Siliciclastics Flashcards
Sed Rec
SILICICLASTIC SEDIMENTS COMPOSE THE FOLLOWING
ROCK TYPES:
Breccia
Conglomerate
Sandstone
Mudstone
MOST TERRIGENOUS GRAINS ARE RICH IN what?
QUARTZ
TRANSPORTATION TO THE DEPOSITIONAL SITE CAN OCCUR BY A VARIETY OF AGENTS SUCH AS:
- WIND, WATER, GLACIER, GRAVITATION, ETC.
- SOURCE AREA FOR SEDIMENTS ARE UPLAND
- EX: MOUNTAIN REGIONS UNDERGOING UPLIFT ± EROSION IN LOWLAND AND COASTAL AREAS
- FINAL SEDIMENT COMPOSITION (MINERALOGY) IS AFFECTED BY DISTANCE OF TRANSPORT AND
- DIAGENETIC PROCESSES
**SEDIMENTARY PARTICLES REQUIRE ENERGY TO MOVE
1. High NRG depositional environments are the
2. Medium-size to fine grains are transported
3. As transport energy drops the size of the smallest particle
carried **
- largest grain sizes
- further away and laid down in lakes, deltas, and oceans
- drops proportionally
The Hjulstrom diagram tells us whether a river will do what?
erode, transport, or deposit
sediment
The Hjulstrom diagram can be used to reconstruct what?
Average water velocity based on grain size.
SEDIMENT TEXTURE: MORPHOLOGY
Morphology (particle shape) is defined by three related but different aspects of grains. What are these?
- Form: refers to the overall configuration of particles and reflects variations in their proportions
- Roundness: is a measure of the sharpness of grain corners (well-rounded to sharp/angular corners)
- Surface texture: refers to small-scale, micro-relief markings such as pits, scratches, or ridges that occur on the surface of the grains.
SEDIMENT TEXTURE: FABRIC
What is Fabric?
a function of grain orientation and packing.
SEDIMENT TEXTURE: FABRIC
Orientation and grain packing controls what?
Bulk density, porosity, and permeability
SEDIMENT TEXTURE: FABRIC
Poorly sorted sediments tend to have what?
lower porosities and permeabilities than well-sorted sediments
MINERALOGY OF SILICICLASTIC SEDIMENTS
**SILICICLASTIC ROCKS ARE COMPOSED OF: **
CLASTS OF PRE-EXISTING ROCKS AND MINERALS, AND CHEMICAL CEMENTS
CLASTS CONSIST OF MAJOR AND ACCESSORY MINERALS, ROCK FRAGMENTS (LITHICS). WHAT ARE THE MAJOR MINERALS (>1-2%)?
Quartz, Feldspar, and chemical cements
CLASTS CONSIST OF MAJOR AND ACCESSORY MINERALS, ROCK FRAGMENTS (LITHICS). WHAT ARE THE ACCESSORY MINERALS?
(<1-2%)
Micas (muscovite & biotite) & heavy minerals,
Rock fragments
MINERALOGY OF SILICICLASTIC SEDIMENTS
Quartz
(makeup, origins, stability,etc)
- up to 65% in sandstone and up to 30% in shales
- mono- or polycrystalline aggregates
- various origins: igneous, metamorphic (quartzite), chert
- mechanically & chemical stable, thus abundant
MINERALOGY OF SILICICLASTIC SEDIMENTS
Feldspar
(makeup, origins, stability,etc)
- 10%-15% in sandstone and up to 5% in shales;
- potassium feldspar (orthoclase, microcline, sanidine)
- plagioclase (albite to anorthite)
- less stable, therefore weathers to clay minerals (kaolinite, montmorillonite, smectite, etc.)
MINERALOGY OF SILICICLASTIC SEDIMENTS
Micas + Heavy minerals
(makeup, origins, stability,etc)
- stable non-opaque: zircon, rutile, anatase, tourmaline)
- metastable non-opaque: amphiboles, garnet, pyroxene, chlorite, epidote, apatite, etc.
