Reading Question
1. What is sediment texture?Sedimentary Texture
• What is sediment texture?
• Size, shape, and arrangement of grains
• Grain size
• Grain shape and surface features
• Porosity and permeability
• Fabric - sedimentary structures
• Reflects, weathering, erosion, transport, depositional environment
Grain Size Scales
• Wide range of particle sizes
• Geometric - Udden-Wentworth
• Values twice as large as previous
• Clay, silt, sand, and gravel
Grain Size Scales
• Phi scale = logarithmic scale - Ø = -log2d
• Sand size and below most abundant
Grain Size Methods
• Grain size measurement - rocks versus sediment
• Rocks
– Manual measurement of clasts
– Thin sections, image analysis
– SEM’s
• Sediment
– Manual measurement
– Sieves - US standard sieves (p.62, table 3.1)
– Pipette analysis - Stokes’ Law (automated techniques)
Grain Size Methods
• Graphic and Statistical analysis
• Histogram - bar graph - quick and easy
– Shape influenced by size categories
– Cannot be used for statistical analysis
• Frequency curve - smoothed out bar graph
– Shape independent of size categories
– Cannot be used for statistical analysis
Grain Size Methods
• Graphic and Statistical analysis
• Cumulative arithmetic curve
– Difficult to see trends
– Can do statistical analysis
• Cumulative probability curve
– Straightens S - easier interpretations
– Can do statistical analysis
– Normal distribution = straight line
– Steeper the slope the better the sorting
Grain Size Methods
• Statistical analysis
• Mode
– The most frequent particle class
– Highest point on frequency curve or max slope on cumulative curve
– Unimodal versus bimodal
Grain Size Methods
• Statistical analysis
• Median grain size
– Size class at which 50% are coarser and 50% are finer
– Cumulative curve intersects the 50% line
– Valuable?
Grain Size Methods
• Statistical analysis
• Mean grain size = average grain size
• Graphic mean
• Very valuable measure
Grain Size Methods
• Statistical analysis
• Standard deviation - degree of deviation from the mean
• A measure of sorting
Grain Size Methods
• Statistical analysis
• Standard deviation - degree of deviation from the mean
• A measure of sorting
Grain Size Methods
• Statistical analysis
• Skewness
– Degree of asymmetry
– Positive = coarse
– Negative = fine Grain Shape
• Sediment grains = complex 3-D shapes
• Form
• Roundness
• Surface conditions
• Transport and depositional environments
Grain Shape
• Form
• Zingg diagrams - shape categories - sphericity
• Settling, transportability, utility?
Grain Roundness
• Roundness
• Wadell equation
• Fourier Analysis
• Visual charts
Surface Texture
• Surface conditions - polished, frosted, etc.
• SEM analysis
• V-shaped percussion
• Upturned plates