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Weathering and the Production of Sediment. Surface Portion of the Geological Cycle. Types of Sedimentary Material. Terrigenous Clastics (TC) Detrital Particles Derived from pre-existing rocks Derived external to the depositional basin .

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Weathering and the production of sediment

Weathering and the Production of Sediment

Surface Portion of the

Geological Cycle



Types of sedimentary material1

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

Types of Sedimentary Material


Types of sedimentary material2
Types of Sedimentary Material

  • Allochemical Particles formed in situ at the site of deposition; of chemical/ biochemical origin

    • Carbonates: ooids, fossil fragments, pellets, lithoclasts

    • Glauconite, phosphate :insitu authigenic/particulate minerals

    • Biogenic sediments: pelagic tests, siliceous and calcareous


Types of sedimentary material3
Types of Sedimentary Material

  • 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


Types of sedimentary material4

Tephra

Types of Sedimentary Material

  • Pyroclasts

    • particles fragmented and transported by volcanic processes

      • Tephra: tuff deposits

      • Volcanic mudflows: lahar and volcanic breccia deposits

Volcanic

Ash



Sedimentary analysis
Sedimentary Analysis

  • Goal:

    • study modern analogue to understand processes

    • identify processes which cause diagnostic characteristic features

    • unravel history

  • Requires description (qualitative, quantitative), analysis (graphical, statistical), interpretation


Describing siliciclastics
Describing Siliciclastics

  • Description

    • Size

    • Texture

    • Fabric

  • Analysis

  • Maturity

    • Textural

    • Compositional


Describing siliciclastics or how to have an intelligent discussion about rocks
Describing Siliciclastics(or how to have an intelligent discussion about rocks)

  • Classification

    • A necessary evil

    • An attempt to organize wide variety into few classes

      • Useful

      • Expect deviations, overlap and some which just don’t fit

  • Foundations

    • Grain type

    • Grain size

    • Transported or in situ

  • Different for each sediment type


  • Describing siliciclastics size
    Describing Siliciclastics-Size

    • Size

      • Gravel and larger (> 2 mm) (conglomerate)

      • Sand (1/16 - 2 mm) (sandstone)

      • Mud (< 63mm = < 1/16 mm) (mudstone)



    Sand becomes sandstone
    Sand becomes sandstone

    • (1/16 - 2 mm)


    Mud becomes shale

    • (< 63mm = < 1/16 mm)


    Siliciclastic rock classification texture

    Descriptive Textural Classification:

    Ternary Plots

    G (gravel >2mm) - S (2mm>sand> 0.063mm)- M (mud<0.063mm)

    significance of gravel (>30%) min. transport energy

    S (sand) - C (clay<0.004mm)- S (0.063mm>silt> 0.004mm

    Siliciclastic Rock Classification: Texture


    Siliciclastic rock classification
    Siliciclastic Rock Classification

    • Mineralogical Classification/terminology

      • Sand ----------->Arenites

      • CGL------------->Rudites

      • MDST----------->Lutites

        textural term mineralogical term

    • Arenites Petrology

      • Ease of analysis and sampling

      • Composition can be interpreted


    Describing siliciclastics size wentworth scale
    Describing Siliciclastics- Size Wentworth scale

    • Udden- Wentworth size scale

      • Udden, 1914; Wentworth, 1922

    • Resolves problems with size classification

      • Cumbersome to discuss size

      • Limiting to restrict to 3 classes

  • Four basic groups + modifiers make more

    • Clay (< 4 mm)

    • Silt (4 mm - 63 mm)

    • Sand (63mm - 2 mm)

    • Gravel (> 2 mm)


  • Siliciclastic rock classification texture1
    Siliciclastic Rock Classification:Texture

    • Descriptive Textural Classification

      • Grain Size

        • Uden-Wentworth grain size scale

        • 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


    Describing siliciclastics size wentworth scale cont
    Describing Siliciclastics- Size Wentworth scale (cont’)

    • 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


    Describing siliciclastics sedimentary texture
    Describing Siliciclastics- Sedimentary Texture

    • Aspects of texture

      • Shape

      • Proportions of clastic: matrix

      • Degree of sorting

      • Surface texture

    • Result of

      • Parent rock type (shape)

      • Weathering

      • Transport history (sorting, shape)

    • Generally for siliciclastics but can be useful for other types


    Describing siliciclastics1
    Describing Siliciclastics

    • Form/Shape

    • Zingg indices

    • spherical (equant), oblate (disk or tabular), bladed, prolate(roller)

    • Roundness

      • Degree of angularity

        • Function of transport history

        • Edges chip off as clasts knock into one another (progressive)

        • Estimate visually or calculate from cross- section

    • Sphericity

      • How closely clast approximates a sphere (equant)

        • Inherited feature! (function of shape formed in weathering)

        • slab may become discoidal but stays flat with time


    Clastic rock classification texture sorting shape
    Clastic Rock ClassificationTexture: Sorting & Shape

    • Sorting: measure of the diversity of grain size

    • A function of grain origin and transport history

    • Clast Rounding: surface irregularity

      • Due to prolonged agitation during transport and reworking


    Describing siliciclastics2
    Describing Siliciclastics

    • Fabric

      • Alignment of elongate clasts

        • Anisotropic (preferred direction) arrangement of particles

        • e.g., shale

  • Surface Texture

    • Pitted or not

    • Folklore says eolian transport leads to etching

      • Yes!

