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Identification and Classification of Sedimentary Rocks. Terrigenous Clastic Sediments and Rocks. Initial Compositional Classification of the Most Common Sedimentary Rocks. T=Terrigenous

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identification and classification of sedimentary rocks

Identification and Classification of Sedimentary Rocks

Terrigenous Clastic Sediments and Rocks

initial compositional classification of the most common sedimentary rocks
Initial Compositional Classification of the Most Common Sedimentary Rocks
  • T=Terrigenous
    • sand mud and gravel weathering products formed at the earth’s surface from exposed, pre-existing ign, meta, and sed rocks; extra-basinal.
  • A= Allochemical
    • chemical or biochemical ppt formed within the basin of deposition (intra-basinal) but subsequently reworked at or near the site of deposition. E.g. particulate carbonate sediment, bioclasts, ooids, etc
  • O= Orthochemical
    • primary chemical ppt formed within the basin without subsequent reworking or transport. Carbonate mud (micrite), phosphate, halite, gypsum, chert, etc

IO= Impure orthochemical

IA= Impure allochemical

initial compositional classification
T: Terrigenous rocks

Most mudrocks, sandstones, and conglomerates. Comprise 65% to 75% of sedimentary strata

IA: Impure Allochemical rocks

E.g.: Very fossiliferous shale, sandy fossiliferous or oolitic limestones. Comprises 10-15% of sedimentary strata

IO: Impure Orthochemical rocks

E.g.: clay-rich microcrystalline limestones. Comprises 2-5% of sedimentary strata

A: Allochemical rocks

E.g.: fossiliferous, oolitic. Pellet, or intraclastic limestone or dolomite. Comprises 10-15% of sedimentary strata

O: Orthochemical Rocks

E.g.: microcrystalline limestone, chert, anhydrite,crystalline dolomite. Comprises 2-8% of sedimentary strata

Initial Compositional Classification
siliciclastic rock classification texture
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
clastic rock classification texture sorting shape
Clastic Rock ClassificationTexture: Sorting & Shape
  • Sorting: measure of the diversity of grain size (see F&P Appendix A)
    • A function of grain origin and transport history
  • Clast Rounding: surface irregularity
    • Due to prolonged agitation during transport and reworking
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
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

The trouble with Sedimentary Rocks

significance of grain size and sorting intrinsic and derivative physical properties
Significance of Grain Size and Sorting: Intrinsic and Derivative Physical Properties
  • Intrinsic Properties
    • Grain Size vs Porosity
    • Sorting vs porosity
  • Derivative Properties
    • Grain size vs permeability
significance of grain size and sorting intrinsic and derivative physical properties10
Significance of Grain Size and Sorting: Intrinsic and Derivative Physical Properties
  • Derivative Properties
    • Grain size vs permeability
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
mineralogical classification sandstone architecture
Mineralogical ClassificationSandstone Architecture
  • F-M-C-P
    • Framework Grains
      • > 0.05mm (particulate residues)
    • Detrital Matrix
      • < 0.05mm (clay, qtz, flds, -CO3, organics, oxides) chemical weathering products
    • Cement
      • post-depositional orthochemical components; ppt from circulating pore fluids (qtz,-CO3, clay, fldsp, oxides, zeolite, salts)
    • Pores;
      • Primary (~40%) or 2ndary due to leaching/dissolution
primary and authigenic components of sedimentary rocks
Primary and Authigenic Components of Sedimentary Rocks
  • Sedimentary rocks are composed of primary and authigenic (post-depositional, secondary) components
  • Both minerals and pores are represented in both the primary and the secondary parts of the rock.
primary and authigenic components of sedimentary rocks14
Primary and Authigenic Components of Sedimentary Rocks
  • Both minerals and pores are subject to profound modification by chemical and mechanical processes in the subsurface (diagenesis).
  • Diagenetic impacts are key to predicting the evolution of fluid flow properties (porosity [Ø], permeability [Κ]; etc.; petrophysical properties)

You should be able to examine a sedimentary rock at various scales (outcrop, hand sample, thin section) and distinguish between primary (depositional; texture, mineralogy, pores) and secondary (diagenetic; mineralogy and pores) features.

mineralogical classification sandstone architecture15
Mineralogical ClassificationSandstone Architecture
  • Framework Grains:
    • relative abundance a function of mineral grain

