Sedimentary Analysis

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 :in situ

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

• Requires description (qualitative, quantitative)

• Analysis (graphical, statistical) interpretation

Describing Siliciclastics

• Description– Size– Texture– Fabric

• Analysis• Maturity

– Textural– Compositional

Describing Siliciclastics-Size• Size

• Gravel and larger (> 2 mm)• Sand (1/16 - 2 mm)• Mud (< 63m = < 1/16 mm)

Silt 4m >x<63mClay <4m

Gravel-Size (Pebbles & Cobbles)(> 2 mm)

Sand

• (1/16 - 2 mm)

• (< 63m = < 1/16 mm)

Mud

Describing Siliciclastics- Size Wentworth scale

• Udden- Wentworth size scaleUdden, 1914; Wentworth, 1922

• Resolves problems with size classificationCumbersome to discuss size

Limiting to restrict to 3 classes

• Four basic groups + modifiers make moreClay (< 4 m)

Silt (4 m - 63 m)

Sand (63m - 2 mm)

Gravel (> 2 mm)

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

Describing Siliciclastics- Size Wentworth scale (cont’)

– Subdivided scale by factor of 2.0039 mm clay.0078 mm very fine silt128 mm = cobbles256 mm = boulders

Logarithmic (base 2) progression!

= -log2(grain diameter in mm)

As grain size increases, phi size decreases

Describing Siliciclastics- Sedimentary Texture

• Aspects of texture– Shape

– 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 historyEdges 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/ShapeZingg indices

spherical (equant), oblate (disk or tabular), bladed, prolate (roller)

ZINGG DIAGRAM

Where:di= intermediate

diameterdl= long diameterds= short diameter

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 Degree of sorting

• Measure of distribution of clast sizes • Well sorted

most clasts fall into one class on the Wentworth scale

• Poorly sortedwide 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 clastics

– Mean: average particle size– Mode: most abundant class size– Median: 50th percentile

Describing Siliciclastics Grain size analysis

• Quantitative analysis– (granulometric analysis)

• Quantitative assessment of % of different grain sizes in clastic sediments

– 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.

Describing Siliciclastics Grain size analysis- graphic analysis

• Plots– Histogram of weight percentage of size fractions– Frequency curve– Cumulative frequency curveWhen 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)

Describing Siliciclastics Grain size analysis- graphic analysis

• Different depositional environments exhibit different grain size distributions

• Glacial sedimentspoorly sorted

• River sedimentsmoderately sorted

• Beach sedimentswell sorted

Statistical/Graphic Presentation of Texture; Granulometry

Refraction Index: immersion oils of known refractive index and comparing the unknown mineral to the oil