Geology Reference
In-Depth Information
4 Microfacies Data: Matrix and Grains
The purpose of this chapter is to outline and discuss
the criteria used in the description of matrix and grains.
Microfacies criteria reflected by fabrics and quantita-
tive microfacies data are discussed in Chap. 5 and 6.
Major constituents seen in thin sections of carbon-
ate rocks are 'grains' and 'groundmass', the latter de-
veloped as fine-grained carbonate 'matrix' or as sparry
calcite representing pore-filling cements or recrystal-
lized matrix. A sparry calcite groundmass is conven-
tionally called 'sparite' (calcite spar), and discussed in
Chap. 7. Grains and matrix are separated at 62.5
m in
terrigenous sediments but at much lower values in car-
bonate rocks. Here the lower limit is defined differ-
ently as larger than 4
m or >20
m or >50
m. SEM
studies reveal the arbitrary character of particle size
boundaries between grains and matrix, but a conven-
tional definition is necessary for thin-section studies.
The physical arrangement of particles, including
their texture and structure, forms a fabric which reflects
the depositional and diagenetic controls on the forma-
tion of carbonate rocks.
terial such as micrite, microspar and calcisiltite. A com-
mon generalization is that 'matrix' is synonymous with
'groundmass', but some authors use these terms to des-
ignate both fine-grained interstitial material (e.g. mi-
crite) as well as coarse interstitial crystal fabrics (e.g.
sparite formed by cementation or neomorphic pro-
cesses, see Sect. 7.6).
This contradicts the limitation of
the term matrix to 'mechanically deposited material be-
tween particles - as distinct from precipitated cement'
(Bathurst 1975). One should be cautious about regard-
ing all fine-grained interstitial material as synsedimen-
tary matrix: Intensive micritization, compaction and
neomorphism of mixed carbonate/siliciclastic deposits
as well as alterations of peloids may lead to the forma-
tion of a carbonate
pseudomatrix
(Dickinson 1970;
Geslin 1994).
Techniques
:
Genetic interpretation and the under-
standing of the properties of fine-grained carbonates
require the examination of polished and/or etched rock
surfaces, thin sections, stereoscan microscope and trans-
mission microscope studies, the investigation of geo-
chemical and isotopical signatures as well as the use of
biomarkers (Sect. 13.3). Conventional thin sections
used in microfacies studies are commonly too thick
(about 30
m) to reveal distinct crystal boundaries. Peels
and ultra-thin sections offer better data (Honjo and
Fischer 1965; Wolf et al. 1967; Lindholm and Dean
1973; Murphy 1986; Tucker 1988).
Micrite textures described only on the basis of light
microscopy may be inadequate. Epifluorescence mi-
croscopy assists in the discussion of organic material
preserved in limestones (Dravis and Yurewicz 1985,
Neuweiler and Reitner 1995).
SEM studies
of broken,
polished and etched surfaces provide an insight into
the variability of micrites. Polished and etched SEM
samples deliver better results than broken surfaces. The
main data which can be retrieved from SEM studies of
fine-grained carbonates are summarized in Box 4.1.
4.1 Fine-Grained Carbonate Matrix:
Micrite, Microspar, Calcisiltite
Fine-grained matrix constitutes a significant part of the
carbonate record. Despite the importance of micro-
crystalline limestones in basin analysis and as reser-
voir rocks and limestone resources, classifying and ge-
netically interpretating microcrystalline carbonates still
involve many problems. However, the advent of the
scanning electron microscope and the data provided by
geo- and biochemical analyses now allow us to take a
closer look at modern carbonate muds and ancient fine-
grained limestones.
Definition:
The term
matrix
denotes the interstitial
material between larger grains. Most definitions refer
to the relatively small size of the crystals and/or par-
ticles bordering intergranular pores. Hence the matrix
of micro-grained limestones includes fine-grained ma-
Major categories:
Micro-grained limestones exhibit
mean sizes commonly between 2
m to about 30
m.
