DIAGENESIS
Christine Perrin
LMTG, Université Paul Sabatier Toulouse, Toulouse,
France
UMR 7207 du CNRS CR2P, Paris, France
Definition
The biological, physical, and chemical processes occurring concomitantly or separately in time and modifying
a sediment during and after its deposit.
Introduction
Reefs have caused a tremendous interest in diagenetic
studies in the past decades, with a particular exponential
development during the 1970s and 1980s, just after the
major discovery of the early submarine cements in modern
coral reef complexes in the late 1960s (Ginsburg et al.,
1967; Macintyre et al., 1968; Land and Goreau, 1970).
In the general topic of carbonate diagenesis, reefs represent highly favorable systems for the study of diagenetic
processes because they are especially susceptible to diagenesis occurring either in marine, mixed, or nonmarine
waters, and as sedimentary systems, they have relatively
clearly defined geometries. Another reason explaining
their scientific interest is their reservoir potential, and from
this point of view also, the precise analysis of reeflimestones diagenesis is of critical importance for the
assessment of their economic significance.
Independently from their age and setting, reefs present
several fundamental specificities as regards carbonate
diagenesis:
1. They are formed with a very high primary porosity.
2. They are lithified at their time of growth, this limiting
the effects of compaction and hence permitting the
preservation of initial porosity.
3. Reef facies are usually composed of chemical unstable
minerals, such as aragonite and high-magnesian calcite
(HMC), which can easily undergo various chemical
and mineralogical changes through interaction with
circulating pore fluids.
The aim of this encyclopedia is to focus on modern reefs.
For this reason, the diagenetic features and processes
described here are related to the early diagenesis in the
sense of syndepositional diagenetic processes and do not
include those depending on burial diagenesis.
General approach and technical methods
Research in carbonate diagenesis has used, and is still
using, a process-based approach, i.e., relying on the identification of processes producing the diagenetic modification, in contrast to the general analogy-based approach
classically used in geology and more particularly in stratigraphy and facies studies.
While laboratory experimentation on precipitation–
dissolution of the different carbonate species under strictly
controlled physicochemical conditions has potentially
proved to be of great help for our understanding of the
mechanisms involved in carbonate diagenesis, a purely
physicochemical approach alone cannot provide the keys
for deciphering the highly diverse diagenetic pathways
and their effects on the studied carbonate system through
time and space. This is mainly due to the fact that the interior of a reef is a strongly dynamic system involving many
interactions of biological, physical, and chemical processes
and important movements of water masses facilitated by the
high porosity and permeability typical of reef facies.
The usual approach is preliminarily based on
a petrographical analysis of thin sections under the optical
microscope, combined with SEM observations and geochemical characterization of the identified diagenetic
products. Complementary observations and analyses
involving various sets of more sophisticated techniques
may be further undertaken depending on the scientific
questions arising from the first results and on the specific
objectives of the study concerned. The improvement and
development of analytical techniques in recent years provide us with a large spectrum of potential analyses
encompassing the fields from biochemistry to mineralogy.
Cementation
Early cements
Early cements in recent reefs
Aragonite cements are frequently developed in Holocene,
including present day and Pleistocene coral reefs. Two distinct types of aragonite cements are commonly recognized: the needlelike or acicular aragonite cement
(Figure 1a) and the botryoidal aragonite (Figure 1b
and c). The first type of cement has been frequently
described in numerous studies of recent reefs and numerous terms have been used to describe the various morphologies of this cement (Ginsburg et al., 1971; Ginsburg and
Schroeder, 1973; Schroeder, 1972; Bricker, 1973; James
et al., 1976; Macintyre, 1977; Harris et al., 1985; James
and Ginsburg, 1979). The acicular crystals are typically
2–10 mm wide and 50–300 mm long with pointed terminations, elongated parallel to the crystal’s c-axis and having
straight extinction (Aïssaoui, 1986). The morphological
varieties of this cement include acicular crystals occurring
as isopachous fringes, bladed crystals, fanlike needles,
and needle meshworks, like those described in Belize
(James and Ginsburg, 1979). The acicular cement frequently forms a relatively loose structure resulting from
the sometimes irregular disposition of crystals. It can also
consist of epitaxial overgrowths of aragonite substrates,
usually aragonitic mollusks or coral fragments
(Figure 1a). The distribution of this cement has often been
shown to be very heterogeneous, even within the same
thin section, with some primary voids completely filled
up with this cement and the neighboring pores entirely
devoid of aragonite needles.
