Analisis Granulometri Sedimen
Acara 4
Tim Asisten
SURVEI DAN PEMETAAN GEOMORFOLOGI
References
References (internet)International Journal of Sediment ResearchJournal of Sedimentary GeologyLibgen.infohttp://en.bookfi.org/Indonesian Journal of Geographyhttp://www.nat-hazards-earth-syst-sci.net (Natural Hazard)Journal Earth Surface Processes and LandformsJournal Landform AnalysisGeomorphology Journal (Elsevier)Journal of Coastal Research
Why sediment analysis important?
Geomorphological processesSource (provenant),
Entrainment, transportation
Facies and Depositional environment
Facies and depositional environments
1. To interpret coastal stratigraphy and sea-levelfluctuations
2. To trace glacial sediment transport and the cycling ofglacial sediments from land to sea
3. By marine geochemists to understand the fluxes,cycles, budgets, sources, and sinks of chemicalelements in nature
4. To understand the mass physical (geotechnical)properties of seafloor sediment, i.e., the degree towhich these sediments are likely to undergoslumping, sliding, or other deformation
Grain-size data are used in a variety of other ways (summarized by Syvitski, 1991 in Boggs, 2006):
Facies and depositional environments
Facies analysis is the interpretation of strata in terms of depositional environments (or depositional systems), commonly based on a wide variety of observations
Facies associations constitute several facies that occur in combination, and typically represent one depositional environment (note that very few individual facies are diagnostic for one specific setting!)
Facies successions (or facies sequences) are faciesassociations with a characteristic vertical order
Walthers Law (1894) states that two different faciesfound superimposed on one another and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time
Taken from McLemore
Facies and depositional environments
Different depositional environments exhibit different grain size distributions
Glacial sediments
poorly sorted
River sediments moderately sorted
Beach sediments well sorted
Folk (1980)
Depositional Environment
Depositional Environment
Depositional Environment
Clastic vs. Carbonate Depositional Systems
Short and Long Depositional Systems
Taken from Christensen
Depositional Environment
Taken from McLemore
Sediment types
Terrigenous Clastics (TC) Detrital Particles
Derived from pre-existing rocks
Derived external to the depositional basin
Transported by surface processes to the site of deposition
Particulate Residues: quartz, feldspar, rock fragments, etc(unaltered rock forming mineral/rock grains)
Secondary Minerals: minerals new-formed in the surface weathering environment: clay minerals, oxides, amorphous silica, etc
Taken from Christensen
Sediment types
Allochemical Particles formed in situ at the site of deposition; of chemical/ biochemical origin Carbonates: ooids, fossil
fragments, pellets, lithoclasts
Glauconite, phosphate :in situauthigenic/particulate minerals
Biogenic sediments: pelagic tests, siliceous and calcareous
Taken from Christensen
Sediment types
Orthochemical Components Chemical Precipitates
Secondary cement
Primary chemical sediments: halite, etc
Organic Particulate Material (detrital organic matter ) terrestrial and particulate
marine pelagic
95% found in mudrocks and indicative of low Eh and low current strength
Laminated Castile Formation basinal evaporites. Dark laminae are calcite plus organic matter; light laminae are gypsum (Peter Scholle)
Coal
Taken from Christensen
Sediment types
Pyroclasts particles fragmented and transported by volcanic
processes Tephra: tuff deposits
Volcanic mudflows: lahar and volcanic breccia deposits
Tephra VolcanicAsh
Taken from Christensen
Sediment analysis
In this part, we only study
clastic sediment
Clastic sediment consists of grains and particles that were eroded from weathered rocks and then were transported and deposited in loose, unconsolidated layers atthe Earths surface.(Thompson and Turk,1997)
Grain size
Ukuran Butir merupakan sifat dasar material sedimen yang mempengaruhiproses transport dan deposisi. Analisis ukuran butir memberikan petunjukpenting mengenai asal muasal sedimen, sejarah sedimen-transport dankondisi pengendapan (Folk and Ward, 1957; Friedman, 1979; Bui et al, 1990)
Grain> 2mm 0,064mm - 2mm 0,004mm 0, 064mm slate
Sediment analysis
Gravel-Size (Pebbles & Cobbles)(> 2 mm)
Sediment analysis
Sand
(1/16 - 2 mm)
Sediment analysis
Mud
(< 63mm = < 1/16 mm)
Taken from Christensen
Methods of measuring grain size data
Boggs, 2006
Describing Clastic sediment
Taken from Christensen
Description Size
Texture
Fabric (kemas)
Analysis
Maturity Textural
Compositional
Different parents, different environments, different paths,different depositional processes
sediment characteristics
Describing Clastic sediment
Defining equations for a sediment with respect to mineralogy andgeometrical properties. (After Griffiths 1967, Pettijohn et al. 1972).
