Sedimentary Petrology GEO 333 Lab (5) Grain Morphology (Grain Shape) 2009 Mansour Al-Hashim

Preview of Lab 4 Classification of sediments Classification of sediments Reading a ternary diagram Reading a ternary diagram Finding Clay : Silt Ratio Finding Clay : Silt Ratio Finding Mud : Sand Ratio Finding Mud : Sand Ratio

Objectives of Lab 5 Principle axes of sedimentary particles Principle axes of sedimentary particles Grain shape Grain shape Roundness Roundness Sphericity Sphericity Form Form

Principle Axes of Sedimentary Particles Each sedimentary particle has three main axes: 1)L (a): Long axis 2)I (b): Intermediate axis 3)S (c): Short axis

Principle Axes of Sedimentary Particles From Cheel (2005)

Principle Axes of Sedimentary Particles From Cheel (2005)

Grain Shape (1) Shape is a fundamental property of particles. Shape is a fundamental property of particles. May provide important information about the history of sediments. May provide important information about the history of sediments. The three shape properties are: Roundness, Sphericity, and Form. The three shape properties are: Roundness, Sphericity, and Form.

Grain Shape (2) Grain shape depends on: 1) The original shape of the particle. Mica particles are usually flat, whereas quartz grains are rounded. 2) The internal structure and hardness. 3) The length of transportation (distance of source area). The longer a grain is transported, the more round it becomes.

Roundness (angularity) 1. A description of the degree of sharpness of the corners and edges of grains. 2. Gives information about the distance of transport, the energy of transporting mechanisms, and the cycles of erosion and redeposition. 3. The simplest way to determine the roundness is by visual comparison with standard forms. 4. The most widely used chart for roundness is Powers’ chart.

Powers’ visual comparison chart After Pettijohn et al. (1973) Roundness classes

Wadell’s Roundness (Rw) 1. The most accurate method, but it involves the greatest effort and time. 2. Rw is the ratio of the average radii of curvature of the corners of a grain to the radius of the largest inscribed circle within the particle. 3. The maximum possible value of Rw is 1.

Wadell’s Roundness (Rw) N : number of corners Rradius of the largest inscribed circle within the particle R : radius of the largest inscribed circle within the particle From Cheel (2005)

Sphericity 1. The degree to which a particle resembles a sphere. 2. May be useful for understanding other properties of the particle (e.g. settling velocity) 3. Stoke’s Law of Settling applies accurately only to spherical particles, and its error increases as the sphericity decreases. 4. Normally given the symbol “ψ”

Wadell’s Sphericity (ψ) Wadell Operational Sphericity (WOS). Wadell (1932)

Sneed and Folk Sphericity ( ψ P ) Also known as maximum projection sphericity (MPS). The most widely-used expression of sphericity. Maximum Projection Sphericity (MPS). Sneed and Folk (1958)

Note Calculating ψ and ψ P requires the measurement of d S, which is impractical for sand-size sediments. Calculating ψ and ψ P requires the measurement of d S, which is impractical for sand-size sediments.

Riley Sphericity (ψ R ) Riley sphericity relies on measurements that can be taken from the two-dimensional view of a sand grain as seen through a microscope. Riley sphericity relies on measurements that can be taken from the two-dimensional view of a sand grain as seen through a microscope.

Riley Sphericity ( ψ R ) From Cheel (2005)

Corey Shape Factor (S.F.) Very similar to MPS. Widely used by engineers to describe the overall shape of grains Corey Shape Factor (CSF). Corey (1949)

Form The geometric form of detrital grains, and is expressed using specific terms. The geometric form of detrital grains, and is expressed using specific terms. The two commonly used methods of describing the form of particles are based on the ratios of dL, dI, and dS. The two commonly used methods of describing the form of particles are based on the ratios of dL, dI, and dS. 1- Zingg diagram (1935) 2- Sneed and Folk classification (1958).

Zingg Diagram From Cheel (2005)

Figure by MIT OCW

Sneed and Folk classification of grain form (1958) From Cheel (2005)

Illenberger Form Diagram 1991 From Illenberger (1991)

Summary Chart

Quiz Find the form, MPS, and CSF for the following grains Grain 1: L= 56, I= 49.5, S=39 (mm) Grain 2: L= 46, I= 36, S= 13 (mm)

Assignment 5 (1) Determine the roundness and Riley sphericity of the given grain. From Folk (1974)

Assignment 5 (2) Using a caliper, measure the L, I, and S axes of the given grains then complete the above table.

References Cheel, R.J., Introduction to clastic sedimentology, Department of Earth Sciences, Brock University, Ontario, Canada. Cheel, R.J., Introduction to clastic sedimentology, Department of Earth Sciences, Brock University, Ontario, Canada. Folk, 1974.Petrology of sedimentary rocks. Folk, 1974.Petrology of sedimentary rocks. Pettijohn F. J., Potter, P.E. and Siever, R., Sand and sandstone, pp Springer-Verlag, Berlin. Pettijohn F. J., Potter, P.E. and Siever, R., Sand and sandstone, pp Springer-Verlag, Berlin. Illenberger, W.K., Pebble shape (and size!). Journal of Sedimentary Petrology, v. 61, p. 756–767. Illenberger, W.K., Pebble shape (and size!). Journal of Sedimentary Petrology, v. 61, p. 756–767.

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