Geology Reference
In-Depth Information
mary strata or associated with vanished evaporites (see
Sect. 12.1.7 for criteria).
Clasts:
Often polymict. Commonly dolomite or de-
dolomite clasts. Weathering of dolomite clasts results
in the formation of a cellular structure (
Rauhwacken
).
Unsorted, variously sized, usually angular and suban-
gular clasts. Usually clast-supported fabric. Variable fit-
ting. Clast-matrix boundary sharp except at the replace-
ment margins of some clasts. Often densely packed.
Very common pre-breccia veining, fracturing common.
Clast
microfacies
types include lime and dolomitic mud-
stones with evidence of former evaporites, and replace-
ment dolomites and dedolomites.
Groundmass
: Fine-grained matrix corresponds to a
microbreccia resulting from internal brecciation of
coarser clasts, or to microcrystalline calcite.
Fossils
: Absent or very rare.
Case studies:
Assereto and Kendall 1977; Beales
and Oldershaw 1969; Middleton 1961; Morrow 1982;
Karakitsios and Pomoni-Papaioanou 1998; Pomoni-
Papaioanou and Dornsiepen 1987; Stanton 1966; Visser
1986.
Internal breccias
Field observations
: Strata-bound, but laterally dis-
continuous. Many internal breccias consist of shallow-
marine platform carbonate rocks overlain by basinal
sediments (e.g. red pelagic limestones). A characteris-
tic sequence (Fig. 5.17) includes from base to top (a)
shallow-water limestones, locally with calcite-cemented
joint, (b) shallow-water with fissures, filled with a brec-
cia of the same lithology and exhibiting a red carbon-
ate matrix, (c) internal breccia consisting of shallow-
water limestones with a red carbonate matrix filled-in
from above, (d) mass-flow breccia consisting of the
same shallow-marine limestones and the same red ma-
trix as in (c), and (e) red pelagic limestones. Common
in shelf-to-slope transition settings.
Clasts
: Usually monomict. Angular and subangular
clasts. Shape irregular to isometric. High to moderate
fitting. Low relative dislocation of clasts. Fracturing of
clasts characterized by several superimposed genera-
tions. The breccia includes fragments of calcite veins
as well as of older internal breccias. Variable
microfa-
cies
types
including all common types of platform car-
bonates.
Groundmass
: Matrix sucked in from above, or ce-
ment. Variously colored, often red.
Fossils
: Common within clasts and also in the ma-
trix.
Other terms used for breccias:
Autoclastic breccia
(Fairbridge 1978), dilatation breccia (Roehl 1981).
Case studies:
Richter and Füchtbauer 1981; Fücht-
bauer and Richter 1983; Herbig 1985; Wächter 1987.
Tectonic breccias
Movement along a fault surface at high levels within
the crust generally results in the fracturing and break-
ing of wall rock fragments and crystals due to cataclasis.
Products of this process are fault breccias (Pl. 26/5),
microbreccias (with largest fragments < 1.0 mm) and
fine-grained gouges (consisting of fragments diminu-
ated to 0.1-100
m).
Many tectonic breccias are calcite cemented. The
terminology of fault-related breccias was discussed by
Wise et al. (1984). Fissures filled with breccias are of-
ten related to synsedimentary tectonics. Other tectoni-
cally controlled breccias are internal breccias and shear
breccias formed.
Shear breccia
Field observations:
Poorly stratified and discontinu-
ous. Shear faces crossing the breccia and crushed clasts.
Marine fissure-fill breccias
Field observations:
Fissure-fill breccias occur in
karst cavities (see Sect. 15.2.1) and in vertical and ob-
lique submarine dikes. Commonly in strata underlying
strata with internal breccia.
Clasts
: Monomict. Angular or subrounded. Clasts
usually small, cm-sized. Unsorted. Clast/matrix bound-
ary sharp. Poor fitting.
Microfacies:
Lithoclastic floatstones.
Groundmass
: High matrix percentage consisting of
recrystallized fine-grained calcite.
Fossils
: Broken and worn mollusk shells and echi-
noderms may be associated with breccia clasts.
Case studies
: Schlager 1969; Richter and Fücht-
bauer 1981.
Fig. 5.17.
Schema of the development of internal breccias.
Fractured shallow-marine limestones are overlain by inter-
nal breccia, followed by mass-flow breccia and pelagic lime-
stones. Modified from Füchtbauer and Richter (1983).
