Geology Reference
In-Depth Information
be differentiated by the intensity and distribution of bur-
rowing and bioturbation (compare Pl. 19/7 with Pl. 19/
6). Fenestral fabrics are important environment-sensi-
tive MFT indicators. Have a look at Pl. 20/1, Pl. 20/2
and Pl. 20/3! These photographs illustrate the differ-
ences with respect to the geometry and frequency of
open-space structures.
Sedimentation on
carbonate ramps
is strongly con-
trolled by lateral variations in hydrodynamic conditions
resulting in a wide range of sedimentary structures (e.g.
bedding types, channels, erosion marks) and lithofa-
cies textures (Dunham types). A specific characteristic
is the close occurrence of areas with in situ sedimenta-
tion and areas dominated by the deposition of re-
sediments and transported grains. These differences in
depositional patterns and sedimentary structures assist
greatly in defining environment-sensitive MFTs for car-
bonate ramps (Bachmann 1994; Rüffer 1995).
greatly alter depositional microfacies and make MFT
establishment more difficult.
On the other hand, the combination of compositional
and textural microfacies criteria with diagenetic crite-
ria can assist in defining MFTs. Early diagenetic ma-
rine and meteoric carbonate cements indicate specific
conditions within different parts of a reef complex char-
acterized by different microfacies types (Städter and
Koch 1987). Diagenetic microfacies is commonly de-
fined using observations from petrographic (thin sec-
tion, SEM, CL) and isotopic analyses, as well as from
outcrop and core data, focusing on carbonate cements,
porosity types and diagenetic textures (Chap. 7). Com-
bination of these criteria results in the definition of
'diagenetic microfacies types'
that are critical to the
development of reservoir models in carbonate sedi-
ments (see Chap. 17).
How many microfacies types are reliable?
A survey of about hundred papers shows that car-
bonates of different depositional environments are de-
scribed by different numbers of microfacies types. Per-
itidal carbonates have been characterized by only a few
MFTs (up to 5); lagoonal carbonates by up to 10 MFTs.
Reef complexes including back-reef, central reef and
forereef were described by 10 to 15 MFTs. Carbonate
ramps and carbonate platforms have been differenti-
ated into up to 22 MFTs. Platform margin carbonates
(excluding reef carbonates) display 5 or less MFTs.
Similar low numbers are reported from carbonates
formed on open-marine slopes, but the number of MFTs
is highly variable because of differences in the fre-
Diagenesis and microfacies types
Diagenesis may or may not change textural and com-
positional criteria used for the definition of MFTs. Com-
pare the samples depicted in Pl. 28/1 and Pl. 28/2! The
original structures of the skeletal grains of the reef lime-
stone (Pl. 28/1) are strongly altered by recrystalliza-
tion. The depositional microfacies is, however, still rec-
ognizable. The constituents of the lagoonal limestone
in Pl. 28/2 are, in contrast, considerably obliterated by
burial cementation and dolomitization, making MFT
definition difficult (another example is shown in Fig.
7.21). Solution enlargement of interparticle pores (Pl.
29/1) and selective dissolution of grains (Pl. 29/8) can
Plate 100 Defining Microfacies Types
The interpretation of depositional and environmental constraints from microfacies data requires a synopsis of
those elements that are characteristic of the sediment. A generalization of microfacies types leads to categoriza-
tion of common microfacies data into standard microfacies types (Sect. 14.3). The plate displays two samples
of a skeletal grainstone from a carbonate platform which are used to demonstrate the methodology used to define
microfacies types (MFTs).
1
This sample
A
yields calcareous algae (udoteacean green algae:
Neoanchicodium
- N; dasyclad green algae - D), foramin-
ifera (fusulinid larger foraminifera - F, palaeotextulariid smaller foraminifera - P), echinoderms (crinoids - C, echinoids -
E), and gastropods (G). Composite grains are represented by microbial and encrusted aggregate grains (AG). Sorting is
moderate. Interparticle pores are occluded by carbonate cements. The MFT is typified not by the depositional texture
(moderately sorted skeletal grainstone) but by the dominant skeletal grain category (
Neoanchicodium
) and the association
of the alga
Neoanchicodium
, microbial and encrusted aggregate grains, echinoderms and fusulinids. Early Permian (As-
selian) platform carbonates: Zweikofel, Carnic Alps, Austria.
2
Sample
B
contains calcareous algae (
Neoanchicodium
- N, dasyclads - D), foraminifera (fusulinids - F), and echinoderms
(crinoids - C). Large irregularly shaped micritic grains are aggregate grains (AG). Sorting is moderate. Interparticle pores
are filled with thin rims of marine cements (arrow) and gray crystal silt (CS). The microfacies criteria agree with those of
sample A. A striking difference exists with regard to the type of interparticle cement The coincidence of compositional
criteria attribute the sample to the same MFT as sample A. The different cement types indicate a different diagenetic
history for limestone beds represented by the samples A and B, but not a different depositional history. Same locality as -> 1.
