Page 21 - Sedimentology and Stratigraphy

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Stratigraphy
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Stratigraphy
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26.2.2009 8:14pm Compositor Name: ARaju
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8 Terrigenous Clastic Sediments: Gravel, Sand and Mud 26.2.2009 8:14pm Compositor Name: ARaju
limestone bedrock. Recognition of the variety of clasts
can therefore be a means of determining the source of
a conglomeratic sedimentary rock (5.4.1).
2.2.2 Texture of conglomerate
Conglomerate beds are rarely composed entirely of
gravel-sized material. Between the granules, pebbles,
cobbles and boulders, finer sand and/or mud will often
be present: this finer material between the large clasts
is referred to as the matrix of the deposit. If there is a
high proportion (over 20%) of matrix, the rock may
Fig. 2.4 A conglomerate (or breccia) made up of angular be referred to as a sandy conglomerate or muddy
clasts. conglomerate, depending on the grain size of the
matrix present (Fig. 2.5). An intraformational con-
some circumstances it is prudent to specify that a glomerate is composed of clasts of the same material
deposit is a ‘sedimentary breccia’ to distinguish it as the matrix and is formed as a result of reworking of
from a ‘tectonic breccia’ formed by the fragmentation lithified sediment soon after deposition.
of rock in fault zones. Mixtures of rounded and angu- The proportion of matrix present is an important fac-
lar clasts are sometimes termed breccio-conglomer- tor in the texture of conglomeratic sedimentary rock,
ate. Occasionally the noun rudite and the adjective that is, the arrangement of different grain sizes within
rudaceous are used: these terms are synonymous it. A distinction is commonly made between conglom-
with conglomerate and conglomeratic. eratesthatareclast-supported (Fig. 2.6), that is, with
clasts touching each other throughout the rock, and
those which are matrix-supported (Fig. 2.7), in
2.2.1 Composition of gravel and
conglomerate which most of the clasts are completely surrounded
by matrix. The term orthoconglomerate is some-
times used to indicate that the rock is clast-supported,
A more complete description of the nature of a gravel or and paraconglomerate for a matrix-supported tex-
conglomerate can be provided by considering the types ture. These textures are significant when determining
of clast present. If all the clasts are of the same material the mode of transport and deposition of a conglomer-
(all of granite, for example), the conglomerate is con- ate (e.g. on alluvial fans: 9.5).
sidered to be monomict.A polymict conglomerate is The arrangement of the sizes of clasts in a conglom-
one that contains clasts of many different lithologies, erate can also be important in interpretation of deposi-
and sometimes the term oligomict is used where tional processes. In a flow of water, pebbles are moved
there are just two or three clast types present. more easily than cobbles that in turn require less energy
Almostany lithologymay befound asa clast ingravel
and conglomerate. Resistant lithologies, those to move them than boulders. A deposit that is made up
of boulders overlain by cobbles and then pebbles may be
which are less susceptible to physical and chemical
interpreted in some cases as having been formed from a
breakdown, have a higher chance of being preserved
flow that was decreasing in velocity. This sort of inter-
as a clast in a conglomerate. Factors controlling the
pretation is one of the techniques used in determining
resistance of a rock type include the minerals present
the processes of transport and deposition of sedimentary
and the ease with which they are chemically or phys-
rocks (4.2).
ically broken down in the environment. Some sand-
stones break up into sand-sized fragments when
eroded because the grains are weakly cemented 2.2.3 Shapes of clasts
together. The most important factor controlling the
varieties of clast found is the bedrock being eroded in The shapes of clasts in gravel and conglomerate are
the area. Gravel will be composed entirely of lime- determined by the fracture properties of the bedrock
stone clasts if the source area is made up only of they are derived from and the history of transport.