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)

• 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 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