13.5 Late, Deep-Burial Diagenesis
phase diagenetic history (e.g. Swennen et al. 1996; Muchez
et al. 1995). In Pennian to Tertiary sediments of the Eastern
Alps, several generations of deep-burial carbonate cements
have been identified (Zeeh et al. 1997) which are related to
saline fonnation waters or metamorphic waters. Some of
these fluids must have come from crystalline massifs to the
north ofthe Alps. In Cretaceous hydrocarbon reservoir rocks
ofthe Rocky Mountains, both basin-wide fluid flow and flow
systems restricted to smaller compartments in the subsurface
have been identified (Surdam et al. 1996).
Late pervasive dolomitization. The dolomitization of
thick, deeply buried calcium carbonate deposits has
been frequently discussed. The main problem is that
the Mg content of normal marine pore waters and compaction flow is not sufficient for transforming large
volumes of calcium carbonate into secondary dolomite
(pervasive dolomitization). Mg-rich bittem salts are
rare in the subsurface, and small quantities of Mg released in conjunction with the transformation ofunstable to stable clay minerals (cf. Sects. 13.3.7) are, at
least partly, consumed by the formation of chlorite.
Thus, pervasive dolomitization requires another large
Mg source.
Early dolomitization of marine lime mud and
calcarenites can by explained by downward Mg transport from the sea water reservoir by diffusion and
some advection (cf. Sect. 13.4.2). This process continues during slow sediment aggradation and may generate thick dolomite sequences. In contrast, late deepburial dolomitization requires another system providing the large amounts of Mg necessary. Some authors
have mentioned convective flow or deep meteoric water circulation. In both cases, Mg-rich brine must be
available to be transported to the site of dolomitization.
It is now the deep meteoric water circulation which
is most favored. Near-surface or buried evaporites are
dissolved in topographically high parts of the basin,
e.g. in front of an overthrust belt, and their solutes migrate basinward until they are forced by impermeable
barriers to ascend again to the surface (Fig. 13.25c).
This kind offluid flow normally moves very slowly (a
few mm up to 1 m per year; see also Pinti et al. 1997),
because the hydraulic gradients and the permeabilities
ofthese flow systems are low. Buried platform carbonates on the opposite side of the basin can be
dolomitized in this way, particularly along the principal, ± vertical flow conduits and basinward dipping
permeable strata into which the fluids can penetrate.
This late dolomitization is very common and forms
coarse-crystalline dolomite, thereby destroyingprimary
sedimentary structures in contrast to early, fine-grained
dolomitization which can preserve these structures.
Some features of dolomitization as observed in thin
sections are shown in Fig. 13.27.
In the example ofthe Cambrian Cathedral Fonnation in the
Canadian RockyMountains (Fig. 13.25c; Yao andDemicco
1997), a basin-wide meteoric flow system is assumed following the topographic gradient from an uplifted magmatic arc
691
to the distal craton. The deeply circulating water probably
dissolved evaporites along its flow path and reached temperatures of 100-200°C, salinities of 10-25 wt%, and had a
CaJMg ratio of 1:2 (detennined from fluid inclusions). At the
transition from the "shale basin" to the carbomate platfonn,
which was characterized by fractures and later overprinted by
tectonism, the brine ascended and fonned vertical pipes and
dikes of secondary dolomite. Then the fluids penetrated into
stratafonn sheets ofthe platfonn tens of meters thick, thereby
cooling. Dolomitization did not reach the most distal, landward parts of the platfonn. The massive replacement dolomization (2nd generation) ofthe fonner calcareous and partially dolomitized platfonn (1st generation) is assumed to
have taken place in the time period from the Middle Silurian
to the Late Devonian (i.e., in -60Ma).
Regional deep-burial dolomitization caused by long-distance, topographically driven fluid flow, has also been reported from the Devonian of western Canada (Mountjoy et
al. 1997). Here, the geohistory and diagenetic evolution display two phases of subsidence, the first one reflecting a passive margin setting, the second one orogenetic compression.
