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02_416M-Grain Parameters_handout

02_416M-Grain Parameters_handout - Grain Parameters Lecture...

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Grain Parameters Lecture Outline I.Grain-size analysis & statistical parameters II.Sediment / Rock textural classification III.Grain shape IV.Grain roundness V.Textural maturity VI.Grain surface features VII.Sediment / rock fabric Reading Assignment: Boggs, Chapter 3 1
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Why Do Grain-Size Analysis? Sediment / rock description = communication Sediment / rock name = classification = communication Information on source area, transport, and depositional environment 2
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How To Measure Grain Size 3
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Full range of weathering products in transport Sediment from physical weathering Sediment from chemical weathering Ions from chemical weathering Wentworth Scale addresses sediment Sediment ranges from clay to boulder Uses geometric scale because grain size ranges from <0.004 mm to m Establishes size-classes (cobble, sand, silt, etc.) Standard sieve sizes for silt to pebble Also uses a Phi Scale to make units of equal value Phi = -log2d 4 Grain Size: Udden-Wentworth Scale
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5 Millimeters Microns Φ (Phi Scale) Wentworth Size Class Conglomerate (rounded clasts) Breccia (angular clasts) Sandstone Mudstone Lithification conversion of unconsolidated sediments into rock
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6 Text Version – Note sieve mesh size Table 3.1, p. 53.
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How To Display The Grain-Size Data Record weights Always plotted as Coarse to Fine Histogram Frequency Curve Cumulative Frequency Curve Semi-Log Plot 7
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“Average” grain size Mode - most frequently occurring size class Median – 50% Mean – arithmetic average Sorting Skewness Kurtosis 8 Statistical Treatment of Grain-Size Data
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9 “Average” Grain Size φ84 φ16
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Why Sample at 16 & 84%; 5 & 95%? 1 sd 1 sd 68% 2 sd 2 sd 90% 16 % 84% 5% 95% 10
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Significance of Average Grain Size Source Area – What size did the grains start at? Weathering history? Transport History Grain size directly correlates to energy level Selective transport Proximal to Distal fining Depositional Environment Energy level Calculation of flow speed in water & air from grain size Depositional process of sediment layers Segregation of grain populations on bedforms Bedload vs. suspended load 11
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12 Sorting – How Uniform Is The Population?
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13 Sorting in Real Sediment
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14 Visual Estimates of Sorting Fig 3.3, p. 58.
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Significance of Sorting
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