Conglomerate  is a coarse-grained
clastic sedimentary rock that is
composed of a substantial fraction of
rounded to subangular gravel-size
clasts, e.g., granules, pebbles,
cobbles, and boulders, larger than 2 mm
in diameter. Conglomerates form by the
consolidation and lithification of
gravel. Conglomerates typically contain
finer grained sediment, e.g., either
sand, silt, clay or combination of them,
called matrix by geologists, filling
their interstices and are often cemented
by calcium carbonate, iron oxide,
silica, or hardened clay.
The size and composition of the
gravel-size fraction of a conglomerate
may or may not vary in composition,
sorting, and size. In some
conglomerates, the gravel-size class
consist almost entirely of what were
clay clasts at the time of deposition.
Conglomerates can be found in
sedimentary rock sequences of all ages
but probably make up less than 1 percent
by weight of all sedimentary rocks. In
terms of origin and depositional
mechanisms, they are closely related to
sandstones and exhibit many of the same
types of sedimentary structures, e.g.,
tabular and trough cross-bedding and
graded bedding.
Classification
Conglomerates may be named and
classified by the:
Amount and type of matrix present
Composition of gravel-size clasts they
contain
Size range of gravel-size clasts present
The classification method depends on the
type and detail of research being
conducted.
A sedimentary rock composed largely of
gravel is first named according to the
roundness of the gravel. If the gravel
clasts that comprise it is largely
well-rounded to subrounded, it is a
conglomerate. If the gravel clasts that
comprise it are largely angular, it a
breccia. Such breccias should called
sedimentary breccias to differentiated
them from other types, e.g., volcanic
and fault, of breccias. Sedimentary
rocks that contain a mixture of rounded
and angular gravel clasts are sometimes
called breccio-conglomerate.
= Texture=
Conglomerates are rarely composed
entirely of gravel-size clasts.
Typically, the space between the
gravel-size clasts is filled by a
mixture composed of varying amounts of
silt, sand, and clay, known as matrix.
If the individual gravel clasts in a
conglomerate are separated from each
other by an abuncance of matrix such
that they are not in contact with each
other and float within the matrix, it is
called a paraconglomerate.
Paraconglomerates are also often
unstratified and can contain more matrix
than gravel clasts. If the gravel clasts
of a conglomerate are in contact with
each other, it is called a
orthoconglomerate. Unlike
paraconglomerates, orthoconglomerates
are typically cross-bedded and often
well-cemented and lithified by either
calcite, hematite, quartz, or clay.
The differences between
paraconglomerates and orthoconglomerates
reflect differences in how they are
deposited. Paraconglomerates are
commonly either glacial tills or debris
flow deposits. Orthoconglomerates are
tyipically associated with aqueous
currents of some sort.
= Clast composition=
Conglomerates are also classified
according to the composition of their
clasts. A conglomerate or any clastic
sedimentary rock that consists of a
single rock or mineral is known as
either a monomict, monomictic,
oligomict, or oligomictic conglomerate.
If the conglomerate consists of two or
more different types of rocks, minerals,
or combination of both, it is known as
either a polymict or polymictic
conglomerate. If a polymictic
conglomerate contains an assortment of
the clasts of metastable and unstable
rocks and minerals, it called either a
petromict or petromictic conglomerate.
In addition, conglomerates are
classified by source as indicated by the
lithology of the gravel-size clasts If
these clasts consist of rocks and
minerals that are significantly
different in lithology from the
enclosing matrix and, thus, older and
derived from outside the basin of
deposition, the conglomerate is known as
an extraformational conglomerate. If
these clasts consist of rocks and
minerals that are identical to or
consistent with the lithology of the
enclosing matrix and, thus,
penecontemporaneous and derived from
within the basin of deposition, the
conglomerate is known as an
intraformational conglomerate.
Two recognized types of type of
intraformational conglomerates are
shale-pebble and flat-pebble
conglomerates. A shale-pebble
conglomerate is a conglomerate that is
composed largely of clasts of rounded
mud chips and pebbles held together by
clay minerals and created by erosion
within environments such as within a
river channel or along a lake margin.
