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Sandstone Cementation (Quartz Cement (euhedral overgrowth (angular…
Sandstone Cementation
Quartz
Cement
euhedral overgrowth
angular crystal growths
superimposed atop remnant quartz grains
interlocks
grains together
decreases porosity
by filling porespace
optical continuity
- goes extinct with grain it surrounds
dusty rough pitted rim
= remnant of rounded original sand grain (detrital grain)
Formation
qtz cement
Quartz sourced from surrounding rock
Qtz dissolved during chemical compaction
pressure solution at long/suture grain contacts
Dissolved quartz diffuses away
Reprecipitates as cement nearby
Secondary porosity
space within dissolved grains
can represent dissolution of an entire crystal?
i.e. space formed inside dissolving fsp
oversized pores
anomalously large pore (similar size to grains)
form via dissolution of unstable grains
form during burial
Cement Types
Carbonate
dolomite CaMg(CO3)2
calcite CaCO3
Silica (quartz) SiO2
Clay e.g. kaolinite
Pressure Solution
=
Dissolution at grain contacts
during deep burial
--> Cement precipitation occurs elsewhere
Dissolved material must go somewhere
to maintain mass balance
Cement
derived
often from surrounding rock
advection of material from elsewhere
Depositional setting = control
quartz = grain-grain disolution qtz - reprecipitation
calcite - local detrital grains
Cementation
precipitation of dissolved material
which joins grains together
minerals dissolved during deep burial by
pressure solution
Varying extent cementation
Low - Porous
sandstones
Little to no cement
Compacted - point and long contacts
Lotsa porosity between original grains
Extensively cemented
Cement fills pore space
No floating grains
Zero porosity between original grains
Impermeable
Mineralogically mature (fsp/qtz)
Cement
=
Precipitate filling space between grains
Mineral in solution in pore water
Crystallises out during burial
Prevents further compaction
closes porosity
Cement forms after deposition
Limestones - near surface
Sandstones - during deep burial
Cement decreases porosity
Chemical compaction - to dissolve mineral
Quartz cement fills pores