- stable opaque: hematite & goethite
- metastable non-opaque: magnetite, ilmenite
MINERALOGY OF SILICICLASTIC SEDIMENTS
Rock Fragments
(makeup, origins, stability,etc)
- up to 10%-15%;
- could be any igneous, metamorphic, or other sedimentary rocks
- stability varies
MINERALOGY OF SILICICLASTIC SEDIMENTS
What are the chemical cements?
(makeup, origins, stability,etc)
- Silicates (quartz, μquartz, opal, feldspars)
- Carbonates (calcite)
- Fe-oxide (hematite goethite)
- Sulfates (gypsum, anhydrite, barite)
What is Breccia?
(sharpstone)
- lithified rubble made up of angular clasts > 2 mm
- Generally indicates clasts have not traveled far from their source or were transported by a non-fluid medium (e.g., gravity or glacial ice)
What is Conglomerate?
(roundstone)
- lithified gravel made up of rounded to subrounded clasts > 2 mm
- Rounded clasts may indicate considerable distance of transport from source
- Rounding time varies with the lithology of the clast
- EX: limestone clasts will become round a short distance from their source whereas quartzite will require much greater transport
BRECCIA AND CONGLOMERATE COMPOSITION
What makes up the particle composition?
- mineral fragments (major components: Q, ; 5% or more);
- mineral clasts (accessory constituents: garnet, Px, Amf, muscovite, zircon, Mgt*; <5%)
- rock fragments: are typically the most abundant and interesting
- They can be any variety of igneous (rhyolite), metamorphic (quartzite, slate, marble), or sedimentary rock (limestone)
BRECCIA AND CONGLOMERATE COMPOSITION
What is texture composition?
- Interstitial space between framework grains can be empty (pores), filled with finer-grained
- detrital matrix, occupied by cement, fluid (water or oil), or natural gas
- breccia & conglomerate are best studied in the field
- Difficult to use for stratigraphic correlations because of the lack of fossils
- Texture and composition provide insights about provenance, depositional environment, paleogeography, and tectonic settings
SILICICLASTIC ROCKS
Extraformational Source:
(where found, shape, framework, etc)
- clasts are “exotic”(derived from outside the depositional basin from a distant source)
- clasts are normally very well rounded and well sorted
- framework clasts differ markedly from matrix in composition
SILICICLASTIC ROCKS
Intraformational Source:
(where found, shape, framework, etc)
- clasts are derived from within the depositional basin
- clasts are eroded from the same sedimentary rock unit of which they are part
- framework grains are identical with those of the matrix (e.g., mud clasts)
ORTHOCONGLOMERATE IS FROM WHICH SOURCE?
Extraformational
Characteristics of orthoconglomerate:
- All clasts are in contact and support each other
- Likely little to no matrix between clasts (open framework)
- May have matrix of finer sediment between clasts (closed framework)**
PARACONGLOMERATES ARE FROM WHICH SOURCE?
Extraformational
Characteristics of Paraconglomerate:
- Most clasts are not in contact (i.e., the matrix supports the clasts)
- such conglomerates are typical for deposits of debris flow in which gravel size clasts were less abundant compared to the finer grain size
What is a Diamictite?
- a paraconglomerate composed of nonsorted-to-poorly sorted gravel-size grains suspended in a mud matrix
- glacial origin: meltwater flow, moraines, or ice rafted sediments
Orthoconglomerates can be oligomictic or polymictic. Describe each one.
- Oligomictic: Clasts are made up of only one rock type. suggests source was either nearby or extended over a wide geographic area.
- Polymictic: Clasts include several different rock types. Conglomerates include clasts from a broad-variety of source rocks, possibly derived over a wide geographic area or a smaller, but geologically more complex.
What is Sandstone?
the indurated equivalent of unconsolidated sand
Sandstones make up what percentage of Earth’s sedimentary rocks?
20-25%
What is Sandstone classification based on?
the percentage of matrix and composition of sand framework grains
* Q, F, and lithics (L) (i.e. rock fragments)
Arenites and wacke can be further subdivided using the QFL plot. What is the Q: F: L ratio?
