Geology Reference
In-Depth Information
A
Diagenetic
Location
Pore Filling
Processes ~ Time
environment
needed
Meteoric
Above water table,
Pores filled with
Solution zone (soil): Extensive solution; removal 10
3
- 10
5
vadose
between land surface
freshwater and/
of aragonite; formation of vugs.
years
environment
and meteoric
or air
Precipitation zone (near surface): Minor cementation
phreatic zone
Solution zone (e.g. sinkholes, caves): Solution; 10
3
-10
5
Meteoric
Below water table,
Pores filled
phreatic
may tend downwards
with freshwater
formation of molds and/or vugs. Active zone (upper part
up to
environment
100s of meters
of meteoric phreatic environment): Dissolution of 10
6
- 10
7
aragonite and Mg-calcite; rapid and diverse cementation;
years
precipitation of calcite; creation of molds and vugs.
Stagnant zone (deeper part and in arid climates): Little
cementation; stabilization of aragonite and Mg-calcite
Marine
On the shallow or
Pores filled
Shallow-marine environment: Waters oversaturated with 10
1
- 10
4
phreatic
deep sea floor or
with marine
respect to CaCO
3
; rapid cementation by aragonite and
years
environment
just below
water
Mg-calcite; diverse cement types. Deep-marine and
cold-water environments: Waters undersaturated with
respect to CaCO
3
; strong dissolution of aragonite and
calcite at two dissolution levels
Shallow burial (first few meters fo tens of meters) and 10
6
-10
8
Burial
Subsurface beneath
Pores filled with
environment
reach of surface-
brines of varying
deeper burial (sediment overburden of hundreds to
years
related processes,
salinity, from
thousands of meters): Physical compaction; chemical
down to realm of low-
brackish to
compaction (pressure solution); cementation; porosity
grade metamorphism.
highly saline
reduction
May tend downwards
1000s of meters
B
Fig. 7.2.
Major diagenetic environments.
A
- Simplified scheme. Many of the studies dealing with the diagenesis of carbon-
ate rocks are environment-specific and concentrate on processes affecting particular hydrogeochemically defined diagenetic
environments. Carbonate diagenesis operates in the
meteoric
environment
, the
marine
environment
and the
burial
environ-
ment
. In the
meteoric environment
pore space is occupied by freshwater and air (
meteoric vadose zone
above water table;
hatched) or by freshwater (
meteoric phreatic zone
). The
marine-vadose
zone
at the land-sea boundary and the
mixing zone
in
coastal areas and shallow near-coastal subsurface exhibits meteoric and marine criteria. In the
marine phreatic environment
water is supersaturated with respect to CaCO
3
in shallow seas and undersaturated in cold and deep seas. The subsurface
burial environment
comprises the subsurface beneath the reach of surface-related processes down to the realm of low-grade
metamorphism. Conventionally
shallow burial
and
deep burial
are differentiated. The term
near-surface diagenesis
refers to
processes at or close to the sea floor and in the meteoric environment within the reach of surface-related processes related to
depositional or weathering interfaces. Here, cementation is highly facies-specific. The terms eogenic, mesogenic and telege-
nic, introduced by Choquette and Pray (1970), refer to early near-surface, burial and uplift/unconformity-related processes
(Sect. 7.3.2).
B
- Major processes occurring in different diagenetic environments. The time involved in diagenetic processes varies signifi-
cantly in different diagenetic zones. Early diagenetic solution/precipitation processes in meteoric vadose and shallow marine
phreatic environments need far less time than late diagenetic deeper burial diagenesis, which can last millions of years.
Similarly, unconformity-related meteoric phreatic processes may continue over very long time intervals. Early cementation
in intertidal and shallow subtidal environments occurs within a range of almost recent to several tens to a few thousand years.
Synsedimentary botryoidal cements on marginal slopes of platforms may grow over several tens of years, resulting in syn-
sedimentary stabilization of steep carbonate slope deposits at or above angles of repose (Grammer et al. 1993).
