number 10, 2.0 mm (-1ø) sieve to separate the sand and gravel. The size distribution within the gravel fraction was determined by sieving.

The sand fraction was dry-sieved at whole phi intervals using a Ro-Tap shaker. The fine fraction was analyzed by Coulter Counter. To mitigate biologic or chemical changes, storage in the Mason jars prior to analysis never exceeded five days. The gravel, sand, and fine fraction data were processed by computer to generate the distributions, statistics, and data base (Poppe and others, 1985). One limitation of using a Coulter Counter to perform fine fraction analyses is that it has only the ability to "see" those particles for which it has been calibrated. Calibration for this study allowed us to determine the distribution down to 0.7µ or about two-thirds of the 11ø fraction. Because clay particles finer than this diameter and all of the colloidal fraction were not determined, a slight decrease in the 11ø (and finer) fraction is present in the size distributions.

References

Poppe, L. J., Eliason, A. H., and Fredericks, J. J., 1985, APSAS: An automated particle-size analysis system: U.S. Geological Survey Circular 963, 77 p.

Shideler, G.L., 1976, A comparison of electronic particle counting and pipette techniques in routine mud analysis: Journal of Sedimentary Petrology, v. 42, p. 122-134.

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