Geology Reference
In-Depth Information
Box 4.5.
Significance of micritic limestones - a first glance.
Paleoenvironmental proxies
Water energy levels:
Microcrystalline carbonates are tradionally thought to represent mud accumulated in low-
energy quiet waters on the sea floor. However, fine-grained calcite also forms as an internal precipitate or sediment
in cavities below the sediment/water interface, especially in reef carbonates. This internal micrite can invalidate
interpretations relating the amount of micrite matrix to the degree of water energy. Another pitfall is that fine-
grained carbonates can be deposited and preserved in high-energy environments, if organisms (e.g. microbes) bind
the sediment (Sect. 12.1.1).
Changes in sediment input:
The formation of accretionary organomicrites requires low or moderate rates of sediment
input.
Bottom conditions:
Differences in organisms living on and within the sediment, and favoring soft, solid and hard
bottoms are indicators of substrate consistency Sect. 12.1.3).
Depositional settings of finegrained carbonates
Lagoons:
Limestones formed in lagoons can be differentiated from open-marine micritic carbonates by dominating
microfacies types and specific associations of fossils (Sect. 10.3).
Reefs:
Fine-grained carbonates originate in reefs inside framework and solution cavities. Different parts and growth
stages of framework reefs are characterized by the specific microfabrics of these internal sediments, which correspond
partially to allomicrites, but also include biologically induced automicrites. Mud mounds exhibit diagnostic microfacies
types (Sect. 16.2.5.1).
Periplatforms:
The transitional character of fine-grained periplatform carbonates accumulating on slopes or in basins
is reflected by specific microfacies types. Diagnostic criteria are platform-derived grains associated with pelagic
biota as well as diagenetic fingerprints (Sect. 15.7.5).
Slopes:
Steep ancient and modern slopes
were stabilized by biologically induced automicrites that exhibit specific
microfacies patterns (Sect. 15.7.4).
Pelagic micrites:
Microfacies as well as diagenetic criteria and geochemical signals characterize depositional settings
and assist in the understanding of the ooze/chalk/limestone transition (Sect. 7.1.4 and 15.8).
Global secular variations
Temporal changes in the dominating mineralogy:
The dominant mineralogy of the mud precursors of micrites has a
strong influence on the resulting microfabric. A distinction between the aragonite-dominated-precursor (ADP) and
calcite-dominated precursor (CDP) micrites can be made by recognizing aragonite relicts, differences in crystal
surfaces and crystal size, differences in microfabrics, as well as on the basis of geochemical data. The study of the
Phanerozoic record of micrites and other carbonate constituents points to the existence of secular variations in
carbonate mineralogy and global changes in the chemistry of the oceans over time.
Technological properties
Reservoir rocks:
The microporosity of micrites is one of the reasons that giant hydrocarbon fields are related to fine-
grained limestones, not only to genuine chalks in the North Sea but also to chalky micritic limestones, e.g. in the
Middle East (Sect. 17.1.2).
Limestone resources:
Many micrite-dominated reefs, particularly mud and reef mounds, are characterized by the
exceptional purity of the limestones, which makes the carbonates very suitable for industrial use. Micritic limestones
with high intergranular porosity have a high aptitude for mechanical disintegration and grainy destruction, which
are useful properties for the production of high-quality technical chalk. Many technological properties of fine-
grained carbonates are controlled by the sedimentary fabric and the course of diagenesis (see Chap. 18).
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Further reading
: K033, K078
