Geology Reference
In-Depth Information
elements, life habits, the trophic composition of paleo-
communities and trace fossil assemblages. Microfacies
data play an essential role in evaluating the results of
these reconstructions.
must be evaluated separately. Automicrites (e.g. micro-
bialites) can originate in low-energy as well as in high-
energy environments.
Grains
Oncoids
originate in low-energy and high-energy
environments (Box 4.10). Low-energy environments
are characterized by single or multiple micritic oncoids
exhibiting discontinuous growth layers, often forming
floatstones and wackestones. High-energy oncoids are
small, exhibit rather regular laminations, and commonly
form packstones and grainstones (rudstones).
Rhodoids
formed under low-energy and high-energy
conditions differ significantly in shape, external and
internal growth, as well as the type of ecological suc-
cessions (Sect. 4.2.4.2).
Ooids
are formed in high-energy and low-energy
environments (Sect. 4.2.5; Fig. 4.20). High-energy en-
vironments are indicated by concentric (tangential) or
micritic ooids, and broken or distorted ooids (Fig. 4.23).
Low-energy environments are characterized by radial
(radial-fibrous) ooids or asymmetrical (eccentric oo-
ids; Fig. 4.23). The maximum and average ooid size
and the thickness of ooid coatings can be used as an
approximate measure of water energy (Fig. 4.25).
12.1.1.1 Hydrodynamic Energy Levels
Energy levels are reflected by microfacies criteria
summarized in the following text. The recognition of
energy levels based on depositional textures is discussed
in Sect. 12.1.1.2. Fig. 12.1 gives an idea of the ideal
relations between the winnowing of fine-grained par-
ticles, sorting and rounding of grains and the intensity
of hydrodynamic processes.
Microfacies are proxies for hydrodynamic depositional
conditions of carbonate sediments.
Matrix
Micrite:
The occurrence and amount of seafloor mi-
crite (Box 4.2) as well as the micrite : grain ratio are
generally regarded as an indication of low-energy depo-
sitional environments. Limitations to this concept are
discussed in Sect. 4.1.6. Allomicrites and automicrites
Fig. 12.1.
Winnowing, sorting and rounding
are controlled by the intensity of hydrodynamic processes. Winnowing of lime
mud taking place at low energy levels may be absent (A) in low-energy environments, incomplete (B) in environments with
fluctuating energy conditions, or complete (C and D) in high-energy environments. An increase in the intensity of wave
action and currents leads to changes from unsorted (C) to sorted (D) sediments with layers of different grain size. An increase
in rounding (E) may reflect an increase in water energy, e.g. in the surf zone. Sorting and rounding of carbonate grains will
decrease (F and G) when extreme water energy (e.g. severe storms) affects the sediment. The example shown refers to shelly
limestones. Black areas between skeletal grains represent fine-grained matrix (e.g. micrite), white areas between skeletal
grains intergranular pores filled with calcite cement. After Folk (1962).
