Post on 22-Dec-2015
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