2. • Grain size is a fundamental attribute of siliciclastic sedimentary rocks
and thus one of the important descriptive properties of such rocks.
• variation in grain size reflects :
(i) weathering & erosion process
(ii) Transport process
Some aspects for grain size analysis:
1. Techniques for measuring grain size and expressing it in terms of some type of
grade scale( Phi scale ),
2. Methods for summarizing large amounts of grain size data and presenting
them in graphical or statistical form so that they can be more easily evaluated
(cumulative graph), and
3. Interpretation of those data or significance.
3. (i) GRAIN SIZE MEASURMENT & GRADE SCALES:
Grain size can very form few microns to few meters.Logarithmic or
geometric scales are more useful for expressing size than are linear
scales.
(i) Udden-Wentworth scale (Udden in l898,Wentworth in 1922):
The scale rages from <1/256 mm (0.0039 mm) to >256 mm and is
divided into four major size categories (clay, silt, sand, and gravel)
(ii) Phi scale(Krumbein in l934):
φ =−log2 𝑑 ( d= diameter of the particles)
Lets say , d= 2 mm, then φ= -1
and for d= 1
2 mm, then φ= 1
7. Histogram—a bar diagram shows how individual weight
percent can be plotted as a function of grain size to
yield a grain-size
A frequency curve is essentially a histogram in which a
smooth curve takes the place of a discontinuous bar
graph.
A grain-size cumulative curve is generated by
plotting grain size against cumulative weight percent
frequency. This curve allows to calculation of several
important grain-size statistical parameters.
8. MATHEMATICAL MEASURMENT
• Average Grain Size Three mathematical measures of average grain size are in
common use.
• The mode is the most frequently occurring particle size in a population of
grains. Siliciclastic sediments and sedimentary rocks tend to have a single
modal size, but some sediments are bimodal, with one mode in the coarse
end of the size distribution and one in the fine end. Some are even
polymodal.
• The median size is the midpoint of the grain-size distribution. Half of the
grains by weight are larger than the median size, and half are smaller. The
median size corresponds to the 50th percentile diameter on the cumulative
curve .
• The mean size is the arithmetic average of all the particle sizes in a sample.
The true arithmetic mean of most sediment samples cannot be determined
because we cannot count the total number of grains in a sample or measure
each small grain
9. SORTING& STANDARD DEVIATION
The SORTING of a grain population is a measure of the range of
grain sizes present and the magnitude of the spread or scatter of
these sizes around the mean size. The mathematical expression
of sorting is STANDARD DEVIATION. In conventional statistics,
one standard deviation encompasses the central 68 percent of
the area under the frequency curve. That is, 68 percent of the
grain-size values lie within plus or minus one standard deviation
of the mean size
STANDARD DEVIATION Vs SORTING
Value of standard
deviation is low ,
well sorted
10. SKEWNESS
The frequency curves of such non normal populations are not perfect bell-shaped curves, such as the
example shown. Instead, they show some degree of asymmetry, or skewness. The mode, mean, and median in
a skewed population of grains are all different. Populations that have a tail of excess fine particles are said to
be positively skewed or fine skewed, that is, skewed toward positive phi values. Populations with a tail of
excess coarse particles are negatively skewed or coarse skewed.(Folk 1974)
11. KURTOSIS
• Grain-size frequency curves can show various degrees of sharpness or peakedness. The degree of
peakedness is called kurtosis.
12. (iii) Application and Importance of Grain-Size Data
1. To interpret coastal stratigraphy and sea-level fluctuations.
2. To trace glacial sediment transport and the cycling of glacial
sediments from land to sea.
3. By marine geochemists to understand the fluxes, cycles, budgets,
sources, and sinks of chemical elements in nature.
4. To understand the mass physical (geotechnical) properties of
seafloor sediment, that is, the degree to which these sediments are
likely to undergo slumping, sliding, or other deformation.
