13.5 Late, Deep-Burial Diagenesis
geohistory of these sandstones (e.g. carbonates, anhydrite and illite); earlier cements were partly dissolved and reprecipitated.
Rotliegend sandstones in horst position in the North
Sea (Leman field, maximal burial depth 3.5-4 km,
present depth ~2 km) were affected by fluids from
both the underlying coal seams and the overlying
evaporites as well as by late meteoric flow. After basin
inversion, the low-pressured Rotliegend sandstones
seem to have acted as drains for fluids.
These results and more details about this example have been
reported by Gaupp et a1. (1993), Platt (1994), Sullivan et a1.
(1994), Amthor and Okkerman (1998). Radiometric dating of
the extensive authigenic illite in Carboniferous (horst position) and Rotliegend sandstones (graben) revealed a slowly
prograding illitization front (0.2 mmlyear between ~200 and
150 Ma) from the horst to the graben (Zingmann et a1. 1998).
The transgressive Zechstein conglomerate, ~3 m thick in
aborehole in Belgium, is affected by late pervasive replacement and pore-filling ankerite (De Craen and Swennen
1992). The Fe-rich fluids are probably derived from elastics
of the underlying Carboniferous. Deeply buried Carboniferous (Westphalian) sandstones ofthe coal district of Aachen,
Germany, are rich in dolomite, ferroan carbonates, siderite,
and quartz. They were later uplifted and experienced a second phase ofmeteoric flow (Swennen et a1. 1996).
Further descriptions ofthe diagenetic processes in various
rock types and tectonic settings inelude Muchez et a1. (1995),
Khanna et a1. (1997), Land (1997), Munz et a1. (1998),
O'Reilly et a1. (1998) and references listed above.
13.5.5 Cement Sources and
Cement Mineral Sequences
Cement sources . In the past the question after the
sources for specific cement minerals could not always
answered with confidence. In the shallow meteoric
regime, import and export of dissolved matter is normally no problem because large volumes of water pass
the sediment. In addition, considerable amounts of
solids are dissolved and reprecipitated within the sediment considered. Sabkha brines may provide highly
soluble salts which are not available in humid climates.
Our question is more difficult to answer for the cement sources at greater burial depths. There is no
doubt that pressure solution and short-distance molecular diffusion are very effective mechanisms in local
redistribution of carbonate and silica, but quantitative
investigations have shown that the mass balance between dissolved and reprecipitated matter in certain
strata is often either negative or positive. The various
reactions releasing water and dissolved constituents in
the deep subsurface do not provide enough mass to
account for the large volumes of cement observed in
many lithified rocks. Even in the case of relatively easily soluble carbonates, ab out 100 000 volumes of pore
water are needed to fill one volume of pore space
(Bathurst 1975). Therefore, the import of carbonate
695
from the meteoric zone or the dissolution and import
of carbonate from the deeper subsurface are often
quoted as a mechanism for providing additional carbonate, for cementation. The latter process may be promoted by the release of carbon dioxide from the decomposition of organic matter and kerogen.
Particularly the source of silica for quartz cement,
which is the most important cement type besides carbonate, has been frequently discussed in the literature.
Quartz sandstones, for example, may import or export
silica. Shallowly buried sandstones may receive silica
from meteoric water rich in silica due to intensive
chemical weathering. Deeply buried sandstones have
to import silica from under- and overlying shales if
pressure solution cannot account for all the silica
needed for quartz cement. In some regions, convective
flow in large circulation cells (Fig. l3.l6t) may transfer silica in considerable quantities from deeper and
hotter to more elevated and cooler portions of a sand
body.
Mass balances reveal that thousands of pore water volumes
are required to supply alone the Si0 2 needed for the quartz
overgrowths in the sandstones (e.g. Lynch 1996; see also
Bj0rlykke and Gran 1997, Sect. 31.3.2). The amountofwater
necessary to produce the diagenetic modifications of sandstones normally is much larger than that present in the entire
formation.
The same is true ofpervasive dolomitization (see above).
Sulfate cements such as anhydrite, barite, and celestite, as
weIl as base metals are often not available in the sedimentary
sequence investigated. These constituents must have come
from distant sources, often transported along fracture zones.
It has been frequently observed that cementation is more intense elose to faults and fractures than farther away from
these fluid conduits.
So, the answer to our question is, at least in a number of
cases, that long-lasting and long-distant fluid flow delivers
considerable amounts of solutes which cannot be derived
from the neighborhood of the site studied.
Cement Mineral Sequences. The previous discussion
on the various realms of diagenesis and systems of
mass transport in different rocks and rock sequences
demonstrates that a comrnon rule for the succession of
cement minerals (diagenetic mineral paragenesis ) cannot exist. In addition, the numerous mineral reactions
involved, including the dissolution and replacement of
earlier cement by later generations of mineral phases
(cf. Fig. l3.22c) and recrystallization, render it difficult in many cases to establish the true, complete cement sequence. Differing coeval cements and cement
successions, caused by primary differences in grain
size and fabric, skeletal structures, pore sizes and permeability etc., enhance this problem. Early cements of
the vadose and phreatic meteoric regime, in particular,
vary greatly in response to climate change.