DIAGENESIS
309
Christine Perrin
LMTG, Université Paul Sabatier Toulouse, Toulouse,
France
UMR 7207 du CNRS CR2P, Paris, France
Definition
The biological, physical, and chemical processes occurring concomitantly or separately in time and modifying
a sediment during and after its deposit.
Introduction
Reefs have caused a tremendous interest in diagenetic
studies in the past decades, with a particular exponential
development during the 1970s and 1980s, just after the
major discovery of the early submarine cements in modern
coral reef complexes in the late 1960s (Ginsburg et al.,
1967; Macintyre et al., 1968; Land and Goreau, 1970).
In the general topic of carbonate diagenesis, reefs represent highly favorable systems for the study of diagenetic
processes because they are especially susceptible to diagenesis occurring either in marine, mixed, or nonmarine
waters, and as sedimentary systems, they have relatively
clearly defined geometries. Another reason explaining
their scientific interest is their reservoir potential, and from
this point of view also, the precise analysis of reeflimestones diagenesis is of critical importance for the
assessment of their economic significance.
Independently from their age and setting, reefs present
several fundamental specificities as regards carbonate
diagenesis:
1. They are formed with a very high primary porosity.
2. They are lithified at their time of growth, this limiting
the effects of compaction and hence permitting the
preservation of initial porosity.
3. Reef facies are usually composed of chemical unstable
minerals, such as aragonite and high-magnesian calcite
(HMC), which can easily undergo various chemical
and mineralogical changes through interaction with
circulating pore fluids.
The aim of this encyclopedia is to focus on modern reefs.
For this reason, the diagenetic features and processes
described here are related to the early diagenesis in the
sense of syndepositional diagenetic processes and do not
include those depending on burial diagenesis.
General approach and technical methods
Research in carbonate diagenesis has used, and is still
using, a process-based approach, i.e., relying on the identification of processes producing the diagenetic modification, in contrast to the general analogy-based approach
classically used in geology and more particularly in stratigraphy and facies studies.
While laboratory experimentation on precipitation–
dissolution of the different carbonate species under strictly
controlled physicochemical conditions has potentially
proved to be of great help for our understanding of the
mechanisms involved in carbonate diagenesis, a purely
physicochemical approach alone cannot provide the keys
for deciphering the highly diverse diagenetic pathways
and their effects on the studied carbonate system through
time and space. This is mainly due to the fact that the interior of a reef is a strongly dynamic system involving many
interactions of biological, physical, and chemical processes
and important movements of water masses facilitated by the
high porosity and permeability typical of reef facies.
The usual approach is preliminarily based on
a petrographical analysis of thin sections under the optical
microscope, combined with SEM observations and geochemical characterization of the identified diagenetic
products. Complementary observations and analyses
involving various sets of more sophisticated techniques
may be further undertaken depending on the scientific
questions arising from the first results and on the specific
objectives of the study concerned. The improvement and
development of analytical techniques in recent years provide us with a large spectrum of potential analyses
encompassing the fields from biochemistry to mineralogy.
Cementation
Early cements
Early cements in recent reefs
Aragonite cements are frequently developed in Holocene,
including present day and Pleistocene coral reefs. Two distinct types of aragonite cements are commonly recognized: the needlelike or acicular aragonite cement
(Figure 1a) and the botryoidal aragonite (Figure 1b
and c). The first type of cement has been frequently
described in numerous studies of recent reefs and numerous terms have been used to describe the various morphologies of this cement (Ginsburg et al., 1971; Ginsburg and
Schroeder, 1973; Schroeder, 1972; Bricker, 1973; James
et al., 1976; Macintyre, 1977; Harris et al., 1985; James
and Ginsburg, 1979). The acicular crystals are typically
2–10 mm wide and 50–300 mm long with pointed terminations, elongated parallel to the crystal’s c-axis and having
straight extinction (Aïssaoui, 1986). The morphological
varieties of this cement include acicular crystals occurring
as isopachous fringes, bladed crystals, fanlike needles,
and needle meshworks, like those described in Belize
(James and Ginsburg, 1979). The acicular cement frequently forms a relatively loose structure resulting from
the sometimes irregular disposition of crystals. It can also
consist of epitaxial overgrowths of aragonite substrates,
usually aragonitic mollusks or coral fragments
(Figure 1a). The distribution of this cement has often been
shown to be very heterogeneous, even within the same
thin section, with some primary voids completely filled
up with this cement and the neighboring pores entirely
devoid of aragonite needles.
DIAGENESIS
309