In total, five episodes of fracturing and three dissolution
events, including stylolite fonnation and dedolomitization, as
weil as several generations of dolomite (replacement dolomite, saddle dolomite, fracture-filling dolomite) were identified. The hydrothennal fluids mainlyused fracture zones and
penetrated prior to or after hydrocarbon migration. Fe-poor
calcite is, as in many other instances, the latest cement mineral. Another Canadian example are Devonian reef complexes of Alberta which were affected by pervasive
dolomitization in response to long-distance flow (Rostron et
al. 1997).
Another example of late, regional dolomitization is the
Lower Carboniferous of southern Ireland (Hitzman et al.
1997). Here, the study of C and Sr isotopes has revealed that
the north-directed, moderately hot and saline fluid flow
seems to have changed laterally in its composition.
Saddle (or baroque) dolomite is a common cement mineral in deep-burial carbonates and sandstones. It is characterized by irregularly curved crystal boundaries and sweeping
extinction under the microscope. Fluid inclusions have
shown that saddle dolomite mostly fonns at temperatures
between 90 and 160°C from fluids with a salinity greater
than that of sea water (Spötl and Pitrnan 1998; cf. Fig.
13.25d).
Further publications dealing with pervasive dolomitization and dedolomitization (the process of calcite replacing dolomite) include Chilingar et al. (1979), Land
(1985), Lee and Friedman (1987), Füchtbauer (1988), Moore
(1989), Tucker and Wright (1990). Experiments and theoretical considerations have shown that high temperatures
(e;80°C) and a low Ca++/Mg++ favor dolomitization
(Usdowski 1994).
13.5.4 Geohistory and Diagenesis (Case Studies)
The highly variable burial his tory and regional differences in the diagenetic evolution of certain strata has
been documented by a considerable number of case
studies. Sophisticated methods (cf. Sect. 13 .8.1),
which were not available for earlier investigations,
have allowed a much better identification and understanding of the complex processes operating in diage-
phase diagenetic history (e.g. Swennen et al. 1996; Muchez
et al. 1995). In Pennian to Tertiary sediments of the Eastern
Alps, several generations of deep-burial carbonate cements
have been identified (Zeeh et al. 1997) which are related to
saline fonnation waters or metamorphic waters. Some of
these fluids must have come from crystalline massifs to the
north ofthe Alps. In Cretaceous hydrocarbon reservoir rocks
ofthe Rocky Mountains, both basin-wide fluid flow and flow
systems restricted to smaller compartments in the subsurface
have been identified (Surdam et al. 1996).
Late pervasive dolomitization. The dolomitization of
thick, deeply buried calcium carbonate deposits has
been frequently discussed. The main problem is that
the Mg content of normal marine pore waters and compaction flow is not sufficient for transforming large
volumes of calcium carbonate into secondary dolomite
(pervasive dolomitization). Mg-rich bittem salts are
rare in the subsurface, and small quantities of Mg released in conjunction with the transformation ofunstable to stable clay minerals (cf. Sects. 13.3.7) are, at
least partly, consumed by the formation of chlorite.
Thus, pervasive dolomitization requires another large
Mg source.
Early dolomitization of marine lime mud and
calcarenites can by explained by downward Mg transport from the sea water reservoir by diffusion and
some advection (cf. Sect. 13.4.2). This process continues during slow sediment aggradation and may generate thick dolomite sequences. In contrast, late deepburial dolomitization requires another system providing the large amounts of Mg necessary. Some authors
have mentioned convective flow or deep meteoric water circulation. In both cases, Mg-rich brine must be
available to be transported to the site of dolomitization.
It is now the deep meteoric water circulation which
is most favored. Near-surface or buried evaporites are
dissolved in topographically high parts of the basin,
e.g. in front of an overthrust belt, and their solutes migrate basinward until they are forced by impermeable
barriers to ascend again to the surface (Fig. 13.25c).
