Geology Reference
In-Depth Information
high percentage of reef-derived material), SMF 6
(reef talus, fossils and lithoclasts) and SMF 24 (con-
sisting of coarse micrite clasts and laminated clasts,
'flat pebble conglomerate'). All these SMF types also
develop other textures and are, therefore, also in-
cluded in Box 14.7.
--
Concentrations of shells and echinoderms (mostly
crinoids):
Shell beds and coquinas are represented
by SMF 12. The fossils form packstone, rudstone
and floatstone fabrics. The type needs further sub-
division.
--
Reef limestones
are assembled in SMF 7 charac-
terized by sessile reefbuilders and framestone, baffle-
stone or bindstone fabrics. These limestones require
specific investigations of the role of organisms in
the formation of reefs, growth fabrics, internal sedi-
mentation and cement types (see Sect.16.2).
lasts may create problems in the accurate classifica-
tion of SMF Types. The SMF Types SMF 8, SMF 9,
SMF 10 and SMF 11 often exhibit a multigrain com-
position without a pronounced quantitative domi-
nance of a specific grain category. Your problem may
be related to this shortcoming in the SMF classifi-
cation.
-- Finally, your assignment may not fit because you
have relied on the wrong depositional model. Con-
sider the possibilities of the carbonate ramp model
(Fig. 14.3). Perhaps you will find the microfacies
characteristics of your sample in one of the RMF
Types listed in Sect. 14.3.4.2 and summarized with
regard to their more common distribution in Fig.
14.30. Note that several of these RMF Types corre-
spond in their composition and texture to SMF Types,
but that the distributional pattern of some of these
corresponding types differs from that on carbonate
platforms.
• Distinguish remaining samples according to textural
types using the Dunham classification.
• Use Box 14.7 to differentiate the samples, consider-
ing common and abundant grain types. Note that
samples with identical grain composition can show dif-
ferent depositional textures.
14.4 Dynamic Microfacies Types and
Environmental Changes
The concept of microfacies types includes a strong ten-
dency to categorize. Strict categorization, however, may
obscure the question of whether or not the microfacies
data set consists of components that vary in a continu-
ous fashion, or of discontinuous grain associations. If
the spatial distribution is discontinuous, the stratigraphic
or geographical representation of various microfacies
types as a mosaic of 'cells' is justified and can provide
a basis for the interpretation of sedimentary environ-
ments. This approach is followed by the SMF concept.
However, if the distribution is continuous, the facies
mosaic gives only a rough picture of nature. This is
specifically important in the facies interpretation of
carbonate ramps where environmental changes are pro-
gressive and gradual rather than abrupt.
• Compare your favorite SMF designation with the
definition of the SMF Type and with the pictures of the
examples. But note that the pictures can not show the
whole spectrum of microfacies variations.
• Check the occurrence of the SMF Type in the con-
text of the rimmed platform facies model (Fig. 14.29)
and think about the reliability of attributing your sample
via the SMF classification to a specific facies zone. If
you find that your sample can not have originated in
the facies zone indicated by the model, have faith in
your interpretation and consider the following possi-
bilities:
-- The composition and texture may not reflect cri-
teria caused by autochthonous deposition, but rather
criteria caused by transport from the original depo-
sitional site to other facies belts. Oolite beds inter-
calated within basinal sediments and representing
turbidites or debris flow deposits may display all the
characteristic criteria of SMF 15. Concentrations of
shells (SMF 12) are often allochthonous deposits
formed far from the living sites of the organisms.
Storms can transport skeletal grains in offshore and
onshore directions, and form multigrain bio- and
lithoclastic calcarenites distant from the carbonate-
producing area.
-- Different mixtures of various skeletal grains,
coated grains, composite grains, peloids and intrac-
Most grain analyses are descriptive with little analy-
sis of relationships in grain composition or grain abun-
dance of samples. One way to detect relationships and
patterns in grain-type distributions are ordination meth-
ods that explore relationships between the components
and detect and analyze relays in carbonate rocks. Re-
lays are spatial or temporal systematic shifts in the rela-
tive importance of component grains answering to uni-
directional changes of major environmental conditions
(Hennebert and Lees 1985). The application of ordina-
tion methods to the study of relay phenonema facili-
tates the recognition of environmental gradients
(Hennebert and Lees 1991).