      • No!


  • Describing siliciclastics clastic matrix
    Describing Siliciclastics Clastic: matrix

    • Clasts

      • Fragment which makes up a sedimentary rock

    • Matrix

      • Finer- grained material which lies between the clasts

    • Relative difference!

      • Boulder/ cobble or sand/silt


    Describing siliciclastics degree of sorting
    Describing Siliciclastics Degree of sorting

    • Measure of distribution of clast sizes

      • Well sorted

        • most clasts fall into one class on the Wentworth scale

      • Poorly sorted

        • wide range of clast sizes

  • Due to origin and transport history

    • Greater distance (or repeated agitation of sediment), better separation of sizes

  • Qualitative (visual) and quantitative methods


  • Statistical graphic presentation of texture grain size sorting
    Statistical/Graphic Presentation of Texture: Grain Size/Sorting

    • Quantitative assessment of the % of different grain sizes in a clastic rock

      • Mean: average particle size

      • Mode: most abundant class size


    Describing siliciclastics grain size analysis
    Describing Siliciclastics Size/SortingGrain size analysis

    • Quantitative analysis

      • (granulometric analysis)

        • Quantitative assessment of % of different grain sizes in clastic sediments and sedimentary rocks

      • Useful in interpretting depositional history of clasts, especially in modern environments

    • Technique used varies with grain size

      • Direct

      • Indirect


    Describing siliciclastics grain size analysis techniques
    Describing Siliciclastics Size/SortingGrain size analysis- techniques

    • Gravel

      • direct measurement in the field

      • measure all within a quadrant

        • meter is used for pebbles, cobbles

    • Sand

      • pass through a stack of sieves with mesh keyed to 

        • weigh contents of each sieve, get distribution by wt.

    • Sandstones and Conglomerates (∑2d/n)/N

      • n=#grains cut by view; d = diameter of field of view; N = total # views counted

    • Coarse silt and finer

      • based on Stokes Law

        • particle will settle through water at a predictable rate

      • pipette

      • sedigraph (X-ray the sediment/ water solution)


    Describing siliciclastics grain size analysis graphic analysis
    Describing Siliciclastics Size/SortingGrain size analysis- graphic analysis

    • Plots

      • Histogram of weight percentage of size fractions

      • Frequency curve

      • Cumulative frequency curve

        When 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)

      • symmetry (skewness)

      • flatness of curve (kurtosis)


    Describing siliciclastics grain size analysis graphic analysis1
    Describing Siliciclastics Size/SortingGrain size analysis- graphic analysis

    • Different depositional environments exhibit different grain size distributions

      • Glacial sediments

        • poorly sorted

      • River sediments

        • moderately sorted

      • Beach sediments

        • well sorted



    Describing siliciclastics grain size analysis graphic analysis2
    Describing Siliciclastics Size/SortingGrain size analysis- graphic analysis

    • No unique solutions!

    • Need additional data

      • field observations

      • large- scale sedimentary relationships

      • sedimentary structures

      • facies associations

  • If sediments are eroded from rocks previously deposited, then misleading data can result

    • e.g., river (mod. sorting) may be transported sediments eroded from old beach rock (well sorted)


  • Significance of grain size sorting and rounding interpretive
    Significance of Grain Size, Sorting and Rounding : Interpretive

    • Textural Maturity

      • Kinetic energy during transport and reworking

      • Transport history

      • Dispersal patterns

      • Caveat emptor!

        • Mixed sources

        • Biogenic reworking


    Describing siliciclastics maturity of siliciclastic material
    Describing Siliciclastics InterpretiveMaturity of Siliciclastic Material

    • Extent to which material has changed when compared to the starting material (e.g., granite) from which it was derived

    • Textural

    • Compositional/ mineralogical

      • Generally linked

        • High textural maturity leads to high compositional maturity


    Textural maturity
    Textural maturity Interpretive

    • Clue to

      • Erosion, Transport, Depositional history

  • Independent of composition!

  • Generalizations

    • Maturity increases with energy input (same source)

      • higher downstream

      • Relative to starting material!

      • clean sandstone can have high maturity (if rounded)

      • Comparisons from different sources uncertain

      • different starting grain size and shape distributions not comparable


    Compositional maturity
    Compositional maturity Interpretive

    • Measure of proportion of resistant or stable minerals present in the sediment, to less resistant minerals

      • Sandstone with high maturity has mostly quartz

      • Strongly influenced by composition of source rock area

        Resistant Less resistant

        Quartz Feldspar

        Chert clasts Most other minerals

        Zircon Lithic clasts


    Cycles of sedimentation
    Cycles of Sedimentation Interpretive

    • First cycle

      • Material is eroded, transported, deposited

    • Additional cycles

      • Burial, lithification, uplift, exposure, transport

      • Redeposition - second cycle of sedimentation

      • Increasing clastic detrital textural and mineralogical maturity with each cycle

    • Resistant minerals

      • Can survive repeated weathering, erosion, transport

      • Quartz, lithic fragments of chert, zircon (highly resistant)


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