Weathering and the Production of Sediment

1. Weathering and the Production of Sediment Surface Portion of the Geological Cycle

2. Types of Sedimentary Material

3. 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

4. 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

5. 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

6. 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

7. Terrigenous Sediment

8. 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

9. Describing Siliciclastics • Description • Size • Texture • Fabric • Analysis • Maturity • Textural • Compositional

10. 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

11. Describing Siliciclastics-Size • Size • Gravel and larger (> 2 mm) (conglomerate) • Sand (1/16 - 2 mm) (sandstone) • Mud (< 63mm = < 1/16 mm) (mudstone)

12. Conglomerate and Breccia (> 2 mm)

13. Sand becomes sandstone • (1/16 - 2 mm)

14. Mud becomes shale • (< 63mm = < 1/16 mm)

15. 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

16. 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

17. 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)

18. 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

19. 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

20. 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

21. 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

23. 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

24. 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!

25. 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

26. 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

28. 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

29. 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

30. 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)

31. 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)

32. 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

33. Statistical/Graphic Presentation of Texture; Granulometry

34. 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)

35. 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

36. 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

38. 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

39. 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

40. 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)