Page 9 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks

Introduction

Terrigenous clastic sediments arc made up of transported fragments Tabl e I. Grain-size clas sification of sediments
derived from the weathering of pre-existing igneous, sedimentary or Size in mm Class term Grain size terms for rock
metamorphic rocks. These rocks are classified initially according to of class
grain size, using the Udden-Wentworth scale (Table 1). boundary
It is those terrigenous sediments of intermediate grain size the
coarser siltstones, sandstones and finer conglomerates and breccias­ boulders
that arc most usefully studied using the petrographic microscope, since 256 rudite
the grain types can be identified by this means. The principal cobbles rudaceous rock
component grain types are quartz, f e ldspar and rock fragments. The 64 pebbles conglomerate
matrix of such sediments may be the fine-grained weathering products 4 brecci a
of the source rocks, such as clay minerals, or it may be a secondary granules
cement. 2
Clays and shales arc too fine-grained for study using the petro­ very coarse sand
graphic microscope and must be examined by electron microscopy or I
X-ray diffraction. The components of coarser conglomerates and coarse sand
breccias can usually be identified with the aid of only a hand lens. 0.5(!) medium sand arenite
The shape and roundness of the components of terrigenous clastic 0.25(.!) arenaceous rock
rocks arc important in describing sedimentary textures. Categories of fine sand sandstone
roundness for grains of high and low sphericity are shown in Fig. A. 0.125(A)
Sedimentary textures arc discussed on p. 24. very fine sand
0.0625(��)
coarse silt
o.o312Ul>
medium silt argillite
O.Ol56{.f4) siltstone argillaceous rock
fine silt mudstone
0.0078( 1 !s> mud rock
very fine silt shale
0.0039(l t6)
clay claystone

5. Well-rounded 1. Angular 0.
0
.
.
.
.
0

.

Fig. A Categories of roundness for grains of low and high s p hericity (after Pettijohn e t a!., 1973)

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