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and
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Stratigraphy
Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
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Terrigenous Clastic Sediments: Gravel, Sand and Mud
. 2 pebbles in a conglomerate can be used to indicate
the direction of the flow that deposited the gravel.
If a discoid clast is also elongate, the orientation of

# $ & + & $ 2 ++ tend to have their long axis oriented perpendicular
the longest axis can help to determine the mode of
deposition: clasts deposited by a flow of water will
%8
7 '$
to the flow, whereas glacially deposited clasts (7.3.3)
will have the long axis oriented parallel to the
ice flow.
2.3 SAND AND SANDSTONE
Sand grains are formed by the breakdown of pre-
existing rocks by weathering and erosion (6.4 &
( ! '
6.5), and from material that forms within the deposi-
2 2 ++ . 2 tional environment. The breakdown products fall

# + $ & & $ into two categories: detrital mineral grains, eroded
from pre-existing rocks, and sand-sized pieces of rock,
Fig. 2.8 The shape of clasts can be considered in terms of or lithic fragments. Grains that form within the
four end members, equant, rod, disc and blade. Equant and depositional environment are principally biogenic in
disc-shaped clasts are most common.
origin, that is, they are pieces of plant or animal,
but there are some which are formed by chemical
reactions.
Sand may be defined as a sediment consisting pri-
marily of grains in the size range 63 mmto 2 mm
and a sandstone is defined as a sedimentary rock
with grains of these sizes. This size range is divided
into five intervals: very fine, fine, medium, coarse
and very coarse (Fig. 2.2). It should be noted that
this nomenclature refers only to the size of the parti-
cles. Although many sandstones contain mainly
quartz grains, the term sandstone carries no implica-
tion about the amount of quartz present in the rock
and some sandstones contain no quartz at all.
Similarly, the term arenite, which is a sandstone
Fig. 2.9 A conglomerate bed showing imbrication of clasts with less than 15% matrix, does not imply any parti-
due to deposition in a current flowing from left to right.
cular clast composition. Along with the adjective
arenaceous to describe a rock as sandy, arenite has

its etymological roots in the Latin word for sand,
‘arena’, also used to describe a stadium with a sandy

floor.
2.3.1 Detrital mineral grains in sands
and sandstones
A very large number of different minerals may occur
Fig. 2.10 The relationship between imbrication and flow in sands and in sandstones, and only the most com-
direction as clasts settle in a stable orientation. mon are described here.