Sedimentary Textures - Michigan Technological...
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Sedimentary TexturesOct. 3, 2007
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Grain Size
Size reflects a variety of things
Sedimentologists are interested in 3 thingsmeasurement + expressionpresentationwhat it means
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1) Measurement
Udden-Wentworth: used to classify sediment diameters
each successive size class is half as large as the previous
A
2 A
1/2 A
Have grain size class ‘A’
The next larger class is twice the diameter
The next finer class is half the diameter
very-coarse/coarse sand boundary 1 mm
coarse/med sand boundary 0.5 mm
granule/very-coarse sand boundary 2 mm
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Phi-scale
ɸ = -log2d
ɸ lower d (mm) class-8 256 boulder-6 64 cobble-2 4 pebble-1 2 granule4 0.125 sand8 0.0039 silt14 0.00006 clay
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Measuring Size
pebbles -> bouldershttp://www.wpclipart.com/tools/tape_measure_2.pnghttp://www.soil-net.com/album/Soils_Rocks/slides/Rock%20Conglomerate.jpg
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Measuring Size
Granule -> Silthttp://invam.caf.wvu.edu/methods/spores/extractions/small-sieves.jpghttp://rockyweb.cr.usgs.gov/frontrange/virtour/images/sieves.jpg
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Measuring Size
Silts & Clayshttp://www.uasb.org/discover/settling.jpg
Settling method
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Measuring Size
Sophisticated method for fine sediments..
http://www.malvern.com/ProcessEng/systems/laser_diffraction/technology/technology.htm
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Rock?
not loose particleshttp://www.photo-mark.com/webpix/ds/Sandstone.jpg
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2) Presentation
Histogram: grain size vs weight %http://gpc.edu/~janderso/historic/labman/sievean.htm
larger finer+ɸ-ɸ
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2) Presentation
Frequency curvesmooth curve fitted to histogramhttp://gpc.edu/~janderso/historic/labman/sievean.htm
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2) Presentation
cumulative arithmetic curve (cumulative weight %)
http://gpc.edu/~janderso/historic/labman/sievean.htm
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2) Presentation
cumulative probability curvea normal distribution = straight line
http://gpc.edu/~janderso/historic/labman/sievean.htm
black line = normal distribution
red line = plotted datacan easily see how it
deviates
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2B) Mathematical Presentation
Lots of data = lots graphsUse mathematical methods instead
see Table 5.3, page 85 of text
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Grain Size
Mode: most frequently occurringThis data is bimodal
http://gpc.edu/~janderso/historic/labman/sievean.htm
Modes identified with
stars
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Cumulative Frequency Curve
Same data as previous slide, but plotted differently
Here the steepest slopes of the curve
represent the modes
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Grain Size
Median Size50% of sample is coarser than 1.5 phi
50% of sample is finer than 1.5 phi
http://gpc.edu/~janderso/historic/labman/sievean.htm
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Grain Size
Mean (average) SizeMean, Median & Mode are only equal in a normal distribution
M= ɸ16 + ɸ50 + ɸ84
3
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Sorting
Well Sorted Poorly Sorted
σ1 = ɸ 84 - ɸ16 ɸ95 - ɸ5+
4 6.6
Base of curve occupies small range of grain sizes
Base of curve occupies wide range of grain sizes
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Skewness
degree of asymmetry
compared to a normal distribution, a Positive skewed sample has an excess of fine particle
http://gpc.edu/~janderso/historic/labman/sievean.htm
Positive skew: fine grained tail(larger Φ)
Negative skew: coarse grained tail
(smaller, or negative Φ)
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Kurtosis
“peakedness” of frequency curvesdegree of sorting of central population vs edges
http://gpc.edu/~janderso/historic/labman/sievean.htm
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Method of Moments
Grain Size paramtersMeanStandard DeviationSkewnessKurtosis
Can be obtained mathematically
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4) Importance
Descriptive of the rock itselfEconomically important
sorting, shape etc.. are linked to porosity & permeability
Depositional Environmenthopefully this tells us something about where the sediment came from
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Particle Shape
Parent rockWeatheringTransportBurial
FormRoundnessTexture
Depends on... Described by...
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Form
SphericityDepends a lot on composition
Equant Platy
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Roundness
http://maps.unomaha.edu/maher/ESSlectures/ESSlabs/lab6sediments.html
Depends on abrasion history, clast size & composition
Larger clasts round faster than small onesSofter clasts round faster than hard ones
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What environments do the most rounding of grains...
http://www.seedmagazine.com/news/uploads/singingdune.jpg
http://jan.ucc.nau.edu/~rcb7/Wavetrainsore.jpg
http://www.cc.gatech.edu/cpl/projects/graphcuttextures/data/interaction/LittleRiver.jpg
for sand sized quartz
< <
for pebbleshttp://members.aol.com/Mmcbs3/mississippi-
grandrapids1.jpg
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Surface texture
mechanicalchemicaltectonic
http://www.nvcc.edu/home/cbentley/shenandoah/sauk.jpg
SEM image of a rounded sand grain showing extreme “frosting” indicating wind transport
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Fabric
Grain OrientationGrain Packing
affects bulk densityporositypermeability
http://www.umt.edu/geosciences/faculty/hendrix/g100/imbrication.jpg
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Is this useful?
Individual calculations or features may not be diagnostic of an environmentbut combining a variety of these grain analyses along with observations of sedimentary structures can help narrow down the possibilities
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