Geology Reference
In-Depth Information
8.3.2.2 Discussion
What do mudstones and wackestones indicate about
paleoenvironmental conditions?
Mudstones are per
definition 'muddy carbonate rocks containing less than
10 per cent grains'. Grains and grain-producing organ-
isms are rare, possibly reflecting unfavorable environ-
mental conditions. The sparsity of grains, however,
could be a matter of scale and method: Many pelagic
mudstones correspond to densely packed wackestones
or even to packstones if studied with the aid of ultra
thin-sections or the stereoscan electron microscope.
Linking the deposition of mudstones and wackestones
to low-energy settings may be a misinterpretation, be-
Dunham's rock names are generally accepted despite
some problems in the difficulty of unequivocally dif-
ferentiating of the textural categories and their inferred
environments. The greatest problem is that the classifi-
cation strongly suggests a 'depositional' character of
the textures. It must be kept in mind, however, that lime-
stones exhibiting mudstone, wackestone, packstone and
even grainstone fabrics could also be products of di-
agenetic processes, such as micritization, cryptocrys-
talline calcite cementation or neomorphism.
Plate 43 Allochthonous Carbonates. Basic Depositional Fabrics: GrainSupport and MudSupport
Major criteria for classifying allochthonous limestones are the depositional fabric, the abundance of various
grain categories as discussed in Chapt. 4, and the type of fine-grained matrix and carbonate cements associated
with the grains.
The principal feature of the classification proposed by Dunham (1962) is the distinction between grain-sup-
port (particle-support) and mud-support (matrix-support) describing the basic depositional fabrics:
Grain-sup-
port
(-> 1) is defined as a fabric with little or no mud and abundant sand-sized grains that are in three-dimen-
sional contact and able to support each other. The fabric is comparable to that of grains in a pile of sand.
Mud-
support
(-> 2) is defined as a fabric whose sand-sized grains (at least 10% of the total rock volume) 'float', i.e.
are embedded in, and supported by the muddy matrix. The fabric resembles pebbles within a mud puddle.
Distinguishing grain-supported packstone from mud-supported wackestone may be difficult, because thin
sections of both rock types can show grains 'floating' within a matrix and apparently not in contact. Because of
the two-dimensional aspect of thin sections, irregularly shaped grains of a grain-supported fabric often do not
appear to be in contact along every surface. Hence the point contact of grains is not necessarily a decisive factor
in defining a 'packstone'. The degree of grain packing, grain density, and number of grains can be measured in
thin sections using various methods (see Sect. 6.1.1.1). However, the number of grains alone is not conclusive
for distinguishing between grain- and mud-support because even 20-30% of irregularly shaped large grains may
form a self-supporting framework. Helpful but not unequivocal criteria in recognizing grain-support are: (a) lack
of carbonate mud, (b) floored interparticle pores, (c) embowed grains, (d) close and overly close packing, and
(e) abundant interparticle cement.
The grain-/mud-support concept is fundamental in interpreting depositional controls (e.g. hydrodynamic con-
ditions).
The distinction between grain-supported and mud-supported fabrics reflects major differences in primary
porosity types (Sect. 7.3) and, in turn, the potential of limestones as reservoir rocks (Sect. 17.1.5.2).
1
Grain-support
fabric, indicated by absence of mud, close packing of grains, and abundance of carbonate cement in
interparticle pores. Most particles are aggregate grains, followed by some micritized bioclasts (foraminifera, F; mollusks,
M), very small gastropods (G) and peloids. The grains are equally sized and very well sorted (standard deviation 0.35),
see Fig. 6.3. The calcite cement in the interparticle pores appears white. The fine-grained lithoclastic
grainstone
forms a
capping bed overlying a rudist patch reef. The capping beds correspond to mobile midshelf grain flats. SMF 17. Mid-
Cretaceous (Edwards Limestone, Comanche Formation, Albian): Belton, central Texas.
2
Mud-support
fabric indicated by grains 'floating' in lime mud. Skeletal grains are echinoderms (E), thin-shelled bivalves
(filaments, FI), double-valved bivalves (B) and foraminifera. Note that geopetally filled cavities (black arrows) form a
distinct horizon, possibly indicating shallow-burial dissolution. Aragonitic shells are dissolved and infilled with sediment
(ammonite mold: white arrow). Original calcitic grains (echinoderms, filaments) are well preserved. The sample would
be classified as echinoderm
floatstone
according to the echinoderm fragments, which are larger than 2 mm and comprise
more than 10% of the rock. This name would fail to document the conspicuous bimodality of the sample and specific
composition of the matrix. The total rock fabric is better described as 'crinoid floatstone with a fine-grained skeletal
wackestone matrix rich in filaments'. The red nodular limestones originated in a moderately deep pelagic basin. Late
Triassic (Hallstatt limestone, Carnian): Rötelstein, Styria/Austria.
