Geology Reference
In-Depth Information
cias formed in upper slope environments often exhibit
a bimodal grain size composition, with larger clasts and
interstitial well-sorted sand-sized particles (peloids,
ooids, skeletal grains, Pl. 26/7). Size and sorting are
essential criteria in discriminating genetic conglomer-
ate types.
Shape
and surface of carbonate clasts:
The form
may vary from sharply angular to more or less rounded,
the shape can be isometric, platy, or tabular. Many round
conglomerate pebbles exhibit biogenic encrustations
and borings indicating interruptions and resting times
during transport processes (Pl. 26/7).
Roundness
: Per definition breccia clasts are angu-
lar, but carbonate breccias display all transitions pos-
sible from predominantly angular to variously rounded
clasts, which relate the latter samples to conglomerate
categories (see Pl. 26/3). The roundness of breccia clasts
is determined with the help of comparison charts that
Plate 26 Carbonate Breccias
Carbonate breccias and conglomerates, made of angular or rounded rock fragments embedded within a matrix
and/or cemented by various minerals, are common in marine and non-marine sequences. Integration of field and
laboratory data is necessary for classifying and interpreting these rocks. In polymict breccia, clasts and matrix
are not genetically related, and result from transport and depositional processes (e.g. submarine landslide, fluvial
transport and deposition, subaerial and subaqueous debris flows (-> 3), slumping and volcanic phenom-
enons (-> 2). Mechanisms triggering mobilization are gravity, earthquake and water loading that induce over-
pressure, storms (producing flat-pebble conglomerates), tides, oversteepening of slopes (-> 7), gas hydrate de-
stabilization, and glaciation processes. In autoclastic or intraformational breccias, clasts and matrix are geneti-
cally related. In terrestrial or meteoric-vadose environments, these breccias result from in situ deformation or
reworking processes (dehydration, karstification).
1
Polymict stylobreccia
consisting of tightly packed limestone clasts. The scarce marly matrix is a residue product of
pressure solution. The breccia represents a massflow deposit formed by submarine gravity flows and modified by subse-
quent pressure-solution. Rounding of clasts is caused by burial solution, not by transport. Arrows point to solution seams.
Note the different microfacies of the clasts: The dark clasts are pelagic bioclastic wackestone with filaments, ostracods
and radiolaria. Light clasts are bioclastic packstones. Early Jurassic (Adnet Limestone): Adnet near Salzburg, Austria.
2
Matrix-supported agglomerate
(volcanic breccia) formed by submarine volcanic explosion followed by admixture and
massflow transport of volcanic clasts (V) and rounded carbonate extraclasts (E). Middle Triassic (Ladinian): Mt. Agnello,
Southern Alps, Italy.
3
Clast-supported monomict limestone breccia.
The clasts exhibit different roundness/sphericity values: subangular, sub-
rounded and rounded clasts (fields B, E and I in the Krumbein chart, cf. Fig. 4.30). The clasts are pelagic bioclastic
wackestones and packstones. Calcite-filled microfractures are limited to the clasts indicating fracturing prior to breccia-
tion. Interclast voids are filled with several generations of marine fibrous and blocky calcite cements. The breccia is
explained as mass-flow breccia resulting from submarine slides of semiconsolidated pelagic sediments on a steep slope of
a drowned Triassic reef, triggered by tectonic activities (Hudson and Jenkyns 1969; Böhm et al. 1995). Because of the
attractive appearance of the red limestone clasts within the white sparry calcite, the breccia has been widely used as
decorative stone. Early Jurassic ('Scheck breccia', Piensbachian/Toarcien): Adnet near Salzburg, Austria.
4
Polymict clast-supported microbreccia
(sandy lithoclast packstone) consisting of angular (gray) dolomite clasts, (dark)
limestones clasts and (white) quartz grains. Note the absence of matrix. Debris-flow deposit. Early Cretaceous: Mosaic
material of a Roman building, Kraiburg, southern Bavaria, Germany.
5
Monomict tectonic fault breccia.
Note the angularity and the complex and strong fracturing of individual clasts as well as
the whole rock. Microfractures differ in width, filling and orientation. Some calcite veins clearly postdate brecciation.
The matrix consists of small broken and sheared rock fragments bound together by sparry calcite. Late Triassic (Dachstein
Limestone, Norian): Northern Alps, Austria.
6
Polymict breccia consisting of extraclasts
(left: shallow-marine algal wackestone with dasyclads - AW, bottom right:
microbioclastic packstone - P) and alveolinid foraminifera. Interpretation: Massflow deposit. Redeposition of shallow-
ramp bioclasts and lithoclasts in deeper ramp areas caused by regressive events (Kulbrok 1995). Early Tertiary (Eocene):
St. Paul Monastery, Wadi Araba, eastern Egyptian Desert.
7
Polymict massflow breccia.
The material was eroded from reefs formed on the upper slope of a platform margin. The
sample is part of a megabreccia consisting of meter-sized reef- and slope-derived blocks.. Deposition took place in a
proximal position on the foreslope. Most clasts are boundstones except for the crinoid packstone (P) representing slope
facies. Note biogenic encrustations (arrows) indicating breaks during the transport on the slope. GP: Upside-down geo-
petal structure (arrow). The interstitial grainstone 'matrix' (GM) represents platform-derived material infilled subsequent
to the deposition of the clasts. Fracturing postdates brecciation. Middle Triassic (Ladinian): Seiser Alm, Dolomites, Italy.
