Geology Reference
In-Depth Information
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Further reading
: K071
The Latin word
conglomeratus
means heaped, rolled
and pressed together. The study of breccias and con-
glomerates requires integrating field observations and
laboratory data. Microfacies analysis provides a pow-
erful means of deciphering the provenance of the clasts
and understanding the relationships between deposi-
tional and tectonic controls on recycled sediments.
5.3.3.1 Terminology
A variety of terms is applied to rocks composed of car-
bonate clasts. Most terms recognize the rock to be ge-
netically interpretated. Breccias related to seismic
events (see Sect. 12.4) and impact breccias caused by
meteorites will be discussed later.
Impact breccias
are
generally characterized by strongly fractured clasts and
a large amount of groundmass consisting of fragmented
particles. The rock is twisted, sheared and heavily
veined. Cement filled vugs are common (Martinez et
al. 1994; Warme and Kuehner 1998).
Other terms focus on the dimensions of the clasts,
which vary widely:
Microbreccias:
containing mm- and cm-sized car-
bonate clasts are often described within the framework
of common limestone classifications using the terms
intra- or lithoclast, and indicating size ranges by the
use of 'calcirudite', 'rudstone' or 'floatstone' (e.g. 'poly-
mict intracalcirudite'). The boundary between lithoclast
limestones and breccias is artificial, some authors pro-
pose a boundary of about 2 mm. Some microbreccias
are extremely diverse with regard to the microfacies
types of the carbonate clasts. Auley-Charles (1967) for
example, reported a Miocene microbreccia consisting
of more than 30 limestone types. Microbreccias are
common constituents of slope deposits (Pl. 114/1, 3)
or represent distal debris-flow deposits (Pl. 26/4).
Megabreccia
are coarse matrix-supported breccias
containing individual angular blocks up to a meter to
more than 100 meters in size. The breccias result from
sedimentary and tectonic processes and develop down-
slope by large thrusts connected with gravitational
slidings, e.g. mega-debris flow deposits (Cook et al.
1972; Mountjoy et al. 1972; Leigh and Hartley (1992).
They are formed on a broad range of slope angles and
contains coarse-grained as well finer-grained breccias.
The latter represent a specific Standard Microfacies
Type (Pl. 114/1, 3). Explanations for the formation of
megabreccias are the release of pore-water overpres-
sure of confined aquifer horizons beneath the sea floor
(particularly during falls of sea level) or a catastrophic
collapse of high-angle oversteepened carbonate plat-
5.3.3 Carbonate Breccias and
Conglomerates
Carbonate breccias and conglomerates are sedimentary
rocks characterized by limestone or dolomite fragments
embedded in a groundmass consisting of fine-grained
matrix or/and cements. Breccias are characterized by
abundant angular clasts (usually > 50%), conglomer-
ates by abundant well-rounded clasts of pebble- or
gravel size.
The word breccia originated in Italy where it has
long been used for the crushed rubble used in walls.
