Geology Reference
In-Depth Information
exposure surfaces. Discontinuity surfaces and expo-
sures surfaces are recorded by characteristic microfacies
criteria. Definitions of these criteria have been dis-
cussed in Chap. 4 and 5.
member consists of mudstone, wackestone or pack-
stone, often with a basal bio- or intraclastic grainstone
as a transgressive lag on top of the preexisting succes-
sion. The lower intertidal member exhibits thin-bed-
ded, variably bioturbated mudstones, and bioclastic,
peloidal and sometimes oolitic grainstones, locally with
stromatolites. The upper intertidal and the supratidal
member is usually a microbially laminated argillaceous
mudstone/bindstone with fenestral fabrics and thin
interbeds of intraclastic packstones and laminae con-
sisting of peloidal or bioclastic grainstone. In arid cli-
mates, the inter- and supratidal members may be rep-
resented by nodular to wavy beds of anhydrite.
• The
high-energy type
also exhibits subtidal bio-
turbated limestones, predominantly bio- and intraclastic
grainstones, at the base, but the lower intertidal mem-
ber consists of cross-bedded and cross-laminated bio-
clastic and/or locally oolitic grainstones, representing
beach deposits. The upper intertidal and the supratidal
members are characterized by desiccation-cracked mi-
crobial laminites.
Both subtypes may exhibit a pedogenic capping ho-
rizon (paleosol, calcrete); the units may be separated
by subaerial karst surfaces or erosion surfaces formed
during environmental shifts.
Shallowing-upward cycles of carbonate platforms
Many Precambrian and Phanerozoic carbonate plat-
forms are well stratified. The stratification is caused
by stacking of meter-scale depositional sequences dis-
playing a shallowing-upward trend. The shallowing-
upward sequences consist of repetitions of subtidal fa-
cies bounded by peritidal facies (
peritidal cycles
; e.g.
Lofer cycle, Sect. 16.1.1.3), subaerial exposure surfaces
(
diagenetic cycles
), and/or marine flooding surfaces
(
subtidal depositional cycles
). More or less regular rep-
etitions of sequences in platform and peritidal carbon-
ates imply that cyclic processes (e.g. periodic changes
in flooding through transgressions) controlled carbon-
ate production and deposition.
Diagenetic cycles
originate in shallow and in deeper
marine environments. In shallow environments, they
consist of subtidal wackestones and grainstones, over-
lain by a thin cap of vadose cements, pisoids and vari-
ous evidence of calichification (e.g. Latemar cycle,
Sect. 16.1.1.3).
Subtidal depositional cycles
are characterized by a
coarsening-upward trend in high-energy structures such
as grain size, bed thickness and cross-bedding, and the
lack of intertidal sediments or subaerial exposure. These
cycles are a dominant cycle type on unrimmed shelves
and ramps (Calvet and Tucker 1988; Osleger 1991).
In deeper open shelf environments, cycles capped
by hardgrounds can originate from a change in water
depths and reduced sedimentation. Sediment accumu-
lation takes place when the shelf was below the depth
of wave abrasion, cementation at times when the shelf
was above wave base and subject to water pumping.
An example of this are Tertiary temperate limestones
of southern Australia (Bone 1991).
Meter-scale cycles dominated many shallow-water
successions throughout the history of carbonate rocks.
These high-frequency depositional cycles are funda-
mental sequence-stratigraphic units of carbonate plat-
forms (parasequences; Sect. 16.1.2.1) and basic ele-
ments in the interpretation of transgressive-regressive
processes.
Common trends pointing to shallowing-upward in
peritidal cycles
starting with shallow subtidal sediments
at the base are
• upward decrease in grain-supported depositional
textures, in grains formed in high-energy environments
(e.g. ooids), and in normal marine biota (suggesting
increased restriction of environmental conditions),
• increase in the evidence of meteoric-vadose diagen-
esis, in the abundance of admixtures of terrigenous
quartz and silt, and of extraclasts.
Microfacies of shallowing-upward cycles
. Ideal shal-
lowing-upward cycles consist of distinct microfacies
exhibiting criteria of shallow subtidal, intertidal and/
or supratidal deposition and evidence of subaerial ex-
posure (e.g. desiccation cracks, pedogenic structures,
pisoids, karst criteria). The diagnostic criteria of these
depositional environments have been discussed in Sect.
15.5.1.2 and are summarized in Box 15.1.
Microfacies criteria pointing to shallowing-upward
conditions are mud peloids, oncoids, rhodoids and the
types of fenestral fabrics (Sect. 5.1.5) as well as in-
creasing meteoric-vadose cementation. Changes in the
frequency of mud peloids within bedded micritric lime-
stones indicate fluctuations in water energy and ero-
sion of the sea bottom and can reflect cyclic shallow-
ing patterns (Carss and Carozzi 1965; see Sect. 4.2.2).
Peritidal shallowing-upward successions.
Pratt et
al. (1992) distinguished two types:
• The
low-energy type
is characterized by a subtidal,
burrowed, variably argillaceous lowest member. The
