|Open-File Report 2005-1001|
|OFR 2005-1001 Home / Procedures / East-Coast Database / GIS Data Catalog|
Geological Survey Open-File Report 2005-1001
USGS East-Coast Sediment Analysis: Procedures, Database, and GIS Data
Coarse Fraction Analysis (Gravel Plus Sand)
Dry the coarse fractions in a convection oven at, or slightly under 100oC. When the samples are dry (overnight is usually sufficient), place them in a desiccator to cool before weighing them. Gross coarse weight minus the weight of the beaker gives net coarse weight.
Much of the biogenic calcite found in a sample is commonly in the form of coarse pelecypod or gastropod shell fragments. Rather than treating the bulk sample with acid, it is often easier and less time consuming to manually remove this carbonate from the washed coarse fraction and to re-weigh the decalcified portion to determine the new net coarse weight. The weight of the carbonate must be subtracted from the net sample weight before entering the data into the computer.
The grain-size distribution within the coarse fraction can be determined by use of a Rapid Sediment Analyzer (that is, settling tube) (Schlee, 1966). However, if this fraction weighs less than 10 g or if it contains greater than about 2% foraminifera, which will not settle properly in a sedimentation column, then wire-mesh sieves must be employed. If the coarse-fraction grain-size distribution is to be determined by sieving, assemble a bank of sieves, which generally consists of a cover, the -1, 0, 1, 2, and 3 phi sieves, and a catch pan. The bank of sieves is agitated in a mechanical shaker for a minimum of 15 minutes. Although Mizutani (1963) and McManus (1963, 1965) recommend a sieving time of 35 minutes for 8-inch diameter sieves, Royse (1970) suggests 10 minutes as an adequate sieving time. After sieving, weigh the individual sand and gravel phi fractions and record the weights from each phi class. Net coarse weight minus the gravel weight gives the sand weight. If material coarser than 2 mm is present, assemble a bank of gravel-fraction sieves (the cover, the -5 through -2 phi sieves, and a catch pan); sieve to separate, weigh each phi class, and record the weights. Calculate the relative percentages of the sand-fraction phi classes and the relative percentages of the gravel-fraction phi classes. Before 2005, the sieve data were normally entered directly into a computer using the program RSAM to obtain a complete grain-size distribution. Currently , the sieve data are entered into the GRAINSIZE.XLS spreadsheet and a function of this Microsoft Excel file calculates the relative percents.
Sieving efficiency increases with reduction in load (that is, smaller sample size) and the maximum sieve load diminishes with mesh size, but it is the quantity of finer near-mesh size particles that actually determines sieving efficiency (Royse, 1970). Twenhofel and Tyler (1941) recommended 40 g of sandy sediment as the maximum load for 8-inch diameter sieves. When selecting a procedure, it is important to remember that sieves sort material not only according to size, but also according to the shape and roundness of the particles (Sahu, 1965; Kennedy and others, 1985).
The settling tube (Zeigler and others, 1960; Zeigler and others, 1964; Schlee, 1966, Syvitski and others, 1991) provides a means for rapid analysis of sand-sized material by settling the grains through a column of water. One common settling tube design is based on using the pressure differential between two columns of water that have a common head. The pressure change caused by the introduction of sediment within one of the columns is measured by a Validyne DP-103 variable-reluctance wet-wet pressure transducer and amplified by a Validyne Model CD-23 frequency demodulator. Results are relayed to a Dell Dimension personal computer running under Windows 98 and equipped with an ADAC Corporation 5500HR-1 analog-to-digital data acquisition board and associated ADLIBWIN and ADLIBPC data acquisition software drivers. As the sediment settles past the pressure transducer port, the pressure differential decreases with time. In accordance with Stoke's Law, sedimentation rate is a function of grain size, one can interpret the sand-fraction grain size distribution from the variation in pressure differential. In addition to the settling tubes that use pressure transducers, other devices incorporate a balance, and measure changing weight with time as the sediment settles.
If the sand-fraction grain-size distribution is to be determined by settling tube and the program RSA2000, the gravel-fraction distribution, if present, must still be determined by sieve analysis. To use a settling tube with a pressure transducer, a warm-up and stabilization time of about an hour is required for the transducer and associated electronics. During this time the operator should check the system pressure lines for air bubbles and the settling column water level and turn on the computer and initialize the program RSA2000. Remove all air bubbles from the lines; raise the water level to 3-5 mm above the actual settling tube.
A micro-splitter or cone-and-quartering is used to obtain a sample of about 10 g. Smaller samples generate too weak a signal from the pressure transducer; a weak signal may be diluted by electrical and mechanical noise causing greater error in the reproducibility of the data. Samples larger than 10 g are more apt to form density currents down the sides of the settling tube or, upon introduction, to form clumps of finer sediment which act as much larger particles. The sample must be dampened with just enough water to afford enough inter-grain cohesion to insure simultaneous introduction of the sample, and to prevent air from becoming trapped in the sample, which may slow the settling rate of the particles.
Sample introduction into the settling tube simply involves placing the split into a tablespoon, dampening the sample with an eyedropper, and pouring the sample in one smooth, rapid motion from a height of about 3 cm down the center of the settling tube. If the operator misses the center and introduces the sample near the wall of the tube, a density current that flows down the side of the tube will form and the sample will appear to be coarser than its true distribution. If the operator does not introduce the entire sample simultaneously, the sample will appear to be finer than its true distribution. The operator is encouraged to run a few standards to check for equipment problems and practice sample introduction. This exercise will help the operator to produce more accurate and reproducible analytical results. After the sample has settled to the bottom of the tube and the pressure differential returns to approximately zero, a hard copy of the sand-fraction grain-size distribution that is generated by the RSA2000 program can be printed, and the data can be saved to disk.
Modifications Since 2005
The program RSAM is no longer used during sieve analysis. Coarse fraction data are entered directly into the GRAINSIZE.XLS spreadsheet. This Microsoft Excel spreadsheet generates normalized distributions by combining the whole-phi-interval data from the gravel, sand, silt, and clay fractions obtained during typical sediment grain-size analyses.
|Fine Fraction Analysis|