Geology Reference
In-Depth Information
Type II,
representing
alternate deposition in agitated
and quiet water
, is characterized by a mixture of quiet-
and agitated water criteria. Microcrystalline calcite
matrix comprises more than 50% of the rock. The ad-
mixture of > 50% transported grains is considered as
evidence for intermittent water energy. Fossil assem-
blages are more highly complex than those of Type I.
Subtype II-1 (Pl. 44/2) and II-2 (Pl. 44/3) differ in the
size of transported grains (fine- to medium-grained
grains and coarse- to very coarse transported grains,
respectively). Grains are subangular to rounded. Sub-
type II-3 (Pl. 18/2) is characterized by alternating lami-
nae composed of lime mud and coarser clastic particles.
Clay content is below 15%.
Type III
describes deposition in
slightly agitated
water
. The size of the transported grains varies from
silt through fine-grained sand. Subtype III-1 (Pl. 118/
1) corresponds to a calcisiltite, subtype III-2 to a very
fine-grained calcarenite and subtype III-3 (Pl. 121/1)
to a fine-grained calcarenite. Matrix (homogeneous mi-
crocrystalline in III-1 and III-2; inhomogeneous and
varying in size) may constitute < 50 percent of the rock
bulk. Type III includes nonfossiliferous, sparsely or
moderately fossiliferous limestones (III-1 and III-2) as
well as limestones with abundant and diverse fossils
(III-3). Grains are subrounded to rounded. No conspicu-
ous clay content.
Type IV
indicates deposition in
moderately agitated
water
. The limestones are medium-grained to very
coarse-grained calcarenites. The matrix is poorly sorted.
Fossils are more common than in Type III. Subtypes
(IV-1: Pl. 122/1; IV-2: Pl. 118/3; Fig. 4.32; IV-3: Pl.
45/3) differ predominantly in the grain size of trans-
ported particles which are generally broken and abraded
fossils. Grains are subrounded to well rounded. No con-
spicuous clay content.
Type V
characterizes deposition and growth in
strongly agitated water.
Fossil fragments and rock de-
bris are coarse-grained and lie within the range of
calcirudites. Subtype V-1 (Pl. 115/2) corresponds to a
limestone consisting of subrounded to rounded gravel-
sized fossils and rock fragments. Subtype V-2 (Pl. 9/7)
is described as being 'composed predominantly of con-
glomeratic and brecciated rock fragments'. Grains are
angular to well rounded. Some clay may be present.
Subtype V-3 corresponds to a 'reef' limestone with in
situ frame-building organisms'.
points of EI log
max
and EI log
min
of a geological sec-
tion. The field between these points is called the
differ-
ential energy index field
(DEI field). It describes the
textural variation of carbonate rocks. A wide DEI field
indicates large variations, a narrow field smaller varia-
tions.
In contrast to Plumley et al. (1962), Catalov distin-
guished only four limestone types: Type I (deposited
in quiet water), type II (deposited in slightly agitated
water), type III (deposited in moderately agitated wa-
ter), and type IV (deposited in strongly agitated water).
The differences are indicated by different degrees of
winnowing as well as frequency, size, sorting and
roundness of grains; type and frequency of terrigenous
admixtures, fossil associations and substrate (muddy,
sandy). The main difference between the classification
of Plumley et al. and Catalov is that the frequency and
size of grains as well as fossils are much more pro-
nounced in Catalov's classification.
Discussion:
The original and the modified Energy
Classification have proved successful in describing
lithologic variations in time and space, changes in en-
ergy levels during time as well as in correlating out-
crops and boreholes (e.g. Bolliger and Burri 1970;
Skupin 1973; Rasser 1994). Assessment of subgroups
may sometimes be difficult, if information on non-car-
bonate contents of the limestones is missing. Pitfalls in
the use of the Energy Index can arise because
• the size of grains depends on water energy, but also
on the original size ranges (e.g. of fragmented green
algae), the growth rates of organisms (e.g. in oncoid
formation) or the cementation stages of grains,
• the abundance of detrital quartz and clay is not nec-
essarily dependent on water energy. Quiet-water car-
bonates may exhibit much higher quartz contents than
suggested by Plumley et al., due to eolian input,
• the occurrence of microcrystalline calcite matrix
does not prove a priori quiet-water conditions, because
mud can be fixed by organisms also in high-energy set-
tings as well,
• the use of 'characteristic fossils' in determining en-
ergy index types is dangerous as long as only groups or
higher taxonomic categories are considered.
Energy levels and resulting limestone types are not
necessarily related to water depth: Quiet-water Type I
limestones may occur in either deep (e.g. basinal) or
shallow (e.g. lagoonal) water. Intermittently agitated
carbonates of Type II should be more common in the
transition zone between deep water (depositional in-
terface below wave base) and very shallow water, where
Modified energy classification:
While studying Tri-
assic limestones of Bulgaria, Catalov (1972) differen-
tiated between the
energy index log of the energy
minima
and the
energy index log of the energy maxima
.
These logs are obtained by connecting the respective
