13.5 Late, Deep-Burial Diagenesis
commonly well aerated, but oxidation also affects the
upper part of the saturated, phreatic zone. As a result,
primary sediment partic1es containing ferrous iron and
manganese are oxidized and partially replaced by
oxyhydrates of iron and manganese (e.g., limonite,
goethite). These iron minerals can be later transformed
into hematite forming coatings around larger grains
(red-bed diagensis; cf. Sect. 6.3). While unstable silicate minerals tend to be transformed into kaolinite in
humid c1imates, arid conditions may lead to the
neoformation of potassium feldspar.
Sands in deeper marine environments, on the other
hand, are affected neither by meteoric waters nor by
carbonate-supersaturated sea water. Since most oftheir
common mineral phases (quartz, feldspar, c1ay minerals) are more or less in equilibrium with sea water,
there is little cause for reactions between the sand partic1es and the marine pore fluid during early diagenesis
(cf. Fig. 13.20). Above or near the redox boundary
(Fig. l3.23a,b), some glauconitic minerals and ironrich chlorites and chamosites may form (cf. Sect. 6.1).
In the sulfate reduction and methanogenesis zones,
pyrite and various carbonates may locally fill the pore
space.
Where relatively thin sand layers and sand bodies
act as drains for upstreaming compaction flow (Sect.
13.2.3) supersaturated with respect to carbonate and
silica, small amounts of quartz cement and possibly
larger volumes of carbonate may precipitate. However,
cooling of the ascending pore fluids may reduce precipitation because the solubility of carbonate increases
with lowering temperature. Sands in direct or indirect
contact with evaporative brines frequently exhibit sulfates, halite, and zeolites as early pore cement. After
uplift and exposure to subaerial weathering, former
cements of deeper burial are altered, e.g. siderite to
goethite or felds pars to c1ay minerals.
These processes are described for various sediments and basin settings in numerous pub1ications (see, e.g., Füchtbauer
1988; Bj0r1ykke 1984, 1991; Hesse 1989b; Muchez et al.
1992; Houseknecht and Pittrnan 1992; Burley and
MacQuaker, Gaupp et al. 1993; Crossey et al. 1996; Gaupp
1996; Cookenboo and Bustin 1999). Further references are
listed below).
13.5 Late, Deep-Burial Diagenesis
13.5.1 Trend to a Final, Inactive Stage?
Meteoric water circulation, being frequently a major
mechanism in early diagenesis, particularly in well permeable sands and bioc1astic grainstones (cf. Sect.
l3.4.4), normally ceases in deeply buried sediments
sealed by overlying low-permeable strata.
At deep burial depths the sediments are subjected to
increased temperature and ejJective stress which promote the dissolution of several major mineral species
689
(e.g. that of quartz). Other minerals that are stable at
surface temperature become unstable and tend to transform into new minerals and to release some water (Fig.
l3 .19). Pressure solution and diffusion transport principally act without fluid flow (advection). However,
their effects slow because of the increasing contact
areas between neighboring grains (cf. Fig. l3.11d) and
decreasing diffusion coefficients.
With increasing temperature and burial depth, the
zone of microbial methanogenesis is followed by the
zone ofthermocatalytic decarboxylation (Fig. 13.22b;
cf. Sect. 14.2). This process is generally ascribed to
late diagenesis. Its products are methane, carbon dioxide, and various hydrocarbons. In addition, siderite
may form in minor quantities. If the decomposition of
kerogen releases some sulfur, small quantities of late
diagenetic pyrite may result. Thermocatalysis tends to
end when all reactive organic matter is consumed. Solute transport by the various flow mechanisms described in the previous sections becomes less effective
as a result of decreasing rock matrix permeability.
All this may lead to the conc1usion that late
diagenesis generally operates very slowly and finally
approaches a ± inactive final stage, unless substantially
increased temperature and pressure conditions trigger
the on set ofmetamorphism (see below). The approach
to an inactive diagenetic stage is certainly true for
many cases where the rocks have become more or less
sealed against influences from outside and have not
been affected by substantial subsequent tectonism (e.g.
subsidence or uplift).
13.5.2 Reactivation of Late-Stage Diagenesis
On the other hand, many examples of the diagenetic
his tory of rocks have been described where late
diagenesis has been reactivated by continued or renewed fluid flow and the import of low-salinity or
evaporitic brines from overlying, laterally located, or
underlying strata (cf. Sect. l3.3.7). This is reflected by
formation waters of markedly varying composition
(Sect. l3.3.6), by late-stage cementation and secondary
porosity, as well as by cement minerals formed coeval
with or later than hydrocarbon migration (cf. Sect.
13.3.7). Where pore fluids of differing composition
mix, as stated earlier, a new suite of cement minerals
can be generated andlor older cements and primary
minerals are dissolved or replaced by other cements.
This mechanism has been quoted as one of the most
important pro ces ses in late diagenesis.
