Identification and Classification of Sedimentary Rocks

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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= Allochemicalchemical 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, etcO= Orthochemicalprimary chemical ppt formed within31097
Identification and Classification of Sedimentary Rocks

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

2. 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

3. 3 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

4. 4 Clastic Rock Classification Texture: 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

5. 5 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

6. 6 Siliciclastic Rock Classification: Texture Descriptive Textural Classification: Ternary Plots (see appendix B in Fichter and Poche) of 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

7. 7 Textural Nomenclature ? TC?s

8. 8 Classification of Mudrocks

9. 9 Classification of Gravel/CGL

10. 10 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

11. 11 Mineralogical Classification Sandstone 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

12. 12 Mineralogical Classification Sandstone 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

13. 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. 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 [?], permeability [?]; 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. 15 Mineralogical Classification Primary Sandstone 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

16. 16 Data Plots and Sandstone 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

17. 17 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

18. 18 Sandstone Classification Complete Arenite name (Folk,Andrews, and Lewis, 1970): (sorting term), (size term): (cement), (prominent non-detrital grain type), (prominent detrital acc grain type), (named arenite) e.g Moderately Sorted, Medium-grained: Calcite Cemented, Glauconitic, Micaceous, Quartz Arenite

19. 19 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.


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