Clastic sediment : : Texture
ADALAH SUATU KENAMPAKAN YANG BERHUBUNGAN DENGAN UKURAN DAN BENTUK BUTIR SERTA SUSUNANNYA (PETTIJH0N, 1975).
Tekstur
Ukuran butir(grain zsize)
Bentuk (shape)
Kebundaran(Roundness)
Kepipihan(Sphericity)
Pemilahan(sorting)
Clastic sediment :Texture
Texture- refers to the size, shape, arrangement of the grains the make up the rock.
Grain size- grain diameter (boulders, pebbles, cobbles, sand, silt, or clay).
Shape- is described in terms of sphericity
Roundness or (angularity) refers to the sharpness or smoothness of their corners.
Taken from Santos
Clastic sediment Classification:Texture
Descriptive Textural Classification Grain Size
Udden-Wentworth grain size scale. Udden (1898) modified by Wentworth (1922)
Because of this wide range of particle sizes, logarithmic or geometric scales are more useful for expressing size than are linear scales.
Phi ()=-log2 (grain diameter in mm)
naturally occurring groups;
Gravel ~ rock fragments,
Sand ~ individual mineral grains (particulate residues)
Clay ~ chemical weathering products (clay minerals, etc.)
Mud ~ particulate residues +/-chemical weathering products
Taken from Christensen
Clastic sediment Classification:Texture
Subdivided scale by factor of 2 .0039 mm clay
.0078 mm very fine silt
128 mm = cobbles
256 mm = boulders
Logarithmic (base 2) progression!
= -log2(grain diameter in mm)
As grain size increases, phi size decreases
Taken from Christensen
Clastic sediment : Roundness
Clastic sediment : Sorting
PEMILAHAN/SORTING adalah keseragamandari ukuran besar butir menyusun suatubatuan sedimen, artinya bila bila semakinseragam ukurannya dan besar butirnya makapemilahan semakin baik.
A function of grain origin and transport historyClast Rounding: surface irregularityDue to prolonged agitation during transport and reworking
Clastic sediment : Sorting
Taken from Christensen
Clastic sediment : Sorting
Taken from Christensen
what do shape, size, sorting tell us?
They reflect: Derivation (what were the original
rocks, parent rocks) Process and Climate (during
formation, Transport history, weathering)
Post-depositional factors (diagenesis/lithification)
How we get the data?
Field measurement
(> 16 mm)
Laboratory analysis (
How we get the data?: Field measurement
Field measurement (>16 mm)
Pengukuran panjang,lebar dan tinggi butirsedimen.
Pengukuran bentuk butirdilakukan denganmembandingkan bentukmaterial sedimen sungaidengan komparatorkelengkungan batuan.
How we get the data?: Laboratory analysis
record the weight of each sieve with its retained sediment.
Place the sieve stack in the mechanical shaker and shake for 10minutes.
Carefully pour the dry sediment sample into the top sieve and place the cap over it
Sieve analysis; Dry sample
Sediment analysis
Sediment analysis
Requires description (qualitative, quantitative)
Analysis (graphical, statistical) interpretation
Describing Clastics Grain size graphic analysis
Plots Histogram of weight percentage of size fractions Frequency curve Cumulative frequency curveWhen plotted, grain size increases from right to left, fines to right, coarse to left
Graphically represent grain size distribution mean grain size standard deviation from a normal distribution (sorting-
sortasi) symmetry (skewness-kemencengan) flatness of curve (kurtosis-keruncingan)
Taken from Christensen
Statistical/Graphic Presentation of Texture: Grain Size/Sorting
Quantitative assessment of the % of different grain sizes in a clastics
Mean: average particle size Mode: most abundant class size Median: 50th percentile
Taken from Christensen
Statistical/Graphic Presentation of Texture: Grain Size/Sorting
Statistical/Graphic Presentation of Texture: Grain Size/Sorting
Conventional phi scale showing grain size increasing to the left and decreasing to the right
Grading Curves
Why measure grain size?
