Page 80 - Sedimentology and Stratigraphy

Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
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26.2.2009 8:16pm Compositor Name: ARaju
Terminology for Sedimentary Structures and Beds 67
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Fig. 4.36 Terminology used for sets and
co-sets of cross-stratification.

In common with many other fields of geology, /
there is some variation in the use of the terminology
to describe bedforms and sedimentary structures. The
approach used here follows that of Collinson et al. %
(2006). Cross-stratification is any layering in a
sediment or sedimentary rock that is oriented at an
angle to the depositional horizontal. These inclined & #
strata most commonly form in sand and gravel by
the migration of bedforms and may be preserved if
there is net accumulation. If the bedform is a ripple
the resulting structure is referred to as cross-lamina-
tion. Ripples are limited in crest height to about
30 mm so cross-laminated beds do not exceed this $

thickness. Migration of dune bedforms produces
cross-bedding, which may be tens of centimetres to
tens of metres in thickness. Cross-stratification is
the more general term and is used for inclined strati- '
fication generated by processes other than the migrat-
ion of bedforms, for example the inclined surfaces
Fig. 4.37 Lenticular, wavy and flaser bedding in deposits
formed on the inner bank of a river by point-bar
that are mixtures of sand and mud.
migration (9.2.2). A single unit of cross-laminated,
cross-bedded or cross-stratified sediment is referred
to as a bed-set. Where a bed contains more than
one set of the same type of structure, the stack of sets
is called a co-set (Fig. 4.36). the sands due to either current or wave activity then
Mixtures of sand and mud occur in environments an array of sedimentary structures (Fig. 4.37) may
that experience variations in current or wave activity result depending on the proportions of mud and sand.
or sediment supply due to changing current strength Flaser bedding is characterised by isolated thin
or wave power. For example, tidal settings (11.2) dis- drapes of mud amongst the cross-laminae of a sand.
play regular changes in energy in different parts of the Lenticular bedding is composed of isolated ripples of
tidal cycle, allowing sand to be transported and depos- sand completely surrounded by mud, and intermedi-
ited at some stages and mud to be deposited from ate forms made up of approximately equal proportions
suspension at others. This may lead to simple alterna- of sand and mud are called wavy bedding (Reineck &
tions of layers of sand and mud but if ripples form in Singh 1980).