Availability, Chemical Stability, Mechanical Durability

  • Anything Possible, most common:
    • Qtz :
      • mono, poly, ign, meta, qtzite, chert, volc, etc; mech & chem stable, abundant
    • Feldspar:
      • K-spar (sandine, microcline), Plag (Na-Ca), stains (Amaranth soln), abundance and mechanical stability (variable)
    • Rock Fragments:
      • all kinds (including limestone/dolomite RF’s) ; abundant, variable stability
mineralogical classification sandstone architecture16
Mineralogical ClassificationSandstone Architecture
  • Framework Grains
    • Accessory Minerals:
      • Mica
      • ZTR; zircon, tourmaline, rutile: stable heavies
      • Unstable heavies: Amph, Pyx, Chl, Garn, Epid
      • carbonate allochems
      • non-detrital/orthochem; glauconite (iron-rich clay after fecal pellets) and phosphate (colophane, apatite); unusual oceanographic conditions
main authigenic components of sandstone
Main Authigenic Components of Sandstone
  • Carbonate cement (calcite, dolomite, ankerite, siderite)
  • Clay minerals (kaolinite, illite)
  • Quartz
  • Feldspar (albite)
  • Zeolite
data plots and primary sandstone composition classification
Data Plots and Primary Sandstone Composition Classification
  • TC’s with >50% grains & > 0.05mm
    • Arenites Ternary Diagram Q - F - R(L)
    • Q= mono and polycrystalline (not chert) quartz
    • F= monocrystalline feldspar
    • R (L)= rock (or lithic) fragments

Normalized, 3 phase classification: Q=q/q+f+r; F=f/q+f+r; R=r/q+f+r

data plots and sandstone classification
Data Plots and Sandstone Classification
  • Normalized, 3 phase classification
    • Q= q/q+f+r
    • F= f/q+f+r
    • R= r/q+f+r
  • 7 types of “normal” Arenites
    • others = “mineral” arenite, i.e. mica-arenite, magnetite-arenite
interpretation of sandstone composition
Interpretation of Sandstone Composition:
    • a relative measure of how extensively and thoroughly a sediment (sand size and larger) has been weathered, transported and reworked toward its ultimate end product, quartz sand.
interpretation of sandstone composition21
Interpretation of Sandstone Composition:
  • Provenance Basin Analysis and Paleotectonic Reconstructions
      • QFL plots for SUBQUARTZOSE (<75% quartz) Sst
      • Heavy Minerals (and other things) for QUARTZOSE (>75% quartz) Sst
importance of sandstone composition
Importance of Sandstone Composition:
  • Provenance
  • Can be interpreted in terms of tendency to- wards chemical and physical alteration during diagenesis
    • Quartzose sandstone experiences less physi- cal alteration and, mainly, depth related chemical alteration/loss of porosity due to cementation
    • Subquartzose sandstone is more subject to chemical and physical (compaction) loss of porosity and formation of authigenic cements
  • Most abundant sedimentary rock type
  • Source of much organic material precursor to fossil fuels
  • Good indicators of chemical/biological conditions at the site of deposition
  • Impermeable physical properties are important for subsurface fluid flow
  • Most effectively studied using SEM/XRD
  • Both primary and secondary minerals in sedimentary rocks
sheet silicates the mica s and clay minerals
Mica and clay minerals are Phyllosilicates

Sheet or layered silicates with

Two dimensional polymerization of silica tetrahedra

Common structure is a Si205 layer

Sheet Silicates: the Mica's and Clay Minerals



sheets of silica tetrahedra

structure of phyllosilicates
Structure of Phyllosilicates
  • Octahedral layer
    • Layer of octahedral coordinated
      • magnesium (brucite layer) or
      • Aluminum (gibbsite layer)
    • Makes up the other basic structural unit

Kaolinite: Al2Si2O5(OH)4

1:1 tetrahedral – octahedral


the major clay mineral groups
The Major Clay Mineral Groups
  • Kaolinite group:
    • 1:1 TO clay minerals
  • Mica (illite) group:
    • 2:1 TOT clay minerals
    • Expandible clays:
      • Smectite- montmorillonite complex 2:1 clay minerals
  • Chlorite
    • Fe- and Mg-rich TOT clays
2 1 tot phyllosilicates and true mica
Muscovite and Biotite:

Macroscopic model for clay minerals

TOT sandwich with an (K+) olive

Electrostatically neutral, stable

2:1 TOT Phyllosilicates and “True” Mica
2 1 tot phyllosilicates with charge deficiencies
Illite (relationships also relevant to other clays):

Muscovite with an attitude (charge) problem

More Si +4

Less Al +3

Less K+

Unit cell charge imbalance

Results in

Fine grain size

Colloidal size particles

High surface area to volume;

High surface electrostatic reactivity

Flocculation (particles stick together)

Cation exchange capacity (CEC)

2:1 TOT Phyllosilicates With Charge Deficiencies
clay mineral physical properties
Clay Mineral Physical Properties
  • Fine particle size (<2-4µm)
    • High surface area to volume
    • Electrostatic charge in
      • Interlayer (between “T” and “O” layers)
      • Exterior surfaces due to broken bonds
    • Other ions and polar molecules are attracted to and held by clay mineral particles
clay minerals water sorption and cec
Clay Minerals, Water Sorption, and CEC
  • Water sorption (electrostatic attachment to clay size particles) effects
    • Engineering properties of clay-rich Earth materials
      • Plasticity
      • Expansion/contraction with changes in humidity
    • Chemical properties of clay-rich Earth materials
      • Exchange of metal cations (and nitrogen compounds NH4+) with natural waters (ground water, etc);
      • CEC