1. Grain size is important to determining the strength of currents that transported the sediment. Therefore, we need a precise measurement of size to quantitatively interpret paleohydraulicconditions.
2. Sorting reflects the ability of the transport mechanism to segregate grains by size.
3. Skewness reflects the ability of the transport mechanism to selectively remove coarse or fine grain sizes.
4. It appears that grain size distributions have very specific interpretations in terms of how the sediment moved while it was in transport.
5. We need basic descriptors of sediment size to allow us to communicate with others.
6. Grain size and various properties of its distribution are important in determining a sediment's porosity and permeability.
Statistical/Graphic Presentation of Texture; Granulometry
Taken from Christensen
Graphic Methods (Folk & Ward, 1957)
Statistical/Graphic Presentation of Texture; Granulometry
Statistical/Graphic Presentation of Texture; Granulometry
Schematic illustration of the various types of skewness. Note that dashed lines indicate the symmetrical distribution for comparison with fine and coarse skewed frequency curves. M is mean, Md is median and Mo is mode.After Friedman and Sanders (1978)
Statistical/Graphic Presentation of Texture; Granulometry
Skewness and Kurtosis
Statistical/Graphic Presentation of Texture; Granulometry
2. Gradistat 4.0Dibuat oleh Dave Thornley dan John Jack dari PostgraduateResearch Institute for Sedimentology di University of Reading,UK.Program ini dikembangkan dengan Ms. Excel dengan outputtabel dan grafik untuk memudahkan dalam uji statistik dananalisis ukuran butir.Uji Statistik Gradistat meliputi nilai mean, modus, median,standar deviasi, skewness, kurtosis dan rangeHasil perhitungan Statistik menggunakan Gradistat meliputidistribusi ukuran butir, tekstur, sortasi butir.
Describing Clastics Grain size graphic analysis
Example: Sediment data
Screen Opening (phi)
Weight of Beaker with sand (grams)
Weight of beaker empty (grams)
Weight of Sand (grams)
Cumulative Weight (grams)
Weight Percent
Cumulative Weight Percent
-1 5.27 2.32 2.95 2.95 8.83 8.83
0 7.27 2.32 4.95 7.9 14.81 23.64
1 15.66 2.3 13.36 21.26 39.98 63.61
2 11.86 2.31 9.55 30.81 28.58 92.19
3 4.44 2.31 2.13 32.94 6.37 98.56
4 2.78 2.32 0.46 33.4 1.38 99.94
5 2.31 2.29 0.02 33.42 0.06 100
Describing Clastics Grain size graphic analysis
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16
-1 0 1 2 3 4 5
Histogram
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Cumulative frequency curveon an arithmetic scale
Hjulstrm diagram
The Hjulstrm diagram showing the water velocity at which entrainment and deposition occur for particles of a given size in well-sorted sediments. Source: Adapted from Hjulstrm (1935)
Relative bed stability (RBS)
Keterangan :Vc = Kecepatan kritisVb = Kecepatan dasar aliran sungai
Nilai kecepatan kritis (Vc) dapat diperoleh menggunakan rumus berikut:
dengan d merupakan rata-rata ukuranpartikel (mm). Sedangkan nilai kecepatandasar aliran sungai (Vb) dapat dihitungmenggunakan rumus :
dengan V merupakan kecepatan rata-rata aliran sungai (m/s).
Nilai yang diperoleh dari perhitungankemudian dianalisis menggunakan kurvahjulstrom sebagai dasar penentuanproses dominan yang terjadi padasegmen sungai yang dikaji.
