Page 36 - Sedimentology and Stratigraphy

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Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
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Nichols/Sedimentology
Nichols/Sedimentology
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9781405193795_4_0
9781405193795_4_0
Stratigraphy
Stratigraphy
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ARaju
ARaju
Name:
26.2.2009 8:14pm Compositor Name: ARaju
26.2.2009
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8:14pm
8:14pm
26.2.2009
Textures and Analysis of Terrigenous Clastic Sedimentary Rocks 23
therefore used for investigating the form of clay in areas that experience intermittent flows, such as
minerals and their relationship to other grains in a tidal environments (11.2).
rock. The distinction between clay minerals deposited
as detrital grains and those formed diagenetically
(18.3.1) within the sediment can be most readily 2.5 TEXTURES AND ANALYSIS
made using an SEM. OF TERRIGENOUS CLASTIC
An X-ray diffractometer (XRD) operates by firing SEDIMENTARY ROCKS
a beam of X-rays at a powder of a mineral or disag-
gregated clay and determining the angles at which The shapes of clasts, their degree of sorting and the
the radiation is diffracted by the crystal lattice. The proportions of clasts and matrix are all aspects of the
pattern of intensity of diffracted X-rays at different texture of the material. A number of terms are used in
angles is characteristic of particular minerals and the petrographic description of the texture of terrige-
can be used to identify the mineral(s) present. X-ray nous clastic sediments and sedimentary rocks.
diffractometer analysis is a relatively quick and easy Clasts and matrix The fragments that make up a
method of semi-quantitatively determining the sedimentary rock are called clasts. They may range in
mineral composition of fine-grained sediment. It is size from silt through sand to gravel (granules, peb-
also used to distinguish certain carbonate minerals
bles, cobbles and boulders). A distinction is usually
(3.1.1) that have very similar optical properties.
made between the clasts and the matrix, the latter
being finer-grained material that lies between the
clasts. There is no absolute size range for the matrix:
2.4.5 Clay particle properties
the matrix of a sandstone may be silt and clay-sized
material, whereas the matrix of a conglomerate may
The small size and platy shape of clay minerals means
be sand, silt or clay.
that they remain in suspension in quite weak fluid
flows and only settle out when the flow is very slug- Sorting Sorting is a description of the distribution of
gish or stationary. Clay particles are therefore present clast sizes present: a well-sorted sediment is composed
as suspended load in most currents of water and air of clasts that mainly fall in one class on the Went-
but are only deposited when the flow ceases. worth scale (e.g. medium sand); a poorly sorted
Once they come into contact with each other clay deposit contains a wide range of clast sizes. Sorting
particles tend to stick together, they are cohesive. is a function of the origin and transport history of
This cohesion can be considered to be partly due to the detritus. With increased transport distance or
a thin film of water between two small platy particles repeated agitation of a sediment, the different sizes
having a strong surface tension effect (in much the tend to become separated. A visual estimate of the
same way as two plates of glass can be held together sorting may be made by comparison with a chart
by a thin film of water between them), but it is also a (Fig. 2.15) or calculated from grain-size distribution
consequence of an electrostatic effect between clay data (2.5.1).
minerals charged due to incomplete bonds in the Clast roundness During sediment transport the
mineral structure. As a result of these cohesive prop- individual clasts will repeatedly come into contact
erties clay minerals in suspension tend to flocculate with each other and stationary objects: sharp edges
and form small aggregates of individual particles tend to be chipped off first, the abrasion smoothing
(Pejrup 2003). These flocculated groups have a the surface of the clast. A progressive rounding of the
greater settling velocity than individual clay particles
edges occurs with prolonged agitation of the sediment
and will be deposited out of suspension more rapidly.
and hence the roundness is a function of the transport
Flocculation is enhanced by saline water conditions
history of the material. Roundness is normally
and a change from fresh to saline water (e.g. at the
visually estimated (Fig. 2.16), but may also be calcu-
mouth of a delta or in an estuary: 12.3 & 13.6)
lated from the cross-sectional shape of a clast.
results in clay deposition due to flocculation. Once
clay particles are deposited the cohesion makes them Clast sphericity In describing individual clasts, the
resistant to remobilisation in a flow (4.2.4). This dimensions can be considered in terms of closeness to
allows deposition and preservation of fine sediment a sphere (Fig. 2.16). Discoid or needle-like clasts have