Page 39 - Sedimentology and Stratigraphy

Final
Proof
02
Final Proof page 26
Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
and
Stratigraphy
9781405193795_4_0
26.2.2009
26.2.2009 8:14pm Compositor Name: ARaju
page
26
Name:
ARaju
8:14pm
Compositor
Terrigenous Clastic Sediments: Gravel, Sand and Mud
26 Nichols/Sedimentology and Stratigraphy 9781405193795_4_0 02 Final Proof page 26 26.2.2009 8:14pm Compositor Name: ARaju
reworked from older sediment. For examples, a river 2.5.3 Maturity of terrigenous
transporting material eroded from an outcrop of older clastic material
sandstone formed in an aeolian environment will
deposit very well-sorted material. The grain-size dis- A terrigenous clastic sediment or sedimentary
tribution characteristics would indicate deposition by rock can be described as having a certain degree of
aeolian processes, but the more reliable field evidence maturity. This refers to the extent to which the
would better reflect the true environment of deposi- material has changed when compared with the start-
tion from sedimentary structures and facies associa- ing material of the bedrock it was derived from.
tions (5.6.3). Maturity can be measured in terms of texture and
Granulometric analysis provides quantitative infor- composition. Normally a compositionally mature
mation when a comparison of the character is sediment is also texturally mature but there are
required from sediments deposited within a known exceptions, for example on a beach around a volcanic
environment, such as a beach or along a river. It is island where only mineralogically unstable compo-
therefore most commonly used in the analysis and nents (basaltic rock and minerals) are available but
quantification of present-day processes of transport the texture reflects an environment where there has
and deposition. been prolonged movement and grain abrasion by the
action of waves and currents.
2.5.2 Clast-shape analysis
Textural maturity
Attempts have been made to relate the shape of The texture of sediment or sedimentary rock can be
pebbles to the processes of transport and deposition. used to indicate something about the erosion, trans-
Analysis is carried out by measuring the longest, port and depositional history. The determination of the
shortest and intermediate axes of a clast and calcu- textural maturity of a sediment or sedimentary rock
lating an index for its shape (approaching a sphere, can best be represented by a flow diagram (Fig. 2.18).
a disc or a rod: Fig. 2.8). Although there may be Using this scheme for assessing maturity, any sand-
some circumstances where clasts are sorted accord- stone that is classified as a wacke is considered to be
ing to their shape, the main control on the shape of texturally immature. Arenites can be subdivided on
a pebble is the shape of the material eroded from the the basis of the sorting and shape of the grains. If
bedrock in the source area. If a rock breaks up into sorting is moderate to poor the sediment is considered
cubes after transport the rounded clasts will be to be submature, whereas well-sorted or very well-
spherical, and if the bedrock is thinly bedded and sorted sands are considered mature if the individual
breaks up into slabs the resulting clasts will be grains are angular to subrounded and supermature if
discoid. No amount of rounding of the edge of a rounded to well-rounded. The textural classification of
clast will change its fundamental dimensions. the maturity is independent of composition of the
Clast-shape analysis is therefore most informative sands. An assessment of the textural maturity of a
about the character of the rocks in the source area sediment is most useful when comparing material
and provides little information about the deposi- derived from the same source as it may be expected
tional environment. that the maturity will increase as the amount of energy

4 (
" (

,
$ & !

C.*F G
H C2 * G9
H
E.*F G9 H E2 * G 5
H
Fig. 2.18 Flow diagram of the determi-
nation of the textural maturity of a terri-

genous clastic sediment or sedimentary
rock.