Acara 4 Granulometri

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 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


Different depositional environments exhibit different grain size distributions

Glacial sediments

poorly sorted

River sediments moderately sorted

Beach sediments well sorted

Folk (1980)

Depositional Environment


Clastic vs. Carbonate Depositional Systems

Short and Long Depositional Systems

Taken from Christensen


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


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


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


Sediment types

Pyroclasts particles fragmented and transported by volcanic

processes Tephra: tuff deposits

Volcanic mudflows: lahar and volcanic breccia deposits

Tephra VolcanicAsh


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)


Methods of measuring grain size data

Boggs, 2006

Describing Clastic sediment


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


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


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


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)


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



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


Graphic Methods (Folk & Ward, 1957)


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)


Skewness and Kurtosis


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.


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


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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


Transport Modes and Particle Entrainment


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


Transport Modes and Particle Entrainment


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


Step by step Graphical Method (Manual)

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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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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

Acara 4 Granulometri

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