Geology Reference
In-Depth Information
Fig. 11.3.
Bioclastic echinoderm-shell packstone
used as an example for discriminating microfacies types. B - brachiopod
shells, C - crinoid fragments, E - echinoid spine, M - mollusk shells. Late Triassic (Rhaetian): Steinplatte near Waidring,
Tyrol, Austria. Scale is 1 mm.
tested on data sets collected from modern carbonates.
Compositional and distributional data for shallow-ma-
rine carbonates of the Great Bahama Bank (Purdy
1963a, 1963b) have been used several times to test the
value of multivariate discrimination methods (Gill
1993). Frequency data for twelve sedimentary and bio-
genic constituents per sample, derived from point-
counting of thin sections of plastic-impregnated sedi-
ment samples, were grouped using agglomerative clus-
ter analysis to obtain a grouping of constituents and
factor analysis to distinguish the main facies groups.
The statistical division into facies groups (correspond-
ing roughly to microfacies types) showed good agree-
ment with the sediment grouping based on geological
field data, except for those carbonates characterized by
high amounts of carbonate mud.
Presence or absence data, multistate nominal data,
and binary data obtained by dichotomizing each vari-
able about its mean value (Shi 1993) are often regarded
as sufficient to present meaningful classifications of
multivariate facies data (Schott 1983, 1984; Caracuel
et al. 1998).
Microfacies types and sedimentary fabrics and struc-
tures
MFTs based on textural and compositional criteria
should be tested with respect to variations caused by
differences in depositional fabrics. Specific biofabric
features (Sect. 5.1.2) including grain orientation (Pl.
18/1, 5) and small-scale cross-bedding (Pl. 18/3) may
be useful MFT criteria. Graded bedding layers may ex-
hibit different MFTs, particularly if textural criteria are
considered as being of higher importance than grain
composition (see Pl. 17/1). Biolamination (Pl. 18/4) and
lamination fabrics caused by bottom currents (Pl. 18/
7) are other MFT characteristics. Wackestone MFTs can
