Geology Reference
In-Depth Information
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Further reading: K020, K177
units. The presence of frameworks and reefs assists in
coping with a rapidly rising sea level. The absence of
frameworks may promote high rates of sediment re-
working and export.
Sea-level fluctuations influence growth potential and
the abundance of biogenic carbonate production on
platforms. The frequency and growth potential of sedi-
ment-producing organisms may be different during
transgressive and regressive phases. During transgres-
sions large areas are flooded and biogenic production
is high. During regressive phases, when large areas have
emerged, the productive depth range is small.
• The composition of skeletal grain associations de-
pends strongly on the relative frequency and the tapho-
nomic stability of skeletal grains.
• The amount of biogenic sediment production influ-
ences the growth rates of platforms. Sediment produced
on the platform is either deposited on the platform or
exported. Maximum sediment production will normally
occur, where the environmental parameters meet best
the ecologic requirements of sediment producers and
where environmental parameters remain constant over
a longer time.
• The early diagenesis of platform sediments is influ-
enced by biofabrics that control the primary perme-
ability and porosity, and by the mineralogy of skeletal
grains. Sediments consisting mainly of aragonite and
High-Mg calcite have a higher diagenetic potential than
sediments dominated by Low-Mg calcite. Resulting
differences in dissolution and reprecipitation will be
particularly conspicuous in platforms and inner ramps
affected by meteoric diagenesis.
15.6 Carbonate Platforms and
Ramps
Facies models and microfacies types of rimmed car-
bonate platforms and carbonate ramps were discussed
in Cha pter 14. The objective of this section is to dem-
onstrate the potential of microfacies analyses for dif-
ferentiating and interpreting platforms and ramps by
using selected case studies.
15.6.1 Ecological Controls on Platforms and
Ramps
The Pomar Model: Carbonate-producing organisms
and depositional profiles
The geometry and accumulation of sediment on car-
bonate platforms depend strongly on the biota living
on the platforms, and the interplay of benthic organ-
isms and physico-chemical environmental factors (Fig.
15.12, Fig. 15.13):
Ecologic factors control the location and rate of
organic sediment production . The depth of the main
carbonate production and its position in relation to plat-
form margins affects the geometric evolution of plat-
forms. Both parameters depend on the ecology of car-
bonate-producing organisms as reflected by their life
habit. Carbonate production depth is controlled by il-
lumination, and the lateral distribution of organisms
across the platform is dependent on hydrodynamics,
substrate and nutrients.
The presence or absence of a biogenic framework
influence the stabilization and geometry of platform
margins, and the composition and location of facies
Facies models of platforms and ramps describing
the depositional zones of shallow-marine to deep-ma-
rine transects were discussed in Chap. 14. A new model,
developed by Pomar (2001a), emphasizes the interac-
tion between the different sediment types being pro-
duced by organisms and the hydraulic energy acting in
the diverse loci of production (Fig. 15.13).
The basic ideas of the model are:
(1) The differences between rimmed platforms, non-
rimmed platforms, distally steepened ramps and homo-
clinal ramps, can be considered as a balance between
(a) the type (basically the grain size) of the sediment
produced, (b) the locus of the sediment production, and
(c) the hydrodynamic energy.
• The carbonate production (sediment supply) of plat-
forms and ramps depends mainly on biological sys-
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