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