Page 20 - Sedimentology and Stratigraphy

Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 7
02
Nichols/Sedimentology
Final
9781405193795_4_0
Nichols/Sedimentology
02
9781405193795_4_0
Final
Proof
Proof
Stratigraphy
and
and
Stratigraphy
page
Compositor
Compositor
8:13pm
Name:
ARaju
ARaju
Name:
8:13pm
7
7
page
26.2.2009
26.2.2009 8:13pm Compositor Name: ARaju
26.2.2009
Gravel and Conglomerate 7
$$ ! $ the natural distribution of sedimentary particles and in
a simple way it can be related to starting with a large
( block and repeatedly breaking it into two pieces.
)*+ ,-
Four basic divisions are recognised:
.)- ,/
clay (<4 mm)
+0 ,+ silt (4 mmto 63 mm)

gravel/aggregates (>2.0 mm)
1) ,* sand (63 mm or 0.063 mm to 2.0 mm)
.+ ,0 The phi scale is a numerical representation of the
- ,1 Wentworth Scale. The Greek letter ‘f’ (phi) is often
0 ,) used as the unit for this scale. Using the logarithm
4 base two, the grain size can be denoted on the phi
) ,. scale as
& 5
. 2 f ¼ log 2 (grain diameter in mm)

2 * . The negative is used because it is conventional to

2 )* ) represent grain sizes on a graph as decreasing from
left to right (2.5.1). Using this formula, a grain diam-
2 .)* 1 eter of 1 mm is 0f: increasing the grain size, 2 mm
& 5
2 2+1 0 is 1f, 4 mm is 2f, and so on; decreasing the grain

size, 0.5 mm is þ1f, 0.25 mm is 2f, etc.
2 21. *

2 2.*+ +

2.2 GRAVEL AND CONGLOMERATE
2 22/- /
& 5
2 2213 - Clasts over 2 mm in diameter are divided into gran-
5 ules, pebbles, cobbles and boulders (Fig. 2.2). Consol-
idated gravel is called conglomerate (Fig. 2.3) and
Fig. 2.2 The Udden–Wentworth grain-size scale for clastic when described will normally be named according to
sediments: the clast diameter in millimetres is used to define the dominant clast size: if most of the clasts are
the different sizes on the scale, and the phi values are between 64 mm and 256 mm in diameter the rock
log 2 of the grain diameter. would be called a cobble conglomerate. The term
breccia is commonly used for conglomerate made
up of clasts that are angular in shape (Fig. 2.4). In
2.1.2 The Udden–Wentworth
grain-size scale
Known generally as the Wentworth Scale, this is the
scheme in most widespread use for the classification of
aggregates particulate matter (Udden 1914; Went-
worth 1922). The divisions on the scale are made on
the basis of factors of two: for example, medium sand
grains are 0.25 to 0.5 mm in diameter, coarse sand
grains are 0.5 to 1.0mm, very coarse sand 1.0 to
2.0 mm, etc. It is therefore a logarithmic progression,
but a logarithm to the ‘base two’, as opposed to the ‘base
ten’ of the more common ‘log’ scales. This scale has
been chosen because these divisions appear to reflect Fig. 2.3 A conglomerate composed of well-rounded pebbles.