Syn- and post-depositional faulting in conjuction
with overpressured systems, differential subsidence
and compaction, as well as other tectonic processes
can bring about a change from the normally c1osed,
deep-burial diagenetic system to an open one. Fracturing, uplift and emergence of part of the basin filling
can reactivate meteoric flow. Ifthe new fluid conduits
commonly well aerated, but oxidation also affects the
upper part of the saturated, phreatic zone. As a result,
primary sediment partic1es containing ferrous iron and
manganese are oxidized and partially replaced by
oxyhydrates of iron and manganese (e.g., limonite,
goethite). These iron minerals can be later transformed
into hematite forming coatings around larger grains
(red-bed diagensis; cf. Sect. 6.3). While unstable silicate minerals tend to be transformed into kaolinite in
humid c1imates, arid conditions may lead to the
neoformation of potassium feldspar.
Sands in deeper marine environments, on the other
hand, are affected neither by meteoric waters nor by
carbonate-supersaturated sea water. Since most oftheir
common mineral phases (quartz, feldspar, c1ay minerals) are more or less in equilibrium with sea water,
there is little cause for reactions between the sand partic1es and the marine pore fluid during early diagenesis
(cf. Fig. 13.20). Above or near the redox boundary
(Fig. l3.23a,b), some glauconitic minerals and ironrich chlorites and chamosites may form (cf. Sect. 6.1).
In the sulfate reduction and methanogenesis zones,
pyrite and various carbonates may locally fill the pore
space.
Where relatively thin sand layers and sand bodies
act as drains for upstreaming compaction flow (Sect.
13.2.3) supersaturated with respect to carbonate and
silica, small amounts of quartz cement and possibly
larger volumes of carbonate may precipitate. However,
cooling of the ascending pore fluids may reduce precipitation because the solubility of carbonate increases
with lowering temperature. Sands in direct or indirect
contact with evaporative brines frequently exhibit sulfates, halite, and zeolites as early pore cement. After
uplift and exposure to subaerial weathering, former
cements of deeper burial are altered, e.g. siderite to
goethite or felds pars to c1ay minerals.
These processes are described for various sediments and basin settings in numerous pub1ications (see, e.g., Füchtbauer
1988; Bj0r1ykke 1984, 1991; Hesse 1989b; Muchez et al.
1992; Houseknecht and Pittrnan 1992; Burley and
MacQuaker, Gaupp et al. 1993; Crossey et al. 1996; Gaupp
1996; Cookenboo and Bustin 1999). Further references are
listed below).
13.5 Late, Deep-Burial Diagenesis
13.5.1 Trend to a Final, Inactive Stage?
Meteoric water circulation, being frequently a major
mechanism in early diagenesis, particularly in well permeable sands and bioc1astic grainstones (cf. Sect.
l3.4.4), normally ceases in deeply buried sediments
sealed by overlying low-permeable strata.
At deep burial depths the sediments are subjected to
increased temperature and ejJective stress which promote the dissolution of several major mineral species
689
(e.g. that of quartz). Other minerals that are stable at
surface temperature become unstable and tend to transform into new minerals and to release some water (Fig.
l3 .19). Pressure solution and diffusion transport principally act without fluid flow (advection). However,
their effects slow because of the increasing contact
areas between neighboring grains (cf. Fig. l3.11d) and
decreasing diffusion coefficients.
With increasing temperature and burial depth, the
zone of microbial methanogenesis is followed by the
zone ofthermocatalytic decarboxylation (Fig. 13.22b;
cf. Sect. 14.2). This process is generally ascribed to
late diagenesis. Its products are methane, carbon dioxide, and various hydrocarbons. In addition, siderite
may form in minor quantities. If the decomposition of
kerogen releases some sulfur, small quantities of late
diagenetic pyrite may result. Thermocatalysis tends to
end when all reactive organic matter is consumed. Solute transport by the various flow mechanisms described in the previous sections becomes less effective
as a result of decreasing rock matrix permeability.
All this may lead to the conc1usion that late
diagenesis generally operates very slowly and finally
approaches a ± inactive final stage, unless substantially
increased temperature and pressure conditions trigger
the on set ofmetamorphism (see below). The approach
to an inactive diagenetic stage is certainly true for
many cases where the rocks have become more or less
sealed against influences from outside and have not
been affected by substantial subsequent tectonism (e.g.
subsidence or uplift).
13.5.2 Reactivation of Late-Stage Diagenesis
On the other hand, many examples of the diagenetic
his tory of rocks have been described where late
diagenesis has been reactivated by continued or renewed fluid flow and the import of low-salinity or
evaporitic brines from overlying, laterally located, or
underlying strata (cf. Sect. l3.3.7). This is reflected by
formation waters of markedly varying composition
(Sect. l3.3.6), by late-stage cementation and secondary
porosity, as well as by cement minerals formed coeval
with or later than hydrocarbon migration (cf. Sect.
13.3.7). Where pore fluids of differing composition
mix, as stated earlier, a new suite of cement minerals
can be generated andlor older cements and primary
minerals are dissolved or replaced by other cements.
This mechanism has been quoted as one of the most
important pro ces ses in late diagenesis.
Syn- and post-depositional faulting in conjuction
with overpressured systems, differential subsidence
and compaction, as well as other tectonic processes
can bring about a change from the normally c1osed,
deep-burial diagenetic system to an open one. Fracturing, uplift and emergence of part of the basin filling
can reactivate meteoric flow. Ifthe new fluid conduits