Grain size distribution is one of the most important characteristics of sediment. This is true because grain size is a powerful tool for describing a site’s geomorphic setting, interpreting the geomorphic significance of fluid dynamics in the natural environment, and distinguishing local versus regional sediment transport mechanisms as well as because grain size is a dominant controlling factor in sediment geochemistry. Cations derived from mineral weathering and pollution sources are preferentially adsorbed onto clay, which has the highest surface area to volume ratio of any particle size class.
The grain size distribution of a sediment sample is determined by any one of a number of techniques or combination of techniques depending on the range of sizes present in a sample. For this procedure, “fines” are any particle smaller than 63.5 microns. Sand is 63.5 microns to 2 mm in diameter. Gravel is larger than 2 mm.
Characterizing the physical properties of sediment is important in determining its suitability for various uses as well as studying sedimentary environments and geologic history.
The physical properties of sediment can be described by several parameters. Grain size is the most important of these and is the main way in which sediment (and clastic sedimentary rocks) is classified. Other commonly used properties of sediment are sorting and shape (roundness and sphericity). All of these properties are important in describing sediment and determining its suitability for various uses, such as a construction aggregate or fill for a beach volleyball court.
Standard grain size terms of sedimentary particles and the arbitrary boundaries between them are as follows:
Grain size analysis procedure:
1.) Label two plastic trays with the sample number and time of sampling.
2.) Open a ziplock bags we collected on the boat and use a large table spoon to transfer two representative, similar size and samples onto the two plastic trays.
3.) Weigh both trays and put one tray into the oven @ 50oC. Enter weights on spreadsheet.
4.) Assemble the two sieves with the coarse one up over a sink in the lab.
5.) Transfer the sediment from the 2nd tray onto the top sieve and use tap water to flush the finer particles through the sieves. Shake the sieves during this process.
6.) The fraction <63µm will go through both sieves and will be flushed down the drain. The fraction >63µm, but <2mm will remain on the bottom sieve, and the fraction >2mm on the top sieve.
7.) Label 2 additional trays with sample number and ‘coarse’ and ‘medium’, respectively.
8.) Use a brush and perhaps water to transfer those coarse and medium fractions onto the trays.
9.) Clean sieves etc.
10.) Put samples into oven and let dry overnight.
11.) Weigh all fractions the next days and enter data on spreadsheet.
12.) Clean the trays – so that they can be reused.
13.) Determine relative weight fractions of clays and silt (<63um, fine), sand (<2mm, >63um, medium), and gravel (>2mm, coarse). Calculate fine, medium and coarse fractions from the spreadsheet. Express the results in %.