Page 18 - Sedimentology and Stratigraphy

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Stratigraphy
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9781405193795_4_0
9781405193795_4_0
Stratigraphy
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Compositor
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ARaju
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26.2.2009
5
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8:13pm
26.2.2009
26.2.2009 8:13pm Compositor Name: ARaju
Nichols/Sedimentology
Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 5
2
Terrigenous Clastic Sediments:
Gravel, Sand and Mud
Terrigenous clastic sediments and sedimentary rocks are composed of fragments that
result from the weathering and erosion of older rocks. They are classified according to the
sizes of clasts present and the composition of the material. Analysis of gravels and
conglomerates can be carried out in the field and can reveal where the material came
from and how it was transported. Sands and sandstones can also be described in the field,
but for a complete analysis examination under a petrographic microscope is required to
reveal the composition of individual grains and their relationships to each other. The finest
sediments, silt and clay, can only be fully analysed using scanning electron microscopes
and X-ray diffractometers. The proportions of different clast sizes and the textures of
terrigenous clastic sediments and sedimentary rocks can provide information about the
history of transport of the material and the environment of deposition.

2.1 CLASSIFICATION OF SEDIMENTS pre-existing rocks. The clasts are principally detritus
AND SEDIMENTARY ROCKS eroded from bedrock and are commonly made up
largely of silicate minerals: the terms detrital sedi-
A convenient division of all sedimentary rocks is ments and siliciclastic sediments are also used for
shown in Fig. 2.1. Like most classification schemes this material. Clasts range in size from clay particles
of natural processes and products it includes anoma- measured in microns, to boulders metres across.
lies (a deposit of chemically precipitated calcium Sandstones and conglomerates make up 20–25% of
carbonate would be classified as a limestone, not the sedimentary rocks in the stratigraphic record and
an evaporite) and arbitrary divisions (the definition mudrocks are 60% of the total.
of a limestone as a rock having more than 50%
Carbonates By definition, a limestone is any sedi-
calcium carbonate), but it serves as a general frame-
mentary rock containing over 50% calcium carbo-
work.
nate (CaCO 3 ). In the natural environment a
Terrigenous clastic material This is material that principal source of calcium carbonate is from the
is made up of particles or clasts derived from hard parts of organisms, mainly invertebrates such