Geology Reference
In-Depth Information
MICROFACIES INTERPRETATION
The second part of this topic explains the characteristics of microfacies types (Chap. 11), discusses diagnostic
features of paleoenvironmental conditions (Chap. 12) and the importance of integrated facies analysis (Chap. 13),
examines the potential of generalized facies models and standard microfacies (Chap. 14), and illustrates the
significance of microfacies in the recognition of depositional settings (Chap. 15) and specific sedimentation
patterns (Chap. 16).
11 Summarizing Microfacies Criteria: Microfacies Types
Microfacies studies aim for the recognition of overall
patterns that reflect the history of carbonate rocks, by
means of a thorough examination of their sedimento-
logical and paleontological characteristics. The previ-
ous chapters have shown which thin-section criteria are
used in describing the microfacies of limestones. The
evaluation of microfacies in the context of facies inter-
pretation requires a synopsis of microfacies data ob-
served in various samples into a microfacies type
(MFT). Microfacies types and facies associations are
fundamental to the development of models for carbon-
ate sedimentation.
sitional fabrics (Chap. 5) and the ability to attribute thin-
section fossils to major systematic groups and taxo-
nomic units.
Identical biota and comparable depositional envi-
ronments give rise tof
time-specific microfacies types,
some of which have been generalized, for example the
facies types developed for Devonian shelf and reef car-
bonates (Hladil 1986; Machel and Hunter 1994). Simi-
lar or identical criteria in individual microfacies types
from limestones not restricted to specific time inter-
vals are used to define
standard microfacies types
(SMF
types), which describe major depositional and biologi-
cal controls and suggest major depositional settings
(Wilson 1975). The SMF concept and its application in
reconstructing depositional systems are discussed in
Sect. 14.3.
11.1 MFT Concepts
Microfacies, microfacies types and standard micro-
facies types
The microfacies recorded by a thin section is not
necessarily identical to a microfacies type. The approach
to microfacies is largely descriptive, and microfacies
types should be defined by those microfacies criteria
whose existence and abundance are determined by spe-
cific environmental factors and are linked to specific
depositional settings. This requires careful selection of
the textural and compositional criteria used.
Basic prerequisites for defining MFT are a clear-
cut discrimination of grain categories discussed in Sect.
4.2, an understanding of limestone classifications based
on textural criteria (Sect. 8.3), the recognition of depo-
Microfacies types, microfacies variability and facies
hierarchy
Texture, as well as qualitative and quantitative mi-
crofacies characteristics, may vary considerably within
a limestone bed, both laterally and vertically. Variabil-
ity depends partly on sample size and sampling proce-
dures (see Sect. 3.1.2.1), but may be real and reflect
small-scaled depositional events or rapidly fluctuating
environmental conditions. Several MFT may occur
within a single thin section (Morrow and Webster 1991;
see Pl. 127) or in samples taken within lateral distances
of only a few tens of centimeters (Egenhoff et al. 1999).
The evaluation of microfacies variability requires pre-