This kind offluid flow normally moves very slowly (a
few mm up to 1 m per year; see also Pinti et al. 1997),
because the hydraulic gradients and the permeabilities
ofthese flow systems are low. Buried platform carbonates on the opposite side of the basin can be
dolomitized in this way, particularly along the principal, ± vertical flow conduits and basinward dipping
permeable strata into which the fluids can penetrate.
This late dolomitization is very common and forms
coarse-crystalline dolomite, thereby destroyingprimary
sedimentary structures in contrast to early, fine-grained
dolomitization which can preserve these structures.
Some features of dolomitization as observed in thin
sections are shown in Fig. 13.27.
In the example ofthe Cambrian Cathedral Fonnation in the
Canadian RockyMountains (Fig. 13.25c; Yao andDemicco
1997), a basin-wide meteoric flow system is assumed following the topographic gradient from an uplifted magmatic arc
691
to the distal craton. The deeply circulating water probably
dissolved evaporites along its flow path and reached temperatures of 100-200°C, salinities of 10-25 wt%, and had a
CaJMg ratio of 1:2 (detennined from fluid inclusions). At the
transition from the "shale basin" to the carbomate platfonn,
which was characterized by fractures and later overprinted by
tectonism, the brine ascended and fonned vertical pipes and
dikes of secondary dolomite. Then the fluids penetrated into
stratafonn sheets ofthe platfonn tens of meters thick, thereby
cooling. Dolomitization did not reach the most distal, landward parts of the platfonn. The massive replacement dolomization (2nd generation) ofthe fonner calcareous and partially dolomitized platfonn (1st generation) is assumed to
have taken place in the time period from the Middle Silurian
to the Late Devonian (i.e., in -60Ma).
Regional deep-burial dolomitization caused by long-distance, topographically driven fluid flow, has also been reported from the Devonian of western Canada (Mountjoy et
al. 1997). Here, the geohistory and diagenetic evolution display two phases of subsidence, the first one reflecting a passive margin setting, the second one orogenetic compression.
In total, five episodes of fracturing and three dissolution
events, including stylolite fonnation and dedolomitization, as
weil as several generations of dolomite (replacement dolomite, saddle dolomite, fracture-filling dolomite) were identified. The hydrothennal fluids mainlyused fracture zones and
penetrated prior to or after hydrocarbon migration. Fe-poor
calcite is, as in many other instances, the latest cement mineral. Another Canadian example are Devonian reef complexes of Alberta which were affected by pervasive
dolomitization in response to long-distance flow (Rostron et
al. 1997).
Another example of late, regional dolomitization is the
Lower Carboniferous of southern Ireland (Hitzman et al.
1997). Here, the study of C and Sr isotopes has revealed that
the north-directed, moderately hot and saline fluid flow
seems to have changed laterally in its composition.
Saddle (or baroque) dolomite is a common cement mineral in deep-burial carbonates and sandstones. It is characterized by irregularly curved crystal boundaries and sweeping
extinction under the microscope. Fluid inclusions have
shown that saddle dolomite mostly fonns at temperatures
between 90 and 160°C from fluids with a salinity greater
than that of sea water (Spötl and Pitrnan 1998; cf. Fig.
13.25d).
Further publications dealing with pervasive dolomitization and dedolomitization (the process of calcite replacing dolomite) include Chilingar et al. (1979), Land
(1985), Lee and Friedman (1987), Füchtbauer (1988), Moore
(1989), Tucker and Wright (1990). Experiments and theoretical considerations have shown that high temperatures
(e;80°C) and a low Ca++/Mg++ favor dolomitization
(Usdowski 1994).
13.5.4 Geohistory and Diagenesis (Case Studies)
The highly variable burial his tory and regional differences in the diagenetic evolution of certain strata has
been documented by a considerable number of case
studies. Sophisticated methods (cf. Sect. 13 .8.1),
which were not available for earlier investigations,
have allowed a much better identification and understanding of the complex processes operating in diage-