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 –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 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 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 Material Pyroclasts – particles fragmented and transported by volcanic processes Tephra: tuff deposits Volcanic mudflows: lahar and volcanic breccia deposits Tephra Volcanic Ash

Terrigenous Sediment

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 Description –Size –Texture –Fabric Analysis Maturity –Textural –Compositional

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 Size Gravel and larger (> 2 mm)(conglomerate) Sand (1/ mm)(sandstone) Mud(< 63  m = < 1/16 mm)(mudstone)

Conglomerate and Breccia (> 2 mm)

Sand becomes sandstone (1/ mm)

(< 63  m = < 1/16 mm) Mud becomes shale

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 silt> 0.004mm

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 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  m) Silt (4  m - 63  m) Sand (63  m - 2 mm) Gravel (> 2 mm)

Siliciclastic Rock Classification:Texture Descriptive Textural Classification –Grain Size Uden-Wentworth grain size scale Phi (  )=-log 2 (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’) –Subdivided scale by factor of mm clay.0078 mm very fine silt 128 mm = cobbles 256 mm = boulders Logarithmic (base 2) progression!  = -log 2 (grain diameter in mm) As grain size increases, phi size decreases

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 Siliciclastics 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 Form/Shape Zingg indices spherical (equant), oblate (disk or tabular), bladed, prolate (roller)

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

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 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 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 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 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 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. 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) Sandstones and Conglomerates (∑2d/n)/N n=#grains cut by view; d = diameter of field of view; N = total # views counted

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, f ines 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 analysis Different depositional environments exhibit different grain size distributions Glacial sediments poorly sorted River sediments moderately sorted Beach sediments well sorted

Statistical/Graphic Presentation of Texture; Granulometry

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 Textural Maturity –Kinetic energy during transport and reworking –Transport history –Dispersal patterns –Caveat emptor! Mixed sources Biogenic reworking

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 –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 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 ResistantLess resistant QuartzFeldspar Chert clastsMost other minerals ZirconLithic clasts

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)