Sedimentology Lab 2.pdf - University of Sulaimani

University of Sulaimani
Practical Sedimentology
College of Science
3rd Year Geology
Department of Geology 2009-2010
Lab. No.2
Terrigenous Clastic sediments
Gravel, sand and mud
The components of sediments and sedimentary rocks:
Sediments and sedimentary rocks can be classified by either their constituents or their mode of
origin or a combination of the two. The classification as follows:
- Terrigenous clastic material
- Carbonates
- Evaporites
- Volcanoclastic sediments
- Others
Terrigenous clastic material:
A threefold division on the basis of grain size is used:
Gravel >2 mm Sand (2 - l/16) mm Mud (clay and silt) < l/16 mm (< 63µm)
Measurements:
Wentworth scale; is a scale which made on the basis of factor of 1/2 or 2, means adjacent
classes differ by a factor of 1/2 or 2. (e.g. 1, 0.5, 0.25…etc.)
Phi scale (φ): is a numerical representation of the Wentworth scale. Phi values are
dimensionless numbers that increase with decreasing particle size. Phi diameter is computed by
taking the negative log of the diameter in millimeters.
φ = -log 2 d
d: diameter of the particle (mm).
Sieve mesh: which is the number of openings per inch of screen (Lambe, 1951) or between
the wires are square openings, the number of which per unit length give the mesh number
(Tucker, 1988).
Grain size analysis (Granulometric analysis):
Quantitative assessment of the percentage of different grain sizes in clastic sediments and
sedimentary rocks is called granulometric analysis. The data and measurements of the shape of clasts
can be used in the deposition and interpretation of clastic sedimentary material.

• Gravels; are normally assessed by direct measurement in the field.
• Sands; Sieves and microscopic study. By weighting the contents of each sieve the distribution
by weight of different size fractions can be determined.
• Silt and clay; most laboratory techniques employed in the granulometric analysis of silt and
clay size particles are based on settling velocity relationships predicted by Stokes' law.
Sieve analysis methodology:
1- Heap 500gm of sand and gravel into a cone.
2- Cut the cone into quarters with a spatula, reject the first and third or second and fourth quarters,
3- Separate the sand from gravel by sieve 2mm.
4- Separate gravels into its categories (4, 8,16mm).
5- Using the same method coning and quartering, you should have a sample weighting about 50gm.
6- Use a set of sieves, in which at the top coarser and downward to finer (1, 0.5, 0.25, 0.125, and
0.0625).
7- Then the set of sieves vibrated mechanically by using a shaker for 15 minutes.
8- Weight the remained sand in each sieve separately.
The results from the analyses are plotted in one of these forms; either a histogram of the weight
percentages of each of the size fractions, a frequency curve or a cumulative frequency curve.
The coarse sizes plot on the left and the finer material on the right of the graph. Then statistical
parameters are calculated:
Mean grain size, mode, median, sorting (standard deviation from a normal distribution), skewness
(an indicator of whether the grain size histogram is symmetrical or is skewed to a higher
percentage of coarser or finer material) and kurtosis (a value which indicates whether the
histogram has a sharp or a flat top).
Exercises:
1- Convert φ 6 to mm size and 16 mm to φ scale using the equation.
2- Conduct the sieve analysis for a specimen with the weight of 500gm, (or the result of sieve analysis
as in the table below)
3- Name the sample according to the gravel-sand-silt triangle.
Grain size (mm) φ Wt. (gm) Wt. %
8-4 50
4-2 40
2-1 65.6
1-0.5 164
0.5-0.25 82
0.25-0.125 82
0.125-0.0625 16.4