Sedimentology Grain Size Sorting

SEDIMENTOLOGY & STRATIGRAPHY

SEDIMENT GRAIN SIZE & SORTING BY ASSOC. PROF DR. CHOW WENG SUM

Grain Size Distribution • -indicate the availability of material of that particular grain size to be supplied to that area where the process is taking place - sediment reflects both the hydrodynamic condition in the area of deposition and the grainsize population of sediments available from the source area

Grain Size • - normally mean grain diameter; but most grains are not spherical • - use the term norminal diameter (dn) which is defined as the diameter of a sperical body which has the same volume as the grain Sand and gravels are analysed by mechanical sieving Fine silt and clay are analysed by hydrometer analysis

Principles Involved In Sieve Analysis & A Sedimentation Balance

Hydrometer Analysis

Grain Size Distribution • Wentworth Scale is based on logarithms to the the base 2 f =-log2d (where d is the grain size) For example, if d=256mm, f =-log10256/log102 =-2.408/0.301 =-8

Grain Size Classification Of Clastic Sediments

Surface/Volume Ratio

SETTLEMENT OF PARTICLES • Density, shape and size are important factors • All else being equal a large particle will settle out faster than a small particle • The frictional force that resists the particle’s fall acts only on the surface of the particle; the smaller the particle, the greater its ratio of surface area to volume and weight. • The larger particle with more weight per unit surface, needs more force per unit weight to move it.

SETTLEMENT OF PARTICLES • Shape also affects ease of transport • A perfect sphere has more volume and weight per unit surface than any other shape • Less spherical a particle , the easier a stream can carry it • For any given shape and size, a denser particle will settle faster because its weight-to surface ratio is higher

Grain Size Distribution • Grain size distribution is presented in a cumulative curve The steeper the curve, the better the sorting Mean M= (f 16+f 50+f 84)/3 Sorting SO1=(f 84- f 16)/4 +(f 95-f 5)/6.6 Skewness Sk=(f 16+f 84-f 50) + (f 5+f 95-2f 50) 2(f 84- f 16) 2(f 95-f 5) Kurtosis KG= (f 95-f 5) 1.44(f 75-f 25)

Presentation Of Grain-Size Distribution Data

Grain Size Distribution • Kurtosis is the spread of extreme ends of a grain size distribution curve in relation to the central part • Median diameter= diameter of that fraction of grains with a size such that 50% by weight of the sample grains are smaller than it, and 50% are larger Mean diameter= arithmatically calculated average grain size

CUMULATIVE FREQUENCY CURVES- RIVER SAND & BEACH SAND

CUMULATIVE FREQUENCY CURVE OF BEACH SAND

Sorting • Sorting refers to the selection,during transport, of particles according to their sizes, specific gravities, and shapes. Deposits that contain only a small range of particle sizes are well sorted Diamicton: any non/poorly sorted terrigenous sediment that consists of sand and/or larger particles in a muddy matrix Particle-supported : framework particles are abundant enough to be in contact with one another Matrix-supported: the matrix is so abundant that the coarser particles are not in contact (mud-supported, silt-supported or clay-supported)

Fabric Mixtures

Sorting Classes

Skewness • In completely symmetrical distribution curves, the mean (M) and median (Md) diameters will coincide. If the sample has a wide spread (tail) towards the fine grain sizes,and a relatively sharp delimitation at the large grain-size end, the sample is positively skewed, which is characteristic of fluvial sediments. This is because there is a fairly definite upper limit to the grain size which can be carried as the bottom load

Sorting Vs Skewness

SCATTER PLOT, SKEWNESS VS SORTING FOR BEACH & RIVER SANDS

Characteristics Of Various Sediments Eolian deposit--- +ve skewness, very well sorted Fluvial deposit--- +ve skewness Beach deposit--- -ve skewness, well sorted Further out beyond beach—poor sorting Sedimentary deposit from suspension- +ve skewness, poor sorting Mud flows--- -ve skewness Glacial and till deposit--- extremely poor sorting

Grain Shape- Roundness • Roundness is a property of surface shape-whether it is smooth or angular Roundness=Sr/R n Roundness can be defined as the sum of all (n) radii (r) of circles which can be inscribed by a section through the grain divided by the radius (R) of the inscribed circle - difficult to measure - use visual scale instead

Roundness

Roundness

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