The Q: F: L ratio is an index of compositional maturity, reflecting
the difference between sand with other soft, unstable rock
fragments and Fp
- Q, F, L = quartz, feldspar, lithic major constituent (>5%); f, l = feldspar, lithic accessory abundance (<5%)
DESCRIBE QUARTZ ARENITES
(texture, composition, color, size, etc)
- Super mature texture and composition
- Sedimentary structures = small, medium, and large scale cross stratifications
- Aeolian quartz arenites are commonly red through the presence of finely disseminated hematite (Fe 2 O3 ) which coats grains
What is the origin of Quartz Arenites?
- First-cycle deposits derived from primary crystalline or metamorphic rocks
- Product of multiple recycling of quartz grains from sedimentary rocks
What is the depositional environment of Quartz Arenites?
Typical of, but not restricted to, high-energy shallow-marine environments and aeolian (wind-blown) sand-seas in desert
DESCRIBE FELDSPATHIC ARENITES
(texture, composition, color, size, etc)
- Less mature texturally and compositionally compare to Q-arenites
- Bedding may range from structureless to parallel laminated or cross-bedded
- White, gray, or pink color of Fp imparts a similar tint to Fp-arenites
What is the depositional environment of Feldspathic Arenites?
Semi-arid and glacial climates favor Fp-arenites (arkoses) formation
(under humid conditions, Fp would weather to clay minerals)
What is the origin of Feldspathic Arenites?
derived from granites and gneisses that were either weathered in situ
(little movement) or when stratified and cross-bedded implies
substantial sediment transport
What is the occurrence of Feldspathic Arenites?
Cratonic or stable shelf settings, associated with conglomerates,
shallow-water Q-arenites or lithic arenites
DESCRIBE LITHIC ARENITES
(texture, composition, color, size, etc)
- Immature to sub-mature composition: Implies high rates of sediment production followed by short to moderate transport distances
- Sedimentary structures = irregularly bedded, cross-stratified, ripple marks
- Colors range from light gray, salt-and-pepper to medium, even dark gray
What is the origin of Lithic Arenites?
Form under conditions favoring the production and deposition of large volumes of relatively unstable materials (mudrock, volcanic grains, muscovite, some Fp)
What is the depositional environment of Lithic Arenites?
Characteristic of deltaic coastal plains, and may be deposited in nearshore marine environments, swamps, or marshes
DESCRIBE WACKES
(matrix, sand fraction, structure, etc)
- Hard, light to dark grey immature rocks as they are made of >15% matrix)
- Matrix: Chlorite, sericite, and silt-sized grains of Q + Fp. Fine-grained sediment deposited along with the sand fraction or diagenesis
- Sand fraction: Q»_space; L, Fp
- Sedimentary structures: sole structures, graded bedding, upward changes
in grain size characteristics to turbidites
What is the origin of Wackes?
BRUH..
Deposited by turbidity currents in deep water basins off continental margins,
in back-arc and fore-arc basins
What is the occurrence of Wackes?
Dominant sandstone of the Archean, when nongranitic volcanic arcs were bordered by deep troughs
What is cementation?
Involves μ-quartz, chalcedonic quartz and opaline silica
* carbonates may also be involved
What is Authigenesis?
Is the process in which NEW mineral phases are crystallized in
the sediment following one of the three pathways.
What are the three Authigenesis pathways?
- Reactions involving phases already present in the sediment (rock)
- Reaction between the primary sediment and other components introduced by circulating solutions
- Through precipitation of materials introduced in the fluid phase
What is Feldspar authegenesis?
- When pore waters are rich in Na +, K+, Al 3+, and Si 4+
- Hydrolysis & dissolution of metastable grains (volcanic ash) or transformation of plagioclase-Fp
What is Clay authigenesis?
- Illite & kaolinite are the most common authigenic clays in sandstones
- Occur as clay rims (early diagenesis) and pore-filling cements (late diagenesis), reducing porosity & permeability
DIAGENETIC ENVIRONMENTS
What is Early diagenesis?
- Processes taking place from deposition and into the shallow burial realm
- Time frame = 1,000 to 100,000 yrs
- Depth = 1 to 100 m
DIAGENETIC ENVIRONMENTS
What is Late diagenesis?
- Processes affecting the sediments at deeper levels, during and after uplift
- Time frame = million of years
- Depth = 100 to 10,000 m
