Grain Size

grain size analysis
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  GRAIN SIZE ANALYSIS Why do grain size analysis? 1) Purely descriptive, quantitative measure of sediment 2)Grain size distribution may be characteristic of sediment deposited in certain environments 3)Size distributions may provide data about transport, deposition and diagenesis of the sediment. 4)Size is important in other properties like porosity and permeability  GRAIN SIZE ANALYSIS We are primarily concerned with 3 things: 1)Measuring techniques and the expression of data in terms of a scale 2)Methods for summarizing and displaying grain size data. 3)Interpretation of the genetic significance of these data 1) We use the Udden  –   Wentworth Scale, also called the phi (   )  scale. It is a logarithmic scale because small changes are more important at small sizes than at large sizes.  GRAIN SIZE ANALYSIS 1)The Udden  –   Wentworth Scale phi (   )  = - log 2  S  Why the negative log transformation? 1)If N= a x , then X = log a  N and 2)1/N = a -x , then X = -log a  1/N 3)Since most grains are less than 1 mm we can avoid using negative phi numbers most of the time 4)A log scale makes phi increments more meaningful and useful at large grain sizes. Example  –   An arithmetic increase from 1 mm to 2 mm is a doubling but a 1mm increase from 256 to 257 mm is tiny and isn’t useful in describing large particle differences
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