Diagenesis of Siliciclastics

Diagenesis of Siliciclastics
What is Diagenesis?
Diagenesis is the physical and chemical processes
that convert sediment into sedimentary rock.
This conversion is due to an increase in
temperature, an increase in pressure, and changes
in pore-water composition.
Initially, siliciclastic rocks form as unconsolidated
deposits of gravel, sand or mud. After diagenesis
gravel is lithified to conglomerates, sand is
lithified to sandstone, and mud into shale.
When Does Diagenesis Occur?
Diagenesis normally occurs at a temperature
below ~300oC.
 Some controlling factors of diagenesis are:
composition, pressure, temperature, grain
size, porosity/permeability, and the amount
of fluid flow.
 After deposition has taken place, diagenesis
can begin almost immediately.
Three Stages of Diagenesis
– earliest stage which
takes place at very shallow depths
 Mesodiagenesis – occurs during deep
 Telodiagenesis – uplift of buried
sediment into the system of meteoric
 Eodiagenesis
Eodiagenesis occurs mainly under the
conditions of a depositional environment.
 The principal changes that occur are
bioturbation, mineralogical changes, and
 A common result of eodiagenesis is the
formation of mottled bedding due to
Burrows dug by organisms in the sediment
The processes that take place in
mesodiagenesis are: physical and chemical
compaction, cementation, dissolution by
pore fluids, mineral replacement, and clay
mineral authigenesis.
 Physical compaction – results in porosity
reduction, bed thinning, and tighter grain
Compaction Due to Pressure
Factors that control compaction are: grain
shape, sorting, original porosity, and pore fluid
Continued Mesodiagenesis
Chemical compaction – results in partial
dissolution of silicate grains
 Cementation – results in a reduction of
porosity thus bringing about lithification.
Carbonate and silica are the most common
 Dissolution – almost the opposite of
cementation; leads to an increase in porosity
Diagenesis: cementation
Cementation is the process in which
chemical precipitates (in the form of new
crystals) form in the pores of a sediment
or rock, binding the grains together.
Common cements: quartz, calcite and
Less Common: aragonite, gypsum, and
uncrossed polars
©K. Simpson
Pressure solution produces locally derived
Many cements consist of new minerals
previously in solution in the fluid phase.
Cementation reduces porosity by filling in
the pore spaces between the grains.
crossed polars
©K. Simpson
Photomicrograph of a dolomite-cemented siltstone in crossed and
uncrossed polars. The cement between the grains can be easily seen
Mesodiagenesis continued
Mineral replacement – results in partial to
complete replacement of some silicate
grains and clay matrix by new minerals
 Clay mineral authigenesis – alteration of
one kind of clay mineral to another (i.e.)
smectite clays will transform to illite at
temperatures ranging from 55-200oC.
Diagenesis: replacement
Opalized wood
Although there are many
replacement phases, dolomite, opal,
quartz, and illite are some of the
most important phases
Replacement occurs when a newly formed
mineral replaces a preexisting one in situ.
Replacement may be:
•neomorphic: where the new grain is the same phase as the old grain, or is a
polymorph of it (i.e. albitization; replacing a grain with a more Na-rich
plagioclase grain).
•pseudomorphic: where the old grain is replaced with a new mineral but the
relict crystal form is retained,
•allomorphic: an old phase is replaced with a new phase with a new crystal
Rocks that have gone through deep burial may be
uplifted due to erosion or mountain building
Sedimentary rocks will experience dissolution,
replacement, and oxidation.
The results include: solution of carbonate cements,
alteration of feldspars to clay minerals, oxidation
of iron carbonate minerals to iron oxides and
pyrite to gypsum, and solution of less stable