Sedimentary Petrology GEO 333 Lab4 Classification of Unconsolidated

Sedimentary Petrology GEO 333 Lab(4) Classification of Unconsolidated Sediments 2009 Mansour Al-Hashim

Preview of Lab 3 n n n n n Textures of sedimentary rocks Grain size Roundness and sorting Maturity Color Cohesiveness (firmness) Mineral composition and sedimentary structures Framework and matrix Cements in sedimentary rocks

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

Classification of Sediments (1) n Classification of detrital sediments is based on particle size. The major size categories are gravel, sand, and mud. n Boundaries between these categories are in accord with those of Udden-Wentworth grade scale. n

Classification of Sediments (2) The problem is that sediments are NOT always very well sorted. n There is always a spread of sizes around the mean size. n So what does one do about naming mixed sediments? ? n For the sake of that purpose, various classifications have been proposed. n

Classifications of sediments that contain mixtures of size classes Figure by MIT OCW

From Long (2006), based on Folk (1954)

Reading a Ternary Diagram n A ternary diagram is a triangle. n Each of its three apexes represents a specific component, such as gravel, sand, or mud. n In the following example the apexes are labeled A, B, and C.

Composition A From csmres. jmu. edu

Composition B From csmres. jmu. edu

Composition C From csmres. jmu. edu

A Complete Ternary Diagram From csmres. jmu. edu

Example (1) • What is the composition of the points 1 -4 plotted on the diagram? From csmres. jmu. edu

Solution Point 1 (60% A) (20% B) (20% C) = 100% Point 2 (25% A) (40% B) (35% C) = 100% Point 3 (10% A) (70% B) (20% C) = 100% Point 4 (0. 0% A)(25% B) (75% C) = 100% Note: the total percentages of compositions that make up a specific sediment must be 100%.

Notes n In common practice, "A Scale", "B Scale", and "C Scale" are not always shown on the diagram. n Sometimes, these diagrams even lack the percent numbers. n Here, we will learn how to read such diagrams.

Can you read it now? ? From csmres. jmu. edu

Triangle (A) Folk (1954)

Triangle (B) Folk (1954)

Triangle (B) Based on Folk (1954)

When do we use triangle A and B? Triangle (A): n Sand, Silt, and Clay n No Gravel n Clay : Silt Ratio Triangle (B): n Gravel, Sand, and Mud (Silt + Clay) n Mud : Sand Ratio

Finding Clay : Silt Ratio Example (1) Sand= 25%, Silt= 25%, Clay= 50% Clay : Silt 50 : 25 50/25 : 25/25 2 : 1

Finding Mud : Sand Ratio Example (2) Gravel= 10%, Sand= 80%, Mud= 10% Mud : Sand 10 : 80 10/10 : 80/10 1 : 8

Assignment (4) n Classify the following sediments using the ternary diagrams (A) or (B) Sediment 1 Gravel=70 %, Sand= 20 %, Mud= 10 % Sediment 2 Sand= 60 %, Clay= 40 %, Silt= 0 % Sediment 3 Gravel=30 %, Sand= 50 %, Mud= 20 % Sediment 4 Sand= 10 %, Clay= 15 %, Silt= 75 % n Find the Mud : Sand or Clay : Silt for all the above cases.

References n Long, D. 2006. BGS detailed explanation of seabed sediment modified folk classification. n Folk, R. L. , 1954. The distinction between grain size and mineral composition in sedimentary rock nomenclature. Journal of Geology 62 (4), 344 -359. n csmres. jmu. edu

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