13.8 Special Methods and Processes in Diagenesis
incoming pore water may ultimately leave the accretionary
wedge via diffusive flow and less than 1 % via focused flow
along the decollernent; roughly 1 % is subducted (Saffer and
Bekins (1998). Dewatering of smectite at burial depths in
excess of 1 km at least contributes to the freshening of the
escaping pore water.
The submarine vents discharge not only water with
elevated temperatures, but also pore fluids reflecting
the chemie al properties of the sulfate reduction zone
and the zone ofmethanogenesis (cf. Sect. 13.4). Ifthe
subducting sediments have been deposited rapidly and
are rich in organic matter, the expelled pore water may
contain hydrogen sulfide andJor methane in considerable quantities (Fig. 13.32). Some of the methane is
oxidized near the surface and gives rise to the precipitation of calcium carbonate cement (and barite) which
may lithify sand and form chimneys at the vents. Near
13.9 Summary (Chemical Diagensis)
Major processes in chemie al diagenesis and rock
lithification are dissolution and altering of thermodynamically unstable pre-existing minerals,
replacement of older and precipitation of new
mineral phases.
Dissolved matter is transported in the subsurface
by molecular diffusion (over short distances)
and by various flow systems (cf. Sect. 13.2.9),
partially at a basin-wide scale.
Early diagenesis (eodiagenesis), particularly that
of continental or emerged marine deposits, is
mainly controlled by the depositional environment (e.g. humid vs. arid, oxidizing vs. reducing), as well as by topographically driven meteoric flow, pedogenesis, or sabkha brine migration. One can distinguish between vadose,
phreatic, and marine cements.
Sandstones may experience a kind of red-bed
diagenesis with the formation of hematite,
anhydrite and c1ay minerals.
Submerged muddy sediments with organic matter may be strongly affected by sulfate reduction
and methanogenesis (formation of pyrite and
carbonate concretions).
The compaction and lithification of chalk and
biosiliceous sediments often takes place in distinct steps.
The deeper burial diagenesis is characterized by
pressure solution at grain contacts (beginning at
705
these vents, faunal communities, including giant c1ams
and large tube worms adapted to methane, hydrogen
sulfide, and warm water, were observed.
Such phenomena can also be expected in ancient
subduction zones. In addition, overpressuring of sediments may cause liquefaction and fluidization. As a
result, sediment is forced upward to form mud diapirs
and mud volcanoes, or it is injected vertically and laterallyas dikes and sills into neighboring deposits (cf.
Sect. 12.5.2). In this way, chaotic structures of large
dimension can form which resemble phenomena
known from thick debris flows or tectonic melange
zones. The dewatering of accretionary prisms is an
important process in the deformation, stress release,
diagenesis and subsequent metamorphism of sediments
at convergent plate boundaries.
~ 1 km burial depth for quartz, but earlier in carbonates) and continued cementation. Quartz
and ferroan carbonates are most abundant. Unstable silicate minerals (e.g. smectites and
zeolites) transform to more stable phases,
mostly under the release of Si0 2 and water.
- Mixing of pore fluids of differing composition
can lead to secondary porosity and the precipitation of new mineral phases. Hot evaporitic
brines from nearby or distant sources, or renewed meteoric water circulation, aided by
fracturing, may reactivate the diagenetic system. They enable, among other effects, regional
(pervasive) dolomitization, late sulfate cements,
and base metal concentrations.
- Uplift and exhumation of deeply buried strata is
frequently accompanied by meteoric water circulation, dedolomitization, and final Fe-poor
blocky calcite cementation.
- The matrix permeability of most rock types is
drastically reduced by diagenesis, whereas
some porosity can often be maintained.
- The thermal evolution of basin fillings controls
the maturation of organic matter and the
prograde or retrograde solubility of minerals.
- The dewatering of accretionary complexes creates specific phenomena along subduction
zones.
incoming pore water may ultimately leave the accretionary
wedge via diffusive flow and less than 1 % via focused flow
along the decollernent; roughly 1 % is subducted (Saffer and
Bekins (1998). Dewatering of smectite at burial depths in
excess of 1 km at least contributes to the freshening of the
escaping pore water.
The submarine vents discharge not only water with
elevated temperatures, but also pore fluids reflecting
the chemie al properties of the sulfate reduction zone
and the zone ofmethanogenesis (cf. Sect. 13.4). Ifthe
subducting sediments have been deposited rapidly and
are rich in organic matter, the expelled pore water may
contain hydrogen sulfide andJor methane in considerable quantities (Fig. 13.32). Some of the methane is
oxidized near the surface and gives rise to the precipitation of calcium carbonate cement (and barite) which
may lithify sand and form chimneys at the vents. Near
13.9 Summary (Chemical Diagensis)
Major processes in chemie al diagenesis and rock
lithification are dissolution and altering of thermodynamically unstable pre-existing minerals,
replacement of older and precipitation of new
mineral phases.
Dissolved matter is transported in the subsurface
by molecular diffusion (over short distances)
and by various flow systems (cf. Sect. 13.2.9),
partially at a basin-wide scale.
Early diagenesis (eodiagenesis), particularly that
of continental or emerged marine deposits, is
mainly controlled by the depositional environment (e.g. humid vs. arid, oxidizing vs. reducing), as well as by topographically driven meteoric flow, pedogenesis, or sabkha brine migration. One can distinguish between vadose,
phreatic, and marine cements.
Sandstones may experience a kind of red-bed
diagenesis with the formation of hematite,
anhydrite and c1ay minerals.
Submerged muddy sediments with organic matter may be strongly affected by sulfate reduction
and methanogenesis (formation of pyrite and
carbonate concretions).
The compaction and lithification of chalk and
biosiliceous sediments often takes place in distinct steps.
The deeper burial diagenesis is characterized by
pressure solution at grain contacts (beginning at
705
these vents, faunal communities, including giant c1ams
and large tube worms adapted to methane, hydrogen
sulfide, and warm water, were observed.
Such phenomena can also be expected in ancient
subduction zones. In addition, overpressuring of sediments may cause liquefaction and fluidization. As a
result, sediment is forced upward to form mud diapirs
and mud volcanoes, or it is injected vertically and laterallyas dikes and sills into neighboring deposits (cf.
Sect. 12.5.2). In this way, chaotic structures of large
dimension can form which resemble phenomena
known from thick debris flows or tectonic melange
zones. The dewatering of accretionary prisms is an
important process in the deformation, stress release,
diagenesis and subsequent metamorphism of sediments
at convergent plate boundaries.
~ 1 km burial depth for quartz, but earlier in carbonates) and continued cementation. Quartz
and ferroan carbonates are most abundant. Unstable silicate minerals (e.g. smectites and
zeolites) transform to more stable phases,
mostly under the release of Si0 2 and water.
- Mixing of pore fluids of differing composition
can lead to secondary porosity and the precipitation of new mineral phases. Hot evaporitic
brines from nearby or distant sources, or renewed meteoric water circulation, aided by
fracturing, may reactivate the diagenetic system. They enable, among other effects, regional
(pervasive) dolomitization, late sulfate cements,
and base metal concentrations.
- Uplift and exhumation of deeply buried strata is
frequently accompanied by meteoric water circulation, dedolomitization, and final Fe-poor
blocky calcite cementation.
- The matrix permeability of most rock types is
drastically reduced by diagenesis, whereas
some porosity can often be maintained.
- The thermal evolution of basin fillings controls
the maturation of organic matter and the
prograde or retrograde solubility of minerals.
- The dewatering of accretionary complexes creates specific phenomena along subduction
zones.