Transcript Sedimentary Textures

Sedimentary Analysis
Types of Sedimentary Material
• Terrigenous Clastics (TC)
– Detrital Particles
– Transported by surface
processes to the site of
– Derived from pre-existing rocks
deposition
– Derived external to the
• Particulate Residues: quartz,
depositional basin
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
(< 63mm = < 1/16 mm)
Silt
4mm >x<63mm
Clay
<4mm
Gravel-Size (Pebbles & Cobbles)
(> 2 mm)
Sand
• (1/16 - 2 mm)
Mud
• (< 63mm = < 1/16 mm)
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)
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 silt
128 mm = cobbles
256 mm = boulders
Logarithmic (base 2) progression!
 = -log2(grain diameter in mm)
As grain size increases, phi size decreases
Describing SiliciclasticsSedimentary 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
• 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
Where:
di= intermediate
diameter
dl= long diameter
ds= short diameter
ZINGG DIAGRAM
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 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 clastics
– Mean: average particle size
– Mode: most abundant class size
th
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 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)
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
Refraction Index: immersion oils of known refractive index
and comparing the unknown mineral to the oil