Geology Reference
In-Depth Information
'Clasts' and 'groundmass':
The clast-like mottled
areas are of different size, have irregular and transi-
tional outlines and appear poorly sorted. The calcite
crystals of the mottles are larger than those of the in-
ter-mottle areas. The proportions of 'clasts' and 'ma-
trix' differ widely. No fitting and no veined 'clasts'.
The 'groundmass' is more frequently affected by late
diagenetic dolomitization than the 'clasts'. This brec-
cia type may appear either 'clast' or 'matrix' supported.
Case studies
: Bathurst 1959; Blount and Moore
1969; Solomon 1989.
Note that the term 'pseudo-breccia' is also applied
to breccias consisting of in-place clasts, separated by a
network of joints and interpreted as being the result of
earthquake loading. Similar breccias can also be caused
by wave-induced processes including pseudo-cracking,
crack collapse and heterogeneous liquefaction (Bou-
chette et al. 2001).
Stylobreccia caused by variable pressure solution
Field observations:
Massive or indistinctly bedded
limestones, often displaying a 'nodular' fabric.
Clasts
: The 'clasts' are 'idens' (see Sect. 7.5.2) and
comprise resedimented lithoclasts and fossils affected
by pressure solution (Pl. 26/1). They are separated by
thin residual clay seams or microstylolites. Many
'clasts' appear subrounded or subangular. Clast sizes
highly variable. Fabric appears clast-supported
Micro-
facies
types highly diverse. A specific case are
stylo-
conglomerates
formed by selective pressure solution
of lime mudstones (Pl. 47/6).
Groundmass
: More or less absent or restricted to
small amounts of residual micrite.
Fossils
: Common.
Case studies
: Logan and Semeniuk 1976; Steiger
1981.
Fig. 5.18.
Internal breccia
. The bivalve coquina consisting
of
Halobia
shells is brecciated and fractured. Note the corre-
sponding boundaries of many 'clasts' (white arrows) and cal-
cite-filled cracks. Late Triassic (Hallstatt limestone, Norian):
Obersalzberg, Berchtesgaden, Germany. Scale is 1 mm.
Clasts
: Usually monomict. Closely packed and dis-
sected by shear planes. Low fitting. Relative disloca-
tion of clasts high. Different generations of fractured
sometimes difficult to separate. Flat to lens-shaped
clasts. Clasts closely packed.
Groundmass:
Groundmass consists of cataclastic
clasts or cement.
Fossils
: If present poorly preserved.
Other terms used for shear breccias:
Tectonic brec-
cias (Blunt and Moore 1969).
Case studies:
Roehl 1981.
5.3.3.4 Carbonate Conglomerates
Most authors distinguish
conglomerates sensu strictu
(formed by extraclasts) and synsedimentary
soft con-
glomerates
(formed by intraclasts). The latter comprise
flat-pebble conglomerate produced by erosional rip-
up of intertidal and supratidal laminites during storm
events.
Descriptive parameters of conglomerates are fab-
ric, stratification and grading, size and sorting of the
pebbles, lithologic uniformity or variability, and the
amount of matrix. The composition of the groundmass
is variable (carbonate, siliciclastics, matrix and/or ce-
ment).
Orthoconglomerates
are characterized by clast-
Diagenetic breccia
Pseudobreccia caused by patchy recrystallization
Field observations:
Fine-grained bedded dolomites
or limestones displaying a distinct irregularly distrib-
uted mottled texture, which mimics the appearance of
breccia. The mottles differ in darker color from the ar-
eas in-between. Known predominantly from wacke-
stones and packstones.
