weathering and the production of sediment
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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
TephraTypes 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)
slide14
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 Grain 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 Grain 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 Grain 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 Grain 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 Grain 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 Maturity 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
  • 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
  • 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
  • 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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