Sed Pet Week III

sedimentary structures, sandstone compositions

Liquefied flows

Very concentrated dispersions of grains in a fluid. Usually result from shock of granular sediment (earthquake). Grains kept in suspension by fluid pore pressure and upward movement of expelled fluid

Grain flows

Characterized by grain to grain collisions; little friction so they only occur on steep slopes where the angle of initial yield is exceeded

Debris flows

Slurry like flows in which large particles are set in fine-grained matrix. Matrix has yield strength that helps to support grains. It lubricates grain irregularities so debris flows can occur on gentle slopes. Coarser grains carried on top

seen commonly in continental margins (subaqueous)

Cohesive freezing

Internal cohesion causes a fluid to stop moving/move very slowly

Sandstone formation

Sed rock → pedogenesis (soil) → erosion → transport → deposition/burial

Processes of transport

mechanical breakage, chemical weathering, authigenic input, hydraulic sorting, burial diagenesis

Goldich weathering series

Silicate minerals with higher polymerization tend to be most stable at earth’s surface (quartz, muscovite, feldspar). Those that form at higher temps and pressures are less stable and this more susceptible to weathering (olivine, pyroxenes)

Interference colors differ due to

Birefringence * thickness of thin section. Light is split in different directions when entering a mineral

Sodium cobaltinitrite

stains potassium-bearing minerals yellow

Most common way clay minerals are formed

Formed through weathering of feldspar. Most are product of hydrolysis

Sandstone types

quartz-sandstones, feldspathic sandstones, lithic sandstones, muddy sandstones, mudstones

Greywacke

muddy sandstone that is grey in color (dirty sandstone)

Quartz subdivisions

Monocrystalline (single quartz crystal in a grain). Usually volcanic, not undulatory

Polycrystalline (multiple quartz crystals in a grain). Mature sandstones, undulatory

Foliated (quartz crystals are squashed and elongated). Seen in metamorphic grains, polycrystalline quartz

Chert (cryptocrystalline, very small crystals). Speckled appearance under microscope

generally optically clear, conchoidal

Feldspar subdivisions

Potassium feldspar (carlsbad twinning) orthoclase. Turns yellow with staining

Plagioclase feldspar (polysynthetic twinning) albite. Doesn’t turn yellow with staining

Unidentified (hard to tell)

usually have close to 90 degree cleavage in 2 directions

Zoning in plagioclase

Early plagioclase rich in calcium (anorthite)

Late plagioclase rich in sodium (albite). Show more zoning, center of grain is often more calcium-rich and fines outward to sodium rich

Lithic subdivisions

Sedimentary - (mudstone, siltstone, shale, sandstone, carbonate)

Metasedimentary - (foliated, massive)

Metamorphic (look for deformation)

Plutonic (granite, pyroxenes, porphyry)

Volcanic (basalt, feldspar)

Euhedral, subhedral, anhedral

How well formed crystal faces are; euhedral are well formed, anhedral are irregular and have few faces. subhedral are somewhere in between

Undulatory extinction

Because the c-axis of the grain is bent due to heavy deformation, the grain will go extinct at different times (quartz)

Optic axis (hexagonal and tetragonal)

Same as the c axis (vertical). When looking down this, the mineral will go extinct (no light splitting)

Controls on sandstone composition

• Source composition (dependent on tectonic setting, eg. quartz varieties)

• Modification during weathering (climate)

• Modification during transport (sorting, compositional
  stabilization)

• Modification during burial (introduction of cements,
  deletion of labile grains)

Dolomite shape in xpl

rhomb, with warm, pale brown colors (high birefringence)

Quartz grades

Lower grade (lower metamorphic) quartz tends to be undulatory. Polycrystalline, less clear

Upper grade (upper metamorphic, volcanic) quartz tends to be less so. Monocrystalline, optically clear

Quartz and dolomite replacement

Quartz dissolves under basic conditions, precipitates in acidic conditions

Dolomite precipitates under basic conditions, dissolves in acidic conditions

dolomite replaces the quartz

Stable craton provenance

Associated with little metamorphic activity; quartz rich

Continental block provenance

Chunk of continental crust (granite). Generally feldspar and quartz rich. Seen in rift-related settings (east Africa), and transform faults

Recycled orogen provenance

Foreland fold thrust belts are common; limestone and surface sedimentary rocks are visible and deformed, but basement metamorphic layers are rarely visible. Basal layers are not deformed, and so the sediment that is produced will be mainly sedimentary (sand-sized lithics and quartz; chert, flint, siltstone, sandstone, dolomite)

Magmatic arc provenance

Arc origin sources show mainly lithic-volcanics.

Weathering in hot, moist climates

Hydrolysis happens faster because heat is a catalyst. Weathering happens much more quickly, chemically and physically. Polycrystalline quartz is broken down into monocrystalline grains, feldspar and lithics are removed

Weathering in cold, dry climates

Hydrolysis happens more slowly due to less heat. Weathering happens slowly because there are fewer catalysts to speed up the process. More feldspar and lithics survive in sandstones

Larger lithic grains are more likely to

Survive in transport. Smaller lithic grains are weaker and break into their constitutive parts. Finer sand grains also mean lower presence of feldspar (downstream fining)

Coarser sandstones have higher presence of

lithics than fine grained sandstones. It is important to collect sandstone with similar grain sizes when comparing them

Sandstone analysis considerations

1. look at the fabric of the sample (matrix/grain support, grain contact, overgrowths, etc.)

2. look at grain texture (size, shape, sorting, rounding)

3. look at composition of framework grains

4. look at composition and nature of interstitial material (diagenetic contributions)

Overgrowths

layers of new quartz that form on top of existing sand grains during the diagenetic process. Layers preserve the original rounded detrital quartz grain. Overgrowths can become abraded if sandstone is weathered

Textural anomalies

Irregularities in grain sorting that indicate the sediment must have undergone different processes before being deposited

ex:

well-rounded sand grains floating in silt or clay matrix
extremely well-sorted but very angular grains
finer grains better rounded than coarser grains

Types of cement in siliciclastic sandstones

Most common: carbonate, silicate (qtz), clay

less common: sulfate, halite, iron oxides

Types of authigenic grains in siliciclastic sandstones

feldspar, hematite, glauconite, zeolites (volcaniclastics)

How to recognize siliciclastic type from quartz

see image

Feldspar varieties (complex)

see image

Lithic fragments are the most direct evidence of

provenance types (where the rock originated). However, they can be fine grained and hard to interpret

Factors influencing presence/type of lithics

– Source type
– Distance from source (e.g., schist fragments restricted to close to source)
– Generally more prevalent in coarser fractions
– Durability influenced by climate
– Some are highly susceptible to weathering and diagenesis (e.g. volcanic lithics)
– Intraclasts derived from within basin (mud chips)

Volcaniclastics

Very volumetrically important. Eruption type can be related to composition; silicic is indicative of explosive eruptions (felsic) while mafic is indicative of quiet eruptions

Tuff types

Vitric (glassy appearance; indicative of pumice, flow banding)

Crystal (crystals may be any volcanic material)

Lithic tuff (variety of lithic fragments can be incorporated, either during eruption or subsequent reworking of volcanic sand)