Flat-pebble conglomerates are
conglomerates that consist of relatively
flat clasts of lime mud created by
either storms or tsunami eroding a
shallow sea bottom or tidal currents
eroding tidal flats along a shoreline.
= Clast size=
Finally, conglomerates are often
differentiated and named according to
the dominant clast size comprising them.
In this classification, a conglomerate
composed largely of granule-size clasts
would be called a granule conglomerate;
a conglomerate composed largely of
pebble-size clasts would be called a
pebble conglomerate; and a conglomerate
composed largely of cobble-size clasts
would be called a cobble conglomerate.
Sedimentary environments
Conglomerates are deposited in a variety
of sedimentary environments.
= Deepwater marine=
In turbidites, the basal part of a bed
is typically coarse-grained and
sometimes conglomeratic. In this
setting, conglomerates are normally very
well sorted, well-rounded and often with
a strong A-axis type imbrication of the
clasts.
= Shallow marine=
Conglomerates are normally present at
the base of sequences laid down during
marine transgressions above an
unconformity, and are known as basal
conglomerates. They represent the
position of the shoreline at a
particular time and are diachronous.
= Fluvial=
Conglomerates deposited in fluvial
environments are typically well rounded
and well sorted. Clasts of this size are
carried as bedload and only at times of
high flow-rate. The maximum clast size
decreases as the clasts are transported
further due to attrition, so
conglomerates are more characteristic of
immature river systems. In the sediments
deposited by mature rivers,
conglomerates are generally confined to
the basal part of a channel fill where
they are known as pebble lags.
Conglomerates deposited in a fluvial
environment often have an AB-plane type
imbrication.
= Alluvial=
Alluvial deposits form in areas of high
relief and are typically coarse-grained.
At mountain fronts individual alluvial
fans merge to form braidplains and these
two environments are associated with the
thickest deposits of conglomerates. The
bulk of conglomerates deposited in this
setting are clast-supported with a
strong AB-plane imbrication.
Matrix-supported conglomerates, as a
result of debris-flow deposition, are
quite commonly associated with many
alluvial fans. When such conglomerates
accumulate within an alluvial fan, in
rapidly eroding environments, the
resulting rock unit is often called a
fanglomerate.
= Glacial=
Glaciers carry a lot of coarse-grained
material and many glacial deposits are
conglomeratic. Tillites, the sediments
deposited directly by a glacier, are
typically poorly sorted,
matrix-supported conglomerates. The
matrix is generally fine-grained,
consisting of finely milled rock
fragments. Waterlaid deposits associated
with glaciers are often conglomeratic,
forming structures such as eskers.
Examples
An example of conglomerate can be seen
at Montserrat, near Barcelona. Here,
erosion has created vertical channels
that give the characteristic jagged
shapes the mountain is named for. The
rock is strong enough to use as a
building material, as in the Santa Maria
de Montserrat Abbey.
Another example, the Crestone
Conglomerate, occurs in and near the
town of Crestone, at the foot of the
Sangre de Cristo Range in Colorado's San
Luis Valley. The Crestone Conglomerate
consists of poorly sorted fanglomerates
that accumulated in prehistoric alluvial
fans and related fluvial systems. Some
of these rocks have hues of red and
green.
Conglomerate cliffs are found on the
east coast of Scotland from Arbroath
northwards along the coastlines of the
former counties of Angus and
Kincardineshire. Dunottar Castle sits on
a rugged promontory of conglomerate
jutting into the North Sea just south of
the town of Stonehaven.
Conglomerate may also be seen in the
domed hills of Kata Tjuta, in
Australia's Northern Territory.
In the nineteenth century a thick layer
of Pottsville conglomerate was
recognized to underlie anthracite coal
measures in Pennsylvania.
Metaconglomerate
Metamorphic alteration transforms
conglomerate into metaconglomerate.
See also
Puddingstone
Jasper conglomerate
References
External links
Conglomerate at Cushendun, Northern
Ireland
