Geology Reference
In-Depth Information
Fig. 4.32.
Morphometry values of carbonate grains coupled with grain size and sorting offer the possibility in differentiating
environmental conditions.
The sample (Late Jurassic Sulzfluh limestone: Graubünden, Switzerland) exhibits aggregate grains
(AG), intraclasts (IC), extraclasts (EC), small ooids (O) and skeletal grains including benthic foraminifera (F), gastropod
shells (G) and dasyclad fragments (D) enclosed within cortoids (C). Aggregate grains, intraclasts and extraclasts differ in
morphometric
criteria
: Roundness of aggregate grains is low, their sphericity is high (fields B and G of Fig. 4.30). Intraclasts
are rounded and well-rounded and show different sphericity (fields E, O and S of Fig. 4.30). Extraclasts have low roundness
and low sphericity values mostly corresponding to field L of the comparison chart. Similar differences exist for
grain size:
Aggregate grains range between 0.20 and 2 mm, most grains are smaller than 0.50 mm. Intraclasts have sizes between 0.50
and 1 mm, extraclasts between 0.50 and 5 mm.
Sorting
of the sediment, estimated using the comparison charts depicted in
Fig. 6.2 and Fig. 6.3, is poor to very poor. Relative frequency of grains expressed by
constituent ranking
(see Sect. 6.2.1.4)
places aggregate grains in the first position, followed by intraclasts, extraclasts and skeletal grains. The
petrographic diversity
(Sect. 6.2.1.4) of the skeletal grains is low. The sediment is characterized by a high
compositional maturity
(Sect. 6.2.1.4),
most grains are reworked particles. Scale is 1 mm.
Interpretation:
The variations in roundness and sphericity of grains, the wide ranges in grain size as well as poor sorting
indicate mixing and reworking of grain populations. Roundness and sphericity of aggregate grains correspond to their origi-
nal shape and indicate only little transport. In contrast the morphometric data of intra- and extraclasts point to transport of
particles within the depositional environment. The fenestral fabric of the large extraclast in the center suggests a source in a
low-energy protected environment. The depositional fabric of the sample (lithoclastic grainstone), grain size ranges and
sorting correspond to the Energy Index IV-3 (see Sect. 12.1.1.2) characterizing a sedimentation of reworked grains in a
moderately agitated environment. Shallow water depth of grain source areas is indicated by the dasyclad fragments (
Clypeina
)
and by the lituolinid foraminifera (see Pl. 62/5, 6 and Sect. 10.2.2.1). All microfacies criteria attribute the sample to a
differentiated carbonate platform. Poor sorting, mixing of resediments and the deposition of eroded grains together with finer
grains point to a deposition within tidal channels.
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Further reading
: K107