Flow/Grain Interaction: Particle Entrainment and Transport
Forces acting on particles during fluid flow
Inertial forces, FI, resisting grain movement
FI = gravity + friction + electrostatics
Mobility Forces, Fm, inducing grain movement
Fm= fluid drag force + Bernoulli force + buoyancy
www.geology.wmich.edu
Critical Threshold for Particle Entrainment
53
Fm > Fi Hjulstrom Diagram
Empirical relationship between grain size (quartz grains) and current velocity (standard temperature, clear water)
Defines critical flow velocity threshold for entrainment
As grain size increases entrainment velocity increases (sand size and > particles)
For clay size particles electrostatics requires increased flow velocity for entrainment
(gray area is experimental variation)
Relationships for a specific flow depth, ~ 1meter*
*F=Mawww.geology.wmich.edu
Transport Modes and Particle EntrainmentSuspension Saltation - Traction
With a grain at rest, as flow velocity increases
Fm > Fi ; initiates particle motion
Grain Suspension (for small particle sizes, fine silt; Fi
U (flow velocity) >>> VS (settling velocity)
Constant grain Suspension at relatively low U (flow velocity)
Wash load Transport Mode
www.geology.wmich.edu
Transport Modes and Particle Entrainment
With a grain at rest, as flow velocity increases
Fm > Fi ; initiates particle motion
Grain Saltation : for larger grains (sand size and larger)
When Fm > Fi U > VS but through time/space U < VS
Intermittent Suspension
Bedload Transport Mode
www.geology.wmich.edu
Transport Modes and Particle Entrainment
With a grain at rest, as flow velocity increases
Fm < Fi , but fluid drag causes grain rolling
Grain Traction : for large grains (typically pebble size and larger)
Normal surface (water) currents have too low a U for grain entrainment
Bedload Transport Mode
www.geology.wmich.edu
Step by step Graphical Method (Manual)
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-1 0 1 2 3 4 5
Cumulative frequency curveon an arithmetic scale
Perhitungan statistik sediment secara manual dilakukan dengan membuat kurva frekuensi kumulatif. Satuan ukuran butir yang digunakan adalah phi ()
Step by step Graphical Method (Manual)
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-1 0 1 2 3 4 5
Cumulative frequency curveon an arithmetic scale
Plot nilai phi pada nilai kumulatif tertentu sesuai rumus untuk perhitungan. Misal akan menhitung Mean, maka nilai phi yang dicari pada 16, 50, dan 84. Plot pada grafik dan tarik ke axis, baca nilai phi pada nilai kumulatif tersebut. Gunakan KERTAS MILIMETER BLOK untuk menggambar kurva frekuensi kumulatif.
16 = -0,5 50 = 0,3 84 = 1,7
Cara ini juga digunakan untuk menghitung parameter statistik yang lain.
Step by step using GRADISTAT (Digital)
Rubah nilai dari ukuran butir pada Gradistat sesuai dengan ukuran butir yang kita miliki
Masukkan berat (dalam bentuk persentase) dari tiap ukuran butir kedalam tabel
Klik Calculate statistics untuk memperoleh nilai statistik dari Sampel
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Step by step using GRADISTAT (Digital)
Ringkasan nilai statistik dapat dilihat pada tab Sample Statistics
Metode perhitungan yang digunakan adalah Folk and Ward Method dan Method of Moments
Hasil Praktikum
(Manual and Digital (GRADISTAT)1. Histogram of weight percentage of size
fractions and frequency curve2. Cumulative frequency curve3. Mean grain size 4. Standard deviation from a normal distribution
(sorting)5. Symmetry (skewness)6. Flatness of curve (kurtosis)7. Mean plot on the Hjulstrm diagram to know
the water velocity at which entrainment and deposition occur for particles of a given size
Guideline Bahasan dan Hasil Praktikum
1. Bagaimana karakteristik ukuran butir rata-rata (mean), sortasi (sorting), kemencengan (skewness), dan keruncingan (kurtosis) ukuran butir sedimen Anda? Bagaimana kaitannya dengan karakteristik geomorfologi yang membentuk sedimen tersebut?1. Sortasi-mekanismen transportasi sedimen?2. Kemencengan (Skewness) kemampuan
mekanisme aliran secara selektif dalam memisahkan partikel halus atau kasar?
3. Keruncingan (kurtosis)- distribusi ukuran butir2. Seberapa besar kecepatan aliran yang dibutuhkan
berdasarkan data sedimen Anda untuk terjadinya erosi, transportasi dan deposisi? Lihat Hjulstrm diagram.
Terima kasih atas kesabarannya