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Identification and Classification of Sedimentary Rocks

Identification and Classification of Sedimentary Rocks

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Identification and Classification of Sedimentary Rocks

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  1. Identification and Classification of Sedimentary Rocks Terrigenous Clastic Sediments and Rocks

  2. 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

  3. 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

  4. 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

  5. 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

  6. Statistical/Graphic Presentation of Texture; Granulometry

  7. 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

  8. 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

  9. 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

  10. Significance of Grain Size and Sorting: Intrinsic and Derivative Physical Properties • Derivative Properties • Grain size vs permeability

  11. 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

  12. 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

  13. 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.

  14. 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.

  15. 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

  16. 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

  17. Main Authigenic Components of Sandstone • Carbonate cement (calcite, dolomite, ankerite, siderite) • Clay minerals (kaolinite, illite) • Quartz • Feldspar (albite) • Zeolite

  18. 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

  19. 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

  20. Interpretation of Sandstone Composition: • MATURITY – • 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.

  21. 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

  22. 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

  23. Mudrocks • 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

  24. 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 Phyllosilicates Si2O5 sheets of silica tetrahedra

  25. 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 sheets

  26. 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

  27. Muscovite and Biotite: Macroscopic model for clay minerals TOT sandwich with an (K+) olive Electrostatically neutral, stable 2:1 TOT Phyllosilicates and “True” Mica

  28. 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

  29. 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

  30. 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