N eglecting these early cement variations, nevertheless the following major cement sequences can be frequently observed:
geohistory of these sandstones (e.g. carbonates, anhydrite and illite); earlier cements were partly dissolved and reprecipitated.
Rotliegend sandstones in horst position in the North
Sea (Leman field, maximal burial depth 3.5-4 km,
present depth ~2 km) were affected by fluids from
both the underlying coal seams and the overlying
evaporites as well as by late meteoric flow. After basin
inversion, the low-pressured Rotliegend sandstones
seem to have acted as drains for fluids.
These results and more details about this example have been
reported by Gaupp et a1. (1993), Platt (1994), Sullivan et a1.
(1994), Amthor and Okkerman (1998). Radiometric dating of
the extensive authigenic illite in Carboniferous (horst position) and Rotliegend sandstones (graben) revealed a slowly
prograding illitization front (0.2 mmlyear between ~200 and
150 Ma) from the horst to the graben (Zingmann et a1. 1998).
The transgressive Zechstein conglomerate, ~3 m thick in
aborehole in Belgium, is affected by late pervasive replacement and pore-filling ankerite (De Craen and Swennen
1992). The Fe-rich fluids are probably derived from elastics
of the underlying Carboniferous. Deeply buried Carboniferous (Westphalian) sandstones ofthe coal district of Aachen,
Germany, are rich in dolomite, ferroan carbonates, siderite,
and quartz. They were later uplifted and experienced a second phase ofmeteoric flow (Swennen et a1. 1996).
Further descriptions ofthe diagenetic processes in various
rock types and tectonic settings inelude Muchez et a1. (1995),
Khanna et a1. (1997), Land (1997), Munz et a1. (1998),
O'Reilly et a1. (1998) and references listed above.
13.5.5 Cement Sources and
Cement Mineral Sequences
Cement sources . In the past the question after the
sources for specific cement minerals could not always
answered with confidence. In the shallow meteoric
regime, import and export of dissolved matter is normally no problem because large volumes of water pass
the sediment. In addition, considerable amounts of
solids are dissolved and reprecipitated within the sediment considered. Sabkha brines may provide highly
soluble salts which are not available in humid climates.
Our question is more difficult to answer for the cement sources at greater burial depths. There is no
doubt that pressure solution and short-distance molecular diffusion are very effective mechanisms in local
redistribution of carbonate and silica, but quantitative
investigations have shown that the mass balance between dissolved and reprecipitated matter in certain
strata is often either negative or positive. The various
reactions releasing water and dissolved constituents in
the deep subsurface do not provide enough mass to
account for the large volumes of cement observed in
many lithified rocks. Even in the case of relatively easily soluble carbonates, ab out 100 000 volumes of pore
water are needed to fill one volume of pore space
(Bathurst 1975). Therefore, the import of carbonate
695
from the meteoric zone or the dissolution and import
of carbonate from the deeper subsurface are often
quoted as a mechanism for providing additional carbonate, for cementation. The latter process may be promoted by the release of carbon dioxide from the decomposition of organic matter and kerogen.
Particularly the source of silica for quartz cement,
which is the most important cement type besides carbonate, has been frequently discussed in the literature.
Quartz sandstones, for example, may import or export
silica. Shallowly buried sandstones may receive silica
from meteoric water rich in silica due to intensive
chemical weathering. Deeply buried sandstones have
to import silica from under- and overlying shales if
pressure solution cannot account for all the silica
needed for quartz cement. In some regions, convective
flow in large circulation cells (Fig. l3.l6t) may transfer silica in considerable quantities from deeper and
hotter to more elevated and cooler portions of a sand
body.
Mass balances reveal that thousands of pore water volumes
are required to supply alone the Si0 2 needed for the quartz
overgrowths in the sandstones (e.g. Lynch 1996; see also
Bj0rlykke and Gran 1997, Sect. 31.3.2). The amountofwater
necessary to produce the diagenetic modifications of sandstones normally is much larger than that present in the entire
formation.
The same is true ofpervasive dolomitization (see above).
Sulfate cements such as anhydrite, barite, and celestite, as
weIl as base metals are often not available in the sedimentary
sequence investigated. These constituents must have come
from distant sources, often transported along fracture zones.
It has been frequently observed that cementation is more intense elose to faults and fractures than farther away from
these fluid conduits.
So, the answer to our question is, at least in a number of
cases, that long-lasting and long-distant fluid flow delivers
considerable amounts of solutes which cannot be derived
from the neighborhood of the site studied.
Cement Mineral Sequences. The previous discussion
on the various realms of diagenesis and systems of
mass transport in different rocks and rock sequences
demonstrates that a comrnon rule for the succession of
cement minerals (diagenetic mineral paragenesis ) cannot exist. In addition, the numerous mineral reactions
involved, including the dissolution and replacement of
earlier cement by later generations of mineral phases
(cf. Fig. l3.22c) and recrystallization, render it difficult in many cases to establish the true, complete cement sequence. Differing coeval cements and cement
successions, caused by primary differences in grain
size and fabric, skeletal structures, pore sizes and permeability etc., enhance this problem. Early cements of
the vadose and phreatic meteoric regime, in particular,
vary greatly in response to climate change.
N eglecting these early cement variations, nevertheless the following major cement sequences can be frequently observed: